Improvement of PVC wearability by addition of additives
Introduction
Polyvinyl chloride (PVC) is now the second most commonly used plastic material in terms of volume. Great achievements have been made in modifying its structure, behavior, and properties by including miscellaneous additives. Aspects of mechanical, thermal and electrical properties of PVC composites were intensively studied. However, knowledge of the effects of fillers on wear resistance of PVC composite materials are limited and wear data are not available.
Wear is generally understood as progressive loss of material from the surface of a solid body caused by a mechanical action, i.e., the contact and relative motion with a solid, liquid, or gaseous phase. Since at least two components of a system are involved in wear, it is not a pure material characteristic, but only a system characteristic. For rigid polymers, the simplest physical concept of abrasive wear is that asperities of the harder surface penetrate the polymer and remove material by shearing or cutting [1]. Later studies [2]showed that the roughness of the counterface has a profound effect on the wear of some polymers.
Polymers usually have poor resistance to abrasive sliding attack because of their relatively low levels of hardness and strength, high plasticity, and low thermal conductivity. Fillers will certainly improve these properties therefore promote the wear resistance. The mechanisms revealing how fillers reduce wear are not well established. At present, there appear to be two broad explanations. One of them stems from the observation that excess filler concentration is noticed on the composite surface after prolonged sliding. Based on this observation the bulk of the load is supported by the concentrated filler resulting in increased wear resistance of the composite [3]. The second suggestion is that fillers may improve the adhesion of the transfer film to the counterface and thereby suppress the wear process.
While the first explanation might be accepted easily by common sense, the second is complicated. In a broad sense, it is considered to be either physical or chemical in nature. Physical interaction involves van der Waals forces and is comparable in strength with the forces between molecular chains within the polymer itself. The chemical interaction, which is promoted by the high temperature generated at the sliding interface, may increase the adhesion of the transfer film. Some studies proved that there exists oxidation or chemical decomposition of unfilled and filled polymers in sliding contact [4]. However, the evidence for chemical effects in improving the adhesion of transfer film to the counterface is as yet inconclusive.
Since a large number of fillers are abrasive, they tend to abrade the counterface. The effect of the fillers to the resistance of wear depends on many factors, such as the shape, size, relative hardness and aspect ratio of the filler particles, the content of filler in the composite, the sliding speed of the specimen on the counterface, the load exerted on the specimen, the environmental condition and so on 5, 6. The initial counterface roughness also seems to affect the transfer film and so the wear rate. It has been stated in the literature [7]that there is an optimum roughness for minimum wear. A direct correlation between these variables and the wear rate is often difficult to establish because of the breakdown and accumulation of the filler particles in the contact zone 8, 9and the work hardening of the counterface [10]. Many composites exhibit pronounced environmental sensitivity, which is attributed to specific filler–environment interactions 11, 12. A collection of data abstracted from the literature by Arkles et al. [13]shows no discernible pattern of behavior in this respect.
The objective of this work is not to correlate these variables but to investigate the wear behavior of PVC filled with different materials in varying proportions and combinations, and to explore a practical means of obtaining high wear-resistant PVC composites. The chosen fillers for this study include inorganic materials such as Si, SiC, Al2O3, CaCO3, B4C, wollastonite fiber, fly ash, and organic materials such as carbon black, graphite, ABS, PTFE, etc.
Section snippets
Supporting equipment
An analytical balance (Model 200, maximum sensitivity 0.0001 g, Sartorius, Long Island, NY) was used for the sample mass measurement. PVC and filler powders were mixed by a Turbula Shaker–Mixers (Type T2C, Impandex, NJ). Specimens of the composites were molded by a press (Laboratory Press, Fred S. Carver) and a die made of steel. Electrical heating tape was wound on the outside surface of the die to supply heat for melting the powder mixture. The heating temperature was controlled by a
Loss of material after wear
Mass loss of pure PVC after wear under the above mentioned testing conditions was 0.0190 g. The wear of all the PVC composites was expressed as a relative mass loss, which is calculated by the following equation:where, X is the relative mass loss, WC is the mass loss of composite and WPVC is the mass loss of pure PVC.
All wear results were plotted in Fig. 3(a) to (c).
It is seen from Fig. 3 that the composites of PVC/Al2O3/coupling agent (Fig. 3(a)) and PVC/SiC/ABS (Fig. 3(b)) have a
Conclusions
The addition of various fillers to PVC matrix modifies the wear-resistance of the composites. SiC and Al2O3 show the highest improvement on the wear resistance. Si and wollastonite enhance the wear resistance obviously. Fly ash and B4C reduced wear only when 10% fly ash or 7% of B4C was included. CaCO3 and SiO2 have a negative effect on wear resistance. Carbon, graphite, and PTFE are not compatible with PVC.
Blending of ABS to the composite improves the strength of the composite. Titanium
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