SPE Library

The authors have earlier identified crack patterns that are generic to a range of rubber-modified polymers as well as homogeneous and other heterogeneous systems. This is in the form of overlapping parallel cracks that are found repeatedly in different materials. In this study, a complex stress function method has been used to study the micro-mechanics and damage stability that exists in this generic pattern. It is shown that the interaction between the cracks in the overlapping crack pattern can lead to arrest of crack growth. The damage is also modeled as a doubly periodic infinite array of cracks. The differences between the two crack growth criteria namely, stress intensity factor and energy release rate, are explained by considering the effect of the cracks on the overall stiffness of the material.

It has been well recognized that slow crack growth in polyethylene leading to long-term brittle failure is associated with behavior of a process (craze) zone in front of the crack tip. The concept of crack layer (CL) as a system of strongly interacting crack and process zone (PZ) was introduced and analyzed in a number of publications by Chudnovsky with co-authors. The present paper is an extension of the model, which includes: (i) a new approach to determination of PZ size that accounts for stress relaxation along of the PZ boundary and (ii) evaluation of PZ lifetime under variable stress at each stage of CL growth. Analysis of stresses and strains in a vicinity of the crack layer is based on (i) the compatibility condition between bulk polyethylene and PZ material along the PZ boundary and (ii) determination of material elastic and creep characteristics obtained from tests on an original polyethylene and its craze modification. A stepwise process of slow fracture propagation is considered. The time to ultimate failure is evaluated as the duration of slow CL growth from load application to ultimate instability. An experimental procedure to rank various polyethylenes with respect to resistance to fracture propagation (toughness) is discussed.

Defect diagnosis of tires during manufacturing and service is one of the very complicated issues in dealing with the tire manufacture. This is due to the big number of raw materials and processing steps involved in the manufacturing of a tire. Each raw material and process quality parameter is affecting the final quality of the tire. The manufacturing history data of that particular tire is essential, with the raw materials inspection results, as the starting point, and going through all manufacturing steps, to the final inspection results and failure mode in service. This study presents a TIRE Defects Diagnostic eXpert system, which we called (TIREDDX) as a part of an integrated TQM system that can be applied to achieve an effective diagnostic procedure for tires and elastomers defects, which may cause a company a lot of money, effort and time to resolve it.

Environmental factors are known to significantly impact the oxidative failure mechanism of plastics. The chlorine present in potable water as a disinfectant is an oxidant and has been shown to be able to significantly affect the failure mechanism of materials in potable water applications. In this paper, the impact of chlorinated potable water on four polysulfone materials was examined (PSU, PPSU/PSU blend, PPSU and glass-reinforced PSU). The materials were tested in the form of standard commercial insert fittings for plastic piping applications and exposed to continuously flowing aggressive chlorinated potable water at elevated temperature and pressure. The exposure period was chosen as twice the lifetime of the adjoining cross-linked polyethylene pipe (PEX) at the test condition. The exposure is shown not to have impacted the mechanical strength of the fittings when compared to the application pipe. Degradation, attributed to oxidation, of the exposed surface was observed. The morphological and chemical changes were examined using SEM, EDX and EDS. The differences between materials are presented. All materials were found to have excellent oxidative resistance to the chlorinated potable water at the tested condition. The PPSU material is seen to be the most resistant while the PSU materials the least resistant. The PPSU/PSU blend resistance was seen to be between that of the PPSU and PSU materials.

Thermoplastic vulcanizates (TPV’s) have been extensively studied and gained wide acceptance because of their rubber-like properties and thermoplastic processability. Polymer / layered silicate nanocomposites of various types have similarly received much attention as promising high performance materials. Combining these two complementary technologies to form a TPV nanocomposite generates interesting properties that significantly depend on the phase location of the silicate nanoclay reinforcement – whether it lies in the dispersed rubber phase or in the continuous plastic matrix.Our objective is to selectively reinforce either the plastic phase or the rubber phase in some typical TPV’s and observe the distinctive effect of reinforcement partitioning on mechanical and rheological properties.

Williams, Landel and Ferry developed a method of shifting data horizontally along the log time scale by amounts related to a temperature difference to obtain long term properties of the material without requiring testing to those long times. This paper explores the use of the WLF Method to characterize the long-term stress relaxation of thermoplastic vulcanizates. Stress relaxation data is used to predict long-term sealability of these materials in applications such as automotive weatherseals, pipeseals, glazing seals, etc. A master curve is defined for various elastomers and a comparison is made to previous WLF models and to current data to determine its accuracy. The use of this method can allow the prediction of long-term stress relaxation obviating the requirement to submit materials for lengthy and costly testing.

