bluuebunny/crossref_metadata_2025 huggingface dataset:
Consolidation and relaxation behavior of continuous strand random glass mats with thermoplastic binders
Authors: Knight J. C. , Backes D. , Jayaraman K.
Published: April 2004

The consolidation and relaxation characteristics of glass mat reinforcements with a thermoplastic polyester binder have been studied. Attention has been given to the effect of preforming conditions on binder flow around the fibers, and on the rearrangement of fiber tows making up the preform. An instrumented hydraulic press was used to compress stacks of glass mat over a wide range of temperatures (51.7 to 176.7°C) and a range of platen closing speeds (0.02 to 2.00 mm/s). Scanning electron microscopy was used to examine the fiber tows after deformation. Individual fiber tows were observed to flatten during compression, more so at higher temperatures and slower closing speeds. The scanning electron micrographs also reveal changes in binder distribution around the fiber tows that are closely related to the degree of flattening of the fiber tows. This is because small gaps are opened up within the tow as it is flattened. Flow of binder along such gaps is predominantly due to the squeezing force, with only a minor contribution from capillary forces. The redistribution of binder then facilitates further compaction. The extent of binder redistribution is also governed by the binder viscosity, which was found to be a strong function of strain rate and temperature. The shear thinning viscosity of the binder leads to greater compressibility at the highest closing speed of 2 mm/s, although the compressibility of the mats was lower at moderately higher closing speeds. The compressibility was also higher at higher temperatures as expected. The loft was greater for stacks pressed at higher temperatures; this is a consequence of the lower binder viscosity at higher temperatures allowing more rearrangement of the tows. There was no loss of binder from the stacks in these runs.
The use of friction in the shaping of a flat sheet into a hemisphere
Authors: Chu Tun‐Jen , Jiang Kongren , Robertson Richard E.
Published: April 2004

An analysis is given for the use of friction to control the thickness distribution in a hemispherical preform shell. As the friction is changed between a hemispherical punch and an initially flat sheet that is to be formed into a hemispherical shell, the thickness distribution of the resulting shell changes. When the friction is low, the shell tends to be thinner near the pole of the hemisphere than near the equator. As the friction between the sheet and the punch surface increases, the thinning of the shell moves away from the pole toward the equator. The results of the analysis were validated by experiments with a natural rubber sheet that had the deformability necessary to test the predictions of the analysis. The friction distribution necessary to yield a shell of uniform thickness was also derived, and two‐zone and three‐zone combinations of constant friction that yield approximately uniform thickness in the resulting hemispherical shell were also obtained.
Tear resistance of sparsely reinforced elastomer sheets
Authors: Pan Rong , Wati Daniel
Published: April 2004

A family of elastomer composites has been developed which has outstanding tear resistance, and good tensile strength and elongation to failure as measured in a standard tensile test. These composites constitute a family in that they all are sparsely reinforced by a fishnet‐like webbing of reinforcement. The members of the family differ in that the reinforcements can be as flexible as soft polyurethane rod or as stiff as carbon fiber with 3000 strands per fiber. All these composites produce outstanding tear strength and an increase in the elongation to failure in an ASTM Die‐C type of tear test. The intersections of the polyurethane rod reinforcements prove to be effective crack arrestors.
PMR‐15/carbon fiber composites produced from powder‐coated towpreg
Authors: Dugger Jeffrey A. , Hirt Douglas E.
Published: April 2004

An addition‐type thermosetting polyimide, PMR‐15, has a relatively high glasstransition temperature, Tg, of approximately 340°C and attractive high‐temperature composite mechanical properties after postcuring. PMR‐15 prepreg is typically produced by pulling carbon fibers through a methanol solution that contains the PMR‐15 monomers. One of the monomers, methylene dianiline, is highly toxic, thereby making the solution‐coating technique undesirable. To alleviate the toxicity problem in the prepregging step, “preimidized” PMR‐15 is now available in powder form, and this material can be used to produce towpreg using a powder‐coating method. The powder‐coating method that has been used in this work involves the use of an aqueous foam as a carrier medium to continuously deposit the PMR‐15 powder onto carbon fibers. The powder is slurried in a surfactant solution into which nitrogen is dispersed using a foam‐generating device. A predetermined amount of this foam is then applied to a moving carbon fiber tow. The foam is collapsed and the powder fused to the fiber in a set of tubular ovens. The powder appeared to be particularly sensitive to the heat treatment, but towpreg was produced and composites were consolidated successfully. The mechanical properties of the composites formed from powder‐coated towpreg compare favorably with the properties of composites fabricated from prepreg that was drum wound from a methanol solution.
Braided thermoplastic composites from powder‐coated towpregs. Part I: Towpreg characterization
Authors: Ramasamy A. , Wang Youjiang , Muzzy John
Published: April 2004

