Heat Transfer Effects on the Processing — Structure Relationships of Polyetheretherketone (Peek) Based Composites*
Chris N. Velisaris and James C. Seferis t Polymeric Composites Laboratory, Department of Chemical Engineering University of Washington, Seattle, Washington 98195
* Reprinted by permission of the Society of Plastics Engineers t Author to whom correspondence should be addressed
ABSTRACT 1. INTRODUCTION
An analytical methodology capable of describing Carbon fiber reinforced polymer composites are interrelations between thermal processing and polymer rapidly becoming popular alternatives to aluminum and structure for thermoplastic based composite laminates steel in applications that require materials of high was developed. Specifically, this modelling methodology strength with light weight. The carbon fiber reinforcement was used to describe experimental results generated with offers stiffness and strength, while the polymeric matrix a specially designed match die quench mold by processing offers interlaminar shear strength, solvent and damage both neat PEEK polymer and carbon fiber reinforced resistance as well as determines the long term durability laminate samples at different cooling rates. The developed of the composite system as a whole. High performance model accurately predicted temperature profiles for polymer composites have been traditionally made up PEEK laminates of different thicknesses under normal as of carbon fibers as the reinforcement and a high well as extreme quenching conditions of 114°C/sec. performance epoxy as the matrix. However, unmodified surface cooling rates that are possible to generate with epoxy matrices currently in use have several limitations. the quench mold. In general, the modelling methodology Among them are finite shelf life before processing, a is capable of predicting a part's thermal profile during tendency for brittle behavior when fully cured, and processing in terms of the composite's microscopic reprocessing limitations. In light of these difficulties, intrinsic properties (fiber and matrix), composition, and thermoplastic matrices such as Polyetheretherketone lamina orientation. Furthermore, by coupling to the (PEEK), as well as modified thermosetting polymers, are thermal profile description, a previously developed attracting attention, offering solutions to these specific crystallization kinetics model for PEEK polymer and its problems, as well as advantages for automated carbon reinforced composite, a quantitative description manufacturing. As is typical for all polymer systems of structural development during processing was obtained. however, properties such as stiffness, strength, solvent Thus, with this analytical methodology, a skin-core resistance, and toughness, depend on the polymer crystallinity profile, where the crystallinity varies with structure and consequently, the processing method used part-thickness as a result of uneven cooling experienced for that polymer. Accordingly, the development of a during processing, was predicted both for the neat PEEK methodology to characterize the processing-structure- polymer and its carbon reinforced laminate forms. property relationships of a composite system has been Finally, the developed methodology clearly established the central theme of our work, often referred to as the the interplay of both microscopic heat transfer and 'Trinity' of polymer composite usage evaluation /1, 2/. kinetics of crystallization/solidification of the matrix In order to extend development of our methodology to that must be accounted for in predicting the final high performance thermoplastic composite systems, we structure of a carbon fiber reinforced laminate that will, have chosen, for detailed investigations, carbon fiber in turn, govern microscopic and macroscopic reinforced PEEK as an appropriate model system /1 -11/. performance. A semicrystalline thermoplastic such as PEEK can be
