Epoxies, siloxane-modified

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). 

Adherent films would not necessarily require formation of covalent bonds at the Interface, since localized lntermolecul u dispersion forces that u e operative In the adsorption of coatings (with good wetting properties) should provide stable interfacial bond conditions. Among candidate materials which could fulfill the requirements of good adhesion and substrate protection from moisture are epoxy-modified polyurethanes and epoxy-siloxane polymers. 

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. 

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. 

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 ... 

The Tg values determined by DSC for the pure liquid siloxane oligomers were in good agreement with the values determined from DMTA of siloxane-modified epoxies. However, at 0 and 20% TFP content, the siloxane Tg from DMTA was about 16 °C higher than the Tg found by DSC. This suggests that at TFP contents above 20%, the siloxane separates from the epoxy as a purer phase. This point will be discussed further in the next section. Also reserved for later discussion is the depression of the major epoxy transition with an increase of the 2070-40F oligomer. 

In summary of these points, it is seen that the isolation of particles from the epoxy matrix, the effective volume fraction of the elastomeric phase, and strength of the interface interact to control modulus. The morphology which a particular siloxane modifier promotes determines the contribution of any or all of these three factors to the modulus of the modified resin. 

These differences in modulus may be at least partially explained by DSC data such as that in Fig. 10. It is seen that in general the glass transition regions of the ATBN-and CTBN-modified epoxies are broader and have a lower midpoint than those of the control and two siloxane-modified materials. This thermal data suggests that the butadiene oligomers are relatively more miscible with the epoxy and may act as plasticizers. As an additional point, it is likely that the higher molecular weight of the... 

Fracture toughness results for the TFP siloxane-modified epoxies are given in Fig. 11 a. With slight exception, modification with PDMS and 2330-20F gives virtually no improvement in KIC and in fact lowers KIC approximately linearly with weight... 

Our interest in PDMS as an epoxy modifier lies partly in its low Tg relative to the ATBN and CTBN modifiers. Up to this time, however, improvements in K,c through copolymerization of dimethyl siloxane with TFP and DP siloxane require raising the Tg of the siloxane modifier above that of PDMS, as shown by Table 1. It is hoped that increased understanding and control of the synthesis and morphologies of siloxane-modified epoxies will make it possible to retain the low Tg of the modifier while raising the fracture toughness of the resin. The true value of this objective could eventually be shown by measurement of Klc at temperatures below ambient. 

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. 

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