The effect of the use of construction and demolition waste on the mechanical and moisture properties of a wood-plastic composite

Abstract

Recycling of construction and demolition waste (CDW) which contains materials with high resource value such as plastics, metals, wood, glass and concrete has been investigated in recent years due to tightening environmental legislation. Wood-plastic composites as an environmentally friendly material provide a possibility to enhance the recycling of material. The effect of the addition of CDW on the mechanical and moisture properties of WPC is studied in this paper. Three types of wood-plastic composites were manufactured with different amounts and composition of a CDW filler. Their properties were compared to the reference composite manufactured with virgin wood fibers and polypropylene as the polymer. It was found that the addition of the CDW filler weakened the mechanical properties of the composite generally, except for the fact that the Charpy impact strength was improved significantly. Minor variations in the composition of the CDW filler did not have an impact on the mechanical properties, but an increase in the amount of filler increased the deterioration of mechanical properties. For moisture properties, the addition of the CDW filler decreased the water absorption and thickness swelling of the composites. An increase in the filler content improved the thickness swelling more significantly than water absorption.

Introduction

Construction and demolition industry generates a large quantity of solid waste around the world, causing environmental problems due to its uncontrolled disposal [1], [2], [3]. Tightened environmental regulations and laws concerning the recycling of construction and demolition waste (CDW) have been implemented in several countries [4], which has forced the companies to improve and encourage material efficiency and the recycling of wastes. For example, according to the binding legislation set by the EU, 70% by weight of non-hazardous CDW has to be prepared for re-use, recycled, or recovered by the year 2020 [5]. In addition, the new plastic strategy of the EU outlines that all plastic packaging placed on the EU market shall be either reusable or can be recycled by the year 2030 [6].

Variation in the volume and composition of CDW causes challenges for re-use and recycling. CDW consists typically of various materials, including metals, plastics, wood, gypsum, glass, insulation, brick, concrete, and others, many of which can be recycled. Waste materials from new construction are primarily clean and relatively uncontaminated such as unused or damaged raw materials as well as packaging materials [7], whereas demolition wastes are usually dirty or contaminated and mixed with other materials. These differences create specific challenges as well as opportunities for recycling and waste sorting technologies [7], [8], [9].

Wood-plastic composite (WPC) is an environmentally friendly material because the source of the raw material can be either virgin or recycled (non-virgin) materials. The utilization of recycled or waste materials in WPCs reduces the manufacturing costs, energy and depletion of virgin materials. If the waste or recycled materials, especially plastics, are considered as new materials in WPC manufacturing, it is necessary to understand primarily the impact of the waste or recycled materials in WPCs [10]. Generally, composite material is defined to be a multi-phase system consisted of matrix material and reinforcing phase [11]. Typically, WPCs contain polymers as matrix, wood fibers as reinforcement and some additives. Virgin thermoplastic polymers are widely used in WPC manufacturing, but the use of recycled thermoplastics has increased significantly in recent years [10]. The most prevalent polymers are low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP) and polyvinylchloride (PVC). Possible additives include for example lubricants, colorants, ultraviolet stabilizers and flame-retardants [12]. The additives are used to improve the durability of composite [13]. Due to the hydrophilic nature of bio-based fibers and the hydrophobic nature of plastics, the most important additives are coupling agents, which are used to improve the adhesion between fiber and matrix. WPCs are typically used in outdoor applications, such as decking, siding and window framing, where these material are exposed to moisture, freeze–thaw actions and ultraviolet light in sunlight. This has raised concern about their durability, including mechanical properties and moisture resistance [14].

The raw materials and additives used in WPC manufacturing, as well as the processing methods and parameters, correlate with the mechanical, physical and thermal properties of WPCs [15]. The significant advantages of polymer matrix composite materials are their high specific strength and high specific modulus [11] as well as the properties of WPC material can be tailored to meet the needs of a specific application. The thermoplastic polymer matrix has an important role in the formation of the final properties of WPC material. The matrix distribute the applied load uniformly and transfer the loads to fibers. The addition of wood fiber components tends to increase mechanical properties of WPCs, such as the stiffness, while the thermoplastic component improves their moisture resistance [16].

