Epoxy resins with flexibilizers

Another common method of flexibilizing epoxy adhesives is by blending the primary epoxy resin with other, more elastic polymers. Epoxy-nylon, epoxy-polysulfide, and to a certain extent epoxy-urethane hybrids use such a mechanism to provide flexibility. These flexibilizers are important additives for epoxy adhesives even though they may reduce certain... 

Plasticized epoxy resins can be produced by reacting low molecular mass epoxy resins with dimeric fatty acids. These resins (e.g., Epikote 872, Shell) generally exhibit better substrate wetting than the unmodified epoxy resins. This reaction is, however, associated with the formation of ester bonds, and the resins therefore have a poor resistance to alkaline solutions. Instead of fatty acids, carboxy-functional polybutadiene-acrylonitrile elastomers can also be used to flexibilize epoxy resins. 

Fig. 15 a and b. Fracture surfaces of cured epoxy resins with added flexibilizers... 

Polysiloxane with terminal hydroxyl group can be reacted to epoxy resins to give an epoxy resin with a silicone oil flexibilizer [61]. 

Liquid polymers with terminal mercapto groups (LP-2) are obtained when solid polyolefin poiysulfides are reduced. These liquid polymers may be oxidized to solid polymers in the presence of oxidizing agents, such as lead oxides. The liquid polymers are used in caulking compositions and as reactive flexibilizing agents in epoxy resins. 

Similar to composite propellants, flexibilized epoxy or novolac epoxy resins reinforced with fillers or fibers are used for inhibition of fuel-rich propellants. 

Tpo maximize the utility of crosslinked cycloaliphatic epoxy resins in some of the more critical application areas, improved toughness is required. Such improvements can often be made through modification with various flexibilizing agents, but as a rule this improvement is accompanied by a severe degradation of the strength and heat distortion temperature of the cured system. 

The aliphatic epoxy resins formed from reaction with hydrogen peroxide or peracetic acid include epoxidized polybutadiene, epoxidized soya or linseed oil, and epoxidized polyglycols. The resulting products have too low a functionality for use as base polymers. They are almost always used in combination with other epoxy resins to improve properties such as cure rate, flexibility, and heat deflection temperature. Therefore, these resins are often considered to be reactive diluents and flexibilizers. 

TABLE 4.4 Properties of an Epoxy Resin Formulation Based on a Blend of Polyglycol Diepoxy Flexibilized Resin and Standard DGEBA Resin, Cured with Methylene Dianiline5... 

Diluents are higher-MW components than solvents that are also added to the epoxy adhesive formulation to lower the viscosity and modify processing conditions. The primary function of a diluent in an epoxy resin formulation is to reduce its viscosity to make it easier to compound with fillers, to improve filler loading capacity, or to improve application properties. Solvents, certain curing agents, and flexibilized epoxy resins can also lower the viscosity of epoxy adhesive formulations, but this is not their primary function. The effect of various diluents on the initial viscosity of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin is illustrated in Fig. 6.3. 

Functionalized, liquid polybutadiene derivatives have also been developed as hybrid flexiblizers for epoxy resins. Carboxyl-terminated butadiene/acrylonitrile polymers, butadiene homopolymers, and maleic anhydride-amino acid grafted butadiene homopolymers have been used as flexibilizers to impart good low-temperature strength and water resistance to DGEBA-based epoxy adhesives. An epoxy system toughened by polybutadiene with maleic anhydride is claimed to provide a hydrophobic backbone, low viscosity, softness, and high tensile strength and adhesion (Table 7.10). 

Di- or polyfunctional epoxy resins may be considered reactive diluents that provide a degree of resiliency, yet most preserve the inherent properties of the base epoxy resin. When added to an epoxy formulation, they will not reduce functionality, and in some cases an actual increase in crosslink density is noticed. Typical resins of these types are shown in Fig. 8.4. As flexibilizers, these diluents are often blended with the more reactive epoxy resins. They, of course, can also be used as epoxy resins in themselves. 