Thermoplastic vulcanizates (TPVs) have long established themselves as cost effective and durable alternatives to many traditional elastomers such as EPDM rubber and thermoplastic urethanes (TPUs). Cost effectiveness comes from lower specific gravity, while durability comes from the cross-linking of the thermoset rubber. Newly developed TPVs further expand this envelope. This paper compares physical properties, including abrasion resistance, of these newly introduced TPVs, to both ester and ether based TPUs. Data shows these TPVs are equal to or better than the TPUs in flame retardancy and abrasion resistance while at a lower final volumetric and weight cost.

A high level of bipolymer “in-situ” polymerized within a polypropylene random copolymer matrix was synthesized and utilized to achieve interesting elastomeric properties and softness. This versatile new Soft Thermoplastic (STP) is crosslinkable, compatible with other non-olefin polymers and free flowing. Moreover, it is usable with all main compounding technologies, capable of improving properties with a cost-effective solution. Physical-Mechanical characterization and use of this new STP designed as a blend component for product development by compounders are reviewed. Composition properties, in which this grade plays a role as either a component with other polymers or as a base for more elastic thermoplastic vulcanizes, are also presented.

Industrial designers have problems imagining how their designed plastic products might fail.The paper illustrates how our students are educated about the specific structure related properties that might cause failures.A specific course about designing for reliability of plastic products is outlined. Case studies showing failed products are important.Students must be aware of failure causes like: stress concentrations, low mass and/or mould temperatures, highly stressed weld lines, faulty ribbing and incorrect joining.The course deals also with typical failure mechanisms of plastics like: creep and stress relaxation, stable crack extension, chemical attack and environmental stress - cracking.

Chlorinated paraffins are the world's lowest cost secondary plasticizer for flame retardant flexible PVC compounds. Significant quantities are used throughout the world, primarily outside of the United States. In the U.S. the use of chlorinated paraffins has lagged behind the use of phosphate esters in flame retardant flexible PVC. This is principally due to concerns and misconceptions regarding the heat stability and compatibility of chlorinated paraffins. This paper documents the recent progress that has been made in the field of chlorinated paraffin secondary plasticizers with regard to overall economics, heat stability and compatibility in flexible PVC FR compounds.

With the increased critical discussions about potential toxicological effects of phthalates in the midnineties, BASF took a proactive approach to search for alternatives. We are convinced that phthalate plasticizers are suitable for many PVC applications. However, in exposure sensitive applications, such as medical and toys, we felt there was a need to develop a new plasticizer.Different structural classes that could be used as plasticizers for PVC were examined. Based on our knowledge of the physico-chemical requirements and our experience in applications technology, the most promising candidates were selected for further testing. From these results, BASF commercialized Hexamoll® DINCH, the ester of cyclohexanedicarboxilic acid and C-9 alcohols.In a paper presented at Vinyltec in Chicago last year, BASF reviewed the activities of the FDA’s safety assessment of DEHP. The Vinyltec paper went on to discuss the toxicological database and profile for Hexamoll® DINCH. It recommended the use of this new class of plasticizers in exposure sensitive FPVC applications such as medical, toys, packaging and gloves.This paper evaluates the performance of the ester of cyclohexanedicarboxilic acid and C-9 alcohols in flexible PVC and plastisol formulations and compares it to a semi-linear diisononyl phthalate based compound and plastisol. An evaluation comparing DINCH to acetyl tri-n-butyl citrate is ongoing and will be reported at ANTEC.

Through the use of Design of Experiments (DOE), an improved, more efficient heat modifier was evaluated and compared to products commercially available. Experimental results indicate the improved modifier is more efficient and can be used at reduced loading levels over previous modifiers. This modifier performs well in polyvinyl chloride (PVC) and acrylonitrile-butadiene-styrene (ABS) polymer systems. The incorporation of this modifier into these polymer systems can introduce cost savings to the compounder/formulator while potentially opening doors for the use of these polymers in applications where heat performance was previously a limiting factor.

The PVC formulator is constantly challenged to improve performance while lowering cost. This paper reports on a laboratory project where three lubricant systems and three calcium carbonate products were evaluated.A generic rigid PVC formulation, with 1phr TiO2, containing paraffin wax and a one-micron ground calcium carbonate, was compared with formulations containing two different ester lube packages and a finer precipitated calcium carbonate. The ester lubricated, fine precipitated carbonate filled, compounds demonstrated improved impact performance, especially at the lower test temperatures.

