SPE Library
The SPE Library contains thousands of papers, presentations, journal briefs and recorded webinars from the best minds in the Plastics Industry. Spanning almost two decades, this collection of published research and development work in polymer science and plastics technology is a wealth of knowledge and information for anyone involved in plastics.
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Conference Proceedings
Explosion of an ABS Pressurised Air Line
A compressed air line exploded suddenly and caused extensive damage to property, fortunately without casualties. The air line was used as part of a plant for annealing glass, and so was pressurised regularly up to about 10 bar. When the pressure was released, a blast of cold air was directed over the hot glass surfaces. Most of the pipework for the system was made of ABS pipe and solvent-welded fittings. The explosion was caused by catastrophic growth of an internal axial crack in one of the pipes of the system. The inside surface of the pipe was covered with deep cracks, and the first investigators concluded that the cracks had been caused by fatigue. However, a separate investigation of the remains indicated that traces of fluid from the oil of the air pump compressor had created environmental stress cracks.
Fracture Initiation in Polybutylene Tubing in Potable Water Applications
The fracture initiation in polybutylene (PB) tubing is usually observed in the form of a microcrack network within a layer of degraded PB at the inner surface of the tubing exposed to flowing water. Chemical degradation of PB is manifested in a reduction of molecular weight, increase of crystallinity and density of the material, a subtle increase in yield strength, and a dramatic reduction in toughness. An increase in material density, i.e., shrinkage of the degraded inner layer of the tubing constrained by the outer layer of unchanged material results in a buildup of tensile stress due to an increasing incompatibility between the inner and outer layers of the PB tubing. These stresses combined with preexisting manufacturing and service stresses, as well as reduction of toughness, result in fracture initiation. A mathematical model of the foregoing process is presented in this paper. An equation for determination of the critical level of degradation at the onset of fracture is derived. The critical level of degradation leading to fracture depends on the rates of toughness deterioration and buildup of the degradation related stresses as well as on manufacturing and service stresses. A method for the evaluation of the time interval prior to fracture initiation is also outlined.
Reliability Analysis of PB Tubing in Water Distribution Application
A number of water utilities in various states of USA have reported a persistent premature failure of polybutylene (PB) tubing in water distribution system. A methodology of reliability analysis of PB tubing in potable water distribution application and probabilistic model for predictions of future leaks are presented in this paper. The predictive power of the proposed methodology is validated by comparison of predicted number of failures in Harford County Water District, Maryland between 1996 and 2001 with actual events reported by the District in 2002. The proposed methodology also accounts for the effect of preventive replacement program on the reduction of emergence replacements and thus allows a rational planning of replacements. The needs to understand the failure mechanisms for proper interpretation of both descriptive and inferential statistics are also discussed.
Premature Fracture of Noryl Transformer Plugs
Small transformer plugs are widely used for consumer electrical and electronic products, designed to be fitted to mains electricity sockets by the user. Problems occurred when plugs injection moulded in Noryl started cracking at or near the weld zone on first use, exposing the user to danger. A quality audit showed that the base and cover had been moulded in mainland China in two specific weeks of 1999. Although records for that period were not available, those from adjacent batches showed that low tool temperatures (ca 55°C) had been used for the base moulding , which also showed the greatest incidence of brittle cracking. An alternative theory by a Japanese investigator that the material was faulty could not be sustained by EDAX analysis.
Environmental Factors in Performance Forecasting of Plastic Piping Materials
Environmental factors are known to significantly impact the oxidative failure mechanism of materials. For example, the chlorine present in potable water as a disinfectant is an oxidant that has been reported to impact the failure mechanism of materials in potable water applications. In this paper, the relationship between various potable water qualities, with different oxidative potentials, and chlorine induced oxidative failures of plastic piping materials is examined. The primary factors of potable water quality affecting oxidative strength are reviewed. Laboratory exposed pipe samples tested at various water qualities to ultimate failure are examined to determine the impact of water quality on the failure mode. The chlorine in potable water is seen to attack the inner pipe wall causing oxidation and degradation of this inner surface. The stresses on the inner wall lead to micro-crack formation in this degraded layer. These micro-cracks are seen to propagate radially through the pipe wall resulting in a brittle slit type failure. The failure mode is shown to be the same over a range of water qualities. The impact of chlorine is shown to be simply one of oxidation with the rate of degradation primarily related to the oxidative strength of the potable water. For the PEX pipe material examined, it is projected that material performance can significantly exceed the excellent performance predicted based on testing at the aggressive water qualities typically employed in validation testing, depending on the specific water quality of the end use application.