Textile preforms offer the potential for producing low cost, complex shaped, advanced composite structures. For thermoplastic composites in particular, it is advantageous to incorporate the resin in the tow prior to textile processing since it is difficult to impregnate dry fabrics with thermoplastic melts. Incorporating resin in the tows tends to alter the properties of the tows that are critical for textile processibility. In this paper, carbon fiber tows coated with nylon powder are studied. They are characterized for flexibility and friction coefficient, which are critical for textile processibility, and for lateral compressibility, which is critical for composite moldability. These studies reveal that the powder coated tows have higher stiffness, friction coefficient, and bulk factor compared to uncoated and commingled tows. The influence of these towpreg properties on their braidability has been studied and is provided in Part II of this series.
Braided thermoplastic composites from powder‐coated towpregs. Part II: Braiding characteristics of towpregs
Authors: Ramasamy A. , Wang Youjiang , Muzzy John
Published: April 2004

Powder coated tows have a structure that is significantly different from conventional textile fibrous strands. The properties of carbon fiber tows coated with nylon 6 powder have been characterized in Part I of this series of Papers. In this study, braiding experiments have been carried out with powder coated and commingled tows. An approach has been established to estimate the damage inflicted to the towpregs during the braiding operation using a braiding simulator. Braiding studies reveal that the towpreg properties of flexibility and friction coefficient have significant effects on the braidability of these towpregs. These towpreg properties also have similar effects on the towpreg damage assessed with the braiding simulator. Commingled tow is easier to braid than powder coated tow. Wrapping facilitates braiding both types of tows.
Stamp forming of fabric‐reinforced thermoplastic composites
Authors: Hou M.
Published: April 2004

A stamp forming technique has been used to process a fabric woven composite made of glass fibers (GF) and polyetherimide (PEI). A hemispherical mold with a built‐in hold‐down arrangement was designed and used at room temperature to stamp parts from preheated flat preconsolidated laminates. Tensile properties of the material were measured under similar heating conditions as in the relevant stamp‐forming process. Stretch in the fiber direction was found to be smaller than the maximum elastic extension of the glass fibers. Reduction of the angle between the crossing fibers was quite large when the satin woven fabric composite was pulled in the 45° direction. The effect of die geometries and original laminate dimensions on the “shear‐buckling” were studied. The results described the correlations between processing parameters and fiber buckling. Finally, the local strain of fiber bundles was investigated in relation to different directions of fiber orientation.
Thermoplastic nanocomposites filled with wheat straw cellulose whiskers. Part I: Processing and mechanical behavior
Authors: Helbert W. , Cavaillé J. Y. , Dufresne A.
Published: April 2004

Cellulose microcrystals with dimensions of ∼5 nm × 150–300 nm were obtained from wheat straw. To evaluate the reinforcing effect of these fillers within a thermoplastic matrix, composites with a weight fraction of cellulose ranging from O to 30 wt% were processed by freeze‐drying and molding a mixture of aqueous suspensions of microcrystals and poly(styrene‐co‐butyl acrylate) latex. It was found that these microcrystals, or whiskers, bring a great reinforcing effect at temperatures higher than the glass transition temperature (Tg) of the matrix and improve the thermal stability of the composite. The relaxed modulus increased continuously with the filler content, and for a film containing 30 wt% of whiskers, it was more than a thousand times higher than that of the matrix. This effect is discussed with regard to theoretical calculations based on a mean field approach (Halpin‐Kardos model). It is concluded that the great reinforcement observed seems to be due not only to the geometry and stiffness of the straw cellulose whiskers but also to the interactions of the microcrystals, their topological arrangement, and the probable formation of whisker clusters within the thermoplastic matrix, the cellulose fillers probably being linked through hydrogen bonds.
Dynamic mechanical analysis of a uniaxial continuous carbon fiber‐thermoplastic composite subjected to anodic and cathodic currents
Authors: Pakalapati S. N. R. , Gadala‐Maria F. , White R. E.
Published: April 2004

The dynamic mechanical properties of a uniaxial continuous carbon fiber‐thermoplastic composite were found to degrade significantly when subjected to anodic and cathodic currents in the presence of seawater. The composite tested was made of PAN‐based graphite fibers (50 vol%) in Du Pont's J‐2 thermoplastic resin (nylonbased). The shear storage modulus,G′, and the shear loss modulus,G″, of the composite were measured by subjecting a rod‐shaped sample to small amplitude torsional oscillations. The moduli remained constant over time when the sample was in air and when it was submerged in seawater in the absence of applied currents. Both moduli decreased to ∼45% of their initial values when a constant anodic current of density 0.414 μA/cm2was applied for 2 h. A cathodic current of the same density and duration caused the moduli to decrease to ∼63% of their initial values. The moduli remained at these low levels after the current had been turned off. Inspection, under a scanning electron microscope, of the sample exposed to the anodic current revealed numerous cracks in the polymer matrix. In the sample exposed to the cathodic current, the fibers separated from the matrix of the composite.
Compressibility and flow permeability of two‐dimensional woven reinforcements in the processing of composites
Authors: Lekakou C. , Johari M. A. K. B. , Bader M. G.
Published: April 2004

