Wood-PHA Composites: Mapping Opportunities

Polyhydroxyalkanoate (PHA) biopolymers are emerging as attractive new sustainable polymers due to their true biodegradability and highly tuneable mechanical properties. However, despite significant investments, commercialisation barriers are hindering the capacity growth of PHA. In this work, we investigated the market potential for wood plastic composites (WPCs) based on PHAs. We considered the latest global production capacity of PHAs, estimated at 66,000 tonnes/year, and examined the implications of using PHAs for WPC production on the WPC market. Results indicate that a hypothetical usage of the current global PHA production for WPC manufacture would only represent the equivalent of 4.4% of the global WPC market, which is currently experiencing a 10.5% compounded annual growth rate. An economic assessment revealed that a wood-PHA composite as a drop-in alternative WPC product could cost as little as 37% of the cost of its neat PHA counterpart. Thus, WPCs with PHA offer a means to access benefits of PHA in engineering applications at reduced costs; however, further developments are required to improve strain at failure. The successful adoption of wood-PHA composites into the market is furthermore reliant on support from public sector to encourage biodegradable products where recycling is not a ready solution.


Introduction
The scale of global polymer production, which ramped up to 322 million tonnes/year in 2015 [1], is raising serious concerns regarding end-of-life disposal and plastic contamination of the environment. Between 22% and 43% of polymers used worldwide are disposed of in landfill, and approximately 10-20 million tonnes of plastic ends up in oceans each year [2]. Governments are gradually legislating for a stronger emphasis on the circular economy, by introducing bans on landfills and enforcing the use of bioplastics for certain applications, such as shopping bags. Bioplastics are, by definition, sourced from a renewable resource (biosourced) and/or biodegradable. Amongst the bioplastics, polyhydroxyalkanoates (PHAs) have attracted significant interest as promising new sustainable biopolymers due to their combined low environmental impact (true biodegradability and non-toxicity) and high functionality (tuneable mechanical and physical properties).
Commercialisation of PHAs is still in its early stages, and although its global production capacity is one of the fastest growing amongst biopolymers [3], small capacities and volumes are associated with relatively higher costs, which makes it challenging to compete up-front on just price with petroleum-based plastics that are produced on a very large scale. As a relatively new family of Global production is hindered and this limits the extent and opportunity for the development of PHA-based products. In contrast to a 10% annual growth between 2012 and 2014, the bio-based polymer production capacity growth data now shows a 4% annual growth rate from 2014 to 2016, which is almost the same as for the overall global polymer production [3]. Although PHA exhibits the fastest growth rate, the overall lower than expected annual growth rate of bio-based polymers has been reported to be due to several factors [3,5]: (1) the low oil prices, (2) an unfavourable political framework in most countries, (3) a slower than expected growth rate of the capacity utilisation, and (4) societal concern in the adoption of new bio-products. Examples for the latter include genetically modified feedstock, biodiversity loss through monocultures, rainforest depletion by palm oil imports, and the debate about the use of food crops to feed chemicals for industrial applications. These are all examples of the importance of trust in bioproducts by end-users and risks when a raw Global production is hindered and this limits the extent and opportunity for the development of PHA-based products. In contrast to a 10% annual growth between 2012 and 2014, the bio-based polymer production capacity growth data now shows a 4% annual growth rate from 2014 to 2016, which is almost the same as for the overall global polymer production [3]. Although PHA exhibits the fastest growth rate, the overall lower than expected annual growth rate of bio-based polymers has been reported to be due to several factors [3,5]: (1) the low oil prices, (2) an unfavourable political framework in most countries, (3) a slower than expected growth rate of the capacity utilisation, and (4) societal concern in the adoption of new bio-products. Examples for the latter include genetically modified feedstock, biodiversity loss through monocultures, rainforest depletion by palm oil imports, and the debate about the use of food crops to feed chemicals for industrial applications. These are all examples of the importance of trust in bioproducts by end-users and risks when a raw material production is judged within restricted assumed context. However, better communication with end-users, through certifications and quality labels, is expected to assist growth in successful market introduction. In the case of PHA, we postulate that the main two commercialisation barriers are: (1) the production cost of PHA compared to petroleum-based polymers (and indirectly to oil prices), and (2) hindered market adoption due to limited consumer experience with these materials. Both of these factors work against investment in production, which would, in turn, establish supply chains that promote the discovery of benefits and opportunity in market experience. Markets that do not exist cannot be analysed and predicted with any kind of confidence. In the end, it has been repeatedly shown that suppliers and customers must ultimately discover them together [12]. In the present work, we consider the use of novel PHA-based WPCs, as one potential market entry point for PHA-based materials.

