1. Introduction
The global production of plastic materials in 2016 was 335 million tons, and in Europe, plastic demand totaled 60 million tons in the same year. From that amount, different polyolefin (i.e., polyethylene and polypropylene) grades composed 49.1%. In Europe, in the year 2016, 27.1 million tons of post-consumer waste was collected, of which less than a third, 31.1%, was recycled; 41.6% was consumed in energy recovery but still 27.3% went to landfill [
1]. It has been estimated that approximately 4% of global oil and gas production is used as raw material for plastics production, and in addition to that, a similar amount of fossil fuel is consumed to provide the energy needed for the production [
2].
Both incineration and disposal of wastes and energy production needed for producing virgin materials create carbon dioxide emissions. Carbon dioxide is the primary greenhouse gas produced by human activities and has a notable influence in promoting climate change [
3,
4]. All these factors promote recycling materials found in waste streams. Recycling and reusing materials is not only important ecologically but also economically, as the cost of discarding waste has increased and can be expected to continue to increase, so multiple strong drivers promoting the reuse of waste materials exist. During the past few years, in addition to growing environmental awareness, also legislative actions, e.g., Directive 2008/98/EC on waste [
5] (
Figure 1.), have strongly encouraged the development of new uses and applications for discarded materials including waste plastics.
In order to decrease the use of virgin plastics made from non-renewable fossil oil, it would be environmentally sustainable to reclaim plastics from waste streams and to recycle them with minimal effort. The properties of commingled plastics are often poor, so upgrading their performance to meet the requirements of industry and market is crucial. Unfortunately, that is challenging both from economical and technical perspectives [
6]. For recycling of plastic waste, mechanical recycling is the most commonly used process, which typically consists of material collection, sorting, washing and grinding [
7]. If the process enables substitution of virgin polymers in industrial applications, high ecological benefits can be achieved [
8]. The current methods used for the sorting of mixed plastic waste depend on the differences in various properties of plastics. These methods rely on spectral, optic, electrostatic and gravimetric differences of the materials, i.e., their physical and chemical properties [
9]. The most common plastic sorts, polyolefins (i.e., polyethylenes and polypropylene), are close to each other not only chemically but also in density, rendering the simplest mechanical separation methods futile. On the other hand, this indicates that, e.g., polyolefins can be extracted from waste streams simultaneously by e.g., water sink-float method, which is technically simple and inexpensive to operate [
9,
10].
Unfortunately, most polymers are not compatible with each other and also contain impurities, especially if they originate from waste streams. Even in the case of polyolefins, simple mixing of different waste polymers leads typically into an immiscible blend, which has limited or no commercial use due to its inadequate mechanical properties. The immiscible blends lack strong interfacial bonding, resulting also in weak morphological stability, creating an increased risk of phase separation in further processing and use [
6,
11]. In order to overcome these shortcomings, compatibilizing agents can be used. They promote miscibility by lowering interfacial tension between the polymers, hence enhancing the interfacial adhesion. This results in a more stable morphology and more uniform distribution of the dispersed phase [
12]. Wood-polymer composites (WPC) are materials or products consisting of one or more natural fibres or flours and one or a mixture of polymer(s). The natural fibres and flours come from different sources, and different polymers, virgin or recycled, are used. Currently, the main applications of WPC products are decking, cladding, panelling, fencing and furniture [
13]. To improve the mechanical properties of WPCs, good adhesion between the wood fibre and polymer matrix is required. Unfortunately, both components are naturally incompatible due to the hydrophilic nature of wood materials and hydrophobic nature of polymers. Compatibility in the composite can be improved by using either physical or chemical modification of the polymer or wood fibre or by using compatibilizers/coupling agents [
14]. Compatibilizers/coupling agents can improve the performance of WPC significantly. Klyosov [
15] states that properly employed coupling agents can double the tensile and flexural strengths of WPCs, increase the stiffness up to 40% depending on the test method, at least double the impact resistance, and increase the density and decrease the water absorption by two to four times, depending on the duration of immersion.
As WPCs are a mixture of materials by nature, it can be speculated that reusing mixed waste plastics could be easier within composites than as pure raw materials [
16] and also gain environmental benefits compared to the use of conventional virgin plastics [
17]. The use of recycled plastics in WPCs in order to produce more environmentally friendly materials has also drawn the attention of many researchers, suggesting the potential of using recycled plastics. It has been shown that compatibilizers can improve the properties of wood–waste plastic composites [
9,
18,
19,
20,
21,
22,
23,
24,
25,
26], but the focus has not been on evaluating the potential and effects of multiple compatibilizers. In the case of mixed waste plastics, immiscibility of different polymers combined with the natural incompatibility between wood material and the polymer matrix is a challenge in utilizing mixed waste plastics in WPCs. However, the compatibilizers used in WPC are typically maleated/maleic grafted polyolefins [
27,
28], showing improved properties also in WPCs made from PE–PP blends [
29]. Maleic grafted polyolefins can also be used for the compatibilization of polymer blends [
30], suggesting that the use of maleated compounds in WPCs made of mixed waste plastics could provide improved performance.
There are strong environmental and legislative drivers towards utilizing plastic waste with other methods than disposal. Composites are naturally mixtures of different materials, so it is reasonable to expect that it would be easier and less expensive to utilize mixed waste plastics in them [
16]. As noted above, WPCs may be made from recycled plastics, and in general, the properties of WPCs can be improved by the addition of compatibilizers. In this study, the possibility of improving the properties of WPCs made from commingled plastic waste with the addition of compatibilizers is examined in order to find new, feasible means for recycling and use for the millions of tons of plastic waste produced annually. The effect of different levels of addition of compatibilizers and the potential of compatibilizer blends on the properties of WPCs is also evaluated.
