46. ADHESIVES............................................... 1 46.1 Purpose................................................... 1 46.2 Factors to Consider .......................................... 1 46.2.1 Possible Use of Non-Adhesive Treatments ..................... 1 46.2.2 Suitability of Potential Adhesive for Conservation Use ............ 2 46.2.3 Suitability of the Potential Adhesive to the Object ............... 3 46.2.4 Suitability of Potential Adhesive and its Working Characteristics to Proposed Treatment..................................... 3 46.3 Materials and Equipment...................................... 4 46J. 1 Vegetable Adhesives ..................................... 4 A. Starches........................................... 4 1. General Information .............................. 4 2. Wheat Starch Paste From Precipitated Starch - Western Style 3. Wheat Starch Paste From Fresh Starch - Japanese Style (Shin-nori)...................................... 8 4. Wheat Starch Paste - Japanese-Style, Aged (Furu-nori)...... 9 5. Rice Starch Paste................................. 10 6. Modified Starches................................. 11 7. Flour Pastes..................................... 12 B. Vegetable Gums ..................................... 12 1. General Information............................... 12 2. Gum Arabic..................................... 14 3. Gum Tragacanth ................................. 16 4. Agar .......................................... 17 5. Aigin or Sodium Alginate ........................... 19 6. Funori (Japanese Seaweed Adhesive)................... 20 C. Cellulose Derivatives.................................. 21 1. Cellulose Ethers - General Information ................. 21 2. Methyl Cellulose.................................. 25 3. Sodium Carboxymethyl Cellulose (CMC)................ 26 4. Hydroxypropyl Cellulose (HPC)....................... 28 5. Ethyl Hydroxyethyl Cellulose (EHEC) .................. 30 6. Hydroxyethyl Cellulose (HEC)........................ 31 7. Methyl Hydroxyethyl Cellulose (MHC).................. 32 8. Cellulose Esters - General Information ................. 33 9. Cellulose Acetate - General Information................. 34 10. Cellulose Nitrate - General Information................. 38 46.3.2 Proteinaceous Adhesives .................................. 42 A. Collagens.......................................... 42 1. General Information............................... 42 B. Caseins ...........................,............... 45 1. General Information............................... 45 46.3.3 Synthetic Polymer Adhesives ............................... 47 A. Poly Vinyl Acetate Solutions (PVA) ....................... 47 1. General Information............................... 47 B. Poly Vinyl Acetate Dispersions (PVA)...................... 51 1. General Information............................... 51 2. Jade 403 ....................................... 55 3. Jade 454 ....................................... 56 4. Archivart Reversible Adhesive A-1023 ................... 57 5. CM Bond, CM-1.................................. 57 6. CM Bond, CM-2.................................. 58 7. CM Bond, CM-3.................................. 59 8. CM Bond, CM-4____.............................. 59 9. Elvace 40-704 .................................... 60 10. Texicote VJC 555................................. 61 C. Poly Vinyl Alcohol Solutions (PVOH) ...................... 62 1. General Information............................... 62 D. Acrylic Resin Solutions................................ 64 L General Information............................... 64 2. Poly (ethyl methacrylate) poly (methyl acrylate) copolymer (PEMA/PMA) (Acryloid B-72)........................ 67 3. Poly (isobutyl methacrylate) (PiBMA) (Acryloid B-67) ...... 68 4. Poly (n-butyl methacrylate) (PBMA) (Acryloid F-10)........ 69 5. Poly (ethyl acrylate) Poly (methyl methacrylate) copolymer (PEA/PMMA) (Acryloid B-82)........................ 71 E. Acrylic Resin Dispersions .............................. 72 1. General Information............................... 72 2. Rhoplex AC-33................................... 75 3. Rhoplex AC-73................................... 76 4. Rhoplex AC-234................................. . 76 5. Rhoplex 495..........................----....... 77 6. Rhoplex N580.................................... 78 7. Rhoplex N619.................................... 78 8. Rhoplex N1031................................... 79 9. Plextol 360 ....................................... 79 10. Plextol 500 ...................................... 80 11. Plextol 498...................................... 80 12. Lascaux 360 HV (Plextol 360 Base).................... 81 13. Lascaux 498 HV (Plextol 498 Base)____................ 81 14. Texicryl ........................................ 82 F. Proprietary Formulations .............................. 82 1. BEVA 371 - General Information...................... 82 2. BEVA D-8................................... ■ • 84 3. Lamatec........................................ 85 4. Texicryl........................................ 87 5. Fusion 4000 - General Information .................... 88 6. Document Repair Tape - Filmoplast P.................. 89 7. Archival Document Repair Tape ...................... 90 8. Archival Framing Tape ............................. 91 9. Proprietary Resin Aerosol Sprays...................... 91 46.3.4 Heating Equipment ...................................... 92 46.3.5 Diluents.............................................. 92 46.3.6 Miscellaneous Equipment ................................. 92 46.3.7 Mixing/Straining.....................................• • 93 46.3.8 Application Equipment................................... 93 46.3.9 Storage.............................................. 93 46.4 Treatment Variations ......................................... 94 46.4.1 Vegetable Adhesives..................................... 94 A. Starches........................................... 94 1. Preparation ..................................... 94 2. Application...................................... 98 B. Vegetable Gums .................................. • • 99 1. Preparation ..................................... 99 2. Application...................................... 101 C. Cellulose Derivatives.................................. 101 1. Preparation ..................................... 101 2. Application...................................... 103 46.4.2 Proteinaceous Adhesives .................................. 105 A. Collagens......................................... 105 1. Preparation ..................................... 105 2. Application...................................... 105 B. Casein............................................ 106 46.4.3 Synthetic Polymer Adhesives............................... 106 A. Poly Vinyl Acetate Solutions ............................ 106 1. Preparation ..................................... 106 2. Application...................................... 106 B. Poly Vinyl Acetate Dispersions........................... 107 1. Preparation ..................................... 107 2. Application...................................... 107 C. Poly Vinyl Alcohol Solutions ............................ 108 1. Preparation ..................................... 108 2. Application...................................... 108 D. Acrylic Resin Solutions................................ 108 1. Preparation ..................................... 108 2. Application...................................... 108 E. Acrylic Resin Dispersions .............................. 109 F. Proprietary Formulations .............................. Ill 1. Preparation ..................................... HI 2. Application...................................... 112 46.5 Bibliography................................................ H3 46.5.1 General References...................................... 113 46.5.2 Starches.............................................. 116 46.5.3 Gums................................................ H7 46.5.4 Cellulose Derivatives..................................... 118 46.5.6 Proteinaceous Adhesives .................................. 120 46.5.6 Poly Vinyl Acetate Solutions ............................... 121 46.5.7 Poly Vinyl Acetate Dispersions.............................. 122 46.5.8 Poly Vinyl Alcohol Solutions ............................... 122 46.5.8 Acrylic Resin Solutions................................... 123 46.5.9 Acrylic Resin Dispersions ................................. 124 46.5.10Proprietary Formulations ................................. 124 46.6 Special Considerations ........................................ 126 46.6.1 Glossary of Selected Terms................................ 126 46.6.2 Adhesive Application Methods.............................. 127 46. Adhesives, page 1 46. ADHESIVES This outline considers adhesives used by the conservator and some of those encountered by the conservator during treatment. The advantages and disadvantages of a particular adhesive, and its preparation for use in conservation, are also discussed. Consideration is given to the suitability of the adhesive for the following uses: hinging, mending, filling, lining, fixing, consolidation, and sizing. Also considered is the unsuitability of specific stable adhesives for particular applications. Unsuitability of other adhesives for use on art and artifacts on paper is also discussed. Adhesive source and production, chemical and physical properties, and aging characteristics are discussed in section 46.3 Materials and Equipment. Section 46.4 Treatment Variations contains an overview of adhesives in current use, with information on preparation and application. 46.1 Purpose In paper conservation, adhesives are used to adhere reinforcing materials to damaged areas or readhere separated components of an object; to consolidate, fix, size, or provide binders, glazes, and varnishes. Adhesives are also used to construct archival housings for paper objects. (See 23. Consolidation/Fixing/Facing; 25. Mending; 26. Filling of Losses; 29. Lining; and 40. Matting and Framing.) 46.2 Factors to Consider 46.2.1 Possible Use of Non-Adhesive Treatments A. Because of adhesive-substrate interaction, the application of adhesive to a paper object cannot always be considered fully reversible. Also, limitations to practical reversibility of an adhesive can be imposed by the materials of an artwork and/or its condition. In these cases a non-adhesive alternative may be considered, for example placing an object with friable or water sensitive media in a special housing (e.g., rigid support) rather than lining it. B. When temporary housing is required (e.g., for display) non-adhesive methods such as photo corners or sling mats may be preferable to traditional hinging. These methods may also be used in cases when acceptable adhesives prove ineffective in bonding to the object. For example, wheat starch paste will not adhere to resin-coated photographic papers. 46. Adhesives, page 2 46.2.2 Suitability of Potential Adhesive for Conservation Use A. The adhesive selected should demonstrate long term stability and aging characteristics considered acceptable for conservation use. 1. Stability may be judged by natural aging for time-tested, traditional materials and by analysis and accelerated aging for newer materials. Current testing and analysis in the conservation or related technical fields should be consulted when possible. Data from accelerated aging tests should be carefully evaluated to determine validity and appropriateness in governing the choice of a specific adhesive. Whenever possible, aging of materials for use in paper conservation should be evaluated on a paper/adhesive testing system. Although testing programs carried out in another field of conservation (i.e., paintings conservation) may provide valuable information, accurate information for the assessment of adhesives applied to paper should be carried out on paper substrates. (PV) 2. Commercial preparations are recommended with caution because formulations can change without notice. Product literature may not fully disclose chemical composition or aging properties. 3. An adhesive may be available in several grades, with only the highest purity grades suitable for conservation use. 4. Additives in commercially-prepared adhesives may affect long term stability. For example, synthetic resin dispersions with added plasticizers can be unstable since the plasticizers can migrate to adjacent materials causing staining and leaving the adhesive inflexible. Dispersions which are internally plasticized through copolymerization are generally more stable. B. The selection of the potential adhesive should be guided by the principle of reversibility. Materials whose later removal may endanger the physical safety of the object should be avoided. Consideration should be given to whether the adhesive might prevent future treatments. The different levels of reversibility in conservation should be considered when applying adhesives to porous paper substrates. Reversibility may range from complete removeability of the adhesive to only swelling it in order to separate attached parts. Complete solubility of an adhesive layer, with potential penetration of the paper, may or may not be desirable. 