The Effect of Construction and Demolition Waste Plastic Fractions on Wood-Polymer Composite Properties

Global waste generation has grown significantly throughout history and is predicted to increase by tens of percentages in the future unless action is taken¹. In particular, high-income countries have generated more than one-third of the world’s waste although they account for only 16% of the global population¹. The construction sector is a significant producer of this waste due to rapid urbanization and population growth. According to estimates, approximately one-third of global solid waste is formed by construction and demolition projects; however, exact values from different areas are missing². In the European Union (EU), the amount of construction and demolition waste (CDW) is approximately 25%–30% of total waste generation³, and includes valuable and significant secondary raw materials, like plastic. Without organized collection and management, plastic may contaminate and adversely influence ecosystems. In 2016, 242 million tons of plastic waste were generated in the world¹. The share of plastic recycled in Europe was only 31.1%⁴.

Resource scarcity has created a need to change practices toward a circular economy, in which the aims are to use waste as a source of secondary resources and recover waste for reuse. Economic growth and minimized environmental impacts will be created by the circular economy, which is a popular concept in Europe. The European Commission adopted a European Union Action Plan for a circular economy, which set goals and indicators for contributions⁵.

Tighter environmental regulations and laws are contributing to the construction sector putting more effort into waste management and material recycling issues. For example, the European Union (EU) has set targets for material recovery. From the year 2020 onwards, the material recovery rate of non-hazardous CDW should be 70%⁶. The composition of CDW may vary widely across geographical locations but some common characteristics can be identified, including, for example, plastic that is a potential and valuable raw material for wood-polymer composites. The reutilization of plastic is a concrete step towards a circular economy in which virgin plastic polymers are substituted by recycled polymer.

Composite materials are a multi-phase system, consisting of a matrix material and reinforcing phase. Wood-polymer composite (WPC) typically contains polymers as the matrix, wood materials as reinforcement, and additives for improving adhesion, such as coupling agents and lubricants. WPC can be known as an environmentally friendly material because the raw material can be sourced from renewable materials, such as polylactic acid (PLA) and wood. According to the latest innovation⁷, the additives of WPC can be based on renewable sources. Additionally, the source of the raw material can be recycled (non-virgin) materials, which is an ecologically and technically superior alternative⁸. For example, researchers have studied extruded WPC that contains CDW, and found that the properties of CDW–based composites were at an acceptable level⁹. Utilization of recycled raw materials as a component for WPC is also acceptable from the environmental aspect, as proved by several assessments. Overall, it has been demonstrated that utilizing CDW in WPC production can decrease the environmental influences of CDW management¹⁰. In addition, it has been found that using recycled polypropylene (PP) plastic in WPC has the potential to reduce global warming¹¹.

The amount of available recycled polymers will increase in the future. Global plastic production has increased approximately 9% as per year, on average, and it is expected that this increment will continue in the future¹². The most general plastic polymer types are, inter alia, polypropylene (PP) and polyethylene (PE). The shares of total demand for PE and PP were 29.8% and 19.3%, respectively, in Europe in 2017⁴. The global plastic recycling market is expected to grow at an annual growth rate of 5.6% during the period 2018–2026¹³. One of the main applications in which plastics is used is building and construction. For example, almost 20% of the total demand for European plastic was associated with building and construction applications⁴. From an economic perspective, the use of recycled polymers in WPC manufacturing is an interesting alternative, leading to the production of materials with low cost. Previous research has shown that physical effects have a stronger influence on extruded materials made from secondary plastic compared to the corresponding virgin material, but properties depend on the plastic source¹⁴. However, the use of recycled plastic decreases the strength of WPC due to lower compatibility¹⁵. Variation between the structures of plastic polymers causes concerns for re-use and recycling, which contribute to the importance of plastic sorting based on the polymer.

This study intends to assess the utilization of plastic material from CDW as a raw material for WPC. The polymer fractions assessed in the study are acrylonitrile butadiene styrene (ABS), polypropylene (PP), and polyethylene (PE). These are known as universal plastic fractions within CDW. The polymer fractions are treated with general manufacturing processes, such as agglomeration and extrusion, and are tested with universal mechanical property tests. The primary objective of the study is to discover how the properties of WPC would alter if recycled polymers were used as a raw material in matrix instead of primary virgin polymers.

Based on the (local) waste management center (Etelä-Karjalan Jätehuolto Oy), it was shown how plastic-rich CDW is stored. It was demonstrated that a great amount plastic material is included and some examples of CDW plastic polymers were shown. Researchers collected the most suitable polymers for further processing, such as ABS, PP, and PE. The desired polymers (PE, PP, ABS) were identified using portable near infrared (NIR) spectroscopy. WPC product examples were presented in which where collected plastic materials could be utilized as a raw material. The definition of the composite and its advantages were explained.