Secondary material streams have been shown to include potential raw materials for production. Presented here is a protocol in which CDW-plastic waste as a raw material is identified, followed by various processing steps (agglomeration, extrusion). As a result, a composite material was produced, and mechanical properties were analyzed.
Construction and demolition waste (CDW), including valuable materials such as plastics, have a remarkable influence on the waste sector. In order for plastic materials to be re-utilized, they must be identified and separated according to their polymer composition. In this study, the identification of these materials was performed using near-infrared spectroscopy (NIR), which identified material based on their physical-chemical properties. Advantages of the NIR method are a low environmental impact and rapid measurement (within a few seconds) in the spectral range of 1600-2400 nm without special sample preparation. Limitations include its inability to analyze dark materials. The identified polymers were utilized as a component for wood-polymer composite (WPC) that consists of a polymer matrix, low cost fillers, and additives. The components were first compounded with an agglomeration apparatus, followed by production by extrusion. In the agglomeration process, the aim was to compound all materials to produce uniformly distributed and granulated materials as pellets. During the agglomeration process, the polymer (matrix) was melted and fillers and other additives were then mixed into the melted polymer, being ready for the extrusion process. In the extrusion method, heat and shear forces were applied to a material within the barrel of a conical counter-rotating twin-screw type extruder, which reduces the risk of burning the materials and lower shear mixing. The heated and sheared mixture was then conveyed through a die to give the product the desired shape. The above-described protocol proved the potential for re-utilization of CDW materials. Functional properties must be verified according to the standardized tests, such as flexural, tensile, and impact strength tests for the material.
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.
1. Identification and pre-treatment
Material | Polymer / amount |
Wood | CA | Lubr |
CDW-ABS | ABS / 30 | 64 | 3 | 3 |
CDW-PP | PP / 30 | 64 | 3 | 3 |
CDW-PE | PE / 30 | 64 | 3 | 3 |
Table 1: The composition of the studied materials. The name of the sample consists of the included matrix component, recycled acrylonitrile butadiene styrene (ABS), polypropylene (PP), and polyethylene (PE) from the construction and demolition waste (CDW). The amounts of wood, coupling agent (CA) and lubricant (Lubr.) were the same in all samples.
2. Processing of WPC materials with extrusion technology after size reduction treatment
Material | Barrel T °C | Tool T °C | Melt T °C | Melt Pressure (bar) |
Feeding rate (kg/h) |
Avg.Screw speed (rpm) |
CDW-ABS | 181 ± 11.9 | 189 ± 14.7 | 177 | 50 | 15 | 14 |
CDW-PP | 170 ± 10.4 | 207 ± 8.62 | 164 | 37 | 15 | 15 |
CDW-PE | 167 ± 8.51 | 183 ± 10.1 | 164 | 59 | 15 | 13 |
Table 2: Processing parameters of the composite materials. (Values after the ‘±’-mark indicate standard deviations. Avg. = average)
3. Sampling of produced materials and analyzes of properties
To investigate the effect of CDW plastic polymer on the mechanical properties of WPC, three different polymer types as a matrix were studied. Table 1 presents the composition of materials and Table 2 reports the manufacturing processes. The material of CDW-PP requires a higher treatment temperature for tools but, correspondingly, melt pressure was lower compared to the other materials (CDW-ABS and CDW-PE).
Figure 1 presents the flexural strength of material (an average from 20 measurements) as bar charts, including standard deviations as an error bar. The highest flexural strength values were achieved with material containing a recycled ABS polymer in a matrix. Almost congruent high-strength quality was achieved in the material in which recycled PE polymer was used in a matrix. The lowest flexural strengths were achieved with material containing a recycled PP polymer in a matrix. Figure 1 also presents similar results for the flexural modulus of materials, which was measured simultaneously with the strength property. However, even though recycled ABS and PE polymers have congruent results as in the strength tests, the flexural modulus results were different. The recycled PE materials have a significantly lower modulus value compared to the value of recycled ABS polymer.
Figure 1: The flexural properties of the studied materials.
The flexural strength is presented in the solid color filled bars (red, green, and blue) and the flexural modulus is presented using the same colors in pattern-filled bars. The standard deviations are described as error bars. Please click here to view a larger version of this figure.
