Epoxy adhesives impact resistant

Bisphenol A type epoxies such as Epon 828, were also cured with the DIBF OPPI combination. When a modifier, Heloxy 505 (a low viscosity polyepoxide modifier) was added, viscosity was reduced and adhesion to the metal surface and impact resistance were improved as compared with the bisphenol A only. Surprisingly cure was faster than with the Union Carbide cycloaliphatic resins, but charring of the resin during cure was a problem. 

Other effects were more selective. While recycle usually lowered impact strength and heat deflection temperat ire, pre-soaking polyamine recycle surprisingly improved both of these properties. While recycle usually lowered volume resistivity, polyamide recycle improved it. Finally, adhesion of epoxy formulations to the aluminum mold, in spi/ e of wax and silicone mold release agents, was dramatically increased by the use of presoaked recycle, especially in the ai ydrlde system, suggesting unexpected usefulness in epoxy adhesive formulations. 

The epoxy resin is a primary component in any epoxy adhesive formulation, and it is often referred to as the base polymer. However, it is certainly not the only or even not always the predominant component in influencing desirable end properties. Epoxy resins by themselves are often too rigid to provide the required properties such as flexibility, peel and impact strength, and thermal cycling resistance. As a result, they are often modified with other components or hybridized with other types of polymeric resins to provide these functions. 

Hybrid resins have been used to improve the flexibility, thermal shock resistance, elongation, heat distortion temperature, and impact strength of unmodified epoxy adhesives. However, there can also be some sacrifice in certain physical properties due to the characteristics of the additive. These alloys result in a balance of properties, and they almost never result in the combination of only the beneficial properties from each component without carrying along some of their downside. 

Epoxy resins may be blended with certain vinyl polymers to improve the impact strength and peel strength of the adhesive. Polyvinyl acetals, such as polyvinyl butyral and polyvinyl formal, and polyvinyl esters are compatible with DGEBA epoxy resins when added at concentrations of 10 to 20% by weight. The addition improves the resulting impact resistance and peel strength of the cured adhesive. However, temperature and chemical resistance are sacrificed by the addition of the low-glass-transition-temperature vinyl resins. 

Polymerized epoxy adhesives are amorphous and highly crosslinked materials. This microstructure results in many useful properties such as high modulus and failure strength, low creep, and good chemical and heat resistance. However, the structure of epoxy resins also leads to one undesirable property—they are relatively brittle materials. As such, epoxy adhesives tend to have poor resistance to crack initiation and growth, which results in poor impact and peel properties. In sealant formulations, epoxy resins do not often provide the degree of elongation or movement that is required for many applications. 

A good example of a flexible epoxy hybrid adhesive is the epoxy polysulfides. Figure 8.2 shows the improved impact resistance that can be realized by using increasing amounts of... 

FIGURE 8.2 Impact resistance of varying concentrations of polysulfide resin in an epoxy adhesive formulation.6... 

The room temperature cured epoxy adhesives discussed thus far exhibit a general lack of flexibility, especially when considered next to elastomeric sealants. Flexibility is generally desired when the performance requirements include high peel strength, impact strength, and resistance to thermal shock or thermal cycling. 

Resinous modifiers. Epoxy resins have the ability to interact with other resins to form heteropolymers. These are also known as plastic alloys. Improved properties can be achieved with modified systems which are otherwise not possible with a single resin for example epoxies can form plastic alloys with poly(vinyl formal) to improve the impact resistance and peel strength of adhesive formulations. 

Polysulfide polymers have been used successfully to improve the impact resistance of epoxy sealants and adhesives for 20 years. The polysulfide liquid polymers used for this application are the low molecular weight materials,... 

One can rationalize a need for small rubber inclusions in some of the newer approaches to waterborne and high solids epoxy coating systems. Water-thinned epoxy coating compositions are described (48) where the two-component system consists of a nitrile rubber modified epoxy resin in the epoxide component and a styrene/ butadiene/methylmethacrylate latex modifier for an emulsion-based polyamide hardener component. Showing improved adhesion, impact and water resistance, the paint has good wetting characteristics and can be formulated to a high solids content at low viscosity. 

The precedence for nitrile-epoxy alloys in other epoxy applications suggests that similar compositions in coatings should enhance film properties associated with flexibility, impact resistance and adhesion to metals. The testing scheme employed emphasizes these property features. 

The chemical literature reveals relatively new and varied interests for nitrile elastomer-modification of epoxy resins in diverse areas of coatings and primers. Desirable properties such as impact resistance, mandrel bend and adhesion improvement are attainable with little or no sacrifice in critical film properties. In some instances, it is documented that proper elastomer modification of select epoxy coatings will enhance corrosion and moisture resistance. This may relate to better film/substrate adhesion durability. The combined literature, journal and patent, has led to continuing study of nitrile elastomer modified epoxy coatings. 

A fuse manufacturer replaced a 30 min epoxy used in a heat cure process to assemble stainless steel inserts into phenolic holders. By utilizing an aerobic adhesive bonding process, the assembly was complete in 30 s at room temperature. The resulting increase in productivity more than offset the higher adhesive cost. A cyanoacrylate bonding process was even faster, but did not resist aging in a moist atmospheric environment or pass a drop test for impact resistance. 

Uses Epoxy for supported and nonsupported adhesives, chem. corrosion resist, coatings, laminating, container, coil, and powd. coatings Features Good flexibility and impact resist. fast dry in sol n. coatings Properties Gardner 3 solid sp.gr. 1.19 dens. 9.9 Ib/gal EEW 1825 100% solids... 

The properties of existing epoxy adhesives fail to meet a number of requirements of current engineering, bringing the problem of their modification to the forefront because, despite the good adhesion and dielectric properties of the epoxy polymers, their elasticity and resistance to impact loads are not high. 

Epoxy-phenolic finishes are among the most chemically resistant known. 20-30 per cent phenolic resin is used with epoxy resin of molecular weight 1400 and the stoving schedule is 20 min at 180-205° C. Phenolic resins alone give excellent chemical resistance, but cross-linking with epoxy resin improves adhesion and impact resistance. The poor colour of the coatings is due to the phenolic resin. The... 

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