Polyester plastics

The materials are commonly called polyester resins but this simple name does not distinguish between at least two major classes of commercial materials. Also, the same name is used within the unsaturated class to designate both the cured and uncured state. These plastics may be defined by identifying the materials as "unsat-urated polyester resins which, when cured, yield thermoset products as opposed to thermoplastic products." The latter, as exemplified by Dacron and Videne, are saturated polyesters.


Unsaturated polyester resins of commerce are composed of two major components, a linear, unsaturated polyester and a polymerizable monomer. The former is a condensation-type of polymer prepared by esterification of an unsat-urated dibasic acid with a glycol. Actually, most polyesters are made from two or more dibasic acids.

The most commonly used unsaturated acids are maleic or fumaric with limited quantities of others used to provide special properties. The second acid does not contain reactive unsatur-ation. Phthalic anhydride is most commonly used but adipic acid and, more recently, iso-phthalic acid are employed. The properties of the final product can be varied widely, from flexible to rigid, by changing the ratios and components in the polyester portion of the resin.

The polyesters vary from viscous liquids to hard, brittle solids but, with a few exceptions, are never sold in this form. Instead, the polyester is dissolved in the other major component, a polymerizable monomer. This is usually sty-rene; diallyl phthalate and vinyl toluene are used to a lesser extent. Other monomers are used for special applications.

Polyester resins may contain numerous minor components such as light stabilizers and accelerators, but must contain inhibitory components so that storage stability is achieved. Otherwise, polymerization will take place at room temperature.

The most widely used polyester resins are supplied with viscosities in the range of 300 to 5000 centipoise. They are clear liquids varying in color from nearly water-white to amber. They can be colored with certain common pigments, which are available ground in a vehicle for ease of dispersion. Inert, inorganic fillers such as clays, talcs, calcium carbonates, etc. are often added, usually by the fabricator, to reduce shrinkage and lower costs. Resins with thixo-tropic properties are also available.


The liquid resins are cured by the use of peroxides with or without heat to form solid materials. During the cure the monomer copolymer-izes with the double bonds of the unsaturated polyester. The resulting copolymer is thermoset and does not flow easily again under heat and pressure. Heat is evolved during the cure. This must be considered when thick sections are made. Also, a volume shrinkage occurs and the density increases to 1.20 to 1.30. The amount of shrinkage varies between 5 to 10% depending on monomer content and degree of unsat-uration in the polyester. The most popular catalyst is benzoyl peroxide but methylethyl ketone peroxide, cumene hydroperoxide, and others find application.


The development of this field of commercial resins owes a great deal to the commercial production of two other products: The first was the production of low-cost styrene for the synthetic rubber program. Other cheap monomers will work but not nearly as well. Styrene and its homologues have relatively high boiling points and fast polymerization rates. Both properties are important in this field.

The second development was the commercial production of fibrous glass. Polyester resins are not widely used as cast materials nor are the physical properties of such castings particularly outstanding.

The unique property of polyesters is their ability to change from a liquid to a hard solid in a very short time under the influence of a catalyst and heat. This property was not available in any of the earlier plastic materials. Polyesters flow easily in a mold with little or no pressure so that expensive, high-pressure molds are not required. Alternatively, very large parts can be made because the total pressure required to form the material is low.

Glass fibers of fine diameter have high tensile strengths, good electrical resistance properties, and low specific gravity when compared to metals. When such fibers are used as reinforcement for polyester resins (like steel in concrete) the resulting product possesses greatly enhanced properties. Specific physical properties of the polyester resins may be increased by a factor varying from 2 to 10. Naturally the increase in physical strengths obtained will depend upon both the amount of glass fibers used and their form. Strengths approaching those of metals on an equal weight basis are obtained with some constructions.

Fabrication Techniques Hand Layup

This method involves the use of either male or female molds. Products requiring highest physical properties are made with glass cloth. The cloth may be precoated with resin but usually one ply is laid on or in the mold, coated with resin by brushing or spraying, and then a second ply of cloth laid on top of the first. The process is continued until the desired thickness is built up. Aircraft parts, such as radomes, usually require close tolerances on dimensions and resin-to-glass weight ratio. After the layup has been completed, pressure is applied either by covering the assembly with flexible, extensible blanket and drawing it down by vacuum or the mold is so made that a rubber bag can be contained above the part. This is then blown up to apply pressure on the laminate. Thereafter, the cure is accomplished by heating in an oven, by infrared lamps, or by heating means built into the mold.

