Effects of rheological properties and processing parameters on ABS thermoforming

Understanding effects of material and processing parameters on the thermoforming process is critical to the optimization of processing conditions and the development of better materials for high quality products. In this study we investigated the influence of both rheological properties and processing parameters on the part thickness distribution of a vacuum snap‐back forming process. Rheological properties included uniaxial and biaxial elongational viscosity and strain hardening and/or softening while processing parameters included friction coefficient, heat transfer coefficient, and sheet and mold temperatures. The Wagner two parameter nonlinear viscoelastic constitutive model was used to describe rheological behavior and was fit to shear and elongational experimental data. The linear viscoelastic properties along with the Wagner model were utilized for numerical simulation of the thermoforming operation. Simulations of pre‐stretched vacuum thermoforming with a relatively complex mold for a commercial refrigerator liner were conducted. The effects of nonlinear rheological behavior were determined by arbitrarily changing model parameters. This allows determination of which rheological features (i.e., elongational mode, viscosity, and strain hardening and/or softening) are most critical to the vacuum snap‐back thermoforming operation. We found that rheological and friction properties showed a predominant role over other processing parameters for uniform thickness distribution.     

Influence of initial sheet temperature on ABS thermoforming

Understanding the effects of material and processing parameters on the thermoforming process is critical to the optimization of processing conditions and the development of better materials for high quality products. In this study we investigated the influence of initial temperature distribution over the sheet on the part thickness distribution of a vacuum snap‐back forming process. The linear viscoelastic properties along with the Wagner two parameter nonlinear viscoelastic constitutive model were utilized for numerical simulation of the thermoforming operation. Simulations of pre‐stretched vacuum thermoforming with a relatively complex mold for a commercial refrigerator liner were conducted. THe effects of temperature distribution over the sheet on the part thickness distribution were determined to examine process sensitivity and optimization. Effects of the temperature distribution on the material rheology and polymer/mold friction coefficient are primarily responsible for the changes in the thickness distribution. We found that even small temperature differences over the sheet greatly influenced bubble shape and pole position during the bubble growth stage and played a critical role in determining the part thickness distribution. These results are discussed in terms of rheological properties of polymers such as elongational viscosity and strain hardening.     

The effect of composition and the level of crosslinking of the poly(methylmethacrylate) phase on the properties of natural rubber‐poly(methylmethacrylate) semi‐2 interpenetrating polymer networks

The effects of the PMMA content and the cross‐linker level in the poly(methylmethacrylate) component on the dynamic and physico‐mechanical properties of semi‐2 interpenetrating polymer networks based on natural rubber and poly(methylmethacrylate) were determined. The miscibility of the components in these semi‐2 interpenetrating polymer networks was determined using the loss tangent data, obtained from dynamic mechanical thermal analysis and the interphase contents were calculated from modulated scanning calorimetric data. Some component mixing in these semi‐2 interpenetrating polymer networks was evident from these modulated differential scanning calorimetric and dynamic mechanical thermal analysis data. The degree of component mixing increased with cross‐linker level in the PMMA phase. The PMMA content in the semi‐2 IPNs has a significant effect on the tensile and hysteresis behavior of these semi‐2 interpenetrating polymer networks. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effects of ABS rubber particles on rheology,melt failure,and thermoforming

The rubber particles included in rubber modified polymeric materials such as acrylonitrile‐butadiene‐styrene (ABS) polymer and impact modified polymers play an important role in determining their rheological properties, processing behavior, and mechanical properties. In this study both small strain oscillatory shear viscosity in the frequency range from 10^?2 to 10² s^?1 and uniaxial elongational viscosity behavior at two elongation rates ( = 0.1 and 1.0 s^?1) over the range of temperatures from 140°C to 200°C were measured for commercial ABS polymers with different contents and deformability of rubber particles. The influences of rubber content and deformability on rheological properties such as melt elasticity, elongational viscosity, strain hardening and/or softening, the onset of nonuniform deformation, and thermoforming performance were investigated. The Wagner two‐parameter nonlinear viscoelastic constitutive model was used to describe strain hardening behavior, while the Considère criterion was used to determine the onset point of nonuniform deformation. The part thickness distribution obtained through use of a vacuum snap‐back forming process was simulated to investigate the effects of rheological changes associated with different rubber particles on the thermoforming performance. It was found that ABS polymers with larger contents of hard rubber particles exhibited more melt elasticity, stronger strain hardening, a maximum of biaxial elongational viscosity, onset of nonuniform deformation at later time, and better thermoforming performance. Strain hardening and the Considère criterion provide simple, reliable indicators of the thermoforming performance of ABS polymers.     

