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Mechanical Properties of Rotationally Moulded Nanocomposites
Polyethylene montmorillonite and quaternary tallow ammonium chloride modified montmorillonite nanocomposites were prepared by melt blending. The effect of compatibiliser concentration on composite mechanical properties was investigated. The nanocomposite morphology was examined using wide angle X-ray diffraction and scanning electron microscopy. Primarily intercalated structures were observed. The nanocomposites prepared were rotationally moulded using a peak internal air temperature of 200°C and an oven temperature of 300°C and injection moulded samples using a nozzle melt temperature of 220°C. No significant variation was evident in the shrinkage and warpage characteristics of the rotomoulded parts. The tensile, flexural and impact properties of specimens taken from both the rotomoulded and injection moulded parts varied with compatibiliser content. Generally, the properties of the injection-moulded parts were superior to that of the rotomoulded parts.
Interrelationships between Fatigue and Creep Fracture in Poly(ethylene) and Poly(vinyl chloride)
Using a fracture mechanics approach, a dynamic fatigue methodology was developed to accurately predict creep fracture in both poly(ethylene) and poly(vinyl chloride). The predictive methodology applied to cases in which the crack propagated in either a stepwise or continuous manner. The effects of material variables such as molecular weight and impact modifier were also elucidated.
Modelling Impact Fracture and RCP Resistance of Thermoplastics from Cohesive Properties
Developers of thermoplastic materials for pressure pipe applications must design them to resist rapid crack propagation. However, they are usually able only to test them for resistance to impact. The hypothesis that both failure modes are dominated by adiabatic heating leads to good quantitative predictions for each property - and hence to an account of the relationship between them. Here, the model based on this hypothesis is extended to explore the influences on fracture resistance of molecular weight and of thermal property non-linearity.
Residual Strain Characterization Using an Embedded FFB Sensor: Measurements and Simulations
Residual strains in an epoxy specimen are investigated using embedded Fibre Bragg Grating sensors. A novel OLCR technique allows the direct reconstruction of the optical period and provides the strain distribution due to epoxy consolidation along the fibre. The experimental data show an excellent agreement with finite element simulations.
Polyethylene Nanocomposites – Investigating the Tensile Properties of Polyethylene Nanocomposites for Rotational Moulding
Rotational moulding is continuously expanding into new markets which require improved material properties and shorter cycle times. Conventional PE, used in over 80% of the rotational moulding market is a relatively low strength material therefore high part wall thickness, long cycle times and design limitations are imposed.The development of polymer/clay nanocomposites has shown significant improvements in mechanical properties with small additions of organoclay, which have the ability to improve mechanical performances or reduce cycle times.In this work the effect of organoclay on the tensile properties of PE nanocomposites at a range of temperatures was investigated. Tests were also conducted at a range of cross head speeds (CHS) to illustrate the effect of slow rates of extension and instantaneous loading. Tensile testing shows the final nanocomposite properties depend significantly on organoclay loading with improvements of 25% in tensile modulus at room temperature and larger improvements at higher test temperatures being reported. The results also indicate the organoclay loadings investigated had an adverse effect on the tensile yield stress.
Investigation of the Processing Characteristics and Mechanical Properties of Metallocene Catalysed Polyethylene Foams for Rotational Moulding
The object of this work is to investigate the foaming characteristics of three grades of metallocene-catalysed Linear Low Density polyethylenes for rotational moulding using both an exothermic and endothermic chemical blowing agent. This paper reports on the results of ongoing experimental investigations in which rheological and thermal parameters are related to the polymer structure and mechanical properties. Through adjustments to moulding conditions, the significant processing and physical material parameters, which optimise metallocene catalysed linear low-density polyethylene foam structure, have been identified. The results obtained from equivalent conventional grades of Ziegler-Natta-catalysed linear low-density polyethylene are used as a basis for comparison.