The commercial flexibilizing reactive diluents are predominantly glycidyl derivatives of glycols, dimerized acids, or reaction products of dicarboxylic acids with epoxy resins. The properties of certain polyepoxy diluents are summarized in Table 8.3. Some of, but not all, these materials provide good reactivity with aliphatic primary amines at room temperature. 

Heavy, filled construction coatings have been developed where ATBN admixtures with meta-xylylenediamlne constitute an unique flexibilizing hardener for the lower viscosity liquid epoxy resin based on judicious mixtures of bisphenol A and blsphenol F (51). 

The polyether-epoxy flexibilizers which contain epoxy groups along the chain (Scheme 57) impair better mechanical properties of the crosslinked composition in comparison with mixtures of aliphatic epoxy resins and polyether polyols. The improved properties are also obtained using 3,4-epoxycyclohexane-1-carboxylates of polyoxypropylene polyether polyols [38] (Scheme 58). 

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]. 

Epoxy resins are most widely used for low-pressure laminates and are used for some high-pressure types. Combining a resin (or a blend of resins) with a curing agent and, possibly, with accelerators and flexibilizers, produces an epoxy resin system. 

Chem. Descrip. Branched polymer with ether and urethane groups, contg. crosslinking, blocked isocyanate groups Uses Raw material, flexibilizer for epoxy resins, flexible coatings and sealing compds., PU systems Features Solv.-free... 

Features Curing agent for maintenance and marine coatings, overprint varnishes, masonry and pool coatings, adhesives, caulks, grouts flexibilizer for thermoplastic polyamides Properties Gardner 9 max. sol n. dens. 7.6 Ib/gal vise. (G-H) W-Y amine no. 51-57 pot life 18 h (250 g) 59-61% solids in IPA/toluene Use Level 100 phr with epoxy resin of EEW=525 Epikure 3100-HX-60 [Resolution Perl. Prods.]... 

Chem. Descrip. Polyoxyall lene diamine Uses Epoxy curing agent with flexibilizing and toughening props. also in polyamide, polyurea, modified urethane resins, in adhesives, elastomers, foam formulas intermediate for textile and paper treatment chemicals Properties Nearly colorless liq. m.w. 148 sp.gr.1.0154 (25/4 C) vise. 8 cSt flash pt. (PMCC) 265 F distort, temp. 44-47 C (264 psi)... 

Polyamides containing amino groups are also excellent flexibilizers for epoxies since they co-react with the epoxy resin and introduce their inherent thermoplastic properties. 

The use of elastomeric or flexibilizing modifiers occurred and grew with epoxy resins first. Various aspects of toughened epoxy adhesives have been covered in reviews by the present authors (2,3), where the elastomeric modifiers have essentially been carboxylic, liquid and solid butadiene/acrylonitrile polymers. There has not been a systematic review, however, of these and other reactive liquid polybutadiene/acrylonitriles in the burgeoning areas of acrylic, anaerobic and radiation-curable systems. Thus, this paper s intent. 

Reactive flexibilizers such as polysulfldes, urethanes, polybutadienes, and polyesters are copolymerized with epoxies to increase flexibihty, improve adhesion, modify electrical properties, and improve resistance to chemicals. Nonreactive extenders such as thermoplastics, asphaltums, and waxes are used to modify properties and reduce the cost of the compounds. Polyarylene ether sulfone as an extender in epoxy resins increases the fracture toughness fivefold with only a slight drop in Tg. 

The crosslinking density of the flexibilized epoxy resin systems is variable over a wide range, and the chemical incorporation of the flexibilizer precludes the migration frequently observed with plasticizers. Figure 19 shows part of an idealized network structure for epoxy-polyester copolymers. 

The good low-temperature properties of the flexibilized epoxy resin systems are due to the increased mobility of the segments from the main relaxation down to the y relaxation. Epoxy resin pol3nners modified with Empol(dimerized fatty acid) or Hycar (butadiene-acrylonitril copolymer, terminated by carboxyl groups) show notable strength, impact resistance, and temperature-shock resistance down to 20 Yet none of these systems is ductile... 

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