The processing of composites under pressure is described by a viscoelastic model in which the pressure acts on the fibers in an elastic manner and on the liquid matrix in a viscous manner. Following a micro‐mechanics approach, an analytical model has been derived for the nonlinear elastic compression of two‐dimensional, orthotropic, plain woven fiber cloths, containing two empirical constants. The model is validated by experimental data and the empirical constants determined in the compression of a wet, plain woven, glass fiber reinforcement. The effects of the compression of woven cloths during processing are demonstrated by the change of their permeability.
Fiber reinforced nylon‐6 composites produced by the reaction injection pultrusion process
Authors: Cho Bong‐Gyoo , McCarthy Stephen P. , Fanucci Jerome P. , Nolet Stephen C.
Published: April 2004

Reaction injection pultrusion (RIP) combines the injection pultrusion process with reaction injection molding (RIM) techniques to yield one of the more novel methods of thermoplastic matrix pultrusion. An experimental set‐up was designed and built to pultrude nylon‐6 RIM material and continuous E‐glassfiber. Well‐impregnated nylon‐6 composites with 66.5, 68.8, 71.1, and 73.3 vol% fiber were produced. Internal temperature profile within the die was recorded during the process, and physical properties of resulting composites were measured. This paper presents results of the effect of fiber content, die temperature profile and pulling speed variations on internal temperature profile, monomer conversion, and physical properties. The study showed that increasing pulling speed lowered both peak temperature and monomer conversion. Higher die temperatures accelerated the reaction, resulting in a higher exotherm, a higher peak temperature, and a higher monomer conversion within the range investigated. Shear strength, flexual strength, flexual modulus, and transverse tensile strength were proportional to monomer conversion. Flexual modulus increased with higher fiber content within the range observed. Data allow the proper combination of die temperature profile and pulling speed to be selected to achieve a desired level of monomer conversion and physical properties. Results of this study provide basic information required for product design with nylon‐6 composites as well as tool design, selection of processing conditions, and quality control for the process.
Photodegradation of talc‐filled polypropylene
Authors: Rabello M. S. , White J. R.
Published: April 2004

Injection‐molded talc‐filled polypropylene (PP) composites have been exposed to ultraviolet (UV) radiation in the laboratory for periods up to 26 weeks. The extent of chemical degradation has been assessed by means of Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC), and the results correlated with the mechanical properties. In the early stages of exposure, the photo‐oxidation is faster in the talc‐filled composites than in the unfilled polymer, but this trend is reversed for exposures longer than ∼12 weeks. Scanning electron microscopy (SEM) inspection has revealed that surface cracks caused by photodegradation in the filled PP occur in the surface exposed to the UV source only, resulting in much less deterioration in tensile properties when compared with the unfilled polymer which degrades significantly at the unexposed surface also. Measurements of melting temperatures by differential scanning calorimetry (DSC) gave a consistent picture of degradation with that obtained by FTIR and GPC studies. DSC analyses have also shown that an increase in the melting enthalpy for both the unfilled and filled grades occurs during exposure.
Effect of aging on the compressive yielding of PAN‐based carbon fiber/polycarbonate composites
Authors: Nofal M. M. , Zihlif A. M. , Ragosta G. , Martuscelli E.
Published: April 2004

The compressive yield behavior and the effect of aging in boiling water on the mechanical properties of polycarbonate composites reinforced with PAN‐based carbon fibers were studied at different filler contents and over a range of strain rates. The Young's modulus, yield stress, and yield strain are reported as a function of aging time. Other mechanical parameters such as the activation energy and volume of the yielding process were determined through the Eyring theory. The increase of both Young's modulus and yield stress with aging in boiling water is explained by structural changes. The mechanical properties of the composite were correlated with the morphology and its glass transition temperature.
Hydrostatically extruded glass‐fiber‐reinforced polyoxymethylene. I: The development of fiber and matrix orientation
Authors: Hine P. J. , Davidson N. , Duckett R. A. , Clarke A. R. , Ward I. M.
Published: April 2004

This paper describes the determination of fiber and matrix orientation in oriented short‐glass‐fiber‐reinforced polyoxymethylene (POM) composites produced by hydrostatic extrusion. The starting material was random glass fibers (25 wt% and average length 150 μm) in an isotropic POM matrix, and the oriented composites were produced by extrusion through a reducing conical die at 15°C below the polymer melting point: after extrusion the average fiber length was reduced slightly to 133 μm. Fiber orientations were measured using an image analysis method developed at Leeds University, and the matrix orientation was determined using wide angle X‐ray diffraction. The development of fiber orientation with extrusion ratio was found to be close to that predicted by the pseudo‐affine deformation scheme although the fiber orientation was greater than that predicted by the model at low draw ratios and slightly less at the highest draw ratio. The development of the orientation of the crystalline portion of the matrix was found to be always significantly greater than that predicted by the pseudo‐affine scheme.
Dielectric studies of phase separation induced by chemical reaction in thermoplastic modified epoxy
Authors: Vinh‐tung C. , Boiteux G. , Seytre G. , Lachenal G. , Chabert B.
Published: April 2004