Corrugated Sheet Molding

Glass-reinforced polyester sheets are sold in large volume and are made by a relatively simple intermittent or continuous method. The process consists of placing a resin-impregnated mat between cellophane sheets, rolling or squeegeeing out the air, placing the assembly between steel or aluminum molds of the desired corrugation, and curing the assembly in an oven.

Matched Die Molding

This method produces parts rapidly and generally of uniform quality. The molds used are somewhat similar to those employed in compression molding, usually of two-piece, mating construction. The process consists of two steps. A "preform" of glass fibers is prepared by collecting fibers on a screen, which has the shape of the finished article. Suction is used to hold the fibers on the screen; fibers are either blown at it or fall on it from a cutter. Commercial equipment is available for this operation. When the desired weight of glass fibers has been collected, a resinous binder is applied. The preform and screen are then baked to cure the binder, after which the preform is ready for use. The second step is the actual molding. The preform is placed in the mold, the catalyzed resin poured on the preform in correct amount, the mold closed, and the cure effected by heat. Design of the mold is very important and some features are different from those in other molding fields.

Molding cycles vary from 2 to 5 min at temperatures from 110 to 149°C. Trimming, sanding, and buffing are usually required at the flash line.

Premix Molding

This branch of the field provides parts for automotive and similar end uses. The parts are strictly functional and are usually pigmented black. The strength properties required are relatively low except that impact strength must be good. As the name infers, the unsaturated resin is first mixed with fillers, fibers, and catalyst to provide a nontacky compound. The mixers used are of the heavy-duty Day or Baker-Perkins type. The "premix" is usually extruded to provide a "rope" or strip of material easily handled at the press. The fibers used most extensively are cut sisal, but glass and asbestos are also used, and frequently all three are present in a compound. The fillers are clays, carbonates, and similar cheap inorganic materials. A typical premix will contain 38% catalyzed unsaturated resin, 12% total fiber, and 50% filler. However, rather wide variations in composition are practiced to obtain specific end use properties.

The premix is molded at pressures of 103 to 350 MPa and at temperatures ranging from 121 to 154°C. Cure cycles are short, usually from 30 to 90 s. Again, the fact that the resin starts as a liquid makes possible the molding of intricate parts because of ease of flow in the mold. Heater housings for autos are the largest use, but housings of many types and electrical parts are produced in volume. This process provides a cheap but very serviceable molded material.


The strengths obtainable in the finished product are of prime interest to the engineer. However, the numerous forms of reinforcement materials and the variations possible in the polyester constituent present a whole spectrum of obtainable properties. As an example, products are available that will resist heat for long periods of time at temperatures varying from 66 to 177°C; but the higher the temperature, the more expensive the resin.

In general, the commercial resins have good electrical properties and are resistant to dilute chemicals. Alkali resistance is poor, as is resistance to strong acids. The strength-to-weight ratio of polyester parts and their impact resistance are outstanding physical properties. The data in Table P.3 (General Properties of Polyesters) are intended to be illustrative of the properties obtainable by different fabrication techniques.

Available Forms

Probably over 95% of the unsaturated polyester resins sold are liquids in the uncured state. However, certain types are available in solid or paste form for special uses. The cured resins are available in laminate form as corrugated sheeting, which is sold widely for partitions, windows, patio roofs, etc. Rod stock may be purchased for fishing rods and electrical applications. Paper and glass cloth laminates are sold to fabricators. The boat end use is making the material more familiar to the general public. However, a large part of the industrial production is concerned with custom-molded parts. These are perhaps best classified by industries rather than specific products. The aircraft industry uses substantial quantities but automotive end uses are larger. The chemical industry is using increasing amounts in fume ducts and corrosion-resistant containers. The electrical industry uses the material in laminate and molded forms and as an encapsulation medium. Furniture applications are growing. The machinery industry uses moldings as housings and guards; a recent volume use is motor boat shrouds. There are few fields that have not found the material useful in some application.


General Properties of Polyesters

Type of Reinforcement


Glass cloth

Mat or Preform


Parallel Yarn or Roving


Glass content, % by wt






Specific gravity range




1.4 -1.5



Rockwell Hardness







Flexural strength, 1000 psi

13 -17






Tensile strength, 1000 psi







Compressive strength, 1000 psi

18 -23




















Impact, notched, ft-lb/in. notch






Shear strength, 1000 psi




Water absorption (24 h), %







a Corrugated sheet-type laminate. b Total fiber content.

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