13 Vol. 1, No. 1, 1988 Heat Transfer Effects on the Processing - Structure Relationships of Polyetheretherketone (Peek) Based Composites analyzed in terms of our two-phase methodology, where APC2, obtained from ICI were examined in this study the material is described in terms of anisotropic crystalline 119/. and noncrystalline phases that make up the polymer For our crystallization and heat transfer consideration, 112-18/. The addition of carbon fibers to a semicrystalline very little difference was evident in properties assumed polymer system can affect the crystallization kinetics of for PEEK polymer in its neat form and in the APC1 and the polymer and further influence the micromechanics APC2 composites if the proper volume fraction of of both phases /1, 3, 4, 7/. The intrinsic properties of reinforcement was accounted for /1, 3, 4/. Accordingly, the crystalline and noncrystalline phases of matrix and the composite samples will be generally referred to as reinforcement, along with their special arrangements PEEK/CF samples with the volume fraction of fibers that depend on the processing conditions, determine the specified at a nominal 60%. These materials were bulk properties of the composite /1-12/. Accordingly, an examined in terms of their processing-structure relation- important first step in understanding how composite ships through the preparation of neat PEEK resin and properties depend on processing conditions is to composite plaques in the quench match die mold /1, characterize this processing-structure relationship. 3-5/. The volume fraction crystallinity of the neat PEEK A high temperature quench match die mold for specimens processed at various cooling rates in the mold characterizing the processing-structure relationship of was measured with the Density Gradient Technique both neat PEEK and its carbon fiber reinforced (DGT) /I/. It is significant to note, however, that the composites was used extensively in this study /3-5, 10/. crystallization model was developed based on data This mold has capabilities of heating polymer surfaces to generated by differential scanning calorimetry (DSC) 450°C, and cooling those surfaces at rates up to /4, 7/. 120°C/second. In general, composite plaques with Neat PEEK resin plaques were prepared for the rectangular cross sections are formed in the mold. When crystallinity evaluation at cooling rates ranging from forming thick composite specimens at high surface 0.03 to 114°C/second, with a process melt temperature cooling rates through typical consolidation and of 375°C, where cooling rates were taken as the average lamination processes such as compression molding, a value between the approximate glass transition and melt skin-core phenomenon, where the matrix structure is temperatures of PEEK, 144 and 334° C, respectively dependent on the distance from the specimen surface, /19/. The specimens, prepared for skin-core cystallinity can become significant /1, 3, 4, 10). This effect is due to profile evaluation, were either 6 χ 2 χ 0.04 in. thick or 6 the small thermal diffusivity exhibited by thermoplastic χ 2 χ 0.16 in. thick. Specific details on the processing materials, which causes the specimen core to cool at a conditions employed are available elsewhere /3/. For all slower rate than the surface. Typically, a thick semi- samples, the degree of crystallinity reported here was crystalline polymer part quenched at the surface will determined with the DGT. For thin specimens, repre- have a noncrystalline skin and a high crystallinity core, sentative samples for DGT measurements were taken and, consequently, such processed parts display across the entire thickness of those specimens. For the properties that vary from the skin to the core /3/. The thick specimen DGT analysis, samples were cut from presence of carbon fibers can complicate this effect 0.02 inch surface, quarter-plane, and mid-plane plaque through reinforcement influences on the crystallization sections, where the plaque quarter-plane was defined as kinetics and on the bulk thermal diffusivity. In an effort the plane half way between the plaque surface and to characterize this effect, in this work, a heat transfer mid-plane. model was coupled with a nonisothermal crystallization In addition to the neat resin processing, unidirectional kinetics model to provide a generalized methodology for twelve-ply PEEK/CF laminates were consolidated in describing processing of carbon reinforced high order to analyze the heat transfer effects in forming performance thermoplastics, in general, and for PEEK operations. In a typical consolidation procedure, all plies specifically. were first tacked together. Next, the plies were heated from 25 to 375°C at 10°C/minute under contact 2. EXPERIMENTAL pressure and held at 375°C for 10 minutes at 150 psig pressure. Finally, the plaques were quenched at the The neat PEEK resin used in this study was obtained maximum mold cooling rate. For these specimens, the from Imperial Chemical Industries (ICI) as Grade 450P, temperature at the center of each plaque was monitored in powder form /19/. In addition, the carbon fiber as a function of time by the placement of a fine gauge reinforced PEEK (PEEK/CF) composite, both APC1 and thermocouple between the upper and lower 6 plies.
