What are composites?

Composite materials are growing in the number of applications utilizing them as improvements are made to manufacturing techniques and availability of raw materials. Composites consist of two parts: a matrix and a reinforcing material.  The matrix is designed to transfer the load to the reinforcing material (fibers), to protect the fibers, maintain the fiber orientation, and keep the fibers separated from one another.  The reinforcing material is designed to carry the load and provide the strength and stiffness required for structural applications. There are many advantages and disadvantages of composites, depending upon the application these attributes will vary.  In general, composites are lightweight, have high specific stiffness and strength, are easily moldable to complex shapes, have good fatigue resistance, and have low thermal expansion1. These advantages make composites an optimal material choice for advanced applications.  However, composites are expensive and require extensive design and analysis time due to minimal well-proven design rules.  Temperature limits, damage susceptibility, low ductility, and manufacturing times are factors to consider when designing with composites.  The following sections give background into composites their fiber types, matrix types, common tests and fabrication methods. More specifically, thermoplastic composites will be discussed with regard to fabrication, mechanical properties, and chemical properties.

Types of Fibers –

Composite materials are widely used due to their high strength-to-weight ratio as well as their high modulus-to-weight ratio.  Composite mechanical properties can be tailored specifically to each design application. There are three widely used reinforcing materials: carbon fiber, glass fiber, and Kevlar fiber all having unique properties.

Glass fibers are the most commonly used fibers in a polymer matrix composite due to their low cost, high tensile strength, electrical and thermal insulating properties and chemical resistance.  Glass fibers however have a low tensile modulus, have a high density compared to other fiber types, are hard, and have a low fatigue resistance.  Glass fibers are typically formed from long chains of SiO4 tetrahedrons that are melted, extruded, drawn, and coated with sizing before use.  Another disadvantage of glass fibers is there susceptibility to fatigue from water if not properly encased in matrix material.

Carbon fibers are the second most widely used reinforcing material with growing popularity.  Having a high tensile strength-to-density ratio as well as high modulus-to-density ratio carbon fibers are used in many aerospace and technical applications.  Carbon fiber also has low linear thermal expansion, high fatigue strength, and a high thermal conductivity.  Carbon fiber has relatively high electrical conductivity which can be a disadvantage in many applications, but an advantage in some.  Other disadvantages to carbon fiber are its low strain to failure, low impact resistance and its high cost.  Carbon fiber can be manufactured in two ways as shown in Figure 2: using polyacrylonitrile (PAN) or pitch.  PAN carbon fiber is processed in three phases, the first phase, stabilization forms the carbon structure, the second phase, carbonization determines the composition, and graphitization changes the orientation of the graphite structure.  Pitch carbon fiber is a meso-phase in which it is first melt spun then it goes through the same three steps as PAN.  Pitch carbon fiber is more easily graphitized giving it a higher modulus than PAN.

aromatic polyamides.  Nylon is considered an aramid fiber, but is a generic name for a long chain of polyamides.  Kevlar however is a specific network polyamide manufactured from para- substituted aromatic units that is formed from two processes.  The first process is a condensation reaction in which para-phenylene diamine and terephthaloyl chloride are reacted to produce poly-p-phenylene-terephalamide (PPDT).  The PPDT then undergoes an addition reaction to produce linear polymers linking the PPDT mer units.  This precursor PPDT, a nematic liquid, contains the structure of the molecular chains which is then extruded through a spinneret.  The extrusion process forms sheets of molecules that are radially arranged and axially pleated due to bonding.  Kevlar is known for its toughness and its ability to absorb energy.   Because Kevlar is highly oriented, weak hydrogen bonds are formed to adjacent molecules making it highly anisotropic.

There are many different types of reinforcement forms into which fibers can be arranged. These fiber forms have specific applications and purposes due to the importance of the reinforcement on final properties. Each form of reinforcement also plays an important role in the type of processing required.  The fibers are initially made into filaments through a process called spinning, when thousands of fibers are gathered in an untwisted manner they are drawn into tows or rovings. These tows can be oriented in woven fabrics or aligned longitudinally to form unidirectional tapes.  Fabrics have been developed from textile patterns for various handling and mechanical properties.  Fabrics can have a variable openness, or space between the fibers, and variable drape, the ability to hang without creasing.  The simplest weaving pattern is plain weave which is made from interlacing strands in an alternating over under fashion (Figure 3).  The second most common weave pattern is twill weave which has the appearance of diagonal lines due to the over one under two pattern.  There is also basket weave, crowfoot weave, leno weave, and harness satin weaves for various applications based upon the ability to drape and wetting properties.

