The role of the matrix in a fiber-reinforced composite is :
to transfer stresses between the fibers to provide a barrier against an adverse environment, and to protect the surface of the fibers from mechanical abrasion Thermoset polymers, such as epoxies and polyesters, are in greatest commercial use, mainly because of the ease of processing with these materials. Metallic matrices are primarily considered for high temperature applications. We discuss these two categories of matrix in more detail.
Polymeric Matrix
A polymer is defined as a long-chain molecule containing one or more repeating units of atoms, joined by strong covalent bonds. A polymeric material (commonly called a plastic) is a collection of a large number of polymer molecules of similar chemical structure (but not of equal length). In the solid state, these molecules are frozen in space, either in a random fashion (for amorphous polymers) or in a mixture of random and orderly (folded) fashions (for semicrystalline polymers. However, on a submicroscopic scale, various segments in a polymer molecule may be in a state of random excitation. The frequency, intensity, and number of these segmental motions increase with increasing temperature, giving rise to the temperature-dependent properties of a polymeric solid.
Epoxies: principally used in aerospace and aircraft applications Polyester, vinyl esters: commonly used in automotive, marine, chemical, and electrical applicationsPhenolics: used in bulk molding compoundsPolyimides, polybenzimidazoles (PBD), potyphenylquinoxatine (PPQ); for high-temperature aerospace applications (temperature range: 250-4000C)Nylons (such as nylon 6, nylon 6,6), thermoplastic polyesters (such as PET, PBT), polycarbonate (PC), polyacetals: used with discontinuous fibers in injection molded articlesPolyamide-imide (PAI), polyether-ether ketone (PEEK), polysulfone (PSUL), polyphenylene sulfide (PPS), polyether imide (PEI): suitable for moderately high temperature applications with continuous fibersAluminum and its alloys, titanium alloys, magnesium alloys, copper-based alloys, nickel-based superalloys, stainless
steel: suitable for high-temperature applications (temperature range: 300-5000C)Aluminum oxide (Al203), carbon, silicon carbide (SiC), silicon nitride (Si3N4): suitable for high-temperature applicationsPolymers are divided into two broad categories: thermoplastics and thermosets. In a thermoplastic polymer, individual molecules are linear in structure with no chemical linking between them. They are held in place by weak secondary bonds (intermolecular forces), such as Vander Waals bonds and hydrogen bounds. With the application of heat and pressure, these intermolecular bonds in a solid thermoplastic polymer can be temporarily broken, and the molecules can be moved relative to each other to flow into new positions. Upon cooling, the molecules freeze in their new positions, restoring the secondary bonds between them and resulting in a new solid shape. Thus, a thermoplastic polymer can be heat softened, melted, and reshaped (postformed) as many times as desired.
In a thermoset polymer, on the other hand, the molecules are chemically joined together by cross-finks, forming a rigid, three-dimensional network structure. Once these cross-links are formed during the polymerization reaction (also called the curing reaction), the thermoset polymer cannot be melted and reshaped (postformed) by the application of heat and pressure. However, if the number (frequency) of cross-links is low, it may still be possible to soften them at elevated temperatures.
There are two unique characteristics of polymeric solids that are not observed in metals under ordinary conditions, namely, that their mechanical properties depend strongly upon ambient temperature and loading rate (time). Near the glass transition temperature, denoted by Tg, the polymeric material changes from a hard, sometimes brittle (grass like) solid to a soft, tough leather like) solid. Over a temperature range around Tg, its modulus is reduced by as much as five orders of magnitude. Near this temperature, the material is also highly viscoelastic. Thus, when an external load is applied, it exhibits an instantaneous (elastic) deformation followed by a (slow) viscous deformation. With increasing temperature, the polymer changes into a rubberlike solid capable of undergoing large, elastic deformations under external loads. As the temperature is increased further, both amorphous and semiicrystalline thermoplastics achieve highly viscous liquid states, with the latter showing a sharp transition at the crystalline melting point, denoted by Tm Tg. However, for a thermosetting Polymer, no melting occurs; and finally burns at very high temperatures instead, it chars. The glass transition temperature of a thermoset can be controlled by for very highly cross-linked polymers; the glass transition and accompanying softening may not be observed.