14 CJV. Velisaris and J.C. Seferis Science and Engineering of Composite Materials
From the molding results, experimental data at high Physical Properties ' surface cooling rates indicated that the temperature χ-" Processing (.Temperatur* ^reilley profile at the mold surfaces could be approximated as C linear from the melt processing temperature of PEEK, or Constituent Phases^ 375°C, to the temperature of the cooling water, Matrix Reinforcement Initial Constant Surface Cooling Properties Properties measured at 17°C. Once the surface temperature reached Rate, Followed by m r Isothermal Hold the cooling water temperature, the surface temperature \ was maintained constant. Consequently, the slight curvature in mold surface temperature as a function of '^Microscopic Combination^ time, near the cooling water temperature, was neglected. of Properties Based on Composite Form Used, b Predict Temperatur« Profile Through Thickness 3. MODEL DEVELOPMENT for Processed Parts
In order to predict the skin-core crystallinity profile Description of Intrinsic N. Crystallization Kinetics V for general semicrystalline polymer based composite ((Experimental I Theoretical)/ consolidation and lamination procedures, relationships ι Predict Crystallinity for the constituent phase physical properties, the Profile for Different dependence of temperature on time and position, and Processes and Composite Shapes the intrinsic crystallization kinetics of the matrix, are required. First, physical properties of both the matrix and reinforcement phases, such as thermal conductivity, Fig. 1. Schematic outline of the procedure for coupling the density, and heat capacity, must be combined to predict heat transfer and crystallization models for the the bulk composite properties. From these bulk characterization of processing-structure relationships properties, the bulk composite thermal diffusivity can be for carbon fiber reinforced PEEK laminates. derived and that quantity can be incorporated into a general relationship for temperature as a function of time and position, derived for the appropriate shapes and then was assumed to cool until the cooling water and processing conditions. This description for temperature, TQ, was reached throughout. temperature as a function of time and position for the To obtain the appropriate relationships, the heat cooling process can then be combined with a description conduction equation was utilized to describe temperature for the matrix crystallization kinetics to predict the final as a function of time and position, viz.; degree of crystallinity as a function of position for a processed composite specimen. The entire procedure δΤ/öt :«V?T (1) used in predicting the skin-core crystallinity profile, for specimens processed under the observed initial and where boundary conditions with a given thermal processing t = Time (s) profile, is outlined in Figure 1. Τ = Temperature (°C) 2 In accordance with the observed specimen initial and a = Thermal diffusivity (cm /s) boundary conditions, two relationships were used to V = Laplacian operator predict the temperature as a function of time within a plaque for general consolidation and lamination processes For describing our specific process, series solutions were where the observed initial and boundary conditions derived from the one-dimensional form of the heat applied. The first relationship assumed that a plaque was equation, using either the variable and equal boundary initially at a temperature, Tj, throughout. Plaque surfaces conditions with the uniform initial condition, or the were then cooled at a constant rate to the temperature constant and equal boundary conditions with the of the cooling water, TQ. Once at that temperature, the non-uniform, but symmetric initial condition /20 21/. second relationship applied, where the plaque was In both cases, the temperatures at both plaque faces assumed to have an initial temperature distribution, touching the mold were assumed to be equal at all times. increasing from TQ at the surfaces to Tmax at the center, The first relationship, which provides temperature as