The Effect of Coupling Agents on Foaming with Polymer Microspheres in Rotational Molding
The use of polymer microspheres for producing microcellular foams is a new development in rotational molding. In previous studies, some reduction in mechanical properties has been found due to the immiscibility between the polymer shell and the matrix polymer. Coupling agents can act as a molecular bridge at this interface and can also affect bubble growth by altering the rheological properties of the matrix polymer. The influence of different coupling agents on the melt properties of several resins was investigated, as well as the effect of these coupling agents on the mechanical properties of foamed rotationally molded parts.
Manufacturability of Fine-Celled Cellular Structures in Rotational Foam Molding
Any closed-cell polyolefin foam production tends to achieve the highest possible cell size distribution uniformity, cell size reduction, and cell density augmentation. However, the control of the cell size of rotationally foam molded cellular structures formed on the base of a chemical blowing agent (CBA) might be often aggravated by some inherent limitations that are unique to the rotational molding process, which results in coarsercelled final cellular structures being yield. Although a finecelled morphology (cell size < 100 [?m] and cell density > 106 [cells/cm3]) in rotationally molded foams has been closely approached, it has not been actually achieved yet, nor it has been ever clarified whether it is actually achievable in rotational foam molding or not. This study attempts to provide an answer to this fundamental question by focusing on the understanding of the mechanisms governing the formation, growth, shrinkage, and collapse of CBA-blown bubbles in non-pressurized polymer melts originating from extrusion melt compounded foamable resins in a pellet form.
Oil Heating and Cooling Machines for the Rotational Moulding of Plastics
Forced air convection is the most common method for heating and cooling the mould in the rotational moulding process. However, it is generally accepted that this is a very inefficient method of heat transfer and so interest has grown in the use of more direct methods of heating and cooling the mould. This paper reports on an experimental study where oil was used to heat and cool the mould. This method is used commercially in a small sector of the market, but there has never been a detailed study of its effectiveness. The results to be presented in this paper show that oil heating and cooling of the mould offers much higher thermal efficiency and reduced cycle times. The performance of the oil heated machine is compared directly with a conventional hot air oven. It is shown that ovenless rotomoulding machines are more amenable to process control because monitoring equipment on the mould is easily positioned outside the heated environment.
Designing Impact-Modified Polypropylene for Durability
Polypropylene is increasingly being used as a structural load-bearing polymer in durable applications. Elastomer is usually added for impact toughness improvement but few studies address the selection of elastomer morphologies for resistance to failure due to load conditions over long time. This paper discusses the results of screening some polypropylene-elastomer blends for durability using a recently developed notch sensitivity methodology.
A Fatigue Approach for Lifetime Prediction of PE-HD Pipe Grades
Fatigue crack growth (FCG) experiments were conducted on two high density polyethylene (PE-HD) pipe grades with various test specimen configurations such as compact type (CT) and circumferentially notched bars (CNB). The effects of R-ratio and frequency on the FCG behavior were studied. While FCG rates showed a great dependence on R-ratio in terms of stress intensity factor range, the effects of frequency may be considered significant in the low crack growth region. These experimental data were employed for lifetime prediction based on the crack layer theory.
The Effect of Direct Electrical Heating on the Cycle Time and Mechanical Properties of Rotationally Molded Polyethylene Parts
Direct electrically heated oven-less rotational moulding machines have been introduced to the rotational moulding market over the past few years. Whereas some of these systems used composite mould technologies, the MECH™ system developed by PPA Teo of Ireland, employs conventional steel tools heated directly by electrical elements surrounding the mould. The elements are supplied with current though slip rings. This paper will present the results of an initial study into the moulding and cycle time characteristics of the MECH™ system and compares it with two types of conventional hot air rotomoulding machines. Machine efficiencies, as well as heating and cooling times have been compared for the three machines. The effects of processing parameters on the mechanical properties of the mouldings have also been assessed.
The Effect of Cooling Rate on Rotationally Molded Parts
The quality of a rotationally molded part is highly dependent on its cooling rate, perhaps more so than its heating rate. A trade off between part quality and cycle times is often necessary to ensure economical feasibility. This paper presents an overview of the effect of cooling rate on final part quality. The temperature profile through the part wall during cooling is related to the part’s mechanical properties, morphology, shape and general appearance. Six different mold cooling rates are investigated; they range from quiescent cooling (average 2C°/min) to water spray cooling (average 15C°/min).