The mechanical characteristics of a polymeric solid depend on the ambient temperature as well as its value relative to the glass transition temperature of the Polymer. If the ambient temperature is above Tg, the polymeric solid exhibits low surface hardness, low modulus, and high ductility' At temperatures below Tg, the segmental motion in a polymer plays an important role. If the molecular structure of a polymer allows many segmental motions, it behaves in a ductile manner even below Tg. Polycarbonate (PC), Polyethylene terephthalate(PET), and various nylons fall into this category. If, on the other hand, the segmental motions are restricted, as in Polymethylmethacrylate (PMMA), polystyrene (PS) and many thermoset Polymers, it shows essentially a brittle failure. The effect of loading rate on the mechanical properties of a Polymer is opposite to that due to temperature. At high loading rates or short durations of loading, the Polymeric solid behaves in a rigid, brittle (grasslike) manner. At low loading rates or long durations of loading, the same material may behave in a ductile manner and show high toughness values.
The viscoelastic characteristic of a polymeric solid is best demonstrated by creep and stress relaxation tests. In creep tests, a constant stress is maintained on a specimen while its deformation (strain) is monitored as a function of time. In stress relaxation tests, a constant deformation (strain) is maintained while the stress on the specimen is monitored as a function of time. Both tests are performed at various ambient temperatures of interest Typical creep and stress relaxation diagrams exhibit an instantaneous elastic response followed by a delayed, viscous response during both loading and unloading of the specimen.
Softening characteristics of various polymers are often compared by their heat deflection temperatures (HDT). Measurement of HDT is described in ASTM test method D648-72. In this test, a plastic (polymeric) bar of rectangular cross section is loaded as a simply supported beam (inside a suitable nonreacting liquid medium, such as mineral oil. The load on the bar is adjusted to create a maximum fiber stress of either 1.82 MPa (264 psi) or 0.455 MPa (66 psi). The center deflection of the bar is monitored on a dial gauge as the temperature of the liquid medium is increased at a uniform rate of 2 + 0.20C/min. The temperature at which the bar deflects 0.25 mm (0.01 in.) from its initial room temperature deflection is called the heat deflection temperature at the specific fiber stress.
Although HDT is widely reported in the plastics product literature, it should not be used in predicting the elevated temperature performance of a polymer. It is used mostly for quality control and material development purposes. It should be pointed out that HDT is not a measure of glass transition temperature. For glass transition temperature measurements, such methods as differential scanning calorimetry (DSC) or differential thermal analysis (DTA) are used.
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 compositeHigh tensile strength, which controls the intraply cracking in a composite LaminateHigh fracture toughness, which controls ply delamination and crack growth For a polymeric matrix composite, there may be other considerations, such as good dimensional stability at elevated temperatures, and resistance to moisture and solvents. The former usually means that the polymer must have a high glass transition temperature Tg. In practice, the glass transition temperature should be higher than the maximum use temperature. Resistance to moisture and solvent means that the polymer should not dissolve, swell, crack (craze), or otherwise degrade in hot/wet environments or when exposed to solvents. Some common solvents in aircraft applications are jet fuels, deicing fluids, and paint strippers. Similarly, gasoline, motor oil, and antifreeze are common solvents in the automotive environment.
Traditionally, thermoset polymers (also called resins) have been used as a matrix material for fiber-reinforced composites. Starting materials used in the polymerization of thermoset, polymers are usually low-molecular-weight liquid chemicals with very low viscosities. Fibers are either pulled through or immersed in these chemicals before the polymerization reaction begins. Since the viscosity of the polymer at the time of fiber incorporation is very low, it is possible to achieve a good wet-out between the fibers and the matrix without the aid of high temperature or pressure. Among other advantages of using thermoset, polymers are their thermal stability and chemical resistance. They also exhibit much less creep and stress relaxation than thermoplastic polymers. The disadvantages are their limited storage life (before the final shape is molded) at room temperature, long fabrication time in the mold (where the polymerization reaction is carried out to completion and a solid part is obtained), and low strains-to-failure, which also contribute to their low impact strengths.
The most important advantage of thermoplastic polymers over thermoset polymers is their high impact strength and fracture resistance, which in turn impart an excellent damage tolerance characteristic to the composite material. In general, thermoplastic polymers have higher strains-to-failure than thermoset polymers, which may provide a better resistance to matrix microcracking in the composite laminate. Other advantages of thermoplastic polymers are
Unlimited storage (shelf) life at room temperatureShorter fabrication timePostformability (e.g., by thermoforming)Ease of repair by welding, solvent bonding, etc.Ease of handling (no tackiness)Can be recycled In spite of such distinct advantages, the development of thermoplastic matrix has been slower than that of thermoset matrix. Because of their high melt or solution viscosities, incorporation of continuous fibers into thermoplastic matrices is difficult. Commercial engineering thermoplastic polymers, such as nylons and polycarbonate, are of very limited interest in structural applications because they exhibit lower creep resistance and thermal stability than thermoset polymers. Recently, a number of thermoplastic polymers have been developed that possess high heat resistance. They are currently being explored as potential matrix materials for high-performance composite structures.
