Thermal analysis of thermoplastic composites during processing

Unsteady two-dimensional thermal analysis has been performed on PEEK/AS-4 fiber thermoplastic composites. To calculate the crystallinity of the composite, a spherulite growth model was applied. A numerical analysis was carried out with variations in mold cooling rate, the prepreg lay-up, and the composite geometry. The effect of geometry and the cooling rate is significant in the temperature profiles. The degree of crystallinity varies with the cooling rate, but the gradient of crystallinity is small, with the exception of complex geometries at fast cooling rates. The results of numerical calculations are in excellent agreement with the experiments and offer validation of the numerical formulation.     

Molecular entanglement for rigid rod molecular composites: Poly(p-phenylene-2,6-benzbisthiazole)/poly(hexamethylene adipamide)

The entanglement molecular weight necessary for integrity of processed fiber and film for rigid rod molecular composites is discussed. The case presented is one where the matrix flexible coil polymer has a marginal molecular weight for entanglement. The molecular weight of the rigid rod reinforcement was increased to offset this deficiency. No significant phase separation was observed for solutions undergoing high shear. Steady state shear measurements on rigid rod molecule solutions show that the zero shear viscosity results agree very well with recent theoretical rigid rod molecule behavior predictions.     

Two-Dimensional thermal analysis of resistance welded thermoplastic composites

A transient two-dimensional thermal model for resistance welding of thermoplastic composites is presented. A parametric study is conducted that yields insight into the welding process enabling some critical process and material parameters to be identified. Time to melt is predicted by the model and is successfully compared to experimental observations. Local heating and meltthrough can also be explained by the transient thermal model in agreement with experimental observations. Mode I fracture toughness of unidirectional graphite reinforced poly(etheretherketone) resistance welded double cantilever beam specimens are conducted under various process conditions. Experimental results indicate that under optimum process conditions, the interlaminar fracture toughness of the bulk compression-molded thermoplastic composite material can be achieved using resistance heating as a joining technique.     

Dynamic mechanical analysis of thermoplastic composites and resins

The anisotropy of continuous fiber thermoplastic composites limits the number of geometries that can be employed effectively in dynamic mechanical analysis (DMA). Therefore, a Mechanical Energy Resolver, developed for use with elastomers, was modified with a bending fixture using a cantilever beam sample geometry and a high temperature oven. The bending fixture allows determination of composite transition temperatures. Based upon the simple geometry, one can also determine elastic moduli. This test scheme can be coupled with simple shear DMA on melts to determine neat resin properties, including solidification temperature. Results are presented for several semicrystalline and armorphous thermoplastic/graphite fiber laminates and neat resins.     

An analysis of stretch forming of thermoplastic composites

Thermoplastic composite sheet materials have found an increasing number of applications through the use of matched‐die compression molding. The combination of high strength, low weight and moderate manufacturing costs makes the material an attractive alternative to sheet metal stampings in a number of applications. However, a significant range of conventional sheet forming techniques may also prove suitable for these materials, provided the effects of strain rate and temperature can be properly understood and exploited, leading to a further reduction of manufacturing costs and an increasing number of potential applications. In this work, limiting strains in biaxial stretch forming were explored using the hemispherical stretch forming test. The materials tested contain 20, 35, and 40 percent by weight of randomly oriented glass fiber in a polypropylene matrix. The forming tests were performed at temperatures ranging from 75°C to 150°C and at punch speeds of 0.01cm/sec, 0.1cm/sec, and 1cm/sec. A rotationally symmetric, anisotropic material model with rate sensitivity was developed and incorporated into an axisymmetric finite element model of the stretch‐forming process. The model parameters were temperature‐dependent, though the temperature distribution in the formed part was assumed to be uniform. Strain distributions in the formed parts are compared to finite element method results, and the results are good up to the point when localized necking begins to dominate the strain distribution. These forming limit strains are compared with predictions based on Marciniak's imperfection theory, with good results.     