4. Discussion
The physical and mechanical properties of wood-commingled waste plastic composites manufactured by using different compatibilizers were studied. The results indicate that the properties can be improved by using compatibilizers, and different compatibilizers can impact different properties significantly, thus enabling recycling of mixed waste plastics in WPCs.
Moisture resistance could be improved significantly by the addition of the selected compatibilizers. The reduction of water intake was decreased to one third of that of NC with MAM-7. With four of the materials (N525, N416, CM and MAM) at the 7% compatibilizer level, the water absorption at 28 days was more than halved compared to the reference. Of those, only N525 provided the same performance also at the 3% addition level. In the case of thickness swelling, the results were congruent with water absorption, except that N525 swelled more at the 7% addition level (4.33%) than at the 3% level (3.21%). Similar behaviour with an increased MAPP content in WPCs has been reported previously by Ashori and Nourbakhsh [
19]. Interestingly, in both water absorption and thickness swelling, material N525 performed better with 3% compatibilizer level than with 7% level. With all the other compatibilized materials, the swelling decreased by over 40% as the compatibilizer level increased, and a similar trend was found in the case of water absorption. The wood component in composites is mainly responsible for the water absorption and thickness swelling. The compatibilizers improve the properties of WPC due to the formation and increase of ester linkages between the hydroxyl groups of wood and the anhydride part of maleic grafted polymers [
38]. The low performance of N416-3 suggests a lack of linking between the wood and compatibilizers, and thus a higher number of free hydroxyl groups in wood are capable of interacting with moisture. An increase in water absorption and thickness swelling as the compatibilizer content increased was found in N525. The moisture behavior of composites N416 and N525 can also be explained by their compatibilizer composition, as both are also used for polyamide blends, and polyamide is hygroscopic. An increase in water absorption as the amount of compatibilizer increases has been previously reported with recycled PET-PP blends [
39]. Mechanical strength was in general affected and improved only by two compatibilizers, N525 and MAM. A higher level of addition gave better results; especially with MAM, the change was from a 22% improvement to a 60% improvement in tensile strength as the addition level rose from 3% to 7%. A similar trend was evident with flexural strength with MAM; in the case of N525, the improvement was smaller, from 21% to 25%, and not strongly related to the addition level. The increase in strength properties suggests improvement in adhesion between the wood component and the matrix in the composite, and signs of this can be seen in
Figure 3. Both tensile and flexural modulus were decreased by more than 20% with N416, and impact strength was improved by 49% with N416-7. As fillers are generally responsible for the stiffness of composites, this result suggests improved compatibility between PP and PE in the matrix phase. The finding indicates that N416 can be used to modify the properties of composites towards flexibility and elasticity.
The level of addition had an apparent impact on the effect of the compatibilizers. In all cases, the trend was such that a 7% addition level had a stronger effect on the properties than a lower level, or a 7% addition level was needed to cause any notable change in the properties. Relatively high melt flow indexes along with low melt temperatures (
Table 1) suggest that most of the compatibilizers studied are closer to elastomers in physical properties than to PP or HDPE. The compatibilizers with higher melt flow properties, i.e., N416 and CM (composed of Fusabond N416 and Entira EP1754), did not provide improvement in the tensility of flexural strength properties. The increase in viscosity of maleic compatibilizers with an increase in molecular weight has been reported by Kazushige et al. [
40]. As the length of the individual polymer chain is proportional to molecular weight, this indicates that strength properties of the compatibilizer itself with high MFI and low melting temperature are expected to be relatively low on tensile strength and stiffness but higher on impact strength [
41]. In general, impact strength and moisture resistance were strongly increased with an increase in compatibilizer content. In the case of N416 and CM, no notable effect on the strength properties was found, suggesting poor adhesion in the composite. Hence, it can be speculated that 7% addition level increased the performance also by the addition of polymer content in addition to interactions in the matrix phase. It is known, that if cellulosic fibers were totally encapsulated by a hydrophobic polymer, they were protected from moisture. At wood loadings of 40% or higher, the encapsulation of wood fibers in the composite is not complete [
15,
42,
43]. With 7% compatibilizer addition, the fiber content in composites was reduced to 50%, which is relatively close to the 40% threshold.
The best overall single additive solution tested was N525 at the 7% addition level. Also, depending on the raw materials used in the production of the composites, it appears that the mixing of different compatibilizers may be feasible. Good overall performance compared to the reference was found in the composite made with a mixture of two compatibilizers, MAM, at a 7% dosage. This compatibilizer also increased the tensile modulus, providing more stiffness.
Comparison of the best performing materials produced with commercial WPCs (
Table 9) show that the best compatibilization studied improves the performance of the composite close to that of commercial materials. Also, there was no difference of a magnitude between the performance of materials C1 and C2, suggesting that the use of one kind of recycled polymer to replace virgin polymers is feasible. However these materials had a significantly higher moisture absorption than the other materials, which can be attributed to their higher wood content, which is irrelevant to the polymer source. The performance of the N525-7 and MAM-7 made of mixed waste polymers is in the range or above the lowest reported values of the commercial materials, but a substantial improvement is still needed to meet the performance of the best performing commercial materials. It has to be noted that different test standards were used to evaluate the properties of commercial WPCs, making a direct comparison impossible.
A relatively high amount of compatibilizers was studied, and the best performance of the composites was achieved at the highest 7% level of compatibilization. This is in contradiction with some previously published papers showing that the highest performance of WPCs are achieved at compatibilizer levels of 3–4%, and the performance starts to decrease at higher levels [
27,
44]. This suggests that compatibilization of plastic mixtures with wood fibres is more challenging, and to achieve an increase in mechanical performance, a higher compatibilizer content that is typically used is required. The optimization of the amount and type of compatibilization in wood-mixed waste polymer composites still needs further research.