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'Hybrids' extend epoxy performance.

`Hybrids' Extend Epoxy Performance

The 1990 Spring Conference of SPI's Epoxy Resin Formulators Div. in Hollywood, Fla., was devoted to the topic of new "hybrid" formulations, in which epoxies are modified with urethanes, cyanate esters, thermoplastics, liquid rubbers, and polybutenes. The results can be greater flexibility, toughness, strength and heat resistance.


Pacific Anchor Chemical Corp., Los Angeles, says it has developed a new approach for introducing a urethane backbone into a flexible epoxy system, using a urethane acrylate oligomer as the co-reactant rather than a blocked isocyanate as is typical currently. The company is offering a new type of urethane acrylate flexibilizer called Ancarez 300A, which is being used in Europe in epoxy/coal tar waterproof membranes on road and rail bridges. It could also find application in abrasion-resistant floors.

Ancarez 300A reacts directly with aliphatic or cycloaliphatic amine curing agents and reportedly achieves good storage stability in blends with epoxy resin (polyamide and aromatic amine curing agents are not recommended with this product). While the same degree of flexibilization reportedly can be achieved with either Ancarez 300A or a blocked isocyanate, advantages for Ancarez 300A are said to include no free isocyanate release and no yellowing of the finished product on exposure to light. Ancarez 300A also has significantly lower viscosity than the blocked isocyanate, for easier handling. And with one particular Anchor curing agent--a modified cycloaliphatic amine, Ancamine MCA--Ancarez 300A is a more effective flexibilizer than blocked isocyanate, according to the company. Blends of epoxy resin and Ancarez 300A are said to have good storage stability. Polyamides and aromatic amines aren't recommended for use with Ancarez 300A.

When comparing an epoxy flexibilized to about the same degree with 60% Ancarez 300A as one with 70% of a blocked isocyanate, Ancarez 300A provides higher tensile and tear strengths than blocked isocyanate, as well as better abrasion resistance, with Ancamine MCA. Ancarez 300A may show less advantage in tensile properties with other curing agents, but systems with Ancarez 300A are said to invariably give superior abrasion resistance.

Dow Chemical Co., Midland, Mich., is experimenting with a variety of epoxy-urethane hybrid flexibilizers for epoxies. One approach is to synthesize a blocked, isocyanate-terminated urethane prepolymer, which will react with aliphatic amine curing agents. A second experimental system was produced by reacting a urethane prepolymer with an epoxy-functional alcohol to yield an epoxy-capped urethane, which, except for the epoxy end groups, is entirely of urethane character. It also reacts with aliphatic amines. A third approach being investigated is to react an epoxy with an isocyanate to produce an epoxy-capped oxazolidone. This species has a large degree of epoxy character, together with the highly stable oxazolidone ring. All three approaches yield a reactive flexibilizer that gives higher cured [T.sub.g] than a standard polyol epoxide flexibilizing resins (Dow's D.E.R. 732), and the second approach gives significantly improved lap shear strength as well.


Hi-Tek Polymers, Inc., Louisville, Ky. (recently acquired by Rhone-Poulenc, Inc.), presented data indicating that a relatively new family of cyanate ester thermosets can be useful curing agents for epoxies in printed wiring boards, encapsulation of electronic components, film adhesives, and filament wound, pultruded or RTM structural composites.

Hi-Tek's AroCy cyanate ester (CE) monomers and prepolymers are said to be relatively low in toxicity. As little as 20-30% of CE in an epoxy reportedly can yield an attractive combination of handling properties, 355 F [T.sub.g.] toughness, low moisture absorption, dielectric constant of 3.0, and good adhesive properties. CE's reportedly can outperform aromatic amine and anhydride curing agents in epoxy systems that are less expensive than CE homopolymers ($15/lb) or bismaleimides.

CE's are miscible with standard epoxies and are available in liquid, semisolid and hard-resin forms. CE monomer is an effective viscosity reducer. Epoxy/CE blends can be stable from a few days to several weeks at 120 F. Curing of these blends can be catalyzed with octoates, naphthenates or acetylacetonates of copper, zinc, cobalt and manganese. Cobalt and zinc carboxylates or chelates provide pot life of up to 1000 hr at 77 F while promoting gelation in 0.2-2.0 min at 350 F. Compared with aromatic amines and anhydrides, CE's yield higher HDT's (355-392 F) with no compromise of mechanical strength, lower water absorption, and lower dielectric constant (3.0-3.1) and dissipation factor (0.008-0.009).


Shell Development Co., Houston, has developed high-performance epoxies that are modified with thermoplastics. Ronald Bauer, research associate, says significantly improved damage tolerance has been achieved without compromising other critical laminate properties such as compressive strength and hot/wet performance. Two thermoplastics with high glass-transition temperatures but chemically different backbones were used as modifiers at varying levels: polysulfone (Udel P1700 from Amoco Performance Products, Inc., which is just now moving to Atlanta) and polyetherimide (Ultem 1000 from GE Plastics, Pittsfield, Mass.).

The effect of these modifiers on both neat resin and composite mechanical properties of a tetraglycidylamine and a diglycidylether of a stiff-backbone bisphenol were examined. These relatively new resins, Epon HPT Resin 1071 and Epon HPT Resin 1079, were developed specifically for advanced composites (see PT, June '87, p. 50; July '89, p. 27). Bauer says that when they're cured with a new aromatic diamine, Epon HPT Curing Agent 1062, systems are obtained that exhibit high [T.sub.g]'s and lower water absorption and consequently much improved hot/wet performance over tetraglycidyl methylenedianiline (TGMDA) systems cross-linked with diaminodiphenyl sulfone (DDS).

Polysulfone (PSO) and polyetherimide (PEI) thermoplastics were blended with epoxy systems by dissolving both in methylene chloride. Both thermoplastics significantly improved the fracture toughness of neat resin samples, with little or no sacrifice of the epoxy's 350 F heat resistance. Since PEI appeared more effective as a toughener, a carbon fiber prepreg with this system was prepared from a solution, dried, autoclaved and post-cured. Again, good toughening is observed, with little degradation of mechanical performance at elevated temperatures.


BFGoodrich Co.'s Specialty Polymers and Chemicals Div., Cleveland, has developed two new, lower viscosity versions of its Hycar carboxyl-terminated butadiene/acrylonitrile copolymers (CTBN), which are already widely used for toughening epoxies, mainly in adhesives.

The new versions (Hycar 1355x8 and Hycar 1355x13) reportedly provide half the viscosity of standard CTBN's (65,000 cp and 200,000 cp, respectively) for easier formulating, but comparable physical properties in the finished product, and approx. the same acrylonitrile content and solubility parameter.

Amoco Chemical Co., Chicago, discussed the use of its new Actipol epoxidized polybutenes as nonreactive flexibilizers in room-temperature-cured epoxy. These are clear, viscous liquid low-molecular-weight polyolefins with an epoxide group on the end. The epoxide group is said to impart polarity, which makes these products more compatible and more effective in epoxies than standard polybutenes. According to Amoco, relatively low levels (2-6%) of these modifiers can improve tensile strength, flexibility, toughness, and chemical resistance without loss of HDT.
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Author:Fallon, Michael
Publication:Plastics Technology
Date:Aug 1, 1990
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