Poly(lactic acid)/titanium dioxide nanocomposite films: Influence of processing procedure on dispersion of titanium dioxide and photocatalytic activity

Poly(lactic acid)/titanium dioxide (TiO₂) composite films were prepared by direct melt processing using three different procedures (i.e., compression molding, twin‐screw melt extrusion, and melt extrusion and thermoforming). The effect of TiO₂ loading and processing procedures on the phase morphology and on the thermal, mechanical, and barrier properties of the obtained nanocomposites were analyzed respectively by field‐emission scanning electron microscopy‐energy dispersive spectrometry, differential scanning calorimetry, universal testing machine, and water vapor and oxygen permeability measurements. The incorporation of TiO₂ nanoparticles into the poly(lactic acid) matrix increased the crystallinity and improved the barrier properties of the composites. The maximum tensile strength was achieved at the 2% content of TiO₂ for the films produced by compression molding and twin‐screw melt extrusion, whereas the tensile strength for films produced by melt extrusion and thermoforming decreases markedly with an increasing TiO₂ content. The photocatalytic activities of the obtained nanocomposites were investigated by analyzing the degradation of methyl orange. Results confirmed that the processing procedures and the distribution of TiO₂ in the polymer matrix are the key parameters, which rule the photocatalytic behavior of composite films. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Three‐dimensional simulation of thermoforming process and its comparison with experiments

Three‐dimensional solid element analysis and the membrane approximated analysis employing the hyperelastic material model have been developed for the simulation of the thermoforming process. For the free inflation test of a rectangular sheet, these two analyses showed the same behavior when the sheet thickness was thin, and they deviated more and more as the sheet thickness increased. In this research, we made a guideline for the accuracy range of sheet thickness for the membrane analysis to be applied. The simulations were performed for both vacuum forming and the plug‐assisted forming process. To compare the simulation results with experiments, laboratory scale thermoforming experiments were performed with acrylonitrile‐butadiene‐styrene (ABS). The material parameters of the hyperelastic model were obtained by uni‐directional hot tensile tests, and the thickness distributions obtained from experiments corresponded well with the numerical results. Non‐isothermal analysis that took into account the sheet, temperature distribution measured directly from the experiments was also performed. It was found that the non‐isothermal analysis greatly improved the predictability of the numerical simulation, and it is important to take into account the sheet temperature distribution for a more reliable simulation of the thermoforming process.     

Enhanced strain hardening in elongational viscosity for HDPE/crosslinked HDPE blend. II. Processability of thermoforming

Rheological properties and processability of thermoforming were studied for high‐density polyethylene (HDPE) and a blend of HDPE with crosslinked HDPE (xHDPE). Blending the xHDPE, which enhances melt strength and strain hardening in elongational viscosity of HDPE, helps the sheet avoid sagging in thermoforming. Moreover, the product of the blend obtained by vacuum forming has uniform wall thickness. Melt strength and strain hardening of the blend were, however, depressed by a processing history in a single‐screw extruder, whereas reprocessing by a two‐roll mill enhanced the melt strength again. It is considered that the processing history by a single‐screw extruder, in which shear‐dominant flow takes place, depresses the trapped entanglements between network chain of xHDPE and linear HDPE molecules, and results in low level of melt strength. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 79–83, 2002     

Rheological modeling of fracture in plug‐assisted vacuum thermoforming

The fracture of polymeric sheets is one of the practical problems occurring during plug‐assisted vacuum thermoforming. This defect can occur during both the plug‐assist and vacuum‐forming stages. This article focuses on two issues: (1) the origins of fracture creation and (2) the determination of the process parameters needed for removal of the defect. The results of our work not only lead to an understanding of the cause of this problem but also enable us to calculate the parameters that affect the fracture of polymeric sheets during plug‐assisted vacuum thermoforming. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effects of incorporating technique and silver colloid content on antibacterial performance for thermoplastic films

