Adhesives

PVA Adhesives
PVA Adhesives

In bookbinding, glue can sometimes mean paste but paste is never glue.

Generally, glues are animal or PVA/EVA adhesives and pastes are cellulose/starch adhesives. Adhesives should be chosen for their flexibility, strength, non-shrinkage, good aging characteristics and reversibility.

Making your own adhesive can be beneficial in the long term, as, among other factors, most proprietary brands do not list all of their ingredients. However, with any of the following animal, cellulose or starch recipes, it is highly recommended that you use only distilled water. Tap water varies from town to town, or suburb to suburb and can yield a great variation in pH levels. Please bear in mind that the more acidity you add (whether it is in the paper, the boards, the covering materials, or the adhesive) the shorter the lifespan of your book.

Animal Glues

All animal glues are essentially collagen reduced to gelatine. Gelatine has been used as an external paper sizing for centuries and it still maintains the wrinkles in crêpe paper. 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. The lower the Bloom number, the weaker the gel. As a general rule, the paler the powdered animal glue, the higher is the quality (which usually equates to cost) and the less odour it emits when heated.

Nearly all animal glue recipes are best made in a container placed in a double boiler or water-bath so that the mixture is not in direct contact with the heat source. Because of the relatively low temperatures needed for animal glues, other heating suggestions are; placing the glue mix container in an electric frying pan with an adequate amount of water; using an electric baby bottle warmer; or, a leg wax warmer. Genuine gluepots are available from overseas and cost around $200.00.

Do not heat animal glues to boiling point (100°C) as protein strands will breakdown and greatly weaken the glue or even destroy its adhesive abilities. Repeated heating causes yellowing and decreased viscosity and strength.

Isinglass

Isinglass is made from dried fish bits and can be cooked into glue. Today, Isinglass finings are commonly used as a processing aid in the brewing and winemaking industries to accelerate the clarification of beer and wine. As such, genuine Isinglass is sometimes available from good home brewing suppliers, but be aware of artificial/substitute isinglass.

As parchment is so sensitive to moisture, isinglass is also used in that particular area of restoration and conservation. Isinglass also has a greater adhesive strength than many other glues used for parchment repair. Pieces of dried isinglass are soaked to soften and swell and it is then cooked slowly in a double boiler, or water-bath, at 45°C while being stirred. A small amount of gum tragacanth, dissolved in water, is added to the strained isinglass solution to act as an emulsifier. Isinglass can be reactivated with an ethanol-water mixture.

It can also be used to coat tissue or goldbeater's skin. For this use the isinglass is heated with a few drops of glycerine or honey.

Parchment size

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. Heating prior to the addition of alcohol seems to allow more alcohol to be added. However, excess alcohol alters parchment size.

Rabbit Skin

Rabbit skin glue is available in dried and powdered form from some art supplies stores and hardware stores. In a coarser form, it may be found under the label of Joiner’s Pearl Glue or Hide Pearl Glue at specialist woodwork stores. Its advantages are; very fast bonding, very low ‘creep’ (which is the tendency of some glues to move when under stress), and, it may be readily removed a poultice of paste.

Vinyl Acetate Emulsions

Vinyl acetate is synthesised from acetylene and acetic acid. These water-based emulsion adhesives are most often polyvinyl acetate (PVA) homopolymers or vinyl acetate ethylene (VAE) copolymers. Homopolymers require the addition of an external plasticizer to remain flexible. The most common external plasticizer used is dibutyl phthalate. PVA can be internally plasticized by copolymerization with ethylene or poly vinyl alcohol to create EVA/VAE adhesives.

Vinyl Acetate adhesives cure to a semi-soft state, which allows the spine to be more flexible. While bookbinding emulsion adhesives do not crack, they do have relatively weak ‘page-pull strength’ - that is, pages can be pulled out of a ‘perfect-bound’ book fairly easily.

Although it may be from weeks to centuries, all vinyl acetate products eventually degrade and produce acetic acid and other noxious products. The key is finding the adhesive that remains flexible, strong, stable and reversible over the greatest length of time.

Most commercially available adhesives contain: 30–50% polymer solids, 1–3% surface active agent, 0–3% protective colloid, 1–3% initiator, 0–1% modifier, 0–5% plasticizer, 0–1% buffer, and 50–70% water. Solvents (usually toluene), natural gums and starches, as well as various cellulose ethers are used as thickeners. Buffers, if included, might be calcium carbonate or calcium acetate. Fungicides are added to many formulations to reduce the chance of mould growth. These additives can affect aging characteristics radically and may change from batch to batch.

Polyvinyl Acetate (PVA)

PVAs have been commonly used in Europe for the past seventy years and in the US for the last forty years. These periods may assist in deciding what treatments to use when restoring books.

