Siloxane-modified epoxy networks

Fig. 2. Two-step synthesis of siloxane-modified epoxy network. First step, capping of siloxane oligomer with Epon 828. Second step, crosslinking of Epon 828 and capped siloxane with PACM-20...

The dynamic mechanical properties of the siloxane-modified epoxy networks were also investigated. The DMTA curves for the control epoxy network exhibit the two major relaxations observed in most epoxy polymers 39 40,41>. A high temperature or a transition at 150 °C corresponds to the major glass transition temperature of the network above which large chain motion takes place. The low temperature or (5 transition is a broad peak extending from —90° to 0 °C with a center near —40 °C. It has been attributed predominantly to the motion of the CH2—CH(OH)—CH2—O (hydroxyether) group of the epoxy 39-40 2 ... 

Figure 13.11 Paris diagrams for fatigue crack propagation of polyldimethyl-siloxane)-modified epoxy networks. (Rey et a/., 1999 with kind permission from Kluwer Academic Publisher.)... 

GLASS TRANSITION TEMPERATURES OF SILOXANE MODIFIED EPOXY NETWORKS (a)... 

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. 

Figure 5. ATR/FTIR Spectrum of 2% Piperazine Terminated Siloxane Dimer-Modified Epoxy Networks.

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). 

Siloxane-modified networks were prepared for testing via two steps. A linear precursor was generated by reacting the epoxy resin with the siloxane oligomer for one hour under vacuum at 65 °C. PACM-20 was then added, and the mixture was stirred for five minutes under vacuum at 50 °C. Previous studies indicated 151 that reaction between the AEP-terminated siloxane oligomers and the curing agent is not possible, as one would expect. 

The control resin network used in this study was a diglycidyl ether-based epoxy resin crosslinked with a cycloaliphatic diamine. Cooligomeric modifiers were prepared having varying percentages of TFP and DP siloxane and aminoethylpiperazine end groups. Both siloxane and ATBN and CTBN elastomers were used as epoxy modifiers, the latter two having been included to facilitate direct comparisons between modifiers in similarly prepared networks. 

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