The Effect of Thermal Relaxations on the Crack Initiation Resistance of Rotomolded Linear Low Density Polyethylenes
The objective of this paper is to provide a better understanding of how test temperature and test frequency affect the crack initiation and propagation energies of rotomoulded materials. Trials were carried out on a range of rotomolded linear low density polyethylenes. Instrumented impact tests and dynamic mechanical thermal analysis (DMTA) were carried out on each material at a wide range of temperatures and frequencies. It was found that there is a linear relationship between the crack initiation energy of the samples and the tan ? values at all test conditions. By using this relationship it is possible to predict the crack initiation energy of a polymer part over a wide range of conditions by carrying out a small number of tests.
Rotational Molding Process Control
Process control for the rotational molding industry has been continually evolving in recent years. Initially, impact strength, and bubble content in the wall of the molded parts, were used to gauge the level of cure and also to control the process. More recently, the development of the Rotolog process control device has provided a more scientific means to ensuring good process control.This paper highlights some recent work that expands upon the methods previously mentioned, applying new methodology to measure and control the process. The effects of various processing conditions are considered, in particular, with respect to the cooling cycle, and how they relate to process variation. The results outlined provide new processing knowledge that can be used to further develop the control of the rotational molding process.
Performance of a Rotationally Molded Thermotropic Liquid Crystalline Polymer
Thermotropic liquid crystalline polymers (TLCPs) have a number of potentially useful physical properties for rotational molding: excellent chemical resistance, good barrier properties, low coefficient of thermal expansion, high tensile strength and modulus, and good impact resistance. However, it is possible that the nature of the molding process is such that full advantage of these properties cannot be obtained. To determine how well TLCPs perform when rotationally molded a commercially available TLCP, Vectra B 950, was studied under static conditions as well as with a single axis rotational molding unit capable of measuring the internal air temperature. The processing temperature was determined by measuring shear viscosity at several temperatures. The tensile strength and modulus of both statically molded and rotationally molded samples were measured. Samples were evaluated for complete densification by inspecting the fractured surface.
Failure Prediction in Polymer Composite, Sheet Metal Forming Dies
This paper presents a systematic approach to predict damage in a sheet metal forming die fabricated from a composite (aluminum trihydrate-filled polyurethane). The dominant die failure mode is determined based on the mechanics governing the forming process, and the fatigue life is predicted. Both numerical simulations and experiments are performed to verify the method.
Annealing Effects on the Yield and Fracture of Bisphenol-A and 4,4'-Dihydroxydiphenyl Copolycarbonates
The annealing effects on yield and fracture behavior of two different polycarbonates, one bisphenol-A-based (BPA) and one copolycarbonate of BPA and 4,4’- dihydroxydiphenyl (DOD), have been investigated. Annealing increases the yield stress, decreases the tan ? intensity at 80°C, and decreases the resistance to crack growth in both materials. The DOD material shows a slightly increased resistance to annealing compared to the conventional polycarbonate based on relative post-yield stress drops and tearing moduli of the materials.
Assessment of Plastic Failure of Polymers Due to Surface Scratches
This paper is concerned with the evaluation of plastic damage of polymeric substrate under surface scratch deformation. Employing a commercial finite element (FE) package ABAQUS®, FE analysis was performed to study the permanent damage imposed by a spherical indenter as it traverses across the surface of a polypropylene (PP) substrate. As compared to the experimental data, the numerical results make reasonably accurate prediction on the onset of scratch damage. Also, the furnished solutions aid in elucidating the fracture phenomenon encountered and the possible damage initiation during scratch.
Investigating Environmental Stress Cracking with In-Situ Contact Angle Measurements
This paper probes a hypothesis for initiation of environmental stress cracking (ESCR) based on a thermodynamic criterion for localized swelling induced by stress on the polymer. The system chosen for study is polycarbonate with oleic acid. An experimental technique involving contact angle measurements of a sessile drop as a function of stress is presented. A novel technique for contact angle measurements using refraction is also introduced.
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