46. Adhesives, page 3 46.2.3 Suitability of the Potential Adhesive to the Object A. The adhesive should bond well to the object's surface, yet not be so strong as to cause further damage to the object or so weak as to endanger the object. Shrinkage of the adhesive layer on drying should not cause planar distortions in the paper. B. Ideally, the adhesive and its method of application should not alter the appearance of the media or support. Whether aqueous or solvent-based the adhesive should not solubilize media, cause staining in paper, or alter media or paper color. Certain media cannot tolerate the pressure (i.e., pastel or charcoal) or heat (i.e., acrylics or colored pencil) required to attach some adhesives. C. The proximity of the adhesive to the object influences selection. Many adhesives are inappropriate for direct application to an object but are acceptable for constructing a housing (e.g., sink mat). 46.2.4 Suitability of Potential Adhesive and its Working Characteristics to Proposed Treatment A. The adhesive chosen should be appropriately modified considering treatment methods and materials. For example, an adhesive may require dilution for a particular lining process or modification to compensate a fill or size paper. B. The adhesive and its method of attachment should produce a bond whose strength is appropriate to the particular treatment. For example, a cellulose ether may be adequate for hinging a small object while wheat starch paste would generally be better for larger, heavier objects. C. The working and setting times should be appropriate for the proposed treatment. D. Chemical and physical properties of adhesives within a group (e.g., the PVAs) can vary and should be considered for a particular application. For example, higher viscosity materials will form stronger bonds but may require dilution or other methods to enhance penetration. Lower viscosity materials form weaker bonds but may be effective with several applications. 46. Adhesives, page 4 46.3 Materials and Equipment 46.3.1 Vegetable Adhesives A. Starches 1. General Information The adhesive qualities of vegetable starches were recognized in early history. The first recorded use of a starch adhesive dates from the first century A.D. in a description of papyrus manufacture by Pliny the Elder using a paste made from wheat flour. Starch adhesives are now used throughout the world in numerous industrial applications such as papermaking and textile manufacture. Western paper conservation use has been influenced by the Oriental scroll mounting tradition. a. Source Starch adhesives are derived from the roots and seeds of plants such as corn, potatoes, rice, and wheat. The last two are commonly used in conservation. The plant material is processed by a variety of means including treatment with acids, bases, enzymes, and oxidizers. These processes modify a starch's viscosity and "retrogradation" (i.e., stiffening). Depending on the starch type, and the processing method, a vast range of viscosities and adhesive strengths can be produced. b. Chemical and Physical Properties Starches are naturally occurring polymers of glucose. With the empirical formula of (C^^^n, where the exact value of n is unknown. Starch has a more intricate structure than cellulose because its molecules have two distinct areas: 75% has a branched amylopectin molecular structure and 25% has a linear amylose molecular structure. The exact percentages of amylose and amylopectin for each starch is largely responsible for its working properties. "Amylose and amylopectin have different properties, both as dry films and in solution. The highly regular linear structure of amylose allows it to dry from solution to form strong films...Amylopectin, being more amorphous, forms weak films" (Horie 1987, 135-136). Wheat starch contains 18-27% amylose while rice starch contains approximately 17-19% amylose. An AYTEX-P wheat starch representative stated that their wheat starch consistently has an amylose fraction of 25% and that American wheat starches have a consistent range from 23-25%. The 18-27% amylose range reflects world-wide variation. European or Japanese amylose ranges may be different from American wheat percentages.(KN) Vincent Daniels measured the percentage of amylose in aged Japanese 46. Adhesives, page 5 paste, reporting that at two years of aging the amylose is approximately 19%, at four years it is approximately 22%, and at fifteen years it is approximately 24% (Daniels 1988). During paste-making the amylose and amylopectin areas of the molecule behave very differently. The amylose fraction is responsible for the internal strength of a starch, many of its working properties, and for its degree of stiffening upon cooling. Thus, the amylose is responsible for gelatinization (Skeist 1973, 170). Identification: Amylose stains intensely blue in the presence of iodine. Amylopectin stains red to purple (Browning 1977). Wheat starch pastes stain blue/purple with iodine. Physical Form: Vegetable starches are white powders consisting of tiny granules that vary among starch types in form, size, range of size, and marking. Granule sizes range from less than 0.001 mm to 0.15 mm of diameter. The granules are crystalline. Preparation: Pastes for use in conservation are generally prepared by first soaking the starch in water and then cooking it in additional water. Longer cooking time, higher temperatures, and agitation promote the necessary bursting of the granules. Each starch has its characteristic gelatinization range which extends from approximately 55-80t (131-176°F) (Horie 1987, 136). Cooking technique, as well as origin of the starch, affect the characteristics of the resulting adhesive. (See 46.4.1 A. Treatment Variations.) Solubility: Starches do not form true solutions, but rather colloidal dispersions. Cooked starch paste is a mixture of greatly swollen granules, fragments of granules that have burst open, and dissolved starch. Starches swell in cold water and are partially dispersed in hot water. Starches are broken down with starch specific enzymes and are soluble in 2,1 n methyl pyrrolidone. pH: The pH of starches and starch products is not reliably measured by indicator papers, but should be tested with the pH meter. During commercial manufacture, pH is usually kept between 4.0 and 7.5. In the lab, the pH of either the starch-water slurry or the cooked paste can be adjusted easily (Clapp 1987, 145-149; TAPPI 1957, 26). Some conservators use alkaline water to prepare pastes that can serve a dual purpose of adhesion and assistance in alkalization/neutralization (e.g., lining). High pH (above 7.5) favors ready dispersal and slow 46. Adhesives, page 6 settling of the granules. However, above pH 7.5 discoloration may result when the paste film dries (TAPPI 1957, 26). Variations of pH among starch granules or uneven dispersion of any pH adjustor can negatively affect a paste's appearance and performance. Possible Additives: None. Health Hazards: No health hazards. However, as with all fine powders, a dust mask should be worn by those sensitive to airborne irritants. Storage/Shelf Life: Starch powder can be stored indefinitely if kept in an air-tight container in a cool place. Starch pastes are subject to fairly rapid biological attack within a few days of preparation. The deterioration can be slowed somewhat by the addition of a fungicide, but it is recommended that paste be made fresh on a weekly basis to avoid adhesive failure. Because fungicides can cause yellowing of paper over time some conservators avoid mixing a fungicide into their paste by attaching a fungicide-soaked cotton ball or blotter onto the storage container lid. Some conservators do not use any fungicide, but make fresh paste frequently. Others prefer to store their paste in a refrigerator; however, paste "should not be kept at the low temperature of a domestic refrigerator (4xy39.2°F) as it will become granular and lose adhesive qualities" (Paper Conservation News 1989). Oriental and some Western conservators store their paste under water which is changed daily.(KN) Any blending, stirring, or straining of a starch paste before storage may result in more mold spores being introduced into the paste, making it spoil faster. Aging Characteristics Reversibility: Pure starch adhesives remain indefinitely swellable in water and exhibit good reversibility. Starch adhesives of unknown quality found on objects being treated might require starch specific enzymes for their removal. Reversibility may be difficult with thick paste layers; enzymes or mechanical removal may be necessary. "Amylose has been shown to degrade by photo-oxidation and hydrolysis reactions on exposure to ultraviolet, resulting in breaking of the chain and production of organic acids" (Horie 1987, 137). pH: 46. Adhesives, page Appearance: Good quality starch adhesives should not undergo any color change after aging. As encountered by conservators on previously treated objects, appearances can range from invisible to continuous coatings of slightly gray or yellow translucence to crumbly, opaque surfaces in tones ranging from white to gray to yellow/tan. Relative Strength: Some conservators feel that dried starch paste films gradually become brittle. Biological Attack: Starch adhesives are also subject to attack by insects, rodents, and enzymes. Wheat Starch Paste From Precipitated Starch - Western Style This is the primary adhesive for Western paper conservators and the standard against which other adhesives are judged. It is used in numerous applications such as hinging, mending, lining, facing, reinforcement, and consolidation or fixing of media. Wheat starch paste can be very strong, yet at the same time it can be modified and manipulated for very delicate applications. When diluted for delicate work, a well-made paste will not undergo a sudden loss of viscosity, but a gradual and continuous change. This allows a great number of adhesive strengths from one material. a. Source Starches are separated from flour in a wet partitioning step and then dried in the following manner. Wheat flour is kneaded with water producing a stiff mass in which the starch is trapped. After slight aging to allow the gluten and starch to separate from each other, the starch granules are washed out with water. Extraneous fibrous material is caught by a sieve as the starch-water slurry passes through. The starch is concentrated from the slurry by centrifuge and dried. The most commonly used wheat starch in America is Aytex-P which is manufactured by Henkel (formerly by General Mills). It is distributed by several companies. Sources for precipitated starches from Japan include; Harada (Kisa & Co., Ltd.) and Nakamura Co. (available from Conservation Materials, Reno, NV, called Zin Shofu). b. Chemical and Physical Properties Identification: Wheat starch has more small granules than large, with 70% less than 50 microns in diameter. The granules are spherical and saucer-shaped. 46. Adhesives, page 8 Physical Form: Wheat starch is available as a fine, white powder. pH: Varies with method of manufacture (TAPPI 1957). Possible Additives: c. Aging Characteristics (See 46.3.1 A. General Information.) Reversibility: Wheat starch has a higher linear fraction than rice starch and therefore exhibits greater retrogration than rice starch paste (TAPPI 1957, 65-70; Whistler 1965, 350-353, Vol. 2). Wheat Starch Paste From Fresh Starch ■ Japanese Style (Shin-nori) This is the primary adhesive for Japanese conservators. In addition to the functions which parallel those in which Western conservators use wheat starch paste, this adhesive is used for wood to wood bonds in scroll mounting. This paste is felt by its advocates to have a degree of viscosity not matched by paste made from dried starch. (See Wills 1984.) a. Source Japanese-style paste is made from a starch which has been freshly separated from flour in the process of gluten manufacture. This separation process is essentially like that used in the West, except that the starch is not concentrated by centrifuge and dried after separation. Instead, the starch-water slurry is poured into a vat where it is allowed to settle into three distinct layers. The bottom layer is nearly pure wheat starch. Freshly produced starch may be difficult to find in the U.S. b. Chemical and Physical Properties Physical Form: Maintained as a starch-water slurry. Preparation: Paste-making from wet starch is similar to the process used in the West, with individual preferences for, and modifications of, cooking temperature time and degree of agitation. The following modifications sometimes made in traditional Japanese methods are noted but not necessarily 46. Adhesives, page 9 recommended. To reduce viscosity the paste is mixed with aged paste; seaweed gelatin is added to increase elasticity; persimmon extract is added to increase strength and water resistance, and presumably, resistance to bacterial, fungicidal, and vermin attack. Storage/Shelf Life: The undried starch is stored in a cool, dark place with regular changes of its protective cap of water until needed for paste-making. If properly kept, the paste may be stored indefinitely. c. Aging Characteristics Wheat Starch Paste - Japanese-Style, Aged (Furu-nori) Compared to freshly-made paste, this adhesive is reputed to be weaker and more flexible. It is less prone to cause planar distortions in paper supports. Typically used at least eight years after its preparation, it imparts flexibility in multiple lining layers where it is used for secondary and tertiary backings, as well as in scroll linings. This paste is not generally used in the U.S. a. Source The paste is prepared from fresh wheat starch and is made in the coldest months of the year in order that it mature successfully. b. Chemical and Physical Properties Physical Form: Some authors describe properly made aged paste as snow-white, others as pale-beige. The paste is an opaque solid with a crumbly, almost dry texture. It is less viscous than its freshly-made equivalent. Preparation: Specific methods of cooking and aging vary from workshop to workshop. One method is as follows. Several batches of freshly prepared, then cooled, paste are put into a thick-walled ceramic jar. A layer of water is added to cover the paste, an air space is left, and finally the jar is covered to prevent evaporation of the water layer. The jar is stored in a cool, dark place for eight to ten years or longer. Once a year, on an extremely cold winter day, the water layer is poured off and any mold is removed. Fresh water is added and the jar is resealed. For the paste to age properly, various organisms (a tick and several types of fungi) must develop and die in a certain sequence within the top layer of the paste. 