Thermoplastic matrix precursors

Pitches can be derived from coal tar or petroleum and have been discussed as precursor materials for making pitch based carbon fibers (Chapter 4, Section 4.4). Pitches are oligomers and the composition will depend on the exact source and method of processing. A pitch with a high carbon yield and the ability to flow under high pressure should be selected. 

Huttinger and Christ [58] based their studies on liquid phase impregnation of a fiber preform with pitches of different contents of mesophase spherulites. The matrix formed was mainly mesophase because the isotropic mesogenic pitch was pressed out. The composites were stabilized by treatment with air before carbonization of the matrix in order to avoid swelling. After graphitization at 2100°C, flexural strengths up to 650 MPa were measured. 

The composition of a pitch is very complex and, together with the processing conditions, controls the nature of the carbon matrix and it is normal to use a blend of pitches to obtain the requisite properties. The carbonization and graphitization of pitch is considered in 

Chapter 7. A preform can be initially impregnated with an epoxy to provide some interfilament adhesion and then impregnated with a pitch [59]. 

A pitch based resin gives a higher yield than a phenolic resin, is cheaper and when graphi-tized, has greater density (about 1.9 gcm ), but does require high pressures for processing. 

---

Usually, carbon fibers have a diameter of 3-15 n- Carbon fibers may have graphene ribbons parallel to ihe fiber axis. Carbon fibers are produced commercially by pyrolysis of organic precursors such as phenolics, poly(acrylonitrile), or pitch. In general, carbon fibers are chopped, having an initial length of about 0.05-5 cm. Sized fibers are conventionally coated on at least a portion of their surfaces with a sizing composition. The sizing composition effects increased compatibility with the polymeric thermoplastic matrix material (7). 

To ensure sufficient contact between the fibers and the matrix, it is desirable to use a liquid precursor with a low viscosity. Reactive liquids are usually preferred over thermoplastics due to the low viscosity of liquids relative to polymer melts. The reactive liquid is typically a multi-component mixture. The reactive liquid may contain a monomer and an activator, which will cause the monomer to polymerize into a solid polymer matrix. 

Polyimides for microelectronics use are of two basic types. The most commonly used commercial materials (for example, from Dupont and Hitachi) are condensation polyimides, formed from imidization of a spin-cast film of soluble polyamic acid precursor to create an intractable solid film. Fully imidized thermoplastic polyimides are also available for use as adhesives (for example, the LARC-TPI material), and when thermally or photo-crosslink able, also as passivants and interlevel insulators, and as matrix resins for fiber-reinforced-composites, such as in circuit boards. Flexible circuits are made from Kapton polyimide film laminated with copper. The diversity of materials is very large readers seeking additional information are referred to the cited review articles [1-3,6] and to the proceedings of the two International Conferences on Polyimides [4,5]. 

As mentioned earlier, suspensions of particulate rods or fibers are almost always non-Brownian. Such fiber suspensions are important precursors to composite materials that use fiber inclusions as mechanical reinforcement agents or as modifiers of thermal, electrical, or dielectrical properties. A common example is that of glass-fiber-reinforced composites, in which the matrix is a thermoplastic or a thermosetting polymer (Darlington et al. 1977). Fiber suspensions are also important in the pulp and paper industry. These materials are often molded, cast, or coated in the liquid suspension state, and the flow properties of the suspension are therefore relevant to the final composite properties. Especially important is the distribution of fiber orientations, which controls transport properties in the composite. There have been many experimental and theoretical studies of the flow properties of fibrous suspensions, which have been reviewed by Ganani and Powell (1985) and by Zimsak et al. (1994). 

The matrix allows the necessary positioning of the fibers, transfers the load to the fibers and distributes the stress among them, and is also responsible for protecting the reinforcement from the environment. However, the matrix is often the weakest component of a composite. One important parameter for the material properties is the fiber-matrix interface (or interphase), which guarantees the stress transfer from fiber to fiber via the matrix. The interface/interphase is a finite thin layer with its own (very often unknown) physical and chemical properties that depend on the fiber-matrix combination. Because of the low viscosity of the thermoset precursors, they wet the reinforcement better than a thermoplastic polymer. 

The precursor matrix material can be a hydrocarbon gas, a thermoset resin such as a phenolic or furan, or a thermoplastic resin such as a pitch or thermoplastic polymer. 

Ceraplast n. Any reinforced thermoplastic, particularly polyethylene, containing ceramic or mineral particles that have been dispersed in the polymer melt to their ultimate size (no agglomerates) and completely enveloped in resin. Bonding of the envelope to the filler particles and the matrix polymer is aided by the addition of a small percentage of reactive monomer or resin precursor. It is believed that, in the extremely thin transition envelope, there is a smooth gradient of modulus from that of the particulate material to that of the polymer. The mechanical properties of cera-plasts are superior to those in which the same fillers have been conventionally incorporated. 

The sol-gel synthesis of hybrid materials involves the occurrence of hydrolysis and condensation reactions in the presence of an organic polymer. Obviously, the selection of suitable polymer is of fundamental importance for the synthesis of the hybrid materials, as it should exhibit good miscibility with typical sol-gel precursors. The presence of suitable functional groups can facilitate the linkage between the polymer and the inorganic component. Also, the nature of the polymeric matrix is important because different properties of the matrix and, consequently, of the resulting nanohybrid material can be addressed for instance, the polymeric marix can be an elastomer (as in the case of polydimethylsiloxane) or thermoplastic (e.g., polytetrahydrofuran), amorphous, or (partially) crystalline [81]. 

---