Epoxy adhesives heat resistance

Additives to improve heat and/or moisture resistance are also known for epoxy adhesives. In a unique metal-ceramic bonding application, 100-150 mesh glass powder has been used to impart heat (arc) resistance to epoxy adhesives.Heat resistance is improved when the glass melts (>448°C), flows, and adheres to the two substrates. Uretidinedione compounds, e.g., l,3-bis(3-isocyanato-4-methylphenyl)-2,4-uretidinedione (83), have been used in room temperature curing compositions to impart excellent heat and moisture resistance. Adhesive strength was not found to be affected even after heating in steam at 120°C and 2 atm. 

Some specs under which these structural adhe-sivesmay be procured are MIL-A-5090B, "Adhesive Airframe, Metal to Metal MIL-A-8431, "Adhesive, Heat Resistant, Airframe Structural, Metal to Metal MIL-A-8623A, Adhesive, Epoxy Resin, Metal to Metal Structural Bonding and MIL-A-14042, "Adhesive, Epoxy ... 

Butadiene/acrylonitrile copolymer adhesive, food-packaging food containers D-Mannitol adhesive, furniture Urea-formaldehyde resin adhesive, glass Epoxy resin Polyvinyl acetate adhesive, glass fibers Styrene/PVP copolymer adhesive, grinding disks Animal glue adhesive, hair care Polyquaternlum-14 adhesive, heat-resistance Silicone elastomer... 

In addition to epoxy resins, MA has been investigated as a crosslinker for a variety of polymers. For example, poly(oxycyclohexene), polyary-lates, hydroxy-substituted polyspiro resins,and polymethyl-siloxanes have been crosslinked with MA to give thermoplastic adhesives, heat-resistant polyarylates, electrical insulating materials, and other composites. 

Adhesive, synthetic, epoxy resin base, paste form, general purpose Adhesives, Heat-resistant, airframe structural, metal-to-metal Adhesives methods of testing... 

Epoxy resins are also used in special appHcations, such as an overlaying procedure requiring a durable, heat-resistant bond of a difficult-to-bond overlay on a wood-base panel substrate. Metal sheets used as overlays, for example, often require an epoxy adhesive. 

Because the heat distortion temperature of cured epoxy resins (qv) increases with the functionality of the curing agents, pyromellitic dianhydride is used to cross-link epoxy resins for elevated temperature service. The dianhydride may be added as a dispersion of micropulverized powder in liquid epoxy resin or as a glycol adduct (158). Such epoxies may be used as an insulating layer in printed circuit boards to improve heat resistance (159). Other uses include inhibition of corrosion (160,161), hot melt traffic paints (162), azo pigments (163), adhesives (164), and photoresist compounds (165). 

The bisphenol A-derived epoxy resins are most frequendy cured with anhydrides, aUphatic amines, or polyamides, depending on desired end properties. Some of the outstanding properties are superior electrical properties, chemical resistance, heat resistance, and adhesion. Conventional epoxy resins range from low viscosity Hquids to soHd resins. 

This low viscosity resin permits cure at low (70°C) temperatures and rapidly develops excellent elevated temperature properties. Used to increase heat resistance and cure speed of bisphenol A epoxy resins, it has utihty in such diverse appHcations as adhesives, tooling compounds, and laminating systems. A moleculady distilled version is used as a binder for soHd propellants (see Explosives and propellants) and for military flares (see Pyrotechnics). Its chief uses depend on properties of low viscosity and low temperature reactivity, particularly with carboxy-terminated mbbers. 

Such compositions give good heat resistance and adhesion to unprepared metal. Adhesives based on chlorosulfonated polyethylene have also been modified by an epoxy resin [ 146]. 

Fig. 8.5 Epoxy curing through resistive heating of nanocarbons dispersed in the matrix (a) shows a schematic representation of the process (b) experimentally obtained curing cycle and (c) repair of a structural composite panel using the conductive epoxy as resistively curable adhesive [36]. With kind permission from Elsevier (2013).

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. 

Glass transition temperatures were determined refractometrically, as described by Wiley (20), with an Abbe refractometer supplied by Bellingham and Stanley. This instrument can normally be used only at temperatures up to 70°C. The prisms of this instrument were coated with a heat resistant adhesive (epoxy-phenolic resin mixture). After this modification the refractometer can be used at temperatures up to 200°C. 

Used u> increase heat resistance and cure speed of bisphenol A epoxy resins, it has utility in such diverse applications as adhesives, looting compounds, and laminating systems. 

