Polymer Composites

Lesser, A. J.; Weiss, R. A.

doi: 10.1002/pc.10329pmid: N/A

McLeod, M. A.; Baird, D. G.

doi: 10.1002/pc.10330pmid: N/A

This work was concerned with the injection molding of poly(ethylene terephthalate) (PET) reinforced with pregenerated thermotropic liquid crystalline polymer (TLCP) fibrils, where the TLCP had a higher melt processing temperature than PET. These composites, referred to as pregenerated microcomposites, were produced using a two step processing scheme. First, a novel dual extrusion process was used to spin strands of PET reinforced with nearly continuous TLCP fibrils. Second, these strands were subsequently chopped into pellets and injection molded below the melt processing temperature of the TLCP but above that of PET. This allowed the high modulus TLCP fibrils generated in the spinning step to be retained in the injection molded samples. TLCP concentration and strand draw ratio were varied in the composite strands to determine how they affected mechanical properties. It was shown that the best properties were obtained using strands containing 50 weight percent TLCP with draw ratios greater than 50, which were diluted to the desired loading level with a low viscosity injection molding grade of PET. Specifically, these composites had tensile moduli as high as 5.7 GPa when reinforced with 30 weight percent HX1000. Also, it was determined that pregenerated microcomposites had smoother surfaces than glass‐filled PET.

Lee, Sang‐il; Park, Joung‐man; Shin, Dong‐woo; Yoon, Dong‐jin

doi: 10.1002/pc.10331pmid: N/A

The interfacial adhesion and microfailure modes of glass fiber‐reinforced brittle unsaturated polyester/modified epoxy composites were investigated via micromechanical techniques and acoustic emission (AE). Various silane coupling agents caused different degrees of interfacial adhesion and subsequent microfailure modes. In the brittle matrix layer, the number of matrix fragments was significantly influenced by the type of silance coupling agents. The more cracks, the higher the interfacial adhesion under both dry and wet conditions. This is attributed to the chemical and hydrogen bondings in two interphases. The results obtained from microdroplet and fragmentation tests were correlated by associating with the AE technique. The sequential occurrence of mainly three groups of AE were as follows: the first group originated mainly from brittle matrix cracking. The second and the third groups resulted in fiber breakage and ductile matrix cracking and debonding. For dual‐matrix specimens the micromechanical tests provide reliable information with regard to the interfacial adhesion and characterize the microfailure modes when combined with the AE technique.

Riccieri, J. E.; Vázquez, A.; De Carvalho, L. Hecker

doi: 10.1002/pc.10332pmid: N/A

In this work the interfacial properties of polyester/vegetable fiber composites were analyzed by flexural testing. The compressive/tensile (σ) and shear (τ) stresses were determined for each composite in function of the span‐to‐depth ratio (λ). The general behavior of the composites was similar to that of composites reinforced with DuPont Kevlar fiber, i.e., a maximum σ stress value is obtained. Flexural test validity for determining the Young's and shear moduli, E11 and G12, was ascertained. The Young's modulus agreed with that expected from the rule of mixtures for the composites with lowest fiber content. Short beam tests were performed on the composites. The shear stress value was improved by means of the matrix modification. Moisture sorption experiments and dynamic mechanical analysis were also performed on the natural fiber composites in the first Fickian step. Water sorption at 50% RH and 90% RH can be satisfactorily described by using a diffusional model. Water diffusion on parallelepiped samples shows a positive deviation from the Fickian behavior. Fiber capillary flow occurs through the fiber and the debonded matrix/fiber interphase during the initial Fickian step.

Noobut, W.; Koenig, J. L.

doi: 10.1002/pc.10333pmid: N/A

The interfacial behavior of epoxy/glass fiber micro‐composites under cycles of wet and dry environment change was investigated by Fourier Transform Infrared (FTIR) microspectroscopy. The adsorbed water content in the epoxy/fiber interphase under moist conditions is reduced by treating the glass fibers with a silane coupling agent, γ‐aminopropyltriethoxysilane. This results in a significant decrease in the ring‐opening polymerization of epoxy in the epoxy/fiber interphase. It is also found that the wet‐dry cycles cause the significant variation of the residual adsorbed water in the interphase regions. There is an indication that the debonding in the epoxy/silane‐treated fiber interphase is slower than the epoxy/heat‐cleaned fiber interphase.

Robitaille, François; Gauvin, Raymond

doi: 10.1002/pc.10334pmid: N/A

The objectives of this series of papers are to describe the mechanical behavior of textile reinforcements under normal load and to quantify the effects of diverse processing parameters on that behavior. In the first and second papers of the series, experimental compaction and relaxation results were reported; general trends were identified and the effects of changes in the processing parameters were analyzed. In this paper, the results of sequences of successive compaction cycles applied to dry textiles and to textiles saturated in distilled H2O and silicone oil are presented. The reinforcements investigated are produced by assembling tows or rovings following different patterns; it is shown that the resulting heterogeneity, or regular variation of the local fiber volume fraction, can be associated to some particular elements of the mechanical behavior of the reinforcements. The reorganization of the fiber network and the effect of friction at the fiber contacts are demonstrated. Different stages in the reorganization process are identified; each stage is controlled by different parameters and corresponds to a precise behavior. Successive compaction cycles applied to a preform can reduce the void content of the final part.

