Epoxy phenolic molding compounds

The most important resin types used in production of curable molding compounds are phenolic, urea, melamine, unsaturated polyester, epoxide and diallyl phthalate resins. Curable molding compounds built up with these bonding agents are described in DIN 7708" (Phenoplasts and Aminoplasts), DIN 16911 and 169132 (Polyester Molding Compounds and Polyester Resin Mats), and DIN 16912 (Epoxy resin Molding Compounds). There is as yet no standard for diallyl phthalate masses, for the test method see ISO 1385 - 1.02.1977. 

The most common and widely used thermoset molding compounds are classified as follows (a) alkyd, (b) allylic (diallyl phthalate), (c) amino (melamine and urea), (d) epoxy, (e) phenolic, (f) polyester, and (g) silicone. There may be other specialty thermoset resin materials used on specific applications. 

Thermoset molding compounds, when contained within a hardened steel mold, require heat and pressure to be polymerized into a solid mass. Molds may be heated by steam, electricity, or hot oil to temperatures of 280° to 425°F, depending entirely on the type of material and method of molding. Molding pressures may vary from a low of 50 p.s.i. to 15,000 p.s.i. Epoxy materials will mold at 50 p.s.i. whereas, phenolic fabric-filled material may require excessive pressures. Again, the method of molding dictates molding pressures. 

Again, as in many other fields covered in the book, modified epoxies are the most studied systems (toughened epoxies for adhesive coatings and composites). But also rubber-modified phenolics and low-profile unsaturated polyesters for sheet and bulk molding compounds have been extensively studied. 

The resin matrix can be either thermosetting or thermoplastic. Thermosetting resins such as epoxy, polyimide, polyester, and phenolic are used in applications where physical properties are important. Polyester and epoxy composites make up the bulk of the thermoset composite market. Of these two, polyesters dominate by far. Reinforced with glass fiber, these are known as fiberglass-reinforced plastics (FRPs). FRPs are molded by layup and spray-up methods or by compression molding either a preform or sheet molding compound (SMC). 

These resins (Resole or Novolac) are used as curing agents or hardeners for epoxy molding compounds for electronics applications such as computer components. 0-cresol-formaldehyde resins have heen also used to modify phenol-formaldehyde resins, and in laminates. 

The magnitude of the applications for polymeric substrates has been estimated (in tons) for 1987 on a worldwide basis as phenolic resin, 78K epoxy resin, 130K polyester fiber, 1,010 polyimide film, 235 molding compounds, 330 polymers for high-frequency applications, 300 and high-temperature polymers, 1,440 (4). 

All samples were prepared from a commercially available epoxy cresol novolac-phenol formaldehyde novolac-tertia-ry amine based molding compound. Pelletized preforms were heated to 85°C in a RF preheater prior to being transfer molded at 180°C/68 atm. for 90 sec. Molded samples were cooled in air to room temperature and stored in a desiccated environment until testing or subsequent thermal treatment. Post mold curing, PMC, was accomplished in a gravity oven at 175°C for a period of 4 hours. Samples without post mold curing are designated by NPMC. 

The electronics industry desires improved flame suppressant additives for microelectronic encapsulants due to bromine induced failure. Epoxy derivatives of novolacs containing meta-bromo phenol have exhibited exceptional hydrolytic and thermal stability in contrast to standard CEN resins with conventional TBBA epoxy resins. When formulated into a microelectronic encapsulant, this stable bromine epoxy novolac contributes to significant enhancements in device reliability over standard resins. The stable bromine CEN encapsulant took about 30% more time to reach 50% failure than the bias pressure cooker device test. In the high temperature storage device test, the stable bromine CEN encapsulant took about 400% more time to reach 50% failure than the standard compound. Finally, the replacement of the standard resins with stable bromine CEN does not adversely affect the desirable reactivity, mechanical, flame retardance or thermal properties of standard molding compounds. 

From the up-to-date literature and patent review of catalysts used In anhydride and phenolic cured epoxy molding compounds, It Is evident that Imidazoles and their derivatives predominate (Table I). Metal complex, trialkyl or triaryl phosphines and their complexes, Lewis acids such as zinc or stannous octoate are used to a much lesser extent (Table II). There are a few examples of tertiary amines and urea derivatives used. 

The mechanism for organometallics and Lewis acids in phenolic or anhydride cured epoxy molding compounds are still not fully understood. Lewis bases such as imidazoles can be reacted with organic acids to form salts in order to improve latency. Imidazoles are, so far, the most widely accepted as a compatible catalyst family for encapsulating microelectronics. 

When processing (CM, injection molding, extrusion, ICM, etc.) from uncrosslinked (A state) or crosslinked (B state) duroplastic molding compounds, the shearing forces applied upon injection through dies impose orientations upon the macromolecules and, if any are present, upon the reinforcing fibers as well. Because of the low viscosities of uncrosslinked masses such as phenolic, melamine, UP and epoxy resins and the hot mold wall, relaxation of the molecular orientations sets in quickly. As a rule, duroplastics show little or no orientation. Crosslinking fixes this state. 

A second area of potential and perhaps the largest Is molding compounds. Applications are virtually unlimited since almost any property achieved with conventional molding compounds such as phenolic and epoxy can be realized with the polyimides, accompa-led by extension of the desired properties to SSO -SOO . 

Spiral flow (test and mold). There are two types of spiral flow molds— one for the very soft flow encapsulation compound generally associated with the encapsulation grades of the epoxy family of compounds and a spiral flow mold, which is used when testing the high-pressure phenolic, DAP, melamine, urea, epoxy, and thermoset polyester com-poimds. 

Early resin materials used in mold compound formulations were sihcones, phenolic resins, and bisphenol-A or bisphenol-F epoxies. Because of shortcomings in performance, these materials have been displaced by epoxy phenol or cresol novalac resins (ECN resins) and by the biphenyl- and tris(triphenylmethane)-type epoxies (70) (Fig. 20). The high cross-link density of ECN-based materials results in low moisture absorption rate and higher thermal stabihty than... 

Bulk Molding Compoimd, BMC, (Dough Molding Compound in Europe) is produced by first mixing pre-catalyzed liquid resin with fillers, mainly calcium carbonate and talc, in a heavy duty low speed sigma blade mixer. This is compression molded at 500 psi and 300 to 400°F. The resin most commonly used is unsaturated styrene-diluted polyester. Other BMC resins are alkyds, phenolics, urea, melamine, diallyl phthallate, silicones and epoxy. All are highly filled with calcium carbonate, talc, mica or alumina to improve mechanical properties and reduce shrinkage. 

Molding compounds based on epoxy or phenolic resins mixed with fillers are used for the conventional printed-circuit board technologies widely employed in electronics production. These molding compounds belong to the group of thermoset plastics. They consist of tightly crosslinked macromolecules, as reflected in their extremely hard and brittle material behavior at room temperature. 

---