Antibacterial efficacies of various thermoplastics, such as medium‐density polyethylene (MDPE), polystyrene (PS), polyethylene terephthalate (PET) and polyvinyl chloride (PVC) containing nano‐silver colloids were studied under a wide range of testing conditions. The effects of nano‐silver colloid content and the silver‐polymer contact time were of our main interests and quantitatively assessed by shake flask method coupled with a plate‐count‐agar (PCA) technique using Escherichia coli as testing bacteria. Two different methods were used for incorporating the nano‐silver colloids into the thermoplastics, these being spray‐coating and melt‐blending techniques. The experimental results suggested that all neat thermoplastics alone could not generally inhibit the E. coli growth, suggesting that all thermoplastics exhibited nonbactericidal behavior. However, neat PVC appeared to show a retarding effect for the E. coli growth. In addition, coating silver colloid onto all types of thermoplastic substrates could inhibit the E. coli growth up to 99.9% at the optimum silver content of 50 ppm for PS, PET and PVC and of 75 ppm for MDPE. The optimum contact time for all thermoplastics was 150 min. Among the thermoplastics used, PVC exhibited the highest % E. coli reduction, and this was confirmed by the higher silver content via Atomic Absorption Spectrometry (AAS) technique. For a given silver content, the spray‐coating technique could give better dispersion level of silver throughout the thermoplastic films and this led to more effective antibacterial performance as compared with the dry‐blending technique. In PVC sample, the contact angle value appeared to increase with the addition of silver content for both incorporating techniques. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Effect of the unidirectional drawing on the thermal and mechanical properties of PLA films with different L‐isomer content

This work simulates the thermoforming process of two different L ‐isomer content PLA grades (95.6 and 98%), studying the influence on the induced morphology and the variation in the thermal and tensile properties. The thermoforming process was simulated with uniaxial tensile tests performed at different temperatures and strain rates, reporting the tensile behavior before, during, and after the test. The resulting structural changes were analyzed by wide‐angle X‐ray scattering and Fourier transformed infrared spectroscopy. Thermal characterization was carried out with differential scanning calorimetry and dynamomechanical thermal analysis. The results showed that, regardless the L ‐isomer content, drawing at 70°C produced a stable mesomorphic phase that showed greater tensile properties than the original films. This mesomorphic phase, indeed, could be reordered into a crystalline structure under mild annealing conditions (5 min/75°C), if compared with those needed for similar amorphous PLA specimens (60 min/120°C), thus providing a processing opportunity for obtaining thermally stable PLA products at temperatures above 100°C. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Thermoplastic vibration welding: Review of process phenomenology and processing–structure–property interrelationships

Vibration welding offers a robust method for physically joining thermoplastics to fabricate complex hollow assemblies from simpler injection‐molded articles without using an external heat source, adhesives, or mechanical fasteners. Vibration welding involves a complex interplay of several phenomena—solid (Coulomb) friction, melting, high strain‐rate, pressure‐driven, strong (high‐strain) melt flows, solidification, and microstructure development—which ultimately govern the strength and integrity of the weld. Defects in the weld region may lead to catastrophic failure of the welded assembly. In this article, the current understanding of the processing–structure–property relationships in the context of vibration welding of thermoplastics and polymer‐matrix composites is reviewed. Experimental as well as analytical methods of investigation of the vibration welding process phenomenology are presented. The interrelationships between the microstructure in the weld region and the resulting weld strength and fatigue behavior are then discussed in the light of this phenomenological information for neat polymers, filled polymers, polymer blends, and foams. This review is also aimed at identifying the areas requiring further investigation with regard to understanding vibration welding phenomenology and weld structure–property relationships. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Studies on the characteristics of microphase separation and stabilization of interphase for blended systems of high performance polymers

In this report, we review and discuss the results of our recent studies on the characteristics of microphase separation behavior and interphase stabilization for high performance polymer blends. The blends investigated include crystalline/crystalline polymers, crystalline/amorphous polymers, liquid crystalline polymer/thermoplastics, and amorphous/amorphous thermoplastics or thermosetting systems. Most of the blends are either immiscible or partially miscible, and are thermodynamically unstable or meta-stable systems. The macro-properties of these blends are controlled by many factors such as the miscibility, phase morphology and structure, crystallinity, kinetics of crystallization or phase separation processing, and interfacial adhesion of the components. Among these, the microphase and interfacial structures are the most significant factors influencing the ultimate properties of the blends. In order to obtain relatively stable blends, formation of semi-IPN in either the bulk or interphase, and/or the occurrence of crosslinking, transesterification and physical entanglement in the interfacial region will be profitable to the stabilization of the blending systems.The project supported by FORD and NSFC No. 09415312     

