Thermoplastic Composites

During the 1980s and early 1990s, government agencies, aerospace contractors, and material suppliers invested hundreds of millions of dollars in developing thermoplastic composites to replace thermosets. In spite of all of this investment and effort, continuous fiber thermoplastic composites account for only a handful of production applications on commercial and military aircraft.

Before considering the potential advantages of thermoplastic composite materials, it is necessary to understand the difference between a thermoset and thermoplastic. As shown in Fig. 7.51, a thermoset crosslinks during cure to form a rigid intractable solid. Prior to cure, the resin is a relatively low molecular weight semi-solid that melts and flows during the initial part of the cure process. As the molecular weight builds during cure, the viscosity increases until the resin gels, and then strong covalent bond crosslinks form during cure. Due to the high crosslink densities obtained for high performance thermoset systems, they are inherently brittle unless steps are taken to enhance toughness. On the other hand, thermoplastics are high molecular weight resins that are fully reacted prior to processing. They melt and flow during processing but do not form

Polymer Before Processing

Polymer After Processing

Polymer Chains

Polymer Chains

Thermoset

Crosslink

Crosslink

No Branches

Linear Thermoplastic

No Crosslinks

No Crosslinks

Branches

Branches

No Crosslinks

Branched Thermoplastic

No Crosslinks

Fig. 7.51. Comparison of Thermoset and Thermoplastic Polymer Structures1

crosslinking reactions. Their main chains are held together by relatively weak secondary bonds. However, being high molecular weight resins, the viscosities of thermoplastics during processing are orders of magnitude higher than that of thermosets (e.g., 104-107P for thermoplastics vs. 10P for thermosets).31 Since thermoplastics do not crosslink during processing, they can be reprocessed, for example they can be thermoformed into structural shapes by simply reheating to the processing temperature. On the other hand, thermosets, due to their highly crosslinked structures, cannot be reprocessed and will thermally degrade, and eventually char, if heated to high enough temperatures. However, there is a limit to the number of times a thermoplastic can be reprocessed. Since the processing temperatures are close to the polymer degradation temperatures, multiple reprocessing will eventually degrade the resin and in some cases it may crosslink.

The structural difference between thermosets and thermoplastics yields some insight into the potential advantages of thermoplastics. Since thermoplastics are not crosslinked, they are inherently much tougher than thermosets. Therefore, they are much more damage tolerant and resistant to low velocity impact damage than the untoughened thermoset resins used in the early to mid-1980s. However, as a result of improved toughening approaches for thermoset resins, primarily with thermoplastic additions to the resin, the thermosets available today exhibit toughness approaching thermoplastic systems.

Since thermoplastics are fully reacted high molecular weight resins that do not undergo chemical reactions during cure, the processing for these materials is theoretically simpler and faster. Thermoplastics can be consolidated and thermo-formed in minutes (or even seconds), while thermosets require long cures (hours) to build molecular weight and crosslink through chemical reactions. However, since thermoplastics are fully reacted, they contain no tack, and the prepreg is stiff and boardy. In addition, competing thermoset epoxies are usually processed at 250-350° F, while high performance thermoplastics require temperatures in the range of 500-800° F. This greatly complicates the processing operations requiring high temperature autoclaves, or presses, and bagging materials that can withstand the higher processing temperatures. Another advantage of thermoplastic composites involves health and safety issues. Since these materials are fully reacted, there is no danger to the worker from low molecular weight unreacted resin components. In addition, thermoplastic composite prepregs do not require refrigeration, as do thermoset prepregs. They have essentially an infinite shelf life, but may require drying to remove surface moisture prior to processing.

Another potential advantage of thermoplastics is low moisture absorption. Cured thermoset composite parts absorb moisture from the atmosphere that lowers their elevated temperature (hot-wet) performance. Since many thermoplastics absorb only very little moisture, the design does not have to take as severe a structural "knock down" for lower hot-wet properties. However, since thermosets are highly crosslinked, they are resistant to most fluids and solvents encountered in service. Some amorphous thermoplastics are very susceptible to solvents and may even dissolve in methylene chloride, a common base for many paint strippers, while others, primarily semi-crystalline thermoplastics, are quite resistant to solvents and other fluids.

Since thermoplastics can be reprocessed by simply heating above their melting temperature, they offer potential advantages in forming and joining applications. For example, large flat sheets of thermoplastic composite can be autoclave or press consolidated, cut into smaller blanks, and then thermoformed into structural shapes. Unfortunately, this has proven to be much more difficult in practice than originally anticipated. Press-forming processes are limited to relatively simple geometric shapes, because of the extensible nature of the continuous fiber reinforcement. If a defect (e.g., an unbond) is discovered, the part can often be reprocessed to heal the defect, but in practice, such repairs are rarely practical without undesirable fiber distortion and the associated structural property degradation. The melt fusible nature of thermoplastics also offers a number of attractive joining options such as melt fusion, resistance welding, ultrasonic welding, and induction welding, in addition to conventional adhesive bonding and mechanical fastening.

7.15.1 Thermoplastic Consolidation

Consolidation of melt fusible thermoplastics consists of heating, consolidation, and cooling, as depicted schematically in Fig. 7.52. As with thermoset

Temperature (T)

Pressure

Temperature (T)

Heating

Consolidation

Cooling

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