Thermoplastic composite beam structures from mixtures of recycled HDPE and rubber crumb for acoustic energy absorption

The use of recycled rubber crumb in the design and production of thermoplastic-rubber composites as sound absorbers can provide solutions to noise pollution and for the recovery of post-consumer materials from both packaging and waste tyres. The work of this study is concerned with the effect of rubber crumb incorporation in high density polyethylene (HDPE) and also in HDPE glass-fibre composites on acoustic, mechanical and physical properties. Recycled HDPE compounds containing variable concentrations of cured rubber crumb particles were prepared by twin screw extrusion. Thermal analysis has revealed a significant increase in the level of crystallinity of the HDPE component by increasing the rubber content in the mixes. Standard three-point bending and notched impact test specimens were manufactured by injection moulding and large-scale beam samples were produced by compression moulding using an ad-hoc method that allows variation of the through-thickness elastomer content as a means of obtaining composition gradients. The flexural modulus and impact strength varied monotonically with rubber crumb concentration. A Fast Fourier Transform (FFT) technique was used to determine the acoustic performance of the beams over a wide frequency range. The graded structures produced large improvements in acoustic absorption properties in the frequency range 2-6 kHz, notably from composite beams containing 20% rubber and also in some multilayer beams with rubber concentration gradients.