As shown in Table 1.7, the epoxy resin is rarely used unmodified as an adhesive system. Rather, it is used in the form of a compound containing various modifiers and additives to improve properties, such as strength, flow, and heat resistance, and to add or advance other properties that are demanded by the specific application. 

The major problem is that the attainment of properties such as peel, flexibility, and toughness is generally accompanied by the reduction in properties such as heat resistance, chemical resistance, and shear strength. Future chapters discuss how the epoxy adhesive formulator can merge these properties. 

Primary and secondary aliphatic amines react relatively rapidly with epoxy groups at room or lower temperature to form three-dimensional crosslinked structures. The resulting cured epoxies have relatively high moisture resistance and good chemical resistance, particularly to solvents. They also have moderate heat resistance with a heat distortion temperature in the range of 70 to 110°C. Thus, short-term exposures of cured adhesive joints at temperatures up to 100°C can generally be tolerated. 

Pyromellitic dianhydride (PMDA) is a solid having a melting point of 286°C. It contains two anhydride groups symmetrically attached to a benzene ring. Because of the compactness of the molecule, PMDA achieves very high crosslink densities and, therefore, high heat and chemical resistance. PMDA cured epoxy adhesives have a heat distortion temperature on the order of 280 to 290°C. 

The major disadvantages of polymercaptan curing agents are their odor, skinning, and low heat deflection temperature. Progress has been made in the areas of odor and skinning through additives to the adhesive formulation. However, the low heat resistance is an artifact of the epoxy-mercaptan chemistry. 

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-nitrile Nitrile-epoxy adhesives are composed of solid epoxy resin modified with carboxyl-terminated butadiene nitrile (CTBN) copolymer. The CBTN is introduced into die epoxy resin at elevated temperatures. The modification provides toughness and high peel strength without sacrificing heat and chemical resistance. The film adhesives are widely used in the aerospace industry in the construction of jetliners. 

These adhesives are generally based on blends of solid epoxy resins with resole-type phenolic resin. The epoxy resin component is often not the predominant component in the blend, depending on the end properties required. Phenolics are compatible with epoxy resins and will react through the phenolic hydroxyl group. The amount of phenolic resin used is generally much greater than that required to crosslink with the epoxy, so one can debate whether (1) the epoxy toughens the phenolic adhesive or (2) the phenolic increases the heat resistance of the epoxy. 

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. 

Aluminum powder, in particular, is frequently employed at relatively high concentrations in high-temperature epoxy adhesive formulations. The filler provides improvement in both tensile strength and heat resistance, and it increases the thermal conductivity of the adhesive. Aluminum powder fillers also reduce undercut corrosion and, hence, improve adhesion and durability of epoxy adhesive between bare steel substrates. It is believed that this is accomplished by the aluminum filler providing a sacrificial electrochemical mechanism.27... 

The most significant differences in performance properties between a two-component, room temperature curing epoxy adhesive and a one-component, heat-curing type are the heat and chemical resistance. These differences are due primarily to the lower crosslink density or glass transition temperature of the room temperature curing types. 

There are several ways by which the formulator can moderately improve the heat or chemical resistance of room temperature curing epoxy adhesives. Using an elevated-temperature cure or a postcure will, of course, improve the temperature resistance by virtue of improved crosslink density. However, this section describes formulations that have been developed for moderately improved heat resistance after only a cure at room temperature. Optimal (heat-curing) high-temperature and chemically resistant epoxy adhesives are discussed in Chap. 15. 

Other anhydrides such as dodecyl succinic anhydride (DDS A) or adducts of DDS A with polyglycols, can also be used for formulating heat cured epoxy adhesives. These have excellent electrical properties and good thermal shock resistance. Anhydride cured epoxies are also useful for bonding plastics, notably polyester such as Mylar.8... 

Several formulations of elevated-temperature curing epoxy adhesives have been developed with improved thermal resistance and greater toughness. The next section describes the processes and materials that can be used to achieve moderately better heat resistance and toughness. However, formulations with the optimum temperature resistance are discussed in Chap. 15, and tougheners are described in Chap. 8. 

Typically tape or film epoxy adhesives are modified with synthetic thermoplastic polymers to improve flexibility in the uncured film and toughness in the cured adhesive. Epoxy resins can also be blended with phenolic resins for higher heat resistance. The most common hybrid systems include epoxy-phenolics, epoxy-nylon, epoxy-nitrile, and epoxy-vinyl hybrids. These hybrid film adhesives are summarized in Table 13.2, and structural properties are shown in Table 13.3. 

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