In outdoor applications, the weathering properties are essential for the durability of WPC. At the surface of WPC, the wood components absorb water and swell. Water absorption can lead to a degradation of mechanical properties of WPC. As known, moisture absorption and thickness swelling increase with the increase in fiber content. However, the use of coupling agent as additive reduces water absorption and thickness swelling significantly [17]. Therefore, reduced moisture absorption leads to increased durability and dimensional stability [18]. Most of the moisture and mechanical properties of WPCs depend mostly on the interaction developed between wood and the thermoplastic material. Good interaction is essential to transfer stress from the matrix to the fibers and thus improve the mechanical strength of the composites [19]. Mechanical properties and dimensional stability of composites can be achieved by the addition of coupling agent in manufacturing of WPC [17]. Some mechanical properties, such as tensile strength, impact strength, and hardness of WPCs, can be improved with the addition of mineral fillers. On the bending properties, the use of mineral fillers has a negative effect. Furthermore, improved impact strength appears as increased brittleness of WPC [12], [17].

The potential of recycled wood fibers and polymers in WPCs has been widely examined in several studies that indicated recycled fibers and polymers can be used to produce WPCs with mechanical and moisture resistance properties comparable to WPCs made of virgin ones. Mechanical properties are more depending on the fiber content in the composite material than their origin. Kazemi Yasamin et al [20] found that with a fiber loading of 50 wt% composites made from mixed recycled plastics exhibited statistically higher flexural moduli compared with those of mixed virgin plastics. The study of Moreno and Saron [21] revealed that LDPE-waste/recycled wood composites with low fiber content (1.5–10%) had higher elongation and less expressive Young’s modulus compared to composites with fiber content above 10% is opposite. Taufig et al. [22] studied the mechanical properties of WPCs manufactured from recycled PP/PE blends reinforced with different kenaf fiber loadings. It was found that the maximum flexural strength was achieved with 30 wt% of fiber loading whereas the maximum modulus was at 60 wt% of fiber loading, whereas the addition of fiber loading was not observed to improve the impact strength of studied WPCs.

Also the effect of the use of recycled plastics in WPCs has been studied because the use of recycled thermoplastic polymers and natural fibers in WPC manufacturing has been an interesting alternative to produce polymeric materials of low cost, with sufficient properties for application and environmentally adequate [23]. Turku et al [24] studied durability of wood-recycled plastic composites and found that the influence of weathering was greater in composites made of plastic waste compared to a composite made of virgin material. After weathering, the mechanical properties, tensile and flexural, were reduced by 2–30%, depending on the plastic source. The other study of Turku et al. [25] revealed that the use of recycled plastics decreased the strength of WPCs due to incompatibility between the wood flour/plastic blend phases. The research also indicated that wood fibers significantly improved composite modulus, and the stiffness of the composites manufactured from recycled plastic material was remarkably higher compared to the reference made of virgin LDPE.

As well as the utilization of specific waste streams such as construction waste and industrial waste have been the subject of many research [26], [27], [28]. As well as, several studies have revealed that combining of different fillers, such as wood fibers and mineral fillers, in composites can be useful [29], [30], [31]. Hybrid composites combining at least two different type of fillers possess different properties that cannot be obtained with a single type of filler [32]. As known, CDW provide a wide range of recyclable different materials such as fibers, polymers and mineral fillers that can be utilized in composite manufacturing. Therefore, the potential of CDW as a source of recycled raw materials has to be studied more.

In this study, the use of CDW as a filler in composites was studied. Three types of composites were manufactured with different CDW filler contents. The effect of the addition of CDW on the mechanical properties and moisture properties of WPC was studied based on standard methods, and the experimental results were compared with reference values.