Alkyds

Several types of alkyds exist.

Alkyd coatings are used for such diverse applications as air-drying water emulsion wall paints and baked enamels for automobiles and appliances. The properties of oil-modified alkyd coatings depend on the specific oil used as well as the percentage of oil in the composition. In general, they are comparatively low in cost and have excellent color retention, durability, and flexibility, but only fair drying speed, chemical resistance, heat resistance, and salt spray resistance. The oil-modified alkyds can be further modified with other resins to produce resin-modified alkyds.

Alkyd resins are a group of thermosetting synthetic resins known chemically as hydroxy-carboxylic resins, of which the one produced from phthalic anhydride and glycerol is representative. They are made by the esterification of a polybasic acid with a polyhydric alcohol, and have the characteristics of homogeneity and solubility that make them especially suitable for coatings and finishes, plastic molding compounds, calking compounds, adhesives, and plasticizers for other resins. The resins have high adhesion to metals; are transparent, easily colored, tough, flexible, and heat and chemical resistant; and have good dielectric strength.

Alkyd plastic molding compounds are composed of a polyester resin and usually a diallyl phthalate monomer plus various inorganic fillers, depending on the desired properties. The raw material is produced in three forms — granular, putty, and glass-fiber-reinforced. As a class, the alkyds have excellent heat resistance up to about 150°C, high stiffness, and moderate tensile and impact strength. Their low moisture absorption combined with good dielectric strength makes them particularly suitable for electronic and electrical hardware, such as switch-gear, insulators, and parts for motor controllers and automotive ignition systems. They are easily molded at low pressures and cure rapidly.

Alkyds are part of the group of materials that includes bulk-molding compounds (BMCs) and sheet-molding compounds (SMCs). They are processed by compression, transfer, or injection molding. Fast molding cycles at low pressure make alkyds easier to mold than many other thermosets. They represent the introduction to the thermosetting plastics industry of the concept of low-pressure, high-speed molding.

Alkyds are furnished in granular compounds, extruded ropes or logs, bulk-molding compound, flake, and putty-like sheets. Except for the putty grades, which may be used for encapsulation, these compounds contain fibrous reinforcement. Generally, the fiber reinforcement in rope and logs is longer than that in granular compounds and shorter than that in flake compounds. Thus, strength of these materials is between those of granular and flake compounds. Because the fillers are opaque and the resins are amber, translucent colors are not possible. Opaque, light shades can be produced in most colors, however.

Molded alkyd parts resist weak acids, organic solvents, and hydrocarbons such as alcohol and fatty acids; they are attacked by alkalies.

Depending on the properties desired in the finished compound, the fillers used are clay, asbestos, fibrous glass, or combinations of these materials. The resulting alkyd compounds are characterized in their molding behavior by the following significant features: (1) no liberation of volatiles during the cure, (2) extremely soft flow, and (3) fast cure at molding temperatures.

Although the general characteristics of fast cure and low-pressure requirements are common to all alkyd compounds, they may be divided into three different groups that are easily discernible by the physical form in which they are manufactured.

1. Granular types, which have mineral or modified mineral filters, providing superior dielectric properties and heat resistance

TABLE A.5 Properties of Alkyds

2. Putty types, which are quite soft and particularly well suited for low-pressure molding

3. Glass fiber-reinforced types, which have superior mechanical strengths

For each of these distinct types a more detailed description follows.

Granular Types

The physical form of materials in this group is that of a free-flowing powder. Thus, these materials readily lend themselves to conventional molding practices such as volumetric loading, performing, and high-speed automatic operations. The outstanding properties of parts molded from this group of compounds are high dielectric strength at elevated temperatures, high arc resistance, excellent dimensional stability, and high heat resistance. Compounds are available within this group that are self-extinguishing and certain recently developed types display exceptional retention of insulating properties under high humidity conditions.