15 Vol. 1, No. 1, 1988 Heat Transfer Effects on the Processing - Structure Relationships of Polyetheretherketone (Peek) Based Composites a function of time and position for the initial stage of In order to describe temperature as a function of time the cooling process, where plaque surfaces are cooled at and position for neat PEEK and its carbon fiber a constant rate, and the origin is defined at the lower reinforced composites with Equations 2 and 3, the surface of the plaque with thickness 2L, is thermal diffusivities were needed. Microscopically, the quantities that comprise the thermal diffusivity, such as Τ = Tj + Kt + Κ [(x-L)2-L2] I (2a) + (2) thermal conductivity, density, and heat capacity, are all OO functions of position within the nonisotropic, hetero- 16KL2/(COT3) * Σ (-l)n/(2n+l)n * geneous material. However, Equations 2 and 3 apply to a n=ö homogeneous material. Accordingly, it was necessary to exp[-a(2n+l)Vt/(4L2)] utilize a methodology to obtain average, bulk properties cos[(2n+l>(x-L)/(2L)] for the composite thermal diffusivity. These bulk properties were obtained from the physical properties of the neat matrix and reinforcement with our composite where modelling methodology /3, 4, 12/. Κ = Heating rate (Κ <0 for cooling) (°C/s) The neat PEEK resin thermal conductivity, heat L = Plaque half thickness (cm) capacity, crystalline density, and noncrystalline density t = Time (s) values were available from product literature /19/. The Τ = Temperature (°C) references provided little indication of the temperature Tj = Initial temperature (°C) and structure dependence of neat PEEK thermal χ = Position (cm) conductivity and heat capacity. For simplicity, the a = Thermal diffusivity (cm2/s) thermal conductivity and heat capacity values reported as 0.0006 cal/(s*cm2(°C/cm)) and 0.32 cal/(g°C), The second relationship, which provides temperature respectively, were chosen /19/. By analogy to PET, these as a function of time and position for the final stage of values were assumed independent of temperature and the cooling process, where plaque surfaces are kept at a the degree of crystallinity, which resulted in a good constant temperature, and the origin is defined at the approximation /18/. lower surface of the plaque with thickness 2L, is Carbon fiber reinforcement thermal conductivity and heat capacity values were also available in the literature. T T + The transverse value was chosen since heat conduction = o (Tmax-Vli ansin[mrx/(2L)] occurred perpendicular to the fibers for our specific 2 2 2 exp [-an π t/(4L )] (3) process. This value was taken as 0.00102 cal/ (s*cm2(°C/cm)) /22/. The heat capacity of carbon was with assumed a mild function of temperature. For example, it 2L has been reported that from 200 to 300°C, the specific a = (1/L) V f(x*) sin[mrx*/(2L)] dx* (4) heat of carbon increases from 0.28 to 0.32 cal/(g°C) /23/. Accordingly, the heat capacity used in this analysis was chosen as 0.30 cal/(g°C). From the neat matrix and reinforcement properties, where the composite properties were obtained. Typically, these L = Plaque half thickness (cm) composite properties are a function of position through- t = Time (s) out the anisotropic material. For simplicity, however, Τ = Temperature (°C) the physical properties were averaged between the Τ = Initial temperature at the center (°C) max matrix and reinforcement properties. First, a relationship Τ = Final temperature (°C) for the bulk composite thermal conductivity was χ = Position (cm) obtained. One well-known means of averaging composite α = Thermal diffusivity (cm2/s) properties considers two limiting series and parallel arrangements of neat matrix and reinforcement /13, 22/. With these relationships, it must be emphasized that For heat conduction, the highest resistance occurs with a other solutions to accommodate any variations in the series arrangement of matrix and reinforcement. In this specimen geometry and processing procedure can be case, the bulk composite thermal conductivity can be derived in a similar fashion or obtained numerically (21). expressed by Equation 5. This arrangement physically