Compositional analysis of thermoplastic wood composites by TGA

A method has been developed for the quantitative determination of the composition of thermoplastic wood composites. The method involves area measurements of the two peaks in the derivative curve of a tracing produced by the thermogravimetric analysis (TGA) of a thermoplastic composites sample in an inert atmosphere. This may involve the standard, constant heating rate method or the high resolution (Hi‐Res?) TGA method; both produce virtual baseline separation between wood and polyolefin peaks over a wide compositional range. Because this baseline separation suffers from a mutual effect on the thermal degradation of wood and polyolefins, best results are obtained if the parameters relating peak area with polyolefin content are different below and above 40% polyolefin content. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1484–1492, 2004     

Thermal degradation studies on rigid polyurethane foams blown with pentane

The effect of sodium dihydrogenphosphate, trisodium pyrophosphate, and sodium aluminocarbonate on the thermal decomposition of rigid polyurethane (PUR) foams, based on diphenylmethane‐4,4‐diisocyanate, diphenyl‐2,2‐propane‐4,4‐dioxyoligo(ethylene oxide), and oxyalkylenated toluene‐2,6‐diamine, blown with pentane, was studied. Thermogravimetric (TG) data have shown that there is a stabilization effect of additives in the initial stage of degradation, both in nitrogen and air atmosphere, and the decomposition proceeded in two steps up to 600°C. Results of the kinetic analysis by the isoconversional methods of Ozawa–Flynn–Wall and Friedman yielded values of (apparent) activation energy (E_a) and preexponential factor (A). For phosphate‐stabilized PUR samples, E_a remained stable over a broad area of the degree of conversion, while for carbonate‐containing sample two regions of E_a were observed. Further advanced kinetic analysis by a nonlinear regression method revealed the form of kinetic function that was the best approximation for experimental data—for a two‐stage consecutive reaction the first step was the Avrami–Erofeev nucleation‐dependent model, and the second step was a chemical reaction (1st or nth order) model. The integrated thermogravimetric (TG)/Fourier transform infrared (FTIR) technique probed the thermal degradation of modified PURs by analyzing the evolved gases. The solid residue remaining at different temperatures was identified by diffuse reflection FTIR (Kubelka–Munk format). The complex thermal behavior was discussed on the basis of the obtained results—it can be shown that the global stabilization effect is a multistage process whose initial conditions are of critical importance in governing the nature of the entire process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2319–2330, 2003     

Thermal stress induced damage in a thermoplastic matrix material for advanced composites

In this paper, we analyze the basic physics of residual stress development during cooling of thermoplastic composite matrix materials. The analytical model presented here examines development of the stress field during cooling off a cylinder of thermoplastic matrix material. The residual stress field predicted by the model is compared to observations of the differences in microstructure of polymeric materials with different processing histories. The results of the investigation indicate that changes in the matrix related to processing conditions can contribute to damage formation in composites.     

Thermal conductivity studies on Si/SiC ceramic composites

Ceramic heat exchangers are increasingly used in many nuclear power plants. Silicon carbide has been treated as a promising material for heat exchanger application since it has good thermal conductivity and corrosion resistance. In this work, four different types of Si/SiC ceramic composites were prepared by liquid silicon infiltration technique. Thermal conductivities of these ceramic composites at different temperatures are measured by the laser flash thermal conductivity method. Results show that the presence of free carbon and voids are notably affecting the thermal conductivity of these materials.     

Fiber-reinforced cellulosic thermoplastic composites

Steam-exploded fibers from Yellow poplar (Liriodendron tulipifera) wood were assessed in terms of their thermal stability characteristics, their impact on torque during melt processing of a thermoplastic cellulose ester (plasticized CAB) matrix, their fiber–matrix adhesion and dispersion in composites, and their mechanical properties under tension. Fibers included water-extracted steam-exploded fibers (WEF), alkali extracted fibers (AEF), acetylated fibers (AAEF), and a commercial milled oat fiber sample (COF) (i.e., untreated control). The results indicate that the thermal stability of steam-exploded fibers increases progressively as the fibers are extracted with water and alkali and following acetylation. The greatest improvement resulted from the removal of water-soluble hemicelluloses. The modification by acetylation contributed to improved interfacial wetting that was revealed by increased torque during melt processing. Whereas modulus increased by between 0 and 100% with the incorporation of 40% fibers by weight, tensile strength either declined by ⅓ to ½ or it increased by a maximum of 10%, depending on fiber type. AAEF composites produced the best mechanical properties. Fiber–aspect ratio was reduced to an average of 25–50 from ≫ 200 during compounding. The superior reinforcing characteristics of AAEF fibers were also reflected by SEM, which revealed better fiber–matrix adhesion and failure by fiber fibrillation rather than by fiber pullout. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1329–1340, 1999     

Synthesis of rigid rod polymers

Summary Rigid rod polymers containing long rigid side chains have been prepared. The initial approach to this class of materials involved the synthesis of a poly (p-phenyleneterephthalamide) (PPT) backbone with diphenylthiazolothiazole pendant groups. The synthesis and preliminary characterization of the monomer and polymer are reported.     

