DGEBA-type epoxy resin

In adhesive systems, vinyl esters impart low-viscosity, flexibility, and superior wetting characteristics to DGEBA-type epoxy resins. Because of their good chemical resistance they are often found in construction applications such as flooring and grouting. However, their shrinkage is greater than that of any conventional epoxy resins, and often the formulator will have to counteract this. 

Figure 2.12 Reaction schemes for the synthesis of DGEBA-type epoxy resin...

Epichlorohydrin/diaminomethyl/cyclohexane derivative condensates act as effective epoxy adhesive curatives, even after such mild cure conditions as 20 min at 100°C. One example of such a curative is l,3-bis(4-aminomethylcyclohexanemethylamino)-2-propanol. Room temperature overlap shear strengths on steel in excess of 6,700 psi (475kg/cm ) were reported using DGEBA-type epoxy resins. 

Curatives have been prepared via condensation of (meth)acrylonitrile with various hydrazides such as carbohydrazide, oxalyl dihydrazide, and succinyl dihydrazide (Eq. 16). When these cyanoethylated and cyanopropylated hydrazides are mixed with DGEBA-type resins, viscosity increases are noted in 4-5 days at room temperature. However, these partially reacted mixtures are still uncrosslinked and flowable at temperatures near their final cure temperature of around 120°C. At this temperature reaction occurs very rapidly for a variety of epoxide-to-amine ratios. Adhesive compositions of DGEBA-type epoxy resins and the cyanoalkylated hydrazides give extremely strong bonds on substrates such as sheet moulding compound (SMC) and steel. 

Combined tertiary/secondary amine curatives, prepared from ter-tiary/primary diamines and epoxy resins, have been used as accelerators for primary aromatic amine cures of DGEBA-type epoxy resins. For example, diethylaminopropylamine/DGEBA resin adducts have been used in m-phenylenediamine-cured epoxy resin systems to provide relatively low temperature cures (50-160°C). The cured resin systems have good toughness and heat distortion temperatures near 130°C. 

Table 2 Some curing agents for bisphenol A (DGEBA type) epoxy resins.

Commercial cement modifiers used were based on a bisphenol A-type epoxy resin, diglycidyl ether of bisphenol A (DGEBA), which was mixed with three types of hardeners, modified polyamide-amine (MPAA),modified aliphatic polyamine (MAPA).and modified amine (MA) at the hardener contents recommended by the respective manufacturers. The hardener content was expressed as follows ... 

The viscosity of an epoxy resin is dependent primarily on its molecular weight (MW). Low-MW resins typically have a viscosity in excess of 6000 cP, and conventional DGEBA epoxy resins (EEW = 190) have a viscosity around 12,000 cP. Therefore, for applications requiring relatively low viscosity, it is necessary to include other types of epoxy resin or to use diluents to achieve the desired properties. 

These resins are most often characterized by their epoxy equivalent weight (EEW), molecular weight (number of repeating units ri), and viscosity. Table 4.2 shows the relationship between EEW and viscosity. These DGEBA epoxy resins can be used alone or in blends with other DGEBA resins, other epoxy resins, or even other types of polymeric resins. Very often commercial epoxy resin products are actually blends of resins having a broad molecular weight distribution. 

A majority of the world s epoxy resin market consists of the DGEBA type. The liquid DGEBA resins are typically used where low viscosity, high reactivity, and high crosslink density are required. The low viscosity allows them to be conveniently and easily compounded with fdlers and other additives. The relatively high functionality allows them to... 

In adhesive formulations, aliphatic amines are most commonly used to cure the DGEBA type of epoxy resin. Aliphatic amines are not widely used with the non-glycidyl ether resins, since the amine-epoxy reaction is slow at low temperatures. The reaction usually requires heat and accelerators for an acceptable rate of cure. Aliphatic amines are primarily used with lower-viscosity DGEBA resins because of the difficulty in mixing such low-viscosity curing agents with the more viscous epoxy resins. 

The epoxy-PVC plastisol type is a mixture of a plastisol-grade PVC powder, primary PVC plasticizers (e.g., dioctyl phthalate), a liquid DGEBA epoxy resin, thickeners, stabilizers, surfactants, and other additives. The epoxy serves as a secondary plasticizer, acts as a stabilizer (acid scavenger), and helps to fortify the plastisol by crosslinking during cure. 

The butadiene-acrylonitrile tougheners that have been especially successful in both DGEBA and DGEBF types of epoxy resins are... 

As with amidoamine and polyamide cured adhesives, epoxy resins cured with aliphatic amines exhibit tensile shear strength that is dependent on the type of filler and concentration. Table 11.10 shows the effect of filler loading on strength of a simple general-purpose, room temperature curing epoxy adhesive composed of liquid DGEBA epoxy mixed with 10 pph of a tertiary amine. 

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