16 C.N. Velisaris and J.C. Seferis Science and Engineering of Composite Materials approximated the reinforcement and matrix arrangement Xmr = Mass fraction reinforcement in the molded, continuous fiber reinforced composite = Volume fraction reinforcement \x plaques, viz.; The bulk composite density, which was determined !/V Xvm/km + Xvr/kr (5) from the constituent phase densities, was computed assuming that the matrix was completely noncrystalline where /1 /. As a result, the bulk composite density was assumed Bulk composite thermal conductivity 1.534 g/cm3 and 1.579 g/cm3 for the APC1 and APC2, [cal./(s.cm2(° C/cm))] respectively. m Matrix thermal conductivity In addition to the bulk thermal conductivity, the [cal./(s.cm2(°C/cm))] bulk heat capacity was required. The bulk composite Reinforcement thermal conductivity heat capacity was approximated with a mass average of [cal./(s.cm2(°C/cm))] the matrix and reinforcement heat capacities, viz.; vm Volume fraction matrix Volume fraction reinforcement c = υ c + χ Γ vr pb mm pm mr pr (8)
Conversely, the lowest resistance to heat conduction where occurs with the parallel arrangement of matrix and C Bulk composite heat capacity (cal/(g°C)) pb Matrix heat capacity (cal/(g°C)) reinforcement, viz.; Cpm Reinforcement heat capacity (cal/(g°C)) Mass fraction matrix kb " Xvmkm + Xvrkr (6) mm Mass fraction reinforcement Xmr The series and parallel models for the bulk composite thermal conductivity are analogous to similar models for Since the matrix and reinforcement heat capacities the lower and upper bounds of the bulk modulus for a were almost equal in this analysis, Equation 8 was semicrystalline polymer; and consequently, all possible assumed to provide an accurate approximation for the arrangements of matrix and reinforcement within the bulk composite heat capacity. Furthermore, this composite will result in a bulk thermal conductivity prediction for the bulk composite heat capacity can be between these bounds /13/. Equation 5 was selected to utilized to confirm the assumption that was used during model the bulk composite thermal conductivity, since the development of the heat transfer model. For this the series model physically approximated the specific model, the equations that expressed temperature as a reinforcement and matrix arrangement in the molding function of time and position within a composite slab process better than the parallel model. It must be assumed that the heat given off by the polymer during emphasized that the relationship defining bulk thermal crystallization was negligible with respect to the heat conductivity as a function of composition can easily be given off by the slab during the cooling process. Using changed to accommodate variations in specimen micro- the predicted composite heat capacity from Equation 8, mechanics. a PEEK/CF slab cooled from 375 to 20°C, with a high In order to compute the reinforcement thermal crystalline matrix material (25 percent crystalline by conductivity from those of the neat matrix and volume), would release approximately 3 cal/g from the reinforcement, first, the composite volume fraction of crystallization. This enthalpy is negligible when compared the reinforcement was required. The volume fraction to the 110 cal/g released during the entire cooling reinforcement for the PEEK/CF composites was obtained process; and, consequently, the assumption for the from its mass fraction reinforcement, which is nominally derivation of Equations 2 and 3 was validated. 0.60 and 0.68 for the APC1 and the APC2, respectively The predicted bulk composite thermal conductivity, /19/, viz.; density, and heat capacity values were then utilized to compute the bulk composite thermal diffusivity, a^, for use in the series solutions defining temperature as a Xvr" (VPr) Xmr (7) function of time and position for composite plaques, where viz.; = Bulk composite density (g/cm3) = Reinforcement density (g/cm3) % = kb/(PbCpb> (9)
17 Vol. 1, No. 1, 1988 Heat Transfer Effects on the Processing - Structure Relationships of Polyetheretherketone (Peek) Based Composites
TABLE 1: Constituent Phase and Composite Composition for Carbon Fiber Reinforced PEEK/CF Composite
Mass Fxn. Volume Fxn. Density Matrix X__ mm Matrix X„vm_ Phase Ρ (g/cc) (g matrix/g) (cc matrix/cc)
Matrix 1.2626 non cryst. 1.4006 cryst. Reinforcement 1.79 Composite APC-I 1.534 0.40 0.48 APC-2 1.579 0.32 0.38
TABLE 2: Constituent Phase and Composite Properties of Carbon Fiber Reinforced PEEK/CF Composite
Thermal Heat Capacity Thermal Conductivity Diffusivity Phase Cp (cal/(g°C)) k (cal/(s*cm2(°C/cm))) α (cm2/s)
Matrix 0.32 0.0006 0.001485 Reinforcement 0.30 0.00102 0.001899 Composite APC-I 0.31 0.00076 0.001598 APC-2 0.31 0.00081 0.001655