Metal matrix has the advantage over polymeric matrix in applications requiring a long-term resistance to severe environments, such as high temperature. The yield strength and modulus of most metals are higher than those for polymers, which is an important consideration for applications requiring high transverse strength and modulus as well as compressive strength for the composite. Another advantage of using metals is that they can be plastically deformed and strengthened by a variety of thermal and mechanical treatments. However, metals have a number of disadvantages, namely, they have high specific gravities, high melting points (therefore, high process temperatures), and a tendency toward corrosion at the fiber/matrix interface.
The two most commonly used metal matrices are based on aluminum and titanium. Both of these metals have comparatively low specific gravities and are available in a variety of alloy forms. Although magnesium is even lighter, its great affinity toward oxygen promotes atmospheric corrosion and makes it less suitable for many applications. Beryllium is the lightest of all structural metals and has a tensile modulus higher than that of steel. However, it suffers from extreme brittleness, which is the reason for its exclusion as a potential matrix material. Nickel- and cobalt-based superalloys have also been used as matrix; however, the alloying elements in these materials tend to accentuate the oxidation of fibers at elevated temperatures.
Aluminum and its alloys have attracted the most attention as matrix material in metal matrix composites. Commercially, pure aluminum has been used for its good corrosion resistance. Aluminum alloys, such as 201, 6061, and 1100, have been used for their higher tensile strength-weight ratios. Carbon fiber is used with aluminum alloys; however, at typical fabrication temperatures of 500'C or higher, carbon reacts with aluminum to form aluminum carbide (Al4C3), which severely degrades the mechanical properties of the composite. Protective coatings of either titanium boride (TiB2) or sodium are used on carbon fibers to reduce the problem of fiber degradation as well as to improve their wetting with the aluminum alloy matrix. Carbon fiber-reinforced aluminum composites are inherently prone to galvanic corrosion, in which carbon fibers act as a cathode owing to a corrosion potential of 1 V higher than that of aluminum. A more common reinforcement for aluminum alloys is SiC.
Titanium alloys that are most useful in metal matrix composites are alpha, beta alloys (e.g., Ti-6Al-9V) and metastable beta alloys (e.g., Ti-IOV-2Fe-3AI). These titanium alloys have higher tensile strength-weight ratios as well as better strength retention at 400-5000C over those of aluminum alloys. The thermal expansion coefficient of titanium alloys in closer to those for reinforcing fibers, which reduces the thermal mismatch between them. One of the problems with titanium alloys is their high reactivity with boron and A1203 fibers at normal fabrication temperatures. Borsic (boron fibers coated with silicon carbide) and silicon carbide (SiC) fibers show less reactivity with titanium. Improved tensile strength retention is obtained by coating boron and SiC fibers with carbon-rich layers.
Starting materials for epoxy matrix are low-molecular-weight organic liquid resins containing a number of epoxide groups, which are three-membered rings of one Oxygen atom and two carbon atoms. Common starting material is diglycidy, ether of bisphenol A (DGEBA), which contains two epoxide groups, one at each end of the molecule. Other ingredients that may be mixed with the starting liquid are diluents to reduce its viscosity flexbilizers to improve the impact strength of the cured epoxy matrix.
The polymerization (curing) reaction to transform liquid resin to the solid state is initiated by adding small amounts of a reactive curing agent just prior to incorporating fibers into the liquid mix. One such curing agent is diethylenetriamine (DETA). Hydrogen atoms in the amine (NH2) groups of a DETA molecule react with the epoxide groups of DGEBA molecules. As the reaction continues, DGEBA molecules form crosslinks with each other and a three-dimensional network structure is slowly formed. The resulting material is a solid epoxy resin.
If the curing reaction is slowed by external means (e. g., by lowering the reaction temperature) before all the molecules are cross-linked, the resin would exist in B-stage form. At this stage, cross-links have formed at widely spaced points in the reactive mass. Hardness, tackiness, and the solvent reactivity of the B-staged resin depend on the degree of cure advancement. The B-staged resin can be transformed into a hard, insoluble mass by completing the cure later.
