EPOXY-MODIFIED SYSTEMS

The abihty of organically modified ceramics based on alumina, zkconia, titania, or siUca (and mixtures of each) to function as abrasion-resistant coatings has also been studied (62). Eor example, polycarbonate, when coated with an epoxy—aluminosihcate system, experiences a significant reduction in the degree of hazing induced by an abrader, as compared to uncoated polycarbonate. 

Aqueous dispersions are used in fiber bonding, paper coating, friction and abrasive appHcations, and laminates and wood bonding. PhenoHc dispersions improve the strength of latex-contact adhesive appHcations. Epoxy-modified phenoHc dispersions are prepared by dispersion of the phenoHc epoxy resin. The systems are used for baked primer appHcations and bonding requirements. Minimum baking conditions are 20 min at 150°C (25). 

The use of elastomeric modifiers for toughening thermoset resias generally results ia lowering the glass transition temperature, modulus, and strength of the modified system. More recendy, ductile engineering thermoplastics and functional thermoplastic oligomers have been used as modifiers for epoxy matrix resias and other thermosets (12). 

In addition to electrical uses, epoxy casting resins are utilized in the manufacture of tools, ie, contact and match molds, stretch blocks, vacuum-forrning tools, and foundry patterns, as weU as bench tops and kitchen sinks. Systems consist of a gel-coat formulation designed to form a thin coating over the pattern which provides a perfect reproduction of the pattern detail. This is backed by a heavily filled epoxy system which also incorporates fiber reinforcements to give the tool its strength. For moderate temperature service, a Hquid bisphenol A epoxy resin with an aHphatic amine is used. For higher temperature service, a modified system based on an epoxy phenol novolak and an aromatic diamine hardener may be used. 

Cement hydration and epoxy polymerization occur simultaneously to form a structure that is similar to the latex-modified cementitious system. Epoxy systems develop high strength, adhesion and have low permeability, good water resistance and chemical resistance. A major advantage of this system is that it can be cured under moist or wet conditions. According to a recent study, the epoxy-modified mortars can be made without the hardeners with superior properties to those obtained with conventional epoxy mortars [89, 90]. 

Determination Of MBOCA in Human Urine. MBOCA is commercially important as a curing agent for polyurethanes and epoxy resin systems. Since MBOCA was found to be carcinogenic in animals and is a suspected human carcinogen, it is important to have a reliable method available for the determination of MBOCA in the urine of those workers who are potentially exposed to this compound. In a NIOSH publication a GC method was described for the determination of MBOCA in urine. However, since HPLC does not require derivatization and a lower detection limit was expected, the GC method was modified to be performed by HPLC/ . In order to be able to compare both methods, we used the same extraction procedure. The extraction and sample preparation procedures are as follows ... 

A modified BMI-epoxy resin system has been introduced by Shell Chemical Company. The system is a highly reactive blend of a bismaleimide, COM-PIMIDE 1206 (55-60% by weight solution of BMI in DMF), and EPON Resin 1151, a polyfunctional epoxy resin (60). In contrast to many polyimide resins on the market, no free MDA is present in the product. This is an important feature, since MDA has been identified as an animal carcinogen and possibly a human carcinogen. This resin system has been fully evaluated for use in multilayer PCB boards (61). 2-Methylimidazole is recommended as a catalyst. However, if required, the processing window can be widened by using 2-phenylimidazole... 

The carboxyl terminated polybutadiene (C-3000) is about equally effective to CTBN in heat distortion temperature and impact but considerably less effective in strength. From the haze data (the percent haze of ERL-4221 modified with 10 phr of CTBN and C-3000 were 17 and 85% respectively) it is quite clear that this elastomer (C-3000) is highly incompatible with the epoxy-hardener system in the cured state. A 2000 molecular weight polybutadiene elastomer, containing no carboxyl groups, was completely incompatible with the epoxy system and segregated in the cured state. 

Formulators in the adhesives industry do not normally manufacture epoxy resins. Generally, formulators buy epoxy resins, modify them with other materials, do similar compounding to the curative, and then package the product as a complete adhesive system ready for the end user. There are many excellent textbooks6-8 available giving information about the preparation, chemistry, and use of epoxy resins in general applications. It is not the intention here to go into such detail but to focus only on epoxy adhesive systems. 

The inherent thermal stability of the phenol formaldehyde chemistry is preserved but with the crosslinking characteristics of the epoxy groups. However, epoxy novolacs also form very rigid and brittle polymers when fully cured because of their high crosslink density. For this reason, they are often used as modifiers in epoxy adhesive systems rather than as the base polymer. 

The first group, resins that are used primarily to toughen epoxy adhesive systems, is described in Chap. 8. This chapter focuses on the resinous modifiers that are used as alloy blends. Characteristics of commercially available epoxy alloy adhesives are presented in Table 7.1. Tensile shear and peel strengths that are typical of these hybrid adhesives are compared in Table 7.2. 

TABLE 8.6 Formulas for Two-Part CTBN Modified Epoxy Adhesive System Cured Two Week at 25°C18... 

Whereas most room temperature curing epoxy adhesives are cured with aliphatic amines, polyamides, or amidoamines, most elevated-temperature curing epoxy adhesives are cured with aromatic amines, modified aliphatic amines, alcoholic and phenolic hydroxyls, acid anhydrides, Lewis acids, and a host of other curatives. Latent curing agents, such as dicyan-diamide and imidazoles, are typically used in one-component epoxy adhesives systems. 

Adhesives formulated with epoxy-modified latex retain the tack and conformability of the original latex but show improvements in green bond strength and fully cured bond strength. Cured epoxy latex epoxy resin systems also exhibit improved water and chemical resistance over unmodified latex systems. 

Epoxy modified polymer latex systems offer improved handling performance and moisture and chemical strength advantages over unmodified formulations. The wide range of latex polymers and the range of waterborne epoxy dispersions offer the formulator a wide latitude in performance characteristics required by specific applications. 

Overheating of the adhesive is always a critical concern with induction curing. This is especially true when the adhesive cures with a high exotherm, such as epoxy-dicyandiamide systems. Several proprietary epoxy curing agents have been developed that provide lower exotherm yet faster cure rate than typical dicyandiamide reactions.27,28 Tertiary amines and modified polyamines29 are often used to accelerate the cure of dicyandiamide-epoxy... 

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