PVA emulsions with little or no plasticiser, such as woodworking white glue, will become hard and brittle after a relatively short time. Adding a plasticiser will soften the PVA polymer and significantly reduce brittleness.

Adding a plasticiser to PVA (that is, externally plasticising) will generally provide a considerably stronger bond than an internally plasticised copolymer (VAE). This is because the externally plasticised emulsion will harden as plasticiser ‘off-gases’ into the environment or paper stock during aging. In addition, this increase in strength stabilises in time and the loss of plasticiser can be detrimental to other properties, such as flexibility and cold-temperature resistance. Frozen PVA emulsion will lose all adhesive abilities.

In general, the shelf life for most PVA adhesives is nine to twelve months at 5-49°C. PVA suitable for bookbinding is cheaper than EVA.

Vinyl Acetate Ethylene (EVA)

Common usage has caused Vinyl Acetate Ethylene (VAE) dispersion to be misnamed EVA. Strictly speaking, EVA is a solid and VAE is a dispersion. No matter. The binding industry and some conservators, due to its inherent flexibility without external plasticisers, generally prefer ‘EVA emulsions’. Most ‘emulsions’ are technically dispersions.

EVA is considered more stable than formulations containing plasticizers added to the adhesive, that is PVA. EVA is also reversible in water for a longer time.

Cellulose Pastes

The first industrial production of methylcellulose began in Germany during the 1920s. Cellulose ethers are made from wood pulp or cotton linters that have been recrystallised with sodium hydroxide. The “alkali” cellulose then undergoes methylation or etherification. The product is neutralised with acids and the cellulose ether is isolated and purified by extraction of salts and by-products. The product is then dried, milled, and sifted. Viscosity of the final product is determined by the pre-treatment of the cellulose raw material and by subsequent oxidation of the finished product to the desired molecular weight.

All the cellulose derivatives listed are very common in the food and cosmetics industries. Look for the additive numbers 461, 463 and 464.

Cellulose ethers are available as fine to granular powders which range in colour from white to yellow. Each cellulose ether is available in several grades of purity and in a range of viscosities.

In bookbinding and paper conservation, cellulose ethers have been used alone or with starch pastes. Their moisture holding, surfactant, and anti-redeposition properties are used as poultices for removing stains, old adhesives, and other accretions. Dilute solutions are used for sizing or resizing paper. Cellulose ethers have also been used for consolidating flaking media in art and manuscript restoration.

Anti-oxidants or plasticizers (such as glycerine) are possible additives. Like cellulose, all cellulose ethers will suffer from chain breaking through oxidation. This oxidation is accelerated by light exposure. The extent to which degradation occurs varies widely among the many types of cellulose ethers. Sodium carboxymethylcellulose and methylcellulose are the most stable. Hydroxypropylcellulose has been found to have intermediate stability.

Granular grades of cellulose ethers dissolve more easily in water than the finer powders. Granular or coarse types should be added to vigorously stirred water.

Methylcellulose

One of the most common forms of methylcellulose is wallpaper paste. The benefit of using wallpaper paste is that it is readily available as an adhesive, rather than as a food or cosmetic additive. However, besides the additional cost, fungicides and other unknown chemicals are added.

Methylcellulose is a relatively weak adhesive that may not be strong enough in some applications. Methylcellulose has been used to line degraded wood pulp papers where it acts as an easily reversible adhesive which also resizes weak paper surfaces. Methylcellulose has been used to swell scratches and abrasions on leather and for changing the saturation of blanched colours. A 2.5–3% solution of methylcellulose can be used as a poultice to draw up water stains, water-soluble adhesives, etc., where controlling the degree of wetness is important. Methylcellulose is pH-neutral and can be mixed with starch pastes or PVA adhesives. Look for a higher viscosity as an adhesive and a lower one for sizing.

Hydroxypropylcellulose (HPC)

Hydroxypropyl cellulose is a non-ionic cellulose ether adhesive soluble in water, alcohol, acetone, and many other solvents. A 2% solution in ethanol or isopropanol has been used as a consolidant for red rot leathers. It is also soluble in many other solvents, including water. Its higher molecular weight increases tensile strength and elasticity. The viscosity is unchanged over a pH range of 2–11, with the most stable viscosity at pH 6–8.

Once dried, the film is soluble in water, ethanol, and acetone. The propyl groups cause it to be more hydrophobic than methylcellulose, therefore giving it good solubility in polar organics.

HPC is insoluble in water below 40–45°C, therefore prepare a slurry in hot water (above 50°C) and allow to sit for several minutes, maintaining temperature to prevent lumping. Add main volume of cold water and stir until dissolved, approximately ten minutes. Alternatively, add HPC powder to a blender, run approximately ten minutes, or until a lump free solution is obtained. Allow to stand for several minutes before use.