Dielectric measurements were performed in real time to follow the reaction of polymerization in thermoplastic (TP) modified epoxy‐amine (EA) blends. This epoxy system exhibits a phase separation induced by the reaction in the liquid stage of the resin. It results in 3 different types of morphology, made of a TP‐rich phase and an EA‐rich phase. In an electrically stimulated sample, the phase separation results in an interfacial polarization seen by dielectric spectroscopy. This paper shows the possibility of monitoring both the phase separation and the vitrification of each phase. The gelation of the resin is not detected by this technique. By correlating morphological observations with characteristic dielectric responses, the type of morphology can be identified from the permittivity during the phase separation.
Bubble transport through constricted capillary tubes with application to resin transfer molding
Authors: Lundström T. Staffan
Published: April 2004

This paper describes and expounds a theoretical and experimental study of bubble motion through constricted capillary tubes. In the experiment, two liquidfilled capillaries are used. They have unequal radii and are glued together. Gas bubbles are injected into the larger capillary. Then the pressure required to force the bubbles through the constriction is measured for various liquids, bubble lengths, capillary radii and constriction geometry. It appears that the pressure directly follows Young's‐Laplace law for capillary pressure. The results of the study are used to understand the bubble transport through fiber reinforcements, which generally takes place during the manufacturing of composites. The bubbles are carried if: (i) the pressure gradient is high enough, (ii) the surface tension of the liquid is low enough, (iii) the cross‐sectional area of the channels in the reinforcement is sufficiently uniform. The theory reveals that the bubbles are more likely to be trapped on a small scale, i.e. within fiber bundles rather than on a large scale, i.e. between the bundles. It is also concluded that, if the bubbles are trapped at the resin flow front, a converging flow is better for the transport of the voids than a diverging flow.
Snagging resistance of sparsely reinforced flexible elastomer sheets
Authors: Pan Rong , Watt Daniel F.
Published: April 2004

One objective of the work discussed in this paper is to develop a material with high snag resistance, where snagging is the glancing penetration of a composite sheet by sharp objects. The best material of those tested to date is a soft polyure‐thane sheet reinforced sparsely with a web of extruded polyurethane fiber. Stiff carbon fibers sparsely distributed also give good snagging resistance, but closely spaced stiff fibers do not perform well in the ASTM Standard test for snag resistance.
Highly filled thermoplastic composites. II: Effects of particle size distribution on some properties
Authors: Kauly T. , Keren B. , Siegmann A. , Narkis M.
Published: April 2004

The effect of irregularly shaped glass particle size and size distribution on the packing density and flexural mechanical properties of highly‐filled composites with a rubbery thermoplastic matrix was studied. Increasing the particle's median size and size distribution width significantly increases the packing density of the composites. Compression molding causes the glass particles to fracture at a decreasing level with an increasing distribution width. Particle median size, rather than size distribution, affects the mechanical properties; The flexural modulus and strength increase and the ultimate deflection in flexure decreases with a decreasing median size. A “glass network” is formed in the compression molded composites because of the mechanical interlocking of particles. The nature of this continuous glass phase predominates the composites mechanical behavior. The particle's size and shape determine the nature of the glass network and, thus, have a dominating effect on the mechanical properties. The latter are significantly affected by the particle's surface properties. A specific silane treatment of the glass particles acts to reduce the particle/particle friction, resulting in a higher packing density. The treatment also acts as a cohesive liquid to increase the strength of the glass network, and to increase the particle/polymer adhesion, increasing the composites' strength and ductility.
Reprocessing of fiberglass reinforced polyamide 66: Influence on short term properties
Authors: Eriksson P.‐A. , Albertsson A.‐C. , Boydell P. , Eriksson K. , Månson J.‐A. E.
Published: April 2004

The fiber length distribution was found to control the overall short term performance of reprocessed heat‐stabilized short fiberglass reinforced polyamide 66. Length changes, and matrix and interface degradation were studied. Fiber shortening dominates during compounding and during the first injection molding cycle. Further regrinding and remolding has a lesser effect. The short term mechanical strength decreased for reprocessed samples. Using a modified Kelly‐Tyson model, the lower tensile strength of reprocessed samples, compared with virgin samples, can be explained by fiber shortening. Reprocessing had a negligible effect on the strength for both the fiber matrix interface and the matrix of this system. Studies on unreinforced samples confirmed that thermal degradation of the matrix during reprocessing had a negligible effect on short term mechanical performance.
Prediction of mechanical properties of recycled fiberglass reinforced polyamide 66
Authors: Eriksson P.‐A. , Albertsson A.‐C. , Boydell P. , Prautzsch G. , Månson J.‐A. E.
Published: April 2004

