Adhesive Bonding in Severe Applications - Fluoroepoxy Adhesives

Topics Covered

Background

Limitations of Epoxy Adhesives

Halogenated Epoxies

Background

Hydrophobic adhesives have attracted considerable research interest. These adhesives prevent moisture reaching the critical area of the interface between the polymer and metal substrate and so prevent or delay the onset of joint failure. A useful series of moisture resistant materials are based on fluoroepoxy resins. If these are cured with suitable hardeners then adhesives can be produced with very low moisture diffusion coefficients.

Limitations of Epoxy Adhesives

Most structural adhesive joints are based on epoxy adhesives. Unfortunately, these resins can absorb considerable quantities of water, the precise amount and rate of absorption depending on the exact resin structure. A number of effects result from the ingress of moisture to an adhesive joint including plasticisation of the system and disruption of the interfacial region between the substrate and the organic phase. One method of improving the durability of adhesives is to make the bulk adhesive more water resistant. This helps in two ways: it cuts down the rate of diffusion of moisture to the critical interphase between the substrate and the adhesive, and it also reduces the effect on the bulk properties of the adhesive.

Halogenated Epoxies

Recently a number of halogenated epoxy resins with substantial hydrophobic characteristics have been developed. In a joint programme of work between British and American government laboratories, a series of experiments were conducted to characterise the mechanical properties and moisture uptake of these resin systems. Two general forms of curing agent were used in the study, silicone amines and fluoroanhydrides. In all cases elevated temperature cures were required together with catalysts such as tertiary amines to accelerate the cure reactions.

The moisture uptake characteristics of the resins are extremely important so a series of experiments were conducted whereby the weight increase of samples immersed in water at various temperatures was determined. This allowed the diffusion coefficients of the resin systems to be determined and also the maximum moisture uptake as a function of immersion temperature. There is a wide variation in the maximum amount of water absorbed by the resins. Certain systems have a very low absorption at the lower temperature but this breaks down at the higher temperatures. One resin system produces a maximum uptake of moisture of only 0.3% over the complete temperature range.

A further important parameter concerns the thermal resistance of these materials and, as would be expected, the choice of curing agent dominates the eventual glass transition temperature of the resin. The anhydride cured systems have eventual Tg’s above 100°C whereas the siliconeamine cured systems have a Tg around 60°C. The mechanical properties of various resins are listed in Table 1. It is apparent that there is a wide variation between the mechanical properties obtained and the formulation content.

Table 1. Mechanical properties of various halogenated epoxy resins

While some very promising adhesives properties are displayed significant problems such as toughness, still need to be over come.

Primary author : S.J. Shaw and D.A. Tod

Source : Abstracted from Materials World, vol. 2, pp. 523-25, 1994 “Adhesive bonding in severe environments”

For more information on Materials World please visit The Institute of Materials