Curing epoxides

M. Shimbo, M. Ochi and K. Aral, "Internal Stress of Cured Epoxide Resin Coatings Having Different Network Chains", J. of Coatings Technology, Vol. 56, 713, 45-51, June 1984. 

Swelling experiments showed that a lignin epoxide resin of 0.11 epoxy equivalents per lOOg formed a network polymer when cured with DETA, PA, or ATBN. Phase separation was observed in the rubber-toughened lignin epoxide network. Cured epoxides had lignin derivative contents of up to 95%. 

The effect of using different silanes as pretreatment primers on the initial bond strength of a two pack polyamide cured epoxide to mild steel and aluminium is shown in Table 2. 

To determine the wet bond strength coated panels were immersed in distilled water for 1500 h, removed and discs 25.4 mm in diameter stamped from them. The surfaces were wiped with a dry tissue and bonded between two cylindrical test pieces using a polyamide cured epoxide adhesive and immediately placed in a sealed container at 100% RH for the adhesive to cure. After 16 h the specimens were broken on an Instron Universal Test Machine with minimum delay. Recovered values were measured after the panels had dried out at room temperature and humidity for 7 days. Clearly, it is unlikely that the values reported represent the minimum bond strengths, as some drying out is almost inevitable, but the values are directly comparable. 

As it had been shown that silanes were effective as pretreatments for a variety of coatings and particularly so when used as additives, selected silanes were examined as pretreatments and additives in conjunction with a two pack polyamide cured epoxide adhesive (Epikote 828/Versamid 115, 1/1) and a structural polyurethane adhesive based on diphenylmethanediisocyanate and a polyester resin. 

Figure 5. Effect of 100% RH at 2I°C on the bond strength of a glass/aluminium joint—amine cured epoxide adhesive.

Reference to Table 14 will show the effect of increasing levels of APES on the compressive properties of an anhydride cured epoxide/silica microballoon foam, the APES being added on the resin content. The notation w/r (wt% resin) has been used in the tables. Both the yield stress and strain to failure increased steadily with increased silane content, with a corresponding increase in compressive modulus. At the 5 wt% level there was no real increase in yield stress but a marked increase in strain to failure, resulting in a lower modulus. However, at the 4% level the compressive strength was more than double that of the nonsilane control. 

Effect of silane addition level on the compressive properties of an anhydride cured epoxide silica microballoon syntactic foam (APES, cured at 100 C for 4 h, nominal density 0.35 g/cm )... 

Table 16 shows similar data for a polyamide cured epoxide/silica foam at a nominal density of 0.35 g/cm . All the silanes produced an increase in compressive strength and strain to failure, but showed less effect on the compressive modulus. In general this series of foams were stiffer than the anhydride cured series and the silanes showed a different order of merit. Although it cannot be claimed that the level of addition chosen was necessarily optimum in all cases the value of silanes in increasing the compressive strength is obvious. 

In other words the area under the stress/strain curve is a measure of the energy absorbing characteristics. The effect of APES additions on the stress/strain properties of an anhydride cured epoxide syntactic foam at a nominal density of... 

Figure 7 shows the family of curves for anhydride cured foams containing 5 wt% of various silanes and it can be seen that although every silane improved the energy absorbing efficiency to a marked extent, the APES foam was the most efficient, followed closely by MPS and AAMS. Figure 8 shows similar data for the polyamide cured epoxide foams, from which it can be seen that the MPS foam was the most efficient. In these foams the flat response only held up to 25-30% strain. 

Some important trends of the structure-property relationships in this field are well illustrated by a comparison of some stoichiometric, fully cured epoxide-amine networks (Table 10.6). 

It was pointed out earlier that acid and anhydride containing polymers have many possible applications. With the exception of curing epoxide resins and examination of possible biological activity, little has been done to explore uses for the copolymers developed in this study. 

In this paper we address the area of the structure-property relations of epoxy matrices, with specific emphasis on amine-cured epoxides. In attempts to correlate the structure-property relations of amine-cured epoxides, there have been a number of studies on the mechanical properties of these glasses as a function of epoxide amine ratios and the chemical structure of the constituent epoxide and amine monomers.(1-15) Generally, there is no direct correlation between the chemistry of the epoxide and amine monomers and the mechanical properties of epoxies with the exception that as the distance between crosslinks becomes shorter, these glasses become more brittle.(1,10,15) Also, for a specific amine-cured epoxide system, the Tg is always highest for the fully reacted, highest crosslink density glass.(3,6,9,12,... 

In all these curing reactions, the degree of cross-linking, and also the glass transition temperature, increases with increasing conversion. The segments become less mobile, not all groups can react, and a complete network cannot be formed. Consequently, cured epoxide resins do not have the optimum properties expected of ideal networks. 

Table 35-8, Influence of the Kind of Fiber on the Properties of a Cured Epoxide Resin Reinforced with 65% w/ w Fiber... 

M. Shibata, N. Teramoto, Y. Someya and S. Suzuki, Bio-based nanocomposites composed of photo-cured epoxidized soybean oil and supramolecular hydroxystearic acid nanofibers , J Polym Sci Part B Polym Phys, 2009, 47, 669-73. 

Fig. 3.16. Effect of time lapse between gritblasting and bonding in stressed cleavage specimens constructed with an aromatic amine cold-cured epoxide (Ref. 4).

Water depresses the Tg of adhesives. This is worrying particularly for cold-curing epoxides with typical transitions when dry in the range 40°C-50°C, and underlines the need to select adhesives whose TgS do not drop substantially with water sorption. Several attempts have been made to relate the depression of Tg to current concepts of the glass transition(90, 91). The modulus and strength of the cured polymer matrix are also lowered by water-induced plasticisation(34, 41, 95, 101, 102), in a manner akin to the organic plasticisers often used to modify the mechanical properties of adhesives. Brewis et al.(94) showed that for one particular hydrophilic adhesive/aluminium shear lap joint system, the depression of Tg eould be used as a shift faetor to relate the strength/temperature eurve of dry joints to ones with saturated bondlines. 

For cold-curing epoxides wide variations in adhesive material properties are possible, with different combinations of resin, hardener, filler, and the multitude of modifiers. Products which cure at ambient temperature cannot achieve the same performance as is obtained by curing at elevated temperature. For products cured at room temperature their TgS, at 40-50 °C initially, are relatively low and may be lowered even further by absorbed water, in liquid or vapour form. This may also be accompanied by a reduction in strength and modulus. Thus the use of materials with a slow and small water uptake is to be preferred, which implies a fairly highly cross-linked formulation. Such considerations do of course depend upon the performance and durability expectations in service. Whilst the environmental durability of joints can often be improved enormously by the surface pretreatment methods employed (see Chapters 3 and 4), the adhesive must be selected carefully to ensure long term durability in consideration of the modes and duration of loading, and the environmental conditions. Ideally the adhesive should be fairly tolerant of poor surface pretreatment procedures. 

A flexibilised cold-curing epoxide adhesive containing an aromatic amine hardener is likely to be the most successful, particularly if... 

The literature describes a room-temperature curing epoxide made by converting a part of the epoxy function into episulfide by the addition of NMBTT (N-methyl-benzothiazole-2-thione) (74). 

Thermal analysis (DSC) of heat-curing epoxides (Figure 6), such as specialist chipbonding products for the electronics industry, yields additional information about these materials, including heat of polymerization, peak temperature, onset of heat cure, and polymer conversion data. 

The polyamides used to cure epoxide resins are all reactive compounds with free amine groups. They may be amidopolyamines, aminopolyamides... 

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