The impact behavior of poly(vinyl chloride) (PVC) pipe formulations containing either: chlorinated polyethylenes (CPEs) , impact-grade methyl methacrylate butadiene styrene (MBS) , MBS /CPE mixture or an acrylic core-shell impact modifier were evaluated using an instrumented impact tester. The lubricants were adjusted for each modifier to yield similar fusion times. Compounds were extruded on a laboratory-scale twin screw extruder and impact tested at low temperature [- 10°C] conditions. Fusion bowl stability testing was also performed on each blend. The results showed a 50-50% MBS/CPE blend had improved thermal stability and lower extrusion pressures than pure MBS, while retaining good output and comparable impact performance to pure MBS, CPE or acrylic formulations.

The toughness of impact modified poly(vinyl chloride) (PVC) compounds was examined using a modified Charpy test. Increasing impact speed resulted in a quasi-brittle-to-ductile transition in all PVC compounds. In the quasi-brittle region, lower molecular weight PVC modified with 10 phr chlorinated polyethylene (CPE) exhibited a craze-like damage zone that could be described by a modified Dugdale model. Lower molecular weight PVC modified with 10 phr methylmethacrylatebutadiene-- styrene (MBS) impact modifier also exhibited a craze zone and the same intrinsic crazing energy. However, the toughness of the craze and the resistance to fracture depended on the type of impact modifier. Increasing molecular weight of the PVC resin resulted in a more complex damage zone that was not amendable to the Dugdale analysis.

The kinetics and mechanism of fatigue crack growth in poly(vinyl chloride) (PVC) compounds of different molecular weight were studied. The fatigue crack propagation rate of all the PVC compounds followed the Paris law: da/dt=AF?KI 2.7. Fatigue crack propagation rate, as reflected by the pre-factor AF in the Paris law, was highly dependent on molecular weight of the resin, strain rate and temperature. A stepwise mechanism of fatigue crack propagation was observed in all the PVC compounds. Steps were formed by discontinuous growth of the crack through a single craze in the shape of a narrow strip. Step length and lifetime were used to characterize crack propagation.

Elevated process temperatures can accompany current high extrusion rates of rigid poly (vinyl chloride). Various colored weatherable siding compounds were studied in the processing range of 193 C (380 F) to 227 C (440 F). The extruded compounds showed only minor color shifts due to increased melt temperatures. Outdoor exposure through 4 years in Arizona, Florida, and Ohio demonstrated typical color change, but did not exhibit significant color shift relating to the initial processing temperatures. The extruded samples from elevated melt temperatures did demonstrate reduced impact strengths prior to outdoor exposure. Florida and Ohio exposed samples lost impact strength throughout the 5 years of exposure, with the higher temperature processed samples continuing to show lower impacts over time.

Polyethylene is commonly extrusion coated onto a variety of substrates for use in food packaging applications. The greatest utility comes from using polyethylene as a sealant layer. It has been determined that extrusion temperature has a significant effect on the hot-tack and heat seal performance of polyethylene. In the resins evaluated, as extrusion temperature increased, hottack strength decreased while plateau heat seal strength increased. Characterization of coatings applied at various extrusion temperatures has revealed that changes in molecular weight and crystalline morphology determine heat seal and hot tack performance for polyethylene coatings.

It has been known that the properties of nanocomposites are dependent on the degree of dispersion of expandable smectites clays in the polymer matrix. The different states of dispersion are exfoliated, intercalated and immiscible systems. However we and others have demonstrated that within a single system, the dispersion is far from homogeneous. Here we investigate the effect of dispersion of clay in Linear Low Density Polyethylene (LLDPE) nanocomposite films. Cloisite 15A was used as montmorillonite layered silicate (MLS). MLS were precompounded with a carrier resin to produce a master batch. Films of 1 mil thickness were prepared by a blown film extrusion technique. The through thickness dispersion in different films was investigated using x-ray diffraction. The distribution of clays was observed by polarized optical microscopy. The effect of the dispersion on the glass transition of the polymer was studied by differential scanning calorimetry (DSC).

A range of EVA/m-LLDPE/EVA co-extruded films, with Polyisobutylene (PIB) content from 0-20% and Vinyl Acetate (VA) co-monomer content of 6, 12 and 18%, was manufactured using a Killion cast film co-extrusion system. The films were aged at 45°C for up to 28 days, to enable tack (cling) development. The results show that film tack strength improved significantly with ageing. Increased VA concentration in the surface layer also showed significant improvement in film tack strength. The film tensile strength, elongation and tear properties in both MD and TD were not significantly affected by increase in PIB concentration. However, increased VA content showed slight improvement in MD mechanical performance of the films, TD properties were relatively unaffected.