Effect of Acrylic Polymer Dispersions on Water Vapour Permeability and Some Other Physical Properties of Finished Leather
Chapter One
Research Aim and Objectives
This research aims to prepare a formulation of acrylic polymer dispersions suitable for application in leather finishing.
Objectives
The aim of this research was achieved through the following objectives:-
- Preparation of acrylic polymer formulations.
- Retannage of chrome-tanned leather.
- Application of the prepared formulation on the tanned leather.
- Testing of the finished leathers to determine the water vapour permeability and other physical properties.
CHAPTER TWO
LITERATURE REVIEW
Polymer binders are the main components of aqueous finishing preparations. Three chemically different synthetic types of binders are widely used in leather finishing: acrylates, butadiene, and polyurethanes. They all have specific properties depending on their chemical basis. In order to find out which is the right synthetic binder for a certain application, the typical attributes for these binders have to be established. Typical attributes of acrylates include: good light fastness, good heat stability, good hydrolytic stability, good drying properties, good wet properties, good adhesion, poor embossing properties, poor cold flex properties, poor covering properties, low price range. In order to ascertain which type of polymer fulfils best the necessary properties of a finish, the key attributes which make up for the quality of finished leather have to be demonstrated. For the choice of polymers not only the expected character and properties of the leather are of importance but also the behavior of the binder during the finishing operations. These include stability, compatibility with solvents, flow out properties, penetration properties, drying properties, stackability of the leather, ironing- and embossing properties of the finished leather, and smell of the binder. Binders with small particles size have better penetration properties. These are good for impregnation, but less suitable for covering grain defects and not good for embossing. The acrylic binders are very useful for tightening the grain. The harder a product based on the same chemistry, the better it tightens the grain. That means an acrylic with a Shore A hardness 45o would be better than one with 25o. On the other hand the leather becomes slightly firmer. Ever since its acceptance as a binder in leather finishing, the acrylic polymer resins have been enjoying a deserving preference rather as ‗filler-binders‘ for excellent impregnating and grain-sealing properties. As film-forming binders they are also acknowledged for their performance on base coats by virtue of their film-adhesion, grain tightening and sealing effects.
Acrylic polymers have found extensive use in leather finishing. The basic properties that these polymers offer are softness, heat and light stability, and favourable economics. Acrylic polymers have been the choice of basecoats for all types of leathers. The softness of acrylic resins, penetration and adhesion, lightfastness, good molding under the embossing press, and their low cost have contributed to the success of acrylic resins for decades. However, traditional acrylic polymers have been regarded as having high tack, modest fill and coverage, poor embossing plate release, cut-through under harsh conditions (sharp plate and high temperature), and moderate to poor flexibility. These shortcomings were usually remedied by the addition of other resins such as butadienes or polyurethanes or processing additives such as fillers, waxes and crosslinkers. The challenge for acrylic polymers has been to offer soft polymers with reduced tack, improved embossing properties, and physical properties that improve performance of acrylic leather finishes (Campbell and Choi, 1997).
CHAPTER THREE
MATERIALS AND METHODS
Materials
Experimental equipments
- BS- U Viscometer, ii. Hand Pump, iii. Lastometer (Muver, Model 5077-ET), iv. Thermometer,
- Thermostat, vi. Water vapour permeability meter (Muver Model 5011),
Finishing consumables
- Resin Binder ( Nycil, AE 558), ii. Wax (Lepton-Wax A, Basf), iii. Penetrating agent (EE 8044, Pixel Colour), iv. Liquid Syntan (Syntan-Re, Smit-zoom),
- Powdered Syntan (Syntan-SA, Smit Zoom), vi. Bagaruwa (Vegetable Tannin), vii. Wet Blue sheep skins, viii. Toluene.
Methodology
Viscosity measurement of the resin binder
The solution viscosity measurement of the Binder was carried out at 25 oC using toluene as the solvent. 1 g of the solid polymer was dissolved in 50ml of the solvent to give a stock solution of 0.02 g/dl. The stock solution was divided into four portions, one was left that way while the other three were diluted by adding the solvent in the order 5 ml, 10 ml, and 15 ml. 10 ml of the pure solvent was introduced into the viscometer and the elution time, t0 was obtained. This was repeated for each solution and the corresponding elution time was obtained and recorded as t1, t2, t3, and t4 respectively.
CHAPTER FOUR
RESULTS
Examination of the Resin Binder
Viscosity and molecular weight measurement of the resin binder
Viscosity is an integral property of a fluid that offers resistance to flow. It is due to the internal friction of molecules and mainly depends on the nature and temperature of the liquid/solution. Results of solution viscometry of the polymer are presented in table 4.1 below. The elution time measured in seconds of the varied concentrations was transformed into relative viscosity, specific viscosity, and reduced viscosity. The plot (fig 4.1) of reduced viscosity against concentration was extrapolated to the intercept which correlates the intrinsic viscosity of the polymer. The intrinsic viscosity was found to be 227 dL/g, and the calculated average molecular weight of the polymer was 4.03×105 (see appendix II).