A summary of all neat resin, reinforcement, and bulk where, for each process i = 1 or 2, composite properties used in this analysis is presented in Tables 1 and 2. CI: Constant coefficient of the temperature n _1 Finally, a crystallization kinetics model was coupled dependent preexponential factor (s~ K ) with the heat transfer model in order to predict the C2; Empirical parameter, associated with the skin-core crystallinity profile for PEEK resin in specimens temperature dependence of viscosity (K) of both neat PEEK and PEEK/CF composite. According C3i Parameter associated with the free enthalpy to the crystallization model, the nonisothermal crystal- of nucleation (K3) lization kinetics of PEEK resin can be expressed as two vci Time integral expression describing crystal integral expressions in parallel, viz. /3, 4/; nucleation and growth Avrami exponent Xvc/Xvcco = w1Fvcl + w2FVC2 (10) Τ mi· Crystal melt temperature Time from crystallization onset for process i, with ω Volume fraction crystallinity Xvc t; : Equilibrium volume fraction crystallinity
Fvd= 1-exp -Cli/Texp{-[C2i/(T-T +51.6) (11) 0' With the above crystallization model, it has been 2 ni 1 +C3i/(T(TmfT) )]}niti( - )dti possible to explain crystallization behavior of both neat PEEK as well as PEEK/CF, both isothermally as well as dynamically once the appropriate parameters are
w, +w2= 1 (12) determined /3,4/. Furthermore, recent refinements of
18 C.N. Velisaris and J.C. Seferis Science and Engineering of Composite Materials
the expressions utilized in describing crystallization with different fiber volume crystallinity. Further discussions this dual mechanism have made possible predictions concerning these constants may be found in during any temperature profile (cooling, heating Reference /10/. isothermal hold, etc) employing the same parameters as From the crystallization model, master curves of the in the earlier studies /9/. For the present study,however, final volume fraction crystallinity as a function of log since a simple cooling experiment from the melt is (cooling rate) were computed for neat PEEK resin, using described, the earlier simpler form of the crystallization W! = 0.73, and for PEEK composites, using W! = 0.85 model was utilized /4/. and Wj = 0.61. These values were chosen to represent The integral expressions are evaluated simultaneously realistic trends expected with actual composite samples. as a function of temperature, provided that Τ < Tmj, The crystallization curves are shown in Figure 2. Finally, since crystallization cannot occur above the melt temperature for a particular mechanism. Accordingly, for each crystallization process, t· = 0 is defined as the time at which Τ = Tmj. For example, if Tm2 only the second expression is evaluated as the temperature drops from Tm2 to Tml. Once the temperature reaches
Tml, both expressions are evaluated, and the integrations continue until the temperature drops to Tg-51.6.
The weight factors W) and w2 (w2 = l-wj represent the extent that one crystallization process in the dual mechanism occurs over the other. These factors can depend on temperature, cooling rate, and fiber content
/3, 4, 9/. General values for W! and w2, and all other crystallization parameters are summarized in Table 3 /3, 4/. For these calculations, it should be emphasized that Fig. 2. Volume fraction crystallinity as a function of log an equilibrium crystallinity of Xyoo = 0.37 was assumed. (cooling rate), modelled for neat PEEK resin and However, since Cj is constant for both crystallization PEEK resin in carbon fiber composites. Lines represent processes assumed to be taking place in PEEK, new predictions obtained by substitution of the universal values of both Xyoo and Ct may be chosen without constants shown in Table 3, into the general crystal- affecting the results presented in this analysis. This is lization model (Equation 10). particularly important since different equilibrium crystallization values for PEEK have been reported in with these results, the heat transfer model was used to the literature /24, 25/. Finally, for the composite, predict the skin-core crystallinity profiles in neat PEEK different values of w are reported that correspond to polymer and PEEK composites. From the heat transfer