If the application does not require the sophistication of a woven reinforcement, non- woven fabrics called mats can be used.  Non-woven fabrics are less expensive and less precise with lower mechanical properties than a woven fabric would have for the same application. Chopped strand mats can be used for producing simple less durable products where technical fabrics are not needed.  Chopped strand mats are sheets of material made by chopping tows of fibers into small lengths and by joining them together using a binder. Continuous strand mats can also be used for similar applications.  Continuous strand mats have longer strands of material than the chopped mat and therefore have improved mechanical properties. For advanced reinforcements where precision and care has to be taken for orientation, pre-pregs are used.  For applications where the majority of the load will be taken in one direction another method has been developed to optimize fiber orientation.  Unidirectional tapes are now produced to be used in such applications. Unidirectional tape has all of the fibers oriented in one direction, along the length of the roll as shown in Figure 4.  These fibers are coated in the desired final resin amount so no additional resin needs to be added during fabrication. This method forms pre-pregs, or pre-impregnated tapes and fabrics that already contain matrix material.   The resin coating in pre-preg sheets is typically epoxy in a B-stage (partially cured) state.

 

Types of Matrices –

Composites matrix materials are chosen and designed to protect the fibers from the environment, maintain the fiber orientation, transfer the load to the fibers, and to separate the fibers away from themselves.  There are many different types of matrix materials from polymeric to ceramic to metallic.  Each type of matrix is chosen based upon its specific properties and how the matrix reacts with the reinforcement.

THERMOSETTING POLYMERIC MATRICES

Thermosets are typically liquid resins at room temperature.  This liquid resin is then applied to a mold or a material for curing.  The curing process takes place as bonds form and heat is present.  These bonds or crosslinks change the basic nature of the material changing the once liquid resin into a solid material. Once cured the thermoset can no longer be melted. Thermoset polymers have sites along the polymer chain that can be activated to become reactive. These sites react in a way that chemical bonds are formed between adjacent polymer molecules. This crosslinking formation is what cures the material and hardens it.  When crosslinks form they restrict the motion of the polymer molecules to an increasingly greater amount. There are several disadvantages to thermosets, including: processing time, vapors, limited shelf life, inability to reprocess, and poor toughness.

The most common type of matrix material is a polymer.  In a polymer matrix, single molecular units called monomers are linked together into short chains called oligomers that are then bonded together leading to a polymer molecule. Many variations of polymeric matrices exist for predetermined properties.  Some common thermosets include polyester, vinyl ester, epoxy, polyimide, phenolic, and cyano-acrylate.  Polyester resin is the cheapest and most widely used matrix in conjunction with fiberglass.  The curing of polyester is typically initiated with small quantities of catalyst; the catalyst most often used is a type of peroxide, such as methyl – ethylketone peroxide (MEKP).

Epoxies are the second most common thermosetting polymeric matrix materials.  Epoxies are used in many applications other than composites such as coatings and adhesives.  Epoxy is also used in circuit boards due to its low electrical conductivity and high dielectric strength.  Epoxies are formed from an oxirane group which is where crosslinking occurs.  Epoxy has excellent adhesion, shear strength, fatigue resistance, strength and stiffness when compared to polyesters. Epoxy is most commonly paired with carbon fiber while polyester and fiber glass are a common combination.  The increased mechanical properties of epoxy are directly related to an increase in cost.

THERMOPLASTIC POLYMERIC MATRICES

Thermoplastic composite materials are becoming more popular and have increased properties making them used in an increasing number of applications.  Structural thermoplastics in particular have become a prevalent alternative to thermosets due to their enhanced mechanical properties, differences in processing capabilities, and manufacturing benefits.

Thermoplastic resins are solid at room temperature and are processed significantly differently than liquid thermoset resins. With adequate pressures parts are fabricated with low void content and can be recycled.  Thermoplastic composite have the added benefit of being able to be stored at ambient temperatures, so no refrigeration is necessary.  Thermoplastics are heated and

softened for application into a mold and then allowed to cool into the new shape.  Some common aerospace grade thermoplastics include polyetheretherketone, polyetherketoneketone, and polyetherimide.  Even after being formed the first time, thermoplastics can be reformed.  The ability to reform and reshape parts is beneficial in many applications, but care needs to be taken to make sure temperatures are adequate for the design.

Thermoplastics do not have crosslinks and therefore can be reprocessed.  In the heating process of thermoplastics there is no change in molecular weight which can compromise mechanical properties. With the increase in molecular weight in thermosets the mechanical properties also increase (Table I).  In thermoplastics the increased molecular weight makes processing increasingly difficult due to increased melting temperatures. However, thermoplastics have increased fracture toughness making them a desirable candidate for aerospace applications.


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