Dash, B. N.; Rana, A. K.; Mishra, H. K.; Nayak, S. K.; Mishra, S. C.; Tripathy, S. S.

doi: 10.1002/pc.10335pmid: N/A

The development of high performance composites from a cheap natural fiber, jute, as reinforcement is particularly significant from an economic point of view. In this work, jute fiber‐unsaturated polyester(GP) composites having appreciable mechanical properties were prepared by using solution impregnation and hot curing methods. Both unbleached (control) and bleached jute slivers with various percentages of fiber loadings were used to prepare the composites and were named JPH (C) i.e., Jute Polyester Hot Curing (control), and JPH (B) i.e., Jute Polyester Hot Curing (bleached), respectively. Mechanical properties such as tensile and flexural strain, toughness, and moduli of both the grades have been compared. Composites having 60 wt% of jute fiber yielded the best results. JPH (B) showed much better flexural properties than JPH (C), although the tensile properties of the latter were better. The inter‐laminar shear strength (ILSS) of the JPH (B) was found to be higher than JPH (C). The nature of fiber‐resin bonding was studied from scanning electron micrographs of the specimens subjected to tensile and flexural fracture. Dynamic mechanical properties were found to be very high, superior even to those of glass fiber reinforced composites. The flexural storage modulus was found to be 12.3 GPa at 30°C and to decrease slowly with temperature. The major finding in this work is the attainment of high mechanical properties of composite specimens with 60 wt %fiber loading. On a weight and cost basis, bleached jute fibres were found to be better reinforcements than other fibers with usual surface modification by coating or grafting processes.

Padmanabhan, S. K.; Pitchumani, R.

doi: 10.1002/pc.10336pmid: N/A

Curing of catalyzed resin systems is an important and critical processing step in the fabrication of reinforced thermosetting composite materials. Strong uncertainties inherent in the associated process and material parameters, however, pose a stiff challenge to robust commercial manufacturing of quality composites in practice. Although deterministic models have been developed over the years to simulate the cure process, analysis of the effects of the parameter uncertainties on the process performance and the product quality variabilities has been the subject of little attention, and forms the focus of this study. This paper presents a methodology for a systematic analysis of the effects of the process and material parameter uncertainties on the isothermal curing of thermosetting resin systems. A stochastic model is developed, and parameteric studies are presented to systematically examine the effects of the uncertainties in the processing temperature and the kinetic parameters on the process output variabilities. Optimum parameter spaces that minimize the variance of the output parameters are identified, as a first step towards robust manufacturing of composites.

Button, Scott D.

doi: 10.1002/pc.10337pmid: N/A

Most commercial aircraft interior stowage bins are assembled using jigs. These assembly jigs are built to strict tolerances to ensure that the assemblies created in them will perform properly. Over the past decade, commercial aircraft structures assembly has been in transition from a method of assembly jig location to a method of part to part indexing, also known as Determinant Assembly. This method is now being investigated for accurate location of composite sandwich panels and metal attachments for commercial aircraft interiors. A background on Determinant Assembly (DA) as it has developed in structures is introduced. Several methods of coordinating composite sandwich panels to metal parts are discussed. Data that substantiates the use of these methods to maintain the prescribed tolerances is presented graphically. The results of building prototype parts using these techniques are presented. The process capability for DA assembled stowage bins is predicted using Variation Simulation Analysis, and those predictions are compared to results obtained with the current assembly jig method.

Lin, Chao‐ming; Weng, Cheng‐i

doi: 10.1002/pc.10338pmid: N/A

An improved model of the anisotropic flow characteristics of SMC (sheet molding compound) during compression molding is developed. This study is intended to complement our previous paper, which was conducted to determine the anisotropic parameters for short fiber reinforced thermosets SMC (16). Our prior study measured flow viscosities and material anisotropy by means of axisymmetric and plane strain compression molding tests. The current study, in order to identify the superior flow model from the choices (1) isotropic, (2) constant anisotropic and (3) varying anisotropic, applies the finite element method to obtain numerical results, which are subsequently compared with experimental results to determine the flow model with the best fit. The anisotropic parameters of the shear directions are determined by use of normal and planar parameters because SMC is planar isotropic. Six varying anisotropic parameters and six viscosity values are estimated during molding experiments, which are conducted at room temperature so that the polymer does not cure. Two‐dimensional molding numerical analyses are carried out to explain two experimental classes, axisymmetric and plane strain compression molding. The load‐levels predicted by the isotropic model, anisotropic model (parameter values fixed) and anisotropic model (parameter values varying) are compared with the experimentally derived values, the results showing that the varying anisotropic model best fits SMC compression behavior.