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Processing of PMR-15 prepregs for high temperature composites.

Processing of PMR-15 Prepregs for High Temperature Composites

PMR-15 is the most widely used high temperature matrix resin for advanced composite material applications above 232 [degrees] C for four principal reasons: performance, database, material availability, and price. Of the high temperature resins available today, this addition-cure polyimide displays the best overall balance of processing behavior, thermo-oxidative resistance, and retention of mechanical properties in the 260 [degrees] C to 316 [degrees] C range. The databases of engineering properties are the most comprehensive. PMR-15 resins and prepregs are commercial products, routinely offered by several prepreg manufacturers. Finally, the price of PMR-15 prepregs is relatively low--typically ranging from $80 to $150/lb, depending upon the type of reinforcement and quantity purchased.

Applications of PMR-15 composite components for jet engines include compressor blades, bypass ducts, particle separator swirl frames, and nozzle flaps. Other current and potential applications are missile fins, structural wing components, and radar domes.

PMR-15 is one of the best characterized resin systems with respect to its chemistry and cure behavior. Yet, processing can be complex and can appear to require more art than science for achieving satisfactory parts. This article details selected aspects of its chemistry and processing to provide an improved understanding of the known behavior of this material. Processing is discussed in two steps. The first addresses chemical considerations pertaining to prepreg manufacture and quality. The second reviews chemical reaction sequences as they relate to autoclave processing.

Prepreg and Tape


"PMR" stands for in-situ polymerization of monomer reactants. The suffix 15 refers to a formulated molecular weight of 1500. PMR-15 is built from the three monomeric ingredients shown in Fig. 1: the dimethyl ester of benzophenone tetracarboxylic anhydride (BTDE); 4,4'-methylene dianiline (MDA); and the monomethyl ester of norbornene anhydride (NE). To achieve the 1500 molecular weight, the BTDE, MDA, and NE are combined in the molar ratio of n:(n + 1):2, where n = 2.087. For formulation purposes, the molar quantities have been converted to weight percentages in Fig. 1.

PMR-15 impregnating resins are generally manufactured as 40 to 60 wt% methanolic solutions for producig woven fabric and roving prepregs, and as high-solids suspensions in methanol for producing unidirectional fiber tapes. Fabrics and rovings are pulled through the methanolic solution, then through metering bars to remove excess resin, and then through a treater oven to evaporate most of the solvent. Unidirectional tapes are manufactured by simultaneously casting a film of the high-solids suspension on release-coated paper and pressing collimated fiber tows into the film, or by a two-part process involving separate film casting and fiber collimating steps. The amount of methanol retained in prepregs is extremely important. It governs such properties as tack and drape and also influences the extent of low temperature oligomer formation and cured part quality.

Quality Control

The apparent simplicity of PMR-15 is very deceptive. The quality and lot-to-lot uniformity of resins and prepregs depend significantly upon the purity of the raw materials and how they and the prepregs are processed. A manufacturing flow chart is shown in Fig. 2 to emphasize the breadth of quality control (QC) testing of raw, intermediate, and finished materials. A brief discussion of some of the more important QC criteria will give an appreciation of the potential problems that may arise during the curing of PMR-15 composites.

Each of the monomers contains undesirable isomers whose content must be closely regulated. Moreover, the strting anhydrides, BTDA and NA, are moisture sensitive. Free acids, produced by hydrolysis, affect the rate of esterification with methanol, the quality of the esters, and the stoichiometry of the formulated PMR-15 resin. Esterification of the starting anhydrides must be carefully controlled to avoid producing the dimethyl ester of NA and the tri- and tetramethyl esters of BTDA, which are believed to cause excessive voids and blisters in cured laminates. Therefore, their concentration in prepregs must be only several percent at most.

High performance liquid chromatography (HPLC) is the chief technique for monitoring these quality issues. The chromatograms on the left in Fig. 3 compare fresh and aged BTDE. The first group of three peaks is the normal triad for the three naturally produced isomers of the dimethyl ester. The two peaks corresponding to the trimethyl ester are shown at longer elution times. BTDE is obtained and used in methanol solution. Esterification is an equilibrium process that continues at temperatures well below methanol reflux. If the material is to be stored, refrigeration is necessary to quench additional esterification.

NE, in contrast to BTDE, can be easily precipitated as a crystalline solid and so is fairly stable during storage. MDA can be obtained having very different quantities of the 4,4', 2,3', and 2,4' isomers. The latter two isomers are undesirable because their inclusion leads to side-chain branching or kinks in the polyimide molecular backbone.


