Stress‐induced crystallization of biaxially oriented polypropylene

The hot stretching of thick, extruded sheets at high temperatures is a very important process in the production of finished biaxially oriented polypropylene (BOPP) films with special inner structures. Through a simulation of hot stretching in the machine direction (MD) of the processing of BOPP films, it was found that at high temperatures, the stretching ratio greatly influenced the obtained crystalline structure, as observed by differential scanning calorimetry (DSC). Also, in MD hot stretching, the crystallinity increased by an average of 20%. Wide‐angle X‐ray diffraction patterns of extruded sheet samples with and without stretching confirmed the structural changes shown by DSC, and the results proved that β‐crystal modification did not occur during the MD hot‐stretching process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 686–690, 2003     

热成型用透明PP的工业开发

在生产无规共聚聚丙烯(PP)的基础上,添加少量的成核剂,生产出热成型用透明PP;探讨了乙烯含量、成核剂的种类及添加量对专用树脂性能的影响.结果表明:产品具有高透明性,适宜的刚韧平衡性;常温简支梁缺口冲击强度为19～21 kJ/m2弯曲模量1.11 GPa;耐热性能较好,热变形温度85℃.专用树脂的性能达到国外同类产品的水平.     

Dielectric features of two grades of bi‐oriented isotactic polypropylene

The dielectric properties of two grades of bi‐oriented isotactic polypropylene were studied with a variety of techniques: breakdown field measurements, dielectric spectroscopy, thermally stimulated depolarization currents (Is), and direct‐current (dc) conduction I values. Standard polypropylene (STPP) and high‐crystallinity polypropylene (HCPP) films were investigated. Measurements were carried out over a wide temperature range (?150°C/+125°C). The breakdown fields in both materials showed a very small difference. On the other hand, the dielectric losses and dc conduction I values were significantly lower in HCPP. Both materials showed a decrease in the dielectric loss versus temperature in the range 20–90°C; this is favorable for application in alternating‐current power capacitors. The analysis of the dc I value allowed us to find evidence of two main conduction mechanisms: (1) below 80°C in both materials, a hopping mechanism due to the motion of electrons occurred in the amorphous phase, and (2) above 80°C, ionic conduction occurred in HCPP, and hopping conduction occurred in STPP. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42224.     

Melt strength of polypropylene: Its relevance to thermoforming

The melt strength of homopolymer, copolymer and high melt strength (HMS) grades of polypropylene (PP) was measured to assess the sagging resistance of PP for thermoforming applications. Acrylonitrile-butadiene-styrene (ABS), which can be easily processed during thermoforming, was also studied in this work. A polymer with high melt strength is considered to have a better sagging resistance. The melt strength measurements were carried out using a Gottfert “Rheotens” melt strength tester. The melt strength of the polymers increased with decreasing temperature and increasing extrusion rate. ABS generally had the highest melt strength in the low extrusion temperature region approaching the thermoforming region, indicating that it has a good sagging resistance during thermoforming. The HMS PP had significantly higher melt strength than conventional PP grades; therefore HMS PP is expected to have an improved sagging resistance. For conventional PP, melt strength was higher for the lower melt flow index (MFI) grades. A sharp increase in the melt strength was observed for conventional PP at low extrusion temperature, probably due to flow-induced crystallization of the PP. These results indicate that to minimise the sagging problem with conventional PP, low MFI grades should be used and thermoforming should be performed at temperatures close to the melting point of PP.     

Interfacial properties of highly oriented coextruded polypropylene tapes for the creation of recyclable all‐polypropylene composites

The creation of highly oriented, coextruded polypropylene (PP) tapes allows the production of novel, wholly thermoplastic, recyclable “all‐polypropylene” (all‐PP) composites, which possess both a large temperature processing window (>30°C) and a high volume fraction of reinforcement phase (highly oriented PP tapes: >90%). This large processing window is achieved by using coextruded, highly drawn PP tapes. To achieve coherent all‐PP composites the interfacial characteristics following consolidation must be understood. This article investigates the interfacial characteristics of these coextruded tapes by using microcomposite models to create interfaces between tapes of varying draw ratios, drawing temperatures, skin/core ratios, and skin layer thicknesses. The tape drawing parameters are seen to control the interfacial properties in subsequent microcomposite models. The failure mode of these specimens, and hence bond strength, varies with consolidation temperature, and a model is proposed describing and explaining this behavior. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 118–129, 2007     

A comparison of silver‐ and copper‐charged polypropylene feed spacers for biofouling control

Implementation of nanofiltration (NF) and reverse osmosis (RO) processes in treating traditional water sources can provide a steady‐state level of removal that eliminates the need for regeneration of ion exchange resins or granular activated carbon. Moreover, RO can help meet future potable water demands through desalination of seawater and brackish waters. The productivity of membrane filtration is severely lowered by fouling, which is caused by the accumulation of foreign substances on the surface and/or within pores of membranes. Microbial fouling, or biofouling, is the growth of microorganisms on the membrane surface and on the feed spacer as present between the envelopes. The fouling of membranes has demanded and continues to demand considerable attention from industry and research communities. Many of these applications use membranes in a spiral wound configuration that contains a feed spacer. The goal of this project was to develop low‐biofouling polypropylene (PP) spacers through the functionalization of PP by a spacer arm with metal chelating ligands charged with biocidal metal ions, investigate the use of this metal‐charged polypropylene (PP) feed spacers that target biofouling control, and to use some traditional and one novel techniques to autopsy the membranes after filtration to gain a better understanding of the biofouling mechanism and how the modified spacers are affecting it. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Rheological modeling of plug‐assist thermoforming

