Epoxy resins, additives Moisture

Effect of Epoxy Resin Addition on the Moisture Sensitivity of Macro Defect Free Polymer-Cement Composites... 

In addition to the Bisphenol-A backbone epoxy resins, epoxies with substituted aromatic backbones and in the tri- and tetra- functional forms have been produced. Structure-property relationships exist so that an epoxy backbone chemistry can be selected for the desired end product property. Properties such as oxygen permeability, moisture vapor transmission and glass transition temperature have been related to the backbone structure of epoxy resins5). Whatever the backbone structure, resins containing only the pure monomeric form can be produced but usually a mixture of different molecular weight species are present with their distribution being dictated by the end-use of the resin. 

A typical cured aerospace epoxy resin is highly cross-linked and may contain a dispersed nonpolar thermoplastic or rubber which reduces the equilibrium moisture content and hmits the reduction in Tg. As shown in Table 12.2, more advanced systems, such as an epoxy—cyanate ester blend, absorb lower concentrations of water at equilibrium. However, the water is not uniformly distributed so that the epoxy phase is stiU plasticised to the same extent. Figure 12.12 shows that the different component epoxies and/or hardeners are plasticised differentially with the development of additional relaxation peaks. We discussed the different degrees of water molecule hydrogen bonding as a function of network polarity in this chapter. 

Table 3.2 also includes data for the advanced thermoplastic resins (PEEK) and for a thermosetting resin, an end-capped bismaleimide (BMI) called PMR-15. Moisture contents tend to be lower for these advanced materials [12,13]. One way to overcome the environmental sensitivity of epoxide resins is to employ these advanced resins, as demonstrated in Table 3.2. However, changing to other resin systems brings with it other concerns. For example, PEEK relies on crystallinity for its higher temperature performance. Its glass transition temperature is only 143°C and a change in modulus can be observed at that temperature. In addition, higher process temperatures are required both for high performance thermosets and thermoplastics. The consequent higher residual thermal stresses can off-set some of the advantages of a higher service temperature, in comparison with advanced epoxy resins.

J A Hough and F R Jones, Effect of thermoplastic additives and carbon fibres on the thermally enhanced moisture absorption by epoxy resins , in Eleventh... 

The primary problem with epoxy resins appears to be the swellability of the resin in moisture (18). This is likely due to the preponderance of hydrophilic groups in the cured epoxy-adhesive. Urethanes have the additional problem that metal ion catalyzed hydrolysis of the urethane bond can take place. 

In contrast to the condensation polymerisation reaction for epoxy resins, another fast-acting fixing agent. Super Glue , uses an addition reaction to stick objects together. The monomer is CH2= C(CN)COOCHj, methyl cyanoacrylate, and the addition takes place across the carbon-carbon double bond. The polymerisation reaction is initiated by the presence of moisture. 

Nylon-epoxy is typically used in film and tape adhesive applications and has maximum service temperature of 138°C, compared with 177°C for unmodified epoxies. The addition of nylon provides increased flexibility and peel strength compared to unmodified epoxies. Nylon-epoxy is a tough material with excellent tensile lap shear strength and good fatigue and impact resistance. However, the addition of nylon results in deterioration in creep resistance and poor peel strength at low temperatures. Furthermore, the hydrophilic nature of nylon imparts poor moisture resistance in both uncured and cured forms of the resin. Consequently, nylon-epoxies are not as durable as elastomer-epoxies or other thermoplastic modified epoxies. 

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