Determination of Mechanical Properties of Thermoplastics Suitable for Micro Systems

Aircraft and aerospace industry as well as medical and automotive engineering continuously develop smaller and lighter system components. Therefore it is not only important to be able to produce micro parts but also to provide properties for dimensioning and design. This paper explains to what extent mechanical properties from the macro‐range are also valid for micro structures. Thereby the modulus of elasticity as well as stress and strain at the yield point are regarded as relevant properties for the part dimensioning. Systematic investigations with tensile bars of different size show that the geometry‐dependent property changes are obviously not specific for the material classes of amorphous or semi‐crystalline thermoplastics, respectively. That is why a single material testing of micro samples is necessary to determine valid properties for this range. On the other hand the determination known from the classic materials science of plastics that lower temperatures as well as higher loading rates lead to a stiffer material behaviour, is also appropriate for the micro range. The measurements prove that the influence of temperature is much higher than the one of deformation rate. The influence of the production conditions on the mechanical behaviour of test specimen made of semi‐crystalline thermoplastics is demonstrated by using a varied cooling process. The correlation between the resulting different morphological structures and the changes in the mechanical properties is pointed up on the basis of microscopic views.

Micro tensile testing machine at the IKV.     

Uniaxial tensile behavior and thermoforming characteristics of high barrier EVOH‐based blends of interest in food packaging

The uniaxial tensile characteristics of blends of an ethylene‐vinyl alcohol copolymer (EVOH‐32 mol% ethylene) with an amorphous PA and/or a nylon‐containing ionomer, used as barrier layer in multilayer food packaging structures, was assessed in this paper. The stress‐strain behavior of these materials at elevated temperatures and at different strain rates was examined. The stress‐strain curves obtained were used to understand the influence of temperature and strain rate on the uniaxial deformation process of the materials, these being of general importance during typical processing steps including thermoforming. A male mold for deep‐draw was used to assess the thermoforming (biaxial deformation in nature) behavior of extruded sheets at 100, 120, 140 and 150°C, and the results were broadly found to be in agreement with results from simple uniaxial tensile tests. From the preliminary thermoforming results, it was found that EVOH/aPA extruded blends did not improve the poor formability of EVOH alone. In contrast, significant improvement in thermoformability was achieved by blending EVOH with a compatibilized ionomer. Optimum forming capacity was achieved in a ternary blend by addition of a compatibilized ionomer to an EVOH/aPA blend in the range of 140°C–150°C. The ternary blend showed a lower reduction of thickness in the sidewalls, as well as a higher dimensional uniformity in the thermoformed part. Polym. Eng. Sci. 44:598–608, 2004. © 2004 Society of Plastics Engineers.     

Simulation and Empirical Studies of Solvent Evaporation Rates in Vacuum Evaporation Crystallization

Simulation results for continuous vacuum evaporation crystallization obtained by Aspen Plus and experimental results for semi‐batch vacuum evaporation crystallization are presented. In the crystallization experiments, the fixed heat duty was used to compare the water evaporation rates and crystal properties obtained at different pressures. The solution selected was aqueous glycine. It has the ability to form a number of different crystalline polymorphs, which allows it to exhibit a variety of different physical properties while maintaining its chemical properties. X‐ray diffraction results demonstrated that mainly the γ‐crystal form is produced under the conditions applied in vacuum evaporation crystallization.     

Compatibilization and property characterization of polycarbonate/polyurethane polymeric alloys

Semi‐crystalline dendritic poly(ether‐amide)s were synthesized by modifying hydroxyl end‐groups of dendritic poly(ether‐amide) with aromatic urethane acrylate and octadetyl isocyanate. The ratio of these modifiers can adjust the final properties of products to fulfill the requirements of UV‐curable powder coatings. These UV‐curable semi‐crystalline dendritic poly(ether‐amide)s have a T_g in the range of 41–45°C and a T_m of around 120°C. Their thermal behavior and semi‐crystalline properties were studied by DSC and XRD. The photopolymerization kinetics was investigated by Photo‐DSC. The residual unsaturation, thermal stability, and hardness of the UV‐cured films were also studied. The obtained results show that these semi‐crystalline dentritic poly(ether‐amide)s may be used as prepolymers in UV‐curable powder coating systems. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 287–291, 2003     