An experimental study of the mechanical performance of in‐plant recycled fiberglass reinforced polyamide 66 is reported. The fiber length distributions were used to investigate and to predict the influence of process induced fiber shortening on the short term performance of recycled samples compared to that of virgin samples. The results indicate that fiber shortening has a strong influence on strength. Applying a modified Kelly‐Tyson model to the fiber length distribution gave excellent agreement with measured strength. There was no need to vary interface or matrix properties in the theoretical analysis. The effect of reprocessing on these factors does not appear to influence strength within the bounds of the model. The decrease in strength during a continuous in‐plant recycling process is small at a 30 wt% regrind level. Indeed, below 50 wt% regrind, the strength remains within design limits. The impact strength of dry unnotched samples indicated that the resistance is related to the reciprocal fiber length.
Extrudate swell behavior of glass fiber filled polyamide 6
Authors: Wagner Alan , Yazici Rahmi , Kalyon Dilhan M.
Published: April 2004

This paper discusses the extrudate swell behavior of glass‐filled polyamide 6. The key features of the experiments were the facility to extrude directly into a density matched oil kept at extrusion temperature and thus, the ability to measure the diameter of the extrudate of the glass‐filled polyamide immediately upon exit from the die, without the extrudate contacting air and as a function of time in a chamber under isothermal and neutrally‐buoyant conditions. The concentration and orientation of the fibers of the extrudates were also studied using X‐ray radiography in a post‐mortem fashion. A skin/core morphology, where uniaxially aligned fibers constitute the skin zone and the core contains a reduced concentration of fibers, was observed.
Needle‐Punched hybrid nonwovens of flax and ppfibers—textile semiproducts for manufacturing of fiber composites
Authors: Mieck K.‐P. , Lützkendorf R. , Reussmann T.
Published: April 2004

Flax‐nonwoven reinforced polypropylene has become a competitor to textile glass fabric reinforced polypropylene because of its economic and ecological advantages. Suitable application forms are needle‐punched 100% flax or hybrid fabrics of flax/polypropylene manufactured of rough and fine decorticated flax. The construction of the nonwoven influences the strength, handle, matrix compatibility, and flow of the fabric. In this way, composite properties may be tailored to each end‐use. The main application fields are subassemblies exposed to a medium range of stress. The method of hybrid‐nonwoven manufacturing represents a technological alternative to the existing film‐stacking method. Produced with lower technology, the parameters of composites of hybrid‐nonwovens are comparable to properties of composites manufactured by the film‐stacking method. Adhesion characteristics may be improved by addition of compatibilizers. The compatibilizers may be added to the flax‐fiber surface or may be inserted into the polypropylene. All these methods lead to comparable mechanical parameters for the fiber composites.
Process‐induced effects in thin‐film bonding of PEKEKK in metal‐polymer joints
Authors: Ramani Karthik , Tagle John
Published: April 2004

In this work, poly ether ketone ether ketone ketone (PEKEKK) was used as a thermoplastic adhesive for joining metals. Titanium and cobalt chromium alloys were joined to form tensile butt joints. These tensile specimens were used to evaluate bond performance. A controlled thermal processing cycle was used to modify and enhance the polymer crystallinity during bonding. The resulting effects on bond performance were examined. The process window for a thermal bonding process was identified. Factorial experiments were conducted to determine the effects of modifications to adhesive and adherend material, and bonding pressure on bond performance. A titanium alloy and a cobalt chromium alloy were used as the adherends. Understanding changes in the thermoplastic adhesive joint with the variation of process parameters will allow for proper application and processing of thermoplastic structural adhesives.
Influence of fiber surface treatment on the properties of sisal‐polyester composites
Authors: Singh B. , Gupta M. , Verma Anchal
Published: April 2004

The effect of several chemical treatments, viz. organotitanate, zirconate, silane, and N‐substituted methacrylamide, on the properties of sisal fibers used as reinforcement in unsaturated polyester resin (∼50 vol%) was investigated. An improvement in the properties was observed when sisal fibers were modified with surface treatments. Under humid conditions, a decrease of 30 to 44% in tensile and 50 to 70% in flexural strength has been noted. The strength retention of surface‐treated composites (except silane) is high compared with untreated composites. It is observed that N‐substituted methacrylamide‐treated sisal composites exhibited better properties under dry as well as wet conditions. Fractographic evidence such as fiber breakage/splitting and matrix adherence on the pulled‐out fiber surface explains such behavior.
Fatigue behavior of neat and long glass fiber (LGF) reinforced blends of nylon 66 and isotactic PP
Authors: Harmia T.
Published: April 2004