Figure 2 shows the tensile strength and modulus (an average from 20 measurements) as bar charts, including standard deviations as an error bar. The materials, in which recycled ABS and PE were used, have almost congruent tensile strength results but the standard deviation was higher for the material in which recycled ABS was used. The weakest tensile strength was achieved material containing a recycled PP polymer in a matrix. The results of tensile modulus were congruent with the results of flexural modulus, in which the best modulus was achieved with the recycled ABS polymer.
Figure 2: The tensile properties of the studied materials.
The tensile strength is presented in the solid color filled bars (red, green, and blue) and the tensile modulus is presented using the same colors in pattern-filled bars. The standard deviations are described as error bars. Please click here to view a larger version of this figure.
Figure 3 displays the impact strength properties of materials (an average from 20 measurements) as bar charts, including standard deviations as an error bar. The impact strengths of recycled ABS and PP polymers were almost at the same level, but greater impact strength was achieved with the recycled PE polymer, which had the best impact strength property in this study.
Figure 3: The impact strength properties of the studied materials.
The impact strength is presented in the solid color filled bars and the standard deviations are described as error bars. Please click here to view a larger version of this figure.
ABS polymer consists of three monomers, which might increase the favorable behavior within the WPC. For example, the acrylonitrile component contributes strength, butadiene components contribute impact resistance, and styrene components contributes rigidity. PE-based WPC accounts for the largest market share, for examples in North America, and it is easy to nail, screw, and saw. However, PE is manufactured in various polymeric forms, such as high-density polyethylene (HDPE) and low-density polyethylene (LDPE), which have different features. The PP-based WPC had the weakest properties in this study, consistent with the fact that its market share is relatively small. Although it has several superior properties compared to polyethylene, such as being lighter and stronger, it is also more brittle than polyethylene21.
Overall, recycling of composites is the ecologically preferable pathway8, and recycled waste plastic is a suitable raw material for composites, in which performance can be improved using compatibilizers22. The reason for varied mechanical properties might be due to the composition of materials and, in particular, the coupling agent may have a significant effect. The mechanical properties of recycled polymers in WPC were improved with compatibilizers but the effects depend strongly on the agent used and its amount in the structure, causing a large variation between the used agents23. A previous study indicated that the highest performance of PP based WPC was achieved with amounts of compatibilizers at three percentage levels24, which is congruent with the amount used in this study. Thus, the coupling agent used might be more problematic than the level of agent. However, it is generally accepted that the mechanical performance of WPCs is improved when coupling agents are used under optimized conditions25.
Each polymer has individual features in material, demonstrating that separation of polymers increases the value of WPC with the correct additives. In future, novel eco-friendly alternative coupling agents for recycled polymer composites might be used to meet demand, such as the starch gum shown in a new study of Rocha and Rosa26. Additionally, the re-utilization of plastic must make economic sense, and thus also require future action.
The mechanical properties of WPC play an important role in deciding the suitability of these products in various applications. WPC consists of three main ingredients: plastic, wood, and additives. The mechanical properties of fiber-based composites depend on the length of the used fiber, where “critical fiber length” is the term used to indicate sufficient reinforcement25. In addition to the properties of ingredients, the quality of raw materials is the important factor for the performance of WPC. In this study, in particular, where recycled raw materials were used, a lot of attention was placed on the raw materials. This study used materials sourced from CDW which can vary between construction sites, and this variability is a critical factor in the comparison of different studies. Therefore, material must be studied according to the standardized tests that ensure uniform product quality.
In a flexural test, the WPC material experiences compressive stress at the load-bearing side and correspondingly, tensile stress at the opposite end. The test method is based on the standard of wood-based panels (EN 310), illustrating the flexural properties of an extruded profile in actual use. The flexural test will cause compression (on the upper surface) and tensile (on the underside) stress for the material, therefore it is important that the extruded (hollow) profile is symmetrical. Another test for flexural property (for example, the standard EN ISO 17827), where the dimensions of the sample were smaller, will not yield the real valued for the extrusion profile used but will analyze the property of the material without the effect of a hollow profile. It is important to use a standardized distance between the support spans because this has an influence on the results. The flexural strength depends linearly on the support span, in which an increased support span leads to a proportional decrease of the load28.