Solving problems for thermoforming processes in the production of axisymmetric thin walled plastics is investigated in this research work. A nonlinear viscoelastic rheological model with a new strain energy function is suggested for improvement of physical properties of final product. For model validation, a quantitative relation between stress and technical parameters of plug‐assist thermoforming is determined by comparison of theoretical and experimental results. This process with the proposed rheological model could be suggested for prevention from some technical defects such as wall thickness variations, physical instability during inflation‐shrinkage, and warpage exhibited in the final part of a polymeric sheet thermoforming. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 4148–4152, 2006     

Effect of heat‐sealing temperature on the failure criteria of oriented polypropylene/cast polypropylene heat seal

Failure criteria of the heat‐sealed part of oriented polypropylene (OPP) and cast polypropylene (CPP) heat seals made by an impulse type heat‐sealing machine were investigated. Circular notches and precracks were introduced to direct failure to specific areas such as inside the seal, at the border, or at the unsealed part. The notched strength as a function of heat‐sealing temperature revealed that the seals were stronger in the transverse direction (TD) as compared with the machine direction (MD). Tensile failure that occurred inside the heat seal is more sensitive to sealing temperature, whereas that at the unsealed part is immune. The stress intensity factor (K₁) is generally higher along the TD. At high sealing temperatures, i.e. above 150°C, orientation in the OPP film is relaxed and this results in consistently low mechanical properties at this temperature range. This morphology was revealed by studies of crystalline orientation state and FTIR imaging. POLYM. ENG. SCI., 46:205–214, 2006. © 2005 Society of Plastics Engineers     

Steady‐State Modeling and Simulation of an Axial‐Radial Ammonia Synthesis Reactor

A two‐dimensional model was developed for an axial‐radial ammonia synthesis reactor of the Shiraz petrochemical plant. In this model, momentum and continuity equations as well as mass and energy balance equations are solved simultaneously by orthogonal collocation on the finite element method to obtain pressure, velocity, concentration and temperature profiles in both axial and radial directions. For the catalyst particle, the effectiveness factor is calculated by solving a two‐point boundary value differential equation. The boundary conditions for the Navier‐Stokes and continuity equations are obtained by using equations representing the phenomena of gases splitting or joining in different streams and going through holes in a thin wall. The results of the mathematical model have been compared with the plant data and a good agreement is obtained.     

Orientation development in solid‐state extrusion and hot forming of polypropylene tubes

The use of high‐strength polymer in automotive structural components is limited by insufficient understanding of microscopic aspects of deformation for accurate numerical predictions of the mechanical behavior during forming processes. One approach to meeting these critical data needs is a careful examination of the structure property relationships that directly influence formability. Different hot forming processes (solid‐state extrusion, axial feed hot oil tube forming, and biaxial ball stretching test) are utilized in this work for investigating the effect of process conditions on the molecular orientation of polypropylene (PP) tubes. White‐Spruiell representation of orientation factors based on the results form X‐ray diffraction (XRD) patterns is utilized to analyze the development of orientation under extrusion and various forming conditions. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Measurement of polymer‐to‐polymer contact friction in thermoforming

Contact friction plays a critical role in all the major thermoforming processes for polymers. However, these effects are very difficult to measure in practice and, as a result, have received little scientific investigation. In this work, two independently developed test methods for the measurement of elevated temperature polymer‐to‐polymer contact friction are presented, and their results are compared in detail for the first time. One is based on a modified moving sled friction test, whereas the other uses a rotational rheometer. In each case, friction tests were conducted between two plug and two sheet materials. The results show that broadly similar coefficients of friction were obtained from the two test methods. The measured values were quite low (<0.3) at lower temperatures and typically were higher for polypropylene (PP) sheet than for polystyrene (PS). On approaching the glass transition temperature for PS (95°C) and the crystalline melting point for PP (165°C), the friction coefficients rose very sharply, and both test techniques became increasingly unreliable. It was concluded that despite their physical differences, both test techniques were able to capture the highly temperature sensitive nature of friction between polymer materials used in thermoforming. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Coupled thermo‐mechanical analysis for plastic thermoforming

An FEM software ARVIP‐3D was developed to simulate the process of 3‐D plastic thermoforming. The coupled thermo‐mechanical analysis, thermal stress and warpage analysis for plastic thermoforming was carried out by means of this software. Rigid visco‐plastic formula was adopted to simulate the deforming process. During this process, the method of comparing velocity, time and area was adopted as the contact algorithm at different nodes and triangular elements. Sticking contact was assumed when the nodes become in contact with tool surface. The Arrhenius equation and the Williams equation were employed to ascertain the temperature dependence of material properties. In order to analyze the temperature field of plastic thermoforming, the Galerkin FEM code and the dynamic heat conduction boundary condition were adopted; latent heat and deformation heat were treated as dynamic internal heat sources. Based on the above, the model of coupled thermomechanical analysis was established. Assuming that the thermal deformation occurs under elastic conditions, the thermal stress and the warpage following the cooling stage were estimated. Experiments of plastic thermoforming were made for high‐density polyethylene (HDPE). An infrared thermometer was used to record the temperature field and a spiral micrometer was used to measure the thickness of the part. Results of numerical calculation for thickness distribution, temperature field and warpage were in good agreement with experimental results.     

