The most common composites
When people say composite, they usually mean a polymer matrix composite: strong fibers embedded in a plastic resin. Fiberglass boats, carbon fiber bicycles, aircraft panels and wind turbine blades all fit this description. The polymer is relatively light, can be shaped at modest temperatures and costs less to process than metal or ceramic matrices. These are the reasons polymer matrices dominate the field of advanced composites.
The fibers get most of the attention, but the resin does a great deal of work. It also sets limits that the fibers cannot override.
What the matrix does
In a polymer composite the resin:
- Holds the fibers in position so they carry load along the intended paths.
- Transfers load between fibers through shear, particularly around breaks and at the ends of fibers.
- Supports the fibers against buckling when the part is in compression.
- Protects the fibers from abrasion, moisture and chemicals.
- Determines out-of-plane properties, including the strength between layers, which is why interlaminar failure such as delamination is largely a matrix issue.
- Sets the temperature ceiling of the part.
Since the stiffness and strength of the polymer are much lower than those of the fibers, the properties of a laminate along the fiber direction come mainly from the fiber, while properties across the fibers and through the thickness come mainly from the resin.
Thermosets and thermoplastics
Polymer matrices fall into two main groups.
Thermosets cure by a chemical reaction that forms a crosslinked network. Once cured, they do not melt again. They tend to have good dimensional stability and good chemical resistance. Common examples:
- Epoxy. Widely used in aerospace and high-performance parts because of good adhesion to fibers, relatively low shrinkage on cure and a broad range of formulations.
- Polyester and vinyl ester. Cheaper and easy to process, common in marine, construction and corrosion-resistant tanks and pipes. Vinyl esters usually offer better resistance to chemical and water attack than basic polyesters.
- Phenolic. Valued for fire, smoke and heat behavior, used in some interior and fire-sensitive parts.
- Bismaleimide, polyimide and similar resins. Used for higher temperature needs, generally at higher cost and with more demanding processing.
Thermoplastics soften when heated and harden when cooled, and the process can in principle be repeated. Examples include polyamides (nylons), polypropylene and high-performance polymers such as PEEK. They can offer high toughness, faster cycle times and the possibility of welding parts together, but the resins are viscous when molten, which makes fiber wetting more challenging and often requires high processing temperatures and pressures.
Temperature limits
Every polymer has a temperature range where it behaves as a rigid solid and a point where it begins to soften. For amorphous polymers and thermosets, this is called the glass transition temperature. Above it, stiffness drops sharply, and the matrix can no longer support the fibers well, so compressive and shear strength fall even though the fibers themselves are unaffected.
In practice, designers keep the service temperature a margin below the glass transition temperature, and they must consider that moisture lowers it. This is one reason polymer composites are not used in hot sections of jet engines, where high-temperature materials such as ceramic matrix composites or carbon-carbon composites are used instead. The exact limits differ widely between resin systems, so the supplier’s data for the specific formulation should be used.
Moisture, creep and aging
Polymers absorb some moisture from humid air or water, and the amount varies by resin. Absorbed water can reduce the glass transition temperature and some matrix-dominated properties, and it can cause dimensional change. Long-term exposure to ultraviolet light can degrade surface resin, which is why many outdoor parts are painted or coated with a protective layer.
Under sustained load, polymers can creep, meaning they slowly deform. The effect is more pronounced at higher temperature and in matrix-dominated directions. Chemical exposure can also matter, depending on the resin and the fluid.
Toughness
Cured thermosets such as epoxy are stiff but can be brittle. Resin chemists add tougheners, such as small rubbery or thermoplastic particles, to improve resistance to crack growth. Better matrix toughness generally improves damage tolerance and reduces the extent of damage from impact. These matters are part of the broader treatment of how composites fail. Thermoplastics are often tougher by nature, which is one reason they appeal for impact-prone parts.
Fillers and additives
Resins are rarely used neat. Additives may include flame retardants, ultraviolet stabilizers, pigments, tougheners and fillers. Fillers such as glass bubbles, silica or other particles can reduce weight or cost, adjust viscosity or change shrinkage. Nanoscale additives such as carbon nanotubes and graphene sheets are the subject of ongoing development for electrical, thermal or mechanical improvement, though practical results depend heavily on dispersion and cost.
Processing considerations
The resin’s behavior during processing shapes what can be built. A resin that is thin at room temperature can be infused through dry fabric, while one that must be heated to flow suits prepreg or high-pressure molding. Pot life, cure temperature, exotherm and shrinkage all affect the part. These considerations are covered in the discussion of composite layup and curing methods. Cure shrinkage and thermal contraction can leave residual stress that distorts parts.
Bonding to the fiber
The resin also needs to bond to the fiber. Fiber sizing is selected to be compatible with a specific resin family, so changing resin may mean changing fiber. The strength of this bond affects toughness and load transfer, a topic taken up in more detail in the article on the fiber-matrix interface.
Repair and end of life
Thermoset composites can be repaired with patches or scarfed repairs bonded with adhesive, but the repair must be matched to the resin and the structural needs. At end of life, crosslinked thermosets cannot simply be remelted, which makes composite recycling difficult. Thermoplastic composites are more promising in this respect because they can be reheated and reshaped, although fiber length and quality can still suffer.
Choosing a resin
The selection normally follows the demands on the part: operating temperature, chemical environment, required toughness, production volume, cost and the available process. Lower-cost polyester suits large non-critical parts, epoxy suits many structural needs, high-temperature polyimides suit hot structures, and thermoplastics suit high-rate production and parts that need toughness or welding. As always, the resin and fiber must be chosen together.