The PMR-15 chemical reaction is complex, and portions of the reaction sequence are still not completely understood. Figure 4 illustrates the growth of low-molecular-weight amide-acid oligomers through the condensation reactions between NE and MDA and between MDA and BTDE with formation of water and methanol. These reactions occur rapidly in the 80 [degrees] C to 150 [degrees] C temperature range and continue to 200 [degrees] C to 250 [degrees] C. The range from 175 [degrees] C to 250 [degrees] C is also associated with softening and melt flow of the imidized linear polymer. Addition crosslinking reactions begin at approximately 250 [degrees] C and reach their maximum exothermic energy at about 362 [degrees] C. The substantial mass (17.1%) of water and methanol produced during the condensation reactions must be removed efficiently and completely using vacuum to obtain high-quality, void-free cured composites.

Unfortunately, the condensation reaction between NE and MDA is quite facile. It occurs slowly even during refrigeration of PMR-15 resins and prepregs at temperatures as low as 0 [degrees' C. The reaction yields a mononadimide and, on continued aging, a bisnadimide. Both are undesirable because their formation modifies the stoichiometry of the resin system and contributes water, which lowers the expected viscosity or flow behavior during polymerization. Fabricators have associated excessive voids and tendency to delamination with mononadimide concentrations in the prepreg exceeding 3% to 5%. The HPLC chromatograms on the right in Fig. 3 show acceptable and unacceptable amounts of reaction products. The temperature sensitivity with time of this reaction, obtained from HPLC data, is shown in Fig. 5.

The addition-cure reaction is believed to proceed through the Reverse Diel-Alder Reaction, sketched in Fig. 6. Here, the norbornene endcap is postulated to dissociate to produce cyclopentadiene or cyclopentadienyl segments that recombine with maleimide segments to complete the crosslinking reaction. Although experimental evidence is lacking, time-to-time variations in temperature, vacuum, and pressure during cures could produce different amounts of cyclopentadiene, and hence, different degrees or patterns of crosslinking.

Autoclave Processing

PMR-15 composites are fabricated using vacuum bag autoclave and compression molding methods. The autoclave process is more commonly used for large structural parts and will be discussed here. Because cure cycles that work well for thin parts of 10 plies or less do not necessarily produce good parts having 20 or more plies, vacuum bag cure cycles have evolved that yield larger and thicker laminated parts having no voids, surface flaws, or delaminations. Detailed knowledge of the reaction chemistry is certainly important, but effective design of the cure cycle also requires detailed information about the thermodynamic, rheological, and mass transfer characteristics of the system. A healthy appreciation of plumbing is also required, especially as to the effective sizing of vacuum pathways around the prepreg layup, and to vacuum measurement and piping.


Dynamic mechanical analysis has been particularly helpful for understanding the physical processes that accompany the PMR-15 chemical reaction sequences. Figure 7 shows on eof several vacuum bag autoclave cure cycles with a superimposed Rheometrics (RDS II) plot of complex viscosity, [eta] [*], for a woven-carbon-fiber PMR-15 prepeg. The prepereg. The prepeg was heated at 2 [degrees] C/min from room temperature to its cure temperature of 316 [degrees] C. Heating and cooling are necessarily held to low rates in the cure cycle, typically 1.3 to 3.3 [degrees] C/min, to ensure steady-state heat flow and small temperature gradients for adequate thermocouple-based control between the temperature programmer and the true part temperature.

The initial lowering of system viscosity shown on the viscosity curve is due to the melting and redissolving of the solid monomer into the methanol in the prepreg. During this stage, the prepreg stack is maintained at a slight negative pressure of about 10 kPa (3-in Hg). This vacuum helps to maintain good interfacial contract between adjacent plies and prevents disorientation of plies through slippage. Higher vacuum in this temperature range would cause excess resin flow or bleed-out of the reinforcement into the surrounding absorbent bleeder plies and vacuum paths. Relatively large volumes of methanol vaporize during this early stage. Most industry cure cycles maintain an isothermal hold between 66[degrees]C and 82[degrees]C for 1 hr to allow complete removal of the solvent before the system viscosity increases.