46. Adhesives, page 10 A method for making "artificially aged paste" is described by G. Van Steene and L. Masschelein Kleiner 1980, 64-70. Storage/Shelf Life: See Preparation, above. c. Aging Characteristics Vincent Daniels studied the strength of adhesion between new and aged pastes and found no differences (Daniels 1988, 9). However, in Japanese traditional practice, aged paste is considered weaker and more flexible. Rice Starch Paste This is generally considered to be a weaker adhesive than wheat starch paste. However, it is uncertain what the amylose content is for American and European rice starches. Differing amylose percentages and individual working habits of conservators may contribute to contradictory statements regarding properties of rice versus wheat starches. Possible uses are in situations where wheat starch paste would be too strong. Some conservators believe that rice starch paste is not as likely to cause a grayish haze or stain when it dries. a. Source The starch granules are separated from flour or kernels by chemical softening and steeping and then further processed by dewatering and drying, similar to wheat starch processing. b. Chemical and Physical Properties Rice starch has less retrogradation than wheat starch paste (TAPPI 1957, 79). Identification: The granules are polygonal in shape and are the smallest of any common starch, between 4 and 8 microns in diameter. Some conservators believe that this property makes rice starch paste smoother than wheat starch paste. Physical Form: Available as a white powder. Preparation: Generally prepared by soaking the dry powder in water, followed by cooking in additional water. The gelatinization temperature is usually somewhat higher than that of other starches, (about 68-78^) (154-172°F). pH: Usually about 8 since most commercial preparations use alkaline steeping. 46. Adhesives, page 11 Solubility: Some conservators feel rice starch adhesives swell and release sooner than wheat starch. This property can be utilized in mending with wheat starch paste followed by lining with rice starch. This could allow the lining to be applied and possibly removed without disturbing the tear repairs. Storage/Shelf Life: Waxy or glutenous rice starch has great stability against water separation from the paste when stored cold. c. Aging Characteristics Modified Starches (To be expanded.) (The following is from Kirby 1965.) Dextrins are modified starches whose molecular structure has been changed through the use of heat, acid, alkali, or other catalytic conversions. Dextrins have been widely used for stamps, labels, and paper tapes, where the adhesive is moistened for application. a. Source Depending on the manufacturing process used, hydrolytic scission at either the 1-4 or 1-6 glucosidic links is responsible for the molecular modifications of the parent starch. Dextrins have been used as adhesives since the early nineteenth century. The earliest patent was issued in 1867. Starch was spread on iron pans and moistened with a dilute hydrochloric-nitric acid solution. After heating it was dried and used as a gum. Dextrins are often mixed with animal glue, gum arabic, or gum tragacanth. Frequently, blends of different dextrins are used and borax is a common additive to increase tack. There are three major types of dextrins: white, yellow, and British gums. 1) White dextrins are prepared by roasting at 107.2^ (225°F) in the presence of acid. These dextrins are then neutralized with some alkaline material such as ammonia. They are used in 50% concentrations. The color is white. 2) Yellow (or canary) dextrins are prepared by roasting starch with acidic catalysts at high temperature. Colors vary from light yellow to dark brown. Suitable concentrations are between 50-60%. 3) British gums are prepared by roasting starch up to 148.8^ (300°F) without using acid. These dextrins are usually dark colored and exhibit high solubility in warm water. They are used in concentrations of 10-35%. 46. Adhesives, page 12 b. Chemical and Physical Properties Generally, dextrins are much more soluble in water than the source starch because processing has lowered the molecular weight. Dextrins also have a lower viscosity for an equal concentration as compared to starch. Dextrin properties are based on their method of preparation and the parent starch. Possible Additives: Borax can be added to increase tack, rate of bonding, and to minimize wetting. Urea formaldehyde resin is used 5-15% for water resistance coatings. c. Aging Characteristics 7. Flour Pastes (To be expanded.) These are encountered in former linings as historical adhesives. Contain brown chaff and particles. Not currently recommended for use. Vegetable Gums 1. General Information Gums are relatively weak adhesives; however, they function well as protective colloids, preventing agglomeration and settling of finely-divided particles. Because of this property, they have been used, probably since ancient times, as binders in painting media. Gums have been used by watercolorists and miniaturists to saturate and intensify colors, especially to create modelling. Gums were also used to saturate dark areas in prints, particularly lithographs. Gums have a wide variety of commercial uses, particularly in the food and drug industries. Some gums (e.g., gum arabic) are used in adhesive formulations for postage stamps, labels, and envelopes. Gums from fruit trees (e.g., cherry, apricot, and plum) have been used as binding media, glazes, and varnishes on ancient, traditional, and folk objects. Mucilages, related to gums, are plant materials extracted from seeds, roots, and other parts of plants. The term "mucilage" is broadly used, however, to describe general-purpose paper adhesives prepared by cooking gums in water with odorants and preservatives (Davidson 1980, 8-13). Gums have not been widely used in conservation treatments. a. Source Gums occur either as the natural exudants of particular trees and shrubs or as algae. In trees they are produced in response to a "wound" in an effort to seal that wound from microorganism attack. The actual mechanism of gum production 46. Adhesives, page 13 within a tree is not fully understood. In cultivated trees, the bark is incised to stimulate gum production and then it is periodically "tapped." The chief uses of gums are as protective colloids and emulsifying agents in the food industry. Chemical and Physical Properties Gums are complex polysaccharides. Polysaccharides are carbohydrates made of chains of monosaccharide units. A monosaccharide is a sugar unit classed by the number of carbon molecules it contains. Gums are non-crystalline, amorphous colloids. They are readily distinguishable from proteins because they do not have the nitrogen-containing peptide linkages which characterize the latter. In historical literature, gums have often been erroneously referred to as resins. They are readily distinguished from natural resins by their solubility characteristics: resins are typically soluble in organic solvents while gums are typically water soluble. It is worth noting that aqueous solutions made from different gums are not always miscible due to their different chemistries. Identification: Identification tests applicable to individual gums are of limited value in examining paper or media because the small amount of gum present makes obtaining a sample difficult. For qualitative tests to differentiate gums see Glicksman 1969, 530 and Browning 1969, 252. When heated, a gum decomposes completely without melting and is usually charred. Physical Form: Gums are available as "tears" (roupded lumps), flakes, or powders. Finer grades of gum (often collected from cultivated trees) are colorless, partially due to bleaching by the sun. The colors of crude grades range from yellow to brown. Gums are odorless. Preparation: There are various methods of gum solution preparation, all of which involve dissolution of the gum in water. Some preparations recommend initial swelling of the gum in cold water or in alcohol/water. Solubility: Gums appear to dissolve in water, but actually swell and disperse. They are insoluble in organic solvents. There are three solubility types: soluble in water, forming a transparent solution; partially soluble in water; and insoluble in water, forming a gel and possibly a very thick, transparent solution. Good grades leave no residue when dispersed in water. Once dried, they generally disperse again in water. 46. Adhesives, page 14 Health Hazards: Gums are non-toxic and non-flammable. Storage/Shelf Life: Solutions are subject to microbial attack. c. Aging Characteristics Reversibility: Conservators have found that gums used as binders or glazes often remain water-soluble. Some dried gum-based paint films have been observed to become embrittled and cracked. Relative Strength: Gums are considered weak adhesives. Gum films can, however, be more flexible than starch films. Biological Attack: Gums are subject to microbial attack. Gum Arabic a. Source Gum arabic, traditionally the most highly recommended binder in watercolor paints, is the natural exudate of the acacia tree. There are over five hundred species of acacia trees. The exudations are collected, graded by color and size, cleaned, sifted, and often bleached. Gum arable's adhesive properties and its ability to prevent settling of finely ground pigments make it ideal as a watercolor medium. Its shiny, glassy appearance in thick films has been used for modelling by miniature painters. The best gum is said to come from Acacia Senegal. It should be noted that the gum from Acacia arabia is of inferior quality and is rarely used for artist's materials. Gum arable's name seems to be related to early traders rather than its source. b. Chemical and Physical Properties Gum arabic is the slightly acidic salt of a complex polysaccharide. It is the calcium salt of arabic acid. Structurally, gum arabic can be conceived of as a long chain incorporating short, stiff spirals with numerous side branches. The structural features of the gum are: a fairly hydrolysis resistant core and various groups on the periphery of the molecule which are unstable to acids (Whistler 1973). The molecular weight ranges from about 240,000-580,000. Gum arable's moisture content is usually 13-15% (Gettens and Stout 1966, 29). Gum arabic lowers the surface tension of water. Identification: See 46.3.1 B. General Information. 46. Adhesives, page 15 Physical Form: Gum arabic comes in "tears", thin flakes, granules, or powder. It is white to amber in color. The color of the dry form cannot be used to predict the color of the resulting solution. Preparation: Gum arabic is not widely used in conservation. Throughout history, artist's manuals describe a variety of gum solution recipes for use as a painting medium. They involve dissolution of gum "tears" in water by agitation at room temperature, agitation at slightly elevated temperatures, and immersion in rapidly boiling water. Humectants such as ox-gall, honey, sugar, and glycerine were often added to retain moisture in the dried films. One ounce of gum to one quart of water will yield a gum solution appropriate for use as a binder (Dossie 1764). Solubility: Gum arabic is insoluble in oils and most organic solvents. It slowly disperses in glycerine and dissolves in water. It is one of the most water soluble of gums, able to form solutions of greater than 50% concentration. Solubility characteristics of a particular gum will depend on the age of the tree, the amount of rainfall in the region where it is collected, time of exudation, and conditions of storage. Gum arabic is incompatible with gelatin and trivalent metal ions. pH: The pH of aqueous gum arabic is generally acidic with wide variations among samples. Variations can be attributed to the source of the gum and the method of solution preparation. Viscosity is pH dependent with a maximum at pH 7, although high viscosity can be retained over a wide pH range. Possible Additives: See Preparation, above. Added dextrins can be detected with iodine (Gettens and Stout 1966, 28). Health Hazards: Gum arabic is non-toxic and non-flammable. Storage/Shelf Life: In commercial use, the gum is often packed in polyethylene-lined bags or drums. A cool, dry environment is recommended to avoid lumping. Solutions are subject to microbial attack. Aging Characteristics Reversibility: Gum arabic films become most readily water-insoluble under conditions of dry heat aging. Extensive exposure to light alone does not appear to induce such insolubility. The cross-linking believed to cause the water-insolubility is favored 46. Adhesives, page 16 under acidic conditions and is likely related to the amount of heat applied during the gum solution preparation.