This paper focuses on the study of the fatigue behavior of neat and long glass fiber (LGF) reinforced nylon 66/PP‐blends. The fatigue was characterized using Parislaw plots in the stable crack growth acceleration range. The fatigue crack propagation (FCP) is presented as a function of the crack growth per cycle (da/dN), the amplitude of the stress intensity factor ΔK, and of the strain energy release rate ΔG. It was also of interest to compare the order of performance found in fatigue to that in the static fracture test. The fracture surfaces were characterized with SEM to determine the failure mechanisms. Further, thermographic camera recordings were used to study the size of a “heated” area (ΔT = 2°C) that developed around the crack tip during the cyclic loading of LGF‐PP with different amounts of maleic anhydride grafted PP (PP‐g‐MAH). For the neat materials, a different order of performance was detected under static and cyclic loading. This was explained by the different failure mechanisms observed after static and cyclic fracture that were related to different stress states of the specimens during the fracture process. On the other hand, the LGF‐blends showed a similar order of performance during the static and the fatigue test. This was explained by the observation that similar fiber related failure mechanisms occurred in the composite, both after failure caused by the static and cyclic loading, respectively. For the LGF‐PPs with varying PP‐g‐MAH content, the order of performance in fatigue did not correspond to the size of the “heated area” around the crack tip. This was caused by a change in the composite failure mechanisms, which contributed differently to the size of the “heated area” and to the fatigue performance.
Fracture failure processes in polymers. II: Fractographic evidence
Authors: Liu K. , Piggott M. R.
Published: April 2004

Tensile fracture and punch shear failure surfaces have been observed in a scanning electron microscope. In most cases tensile cracks were visible in the shear fracture surfaces and crack initiation points could be found in the tensile fracture surfaces. The punch test imposes very large shear strains on the materials (many thousand percent) and this appears to reorient the chains so that, so long as the chain length is great enough (>4000 repeat units for polyethylene), chain scission is observed, and the failure process at the molecular level is tensile rather than shear. The exceptions are the shorter chain polyethylenes and polypropylene, which are probably truly liquid‐like, with little or no chain scission being involved in tension or shear.
Effect of humidity on the time to breakdown of polymers subject to partial discharges
Authors: Guastavino F. , Tiemblo P.
Published: April 2004

Three polymers, polyethylene terephthalate, a polyimide, and polyamide‐6, were subjected to surface partial discharges up to electrical breakdown under varying ambient humidity conditions. It was found that increasing humidity increases lifetime of the samples until a critical value of relative humidity is attained, after which the polymers' lifetime dramatically decreases. This critical value depends on the hydrophylicity of the polymer and is lower as the ability of the polymer to absorb water increases. The evolution of the electrical time to breakdown with the relative humidity is explained in terms of changes in surface conductivity following variations in the ambient humidity.
Fusion bonding of maleated polyethylene blends to polyamide 6
Authors: Sánchez‐Valdes S. , Yañez‐Flores I. , De Valle L. F. Ramos , Rodriguez‐Fernandez O. S. , Orona‐Villarreal F. , Lopez‐Quintanilla M.
Published: April 2004

Blends of linear low‐density polyethylene (LLDPE) and linear low‐density polyethylene–grafted maleic anhydride (LLDPE‐gMA) were used to promote the adhesion to polyamide 6 (PA) in a three‐layer coextruded film without using an additional adhesive or tie layer. The effect of bonding time and molecular weight (MW) of different maleated polyethylenes on the peel strength of the joints was analyzed. Direct evidence of a copolymer formed in‐situ at the interfaces is also considered. The peel strength of fusion bonded layers of LLDPE/LLDPE‐gMA blends with PA strongly depends on bonding time and molecular weight of the maleated polymer. Tensile properties of three‐layer films, made up of PA as the central layer and LLDPE/LLDPE‐gMA blends as the two external layers, are improved with increases in the maleic anhydride (MA) content in the blend. The in‐situ formation of a copolymer between the MA in the blend and the terminal amine groups of the PA was confirmed by the Molau test, infrared (IR) spectroscopy, and thermal analysis (DSC).
Development of heat‐activated joining technology for composite‐to‐composite pipe using prepreg fabric
Authors: Stubblefield Michael A. , Yang Chihdar , Pang Su‐Seng , Lea Richard H.
Published: April 2004

Owing to the lack of reliable and cost‐effective joining methods, composite materials have not yet achieved their full market potential. The use of heat‐activated thermal couplings offers a quick and cost‐effective method for joining composite‐to‐composite pipe. In this study, a prepreg laminate containing thermoset resins and fiberglass reinforcements is wrapped around the ends of the components to be joined. A shrink tape, constructed of an oriented polyester film, is placed over the prepreg laminate. When heat is applied to the thermal coupling for curing, the shrink tape shrinks and compresses the prepreg to improve adhesion. Tests of the heat‐activated thermal coupling in bending shows an increase of 29% over the currently used butt‐weld method. A finite element model has been developed to assist in determining the most appropriate cure cycle required for different joint configurations. This reduces the need for experimentation when a variable has been changed. Based on the tested prepreg material properties and model, the FEA temperature distribution differs less than 10% from that of experimental results.
Fiber reinforcement and fracture resistance of PC/PBT/LCP ternary in situ composite
Authors: Wong Shing‐Chung , Mai Yiu‐Wing , Leng Yang
Published: April 2004