Wood-PHA Composites as Novel WPCs
Wood plastic composites (WPCs) refer to any composites that contain wood (of any form) and a polymer (thermoset or thermoplastic). The first WPCs date back to the early 1900s, and were composed of phenol-formaldehyde (a thermoset) and wood flour, marketed under the trade name Bakelite. Today, most WPCs focus on wood-thermoplastic composites, with the general understanding that the 'plastic' is a thermoplastic. Examples of WPC products are shown in Figure 2. The growth of the WPC industry involved the interfacing of two industries that historically knew little about each other, and had very different expertise and perspective. The plastics industry has knowledge of plastics processing, and the forestry products industry has more experience and resources in the building and paper product markets. Thus, plastic processors often lacked knowledge about wood, and were reluctant to replace existing fillers (glass, or carbon) with wood fibres, even though they are from a renewable resource, and are less expensive, lighter and less abrasive to processing equipment. The main concerns associated with wood flour/fibres were low bulk density (difficulty to feed into typical plastic processing equipment), low thermal stability above 200 • C, and tendency to absorb moisture [13]. However, these issues were gradually resolved [14]. Thermoplastics were selected that melt or could be processed at temperatures below the thermal degradation temperatures of the wood fibres, such as (in order of market share) specific grades of polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC) [15]. Technological advancements were made in compounding, which facilitated processing and minimised moisture uptake. For instance, a novel patented technology by Scion (Rotorua, New Zealand) can convert high temperature thermomechanical pulp (TMP) fibres into a feedstock that can be readily used by the plastic industry, which was commercialised under the tradename Woodforce [16]. Another proprietary technology developed by Jeluplast (Rosenberg, Germany) consists of mixing wood fibres and the thermoplastic at elevated temperature and pressure to produce a uniform granulate in which each individual wood fibre is fully encapsulated in the polymer [17]. This method enables superior homogeneity and ease of processing and allows for wood contents of up to 70% to be achieved [18]. The WPC market has, since then, grown dramatically to a total volume of 3 million tonnes/year in 2014, and with expectation to double by 2020 [4].
Polymers 2018, 10, x FOR PEER REVIEW 4 of 15 material production is judged within restricted assumed context. However, better communication with end-users, through certifications and quality labels, is expected to assist growth in successful market introduction. In the case of PHA, we postulate that the main two commercialisation barriers are: (1) the production cost of PHA compared to petroleum-based polymers (and indirectly to oil prices), and (2) hindered market adoption due to limited consumer experience with these materials. Both of these factors work against investment in production, which would, in turn, establish supply chains that promote the discovery of benefits and opportunity in market experience. Markets that do not exist cannot be analysed and predicted with any kind of confidence. In the end, it has been repeatedly shown that suppliers and customers must ultimately discover them together [12]. In the present work, we consider the use of novel PHA-based WPCs, as one potential market entry point for PHA-based materials.