(JS) Gum arable solutions dried at temperatures of llO*^ (230^) have been observed to yield insoluble gum (Whistler 1973). Appearance: Water-insoluble gum arabic films discolor to amber or light brown. They often become embrittled. Relative Strength: 3. Gum Tragacanth This gum has been primarily used as a binder for pastels. a. Source Gum tragacanth is extracted from any of the thousands of species of leguminous shrubs belonging to the genus, Astragalus. The exudations are collected from incisions made at the roots or in the bark of the shrub, those from the roots being of higher quality. These exudations seem to result from a transformation of pith cells and not from a plant secretion (as with other gums). The binding strength of gum tragacanth is about eight to ten times that of gum arabic; gum tragacanth is also less brittle. b. Chemical and Physical Properties Gum tragacanth is the slightly acidic salt of a complex mixture of polysaccharides. It is generally believed that it is composed of a water-soluble component called tragacanthin and a water-swellable major component called bassorin (60-70%) along with small amounts of starch and cellulose (Davidson 1980, 11-3). It forms more viscous solutions at lower concentrations than gum arabic. This may be explained by its larger molecular weight (840,000) and its more elongated shape (Masschelein-Kleiner 1985, 59). The moisture content ranges from 12-15%. Specific gravity varies from 1.25-1.384. Identification: Under the microscope, gum tragacanth in water exhibits angular fragments with circular or irregular lamellae and no fragments of lignified vegetable tissue. Under infrared analysis, gum tragacanth exhibits a strong carbonyl absorption at 5.75 /im. An 0.5% gum tragacanth solution forms a yellow, stringy precipitate in 10% potassium hydroxide (Davidson 1980, 11-29). Physical Form: Gum tragacanth comes in the form of coarse crystals, powder, ribbons, flakes, and in solution. Color ranges from white to yellow. Gum tragacanth is more opaque than gum arabic; it has less luster and it is not as glassy or brittle. 46. Adhesives, page 17 Preparation: The gum is first wet with alcohol and then with water. After several hours of swelling, the gelled gum may be shaken with more water. A 2-3% solution will be thick, but can then strained through cheesecloth (Gettens and Stout 1966, 28). Other methods involve mixing a dry blend of the gum into the vortex of an aqueous system. Because gum tragacanth is hydrophilic, care must be taken in mixing solutions to avoid lumping (Davidson 1980, 11-5). Solubility: The gum disperses in water and is insoluble in alcohol. Solutions of uniform consistency are difficult to obtain. Solutions of greater than 0.5% in water form gels. Solution viscosity is pH-dependent with a maximum initial viscosity at pH 8. Gum tragacanth solutions become thin at elevated temperatures. Upon cooling, however, viscosity is regained indicating that heat does not seem to degrade the polymeric structure (Davidson 1980, 11-9). pH: Gum tragacanth is slightly acidic. Decreasing the solution pH has less effect on gum tragacanth's initial viscosity than on other gum solutions. Gum tragacanth is noted for its stability in acids. It has a maximum stable viscosity at pH 5. Possible Additives: Because the pure product is very expensive, it is sometimes adulterated with lesser quality gums and whitened with lead carbonate. Health Hazards: Gum tragacanth is non-toxic and nonflammable. Storage/Shelf Life: Solutions of gum tragacanth have longer shelf lives than other gums. There is virtually no loss of viscosity or microbial growth (Davidson 1980, 11-13). c. Aging Characteristics Agar Agar or agar-bearing algae can be purified to isolate the hydrocolloid agarose, which has been popularized by Richard Wolbers as a gel medium in which enzymes can be suspended for poulticing procedures. Agar is widely used as a microbiological medium and commercially in the food and pharmaceutical industries. 46. Adhesives, page 18 Source Agar, a seaweed colloid, is extracted from marine algae of the class Rhodophyceae from one of two species: gelidium or gracilaria. It is typically manufactured by hot water extraction followed by freezing and filtration. One brand used by conservators for poulticing is Sigma Type VII (Sigma). Chemical and Physical Properties This polysaccharide (a hydrophilic colloid containing sulfur, sodium, and calcium) can absorb up to twenty times its weight in cold water with swelling (Hawley 1977, 20; Horie 1987, 142). It is a mixture of at least two polysaccharides, agarose being one. The viscosity of a given percentage concentration of agar and agaroid dispersions will be dependent on the raw materials and processing conditions. A 1.5% weight solution congeals between 32-39^ (89.6-102.2°F) to a firm, resilient gel which will not melt again below 85^ (185°F); this behavior distinguishes it from other seaweed colloids (Davidson 1980, 7-2). Identification: See 46.3.1 B. General Information. Physical Form: Agar is available in thin membranous pieces or granulated and powdered forms. Its color is white to light yellow. It is either odorless or slightly mucilaginous in smell. Preparation: Agar is usually prepared in boiling water. It is recommended that the agar be allowed to swell in cool water first to prevent scorching. Solubility: Many agars are insoluble in cold water and yet readily dissolve in boiling water. They are insoluble in alcohol. pH: Softening Point/Glass Transition Temperature (TJ: Agar and agaroid gels melt in the range of 60-97^ (140-206.6°F) when there is 1.5% solids. Gelation can occur at concentrations as low as 0.4% (Davidson 1980, 7-5). Possible Additives: Health Hazards: Agar is non-toxic and non-flammable. Storage/Shelf Life: In industry it is stored in polyethylene-lined fiber drums, between 20-25^ (68-77°F). Gel strength decreases with time in warm temperatures. 46. Adhesives, page 19 c. Aging Characteristics 5. Algin or Sodium Alginate a. Source Alginates are extracted from several species of kelp: Macrocystis pyrifera, L. digitata, or L. saccharina (Windholz 1976, 228). They are used industrially as surface sizings and coatings for paper and as food additives. In conservation, they are added to paste or used as suspension media for colorants. b. Chemical and Physical Properties The alginates, usually the sodium salt of alginic acid, are a carbohydrate polymer of anhydromannuronic acid (Browning 1977, 277). Identification: Alginates are extracted from paper in alkaline solution, neutralized with 1% hydrochloric acid, and precipitated as alginic acid by adding one drop of sulfuric acid. An alginate reagent solution of saturated Fe^SO^ will cause a purple-red to brown-black color (Browning 1977, 277). Physical Form: Sodium alginate is a colorless to yellow filamentous or granular powder or solid. Preparation: The sodium salt is dissolved in water. Solubility: The sodium salt dissolves in water to form a viscous colloidal sol (a liquid colloidal dispersion). It is not soluble in alcohol, chloroform, or ether, nor in water/alcohol mixtures of greater than 30% wt./wt. alcohol (Sax 1984, 2407; Windholz 1976, 34). Acidic solutions of less than pH 3 or divalent metal ions produce a precipitate or a gel (Windholz 1976, 34; Browning 1977, 277). Health Hazards: While sodium alginate is used as a food additive, it is also listed on the EPA TSCA (Toxic Substance Control Act) Inventory of 1980. It is a hazard taken intravenously or intraperitoneally, though these routes seem unlikely in conservation use! Storage/Shelf Life: Keep sodium alginate solutions cool to prolong shelf life.(FP) c. Aging Characteristics 46. Adhesives, page 20 Funori (Japanese Seaweed Adhesive) Funori has been used in Japan since 1673 as an adhesive and sizing material (Chapman 1970, 146). Japanese scroll mounters use funori to attach facing papers to paintings and to consolidate flaking paints. a. Source Extracted from marine algae, red seaweed species of the Gloiopeltis family: Gloiopeltis tenax, G. complanata, and G. furcata (Winter 1984, 119; Masuda 1984, 128). Seaweed is gathered, rinsed, and cleaned, then pressed and dried into sheets composed of interlocking yellow-brown strands. This seaweed is gathered in Japan and China, where it is called "Halio" (Chapman 1970, 147). One U.S. source is Aiko's Art Materials (714 N. Wabash Ave., Chicago, IL 60611), which will special order the seaweed from Japan. b. Chemical and Physical Properties The mucilage extract is called funoran. It is a polysaccharide based on galactose units with a high proportion of sulfate groups, distinguishing it from agar (Winter 1984, 120). Unlike agar, it does not gel on cooling (Horie 1987, 142). Physical Form: See Source, above. Preparation: The mucilage is extracted by cooking sheets in water and straining (Koyano 1979, 31). Solubility: Soluble in water. Storage/Shelf Life: Funori in solution keeps under refrigeration for two to three months. Eventually it grows mold. Dried funori keeps indefinitely in a dry place.(FM) Funori solutions spoil rapidly in the summer (Masuda 1984, 128). c. Aging Characteristics Reversibility: Relative Strength: Funori adhesive is less contractile than paste and adheres better to tea ceremony walls (Masuda 1984, 128). 46. Adhesives, page 21 Cellulose Derivatives 1. Cellulose Ethers - General Information In 1912, a process was patented in Germany for reacting cellulose with dimethyl-sulfate in the presence of bases to produce a water soluble cellulose. In 1927, the first industrial production of methyl cellulose began in Germany using gaseous methyl chloride to etherify cellulose. Several years later, commercial production of sodium carboxymethyl cellulose began world-wide (Kennedy et al. 1985, 274). Since then, the processes have been modified and production of a variety of cellulose ethers has expanded to millions of pounds yearly. Commercially, cellulose ethers are used as thickeners, anti-redeposition agents, and protective colloids for liquids in the food, paint, adhesive, and oil-well drilling industries. In paper conservation, cellulose ethers have been used alone or with starch pastes for lining, hinging, and mending. Their moisture holding, surfactant, and anti-redeposition properties may be used to advantage as poultices for removing stains, old adhesives, and other accretions. Dilute solutions have been used for sizing or resizing paper. Cellulose ethers have also been used for consolidating flaking or friable media and as a binder for cellulose powder fills. Because some cellulose ethers become less soluble as water temperature increases, they have been used as facing materials during warm aqueous treatments. a. Source Cellulose ethers are made using glucose from wood pulp or cotton linters that have been prepared by swelling and decrystallizing with sodium hydroxide. The "alkali" cellulose then undergoes methylation or etherification to partially substitute the hydroxyl groups on the anhydroglucose ring with alkyl or hydroxyalkyl groups, such as methyl, ethyl, sodium carboxymethyl, hydroxyethyl, or hydroxypropyl. The product is neutralized with acids and the cellulose ether is isolated and purified by extraction of salts and byproducts. The cellulose ether may be varied further by compounding and surface cross-linking to facilitate dispersion. The product is then dried, milled, and sifted (Kennedy et al. 1985, 276). 