The science of recycled polymer composites

Materials World magazine
,
31 Mar 2017

The future is strong for engineered recycled polymer composites, argues Professor Alma Hodzic.

The science of immiscible recycled polymer blends is not so easy to crack, as the raw materials are post-use and have already been filled with additives of unknown source. Any type of optimisation must account for a large degree of variability in the raw post-use polymer mix, and the synergistic properties must be obtained through the careful selection of reinforcing additives and recycled polyolefins. From raw materials supply to the overall performance of the long profiles, all aspects of this technology must be commercially viable, and the supply chain must conform to the cost of the final product, competing with both lowest (price) and highest (strength) classes of softwood and hardwood species. 

In addition to understanding which polymer blends can be used in the manufacture of recycled polymer composites, it is equally important to have a quality control system that effectively detects polymers and other materials naturally present in such recycled plastic supplies. These may contaminate the final composition either through their inert presence or thermally induced chemical reactions during the melting process. Although a great majority of recycled polymers have been successfully used without such stringent control systems throughout the manufacturing process, for commodity applications and non-structural products in civil and environmental engineering, the formulations that have been researched during long-term academic science projects must be precisely produced in order to achieve their synergistic composite properties that outperform that of hardwood.

Bill Hopkins, Commercial Director of Revaluetech Ltd, UK, claims, ‘We are all about the good old-fashioned manufacturing of engineering structures.’ The technology is expanding into replacing the UK’s coastal groynes, traditionally made of unsustainable ekki and greenheart logs. It has been recognised by the UK Environment Agency that these, and similar types of FSC hardwood have become unsustainable and the supplies will diminish in the near future, according to the Forestry Commission.

Waste not, want not

The properties arising from combining at least three recycled polymers, which can be varied as long as the basic immiscibility is retained, preferably based in low-density polyethylene, are significantly enhanced with the addition of recycled glass fibre. This ensures that strength remains superior to that of the highest-class hardwood, while keeping the cost several times below that of traditional materials in civil engineering structures, including railway sleepers.

Currently, these structurally sound recycled polymer composites are manufactured by Reluma GmbH, Germany. Reluma and Revaluetech have been working closely with a UK-based sustainable building materials group, Tarmac. Both intend to propel manufactured sustainable products into all areas of structural applicability in civil engineering, which is set to establish a new manufacturing plant in the UK built by Tarmac Building Products.

The variation in properties such as strength increases linearly with the increased density in natural materials. However, the synergistic effect of composite constituents achieved by turning post-use polymers into circular economy products expands beyond this simple rule. The compositions have been varied to satisfy two important design criteria, strength and heat deflection temperature, in order to overcome the two most prominent weaknesses in recycled polymer compositions, which have rendered these materials suitable only for commodity products. This researched selection of materials and serial manufacturing production available through re-formative engineering, the trademark of Revaluetech Ltd, could replace unsustainable tropical hardwood with post-industrial and post-consumer polymer waste, in line with stringent engineering standards across the globe.

Environmental costs

The environmental benefits of recycled plastic products are energy and CO2 savings by avoiding the process of oil extraction and polymer manufacturing – expected to be around 95% of the total energy consumed – and reduction in landfill. The most common recycled plastic products find application as ground reinforcement materials, furniture, plastic lumber, bollards, domestic products and non-food grade packaging. Among these, very few have been used in structural engineering products, such as flood defences, bridges, noise barriers, utility poles, overhead lines and other similar civil engineering applications. Using environmental parameters and system boundaries in life cycle analysis (LCA) as funded by Qatar Science and Technology Park, carried out at the University of Sheffield, UK, and Qatar University found that the use of recycled plastics for composite applications could have quantifiable environmental advantages over conventional virgin plastics in many cases. LCA results showed the reduction in overall environmental impact for units optimised for civil and infrastructural applications, because of its significant potential to contribute to the large scale manufacturing of structural components, especially during their life cycle inventory stages. 

With these quantified environmental and materials properties showing the benefits arising from the use of re-formative engineered recycled polymer composites in structurally loaded civil engineering applications, the future holds a strong promise that all types of worldwide forests could be protected and replenished as a result of cost-effective replacement of mass applications in land and sea structures. Trial products have been field tested over the past 16 years in marine and flood defence structures, showing their durability and resistance to weathering. The recycled composites are price competitive, inert, do not pollute potable water, are resistant to damage from seawater and do not damage marine organisms.

Of particular importance for future mass applications is the recycled polymer composite formulation, developed at the University of Sheffield, where it was initially approved by Network Rail to replace the currently used softwood railway sleepers in the UK that have been traditionally treated with creosote to maintain durability in relatively harsh environmental conditions. According to recent advice from the Health and Safety Executive, approvals for professional and industrial creosote/coal tar creosote products have been allowed to continue to date, subject to restrictions on the specification of the products and restrictions on where wood that has been treated with creosote/coal tar creosote is used, despite the general public not being able to use creosote nor coal tar creosote wood preservatives in the UK since 30 June 2003. 

It is expected that with the market penetration of the engineered recycled polymer composites as a viable replacement for all types of wood structures, the use of creosote may be abandoned in the very near future, yielding yet another important environmental impact. The recycled composite railway sleepers developed have been tested using more than 30 international and application specific standard tests, some directly provided by the Network Rail during the project funded by the Knowledge Transfer Partnerships arm of Innovate UK, including full-scale system fatigue tests of up to 30 tonnes applied over several million cycles. Currently, this technology is undergoing international standardisation. 

The commercial opportunity for the application of different recycled composite formulations in civil engineering is massive and will amount to a new multi-billion pound industry in the UK alone. The environmental impact will surpass the financial benefits as the reduction in manufacturing emissions, further restrictions in the use of creosote and the prevention of deforestation will project the immediate benefits over the globe.

Professor Alma Hodzic FIMMM is the Director of Research at Revaluetech Ltd, researching composites for structural applications from recycled polymers.