The steep viscosity increase at 80 [degrees] C to 90[degrees]C is associated with increasing molecular weight through amidization and imidization reactions. The system viscosity is sufficiently high at 140 [degrees] C to 240 [degrees] C, and full vacuum, approximately 100 kPa (30-in Hg), is usually applied to exhaust the condensation volatiles. The rate of imidization begins to decline rapidly at about 130 [degrees] C as molecular and gaseous diffusion processes become hindered by packing of the polymer molecules at or near their ultimate molecular weights. The isothermal hold, shown at 220 [degrees] C, is to allow completion of the imidization reaction before final part consolidation under pressure.

Melt flow of the prepolymer is indicated by the declining viscosity following imidization in the 175 [degrees] C to 250 [degrees] C range. Typical cycles hold the temperature constant at 238 [degrees] C to 243 [degrees] C for 45 to 60 min, during which time pressure is slowly increased to 1.03 to 1.37 MPa to give uniform consolidation before the crosslinking reaction starts at about 249 [degrees] C. Parts are generally held at the 316 [degrees] C cure temperature for about 3 hrs, after which they are cooled slowly to avoid stress relief by microcracking. The part is subsequently heated slowly back to 316 [degrees] C at ambient pressure to complete the remaining few percent of the cure for 6 to 10 hrs.

Process Variables

and Part Quality

This topic may be discussed at great length. However, only one variable, the volatile content, will be examined, because of its extremely significant impact on the usefulness of a PMR-15 part and because the amount of volatile material that must be removed during cure is not widely appreciated.

The condensation reactions shown in Fig. 4 yield 17.1% volatiles. Moreover, as mentioned earlier, PMR-15 prepregs contain additonal methanol to provide drape and tack to an otherwise dry and boardy material. Depending upon the applicatoin, the solvent methanol content in the prepreg will vary from 4% to 12% of the prepreg weight or as much as 25% of the resin weight.

Consider a 20-ply laminate, measuring 91.4 X 94.4 X 0.66 cm, having a 60% carbon-fiber-volume content, and weighing 9.47 kg following complete cure. The prepreg used for such a laminate would have had a wet resin content (resin plus methanol solvent) of about 45% by weight. During the cure cycle, the 20-ply stack of prepreg will have lost 2.255 kg of methanol and water--0.625 kg from the condensation reactions and 1.63 kg from the solvent methanol. This total volatile content represents 19.1% of the original prepreg weight or 42.5% of the PMR-15 resin weight, an even more impressive amount when the approximately 78.2 moles are converted to 2290 liters, at 100 [degrees] C, that have to be exhausted by the vacuum system. It is worth pointing out that volatiles content changes by a factor of 4 as laminate size (surface area) at constant thickness changes by a factor of 2. Thus, halving the edge dimensions of the laminate described above reduces total volatiles content to 0.564 kg or 573 liters.

These examples emphasize the importance of part geometry and mass when designing or selecting a cure cycle for PMR-15 composites. The cure cycle shown in Fig. 7 is a useful starting point but canot be applied without modification for all part configurations. The isothermal hold times at 220 [degrees] C and 243 [degrees] C are particularly sensitive to part thickness and other configurational details, the 45- to 60-min isothermal hold times may prove adequate for curing a 10-ply laminate measuring 45.7 cm on a side. A 30-ply laminate may require 2 to 3 hrs to accomplish the necessary devolatilization and consolidation. Scale-up of laminate size inevitably requires some amount of trial and error to balance the need for more time at 243 [degrees] C to promote more diffusion of gases out of the laminate with less time to avoid the crosslinking that retards diffusion of volatiles. They are also dependent upon practical aspects of autoclave processing involving temperature, vacuum, and pressure application. It is far more difficult to uniformly heat and pressurize parts in a large manufacturing-scale autoclave than one part in a small autoclave used for R&D.


It was mentioned at the beginning of this article that art more than science seems to govern the successful processing of PMR-15 composites. This perception is based chiefly on two factors: an incomplete understanding of the chemistry as it occurs under vacuum and pressure; and the necessity of paying attention to detail during all phases of PMR-15 handling and manufacture. The latter is the more important. Lack of attention to small differences in purity, mass ratios of the monomers, and prepreg handling can unintentionally result in PMR-14, PMR-16, or PMR-something else. Only by processing the material with consistent accuracy can one achieve the excellent performance offered by PMR-15.
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Title Annotation:in situ polymerization of monomer reactants
Author:Kantz, Mel
Publication:Plastics Engineering
Date:Jan 1, 1990
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