Paper Coating Associative Thickeners
These have a long history of use in paints. They consist of an essentially hydro¬philic, water-soluble polymer with strongly hydrophobic terminal groups or side chains. The hydrophobic terminal groups are aliphatic or aromatic hydrocarbons, and they are completely insoluble in water. They are joined to the main polymer backbone by means of a hydrophilic spacer, which ensures that they remain flexible. Their structure is similar to that of surfactants, and they also tend to join together in water to form micelles. This associative interaction between the hydro¬phobic side chains increases the degree of internal coordination of the whole sys¬tem, with the result that the coating color has a very high viscosity at low shear .
The association between the hydrophobic groups is the result of van der Waals’ forces, which are very weak and easily overcome. If the coating color is subjected to shear, they quickly lose their attraction for each other and no longer form a network. The micelles break down and the thickener molecules are ori¬ented in the direction of flow, which causes a reduction in the viscosity of the coating color. However, because the hydrophobic constituents are no longer asso¬ciated with each other, a consequence of the low viscosity of the coating color under high shear is that its water retention is much lower.
The associative thickeners can be divided into three different categories accord¬ing to the chemical composition of the hydrophilic polymer backbone 1. HEUR (Hydrophobic Ethoxylated Urethanes): These thickeners consist of rel-atively short-chain, ethoxylated polyurethanes that have been terminated with hy¬drophobic substituents. The associative interaction between the hydrophobic chain ends causes them to join together to form long chains, and the viscosity increases accordingly. Another feature of these products is that they also interact with the dispersed polymer binder particles. There are variants in which the main polymer backbone consists of alternating hydrophilic and hydrophobic blocks. HEUR-type associative thickeners have long been used in the paints industry but they have been less successful in coating colors because of their lack of affinity for pigments.
Table 3.6 Types and properties of associative thickeners used for coating (source: BASF).
1. 2. HASE (Hydrophobic Alkali-Swellable Emulsions): The hydrophilic polymerbackbone consists of an alkali-soluble polyacrylate, and they do not differ from the other acrylic-based thickeners described above in this respect. The most commonly used monomers are ethyl acrylate, acrylic acid, and methacrylic acid. The hydro¬phobic side chains are attached to the polymer backbone by means of a polyethyl¬ene oxide spacer.
The associative thickening effect can be controlled by varying the ratio of associative monomers to conventional monomers in the polymer back¬bone, the hydrophobicity of the polymer (i. e., the chain length of the aliphatic hydrocarbons), and the number of ethylene oxide units in the spacer. The inter¬molecular association between the hydrophobic side chains themselves and be¬tween the side chains and the dispersed binder particles is responsible for the very high viscosity that can be achieved. The relatively high molar mass of the acrylic polymer backbone and its high affinity for pigments mean that these products can be used in paper coating, but the water retention level is lower than with conven¬tional thickeners.
2. 3. ACT (Associative Cellulosic Thickeners): Products in this category consist of cellulose ethers with hydrophobic substituents. The most important product in this group is hydrophobic modified hydroxyethyl cellulose (HMHEC) which also has a high affinity for styrene-butadiene binders and clay pigments. The applica¬tions open to these products are comparable to those for HASE products. The low water retention can be compensated for by the speed with which the filter cake forms on the surface of the base paper. Associative thickeners bring about a very high increase in viscosity, and so they only need to be added in very small amounts. This is an economic advantage over conventional thickeners, but only if the water retention of the coating color is unimportant. Other products need to be added to boost the water retention of the coating color if, for instance, highly absorbent paper is coated with a precoat or if coarse CaCO3 is used as the pigment.
It has to be noted that the results obtained for water retention with static methods do not correlate with the water retention in practice under dynamic conditions. The great advantage of associative thickeners is that they are much more pseudo-plastic than conventional products, and coating colors have a lower viscosity at high shear (Table 3.7). This gives better runnability, and less blade pressure need be applied in order to control the coat weight. Associative thickeners cause a large, immediate increase in viscosity when the shear applied to the coating is released, and this can lead to a more bulky coating being formed with a superior optical appearance.