Dynamic characteristics of plug‐assist thermoforming process

Plug‐assist thermoforming is a well‐known technique in polymer processing because of its interesting features. The dynamic value of driving‐force for the stretching process is determined based on equilibrium equation. This amount of force is required for applying to a plug to stretch a sheet. It is used for calculation of the required theoretical work and power of a plug‐assist thermoforming process. By using a nonlinear viscoelastic rheological model in the proposed mathematical model, its validity was examined by performing experimental tests on ABS sheets. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

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     

The influence of nucleating agents on the extrusion and thermoforming of polypropylene

A large proportion of thin-gauge containers for the food packaging sector is produced via the thermoforming of extruded thermoplastic sheet (1–4). The production of high quality thermoformed parts is critically dependent on the standard of extruded sheet feedstock used. One method of optimizing the properties of extruded sheet and those of the final thermoformed article is through the incorporation of nucleating agents (5–10). This paper discusses the influence of nucleating agents on the extrusion and thermoforming characteristics of polypropylene. The potential of white titanium dioxide pigment particles as a viable nucleation source for polypropylene is addressed. Evidence is also presented that suggests that nucleating agents may play an important role in controlling the extent of physical change taking place in extruded sheet as a direct result of post-production aging.     

Structure and properties of new thermoforming bionanocomposites based on chitin whisker‐graft‐polycaprolactone

Thermoformable bionanocomposites of chitin whisker‐graft‐polycaprolactone (CHW‐g‐PCL) were synthesized by initiating the ring‐opening polymerization of caprolactone monomer onto the CHW surface under microwave radiation. In this case, the “graft from” strategy contributed to long and dense “plasticizing” PCL tails onto the CHW surface as the key of thermoforming, and, therefore, such bionanocomposites were injection‐molded as the sheets with a structure of cocontinuous phase mediated with the entanglement of grafted PCL chains. The structure and properties of the molded CHW‐g‐PCL sheets were investigated by FTIR, XRD, SEM, DSC, DMTA, contact angle measurement, and tensile test. With an increase of the PCL content in CHW‐g‐PCL, the strength and elongation as well as the hydrophobicity of the nanocomposites increased at one time. This is the first report on the thermoformable polymer‐grafted nanocrystal derived from natural polysaccharide. Moreover, such new bionanocomposites with good mechanical performances could have great potential applications. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of annealing on the microstructure and mechanical properties of polypropylene with oriented shish‐kebab structure

Annealing is thought to be an effective method to promote chain rearrangement in semicrystalline polymers and improve their physical properties. However, little attention has been paid to the annealing of flow‐oriented semicrystalline polymers despite its importance in polymer processing. In this work, the microstructural evolution of injection‐moulded polypropylene with an oriented shish‐kebab structure upon annealing has been explored with differential scanning calorimetry, small‐angle X‐ray scattering and scanning electron microscopy, Fourier transform infrared spectroscopy and dynamic mechanical analysis. The results show that annealing gives rise to a chain rearrangement in both the crystalline and amorphous phases. Accompanied by the growth and perfecting of the kebabs, relaxation of the initially oriented chains in the amorphous phase is observed. Then, the relationship between the structure and the resulting mechanical properties is established. Copyright © 2011 Society of Chemical Industry     

Study on the compatibility of ethylene‐vinyl acetate copolymer/vermiculite in biaxially oriented polypropylene films

A process for preparing expanded vermiculite/ethylene vinyl acetate nanocomposites by ball milling expanded vermiculite (EVMT) in cyclohexane solution of ethylene vinyl acetate copolymer resin (EVA) was presented, and the influence of EVA/EVMT nanocomposites on mechanical, barrier performance of modified biaxially oriented polypropylene (BOPP) films was discussed in this article. By the process, the EVMT is intercalated and exfoliated to obtain a kind of EVMT/EVA nanocomposite, in which EVA serves as both intercalating agent into EVMT and compatibilizer between EVMT and polypropylene (PP); so, when EVMT/EVA was melt blended with PP, platelets of vermiculite can be exfoliated easily and dispersed relatively well in the PP matrix. Compared with original (nonmodified) BOPP, when EVMT loading ranged from 0.1% to 0.5%, both strength and toughness of the modified BOPP films was increased. Moreover, platelets (flakes) of vermiculite dispersed in PP matrix improved barrier properties of modified BOPP films also. The improved barrier properties of BOPP films accorded with Nielsen model. POLYM. COMPOS., 36:78–87, 2015. © 2014 Society of Plastics Engineers