46. Adhesives, page 22 CH20® OH DS - 2 / 0 OH y o \ o® ch:o® Degree of Substitution on Anhydroglucose Units. MS-2 CH20(s)0(b)0H oQ)0(g)OH OH CH2OH Molecular Substitution and Degree of Substitution on Anhydroglucose Units. b. Chemical and Physical Properties ' Each anhydroglucose ring has three hydroxyl groups which may be substituted. The degree of substitution (DS) is therefore three or less. During synthesis, reagent concentration, temperature and duration are factors which control DS. In general, low degrees of substitution produce rather brittle materials while increasing the amount of substitution increases the plasticity of the cellulose ether (Horie 1987, 126). Increasing molecular weight and thixotropy can be achieved when alkyl oxides are used as reactants and side-branching on the hydroxyl group of the new alkyl substituent occurs. The total amount of ether functions substituted by side-branching is referred to as the Molar Degree of Substitution (MS). The basic structures for DS and MS are diagrammed above, with R representing the alkyl or hydroxyalkyl substitution (Kennedy et al. 1985, 275). Viscosity of the final product is determined by the extent of pretreatment of the cellulose raw material and by subsequent oxidation of the finished product to the desired molecular weight. Pretreatment may include chemical additives, grinding, heating, or oxidizing. These decrystallization processes improve accessibility of the cellulose to the reactant and therefore yield a higher DS but result in a reduced end product viscosity (Nicholson 1985, 364). Properties such as pH, refractive index, and gel temperature vary depending on DS, concentration, and the distribution of substituents. Recent analysis conducted by 46. Adhesives, page 23 Gelman for Hercules Incorporated suggests that materials of the same molecular weight and DS/MS can have vastly different performance and that there was no analytical method to ascertain the exact distribution of substituents or predict behavior. Moreover, distribution cannot be controlled during synthesis (Gelman 1985, 296, 299-300). Identification: "A small portion of solid sample is placed in a test tube with benzene (0.5 ml) and 93% sulfuric acid (1 ml), and the tube is warmed carefully in a water bath until an intense yellow color develops and then rapidly turns reddish. The tube is cooled and a layer of alcohol (0.5% ml) is added without stirring. A blue or green ring between the two phases indicates hydroxyethyl or carboxymethyl cellulose; ethyl cellulose gives a violet ring" (Browning 1969, 254). Also see individual ethers. Physical Form: Cellulose ethers are available as fine to granular powders which range in color from white to yellow. Each cellulose ether is available in several grades of purity and in a range of types, varying in viscosity, particle size, and thixotropy. Preparation: Most cellulose ethers used in conservation are prepared by first dispersing the granules in cold or hot water, stirring continuously for a required amount of time, and then allowing the solution to gel. Solubility: The DS and the uniformity of distribution of substituents along the chain influence the extent to which polar solvents may be added to a cellulose ether initially solubilized in water (Nicholson 1985, 364). Increasing substitution increases the solubility of the cellulose ether in organic solvents (Horie 1987, 126). Cellulose ethers are generally soluble in cold water. Sodium carboxymethyl cellulose and hydroxyethyl cellulose are also soluble in hot water. Only hydroxypropyl, ethylhydroxy ethyl cellulose, and ethyl cellulose are initially soluble in polar organic solvents. Possible Additives: Anti-oxidants or plasticizers (e.g., glycerine) are possible additives. See various product literature under headings such as: Chemical Degradation, Plasticity, and Compatibilities. Health Hazards: Cellulose ethers are non-toxic and non-sensitizing (see product literature). However, breathing the powder should be avoided. 46. Adhesives, page 24 Storage/Shelf Life: Protect from oxygen, light, heat, microorganisms, moisture, and extremes of pH. All cellulose ethers are susceptible to oxidative chain breaking, both in storage and in situ, especially with light exposure (Horie 1987, 126-127). Sodium CMC and hydroxalkyl ethers degrade faster than the alkyl celluloses (Horie 1987, 126). Because they are strongly hydrophilic, they should be stored tightly sealed in a dry environment. Empirically, Cellofas B3500 has been found to degrade in solution through acid hydrolysis; however, Cellulose Gum 7H or 7HSP, a more pure product, will not degrade when prepared with deionized water.(CB) Microbiological deterioration reduces viscosity (Hercules, Inc.). It is not advisable to add preservatives known to cause discoloration of paper to cellulose ethers which are intended for permanent contact with works of art. Aging Characteristics Like cellulose, all cellulose ethers will suffer from chain breaking through oxidation. This oxidation is enhanced by light exposure. The extent to which degradation occurs varies widely among the many types of cellulose ethers. Wilt and Feller determined that different types of cellulose ethers underwent dramatically different rates of degradation in heat aging. Sodium carboxymethyl cellulose and methyl cellulose proved to be the most stable followed by ethylhydroxy ethyl cellulose. Hydroxypropyl cellulose was found to have intermediate stability. Generally, cellulose ethers soluble in organic solvents were found to be less stable than those not soluble in organic solvents (Wilt and Feller in press). Reversibility: Unlike the nonionic cellulose ethers, sodium CMC can form irreversible insoluble complexes in the presence of metal ions. All the cellulose ethers can be cross-linked at the hydroxyls under acidic conditions (Horie 1987, 126-127). Long-term resolubility after natural aging is unknown. Indictor, Baer, and Phelan tested two types of methyl cellulose and a sodium carboxymethyl cellulose using dry oven accelerated aging and found them easily reversible after aging. A third type of methyl cellulose they tested was found to be less soluble, but comparable to accelerated aged rice and wheat starch paste (Indictor, Baer, and Phelan 1975, 145). It should be noted that dry oven aging favors the cross-linking reaction.(TJV) Appearance: Wilt and Feller's testing (cited above) showed that dry powder samples yellowed and demonstrated greater color changes than films formed from 2% solutions. 46. Adhesives, page 25 Relative Strength: Biological Attack: The alkyl ethers, methyl, and ethylhydroxyethyl cellulose, are resistent to biodeterioration in solution while hydroxyalkyl ethers such as hydroxyethyl and hydroxypropyl cellulose and sodium carboxymethyl cellulose are susceptible to biological attack (Horie 1987, 128-129). Methyl Cellulose a. Source Methocel A4M, A15C, A4C, A15 (Dow Chemical Co., USA); Culminal (Henkel, Germany, was available through Talas in ..1982 and from Process Materials as Process Materials or Archivart Methyl Cellulose, also available in the U.S. from Aqualon); Methofas (Imperial Chemical Industries, England); other brands from unspecified manufacturers are available; Light Impressions Methyl Cellulose. Synthesized by reacting methyl chloride with alkali cellulose. CH2OCH3 H OH H OH CH2OCH3 b. Chemical and Physical Properties Non-ionic. Average DS ranges from 1.3-2.6. (See product literature for additional information.) Dow-Methocel A is available in the following viscosities (at 20°C/68°F and 2%), 4C (400 cps), 15C (1500cps) and 4M (4000 cps). Identification: "A 1% aqueous solution of methyl cellulose gives no precipitate upon addition of five volumes of 95% ethanol plus three drops of saturated NaCl solution, whereas most other gums do. Methyl cellulose is soluble in ethylene glycol and insoluble in ethyl ether. When heated it chars without melting and produces a smell of burning paper. It is characterized by its. content of methoxyl groups, which can be determined by the Zeisel method" (Browning 1969, 255). Physical Form: See 463.1 C. General Information. Preparation: Disperse powder in water and agitate. 46. Adhesives, page 26 Solubility: Soluble in cold water. Forms a reversible gel on heating to 50-90^ (122-194°F) (Horie 1987, 127). Aqueous solutions can be diluted somewhat with water miscible organic solvents such as ethanol and acetone. Addition of too much solvent will cause methyl cellulose to precipitate out of solution. Neither powder, nor film are soluble in hot water greater than 80^ (176°F).(CB) pH: The pH measurements of various methyl celluloses in solution range from approximately 6.5-7.5. Refractive Index: nD = 1.49 (Horie 1987, 125). c. Aging Characteristics In accelerated aging tests methyl cellulose was found to be non-damaging to silk (Masschelein-Kleiner and Bergiers 1984, 73). Appearance: Methocel A4M on Whatman chromatography paper was tested using artificial aging at 90^ (210.2°F) and 55% RH for sixteen days and found increased strength of papers tested without dramatic decrease in pH or color whiteness (Baker 1984, 59). Relative Strength: Methyl cellulose is a relatively weak adhesive that may not be strong enough in some applications. Sodium Carboxymethyl Cellulose (CMC) a. Source (Often referred to as carboxymethyl cellulose.) Cellulose Gum CMC 7HSP (Hercules, Inc., USA now available through Aqualon); Cellofas B3500 (ICI, England, no longer manufactured but was listed in a Conservation Materials catalog in 1984). Manufactured by reacting chloracetic acid with alkali cellulose in a slurry with an organic solvent, generally a short chain alcohol (Nicholson 1985, 366). 46. Adhesives, page 27 CH.OCH.COjNa H OH H OH CH2OCH2C02Na b. Chemical and Physical Properties Ionic, DS ranges from 0.4-1.2 (Gelman 1985, 263). Hercules Cellulose Gum CMC 7HSP is 1500-2500 cps (1% solids at 25°C/77°F). Contains metal salts, such as sodium, as part of the molecular chain. The approximate sodium content of CMC 7 is 7.0-8.5% (Baker 1984, 55). Identification: "A method for determination of CMC in paper is based on extraction with NaOH solution and treatment of the extract with sulfuric acid to produce glycolic acid which is determined calorimetrically with 2,7-dihydroxynaphthalene" (Browning 1969, 256). Physical Form: An off-white powder. Preparation: Disperse powder in water and agitate. Solubility: Soluble in hot or cold water with maximum hydration under alkaline conditions. In solution, it is compatible with most anionic and non-ionic polymers and gums. Compatibility with salts depends on whether added cations can form soluble salts of carboxymethyl cellulose. Cations forming insoluble salts are aluminum ion, silver, chromium, and zinc (Hercules, Inc.). Limited dilution of an aqueous solution is possible with ethanol. acetone, and other organic solvents. Increasing DS makes the ether more hydrophilic, increasing electrolytic concentration makes it more hydrophobic. pH: The pH of a 2% solution is about 7.5.(CS) The pH of Aqualon cellulose gums in various concentrations range from 4.6-6.3 (Aqualon). Refractive Index: nD = 1.515 (Hercules, Inc.). 46. Adhesives, page 28 Possible Additives: Less pure products, such as Cellofas B3500, contain sodium chloride and sodium glycolate and appear off white to light brown in color. c. Aging Characteristics In 1984, Baker reported that Cellofas B3500 had been used in England as an adhesive and size for over twelve years with no adverse effects having been observed. There is considerable documentation in the literature to predict that sodium CMC is unstable at low pH (Whistler 1973, 704-5). However, because sodium carbonate is formed in small amounts after drying, the pH of the dry film will probably remain quite stable.(CB) Oxidative degradation can occur to films at pH > 9.0 (Hercules, Inc. 1976, 20). In some accelerated aging tests, sodium carboxymethyl cellulose had no discernible damaging effects on silk (Masschelein-Kleiner and Bergiers 1984, 73). Reversibility: Treating a film with cations such as Fe3+or CU2+ will lead to water resistance or insolubility (Hercules, Inc. 1976, 21). Appearance: Some yellowing of Cellofas B3500 and Cellulose Gum CMC 7HSP was found after humid oven accelerated aging and dark storage aging. NaCMC films are non-staining and do not become brittle with age (Baker 1984). Relative Strength: Accelerated aging tests revealed a drop in strength after humid oven aging and dark storage aging. Cellulose Gum CMC 7HSP was found to respond similarly although a smaller degree of loss of strength was noted compared to Cellofas B3500 (Baker 1984). Hydroxypropyl Cellulose (HPC) a. Source Manufactured under the tradename Klucel (Hercules, Inc., USA). Manufactured from alkali cellulose reacted with propylene oxide at elevated temperatures and pressure. 46. Adhesives, page 29 OH OCH2CHCH3 CH2 H OH O / OH H CH2 OCH2CHCH3 OCH2CHCH3 OH b. Chemical and Physical Properties Hydroxypropyl cellulose is non-ionic. Higher molecular weight increases tensile strength and elasticity. (See product literature.) The viscosity is unchanged over a pH range of 2-11, with the most stable viscosity at pH 6-8. Viscosity is lowered by heating the solution and increases rapidly with increasing concentration. It is possible to mix two viscosity types to achieve an intermediate. Klucel is described as having unexpected viscosity effects when combined with anionic or nonionic polymers in aqueous solution. Combined with anionic polymers such as sodium CMC and sodium alginate, it has higher than expected viscosity. Combined with nonionic polymers, such as MC or HEC, it has lower than expected viscosity (Hercules, Inc.). Physical Form: An off-white powder. Preparation: Prepare a slurry in hot water, allow to sit, add cold water and agitate. Solubility: Klucel is soluble in water up to 40°C (104°F) and in polar organic solvents. Heat accelerates dissolution with organic solvents. By first dissolving the cellulose ether in a solvent, it is sometimes possible to add an otherwise incompatible solvent. Once dried, the film is soluble in water, ethanol, and acetone (Hercules, Inc.). The propyl groups cause it to be more hydrophobic than methyl cellulose, therefore giving it good solubility in polar organics; insoluble in water 40-45°C (104- 46. Adhesives, page 30 113°F) and will precipitate out of solution on heating (Horie 1987, 127). Will precipitate at increasingly higher temperatures when water is replaced by other solvents such as ethanol (Hercules, Inc.). pH: 5.0-8.5. Softening Point/Glass Transition Temperature (T^): Softening temperature is 130°C (266°F). Refractive Index; nD = 1.56 (Horie 1987, 125). Storage/Shelf Life: Solutions are susceptible to both chemical and biological degradation, generally resulting in decreased viscosity. Greatest stability is at pH 6-8 and in absence of oxidizing agents (Hercules, Inc.). c. Aging Characteristics See Wilt and Feller, in press. Ethyl Hydroxyethyl Cellulose (EHEC) a. Source Manufactured under the tradenames Ethulose (Chemaster Corporation, Long Island City, NY. No longer available at this address) and Bermocoll (Berol was formerly Modocoll); EHEC is also available from Hercules, Inc, USA and Conservation Materials, USA. Formed by reacting alkali cellulose with ethylene oxide or ethylene chlorhydrin. H H OH CH2OH OH CH2OH b. Chemical and Physical Properties The product is substituted with ethylhydroxyethyl groups. With " DS of ethyl groups about 0.9 and MS of hydroxyethyl groups about 0.8. According to product literature, Ethulose is non-ionic, stable in the presence of dilute acids or alkaline salts. Ethulose 100 is available in viscosities ranging from 50-1200 centipoises. 