Curing time (also called pot life) and temperature to complete the polymerization reaction depend on the type and amount of curing agent. With some curing agents, the reaction initiates and proceeds at room temperature, but with others, elevated temperatures are required. Accelerators are sometimes added to the liquid mix to speed up a slow reaction and shorten the curing time.
The properties of a cured epoxy resin depend principally on the cross-link density (spacing between successive cross-link sites). In general, the tensile modulus, glass transition temperature, and thermal stability as well as chemical resistance are improved with increasing cross-link density, but the strain-to-failure and fracture toughness are reduced. Factors that control the cross-link density are the chemical structure of the starting liquid resin (e.g., number of epoxide groups per molecule and spacing between epoxide groups), functionality of the curing agent (e.g., number of active hydrogen atoms in DETA), and the reaction conditions, such as temperature and time.
The continuous use temperature for DGEBA-based epoxies is 1500C or less. Higher heat resistance can be obtained with epoxies based on novolac and cycloaliphatics, for example, which have a continuous use temperature ranging up to 2500C. In general, the heat resistance of an epoxy is improved if it contains more aromatic rings in its basic chain.
Epoxy matrix, as a class, has the following advantages over other thermoset matrices:Wide variety of properties, since a large number of starting materials, curing agents, and modifiers are availableAbsence of volatile matters during cureLow shrinkage during cureExcellent resistance to chemicals and solventsExcellent adhesion to a wide variety of fillers, fibers, and other substrates The principal disadvantages are its relatively high cost and long cure time. Currently, the primary epoxy resin used in the aerospace industry is based on tetraglycidal diaminodiphenyl methane (TGDDM). It is cured with diaminodiphenyl sulfone (DDS) with or without an accelerator. The TGDDM/DDS system is used due to its relatively high glass transition temperature (240-2600C, compared to 180-1900C for DGEBA systems) and good strength retention even after prolonged exposure to elevated temperatures. Prepregs made with this system can be stored for a longer time period due to relatively low curing reactivity of DDS in the "B-staged" resin. Limitations of the TGDDM system are their poor hot/wet performance, low strain-to-failure and high level of atmospheric moisture absorption (due to its highly polar molecules). High moisture absorption reduces its glass transition temperature as well as its modulus and other mechanical properties.
Specific gravity 1.2-1.3Tensile strength, MPa (psi) 55-130 (8000-19,000)Tensile modulus, GPa (106 Psi) 2.75-4.10 (0.4-0.595)Poisson's ratio 0.2-0.33Coefficient of thermal expansion, 10-6m/m per 0C (10-6 in./in. per 0F)50-80 (28-44)Cure shrinkage, % 1-5 Although the problems of moisture absorption and hot/wet performance are reduced by changing the resin chemistry, brittleness or low strain-to-failure is an inherent problem of any highly cross-linked resin. Improvement in the matrix stran-to-falure and fracture toughness is considered essential for damage tolerant composite laminates. For epoxy resins, this can be accomplished by adding a small amount of highly reactive carboxyl, terminated butadiene-acrylonitrile (CTBN) liquid elastomer which forms a second phase in the cured matrix and impedes its microcracking. Although the resin is toughened, its glass transition temperature, modulus, and tensile strength as well as solvent resistance are reduced. This Problem is Overcome by blending epoxy with a tough thermoplastic resin, such as polyethersulfone, but the toughness improvement depends on properly matching the epoxy and thermoplastic resin functionlities, their molecular weights, etc.
The starting material for a thermoset polyester matrix is an unsaturated polyester resin that contains a number of C = C double bonds. It is prepared by the reaction of maleic anhydride and ethylene or propylene glyco. Saturated acids, such as isophthalic or orthophthalic acid, are also added to modify the chemical structure between the cross-linking sites; however, they do not contain any C = C double bonds. The resulting polymeric liquid is dissolved in a reactive (polymerizable) diluent, such as styrene, which reduces its viscosity and makes it easier to handle. The diluent also contains C = C double bonds and acts as a cross-linking agent by bridging adjacent polyester molecules at their unsaturation points. Trace amounts of an inhibitor, such as hydroquinone or benzoquinone, are added to the liquid mix to prevent premature polymerization during storage.
The curing reaction for polyester resins is initiated b adding small quantities of a catalyst, such as organic peroxide or an aliphatic azo compound, to the liquid mix. With the application of heat (in the temperature range of 107-163 0C), the catalyst decomposes rapidly into free radicals, which react (mostly) with the styrene molecules and break their C = c bonds. Styrene radicals, in turn, join with the Polyester molecules at their unsaturation points and eventually form cross-links between them. The resulting material is a solid polyester resin.
