Plastics,Rubber and Composites: Macromolecular Engineering

Life cycle assessment (LCA) is a sustainability measurement tool that identifies the environmental impacts of a product. The uniqueness of the LCA lies in its methodology, which aggregates all environmental burdens throughout the product's life cycle, providing many variables for optimisation of the product (or process). Having occupied a major market share, the environmental impacts associated with the manufacturing and disposal of polyolefins are quite high. The main theme of this paper is to review the reported LCA studies on polyethylenes and polypropylenes including recycled plastics, biobased materials, which are competing with polyolefins, and polyolefin composites. The widely claimed green product ‘recycled plastic’ is analysed in detail from an LCA perspective, and key points, which determine its sustainability, are discussed. The environmental impacts associated with the manufacturing of polyolefins and their bioalternatives are highlighted. The few published studies of polyolefin composites on different applications are also discussed.

In this study, three different elastomers, namely hydrogenated nitrile butadiene rubber, fluoroelastomer and silicone, have been subjected to two different hard metallised coatings by ion implantation process. The three different elastomers are commonly used in various seal applications, where reduced wear and gas permeability are essential in maintaining seal performance and functionality. Samples of these rubbers have been coated with chromium coating in one set of tests. In the second set of tests, samples of elastomers have been coated with tungsten carbide coating being deposited on all the three different elastomers. Wear, gas permeability and mechanical behaviour of the coated samples were compared with each other and with the control uncoated elastomers. All the coated samples showed good reduction in gas permeability. With the use of metallised coatings, there has been improved resistance to wear in all the coated samples. Adhesion strength and effect of coating on the elastomer have been investigated by mechanical testing. Mechanical tests revealed good adhesion of metal coatings on all the rubber samples, and there was no detrimental effect on the mechanical properties after coating.

This study investigates the influence of two plasticisers, polyethylene glycol (PEG) and tributyl citrate (TbC), on the thermomechanical properties and fracture behaviour of nanosized calcium carbonate blended poly(lactic acid). Various compositions of nanocomposites were compounded and processed using co-rotating twin screw extrusion and compression moulding. DMA analysis shows that adding nano-CaCO₃ reduced the storage modulus (E′) of the nanocomposite while the glass transition temperature (T_g) of the samples was not affected. Furthermore, plasticised poly(lactic acid) (PLA) showed an improvement in elongation at break in all samples, and the impact resistance of the nanocomposites was also improved by 1·6 times with the addition of 20 phr PEG plasticiser and by 1·4 times with the addition of 20 phr TbC plasticiser. Morphological study reveals that the fracture behaviour of PLA-CaCO₃ nanocomposites changed from brittle to ductile after plasticisers were incorporated.

Cure monitoring has received considerable interest for industrial applications that use thermosetting resins and advanced composites. In these applications, the degree of curing is critical for the structures’ high quality and performance. This paper demonstrates the excitation of Lamb waves with low profile surface mounted piezoelectric transducers for the characterisation of symmetric composite laminates at different cure levels. The response signals were processed by extracting their instantaneous characteristics through ensemble empirical mode decomposition. The paper investigates the potential benefit from the use of piezoelectric transducers, which could monitor the life cycle of complex composite structures from the early manufacturing stages to the later aging stages. In these stages, structural monitoring is essential for the prevention of possible catastrophic damage. The response signals successfully captured the different cure levels, and the employed analysis showed good agreement with the increasing cure degree.

The present study outlines a methodology for microstructural characterisation of fibre reinforced composites containing circular fibres. Digital micrographs of polished cross-sections are used as input to a numerical image processing tool that determines spatial mapping and radii detection of the fibres. The information is used for different analyses to investigate and characterise the fibre architecture. As an example, the methodology is applied to glass fibre reinforced composites with varying fibre contents. The different fibre volume fractions (FVFs) affect the number of contact points per fibre, the communal fibre distance and the local FVF. The fibre diameter distribution and packing pattern remain somewhat similar for the considered materials. The methodology is a step towards a better understanding of the composite microstructure and can be used to evaluate the interconnection between fibre architecture and composite properties.

This paper presents the in-plane shear moduli of three-dimensionally (3D) woven angle interlock composites with different proportions of through the thickness binder content. The in-plane shear modulus G_xy was determined via three techniques: the plate twist test, the V notched beam Iosipescu (ASTM D 5379) and the V notched rail shear (ASTM D 7078) test. The results indicate that the plate twist method is suitable for the measurement of in-plane shear modulus for different 3D weave architectures with large repeat unit cells, and that the measurement is more representative of the global material response.

In this study, the compressive behaviour of carbon fibre reinforced plastic quasi-isotropic laminates and sandwich panels with carbon fibre reinforced plastic face sheets and syntactic foam core has been investigated. Experimentally determined open hole strengths have been compared with theoretical predictions obtained by applying a linear cohesive zone model. The unnotched compressive strength has been experimentally determined, and the in-plane fracture toughness has been analytically predicted as input parameters of the model. Buckling phenomena occurred on some specimens, and they have been taken into account. Evaluation of macroscopic failure modes in compression tests on unnotched specimens led to a better understanding on the advantages of the analytical model and on the possibility of applying the model to sandwich structures. The experimental results were in good agreement with the analytical prediction by the Budiansky–Soutis–Fleck cohesive zone model, and the difference between theoretical and experimental open hole strengths of Syncore sandwich panels was <9%.

A three-dimensional progressive failure analysis methodology was developed to predict the strength of double lap bolted joints in [0°/90°/±45°]_2s carbon fibre reinforced plastic laminates. An experimental programme was conducted to verify and validate the proposed computational model. Good agreement was obtained between the experimental data and predictive model. A parametric study was conducted for varied clamping torque and friction coefficient values. The well known effect of those variables on the joint strength was captured. Although the in-plane mode of failure of each individual layer around the fastener hole was predicted, X-ray radiographs have shown that delamination failure is particularly dominant around the washer's outer edge. At present, the proposed model does not account for delamination onset and propagation. Future work will involve implementing cohesive zone elements in regions of interest to capture this interlaminar cracking, a work in which the authors are currently engaged in.

The various methods of self-sensing and self-healing developed within the Composite Systems Innovation Centre, University of Sheffield, are reviewed. Damage sensing using electrical resistance in carbon fibre reinforced composite or using the fibres as optical sensing elements in glass fibre reinforced composite is demonstrated. Amelioration of low level damage is demonstrated in both monolithic composite materials and sandwich structures using direct chemical reactions within the matrix without the use of encapsulants. These reactions can be activated by resistive heating of the material itself. The use of a combination of these techniques could create a truly smart structure able to both sense and repair damage and degradation.