The microstructures, mechanical properties, and fracture toughness of LCP (Vectra B950) reinforced PC/PBT blend with a 60/40 weight ratio have been studied. LCP of varying concentrations were investigated as rigid fillers in matrices of multiphase polymer blends. In this study, differences in microstructures and morphology between samples of two thicknesses (4 mm thick and 6 mm thick) and two geometries (dumbbell and rectangular) were compared using scanning electron microscopy (SEM). Given identical processing conditions, fibrous LCP structures were evident in the 4‐mm‐thick injection molded, dumbbell‐shaped samples, whereas the 6‐mm‐thick rectangular samples displayed spherical dispersion of LCP aggregates that embrittled the preblended ductile matrix. Tensile properties of the dumbbell specimens showed superior strengthening and stiffening whereby the tensile strength increased twofold and the modulus increased fourfold. Plane strain fracture toughness was slightly enhanced as the LCP content increased because of the fiber strengthening effect but the overall fracture performance of the in situ composites was relatively poor compared with PC/PBT. Experimental results were compared with those predicted in composite theory. Simplified micromechanics equations were developed to describe the tensile moduli of injection molded LCP reinforced blends that exhibited a strong skin‐core morphology.
Plastification or melting: A critical process for free radical grafting in screw extruders
Authors: Cartier Hervé , Hu Guo‐Hua
Published: April 2004

This paper shows for the first time that when a monomer is to be grafted onto a polymer backbone by a free radical mechanism in a twin screw extruder, the grafting process occurs mainly, if not exclusively, in the plastification (melting) zone. For this purpose, the free radical grafting of glycidyl methacrylate (GMA) onto polypropylene (PP) and polyethylene (PE) was chosen as model systems. A co‐rotating self‐wiping twin screw extruder of type Werner Pfleiderer ZSK‐30 (L = 42D) was used to process the grafting. Owing to its modular character in terms of barrel arrangement, screw element combination and barrel temperature, the position and length of the plastification zone can be adjusted virtually at will. This allowed us to follow up the grafting not only at the die exit, but also in the plastification zone under different grafting conditions. Our results clearly show that it is in the plastification (melting) zone that the entire grafting process occurs. This length is usually very short in a co‐rotating twin screw extruder like ours. Under the grafting conditions, it varied from 1D to 5D. Thus, any relevant analysis or model of a free radical grafting process carried out in a screw extruder must be based on detailed information generated not only at the die exit, but also and most importantly in the plastification zone. Otherwise, it may lead to incomplete and/or wrong conclusions.
Fluorescence based measurement of temperature profiles during polymer processing
Authors: Migler Kalman B. , Bur Anthony J.
Published: April 2004

We describe a novel noninvasive method to measure temperature profiles during processing. A key feature is the use of a temperature‐sensitive fluorescent dye that is incorporated into the polymer resin at dopant levels. By monitoring spectral features of this dye, we can effectively measure its temperature. A focusing technique that we call “confocal fluorescent optics” then allows us to measure the temperature as a function of position into the resin. We present results of critical tests of the device. Namely we test the spatial resolution and its ability to measure a linear temperature profile in a quiescent polymer. Finally, we present the first on‐line temperature profile measurements, carried out in a circular die at the exit of a twin screw extruder. Temperature gradients on the order of 5°C/mm in a polyethylene resin are observed between the outer wall and the center of the die.
DSC studies on the crystallization characteristics of poly(ethylene terephthalate) for blow molding applications
Authors: Fann Daw‐Ming , Huang Steve K. , Lee Jiunn‐Yih
Published: April 2004

The crystallization behavior of blow‐molded PET bottles, which helps determine the product's transparency, was investigated by DSC dynamic cooling experiments that simulated the cooling that occurs in the injection blow‐molding manufacturing process. Dynamic cooling crystallization of PET optimized the conditions for keeping crystallinity to a minimum during the production of PET bottles. The kinetic parameters of non‐isothermal crystallization behavior were determined, which indicate that the competing processes of nucleation and the growth of PET spherulites during the cooling process are the controlling factors for the degree of crystallinity in the final product. In addition, DSC measurements were employed to obtain information on related aspects, such as the ease of crystallization from glassy and molten states and the crystallinity in the products. An Avrami equation was used for the calculations of the crystallization kinetic parameters.
Experience with a full notch creep test in determining the stress crack performance of polyethylenes
Authors: Fleissner M.
Published: April 2004

A laboratory method to measure the stress crack resistance of polyethylenes was developed and has since been applied in our laboratory for more than twelve years. The experience gathered since our first paper is herewith reported. The creep rupture test of circumferentially notched specimens cut from plaques or pipes has proven to be a rapid and reliable method to evaluate the stress crack performance. Surfactant‐assisted stress cracking was employed to accelerate testing. The stress crack resistance of several polyethylene samples was studied with respect to its dependence on stress, temperature, and environment. The creep rupture behavior at different temperatures could be superposed by horizontal shifting when the stresses were normalized in proportion to the respective bulk yield stresses. The notch tip radius turned out not to be very crucial, and variation of the nominal concentration of the surfactants, nonylphenolpolyglycolethers, scarcely affected slow crack growth. Acceleration of testing by surfactants equalized property differences to a noticeable extent but did not influence the ranking of the materials. The activation energy of crack growth was in the expected range. Defects introduced into the line by butt joint welding were precisely modeled by the full notch creep test.
Cure processing modeling and cure cycle simulation of epoxy‐terminated poly(phenylene ether ketone). V: Estimation of temperature distribution during cure process
Authors: Wang Qiang , He Tianbai , Xia Ping , Chen Tianlu , Huang Baotong
Published: April 2004