46. Adhesives, page 31 Preparation: Add to cold water at a uniform rate, stirring vigorously. Solubility: Soluble in cold or warm water. Insoluble in hot water, hydrocarbons, alcohols, toluene, and xylene. Precipitates out of solution on heating to 40-45°C (104-113°F) (Horie 1987, 127). An aqueous solution can be diluted many times with alcohol, acetone, and other solvents without precipitating. pH: The pH of a 2% solution is 6 (Chemaster Corporation). Storage/Shelf Life: Solutions are resistant to mold and bacteria (Horie 1987, 128). c. Aging Characteristics In Wilt and Feller's testing, HPC was found to have intermediate stability, being less stable than methyl cellulose and sodium carboxymethyl cellulose. (See Wilt and Feller, in press.) Hydroxyethyl Cellulose (HEC) a. Source Natrasol 250 GR and 250 HHR (Hercules, Inc.). Prepared by reacting alkali cellulose with ethylene oxide. CH2OH H OH H OH CH2OCH2 b. Chemical and Physical Properties Hydroxyethyl cellulose is non-ionic, unaffected by cations. Identification: "Hydroxyethyl cellulose is soluble in ethylene glycol and insoluble in ethyl ether; it chars without melting and gives an odor of burning paper" (Browning 1969, 255). Preparation: Add powder to vigorously agitated water. 46. Adhesives, page 32 Solubility: The water soluble range of DS is about 0.8-2.5, with greater DS giving increased solubility. HEC is initially soluble in hot or cold water. Essentially insoluble in organic solvents. Polar or water miscible solvents sometimes affect the solubility; effects vary from swelling to solubility. Its nonionic character allows dissolution in many salt solutions that will not dissolve other water-soluble polymers. It is compatible with more foreign materials than most other water-soluble polymers. Once dried, it is resoluble in water (Hercules, Inc.). pH: 6.5-8.5. Softening Point/Glass Transition Temperature (TJ: Softening range is 135-140°C (275-284°F) (Hercules, Inc.). Refractive Index: nD = 1.51 (Hercules, Inc.). c. Aging Characteristics Also see Wilt and Feller, in press. Based on their findings and those of Howells et al. 1984 and Masschelein-Kleiner and Bergiers 1984, hydroxyethyl cellulose is not recommended for prolonged contact with paper. Appearance: Howells et al., found acrylic dispersions with Natrasol (hydroyethyl cellulose) added as thickener aged poorly and yellowed (Howells et al. 1984). Relative Strength: Masschelein-Kleiner and Bergiers found HEC caused further weakening of impregnated silk after accelerated aging (Masschelein-Kleiner and Bergiers 1984). Methyl Hydroxyethyl Cellulose (MHC) a. Source Manufactured under the tradename of Tylose MH 2000, MH 300 (Kalle Hoechst, West Germany). CH2OCH2CH2OH 0 H OCH, J 1 /' \ H V \ / OH H\ oh y i 1 H OCH3 1 6 CHjOCHjCHjOH 46. Adhesives, page 33 b. Chemical and Physical Properties The Tylose products are methyl celluloses that contain a small amount of hydroxyethyl substitution which raises the thermal gel point from about 55°C (131°F) to about 70^ (158°F). The more polar nature of the hydroxyethyl group allows for the formation of a slightly stiffer gel than is possible with hydroxypropylmethyl cellulose of comparable gelation temperature (Davidson 1980, 3-4). Preparation: Disperse powder in hot water, then add cold water with agitation. Solubility: Soluble in cold water; insoluble in hot water 70^ (158°F) and above. Solutions can be further diluted with alcohol. c. Aging Characteristics Appearance: Yellowed only slightly under accelerated aging conditions (Verdu et al. 1984, 67). Relative Strength: Tylose MH2000 was found to be non-damaging to silk in accelerated aging tests (Masschelein-Kleiner and Bergiers 1984, 73). 8. Cellulose Esters - General Information a. Source In the cellulose molecule, like all adhesives derived from it, the -OH groups on the ring are partially substituted. Esterification is completed (triester) and then hydrolyzed back to the desired free radical content. Some free radical substitution improves solubility and adhesive qualities. Cellulose, an alcohol, is reacted with one of a variety of acids to produce an ester and water. The water is removed to drive the reaction to completion. b. Chemical and Physical Properties Identification: Esters can be readily distinguished from cellulose ethers by the easy saponification of the esters: an unknown material is boiled with methanolic KOH, the alcohol evaporated to a small volume, and the residue warmed with an excess of dilute h2so4, The odor of acetic, propionic, or butyric acid will be detected easily. See specific entries for indicator tests. Many infrared spectra are available. 46. Adhesives, page 34 Molecular Weight: Esters made from cotton, about 700,000-800,000. Esters from wood pulp, about 80,000-400,000. Low MW gives low viscosity. High MW gives high viscosity. Physical Form: Method of film formation is by solvent evaporation. See specific ester entries also. Viscosity: Dependent on MW. Lower viscosity means easier solubility, greater compatibility with other resins and plasticizers, lower MP/Softening Point. Higher viscosity/higher MW means more strength and toughness. Solubility: Soluble in various organic solvents. Possible Additives: Stabilizers against discoloration, degradation, thermal decomposition. Cellulose Acetate - General Information Cellulose acetate was first developed in France in 1869 by the acetylation of cellulose. Industrial production of cellulose acetate began to replace the highly flammable cellulose nitrate as a coating for airplane wings and fuselage fabrics during World War I. It was not manufactured as a film on a large scale until 1930. The properties of cellulose acetate are varied by the degree of acetylation. In the plastics industry it is suitable for both injection molding and continuous extrusion. In paper conservation cellulose acetate has been used in sheet form for the lamination of documents, or in dilute solution as a consolidant for flaking or friable media. Laminating film meeting the National Bureau of Standards (NBS - now called the National Institute Standards and Technology - NIST) specifications is no longer commercially available. However, this film formerly qualified as appropriate archival laminating film. a. Source Cotton linters and purified wood pulps are the two major sources of cellulose for the manufacture of acetate. Acetate from cotton linters is of better color and solution clarity. Cellulose acetate is produced by the acetylation of cellulose with acetic acid in the presence of a catalyst - usually sulfuric acid because it creates the most uniform product. The reaction produces a triacetate. To prepare a product of a low degree of substitution (DS) (with a lowered softening temperature), the triacetate is hydrolyzed to remove some of the acetyl groups, usually to a final acetal value of 52-56% (Windolz 1976). The reaction is carried out via the controlled reversal of the 46. Adhesives, page 35 esterification reaction, by the addition of water and dilute acetic acid. Hydrolysis is stopped by diluting the mixture even further with water. The acetate is subsequently purified by washing with water, and the cellulose acetate flakes are centrifuged and dried (Odian 1981, 672-674) modifying the cellulose-liquid ratio, temperature, catalyst concentration, and solvent produce acetates of varying character. Kodak #4655 and Celanese P911 are two brands which are used in conservation. The basic structure is: Where R1, R11, Rmequals acetate. Cellulose Acetate Repeating Unit. Chemical and Physical Properties Each anhydroglucose unit has three hydroxy! groups which may be substituted. DS is therefore equal to or less than three. The properties of cellulose acetates are dictated by their molecular weight and acetyl content, which is expressed as DS or percent acetyl content. Due to decreased hydrogen bonding and crystallinity relative to cellulose, the cellulose acetates, are thermoplastic. Most are not sufficiently thermoplastic to permit easy processing without the addition of plasticizers. To further lower the softening point these esters are fused with plasticizer under heat and pressure, and then the acetate flake can be processed into products by extrusion and molding. Grades range according to percent acetyl content: plastic, 52-54%; lacquer, 54-56%; film 55.5-56.5%; water-resisting, 56.5-59.0%; triacetate, 60.6-62.5%. 46. Adhesives, page 36 The relationship between percent acetyl content and DS is as follows: 51% acetic acid = 2.2 DS; 53% acetic acid = 2.3 DS; 55% acetic acid = 2.4 DS; 56% acetic acid = 2.5 DS; 59% acetic acid = 2.7 DS; 62% acetic acid = 2.9 DS (Faith et al. 1975, 241). Identification: Cellulose acetate burns slowly, with melting, dripping, and the odors of acetic acid and burning paper. When removed from the flame, it burns slowly with beading at burnt edges. Physical Form: White, odorless, granular flakes, or powder. Clear solution. Preparation: Cellulose acetates used as consolidants are prepared by dispersing the flakes in acetone, ethyl acetate or methyl ethyl ketone (MEK). Solubility: Soluble in acetone, MEK, ethyl acetate, chloroform, and other chlorinated solvents, and various mixtures of organic solvents, depending on the degree of acetylation. Soluble in glacial acetic acid. Insoluble in water and ethanol. Resistant to weak acids, oils, greases, and fats (Faith et al. 1975, 241). Cellulose acetate is less resistant to moisture or water than cellulose nitrate. pH: Softening Point/Glass Transition Temperature (TJ: Laminating film 114.5 ± 2.5t (238.1 ± 36.5°F); flake 60-97^5(140-206°F). Commercial products do not have sharp melting points. Refractive Index: nrj = 1.48-1.50. Possible Additives: Instability in cellulose acetate is caused by the presence of residues of bound sulfuric acid which are very difficult to eliminate during manufacture. Alkaline earth metal salts such as magnesium and calcium may be used to neutralize bound sulphate at the end of hydrolysis. Further addition of an acid acceptor, such as magnesium acetate, ensures that the bound sulphate remains in the salt form. Plasticizers are necessary to lower the melting range and increase tensile 46. Adhesives, page 37 strength. A cellulose acetate film should contain an antioxidant and ultraviolet absorber for maximum stability (McBurney 1954; Wilson and Forshee 1959a). Cellulose acetate does not readily accept additives to modify viscosity or clarity. (CS) Health Hazards: Cellulose acetates are non-toxic (Hawley 1977, 175-176). However, breathing of powders should be avoided. They are flammable and not self-extinguishing, and therefore present a moderate fire risk. Storage/Shelf Life: Protect from extreme heat and excess oxygen, ultraviolet light, moisture, and extremes of pH. Resistant to biodeterioration. Aging Characteristics Films of plasticized cellulose acetate may be considered reasonably stable if they meet the NBS specifications (Wilson and Forshee 1959b). Reversibility: Chemical breakdown of cellulose acetate film or flake involving changes in the acetyl content may cause corresponding changes in resolubility. Hydrolytic degradation is due to acid cleavage of the glucosidic links in the polymer molecule. Chain cleavage is accompanied by deacetylation. Oxidative degradation appears to produce the volatile products of carbon monoxide, water, carbon dioxide, and acetic acid. In turn, these products can accumulate and catalyze degradation of the cellulose acetate in the form of chain breaking (McBurney 1954, 1019-1055). The greater the DS, the greater the stability of the polymer molecule. For laminating film, a DS of 2.4 ±0.1 is recommended (Wilson and Forshee, 1959b, 17). Plasticizers, which may comprise as much as 20-30% of the laminating film, are often the chief contributors to cellulose acetate instability. Some are volatile and many are more easily oxidized than the acetate itself. Plasticizers most resistant to oxidation, including dimethyl phthalate or triphenyl phosphate, extend the stability of cellulose acetate (Wilson and Forshee 1959a). Delamination of a document is performed by immersion in a solvent bath, generally acetone. However, experience gained from performing delamination treatments has shown that degraded cellulose acetate films that will no longer dissolve in-acetone, will often dissolve in acetone/water mixtures, in proportions ranging from 10:1 to 4:1. Dimethyl formamide (DMF) or ethyl acetate may also dissolve deteriorated film. 46. Adhesives, page 38 pH: Acidity is increased considerably by traces of acid catalysts remaining from manufacture. Incorporation of an acid acceptor (i.e., magnesium acetate) retards the process of degradation. Some early cellulose acetate film employed for lamination may degrade, releasing acetic acid and having an acidic, vinegar-like odor. Appearance: For cellulose acetate film or flake, oxidation is accompanied by progressive orange-yellow discoloration (DeCroes and Tamblyn 1952). Cellulose acetate film meeting NBS specifications is resistant to oxidation and discoloration. Degraded cellulose acetate film may exhibit a moderate to severe amount of contraction. Relative Strength: See Wilson and Forshee 1959b for data on tensile strength, edge tear, internal tear, and elongation at break for cellulose acetate laminating film prior to, and following lamination. Biological Attack: Resistant to biological attack. 