These materials have found extensive use as high-grade electrical insulation, especially in the electronics field. One of the major electronic applications for alkyd compounds is in the construction of vacuum tube bases, where the high dry insulation resistance of the material is particularly useful in keeping the electrical leakage between pins to a minimum. In the television industry, tuner segments are frequently molded from granular alkyd compound since electrical and dimensional stability is necessary to prevent calibration shift in the tuner circuits. Also, the granular alkyds have received considerable usage in automotive ignition systems where retention of good dielectric characteristics at elevated temperatures is vitally important.

Putty Types

This group contains materials that are furnished in soft, puttylike sheets. They are characterized by very low pressure molding requirements (less than 800 psi), and are used in molding around delicate inserts and in solving special loading problems. Molders customarily extrude these materials into a ribbon of a specific size, which is then cut into preforms before molding. Whereas granular alkyds are rather diversified in their various applications, putty has found widespread use in one major application: molded encapsulation of small electronic components, such as mica, polyester film, and paper capacitors; deposited carbon resistors; small coils; and transformers.

The purpose here is to insulate the components electrically, as well as to seal out moisture. Use of alkyds has become especially popular because of their excellent electrical and thermal properties, which result in high functional efficiency of the unit in a minimum space, coupled with low-pressure molding requirements, which prevent distortion of the subas-sembly during molding.

Glass-Fiber-Reinforced Types

This type of alkyd molding compound is used in a large number of applications requiring high mechanical strength as well as electrical insulating properties. Glass-fiber-reinforced alkyds can be either compression or plunger molded permitting a wide variety of types of applications, ranging from large circuit breaker housings to extremely delicate electronic components.

Other Types of Alkyd Molding Compounds

Halogen and/or phosphorus-bearing alkyd molding compounds with antimony trioxide added provide improved flame resistance. Other flame-resistant compounds are available that do not contain halogenated resins. Many grades are UL-rated at 94V-0 in sections under 1/16 in. Flammability ratings depend on specific formulations, however, and can vary from 94HB to V-0. Flammability ratings also vary with section thickness.

Glass- and asbestos-filled compounds have better heat resistance than the cellulose-modified types. Depending on type, alkyds can be used continuously to 350°F and, for short periods, to 450°F. Alkyd molding compounds retain their dimensional stability and electrical and mechanical properties over a wide temperature range.

Molding Characteristics

Although full realization of the advantages of molding alkyds is best attained through the use of high-speed, lightweight equipment, nearly all modern compression presses are suitable for use with these materials. Since these compounds are quite fast curing, the press utilized in molding them should be capable of applying full pressure within approximately 6 to 8 s after the mold has been charged. In selecting a press to operate a specific mold for alkyds, the following rule should prove useful: for average draws, the press should furnish about 1500 psi over the projected area of the cavity and lands for molding granular alkyds; about 800 psi for alkyd putty; and about 2000 psi for glass-reinforced alkyd.

Alkyd parts are in successful production in positive, semi positive, and flash molds. In general, the positive and semi positive types are recommended to obtain uniformly dense parts with lowest shrinkage. However, flash molds are frequently used with alkyd putty because of its low bulk factor. In any case, hardened, chromium-plated steel molds are recommended.

The resin characteristics of alkyd molding compounds are such that the material goes through a very low viscosity phase momentarily when heat and pressure are applied. This low viscosity phase makes possible the complete filling of the mold at pressures much lower than those required for other thermosets. Under ordinary conditions, alkyd materials have good release characteristics, and no lubrication is necessary to ensure ejection from the mold.

Applications

High-impact grades of alkyd compounds (with high glass content) are used in military switch-gear, electrical terminal strips, and relay and transformer housings and bases. Mineral-filled grades, which can be modified with cellulose to reduce specific gravity and cost, are used in automotive ignition parts, radio and television components, switch-gear, and small appliance housings. Alkyds with all-mineral fillers have high moisture resistance and are particularly suited for electronic components. Grades are available that can withstand the temperatures of vapor-phase soldering.