Liquid epoxy, adducts

Later, McPherson (39) demonstrated that the Inclusion of a CTBN/liquid epoxy adduct enhanced both the flexibility and adhesion of a dimer acid flexibllized epoxy/brominated epoxy mixture when cured with a combination of chlorendic and nadic-methyl anhydrides. With an epoxy-glass laminate as substrate, such mixtures were coated at 0.5-2.0 mils thickness, cured, sensitized and copper plated (1.5 mils) by an electroless process. 90 peel testing (2 inches/mln. rate, 25 C) gave 14.0 pli peel force where 5.0 pli is considered normal. 

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). 

Other amines, such as aromatic or cycloaliphatic, are less reactive and generally require elevated-temperature cures that result in higher heat distortion temperatures (140 to 150°C). However, aromatic amine adducts of liquid epoxies can be accelerated to cure at room temperature. Aliphatic amines can also be accelerated. 

Suitable curatives for the polysulfide-epoxy reaction include liquid aliphatic amines, liquid aliphatic amine adducts, solid amine adducts, liquid cycloaliphatic amines, liquid amide-amines, liquid aromatic amines, polyamides, and tertiary amines. Primary and secondary amines are preferred for thermal stability and low-temperature performance. Not all amines are completely compatible with polysulfide resins. The incompatible amines may require a three-part adhesive system. The liquid polysulfides are generally added to the liquid epoxy resin component because of possible compatibility problems. Optimum elevated-temperature performance is obtained with either an elevated-temperature cure or a postcure. 

Another chosen example stems from the perceived benefits of combining solid and liquid carboxylic nitrile elastomers in the same modification scheme. This is particularly useful when a degree of tack is required in the system. Table VIII exhibits one approach for preparing such an elastomer-modified solid DGEBA resin. In this instance a resin (epoxide equivalent weight of 650) was prepared by advancing two available liquid epoxy resin adducts — one utilizing a liquid nitrile, the other a solid nitrile elastomer. 

ADDUCTS OF LIQUID EPOXY, CTBN AND DIMER ACID... 

Siebert and Riew [1781 first described the chemistry of rubber particle formation in an admixed model involving CTBN, a DGEBA (diglycidyl ether of bis-phenol A) liquid epoxy resin, and a selective catalyst. They proposed that the composition of the rubber particles in the dispersed phase critically depended on the epoxy—CTBN-epoxy adduct being formed in situ and then further chain-extended and crosslinked with additional resin. 

The liquid nitrile rubbers can be reacted into the epoxy matrix in one of two ways. If a carboxyl terminated material is used it is normally adducted or prepolymerized with an epoxy resin prior to formulating. The resulting epoxy adduct is then placed in the epoxy component of the composition. A second method of incorporating liquid nitrile materials would be to use an amine terminated rubber. In this case the material contributes amine reactivity and functions as a part of the hardener system. These materials were primarily developed to form stable admixes with amine containing epoxy hardeners. 

ATBN liquid polymers cannot be mixed directly into the epoxy resin component of a two-part epoxy adhesive or in a one-part adhesive since crosslinking and shortened shelf life will result. ATBN adducts are, therefore, mixed with the curing agent component of two-component epoxy adhesives.30... 

Adducts can be manufactured from either liquid or solid epoxy resins by using whatever level of LP is appropriate for the properties required for the cured product. The only limitation is that a molar excess of epoxy resin must always be maintained (Rees et al, 1994 Morton International, undated a). Properties of epoxy-polysulphide adducts manufactured by the above route include (Morton International, undated a) ... 

Carboxylic nitrile liquid and solid elastomers are used to prepare elastomer-modified epoxy liquid and solid resins when it is desirable to have the modifier in the epoxy portion of the system. This is effected through alkyl-hydroxy esterification reactions. This is covered in the literature for uncatalyzed liquid resins (9), for tert-amine catalyzed systems (10), for tert-phosphine catalyzed systems (11), for low molecular weight solid resins advanced from the liquid state (12) and for liquid and solid resins where an additional rubber vulcanization step is carried out in addition to the alkylhydroxy ester adducting step (13). Such adduct preparations offer formulation stability with a wide range of hardener types amines, anhydrides, catalytic, Lewis acids/bases. 

Reactive liquid polymers (RLPs) The Uquid rubbers most often used in epoxy adhesives are those based on acrylonitrile-butadiene copolymers or on long-chain polyethers. To act as effective tougheners, however, they cannot be directly added to the formulation but have to be prereacted with some of the epoxy resin, or sometimes with the amino hardeners in two component paste adhesives (adduct fonnatiou). To achieve this, only RLPs with reactive eud groups are used by the formulator. 

The adducting processes for both these types of RLPs are very similar. The functional equivalent weight is known for each polymer and hence they can be blended with a calculated excess of epoxy resin so that the adduct, now an epoxy-terminated polymer, is formed in an epoxy resin solution. As a rule of thumb, this is generally taken as about 1 part by weight of RLP to 3 parts by weight of epoxy resin. Further, it has also been found advantageous to use a blend of liquid and solid epoxies. 

Most of the amines, may irritate skin, possess a noxious odor or emit corrosive fumes. Hence they are mostly sold as modified variations with reduced vapor pressures and tendency to irritate. One approach to formulate amine cross-linkers with higher equivalent weight and lower toxic hazard is to make amine-adducts by reacting standard liquid bisphenol A epoxy with excess of a multifunctional amine. A variety of amines can be used to provide adducts with a range of cure rates and pot lives. A simple epoxy-amine adduct is shown below. In two of the superprimer formulations water-borne epoxy-amine adducts are used as the curing agents [11, 62]. 

Elastomeric modifiers are used to increase the peel strength (toughness) of epoxy resin adhesives. The most commonly used elastomeric materials are functionally terminated polybutadiene resins made by the B.F. Goodrich Company, Chemical Group under the trade name of Hycar Reactive Liquid Polymers. Initially, carboxyl-terminated butadiene acrylonitrile (CTBN) resins were introduced the carboxyl terminated materials are usually adducted with the epoxy resin to improve compatibility and to increase the toughness. 

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