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Tuesday, February 17, 2009

Fundamentals of Resin in a Composite System

Roles Of Matrix Resin In Composite System

The purpose of the composite matrix is to bind the fibers together by virtue of its cohesive and adhesive characteristics, to transfer load to and between fibers, and to protect them from environments and handling.

Resins are divided into two major groups known as thermoset and thermoplastic.
Thermoplastic resins become soft when heated, and may be shaped or molded while in a heated semi-fluid state and become rigid when cooled.
Thermoset resins, on the other hand, are usually liquids or low melting point solids in their initial form.

To produce finished goods, these thermosetting resins are “cured” by the use of a catalyst, heat or a combination of the two.
Once cured, solid thermoset resins cannot be converted back to their original liquid form.
Cured thermosets will not melt and flow but will soften when heated (and lose hardness) and once formed they cannot be reshaped.

The most common thermosetting resins used in the composites industry are unsaturated polyesters, epoxies, vinyl esters and phenolics.
There are differences between these groups that must be understood to choose the proper material for a specific application.

Resin Dominated Properties

Matrix is the 'weak link' in the composite; especially because resins do not presently exist that allow utilization of the stresses that the fibers are able to withstand.

When the composite is under load, resins may micro crack and craze, form larger cracks through coalescence of micro cracks, debond from the fiber surface, and generally break down at composite strains far lower than desired

The matrix resin provides many essential functions;
-the matrix keeps the reinforcing fibers in the proper orientation and position so that they can carry the intended loads,
-distributes the loads more or less evenly among the fibers,
-provides resistance to crack propagation and damage,
-provides all of the interlaminar shear strength of the composite.

Matrix generally determines the overall service temperature limitations of the composite, and may also control its environmental resistance.

Problems with Polyester & Vinyl Ester Resin
The problems of attaining adequate adhesion to carbon and aramid fibers have discouraged the development of applications for polyester or vinyl ester composites that use these fibers.

Although there are applications of high performance fiberglass composites in military and aerospace structures, the relatively poor properties of advanced composites of polyester and vinyl ester resins when used with other fibers, combined with the comparatively large cure shrinkage of these resins, have generally restricted such composites to lower-performance applications.

The development of highly effective silane coupling agents for glass fibers allowed the fabrication of glass fiber reinforced polyester and vinyl ester composites having excellent mechanical properties and acceptable environmental durability

Thermoplastic vs Thermoset

Thermoset composites are hardened to permanent shape by an irreversible reaction (usually cross-linking).

One of the more common class of thermosets used for composite processing is epoxy.

Thermoplastics are also used but less frequently. This class of polymer will soften and melt at high temperatures and re-harden when cooled. This is due to the fact that thermoplastics are long chain polymers that are not cross-linked.

Thermosets are rigid and are usually stiffer, stronger and more brittle than thermoplastics.

Thermoplastics are often selected when high toughness is required.
Table below lists some properties of commonly used polymer matrix materials

Properties of typical polymer matrix materials