Paper Coating Co-binders
These are primarily used to adjust the viscosity and water retention of coating colors to the required levels and to modify their rheology according to the demands of particular coating techniques. But they should also offer some additional advan¬tages over the thickeners in order to justify the higher recipe costs, e. g. binding power, activation of optical brighteners (Table 3.4).
Natural products, like casein, starch, and soy protein used as main binders in the past, also impart the necessary viscosity and water retention to coating colors. However, they were unable to satisfy the increasing demands that were placed on runnability and coating quality, and they have gradually been superseded by synthetic binders. So natural products, principally starch and soy protein, are nowa¬days only used as co-binders. Casein is only used in cast coating because it has some special features which make it difficult to replace with synthetic products. Alginates and hydroxyethyl cellulose (HEC) are mainly used in the United States and Japan, and have not been adopted to any significant extent in Europe.
Carboxymethyl cellulose (CMC) is a versatile product with an all-round range of properties, and is popular in many regions. CMC improves the water retention efficiently. By choosing the optimum grade, the water retention can be adjusted to the individual needs, which are dependent on the coating conditions and the coat¬ing color formulations. In formulations with kaolin clay, the differences in water retention between different CMC grades are much smaller than in colors based on calcium carbonate. One reason for this is that CMC quickly builds up a network structure with clay particles, which imparts extra water retention. With calcium carbonate pigment, the water retention is more dependent on the viscosity of the water phase.
A higher molecular weight and higher viscosity type of CMC is needed to give good water retention for coarse calcium carbonate-based precoat¬ings. Low molecular weight and low viscosity type CMC grades give good perform¬ance for fine clay-based coatings. Lower molecular weight and lower viscosity type CMC grades are closer to Newtonian5 type behavior, while higher molecular weight and higher viscosity type CMC grades are more pseudoplastic. CMC is mechanically stable in high shear conditions and compatible with all common types of coating raw materials.
Synthetic products comprise polyvinyl alcohol (PVOH), polyvinyl pyrrolidone (PVP) in combination with PVOH, acrylic copolymers, and associative thickeners. Due to the high degree of carboxylation of acrylate ester dispersions, they turn into colloidal dispersions upon addition of alkali. Thus the laborious and energy con¬suming dissolving and cooking processes involved in the use of natural binders are avoided. The binding power of PVOH exceeds that of all other binders used in paper coating, nevertheless it has gained only limited acceptance.
This is mainly due to the fact that the application of large amounts of polyvinyl alcohol lead to rheological problems on the coating machine during processing. PVOH is a solid compound, which is composed of a hydrocarbon chain bearing hydroxy groups on every second carbon. Depending on the extent of the polyvinyl acetate hydrolysis, more or less acetyl groups remain attached to the chain. The stereochemical struc¬ture of polyvinyl alcohol, the direction in which the OH groups/acetyl groups point, is already fixed during vinyl acetate polymerization. Like most free radical induced reactions, the PVOH polymer shows an atactic structure.
This means that the functional groups are randomly oriented. Their molecular weight (MW) and degree of hydrolysis primarily characterize polyvinyl alcohols. In practice, further features such as tacticity, branching, average length, and distribution of residual acetyl group sequences play a minor role only. Since PVOH is fully soluble in water, its viscosity under defined conditions is taken as a proportional measure of its molecular weight. Coating grades range from 3 mPa s (very low MW) to 6 mPa s (low MW). PVOHs with viscosities higher than approximately 6 mPa s should not be used on coating machines.
The degree of hydrolysis is based on the measurement of the ester value and indicates how much mole percent of the basic polyvinyl acetate is “saponified” to PVOH. For coating purposes, a degree of hy¬drolysis is selected from a range of 88 % (partially hydrolyzed) through 99 % (fully hydrolyzed). At the same degree of hydrolysis, higher concentrations or lower tem¬peratures lead to an increase in viscosity. Given the same MW, fully hydrolyzed grades display a higher viscosity than do partially hydrolyzed grades due to in¬creased hydrogen bonding. Going from about 97 to 100 mol % hydrolysis, the crystallinity of the polymer increases considerably, which has an impact on the solid state in particular. One apparent change is the reduction in cold water sol¬ubility of the PVOH.
The choice of other monomers for synthetic co-binders is not restricted to acrylic acid, methacrylic acid, and esters such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, and ethyl methacrylate. Other functional monomers such as acrylonitrile, acrylamide, and vinyl acetate can also be used, so there is a great scope for varying the chemical composition of synthetic co-binders. The pro¬portion of carboxylic acids to the other monomers is usually lower, with the result that synthetic co-binders are less anionic and they adsorb more readily on the surfaces of clay pigments. Important differences of synthetic co-binders compared to the thickeners (see below) are their shorter chain length and their low propor¬tion of acids.
This explains the low thickening effect of synthetic co-binders and hence they can be added to coating colors in larger quantities. They are usually added at rates of 0.5–3 parts per 100 parts of pigment, expressed as solids. These relatively high levels of addition and the presence of functional monomers can have a substantial effect on the properties of the coating.