TABLE 3: Model Constants For the Crystallization of Neat PEEK Polymer and PEEK Polymer in the Composite
Model Constants Process 1 Process 2
n. 2.5 1.5 593 615 Tmi 19 Vol. 1, No. 1, 1988 Heat Transfer Effects on the Processing - Structure Relationships of Polyetheretherketone (Peek) Based Composites model, average cooling rates computed as a function of In particular, it gives credence to the physical properties position within a neat PEEK or a PEEK/CF composite used to calculate the matrix thermal diffusivity assuming plaque were incorporated with the crystallization model independence of temperature and structure. In addition, to predict the final crystallization profiles for processed it demonstrated that the matrix and reinforcement specimens. thermal conductivities can be averaged using the series model to yield the composite thermal conductivity. Calculations were also made for predicting temperature 4. RESULTS AND DISCUSSION as a function of time and position for neat PEEK and PEEK/CF composites. These calculations were made for The combined heat transfer and crystallization models typical specimens, 0.20 inch thick, with surfaces that were derived from general energy transport and quenched at the maximum available cooling rate. Results crystallization kinetics principles were compared with are shown in Figures 4 and 5, as plots of temperature as the experimental temperature and crystallinity data generated with our quench match die mold process. Confirmation of the extent in which the heat transfer model predicted the experimental data was obtained directly from the molding experiments with carbon fiber reinforced PEEK laminate. A typical twelve-ply PEEK/CF laminate, that was 0.06 inch thick, was consolidated in the mold with a surface cooling rate of 114°C/second, resulting in a cooling rate of 88°C/second at its center. For this surface cooling condition, the model predicted a central cooling rate of 83°C/second. In addition, as is shown in Figure 3, the plot of temperature measured at the center of the composite specimen as a function of time indicates that the model and the data compare very well. Since this result was obtained with no adjustable TINE S) parameters or further assumption to the model, it validates all assumptions made in the model development. Fig. 4. Predicted temperature as a function of time, and position from the surface to the center, modelled for a 0.20 inch thick neat PEEK plaque with surfaces cooled at 114°C/second. TIME Fig. 3. Predicted temperature as a function of time at the center of a 0.06 inch thick PEEK/CF plaque with ο Fig. 5. Predicted temperature as a function of time, and surfaces cooled at 114 C/second. Line represents the position from the surface to the center, modelled for a prediction obtained from the general heat transfer 0.20 inch thick PEEK/CF plaque with surfaces cooled model, and the corresponding data points are also shown in the figure. at 114° C/second. 20 C.N. Velisaris and J.C. Seferis Science and Engineering of Composite Materials a function of time and the distance from neat PEEK and KINETICS MODEL PEEK/CF plaque surfaces. Since PEEK/CF displays a higher thermal diffusivity than neat PEEK, the PEEK/CF t plaque was predicted to cool at a higher rate in the s 3< center than the neat PEEK plaque. >-in EXPERIMENTAL MODEL From the predicted temperature profiles as a function s§ of time for the 0.20 inch thick neat PEEK and PEEK/CF plaques, cooling rates between the melt and glass transition temperature in Figures 4 and 5 were averaged Ϊ οu for each position, and the crystallinity profiles were > 0.06 predicted using the parallel crystallization model. Results for neat PEEK (W! = 0.73) and PEEK resin in the composite (Wi = 0.61) are shown in Figure 6. Finally, 000 0.032 0.064 0.0S6 0. 