Sodium Carboxymethylcellulose (CMC)

Sodium carboxymethylcellulose is an anionic water-soluble polymer derived from cellulose. It is an adhesive in concentrations of 2.5%-4% and is available in a range of viscosities. When mixed with distilled water it does not need refrigeration, nor does it spoil. Its pH is between 5-8. Sodium CMC may be used on its own for linings or it may be mixed with a starch paste. Sodium CMC is more easily made in a blender. Viscosity increases over a period of one hour.

Starch Pastes

Starches are naturally occurring polymers of glucose. Starch adhesives have been used for thousands of years and are still used throughout the world in numerous industrial applications such as papermaking and textile manufacture. Starch adhesives are derived from the roots and seeds of plants such as corn, potatoes, rice, and wheat.

Starch has a more intricate structure than cellulose because its molecules have two distinct areas: amylopectin and amylose. The exact percentages of amylose and amylopectin for each starch is largely responsible for its working properties. Wheat starch contains 18–27% amylose, while rice starch contains approximately 17–19% amylose. 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.

Cooked starch paste is a mixture of greatly swollen granules, fragments of granules that have burst open, and dissolved starch. 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. And, the cooking technique, as well as origin of the starch, affect the characteristics of the resulting adhesive.

Clag Paste

Clag paste has been around forever. This is a ready-made wheat paste of consistent quality but with a pH value of 3-4 (acidity somewhere between oranges and tomatoes). Be aware that there is also an unknown preservative and fragrance added, so it is not suitable for high-end books or restoration. Furthermore, while a pot of Clag seems useable for ages, it does grow lumps which (just like other lumpy pastes) are the very devil to remove wet from covering materials and very noticeable on your finished work if you ignore them.

Flour

Plain wheat flour creates one of the world’s oldest adhesives after it is mixed with water and cooked.

Modified Starches/Dextrines

Dextrines are modified starches whose molecular structure has been changed through the use of heat, acid, alkali, or other catalytic conversions. Dextrines have been widely used for stamps, labels, and paper tapes, where the adhesive is moistened for application.

Dextrines have been used as adhesives since the early nineteenth century. 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. Dextrines are often mixed with animal glue, gum Arabic, or gum tragacanth. Frequently, blends of different dextrines are used and borax is a common additive to increase tack. There are three major types of dextrines: white, yellow, and British gums.

1. White dextrines are prepared by roasting at 107.2°C in the presence of acid. These dextrines are then neutralised with some alkaline material such as ammonia. They are used in 50% concentrations. The colour is white.

2. Yellow (or canary) dextrines are prepared by roasting starch with acidic catalysts at high temperature. Colours 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°C without using acid. These dextrines are usually dark coloured and exhibit high solubility in warm water. They are used in concentrations of 10–35%.

Generally, dextrines are much more soluble in water than the source starch because processing has lowered the molecular weight. Dextrines 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.

Precipitated Wheat Starch Paste

This is the primary adhesive for paper conservators and the standard against which they judge other adhesives. Wheat starch paste can be very strong, yet at the same time it can be modified and manipulated for very delicate applications. A well made paste can be diluted indefinitely and retain its proportionate strength. High water content of starch pastes may require methods to minimize over wetting and resulting cockling and staining. Such methods include dry linings or blotting the adhesive prior to use. Wheat starch paste does not adhere well to fatty grounds and is not recommended for use with oil media.

Precipitated wheat starch is created when extracting gluten from wheat flour to create “gluten-free flour”. Carefully read the label if you find “gluten-free flour” at a supermarket – it will likely be a blend of rice flour, corn flour, methylcellulose and other chemicals.

Wheat starches are separated from flour in a wet partitioning step when the 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. The starch is concentrated from the slurry by centrifuge and dried.

A disadvantage of wheat starch paste is that it sometimes causes a faint, greyish haze in paper, for example around mended tears. This situation can often be remedied by using rice starch paste or an appropriate cellulose ether.

Most recipes call for wetting the dry starch thoroughly before cooking. Using cold water can help avoid premature thickening and inadequate dispersion. Too high a temperature and/or too long a cooking time can adversely affect viscosity, adhesive strength, and colour of many starches.

Precipitated Rice Starch Paste

Rice starch paste is preferred when very flexible, smooth, and clean adhesion is essential. It is also very successful for adhering fills or inserts.

This is generally considered to be a weaker adhesive than wheat starch paste. Differing growing conditions that result in differing amylose percentages and the 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 greyish haze or stain when it dries.

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°C.

Some conservators feel rice starch adhesives swell and release sooner than wheat starch. This property can be utilised 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.

Rice starch has less stiffening than wheat starch paste.