As it has been explained in other posts, the matrix in composite materials has several functions. They have the function of bonding the fibers together. It transfers any applied loads to the fibers and keeps them in their position and orientation. Finally, it protects them against an adverse environment and protect the fibers from mechanical degradation. Matrix selection is very important as it will determine the maximum service temperature of the final laminate when it is cure. The interlaminar shear strength is an important design consideration for structures under bending loads, whereas the in-plane shear strength is important under torsional loads. The interaction between fibers and matrix is also important in designing damage-tolerant structures.
Thermoplastics or thermosets?
There are two main groups of matrix: thermoplastics and thermosetting.
- Thermosetting or thermosets: the molecules are joined together by strong cross-linking which creates a three-dimensional network structure. They cannot be melted when heat it is applied. Depending of the grade of curing, it can be subjected to higher or lower temperatures.
- Thermoplastics: the individual molecules are not chemically joined together; they are held in place by secondary bond or intermolecular forces. When heat is applied, these secondary bonds brake and the molecules have the ability to move to each other when a force is applied. When they are freeze, then the bonds are restores resulting in a new solid shape.
- Semi-crystalline thermoplastics are so named because a percentage of their volume consists of a crystalline morphology.
- Amorphous: the majority of thermoplastic polymers are composed of a random molecular orientation and are termed amorphous. The molecules are unable to align themselves in an ordered manner, since they are non-uniform or composed of units which have large side groups.
Selection of matrix in composite materials
The primary consideration in the selection of a matrix is its basic mechanical properties. For high-performance composites, the most desirable mechanical properties of a matrix are:
- High tensile modulus, which influences the compressive strength of the composite.
- High tensile strength, which controls the intraply cracking in a composite laminate.
- High fracture toughness, which controls ply delamination and crack growth.
For the composite industry, the most common resins used are polyester, vinilester and epoxy. All of them are thermosets as they have better mechanical properties than the thermoplastics.
Thermoset polymers types
- Polyesters resins: they are the most used resin system in the marine industry. The thermoset polyester matrix is an unsaturated polyester resin with C==C double bonds. It is prepared by the reaction of maleic anhydride and ethylene glycol or propylene glycol.
- Vinylester resins: is an unsaturated vinyl ester resin produced by the reaction of an unsaturated carboxylic acid, such as methacrylic or acrylic acid, and an epoxy. Because of fewer cross-links, a cured vinyl ester resin is more flexible and has higher fracture toughness than a cured polyester resin
- Epoxy resins: are low-molecular-weight organic liquid resins containing a number of epoxide groups, which are three-member rings of one oxygen atom and two carbon atoms. Wide variety of properties, since a large number of starting materials, curing agents, and modifiers are available 2. Absence of volatile matters during cure 3. Low shrinkage during cure 4. Excellent resistance to chemicals and solvents 5. Excellent adhesion to a wide variety of fillers, fibers, and other substrates
- Phenolics: cure by a condensation route with the off-gassing of water. The resulting matrix is characterized by both chemical and thermal resistance as well as hardness, and low smoke and toxic degradation products.
- Cyanate ester: in comparison with the other polymers, it has a high glass transition temperature (Tg = 250ºC). Besides, has better characteristics like lower moisture absorption than epoxies, good chemical resistance, and good dimensional stability. Its mechanical properties are similar to those of epoxies as well as the curing shrinkage. The main field of applications of the cyanate ester resin is in aerospace.
- Bismaleimides: Thermoset polyimides are obtained by addition polymerization of liquid monomeric or oligomeric imides to form a cross-linked infusible structure. On curing, they not only offer high temperature resistance, but also high chemical and solvent resistance. However, these materials are inherently very brittle due to their densely cross-linked molecular structure. As a result, their composites are prone to excessive microcracking. Used in aircraft composites where operation at 230ºC-250ºC is required.
- Polyimides: used when it is needed higher temperatures than bismaleimides can stand (250ºC-300ºC). Polyimides also tend to be hard to process due to their condensation reacting emitting eater during cure and are relatively brittle when cured.
Thermoplastics polymer types
- Polyether ether ketone (PEEK): is a linear aromatic thermoplastic based on the following repeating unit in its molecules. They are known in industry as aromatic polymer composite. PEEK is a semicrystalline polymer with a maximum achievable crystallinity of 48% when it is cooled slowly from its melt. Amorphous PEEK is produced if the melt is quenched. At normal cooling rates, the crystallinity is between 30% and 35%. PEEK has a glass transition temperature of 143ºC and a crystalline melting point of 335ºC. Melt processing of PEEK requires a temperature range of 370ºC–400ºC. The maximum continuous use temperature is 250ºC.
- Polysulfone: has a glass transition temperature of 185ºC and a continuous use temperature of 160ºC. The melt processing temperature is between 310ºC and 410ºC. It has a high tensile strain-to-failure (50%–100%) and an excellent hydrolytic stability under hot–wet conditions. Although polysulfone has good resistance to mineral acids, alkalis, and salt solutions, it will swell, stress-crack, or dissolve in polar organic solvents such as ketones, chlorinated hydrocarbons, and aromatic hydrocarbon.
Other types of matrix in composite materials
Metal matrix
The most common type of metallic matrices are aluminum, magnesium, titanium and copper. Their benefits are its high ductility and fracture toughness, high temperature and fire resistance, high electrical and thermal conductivities and no moisture absorption.
Ceramic matrix
They are used because have advantageous features like high fracture toughness, high specific strength, low thermal expansion and high oxidation resistance. The most common type of ceramic matrix are silicon carbide (SiC), titanium carbide (TiC) boron carbide (B4C), Alumina (Al2O3) and zircona (ZrO2).
Conclusion
There ara a lot of types of matrix in composite materials, the election of them will depend of the final use of the laminate.