The curing time for Polyester resins depends on the decomposition rate of the catalyst, which can be increased by increasing the curing temperature. However, for a given resin-catalyst system, there is an optimum temperature at which all of the free radicals generated from the catalyst are utilized in curing the resin. Above this optimum temperature, free radicals are formed so rapidly that wasteful side reactions occur and deterioration of the curing reaction is observed. At temperatures below the optimum, the curing reaction is very slow. The decomposition rate of a catalyst can be increased by adding small quantities of accelerator, such as cobalt naphthanate (which essentially acts as catalyst for the primary catalyst.
As in the case of epoxy resins, the properties of polyester resins depend strongly on the cross-link density. The modulus, glass transition temperature and thermal stability of cured polyester resins are improved by increasing the cross-link density, but the strain-to-failure and impact energy are reduced. The major factor influencing the cross-link density is the number of unsaturation points in an uncured polyester molecule. The simplest way of controlling the frequency of unsaturation points is to vary the weight ratio of various ingredients used for making unsaturated polyesters. For example, the frequency of unsaturation in an isophthalic polyester resin decreases as the weight ratio of isophthalic acid to maleic anhydride is increased. The type of ingredients also influences the Properties and/or processing characteristics of polyester resins, For example, terephthalic acid generally provides a higher heat deflection temperature than either isophthalic or orthophthalic acids, but it has the slowest reactivity of the three phthalic acids, Adipic acid, if used instead of any of the phthalic acids, lowers the stiffness of polyester molecules, since it does not contain an aromatic ring in the backbone. Thus, it can be used as a flexibilizer for poleester resins. Another ingredient that can also lower the stiffness is diethylene glycol. Propylene glycol, on the other hand, makes the polyester resin more rigid, since the pendent methyl groups in its structure restrict the rotation of polyester molecules.
The amount and type of diluent are also important factors in controlling the properties and processing characteristics of polyester resins. Styrene is the most widely used diluent because it has low viscosity, high solvency, and low cost. Its drawbacks are flammability and Potential (carcinogenic) health hazard due to excessive emissions. Increasing the amount of Styrene reduces the modulus of the cured polyester resin since it increases the space between Polyester molecules. Because styrene also contributes unsaturation points, a higher styrene content in the resin solution increases the total amount of unsaturation and, consequently, the curing time is increased. An excessive amount of styrene tends to promote self-polymerization (i.e., formation of polystyrene) and causes polystyrene-like properties to dominate the cured Polyester resin.
Polyester resins can be formulated in a variety of properties ranging from hard, brittle to soft, and flexible. Its advantages are low viscosity, fast cure time, and low cost. Its properties are generally lower than those for epoxies. The principal disadvantage of polyesters over epoxies is their high volumetric shrinkage. Although this allows easier release of parts from the mold, the difference in shrinkage between the resin and fibers results in uneven depressions (called sink marks) on the molded surface. The sink marks are undesirable for exterior surfaces requiring high gloss and good appearance (e.g., class A quality in automotive body components). One way of reducing these surface defects is to use low-shrinkage (also called low-profile) polyester resins that contain a thermoplastic component (such as polystyrene or polymethyl methacrylate). As curing proceeds, phase changes in the thermoplastic component allow the formation of microvoids that compensate for the normal shrinkage of the polyester resin.
The starting material for a vinyl ester matrix is an unsaturated vinyl ester resin produced by the reaction of an unsaturated carboxylic acid, such as methacrylic or acrylic acid, and an epoxy resin. The C = C double bonds (unsaturation points) occur only at the ends of a vinyl ester molecule, and therefore, cross-linking can take place only at the ends. Because of fewer cross-links, a cured vinyl ester resin is more flexible and has higher fracture toughness than a cured polyester resin. Another unique characteristic of a vinyl ester molecule is that it contains a number of OH (hydroxyl) groups along its length. These OH groups can form physical (hydrogen) bonds with similar groups on a glass fiber surface resulting in excellent wet-out and good adhesion with glass fibers.
Vinyl ester resins, like unsaturated polyester resins, are dissolved in styrene monomer, which reduces their viscosity. During polymerization, styrene also coreacts with the vinyl ester resin to form cross-links between the unsaturation points in adjacent vinyl ester molecules. The curing reaction for vinyl ester resins is similar to that for unsaturated polyesters.
