Invasive trees and recycled plastic a bargain for the environment and housing crisis

21 May 2023

by Joanie Bergh and Jorisna Bonthuys

A new composite building material comprised of invasive Port Jackson trees and recycled plastic waste can not only help alleviate South Africa’s housing shortage but also aid in caring for the environment.

A doctoral study has indicated that the Port Jackson tree has the necessary physical, chemical and mechanical characteristics for the manufacturing of insulation boards that can be used in low-cost housing projects.

This Australian tree, Acasia saligna, is one of the top ten aggressive invaders in South Africa. It infiltrates water catchments, reduces water runoff, increases the severity of wildfires, and threatens native plant species.

In a recent Wood Products Science study by Dr Abubakar Sadiq Mohammed in Stellenbosch University (SU)’s Faculty of AgriSciences, he proved that a combination of composite material from Port Jackson trees and recycled low-grade plastic waste can be used to build better quality low-cost housing. 

Dr Mohammed, who is originally from Ghana, developed a wood-plastic composite that uses a combination of biomass from all the parts of a Port Jackson tree and recycled, low-density polyethylene.

Light polymer is mainly used in packaging, such as plastic bags. Wood plastic composites are usually made from recycled plastic and small wood particles or fibres. Manufacturers typically separate the woody parts of plants that are used in the composites from the non-woody parts, such as bark, leaves and seeds.

The non-woody parts of alien invasive trees are considered polluters rather than suitable components from which composite materials can be made. 

Dr Mohammed chose this dissertation topic due to his worries about how much waste is generated in agricultural processes and the extent of plastic pollution.

“Plastic waste is a known global menace, so the idea was to transform this waste material into something useful,” he says. “I was also interested in developing affordable, eco-friendly alternatives to existing low-cost building materials.”

Smart building material – The composite material made from Port Jackson trees and plastic waste. Photo: Stellenbosch University (SU)

His research forms part of a bigger study project within SU’s Department of Forestry and Wood Science that researches the different uses of alien invasive tree species. 

“People living in RDP houses are often exposed to severe heat and humidity due to the low quality of the building materials used. Over the last decade, the budget for these government-subsidised housing units has remained unchanged at R140 000 each,” says Dr Mohammed. “This often results in the use of the cheapest building materials available.” 

Structurally the houses are made with the bare minimum materials and temperature and sound insulation are left out.

Previously, the government had introduced a program aimed at removing invasive plant species. What eventually becomes of the cleaned biomass, apart from the part used as firewood, is a big source of concern according to Dr Mohammed.

“Typically, the biomass is left in a cleaned-out area to dry out, which poses a fire risk.”

He analysed the physical and mechanical characteristics of the new material, such as its resistance to mould, and determined how fire-resistant it is. His results indicate that wood-plastic composites made from the entire Port Jackson tree have better mechanical properties compared to those of two other invasive species, namely the black wattle and the red eucalyptus.

The low amounts of non-woody plant material, such as seeds, leaves and bark, slows the growth of mould. This, therefore, makes it ideal for covering interior walls and for use as ceiling boards in indoor and high-humidity rooms.

The material is not yet commercially available.

Dr Mohammed’s results have been published in the European Journal of Wood and Wood Products and the Polymers trade journal. He received support from the German Federal Ministry of Education and Research for his study. 

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