Sizing finish properties

As with most things in life, size matters. This is certainly the case when considering the performance properties of finishes on leather. The effect of particle size on finish performance has been considered by a number of workers^{1- 5}. In all these cases, the approach was theoretical, there being little instrumentation that was capable of sizing accurately the materials used. The work also tended to concentrate on the average size of the material with little regard being given to the size distribution in finish materials and what effect this had upon the properties investigated. Possible interactions between materials in a formulated finish were considered in a paper given by Nott at the IULTCS Congress in Philadelphia⁶. Initial studies In the investigations, a Coulter model N4MD instrument, employing laser light to measure the particle sizing, was used to measure both the size and size distribution present in various leather finishes. The size distributions seen in the initial work on three polyurethanes were quite varied. For example, there was a narrow type of distribution where all the particles are of a similar size, as opposed to one which shows a wider distribution. These are both one peak systems, known as unimodal, but it is possible to find multi-modal materials. Effect on properties Nott concluded that it was reasonable to expect the particle size distribution to affect both the physical properties of the finish, eg viscosity, cover and flow out, and also the mechanical properties of the finished leather. It has been suggested that the finish particle size would affect mechanical properties such as gloss^4,3, adhesion⁴, break and handle⁵ and water resistance^4,7. Studies he carried out at BLC were undertaken to assess whether any relationship existed between the finish particle size and the mechanical property of a finished leather. In these tests, a selection of both acrylic and polyurethane resins were used together with a range of different manufacturers' black and white pigments. The finishes made up from these materials were measured to assess their particle size distribution and then used to finish pieces of full grain bovine side leather using one pad and two spray applications of the same formulation; no protective top lacquer was applied. The finished leather was tested for gloss, adhesion and rub fastness, both wet and dry, and flexibility. The results were then related back to the particle size of that finish. When the results of the tests were analysed with respect to the particle size of the applied finish, some relationships between their properties and the average particle size were noted. The particular polymers used had an average particle size of 65nm, while the pigments on their own ranged from 140-4000nm. Penetration is an important factor in obtaining good adhesion. Also, smaller particles can conform to the surface irregularities more closely and this leads to greater mechanical continuity between the finish and the leather. This is for three types of white pigmented acrylic water-based finishes. The average size of the resins was 20-300nm while that of the pigment 300-4000nm. It can be seen that the gloss is reduced as the particle size increases. This seems reasonable because the larger the particles, the rougher the finished surface, as shown on the microscopic level by Talen⁸. Stability of mixtures The experimental results reported above were achieved using relatively simple finish mixes, consisting of one pigment and one polymer emulsion, whereas practical finishes tend to contain more than one resin and at least two pigments. It is known that finishes whose pigmentation is based on white have a tendency to settle on standing. To assess how much standing changed the particle size of a finish, Nott made up a number of finishes consisting of two resins, an acrylate, a urethane, and three pigments, titanium dioxide, phthalocyanine blue and lead chromate yellow. These materials were all from different manufacturers, probably giving the most unstable mix possible due to differing types of stabilising surfactant used by each manufacturer. The mix was diluted to 20% solids and a sample taken for analysis. Further samples were taken at hourly intervals, up to four hours, and after overnight storage. At each measurement point, the mix was stirred in order to reverse gross settlement and to achieve constant colour throughout. It can be seen from this figure that storage of the finish mixture prior to use leads to an increase in the average particle size and to a greater concentration of large particles. This is of practical significance when considered in the light of the fact that many tanners store finishes between finishing packs of the same colour and also that increasing particle size has been shown to lead to a reduction in finish adhesion. This suggests that finish storage can lead to reduced finish adhesion. Further studies applying the stored finish to adhesion at the same time as measuring the particle size distribution and measuring the finish adhesion are shown in Table one. This again shows that finish adhesion is reduced as the particle size increases. These results show the negative effect that finish storage can have on the consistency of leather performance. Conclusions Nott et al showed that the particle size distribution of a finish has an effect upon the properties of the finished leather. This is particularly so in the case of adhesion and gloss.