Thermal analysis of elastomer composites

Thermal analysis of polychloroprene elastomer composites was carried out. Addition of reinforcing fillers such as precipitated silica (Vulcasil‐S), carbon black (FEF N‐550), and short silk fiber led to significant changes in the degradation pattern, depending on their reinforcement and adhesion ability with the elastomer matrix. Attempts were made to correlate the variations of thermal properties with the surface chemistry and the reinforcement characteristics of these fillers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 646–651, 2001     

Determination of filler content in thermoplastic composites by FTIR analysis

FTIR quantitative analytical method is described as an alternative technique for computation of the filler content in polypropylene composites. White rice husk ash (WRHA) was incorporated as a filler material into polypropylene homopolymer. Absorption peaks at 480, 621, and 790 cm^?1 chosen for the quantitative analysis work have been shown to give good linearity with increasing filler contents. © 1995 John Wiley & Sons, Inc.     

Effect of electron-beam radiation on thermoplastic composites

Abstract

The effect of electron beam (EB) radiation on carbon fibre reinforced (CF) thermoplastic (PBT, PPS, PA) composites was investigated. To clarify whether crosslinking could take place without or only with the presence of a crosslinking agent, special attention was paid to the incorporation of this agent into polymer sheets with a carbon fibre content of 50% by volume. The thermal and mechanical properties of the materials before and after exposure under different irradiation doses were evaluated. For materials based on PBT, PPS and PA46, no significant changes in properties after irradiation could be observed. However, CF/PA66 exhibited some changes in the presence of a crosslinking agent after irradiation, which could be related to an irradiation-induced crosslinking reaction. The effect of irradiation on the flexural properties was insignificant but an improvement in the creep behaviour was observed. Non-reinforced PA66 plates were also manufactured and a gel content measurement indicated that crosslinking was successfully induced. Additional studies allowed the changes in the polymer due to this crosslink to be quantified.     

Thermal decomposition behavior and mechanical properties of elastomeric composites based on polyolefinic thermoplastic elastomer and organomontmorillonite

The elastomeric composites based on organomontmorillonite (OMMT) and Santoprene thermoplastic elastomer were prepared by melt processing. Maleic anhydride grafted polypropylene (PP-g-MA) was used as a compatibilizer for the composite system. By adding optimum content of PP-g-MA, the fracture surface of the composites observed by SEM was smoothened as a result from compatibilizing effect. From XRD results, the measured d-spacing data proved a good dispersion of nanoclay along with compatibilizer. Thermal decomposition behavior of the neat components and its composites obtained from simultaneous TG and DSC profiles indicated that the incorporation of OMMT into the matrix polymer improved the thermal stability in air but not in nitrogen. No significant change in thermal stability of the composites with addition of PP-g-MA. The incorporation of clay significantly enhanced in dynamic mechanical and tensile properties of the composites. The dynamic storage modulus, tensile modulus and yield stress of the composites with the presence of PP-g-MA were remarkably improved.     

Press-forming of continuous-fiber-reinforced thermoplastic composites

Process-induced effects on thermoplastic-based composites were investigated for laminates processed under high production rate conditions. Press-formed carbon-fiber-reinforced poly(ether ether ketone) (PEEK) was used as a model system. The morphology and laminate quality were investigated on unidirectional laminates processed at cooling rates from 0.3 to 120°C/s and annealing of 177°C and 300°C. The laminate quality was examined for degree of consolidation, compaction, and fiber-matrix uniformity. Combined calorimetric and density measurements, as well as micrographic techniques, were used for examination of the laminates. The fracture toughness for the laminate was measured as a function of the position over the thickness of a 40-ply thick unidirectional laminate. This study clearly demonstrates the importance of uniform pressure distribution over the laminate to achieve a void free and homogeneous laminate.     

Solidification in anisotropic thermoplastic composites

Heat transfer analysis in thermoplastic composites manufacturing offers several challenges. The melting/solidification process results in the appearance of a molving solid-fluid interface or two-phase zone. The presence of reinforcing fibers with thermal conductivities substantially different than those of matrix meterials causes the composite material to behave anisotropically. Furthermore, an analysis should be adaptable to complex geometries if it is to be useful for practical applications. In this paper, a numerical approach is presented to develop an analysis tool for the processing of thermoplastic-matrix composites. A numerical grid generation technique is employed to determine the temperature distribution within the part while accounting for the heat absorption and liberation during the solidification stage. The influence of anisotropy in the domain is accounted for by considering thermal conductivity as a second-order tensor which varies continuously throughout the domain. Sample part configurations are used to demonstrate the applicability of this technique to thermoplastic composites processing.