Additives and Modifiers

A wide variety of additives are used in composites to modify materials properties and tailor the laminate’s performance. Although these materials are generally used in relatively low quantity by weight compared to resins, reinforcements and fillers, they perform critical functions.

Additive Functions

Additive used in thermoset and thermoplastic composites include the following:

	Low shrink/low profile: when parts with smooth surfaces are required, a special thermoplastic resin, which moderates resin shrinkage, can be added to thermoset resins.
	Fire resistance: Combustion resistance is improved by proper choice of resin, use of fillers or flame retardant additives. Included in this category are materials containing antimony trioxide, bromine, chlorine, borate and phosphorus.
	Air release: most laminating resins, gel coats and other polyester resins might entrap air during processing and application. This can cause air voids and improper fiber wet-out. Air release additives are used to reduce such air entrapment and to enhance fiber wet-out.
	Emission control: in open mold applications, styrene emission suppressants are used to lower emissions for air quality compliance.
	Viscosity control: in many composite types, it is critical to have a low, workable viscosity during production. Lower viscosity in such filled systems is usually achieved by use of wetting and dispersing additives. These additives facilitate the wet-out and dispersion of fillers resulting in lower viscosity (and/or higher filler loading).
	Electrical conductivity: most composites do not conduct electricity. It is possible to obtain a degree of electrical conductivity by the addition of metal, carbon particles or conductive fibers. Electromagnetic interference shielding can be achieved by incorporating conductive materials.
	Toughness: can be enhanced by the addition of reinforcements. It can also be improved by special additives such as certain rubber or other elastomeric materials.
	Antioxidants: plastics are sometimes modified with antioxidants, which retard or inhibit polymer oxidation and the resulting degradation of the polymer.
	Antistatic agents: are added to polymers to reduce their tendency to attract electrical charge. Control of static electricity is essential in certain plastics processing and handling operations, as well as in finished products. Static charges on plastics can produce shocks, present fire hazard and attract dust. The effect of static charge in computer/data processing applications, for example, is particularly detrimental.
	Foaming agents: are chemicals that are added to polymers during processing to form minute cells throughout the resin. Foamed plastics exhibit lower density, decrease material costs, improve electrical and thermal insulation, increase strength-to-weight ratio and reduce shrinkage and part warping.
	Plasticizers: are added to compounds to improve processing characteristics and offer a wider range of physical and mechanical properties.
	Slip and blocking agents provide surface lubrication. This results in reduced coefficient of friction on part surfaces and enhances release of parts from the mold.
	Heat stabilizers: are used in thermoplastic systems to inhibit polymer degradation that results from exposure to heat.
	Ultraviolet stabilizers: both thermoset and thermoplastic composites may used special materials which are added to prevent loss of gloss, crazing, chalking, discoloration, changes in electrical characteristics, embrittlement and disintegration due to ultraviolet (UV) radiation. Additives, which protect composites by absorbing the UV, are called ultraviolet absorbers. Materials, which protect the polymer in some other manner, are known as ultraviolet stabilizers.

Catalysts, Promoters, Inhibitors

In polyesters, the most important additive is catalyst or initiator. Typically, organic peroxide such as methylethylketone peroxide (MEKP) is used for room temperature cured processes, or benzoyl peroxide is added to the resin for heat-cured molding. When triggered by heat, or used in conjunction with a promoter (such as cobalt napthenate), peroxides convert to a reactive state (exhibiting free radicals), causing the unsaturated resin to react (cross-link) and become solid. Some additives such as TBC (tertiary butyl catechol) are used to slow the rate of reaction and are called inhibitors. Accelerators such as DMA (dimethyl aniline) speed curing.

Colorants

Colorants are often used in composites to provide color throughout the part. Additives can be mixed in as part of the resin or applied as part of the molding process (as a gel coat). Also, a wide range of coatings can be applied after molding.

Release Agents

Release agents facilitate removal of parts from molds. These products can be added to the resin, applied to molds, or both. Zinc stearate is a popular mold release agent that is mixed into resin for compression molding. Waxes, silicones and other release agents may be applied directly to the surface of molds.

Thixotropic agents

In some processes such as hand lay-up or spray-up, thixotropic agents may be used. When “at rest”, resins containing thixotropic agents remain at elevated viscosities. This reduces the tendency of the liquid resin to flow or drain from vertical surfaces. When the resin is subjected to shear, the viscosity is reduced and the resin can be easily sprayed or brushed on the mold. Fumed silica and certain clays are common thixotropic agents.

Summary

Additives and modifier ingredients expand the usefulness of polymers, enhance their processability or extend product durability. While additives and modifiers often increase the cost of the basic material system, these materials always improve cost/performance.

Core Materials for Sandwich Structures

Bonded sandwich structures have been a basic component of the composites industry for over 45 years. The concept of using relatively thin, strong face sheets bonded to thicker, lightweight core materials has allowed the industry to build strong, stiff, light and highly durable structures that otherwise would not be practical. This technology has been demonstrated in boats, trucks, and building panels. A 3% weight increase can increase the flexural strength and stiffness by a magnitude of 3.5 times and 7 times respectively if cores and skins are properly chosen. The structure then acts more or less monolithically.

The most common comparison made is that of a composite sandwich to an I-beam. The panel skins, like the flanges of the I-beam, carry the stresses imposed by use. The stresses are transferred between the top and bottom skins through shear stresses that run through the core or web of the I-beam. The purpose of an I-beam is to lessen the weight required to support a given load in bending. Since the highest stresses are carried at the extremities, both the top and bottom of the I-beam, the center section can be much narrower in width in relation to the flanges. In a sandwich structure, the core will generally have the same width and length dimensions as the skins, but can be much weaker than the skins since it primarily experiences shear stresses. Care must be taken in design to ensure that the shear carrying capability of the expected loads does not exceed both the core and the adhesive.

Face sheets can be of almost any material. In the composites industry, the most common face sheets are glass and carbon. The common core materials are foam, syntactic foam, honeycomb, and balsa wood.  Some core materials can be shaped, such as a waffle pattern or corrugation to achieve the desired mechanical properties.