Typical Properties of Cast Thermoset Polyester Resins (at230C)
Specific gravity 1.1-1.4Tensile strength, MPa (psi) 34.5-103.5 (5000-15,000)Tensile modulus, GPa (106psi) 2.1-3.45 (0.3-0.5)Elongation, % 1-5Heat Deflection Temperature, 0C (0F) 60-205 (140-400)Cure shrinkage, % 5-12 Vinyl ester resins possess good characteristics of epoxy resins, such as excellent chemical resistance and tensile strength, and of unsaturated polyester resins such as low viscosity and fast curing. However, the volumetric shrinkage of vinyl ester resins is in the range of 5-10%, which is higher dm that of the parent epoxy resins. They also exhibit only moderate adhesive strengths compared with epoxy resins. The tensile and flexural properties of cured vinyl ester resins do not vary appreciably with the molecular weight and type of epoxy resin or other coreactants. However, the heat deflection temperature and thermal stability can be improved by using heat-resistant epoxy resins, such as phenolic-novolac types.
Bismaleimides(BMIs), PMR- 15 (for polymerization of monomer reactants), and ACTP (for acetylene terminated polyimide) are examples of thermosetting polyimides. Among these, bismaleimides are suitable for applications requiring a service temperature of 127-2320C. PMR and ACTP can be used up to 288 and 3160C, respectively. PMR and ACTP also have exceptional thermo-oxidative stability and show only 20% weight loss over a period of 1,000 h at 3160C in flowing air.
Thermosetting polyimides are obtained by addition polymerization of liquid monomeric or oligomeric imides to form a cross-linked infusible structure. They are available either in solution form or in hot melt liquid form. Fibers can be coated with the liquid imides or their solutions before the cross-linking reaction. 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. One useful method of reducing their brittleness without affecting their heat resistance is to combine them with one or more tough thermoplastic polyimides. The combination produces a semi interpenetrating network (semi-IPN) polymer, which retains the easy processability of a thermoset and exhibits the good toughness of a thermoplastic. Although the reaction time is increased, this helps in broadening the processing window, which otherwise is very narrow for some of these polyimides and causes problems in manufacturing large or complex composite parts.
Physical Properties of Cast Vinyl Ester Resins (at 230 C)
Specific gravity 1.12-1.32Tensile strength, MPa (psi) 73-81 (10,500-11,750)Tensile modulus, GPa (106Psi) 3-3.5 (0.44-0.51)Elongation, % 3.5-5.5 HDT, 0C (0F) 93-135 (200-275)Cure shrinkage, % 5.4-10.3 Bismaleimides are the most widely used thermosetting polyimides in the advanced composite industry. Bismalemide monomers (prepolymers) are prepared by the reaction of maleic anhydride with a diamine. A variety of bismaleimide monomer can be prepared by changing the diamine. BMI monomers are mixed with reactive diluents to reduce their viscosity and other comonomers, such as vinyl, acrylic, and epoxy, to improve the toughness of cured BMI. The handling and processing techniques for BMI resins are similar to those for epoxy resins. The curing of BMI occurs through addition-type homo or copolymerization that can be thermally induced at 170-190 0C.
PEEK has a glass transition temperature of 1430 C and a crystalline melting point of 335 0C. Melt processing of PEEK requires a temperature range of 370-4000C. The maximum continuous use temperature is 250 0C. PEEK is the foremost thermoplastic matrix that may replace epoxies in many aerospace composites. The outstanding property of PEEK is its high fracture toughness, which is 50-100 times higher than epoxies. Another important advantage of PEEK is its low water absorption, which is less than 0.5% at 230C compared to 4-5% for conventional aerospace epoxies. Being semicrystalline, it does not dissolve in common solvents. However, it may absorb some of these solvents, most notably methylene chloride. The amount of solvent absorbed decreases with increasing crystallinity.
Melt processing of PPS requires heating the polymer in the temperature range of 300-345 0C. The continuous use temperature is 2400C. It has excellent chemical resistance.
The glass transition temperatures for K polymer and LARC-TPI are 250 and 265 0C respectively. Both are amorphous polymers, and offer excellent heat and solvent resistance. Since their molecules are not cross-linked, they are not as brittle as thermosetting polymers. They are processed with fibers from low-viscosity solutions much like the thermosetting resins; yet, after imidization, the can be made to flow and be shapeformed like conventional thermoplastics by heating them over their Tg. This latter characteristic is due to the presence of flexible chemical groups between the stiff, fused-ring imide groups in their backbones. In LARC-TPI, for example, the source of flexibility are the carboxyl groups and the meta substitution of the phenyl rings in the diamine-derived portion of the chain.
