Composites from phenolic-type matrices

Thermosets such as phenolic thermosets are brittle at room temperature and usually have poor mechanical properties. However, due to the presence of cross-links, thermosets can be used at higher temperatures, as they have higher softening temperatures and better creep properties than thermoplastics. Thermosets are also usually more resistant to chemical attack than most thermoplastics, among other characteristics (Paiva and Frollini, 2000). In applications where good mechanical properties are required in addition to these intrinsic properties of thermosets, the thermosets can be combined with reinforcements to improve these properties. 

6 Structure of glyoxal-phenol oligomers supported by NMR data (Ramires et at., 2010a). 

Composites based on phenolic resins present flexibility as regards processing, and most of the conventional processes can be used to prepare such composites. 

Macroscale reinforcements and composites have thus far received the most investigation and application. The remarkable advances in the development of these polymeric matrices composites in the twentieth century have made these materials prominent, which is expected to continue despite advances in smaller-scale, mainly nanometric, reinforcements. 

Glass fibers with tailored properties have been developed to meet specific applications, with some designed to be compahble with mihtary specifications for ballistic performance (Taylor, 2010). 

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Carbon Composites. Cermet friction materials tend to be heavy, thus making the brake system less energy-efficient. Compared with cermets, carbon (or graphite) is a thermally stable material of low density and reasonably high specific heat. A combination of these properties makes carbon attractive as a brake material and several companies are manufacturing carbon fiber—reinforced carbon-matrix composites, which are used primarily for aircraft brakes and race cars (16). Carbon composites usually consist of three types of carbon carbon in the fibrous form (see Carbon FIBERS), carbon resulting from the controlled pyrolysis of the resin (usually phenolic-based), and carbon from chemical vapor deposition (CVD) filling the pores (16). 

There are two basic types of processes used to make CAMCs. The first is chemical vapor infiltration (CVI). CVI is a process in which gaseous chemicals are reacted or decomposed, depositing a solid material on a fibrous preform. In the case of CAMCs, hydrocarbon gases like methane and propane are broken down, and the material deposited is the carbon matrix. The second class of processes involves infiltration of a preform with polymers or pitches, which are then converted to carbon by pyrolysis (heating in an inert atmo-sphere). After pyrolysis, the composite is heated to high temperatures to graphitize the matrix. To minimize porosity, the process is repeated untU a satisfactory density is achieved. This is called densification. Common matrix precursors are phenolic and furan resins, and pitches derived from coal tar and petroleum. 

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