Generally, tensile modulus increases with the increasing content PP polymer within wood fiber25. Therefore, we can assume that the composition of materials, including additives such as a coupling agent, were not optimal for this material. The highest variation between the thicknesses of tensile test samples was 0.94 mm; this variation indicates the fastening of samples is a critical step. The testing machine included pneumatic fasteners that cause superfluous force with the various thicknesses of samples. Therefore, the force measurement must be reset at the start of the tensile test in order that the pneumatic fasteners will not distort the results. Alternatively, this troubleshooting could be eliminated by manufacturing homogenous test samples during the sampling phase.
The impact strength test illustrates a different mechanical feature of the material because it measures a momentary strain, while most of the other tests measure the long-term strain of the material. The increasing content of wood fiber decreased the impact strength25. The dimensions of the samples must be measured in all tests, and there may be variations between researchers in the use of measuring devices (e.g., compression force in the use of a caliper or micrometer). Therefore, it is important that the same person measures the dimensions of samples in every test, thereby excluding human errors in the measurements. Another option as a modification technique is to use a device that includes a moment for compression. In addition, the test atmosphere may have an influence on the studied properties. In this study, all studied tests were performed under the same conditions, so the effect of atmosphere was similar, and had a coincident effect for every test. As a future application, the tests could be performed in a room where atmosphere is set to be stable.
Because WPC consists of at least two materials, such as wood and polymer, it can complicate the selection of a standard. For example, there may be suitable standards for wood materials, as well as for polymer materials, that will cause limitations in the selection of an appropriate standard for study. The standard organization has published standards (EN 15534-1:2014+A1:2017) in which test methods for composites made from cellulose-based materials and thermoplastics were characterized. The standard allows researchers who follow the European Standard to act in a universal way in their studies. A complication may arise if a significant portion of researchers follows another standard (e.g., ASTM International), which will cause problems in the comparisons of results. A future development may be a single standard organization whose standards would be valid globally.
The standards of WPCs include detailed instructions for the measuring of properties but the interpretation of these may vary between researchers. Benchmarking between research organizations could unify operation methods but may not be allowed because the research organizations are often restricted institutions dealing with confidential information. Therefore, this kind of visually described work ensures test practices are universal for a wider number of people, thereby restricting the possibilities for misunderstanding.
The authors have nothing to disclose.
The authors acknowledge the support of the LUT RESOURCE (Resource efficient production processes and value chains) research platform coordinated by LUT University and the by the Life IP on waste—Towards a circular economy in Finland (LIFE-IP CIRCWASTE-FINLAND) project (LIFE 15 IPE FI 004). Funding for the project was received from the EU Life Integrated program, companies, and cities.
Agglomeration | Plasmec | TRL100/FV/W | apparatus of turbomixer |
Agglomeration | Plasmec | RFV 200 | apparatus of cooler |
CNC router | Recontech | F2 – 1325 C | CNC machine |
Condition chamber | Memmert | HPP260 | constant climate chamber |
Coupling agent | DuPont | Fusabond E226 | commercial coupling agent additive |
Crusher 1 (crusher/shredder ) | Untha | Untha LR 630 | 10-20 mm sieve |
Crusher 2 (low-speed crusher) | Shini | Shini SG-1635N-CE | 5 mm sieve, granulator |
Extruder | Weber | Weber CE 7.2 | conical counter-rotating twin-screw |
Lubricant | Struktol | TPW 113 | commercial lubricant additive |
NIR spectroscopy | Thermo Fisher Scientific | Thermo Scientific microPHAZIR PC | |
Recycled material ABS from CDW | |||
Recycled material PE from CDW | |||
Recycled material PP from CDW | |||
Sliding table saw | Altendorf | F-90 | circular saw/sliding table saw |
Testing apparatus | Zwick | 5102 | impact tester |
Testing machine | Zwick Roell | Z020 | allround-line materials testing machine |
Wood flour (Spruce) material | |||
WPC example material | UPM Profi | Decking board |