David J. Hitt

Preparation of nickel coated mica as a conductive filler

Electroless plating was used to prepare nickel coated mica fillers. To optimise the conductivity of the filler the nickel coating needed to exceed a certain weight percentage, depending upon the particle size of the mica, and cover the surface of the mica particles. Treatment of the filler in hydrogen improved its conductivity considerably. The fillers were incorporated into an ABS resin to prepare composites as potential electromagnetic interference shielding materials. Increasing the particle size of the mica reduced the critical filler loading required to produce electrically conductive composites. Reducing sample thickness caused a decrease in resistance, due to changes in filler orientation.

Shear viscosity measurements on Polyamide‐12 polymers for laser sintering

Purpose – Laser sintering kinetics and part reliability are critically dependent on the melt viscosity of materials, including polyamide 12 (PA‐12). The purpose of this paper is to characterise the viscosity of PA‐12 powders using alternative scientific methods: constrained boundary flows (capillary rheometry) and rotational rheometry.Design/methodology/approach – Various PA‐12 powders were selected and characterised by both techniques. Measurement of molecular weight was also carried out to interpret the viscosity data.Findings – Results demonstrate conventional pseudoplastic flow in all PA‐12 materials. Zero‐shear viscosity has been quantified by rotational rheometry; a notable observation is the striking difference between virgin/used PA‐12. This is interpreted in terms of molecular weight and chain structure modifications, arising from polycondensation of PA‐12 held at the bed temperature during laser sintering.Research limitations/implications – Accurate zero‐shear viscosity data provide scope for us...

A machine for the biaxial stretching of polymers

A biaxial stretcher which uses an Instron tensile testing machine to drive its stretching mechanism has been designed and constructed. The new stretcher is capable of producing biaxially oriented polymer sheets under controlled drawing and annealing conditions and has enabled a fundamental study to be made of the parameters that influence the properties of biaxially oriented polymer products. The parameters of interest are draw ratio, draw rate and draw temperature, and the subsequent annealing conditions of temperature and time.

Development of a machine for the stretching of polymers

A machine for the equal biaxial stretching of polymers has been developed further to incorporate additional stretching modes of uniaxial (constant width), simultaneous unequal biaxial and sequential. Modifications to the stretching mechanism of the equipment are described and details are given on how oriented sheets are produced under all stretching modes. To complement the latter mechanical properties are presented for oriented sheets of a plasticised poly(vinyl chloride) formulation.

Fracture mechanics approach to compare laser sintered parts and injection mouldings of nylon-12

In comparison to equivalent parts made by injection moulding, components manufactured by laser sintering are often perceived to offer inferior mechanical properties, yet the evidence for this is rarely based upon systematic research studies. In this paper, attempts have been made to conduct a fundamental study of the fracture behaviour of both injection moulded and laser sintered parts, based upon a modification of a standard technique used to determine linear elastic fracture mechanics parameters. The influence of specimen thickness (in the range 2–10 mm) was also included in the experimental plan, which concentrated upon the testing of single-edge notch bending (SENB) beam specimens. Force-displacement characteristics demonstrated significant plastic deformation in nylon (PA12) specimens, so that the J-integral method was used to obtain quantitative fracture parameters, including the energy requirements for crack growth. Comparisons of this parameter showed thickness-dependent ‘geometry-sensitive’ data for each set of samples. For the injection moulded SENB specimens, energy absorption decreased with increasing thickness, a result attributed to the influence of plane-strain-dominated conditions. In contrast, laser sintered samples exhibited increased toughness as specimen thickness increased towards 10 mm; this could not be explained by density or particle melting data and may be interpreted by changes in molecular structure that occur during the additive manufacturing process.

Evaluating PVC Degradation Using UV and Raman Spectroscopies

While the world growth in PVC consumption continues (currently ~35 million tonnes per annum), its poor thermal stability, remains an issue due to restrictions on the use of stabilizers containing heavy metals such as lead. Monitoring degradation in PVC is important, and this work is concerned with the detection of PVC degradation using UV and Raman spectroscopies. The bands which appear in the Raman spectra of PVC samples after thermal degradation at around 1100 and 1500 cm-1, are the result of a resonance Raman effect. Optimization of the PVC degradation assessment process using laser-Raman is crucial to minimize the effects of additive interference and the fluorescent background. The most important factor is the wavelength of the exciting laser source, as it should fall within the absorption range for polyene sequences in degraded PVC. Laser-Raman spectroscopy has been shown to be an appropriate method for the quantitative determination of the polyene length distribution in lightly degraded PVC samples. The application of this method for various degraded PVC samples is reported. Ageing of compounds containing various heat stabilizers were considered, in order to assess their effectiveness. Additives studied included calcium zinc stabilizers and two commercial grades of hydrotalcite (Sorbacid 911 and Alcamizer P93).

Inorganic impact modifier and processing aid for PVC

Abstract An amorphous silica, SIDISTAR has been investigated as an impact modifier for rigid PVC. Optimal dispersion of the silica as primary particles is essential and this can be achieved by incorporating it at an early stage of the mixing process before other fillers are added. This ensures that the PVC grains are coated with SIDISTAR. This material is able to function as an impact modifier via the cavitation mechanism and it will also improve the dispersion of the other ingredients in the compound. It was found that SIDISTAR delayed gelation of the PVC compound, but this could be avoided by minor changes in formulation and processing conditions. Reduction in gelation time resulted in improved impact performance. Furthermore, compounds containing SIDISTAR possessed a wide processing window.

Biaxial orientation of poly(vinyl chloride) compounds Part 2 –Structure–property relationships and their time dependency

Abstract X-ray diffraction and thermomechanical analysis have been used, respectively, to examine structural order and shrinkage behaviour for oriented samples of rigid and flexible poly(vinyl chloride) (PVC). Results were compared with previously measured tensile properties and structure–property relationships explored. X-ray diffraction showed that drawing produces planar crystallite orientation in PVC sheets. If drawing and subsequent annealing conditions are held constant, but draw ratio is varied, there is good correlation between structural order measured by X-ray diffraction and tensile strength. Increased annealing time and temperature improve crystallite order and dimensional stability, while tensile strength is unchanged. The greatest enhancement in tensile strength is achieved by stretching PVC towards its maximum draw ratio at 90°C, but optimum thermal stability of the oriented structure is achieved when higher annealing temperatures are used. Room temperature recovery is observed for flexible PVC when the material has a glass transition temperature below ambient. This can be delayed by increased annealing time and temperature, and by increased draw ratio.

Melt compounding of rigid PVC formulations with hydrotalcites

Abstract Hydrotalcites, compounds of magnesium–aluminium–hydroxycarbonate, are promoted as environmentally safe materials for costabilisation of PVC products. Commercial grades of hydrotalcites have been added to rigid PVC formulations, containing a range of different stabiliser types, to evaluate their contribution to heat stability and their effect on mechanical properties. Hydrotalcites are confirmed to be effective costabilisers for rigid PVC: static thermal stability (through oven testing) and dynamic thermal stability (via torque rheometry) of PVC compounds are modified by their presence. The extent of change is determined by the primary stabiliser type and the grade of the hydrotalcite. Detailed analysis of mechanical properties has been carried out on compression moulded samples and on extruded compounds. With the former, no significant change in tensile or impact performance was observed, for hydrotalcite levels up to 5 phr. Charpy impact data on extruded PVC have shown significant increase in performance of compounds containing an acrylate modifier when hydrotalcites are used.