Effect of an Environmental Stress Cracking Agent on Slow Crack Propagation of Polyethylene
The effect of an environmental stress cracking agent on slow crack propagation in creep and fatigue of polyethylene resins was studied. At 50°C, fatigue and creep behavior of polyethylene in Igepal solutions followed the same stepwise mechanism as in air. The fatigue to creep correlation was probed by increasing Rratio (defined as the ratio of minimum to maximum stress in the fatigue loading cycle) from 0.1 to 1.0. Unexpectedly, 10% Igepal CO-630 solution, an ASTM standard environmental stress cracking agent, retarded fatigue crack propagation at R-ratio 0.1 but accelerated crack propagation at increasing R-ratios, in comparison with experiments in air. In contrast, fatigue and creep experiments in another longer chain Igepal solution, Igepal CO-997, showed no appreciable effect on crack propagation kinetics at different R-ratio compared to air.
Accelerated Testing for Slow Crack Growth in HDPE
Laboratory studies of slow crack growth (SCG) that mimics the field conditions experiments (ambient temperature and relatively low quasi-static load) would take decades and therefore would be useless for PEs ranking and an evaluation of PE life expectance. Thus it is desirable to develop a methodology for prediction of SCG behavior of PE based on a short-term test. An accelerated test for SCG resistance proposed in the present work is based on the analysis of micro-mechanisms and quantitative modeling of SCG. The test provides a comprehensive characterization of the craze material (fibers) creep and time to fiber fracture dependence on stresses, since SCG in PEs is in a large extant controlled by a process zone (PZ), i.e., a craze, formation and rupture in front of the crack. The proposed experimental procedure includes relatively short-term creep tests at various stress levels, which address only the basic properties of a particular polyethylene. The creep characteristics of craze fibers and the stress dependence of the time to craze fiber rupture are employed for SCG modeling described in the previous presentation. The details of the test procedure and examples of the test results are discussed.
Molecular Dynamics Simulation of the Mechanical Properties of Polymers
We investigate the behavior of polymeric materials at the molecular level by the use of computer simulations. We create one and two-phase materials on the computer with varying degrees of chain orientation, second phase concentration, and defects.The simulated materials are subjected to various patterns of tensile force. The true stress developing along the simulation can be calculated and compared with the engineering stress. Free volume can also be monitored and related to the formation of internal micro-cracks during deformation.Computer simulations provide us detailed information about molecular phenomena. This knowledge can be used to create materials with improved properties.
Modeling Damage Patterns in Polymeric Materials
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.
Characterization of Polyethylene Resistance to Slow Crack Growth
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.
An Integrated Intelligent Total Quality Management System for the Diagnosis of Tires’ and Elastomers’ Defects during Manufacturing and Use
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.
Oxidative Resistance of Sulfone Polymers to Chlorinated Potable Water
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.
Phase Reinforcement Effects in TPV Nanocomposites
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.
Application of Time-Temperature Superposition to Stress Relaxation in Elastomers
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.
New Flame Retardant TPVs for Electrical Applications
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.
Versatile New Soft Polyolefin for Compounding with Other Soft Thermoplastics Resins or as a TPV Base Resin
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.
Educating Industrial Design Engineers in Failure Awareness
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.
Improved Chlorinated Paraffin Secondary Plasticizer Compositions
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.
An Innovative Plasticizer for Sensitive Applications
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.
Improved Heat Distortion Modifier for PVC and ABS
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.
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