The world of acrylates

Whether the user is a finisher, or a technologist in the wet-end, the industry cannot escape the grip that acrylates have on the leather-making process. Many tanners describe the panic and fear they feel when they gaze into the future and see how rapidly polymers are taking over the position, that synthetic tanning agents (syntans) and vegetable tanning materials, used to occupy. There is chemically no difference between an acrylate that can be used in finishing (as a resin) or one placed into the wet-end. However, not all resins used in finishing can be used in the wet-end (or vice-versa), as they are formulated depending on their application.

The traditional role in the tannery that acrylates assume, is predominantly in the finishing area – where it used to provide a film-forming material that binds the pigments and fillers. In the wet-end, acrylates are added in the retanning to help fill empty voids, in the grain layer, or in the belly region. The process of selective filling was commonly employed to position a large molecule (that couldn’t fit into the tight butt area) into the looser structured parts of the leather resulting in physical filling of those areas.

The polymers, acrylates, or other new generation molecules are all composed of monomers. If a single type of monomer is used then the resulting molecule is referred to as a ‘homopolymer’. The use of two types of monomers to construct a polymer will result in a ‘heteropolymer’ or co-polymer. Homopolymers consist of monomers (varying in number), and on how the chemist undertakes the polymerisation. Huge, large molecular weight polymers, are constructed by adding the monomers into solution and then starting the reaction in some manner.

Acrylate polymers are polymerised using an addition polymerisation, where an initiator (often a chemical radical) is used to attack the double carbon bond in the vinyl group of the monomer - resulting in a reactive monomer, see Figure 1. The reactive monomer can then attack the vinyl group of the next monomer, starting a chain reaction. A vinyl group is a carbon-carbon double bond onto which one of the carbons also has a carbonyl group (C=O) attached. The vinyl double bond is destroyed during the polymerisation reaction by the initiator.

 

 

In all acrylate polymers, the behaviour of the polymer is dependent on the chemistry and number of the monomers used to construct the polymer. One of the most characteristic features of the polymer is its melting point and the point at which it changes from a ‘glass-like’ state into a rubbery/sticky/flowing state (also known as the glass transition temperature, Tg). Homopolymers with a Tg (well below zero degrees Celsius), are generally sticky at room temperature – useful as finish adhesion promoters – or are polymers that will result in a flexible soft leather – if used in the drum. Table 1 gives a few commonly used acrylate homopolymer Tg values.

Harder polymers, like poly(methyl methacrylate), PMMA – also known as Perspex, are often included into leathers to give firmness and high levels of grain tightness. PMMA can also be added into top coats or into special mud-crack finishing effects, that provide design or durability benefits. PMMA has a Tg of between 105 and 120°C, so is generally solid and glass-like at normal working temperature. 

 

 

In figure 1, the carbonyl group shown, can become an ester (at R2) which results in a lowering of the Tg value, see Figure 2.  Methyl acrylate, ethyl acrylate, propyl acrylate, and butyl acrylate, are listed in increasing chain length, but also in decreasing Tg value (10, -24, -43, -54°C, respectively). In other words, the longer the chain becomes the lower the Tg value and the softer the polymer becomes. The lubricating acrylic syntan, or softening polymers make use of this feature, because these long sidechains prevent the polymer becoming ordered and rigid (or crystalline). These polymers can provide retan and fatliquoring properties, resulting in their increasing use in lightweight leathers. 

 

 

If the carbonyl group is a carboxylic group (-COO), it can be sold to the tanner as an acrylic acid (-COOH) or its salt, e.g. sodium (-COONa). Sometimes very low molecular weight poly(acrylic acid), PAA polymers are sold to the tanner. Looking at the technical data sheets of these polymers will show the tanner, a low pH value (of a 10% solution) of the product (e.g. pH 3.5), and it will say something like can they be used to increase the chromium uptake. Solutions of PAA are often added in the tanning operation to increase chromium uptake as the tanner is simply adding large numbers of carboxyl groups (perfect for chromium). PAA is however not a good idea for intense dyeing though. The large number of carboxyl groups smother the chromium already bound to the collagen and will prevent the dye from fixing.

Figure 1 also shows the site to which another methyl group can attach (R1). The attachment of a methyl group to this location results in the Tg jumping sharply up. Poly(methacrylic acid) has a higher Tg than poly(acrylic acid) with the only difference being the additional methyl group (105, 227°C, respectively). Note again a smaller polyacrylate means closer packing and a harder polymer.

The family of acrylates is large, there are many choices that a tanner can make that will influence the feel and structure of the crust or the final finish. Deeper knowledge of the family will provide more versatility and will lead to a happier customer. – H. Proctor is a pseudonym.

 

 

In the next issue The European Union is currently considering the movement of two substances from the Restricted Substances List (Annex XVII) to the Authorisation List (Annex XIV). The movement means effectively a ban of two phthalate ester mixtures (dihexyl, dioctyl, or didecyl phthalate mixtures). These are massively disruptive reprotoxic agents and are currently used in plastics or finishes, some in the tanning industry. The difference between phthalic acid (unrestricted) and phthalate esters will be explained and how phthalate esters can be avoided in leather.