10. Cellulose Nitrate - General Information Cellulose nitrate was the first major plastic in commercial use, having been formulated in 1832 by Braconnot and produced industrially as early as 1845 in England. Cellulose nitrate was used initially for military explosives (gun cotton) and later plasticized with camphor to produce the first successful synthetic plastic. It was widely used in still negative film and motion picture film until the 1950s. The plastic was patented in England in 1864, and developed independently in America in 1869 where it was marketed as "Celluloid." Industrial lacquer finishes constitute the largest market for cellulose nitrates; explosives and propellants are the second largest market (Kirk and Othmer 1979, 118-143). In paper conservation cellulose nitrate has been encountered as a surface coating and consolidant, as well as a solander box and book cloth coating (known as pyroxylin). Conservators have encountered it on paper substrates used as a general purpose adhesive. (See Skeist 1977, 214 for general purpose nitrocellulose cement recipe.) In the printing industry it has also been used as a binder for inks, especially aniline types because it is tough and pliable. In Dresden, Germany in 1890, a cellulose nitrate product called Zapon was experimentally applied to military maps as a paper strengthener (Marwick 1964). Cellulose nitrate is not currently recommended for use on paper objects. 46. Adhesives, page 39 Source Cellulose nitrate is formed by reaction of cellulose from cotton linters or wood pulp with mixtures of nitric and sulfuric acids. By varying strength of acids, temperature, time of reaction, and acid/cellulose ratio, products showing a wide range of chemical characteristics are obtained. Duco Cement (Dupont), UHU Hart (Lingner and Fischer GmbH), Durofix (Rawlplug), Duco (Decon), Randolph's Universal Cement (Randolph), and H.M.G (H. Marcel Guest, Ltd.) are commercially available cellulose nitrate-based adhesives (Horie 1987, 133; Koob 1982, 31-34). Chemical and Physical Properties Molecular weight approximately 25,000 for adhesives; 120,000 for plastics (Hawley 1977, 614-615). Theoretically it should be possible to replace each of the three hydroxyl groups on the anhydroglucose unit with nitrate groups, producing a calculated 14.14% nitrogen content. Complete nitration is not feasible, however, since the resulting product, cellulose trinitrate, is unstable. In practice, the upper limit of nitration is about 2.9% or 13.8%. Most commercial cellulose nitrates range between 10.9% to 12.2% nitrogen content. The characteristic properties of cellulose nitrate in sheet form depend on molecular weight, degree of nitration, stabilization, film thickness, and plasticizer content. Cellulose nitrate is an inherently unstable substance. (See Aging Characteristics.) Cellulose nitrate in which the nitrogen content is relatively low (10% or less) is a useful lacquer base because of its fast drying properties. Collodion and pyroxylin are plastic forms of low-nitrogen cellulose nitrate which are variable mixtures consisting chiefly of cellulose tetranitrate. Other names for this form are colloxylin, xyloidin, celloidin, and Parloidion (Windholz 1983, 7914). If the nitrogen content is allowed to rise to almost 14% (i.e., cellulose hexanitrate) the product is a high explosive (gun cotton) (Hampel and Hawley 1976, 189). Identification: Cellulose nitrate will give an immediate deep blue color when tested with one drop of diphenylamine solution (6% in concentrated sulfuric acid). On an infrared spectrum it shows prominent bands at 6.1/xm and 11.9/im. It burns rapidly with an intense white flame and the odor of camphor. Physical Form: Yellowish-white matted mass of filaments, having the appearance of raw cotton; colorless liquid to semisolid (solution). It can be safely shipped only when wet with 25-30% water or alcohol. 46. Adhesives, page 40 Preparation: Cellulose nitrate, dissolved in a 50/50 solution of acetone and amyl acetate was recommended in the conservation literature since 1926 as a consolidant and adhesive (Koob 1982, 31-34). Cellulose nitrate film is formed by solvent evaporation. Solubility: Solubility depends on degree of nitration. Low nitrogen form (pyroxylin) is soluble in ether-alcohol mixtures, acetone, methanol, and amyl acetate. pH: Low pH indicates the presence of acid impurities remaining from nitration process of manufacture. Traces of acid will accelerate the hydrolysis of the cellulose nitrate and contribute to instability. Softening Point/Glass Transition Temperature (Tg): Tg is lOO'C (212°F); flash point is 13*t (55^F); autoignition point 170^ (338°F). Refractive Index: nr_> = 1.49-1.51. Possible Additives: Plasticizers such as camphor, tricresyl phosphate, dibutyl phthalate, diocyl phthalate, dibutyl adipate, and castor oil may be present. Eventual volatization of plasticizers contributes to shrinkage and brittleness. Various stabilizers such as light absorbers, peroxide decomposers, inhibitor regenerators, or chain-terminating agents have been tested on cellulose nitrate, but have shown little success in arresting its decomposition (Koob 1982, 31). Health Hazards: According to the literature, cellulose nitrates are non-toxic (Hawley 1977, 615). Breathing of powders, however, should be avoided. Highly flammable. High fire and explosion risk. Degraded cellulose nitrate releases acidic gases of nitrous oxides which are hazardous to humans and objects. Storage/Shelf Life: Exposure to strong light causes cellulose nitrate to become acidic, forming nitric, formic, and oxalic acids, cyanogen and glucose. Will also decompose rapidly when exposed to moderate heat, air, and moisture. Volatile at less than 50% RH; more stable at over 70% RH. Store loosely packed, under refrigeration, away from light and moisture. Resistance to microbial deterioration is excellent. 46. Adhesives, page 41 Aging Characteristics In general, cellulose nitrate adhesive is strong and flexible when new, but after aging shows brittleness, shrinkage, and adhesive failure. Cellulose nitrate is primarily subject to thermal degradation. The reactivity brings about carbon-carbon bond cleavage and the production of aldehydes and ketones. The cleavage reaction results in lowering of chain length, and the addition reaction can result in cross-linking with subsequent insolubility in some formulations. The breakdown of cellulose nitrate in film form by ultraviolet irradiation is greatly accelerated by the presence of oxygen. The denitration reaction produces nitrogen dioxide to nitrogen oxide. These reaction products instigate an autocatalytic process. Cellulose nitrate may undergo hydrolysis by acids or by alkaline reagents, resulting in denitration and reduction in chain length (McBurney 1954). Paper objects stored in unstabilized cellulose nitrate envelopes have been destroyed by the products of decomposition, principally nitric acid. Upon aging, cellulose nitrate adhesive becomes very discolored and brittle, with loss of adhesion. Cellulose nitrate film is capable of spontaneous combustion under certain high temperature conditions, but concern is more reasonably directed toward motion picture film, where there is a high concentration of film packed together in a reel, rather than toward individual sheet negatives, which are generally separated from one another by paper envelopes (Ritzenthaler et al. 1984, 116-117). Reversibility: Dissolves in ketones. Appearance: Discoloration occurs when exposed to light and inflammability increases with exposure. In addition, degradation continues after removal from light. The color changes from clear to yellow to brown. The loss in strength and toughness can be almost complete before any change in hardness is detected (Koob 1982, 31). Biological Attack: Excellent resistance to microbial deterioration. 46. Adhesives, page 42 46.3.2 Proteinaceous Adhesives A. Collagens 1. General Information Adhesives such as gelatin, parchment size, hide glue, bone glue, and fish glue are collagen-based. Adhesives of animal origin have been used since ancient times. Fish glues have been in use since the eighteenth century. North American paper conservators generally use the purest forms, gelatin and parchment size, as consolidants, fixatives, and sizes and in the treatment of parchment. Other bone or hide glues are used by Japanese conservators. Russian isinglass, a pure fish collagen, is used extensively in Eastern Europe. a. Source Collagen is the structural protein of connective tissue, bone, and skin in animals and fish. When these materials are heated in water a partially degraded protein, gelatin, is leached from them. Prolonged boiling also extracts impurities and forms darkly-colored compounds (Mills 1987, 75). Gelatin and parchment size are the result of shorter heating times. Longer heating times produce the less pure adhesives known as hide glue and bone glue. Fish collagen can be processed into either a more pure gelatin product or a fish glue (Skeist 1977, 153). "Photographic quality" gelatin for use in conservation is available through Kodak and Fisher Scientific. Food and pharmaceutical grades of gelatin are also available. Animal collagen primarily contains the amino acids glycine, proline, and hydroxyproline. The amino acid sequences form three separate protein strands which are coiled together and bound by hydrogen bonding interaction. Upon boiling, scission of the hydrogen bonds occurs, the three strands separate and are bound to solvent water (Mills 1987, 75). Upon cooling, the strands will partially reform at a few sites, producing a network of strands linked together like a fishnet.(MB) The hydrolysis of collagen to gelatin may be represented by: Q02 H149 Om N31 + H2 O C102 H151 Oj, N31 (Skeist 1977, 139). b. Chemical and Physical Properties The chemical and physical properties of all collagen-based adhesives are basically consistent and thus are described together in this section. 46. Adhesives, page 43 Identification: The hydroxyproline test is specific for animal-based adhesives. The ninhydrin test is sensitive to all proteins and is commonly used by paper conservators to detect proteinaceous adhesives. The biuret test is also used (Browning 1977, 103-104). Molecular Weight: Animal-based adhesives have molecular weights reported to range from 20,000-250,000. Fish-based adhesives have an estimated molecular weight of 30,000-60,000 (Skeist 1977, 140, 153). Physical Form: Animal and fish-based adhesives are available dry in granulated, sheet, or cake form, as cold liquid glues, or in jelly form. They are graded by viscosity in millipoises and jelly value in Bloom grams (Skeist 1977, 142). The lower the Bloom number, the weaker the gel. For example, Fisher Silver Label Sheet Gelatin has a Bloom number of 130. Fisher G-8 lab grade granular gelatin has a Bloom number of 275. Percent concentration cannot be compared between different sources. The collagen-based adhesives most commonly used in North American paper conservation are gelatin, in granulated or sheet form, and parchment size, which is prepared from parchment scraps. Color ranges from white to pale yellow for gelatin, to darker shades for less pure forms. Preparation: Gelatin is prepared by dissolving the dry adhesive in warm water, or by swelling in cold water, then heating. Parchment size is made by cooking parchment scraps in water and straining. Upon cooling it forms a gel which can be sliced and dried for later use. Solubility: Animal-based adhesives swell in cold water and pass into solution upon heating to 43.3°C (110°F). They are insoluble in organic solvents (Skeist 1977, 142, 153). Gelatin is soluble in glycerol and acetic acid (Windholz 1976, 564). Fish glue is miscible with alcohol and acetone. Gelatin solutions and parchment size can be diluted with ethanol or isopropyl alcohol. pH: Hide glues are generally neutral (pH 6.5 - 7.4). Bone glues are generally slightly acidic, pH 5.8 - 6.3. The pH of fish glues ranges from 5.0 to 8.0 (Skeist 1977, 141, 153). The pH of gelatin and parchment size varies according to grade and/or preparation. Refractive Index: nD = 1.516-1.534 for gelatin (Weast 1978, E-220). 46. Adhesives, page 44 Possible Additives: There are a number of additives which may be present in commercial formulations or may have been added traditionally for use (Skeist 1977). Commercial additives may include preservatives, defoaming agents, wetting agents, dispersing agents, plasticizers, and tannages. Formaldehyde was sometimes added to gelatin as a cross-linking (hardening) agent. Alum was added to alter viscosity and prevent spoilage. Health Hazards: As proteinaceous, natural materials, collagen-based adhesives should pose no health hazards. Storage/Shelf Life: Dry forms of collagen-based adhesives may be stored indefinitely. Liquid or gel forms are susceptible to microbial attack. Refrigeration delays this decay. Aging Characteristics Certain aging characteristics of animal-based adhesives have been studied by Deborah Ann Dyer and Margaret Ann Haupt of the Conservation Programme, Queens University. Their study "An Examination of Animal Glues" was presented as a student paper at the AIC Seventeenth Annual Meeting in May of 1989. Reversibility: In theory, collagen-based adhesives should remain water soluble, especially in warm or hot water and alkaline water solutions.