CHAPTER FIVE
DISCUSSION
Results of solution viscometry of the resin binder are presented in Table 4.1. Viscosity is a measure of the resistance of a fluid to flow. Anderson and Dea, (1967) stated that viscosity is one of the most important analytical and commercial parameters in polymers because it is affected by the size and shape of macromolecules. Viscosity of the resin binder in toluene was found to depend on binder concentration. In this study, the elution time measured in seconds of the varied concentration was transformed into relative viscosity (ῃrel), specific viscosity (ῃsp) and reduced viscosity (ῃred (dl/g). The plot of the reduced viscosity (ῃred) (dl/g) against the concentration (g/dl) was used to obtain the value of intrinsic viscosity [ῃ]. This plot is shown in Figure 4.1 whereby the value of the intrinsic viscosity (227 dl/g) was obtained from the intercept of the reduced viscosity at zero concentration. This is the measure of the inherent property of the binder. The intrinsic viscosity was used to determine the molecular weight of the binder using the famous Huggins equation: [ῃ] = KMa. The calculated viscosity average molecular weight is 403,000, and the details of the calculations are shown in Appendix II. This result showed that the binder is a high molecular weight polymer suitable for use in leather finishing.
CHAPTER SIX
CONCLUSION AND RECOMMENDATIONS
Conclusion
The acrylic resin binder is a hard binder with a very large molecular weight, and has a melting temperature in the range 361-370 oC. This shows that the binder is a high polymer and possesses the properties suitable for application in leather finishing. The effect of the resin formulations on some physical properties of the originally retanned leathers have been studied and it is obvious that the finish had significant effect on such properties as water vapour permeability, lastometer, Shore A hardness, and wet rub fastness. The results showed that the finished leathers were better than their unfinished counterpart. Increasing the quantity of the acrylic resin in the formulations also increased the water vapour permeability and wet rub fastness of the finished leathers except for the lastometer tests and shoreA hardness where there was no specific trend in behaviour. All the formulations showed better and adequate response to all the properties tested when compared to that of the unretanned leathers. Sample A1 (125 g resin offer), however would not be suitable for use in leather finish formulations where wet rub fastness is a priority. Aesthetic properties are very important, but finishes must be durable and standards for upper leathers and must include an assessment of finish properties. This has been highlighted in literatures because of the increasing incidence of worn shoe complaints involving lack of finish fastness, and this report has shown that the quantity of components in finish formulations play an important part in determining wet rub resistance. The effect is probably from the resin binder dispersions.
Recommendations
It is suggested that future development should be aimed at producing components for leather finishes more resistant to water, or means must be found for inactivating the wetting agents present in the finish. While a comprehensive assessment of the effect of all finish components is impossible it is likely that a limited number are of interest to individual tanners who should ensure that new formulations are adequately tested before applying them to bulk production. As finishing plays an important role in water vapour permeability in order to improve the water vapour permeability of leathers, studies should be done on how to improve the water-absorbing capacity of finishing agent. On the other hand, if the water absorbing capacity of finishing agent is too high, the wet rubbing resistance may be decreased. So the work should be done to find a balance to improve the water absorbing capacity without decreasing the wet rubbing resistance of leathers.
REFERENCES
- Abd El-Ghaffar, M.A., El-Sayed, N. H., and Masoud, R. A., (2003). Modification of Leather Properties by Grafting. I. Effect of Monomer Chain on the Physico-Mechanical Properties of Grafted Leather”; Journal of Applied Polymer Science, 89 (6): 1478–1483.
- Alexander, K.C and Chol-Yoo, C., (1997). SB Acrylic Technology: Closing the Performance Gap, Journal of American Leather Chemists Association, 92: 17-20.
- Anton, E., (1998). Structure property relationship of polyacrylate retanning agents”; Journal of American Leather Chemists Association, 93(1): 1-15.
- Anzlover, A., and Zigon, M., (2005). Semi-interpenetrating Polymer Networks with Varying Mass Ratios of Functional Urethane and Methacrylate Prepolymers, Acta. Chem. Slov.52: 230-237.
- Barrere, M., and landfester, K., (2003). High Molecular Weight Polyurethane and Polymer Hybrid particles in Aqueous Miniemulsion, Macromolecules, 36:5119-5125.
- Biemond, G.J.E., Braspenning, K., Gaymans, R.J., (2008). Polyurethanes with Monodisperse Rigid Segments Based on a Diamine-Diamine Chain Extender, Journal of Applied Polymer Science, 107: 2180-2189.
- Braithwaite, T.J., (1997). The Effect of Particle size on the Characteristics and Performance of Emulsion Binders and Topcoats, Journal of the Society of Leather Technologists and Chemists, 62: 82-86.
- Chai, S.L., Fu, L.H., and Tan, H.M., (2008). Synthesis, Application and comparison of Aqueous Polyurethane, Journal of American Leather Chemists Association, 103: 191-199.
- Chai, S.L., Jin, M.M.., and Tan, H.M., (2008). Comparative Study Between Core-Shell and Interpenetrating Network Structure Polyurethane/Polyacrylate Composite Emulsions, European Polymer Journal, 44: 3306-3313.
