Siloxane-modified epoxies, wear

Siloxane containing interpenetrating networks (IPN) have also been synthesized and some properties were reported 59,354 356>. However, they have not received much attention. Preparation and characterization of IPNs based on PDMS-polystyrene 354), PDMS-poly(methyl methacrylate) 354), polysiloxane-epoxy systems 355) and PDMS-polyurethane 356) were described. These materials all displayed two-phase morphologies, but only minor improvements were obtained over the physical and mechanical properties of the parent materials. This may be due to the difficulties encountered in controlling the structure and morphology of these IPN systems. Siloxane modified polyamide, polyester, polyolefin and various polyurethane based IPN materials are commercially available 59). Incorporation of siloxanes into these systems was reported to increase the hydrolytic stability, surface release, electrical properties of the base polymers and also to reduce the surface wear and friction due to the lubricating action of PDMS chains 59). 

This chapter is meant to be an overview of ongoing studies of polysiloxane-modified epoxy resins. Because this research area is still quite young, it is not yet possible to write a standard review article. Presented here is the current status of a collaborative effort encompassing chemistry and synthesis of the modified networks, their morphology, their mechanical properties, and their friction and wear behavior. The earliest work in the synthesis and characterization of siloxane-modified networks was done by Riffle et al. 15). More recent research in the area of chemistry and synthesis has been carried out by Tran 17). 

As will be discussed, incorporation of siloxane oligomers modified the elastic moduli and the fracture properties of the crosslinked epoxy network. Previous work 15) indicated that the surface of these materials was rich in siloxane, which is believed to foster a low energy surface. These characteristic properties have led to our interest in the friction and wear of siloxane-modified epoxies. 

The number of cycles of disk rotation required to initiate the wear track correlated positively with the weight percent of the siloxane modifier in the epoxy. However, the initiation times for the ATBN- and CTBN-modified epoxies showed no significant correlation with the percentage of the incorporated modifier. The initiation of the wear track is assumed to result from the fatigue of the epoxy hence initiation time is related to the surface stresses. Because the surface stresses are inversely related to the elastic modulus as predicted by the Hertzian elastic contact theory 52), the initiation time data at ION load were compared to the elastic moduli of the materials in Fig. 16. The initiation times for the siloxane-modified epoxies were negatively correlated with their elastic moduli while samples modified with ATBN and CTBN showed positive correlations with their moduli. At lower loads the initiation times for the siloxane-modified epoxies increased. The effect of load on the CTBN- and ATBN-modified epoxies was too erratic to show any significant trends. 

The tests in which the epoxy pins were rubbed on steel disks showed that the pins were initially worn by the abrasive action of the asperities on the steel surface. This initial wear correlated with the inverse of the values. During the initial wear, the steel surface was smoothed by the transferred epoxy material. The steady state wear which followed the initial wear was lower in magnitude than the first stage of wear. The highest wear rate was obtained with 15 wt,-% of the dimethyl siloxane modifier. 

The initial and steady state wear rates of the siloxane-modified epoxy pins on the steel disks correlated with the inverse of the KIC values which agrees with previous abrasive wear tests 47>. The steady state wear rates on the smooth glass disks were comparable to those on the steel disks. Thus in both cases the wear mechanism is abrasive wear by the wear particles trapped in the interface between the pin end and the disk. 

In summary, we can first say that there is no significant evidence that the low surface energy siloxane-modified epoxies reduce friction compared with the unmodified epoxy or the ATBN- and CTBN-modified epoxies. Based on the results of the steel ball-on-epoxy experiments, the most significant effect of the siloxane modifiers is the reduction of the elastic modulus associated with large closely spaced domains. The longer initiation times and lowest wear rates observed for the siloxane-modified epoxies were generally associated with a lower modulus. Epoxy modified with the CTBN of 18% AN content also showed lower wear rates with lower modulus but, in contrast with the siloxane-modified resins, had shorter initiation times with lower modulus. 

When polymers slide on machined metal surfaces, it is quite possible that steady-state wear Involves a combination of abrasive, fatigue, and adhesive wear mechanisms. To study fatigue wear, it would be desirable to minimize the contributions of the abrasive and adhesive wear modes. In this paper, the following polymers polycarbonate, polyvinyl chloride, ultra-high molecular weight polyethylene, siloxane modified epoxies, and polylmldes are tested in experiments in which the fatigue wear mode is predominant. 

Yorkgitis and coworkers [214] have chemically modified epoxy resins with functionally terminated poly (dimethyl siloxane), poly(dimethyl-co-methyltrifluo-ropropyl siloxane), and poly (dimethyl-co-diphenyl siloxane) oligomers and have analyzed the morphology, solid-state properties, and friction and wear properties of the system. They have found that the miscibility of siloxane modifiers in epoxy resins can be enhanced by increasing the percentage of methyltrifluoropropyl siloxane or diphenyl siloxane relative to dimethyl siloxane. 

In the fatigue tests, the friction measured in the initial period of sliding during which no wear occurred was not affected significantly by the addition of siloxane or ATBN and CTBN modifiers to the epoxy as shown in Fig. 15 for ION load. At lower loads, the friction coefficients are higher but still show no significant change with the additions of the modifiers. 

Most tests were run for 14 kc of disk rotation. However, some materials did not initiate a wear track within the 14 kc, and the tests were extended, in some cases to over 30 kc. Even these extended tests were usually terminated before the wear track initiated. The modifier which produced the longest initiation times was the 20% TFP siloxane co-oligomer when present at 10 and 15 wt.-% in the epoxy. 

Fig. 17. Wear rates at ION load for the siloxane-, CTBN-, and ATBN-modified epoxies...

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