(CB) In practice, however, they may not. Decreased solubility may be caused by cross-linking, impurities, or additives. Proteolytic enzymes may be used to digest an otherwise insoluble collagen-based adhesive. pH: Alum added to some gelatins in very small percentages (based on the dry weight of the gelatin) is likely to result in a neutral pH due to the buffering capabilities of the gelatin itself. (CB and Tim Barrett) Appearance: Impure collagen-based adhesives may become brittle, shrink, or darken over time. Aged films can show cracking and flaking. Gelatin will darken slightly over time, although the effect is barely noticeable if dilute solutions are used. Prolonged storage of gelatin-sized paper in conditions of high humidity causes breakdown of size with discoloration and weakening of the paper.(SRA) Relative Strength: Collagen-based adhesives may lose strength over time as a result of impurity or microbial attack. 46. Adhesives, page 45 Biological Attack: Collagen-based adhesives are susceptible to microbial attack. Manufacturers caution that alkaline gelatins are more susceptible to microbial attack. Caseins 1. General Information Casein is a traditional adhesive which has been used since ancient times. It varies from the collagen adhesives in source, solubility, permanence, and strength. Casein has been used as a binder for pigments and painting grounds, as well as for paper coatings. It has been recommended in contemporary artist handbooks as a matte fixative for pastel (Wehlte 1975, 466). a. Source Casein is the principal protein in milk. It is precipitated from skim milk by acidification to the isoelectric point, pH 4.5-4.8. Acidification may be achieved by the addition of an acid, the formation of lactic acid during fermentation, or the addition of rennet. A technical grade casein is available from Fisher Scientific. Casein is a phosphoprotein, consisting of amino acids and containing 1% phosphorous (Mills 1987, 76). Unlike collagen, casein does not form any kind of network. It is this linear structure that causes it to be stiff and brittle.(MB) b. Chemical and Physical Properties Identification: The Millon test and tryptophan test are specific for casein (Browning 1977, 105). Molecular Weight: Alpha-casein, the protein which constitutes 75% of the casein proteins, has a molecular weight of 27,600 (Mills 1987, 77). Physical Form: Casein is available as a white, amorphous powder or in granule form (Windholz 1976, 240). Preparation: Casein adhesives may be prepared in one of two ways. The addition of lime to casein results in a water-resistant glue containing calcium caseinate. If lime is not added, but rather sodium salts provide the alkalinity, a non- water-resistant adhesive results. It is the second of these two 46. Adhesives, page 46 methods of preparation that provides the paper adhesive. This casein adhesive is prepared by soaking dry casein in water, adding the alkali, then bringing the mixture to 71.1-82.2°C (160-180T) (Skeist 1977, 168). Solubility: Casein is insoluble in water, but forms a colloidal suspension in alkaline solutions. Organic amines, including alkyl amines, ethanolamine, and morpholine are solvents (Skeist 1977, 160). pH: Refractive Index: Possible Additives: Preservatives and plasticizers are often added to casein adhesives. Hardening agents, such as formaldehyde, are added to promote water-resistance. Additives may also be used to affect viscosity. Commercial formulations may be combinations of adhesives with casein as the major component (Skeist 1977, 160-162). Health Hazards: Casein is generally regarded as safe by the Food and Drug Administration (FDA) (Skeist 1977, 159). Storage/Shelf Life: Liquid casein adhesives are susceptible to microbial attack within a few days of preparation. Proteolytic enzymes naturally present in casein may also cause deterioration (Skeist 1977, 161). Aging Characteristics Reversibility: Casein adhesives may either be initially insoluble by nature of their preparation (see above) or may become insoluble due to cross-linking. pH: Appearance: Casein adhesives yellow over time. Relative Strength: Casein adhesives are quite strong. This is mostly due to the high hydrogen bonding which promotes both cohesive and adhesive strength. (MB) They may lose strength due to microbial or enzymatic attack. Biological Attack: Casein adhesives are subject to microbial and enzymatic attack. 46. Adhesives, page 47 46.3.3 Synthetic Polymer Adhesives A. Poly Vinyl Acetate Solutions (PVA) 1. General Information PVA resins have been commercially available since the 1930s with dispersions available since the 1940s. They are used in conservation primarily as varnishes for paintings, media for inpainting, consolidants for insecure media, adhesives for facings, and as heat or solvent-activated adhesive films. PVA is a thermoplastic, odorless, non-toxic, essentially clear and colorless resin. It has a noncrystalline structure which is relatively branched rather than linear (Skeist 1973, 349, 352). PVA polymers are "among the limited number of polymers adapted by conservators since 1930 which have a common characteristic, they possess 'reasonable' flexibility without the need of added plasticizers" (Stolow, Feller, and Jones 1971, 130). Viscosity grade, solubility, and hardness vary with the MW of the polymer. Methods of adhesion include solvent evaporation, pressure activation, and heat sealing. a. Source Vinyl acetate is synthesized from acetylene and acetic acid (Gettens and Stout 1966, 74). In the past, vinyl acetate has been polymerized in a free radical addition reaction (for description see Skeist 1973, 344-351). It is likely that industrial processes currently use the ionic reaction more than the free radical reaction since the former is more easily controlled.(MB) In the U.S. the principal supplier is the Union Carbide Company, makers of the Vinylite AYA series. Elsewhere other sources include: Gelva (Shawnigan Products Corporation, Canada); Mowilith (Farberwerke Hoechst AG, Federal Republic of Germany); Vinalak (Vinyl Products, Ltd., England); Vinavil (Societa Rhodiatoce, Italy); Rhodopas (Societ6 des Usines Chimiques Rhone-Poulenc, France); Vinnapas (Wacker-Chemie GmbH, Federal Republic of Germany); Lamatec, unsupported adhesive film (Ademco, England) (see 46.3.3 F. Proprietary Formulations) (Feller, Stolow, and Jones 1985, 227). 46. Adhesives, page 48 CH3 ^ P C 0 H 1 I -c-c- I ! H H Chemical and Physical Properties Identification: Dried PVA resin fdms can be identified using infrared spectrophotometry. However, the MW of the resin cannot be determined using this technique; thus, resin grades cannot be distinguished. Small amounts of additives are not detected by IR, because the strong PVA absorption peak in the spectra can mask others (Williams 1988, ARS No. 2313). Molecular Weight: Poly vinyl acetate solid resins come in a range of molecular weights. Those made by the Union Carbide company are known as the AYA series: AYAA, AYAB, AY AC, AYAF, and AY AT. Physical properties vary considerably among poly vinyl acetate resins due primarily to differences in molecular weight (Skeist 1973, 352). These properties include, solubility, viscosity, softening point, heat-seal temperature, tensile strength, and glass transition temperature. (See individual resins below.) The monomer vinyl acetate MW is 46. Molecular weight of AYA series is listed below: AYAA 83,000 AYAB No MW found AYAC 12,500 AYAF 113,000 AYAT 167,000 Viscosity Grade: Viscosity grade is defined as the viscosity in centipoise of a resin solution in toluene at 2\°C (70°F) at a concentration of 20% by weight (Feller, Stolow, and Jones 1985, 126). AYAA 40 cps AYAB 9 cps AYAC Not given AYAF 80 cps AYAT 167 cps 46. Adhesives, page 49 Water Absorption: Percent of water absorbed after sixteen hours at 25°C (77°F) (Union Carbide and Cargon Corp. n.d.). AYAA 1.6% AYAB 2.0% AYAC 2.4% AYAF 1.4% AYAT 1.6% Percent of water absorbed after 144 hours at 25 °C (77°F). AYAA 4.% AYAB 7.3% AYAC 8.3% AYAF 3.6% AYAT 3.6% PVA films are permeable to water vapor and should not be chosen where protection from moisture is first consideration (Gettens 1935, 19). Physical Form: Resins come in beads or dissolved in solvent as concentrated solutions. Resins and their solutions are colorless. Preparation: Suspend resin beads in a cheesecloth bag inside a solvent container. Depending upon the solvent, the beads may need overnight or longer to dissolve at room temperature with occasional stirring. Solubility: At room temperature resins are soluble in acetone, 95% ethanol, isopropanol, cyclohexanone, diacetone alcohol, MEK, methanol, ethyl acetate, trichloro-ethane, benzene, and 1:9 toluene. A small amount of water aids solubility in many solvents. (Partial list only. See Skeist 1973 for complete information.) Softening Point/Glass Transition Temperature (T„): PVA-AYAA Tg: 2l°C (70°F); Softening point is 66°C (150.8°F) PVA-AYAB Tg: 17^ (158°F); Softening point is 45^ (111.2°F) PVA-AYAC Tj 16^ (60.8°F); Softening point is 32°C (89.6°F) PVA-AYAF Tg: 24^ (75.2°F); Softening point is 71°C (170.6°F) PVA-AYAT Tj 26^ (78.8°F); Softening point is S6.5°C (187°F) Refractive Index: no = 1.4665. 46. Adhesives, page 50 Possible Additives: Proprietary formulations may contain additives such as, plasticizers, extenders, fillers, pigments, dyes, thickeners, solvents, and wetting agents. Common plasticizers are dibutyl phthalate and PVOH. Non-solvent diluents are used to modify viscosity and evaporation rate. Resins and gums are sometimes added to modify temperature sensitivity, tack, and water resistance (e.g., rosin and its derivatives, chlorinated diphenyl) (Union Carbide and Cargon Corp. n.d.). Health Hazards: Polymers are non-toxic, but organic solvents present their associated hazards. Storage/Shelf Life: Resins are not measurably affected by sunlight, air, or ultraviolet light, according to Skeist and early conservation literature. However, see Aging Characteristics, below for a more recent appraisal. PVA resins absorb a small amount of moisture (Skeist 1973, 352). Aging Characteristics PVA is a stable resin showing excellent aging properties. These have been reported thoroughly in the conservation literature (see Feller, Stolow, and Jones 1985). The resin has been shown to be highly resistant to deterioration at normal temperatures and to be resistant to reaction with dilute acids and alkalis. The resin does not hydrolyse and has an acid number of 0 (Gettens 1935, 18). If there are additives in the PVA, volatilization or migration of external plasticizer may lead to embrittlement of bond, staining of substrate, insolubility, or darkened color. Reversibility: The branched polymer structure of PVA with additives can continue to bond over time and cross-link making resolubility difficult. The purity of the product directly affects its aging characteristics. Heat or aromatic solvents should reverse a pure PVA film which has been used in a conservation application (e.g., to adhere a lining or to consolidate a paint film). To reverse a known PVA applied as a consolidant, see AIC/BPG/PCC 1988 23. Consolidation/Fixing/Facing, 9. PVA has been shown to be dissolvable from objects after more than thirty to forty years (Cronyn and Horie 1985, 92). The greater challenge to the conservator is reversing an unknown PVA adhesive which has been applied to paper: the adhesive is often no longer soluble in the complete range of solvents mentioned above. Application of heat, ethyl acetate, or acetone and sometimes water can be successful in these instances. 46. Adhesives, page 51 Appearance: PVA resin is not appreciably affected by UV light and does not measurably break down and discolor when exposed to strong sunlight (see Feller, Stolow, and Jones 1985). Newer information shows that PVA is not totally transparent to UV light, according to its spectrum. Since everything is affected by air and light eventually, it may be that some early tests are overly optimistic about the aging properties of PVA. For example, what Skeist considers "unaffected" is probably unacceptable to those in the conservation field.(MB) Relative Strength: PVA resins are flexible and strong (Stolow, Feller, and Jones 1985, 130-133). Biological Attack: Not subject to biological attack. Poly Vinyl Acetate Dispersions (PVA) 1. General Information PVA dispersions were developed in the 1940s, although major technical developments did not occur until the 1950s and 1960s (Skeist 1977, 465). Dispersions consist of minute particles of PVA polymer suspended in water. Polymer dispersions are frequently referred to as emulsions, which is a misnomer. Emulsions are liquid/liquid suspensions, while dispersions are solid/liquid suspensions (DeWitte, Floroquin, and Goessens-Landrie 1984, 32; Feller, Stolow, and Jones 1985, 218; Howells et al. 1984, 36). Unlike PVA solutions, PVA dispersions have low viscosities even at high concentration of solids and at high molecular weights. PVA dispersions form strong, flexible bonds to many materials, although complete film formation may take several months at the proper temperature (Feller, Stolow, and Jones 1985, 221). The minimum film formation temperature (MFFT) varies slightly with different manufacturers' formulations, but in general the MFFT and Glass Transition Temperature (Tg) fall in the range of 0-20