Hybrids epoxy-vinyl

The early aerospace adhesives were primarily based on epoxy resin chemistry. However, unique applications requiring high temperatures and fatigue resistance have forced the development of epoxy-phenolic, epoxy-nitrile, epoxy-nylon, and epoxy-vinyl adhesives specifically for this industry. The aerospace industry has led in the development and utilization of these epoxy-hybrid adhesives. 

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. 

Hybrid glass/carbon epoxy Vinyl ester... 

Scrimp vinyl ester/glass —1— Vinyl ester/glass —Epoxy/glass —it— Epoxy/Kevlar —Epoxy/glass-Kevlar hybrid -—Epoxy/carbon ... 

If R can react with itself or additional components (R contains vinyl, methacryl or epoxy groups, for example), the result of the condensation process is a flexible network of inorganic oxide covalently bonded to organic polymers, namely a hybrid nanocomposite lacking interface imperfections. The properties of this hybrid nanocomposite are intermediate between those of polymers and glasses, and can meet unique requirements. 

Table 14.6 illustrates typical improvements noted in epoxy hybrid formulations with vinyl chloride, acrylic, and styrene butadiene lattices. Tensile strengths of cured, latex-saturated paper substrates are listed in absolute numbers while those of latex-epoxy hybrids are listed as percent increases in tensile strength over that of the latex alone. The mechanisms believed responsible for these improvements are (1) cocuring of the epoxy group with carboxyl and amine functional groups present on the latex backbone and/or (2) homopolymerization of the epoxy catalyzed by the tertiary amine included in some hybrid formulations. 

However, newer adhesives systems having moderate temperature resistance have been developed with improved toughness without sacrificing other properties. When cured, these structural adhesives have discrete elastomeric particles embedded in the matrix. The most common toughened hybrids using this concept are acrylic and epoxy systems. The elastomer is generally a vinyl- or carboxyl-terminated acrylonitrile butadiene copolymer. These adhesive formulations are discussed in detail in Chaps. 8 and 12. 

RTM is compatible with a variety of thermosetting polymers including polyester, vinyl ester, epoxy, phenolic, modified acrylic, and hybrid polyester-urethane. A convenient typical viscosity of thermoset precursors is in the range of 0.2-0.6 Pa s. The RTM process has the following advantages (i) emissions are lower than in open-mold processes such as spray-up or hand lay-up (ii) it can produce parts faster, as much as 5-20 times faster than open-mold techniques (iii) the mold surface can produce a high quality finish (iv) complex mold shapes can be achieved and... 

More recent efforts have focused on developments that create true hybrids. For example, blocked isocyanate prepolymers have been mixed with epoxy resins and cured with amines [68-70]. These blocked prepolymers will react initially with the amines to form amine-terminated prepolymers that cross-link the epoxy resin. Several blocked isocyanates are commercially available. The DESMOCAP (Bayer) llA and 12A products are isocyanates (believed to be blocked with nonylphenol) used as flexibilizing agents for epoxy resins. ANCAREZ (trademark. Pacific Anchor, Inc.) 2150 is a blocked isocyanate epoxy blend used as an adhesion promoter for vinyl plastisols. A one-package, heat-cured hybrid adhesive was reported consisting of isophorone diisocyanate, epoxy resin, and a dispersed solid curative based on the salt of ethylenediamine and bisphenol A [71]. Urethane amines are offered commercially that can be used with epoxy resins to develop hybrid adhesive systems [72]. 

Low-cost injection systems have been developed for RTM systems. The Spartan and VR-2 units, respectively, inject TS polyester and vinyl ester systems with a catalyst content ranging from 0.5—4.5% and can handle closer ratio mixes, as for epoxy or hybrid systems. The two can be combined in a single unit (VR-3). 

A newly developed third class of vinyl ester resins is represented by the even higher quality VE urethanes based on bisphenol A epoxy. Here, the secondary OH groups of the VE react in polyaddition with polyisocyanates to produce urethane bridges, see Eigure 1.3. Vinyl ester urethane resins (VEU resin, vinyl ester urethane-hybrid resins) combine the good mechanical properties (stiffness) of VE resins with excellent high-temperature resistance (T approx. 220 °C and HDT approx. 210 °C) and excellent chemical resistance (the long-term durability of VEUH resin exceeds that of normal VE). 

The thermosetting films are employed for metal-to-metal bonding in aircraft, as well as less demanding appliance, electrical, and automotive requirements. These adhesives are hybrids designed to provide an optimum combination of shear strength, elongation, and heat resistance. Nitrile-phenolic and epoxy-nitrile are the leaders, followed by vinyl butyral-phe-nolic, epoxy-nylon, and epoxy-phenolic. The nitrile rubber in the epoxy-nitrile is a carboxyl-terminated butadiene-nitrile polymer which reacts with the epoxy at the elevated cure temperature. 

The in situ intercalative polymerization associated with the UV curing is a technique which was successfully employed in the synthesis of hybrid films, when fast polymerization of liquid monomers yielded in solid materials with designed properties. It was proved to have high eflftciency for epoxy oligomers, vinyl ethers, oxetanes in the presence of onium salts as photoinitiators [232]. The literature is not abundant in reports on the in situ UV-initiated polymerization of epoxides in the presence of layered silicates [233-237], as compared to data on thermally cured or melt compounded nanocomposites. In some studies, the clays were used either unmodified [235, 237] or organically modified [233, 236, 238] or treated with various reagents able to change their surface properties [239-243]. 

Polyvinyl acetals, which are derived from the reaction of polyvinyl alcohol (a by-product of PVA) with formaldehyde, can be applied from solution as a one- or two-part system. Plasticizers (phosphates) can be added to improve their impact strength. These adhesives are typically used in the structural bonding of metals and auto safety glass. Hybrids of polyvinyl acetals with epoxies are used in structural bonding. Hybrids with phenolics (vinyl phenolic) are used in honeycomb manufacture for the aircraft industry, and also in printed circuits. 

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