A numerical method to estimate temperature distribution during the cure of epoxy‐terminated poly(phenylene ether ketone) (E‐PEK)‐based composite is suggested. The effect of the temperature distribution on the selection of cure cycle is evaluated using a suggested alternation criterion. The effect of varying heating rate and thickness on the temperature distribution, viscosity distribution and distribution of the extent of cure reaction are discussed based on the combination of the here‐established temperature distribution model and the previously established curing kinetics model and chemorheological model. It is found that, for a thin composite (< = 10mm) and low heating rate (< = 2.5K/min), the effect of temperature distribution on cure cycle and on the processing window for pressure application can be neglected. Low heating rate is of benefit to reduce the temperature gradient. The processing window for pressure application becomes narrower with increasing thicknesses of composite sheets. The validity of the temperature distribution model and the modified processing window is evaluated through the characterization of mechanical and physical properties of E‐PEK‐based composite fabricated according to different temperature distribution conditions.
Modeling of the relationship between morphology and viscoelastic behavior of unidirectional fiber‐reinforced polymers
Authors: Alberola N. D. , Benzarti K.
Published: April 2004

A new approach is proposed to predict the relationship between geometric arrangement of fibers within the polymer matrix and viscoelastic behavior displayed by unidirectional fiber‐reinforced polymers. Based on a quantitative morphology analysis of epoxy/glass fibers composites, a four‐phase model taking into account the fiber spatial distribution is developed to rigorously express the reinforcement effect of the polymer matrix over a wide range of fibers and temperatures. Comparisons with other modelings and experiment illustrate the validity of the proposed approach. In addition, changes in the viscoelastic behavior of the epoxy matrix in composite materials resulting from a decrease in the crosslinking degree of such a kind of polymer are, subsequently, assessed through such an approach.
A multiscale model for polymer crystallization. II: Solidification of a macroscopic part
Authors: Charbon Christian , Swaminarayan Sriram
Published: April 2004

In part I of this paper, we presented two efficient front‐tracking methods to simulate the growth of a spherulite within an imposed temperature field. In this second part we present a method that predicts the final microstructure in a macroscopic part by coupling these front‐tracking techniques with (a) a stochastic model for the nucleation of individual spherulites, (b) a cellular model for spherulite impingement and solid fraction evolution and (c) a Finite Difference Method (FDM) for latent heat release and heat diffusion. The method tracks the physical phenomena on several length scales: a course grid for the heat diffusion, a fine grid for solid fraction evolution and a very fine grid for the shape of the individual spherulites and the lamellae within them. To our knowledge this is the first time that fully coupled multiscale model has been applied to the solidification of polymers which gives realistic microstructure evolution, orientation of the different lamellae within spherulites and maps of the solid fraction and temperature fields during solidification. The model provides us with a quantitative predictive tool that can be used to optimize industrial processes.
Microcellular sheet extrusion system process design models for shaping and cell growth control
Authors: Baldwin Daniel F. , Park Chul B. , Suh Nam P.
Published: April 2004

The feasibility of shaping a nucleated polymer/gas solution represents a significant advancement for microcellular plastics process technology. Through proper design of the foaming die, nucleated solution flows can be shaped to arbitary dimensions while maintaining the functional independence of cell nucleation, cell growth and shaping. To maintain funcational independence, stringent pressure and temperature design specifications, which supersede those of conventional foam processing, must be met by the foaming die design. As a means of aiding the design process, a model is developed for predicting pressure losses and flow rates of nucleated polymer/gas solutions. A comparison of the model predictions and the actual foaming die design performance shows good agreement for limited data. These relatively simple models capture the major physics of the complicated two‐phase flow field and provide a sound base from which scale‐up of the foaming die concept can be achieved. The nucleated polymer/gas solution flow models predict highly nonlinear volumetric flow rates contrasting constant flow rates predicted for the neat polymer flow. In addition, a convenient method for classifying nucleated polymer/gas solution flow is presented based on a dimensionless ratio of the characteristic flow rate to the characteristic gas diffusion rate.
The recovery behavior of crosslinked closed cell polyolefin foams
Authors: Rodríguez‐Pérez M. A. , Díez‐Gutiérrez S. , De Saja J. A.
Published: April 2004

In this work an experimental study on the short term creep‐recovery response of a collection of crosslinked low density polyolefin foams, with closed cell structure and different chemical composition, is presented. The effect of the maximum creep strain and the relationships between the chemical composition, the morphology of the foams, and their recovery response are examined. The experimental results show that the main mechanism that controls the behavior of these materials is the viscoelastic recovery of the cell walls.