Creep Epoxy

Water is a natural plasticizer for many polar polymers such as the nylons (23K). polyester resins (239), and cellulosic polymers (240). It strongly shifts in epoxies (241.242). Thus the creep and stress-relaxation behavior of such polymers can be strongly dependent on the relative humidity or the atmosphere. 

Cured Epoxy Requirements. In order to comply with multiple needs identified above, the epoxy pot should have a compressive yield strength of greater than 9000 psi. In addition, for brackish and seawater applications an arbitrary specification of zero creep at 50°C at 1000 psi under seawater for 3000 hours has been established. Applications at higher temperatures would obviously demand a higher zero creep temperature. 

The quasielastic method as developed by Schapery [26] is used in the development of the viscoelastic residual stress model. The use of the quasielastic method is motivated by the fact that the relaxation moduli are required in the viscoelastic analysis of residual stresses, whereas the experimental characterization of composite materials is usually in terms of the creep compliances. An excellent account of the development of the quasielastic method is given in [27]. The underlying restriction in the application of the quasielastic method is that the compliance response of the material shows little curvature when plotted versus log time [28]. Harper [27] shows excellent agreement between the quasielastic method and direct inversion for AS4/3510-6 graphite/epoxy composite. For most graphite/thermoset systems, the restrictions imposed by the quasielastic method are satisfied. 

Epoxy-nylon adhesives are limited to a maximum service temperature of 85°C, and they exhibit poor creep resistance. Possibly their most serious limitation is poor moisture resistance because of the hydrophilic nylon (polyamide) constituent.9 The degradation by exposure to moisture occurs with both the cured and uncured adhesives. 

Polymerized epoxy adhesives are amorphous and highly crosslinked materials. This microstructure results in many useful properties such as high modulus and failure strength, low creep, and good chemical and heat resistance. However, the structure of epoxy resins also leads to one undesirable property—they are relatively brittle materials. As such, epoxy adhesives tend to have poor resistance to crack initiation and growth, which results in poor impact and peel properties. In sealant formulations, epoxy resins do not often provide the degree of elongation or movement that is required for many applications. 

Talc or hydrated magnesium silicate is another mineral that is used to reinforce epoxy adhesives. It has a platelike structure that provides good stiffness and creep resistance at elevated temperatures. It also provides good electrical and chemical resistance characteristics.26 It is relatively inexpensive and disperses well in the resin. 

S. L. Phoenix, P. Schwartz, and H. H. Robinson, IV, Statistics for the Strength and Lifetime in Creep-Rupture of Model Carbon/Epoxy Composites, Composites Science and Technology, 32, 81-120 (1988). 

H. Otani, S. L. Phoenix, and P. Petrina, Matrix Effects on Lifetime Statistics for Carbon Fibre-Epoxy Microcomposites in Creep Rupture, Journal of Materials Science, 26, 1955-1970 (1991). 

The thermal and UV stabilising action of linear, low molec.wt. unsaturated polyesters and epoxy resins in PVC was investigated using short-term tensile and long-term tensile creep testing and calculations of isochronous creep... 

The evolution of the tensile creep compliance of a glassy epoxy resin at different aging times is shown, as an example, in Figure 12.22. The glasses were obtained by quenching the resin from Tg -I- 22°C to Tg — 9°C and were kept at this temperature for different intervals of time. The results obtained show that as the aging time increases, the values of J t) for comparable... 

The contribution of crazing to ten e deformation increases with stress, and therefore with strain rate. This point is illustrated in Fig. 3, which ows changes in mechanism with stress and strain in ABS and in tcai ened epoxy resin. The data are taken from creep tests in which the strain rate increased with time under load, so that the values quoted for low strains are also for low strain rate. The changes in... 

Fig. 3. Rdationsh between volume strain and axial strain dwing creep of (a) AES po rmer, and (b) epoxy rean containii CTBN rubb , at 20 °C. Note m%niQcatk>n of vohime strain scale in (b)...

It has been demonstrated that polydiacetylene single crystal fibres are relatively perfect and have excellent molecular alignment. In consequence they display high values of stiffness and strength and are very resistant to creep. It has been shown that such fibres have considerable promise as reinforcing fibres in an epoxy resin matrix and the study of such composite systems has enabled considerable fundamental information to be obtained concerning the mechanisms of fibre reinforcement. 

The room-temperature tensile properties for the present epoxy networks are independent of or the distribution of M. This independence is ascribed to the restricted motion of the main chains, even above Tg, as deduced from the creep experiments. 

Creep Behavior of Amine-Cured Epoxy Networks Effect of Stoichiometry... 

In some epoxy systems ( 1, ), it has been shown that, as expected, creep and stress relaxation depend on the stoichiometry and degree of cure. The time-temperature superposition principle ( 3) has been applied successfully to creep and relaxation behavior in some epoxies (4-6)as well as to other mechanical properties (5-7). More recently, Kitoh and Suzuki ( ) showed that the Williams-Landel-Ferry (WLF) equation (3 ) was applicable to networks (with equivalence of functional groups) based on nineteen-carbon aliphatic segments between crosslinks but not to tighter networks such as those based on bisphenol-A-type prepolymers cured with m-phenylene diamine. Relaxation in the latter resin followed an Arrhenius-type equation. 

Table I. Composition of Epoxy Specimens for Creep Study...

Figure 1. Typical curves of creep modulus vs. log t at different temperatures (epoxy specimen A-10). Master curve at 169° C shown as solid curve.

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