12B B. 1DB Figure 7 shows a comparison between the crystallinity POSITION(IN) profile predicted for a 0.16 inch thick neat PEEK specimen with surfaces cooled at 23°C/second, and the Fig. 7. Predicted volume fraction crystallinity as a function of degree of crystallinity data from a 0.16 inch thick neat thickness for a 0.16 inch thick neat PEEK plaque, PEEK plaque processed in the quench mold at a with surfaces cooled at 27.7°C/second. One line 28°C/second surface cooling rate. Also shown in Figure represents the prediction obtained using the dual 7 is a crystallinity profile interpolated from the crystallization mechanism model, and the other line experimental master cooling curve /l, 3/, using average represents the prediction obtained using the experi- cooling rates predicted as a function of position with the mental crystallization master curve data. heat transfer model. As Figure 7 indicates, the fit with the data was bounded by both the theoretical model and the experimental crystallization data. What is more 5. CONCLUSIONS impressive about the data, however, is that the crystal- linities were measured by the DGT technique while the An analytical methodology was developed capable of predictions are based entirely on thermal data. Thus coupling heat transfer and crystallization kinetics efforts collectively, these results demonstrate that the modelling to describe the processing of carbon fiber reinforced concepts employed in this work are capable of describing high performance composites. The heat transfer model both the temperature and crystallinity profile, either for provided temperature as a function of time and position a neat matrix molding or a composite lamination for composite specimens in terms of their predicted bulk procedure. thermal diffusivity. The crystallization kinetics model provided crystallinity as a function of temperature and time for PEEK resin. Predicted temperature profiles for carbon fiber reinforced PEEK laminates, during the cooling process, were confirmed with experimental data. Furthermore, PEEK resin crystallinity profiles were predicted for both quenched neat PEEK and PEEK/CF plaques. Ac, is evident from the predicted and measured temperature profile results for both neat PEEK and PEEK composites, the generalized analytical metho- dology has great potential for use in describing processing-structure relationships for general composite lamination techniques. Although, the heat transfer a. 0B- 0. υ model provided by this study was specifically tailored to POSITION««) a given process and specimen geometry, it can be easily Fig. 6. Predicted volume fraction crystallinity as a function of generalized for other process geometries expressing thicknessfor 0.20 inch thick neat PEEK and PEEK/CF temperature as a function of time and position /21/. plaques, with surfaces cooled at 114°C/second. Furthermore, for different composite arrangements (e.g., 21 Vol. 1, No. 1, 1988 Heat Transfer Effects on the Processing - Structure Relationships of Polyetheretherketone (Peek) Based Composites fabric vs cross ply), different combining arrangements of crystalline Thermoplastic-Based Composites", SPE ANTEC '87 Conf. Proc., 1467(1987). the required intrinsic properties could also be employed. 10. C.A. AHLSTROM, "Processing and Characterization Studies Consequently, this work has provided coupled models as of PEEK Matrix Composites", M.S. Thesis, Dept. of Chem. a simple, but general means for predicting heat transfer Engng, University of Washington, Seattle, WA (1987). and crystallization in semicrystalline thermoplastic based 11. E.J. STOBER, J.C. SEFERIS, AND J.D. KEENAN, composite materials. "Characterization and Exposure Polyetheretherketone (PEEK) to Fluid Environments", Polymer, 25,1845(1984). ACKNOWLEDGEMENTS 12. J.C. SEFERIS AND A.R. WEDGEWOOD, "The Material Index Formulation For the Mechanical Properties of The authors would like to express their appreciation Crystalline Properties of Crystalline Polymers", in Composite to Mr. Terry Schneider of Boeing, and to Mr. Robert Systems From Natural and Synthetic Polymers, p. 109, L. Martin and Mr. Ernie Alger of IBM Corporation, for Salmen, A. de Ruvo, J.C. Seferis, and E.B. Stark, eds, Elsevier Sei. Publ. (1986). helpful discussions, and assistance with data acquisition 13. J.C. SEFERIS AND R.J. SAMUELS, "Coupling of Optical and process control. The expert assistance and technical and Mechanical Properties in Crystalline Polymers", Polym. coordination provided to the project by Prof. J.-A. E. Eng. and Sei., 19, 975(1979). Manson as well as updates with recent data of Ms. Carla 14. J.C. SEFERIS AND P.S. THEOCARIS, Eds., Interrelations Ahlstrom of the Polymeric Composites Laboratory is Between Processing, Structure and Properties of Polymeric gratefully appreciated. 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