Crystallization and structure-mechanical property relations in poly(aryl ether ether ketone) [PEEK]

In the development of advanced composite materials, the mechanical behavior of the matrix is of critical importance. The next generation of composite materials will be based on high modulus tough matrices, of which poly(aryl ether ether ketone) [hereinafter, referred to as PEEK] is one of the first crystalline thermoplastics to receive serious attention. As in all crystalline polymers, the matrix is itself a composite material whose properties depend significantly on the crystalline morphology developed during processing. In this contribution, the current understanding of crystallization in PEEK and its influence on mechanical properties is reviewed. Problems yet to be resolved are highlighted.     

Improvement of thermomechanical properties of a DGEBS/DDS system blended with a novel thermoplastic copolymer by realization of a semi‐IPN network

In this article we present the results of a study on the properties of a blend containing a 40:60 amine‐ended copolymer, poly(ether sulfone)—poly(ether ether sulfone), with a system composed of diglycidyl ether of biphenol S and 4,4′‐diaminodiphenyl sulfone (4,4′‐DDS). Five formulations, which varied in amounts of modifier, were characterized through dynamic thermal mechanical analysis and fracture mechanics studies. The morphology of the blends was studied by transmission electron microscopy (TEM). The addition of thermoplastics enhanced the toughness of the system, resulting in an increase in the glass‐transition temperature as a function of the amount of thermoplastics. No phase‐separated morphology was detected by TEM analysis, leading to the conclusion that the formation of a homogeneous semi‐IPN network occurred between the thermoplastics and the thermoset. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3021–3025, 2003     

Study of the thermoformability of ethylene‐vinyl alcohol copolymer based barrier blends of interest in food packaging applications

The thermoforming capacity of a number of blends of an ethylene‐vinyl alcohol copolymer (EVOH‐32, with 32 mol % ethylene) with amorphous polyamide (aPA) and/or Nylon‐containing ionomer with interest in multilayer food packaging structures have been studied. These blends were vacuum‐thermoformed between 100 and 150°C onto male molds of different shapes and areal draw ratios. It was found that EVOH/aPA extruded blends did not improve the inherently poor formability of EVOH alone. In contrast, significant improvements in thermoformability were achieved by blending EVOH with a compatibilized‐ionomer. Optimum forming capacity was achieved in a ternary blend by addition of a compatibilized‐ionomer to EVOH/aPA blends in the range of 140–150°C. Analysis of wall thickness data obtained in the thermoformed parts showed that wall thickness was significantly affected by the ionomer and amorphous polyamide content in the blend. The ternary blend showed a lower thickness reduction in the critical areas, as well as a higher uniformity in the part. A finite element analysis was used to evaluate the wall thickness distribution and the modeling results were compared with the thermoforming experiments. The simulations were performed for the vacuum‐forming process employing a nonlinear elastic material model. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 96: 3851–3855, 2004     

Preparation and properties of catalyzed polyimide/dicyanate semi‐interpenetrating networks for polymer gas membrane with suppressed CO2‐plasticization

The work presents an approach to reduce the plasticization of polymeric membranes caused by condensable gases, and particularly the effect of plasticization caused on polyimides by CO₂ at high pressure. A technical polyimide, Matrimid^®, was chosen as a reference of polyimide membrane and the approach applied consisted of incorporating reactive oligomers to have cross‐linkable mixed systems, which do not plasticize at high CO₂ pressure. Films of semi‐interpenetrating networks (semi‐IPNs) based on Matrimid^® and phenolphthalein dicyanate as cross‐linking monomer in ratios 90/10, 80/20, and 70/30, were prepared using a catalyst to lower the curing temperature from 280 to 180°C. Semi‐IPNs properties such as thermal stability, mechanical properties, glass transition temperatures, or density were measured to characterize the films and were correlated with the dicyanate monomer content. The CO₂ gas permeation behavior of the three semi‐IPNs was studied using a CO₂ feed pressure ranging from 1 to 30 atm. The gas separation properties were mainly explained attending to the density of the films, which depended on the dicyanate content used. In the three catalyzed semi‐IPNs, the CO₂ permeability coefficients remained almost constant all along the investigated range of CO₂ pressure while Matrimid^® treated at 180°C did show a clear tendency to plasticization over a critical feed pressure of about 17 bar. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012