Wood-PHA Composites as Novel WPCs
Wood plastic composites (WPCs) refer to any composites that contain wood (of any form) and a polymer (thermoset or thermoplastic). The first WPCs date back to the early 1900s, and were composed of phenol-formaldehyde (a thermoset) and wood flour, marketed under the trade name Bakelite. Today, most WPCs focus on wood-thermoplastic composites, with the general understanding that the 'plastic' is a thermoplastic. Examples of WPC products are shown in Figure 2. The growth of the WPC industry involved the interfacing of two industries that historically knew little about each other, and had very different expertise and perspective. The plastics industry has knowledge of plastics processing, and the forestry products industry has more experience and resources in the building and paper product markets. Thus, plastic processors often lacked knowledge about wood, and were reluctant to replace existing fillers (glass, or carbon) with wood fibres, even though they are from a renewable resource, and are less expensive, lighter and less abrasive to processing equipment. The main concerns associated with wood flour/fibres were low bulk density (difficulty to feed into typical plastic processing equipment), low thermal stability above 200 °C, and tendency to absorb moisture [13]. However, these issues were gradually resolved [14]. Thermoplastics were selected that melt or could be processed at temperatures below the thermal degradation temperatures of the wood fibres, such as (in order of market share) specific grades of polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC) [15]. Technological advancements were made in compounding, which facilitated processing and minimised moisture uptake. For instance, a novel patented technology by Scion (Rotorua, New Zealand) can convert high temperature thermomechanical pulp (TMP) fibres into a feedstock that can be readily used by the plastic industry, which was commercialised under the tradename Woodforce [16]. Another proprietary technology developed by Jeluplast (Rosenberg, Germany) consists of mixing wood fibres and the thermoplastic at elevated temperature and pressure to produce a uniform granulate in which each individual wood fibre is fully encapsulated in the polymer [17]. This method enables superior homogeneity and ease of processing and allows for wood contents of up to 70% to be achieved [18]. The WPC market has, since then, grown dramatically to a total volume of 3 million tonnes/year in 2014, and with expectation to double by 2020 [4].  With the WPC market growth [4], end-of-life disposal/recovery concerns also increase. Fully recyclable products previously made entirely from PP or PE are now being replaced with WPCs, which are non-recyclable through conventional methods. Still, WPCs are marketed as recyclable products. However, a specific waste legislation classification for WPCs does not exist. The recycling depots face increasing quantities of WPCs that cannot be allocated to a particular area [19]. This issue is primarily due the combination of a renewable and biodegradable material (wood fibres) with a petroleum-based and non-degradable material (e.g., PP, PE), which traditionally require different routes for end-of-life management. In practice, only 'internal recycling', in which production residue is re-used during the manufacturing process, is being carried out by WPC manufacturers. According to one study [19], very limited investigations have been carried out for WPCs on their 'end-of-life recycling', in which the objects are materially or energetically recycled following a certain period of use. Wood-PHA composites offer a unique solution by providing a WPC where both the reinforcement and the matrix ingredients are entirely biosourced and also biodegradable.
The physical and mechanical properties of PHA biopolymers are particularly suitable for the manufacture of WPCs. PHAs exhibit a low melt viscosity. PP and PE grades suitable for injection moulding and extrusion typically exhibit a Melt Flow Index (MFI) in the range of 1 to 10 g/10 min at 230 • C, whereas PHB and PHBV grades of similar molecular weight (M w ), have been reported to have an MFI of 14.7 g/10 min at only 190 • C. During extrusion processing with wood fibres, a higher MFI can lead to a better impregnation of the fibres (wetting) and reduce the amount of mixing and shear intensity required for a homogenous fibre distribution. Initial developments on Wood-PHA composites [20][21][22][23] indicate that properties similar to commercially available PP and PE based WPCs are readily achievable. Table 1 summarises the mechanical properties of Wood-PHA composites in early development, compared to commercially available PE-and PP-based WPC products, showing that the tensile strength and tensile moduli were similar. These early results are promising, especially considering the absence of added compatibilisers, coupling agents, stabilizers, or any other functional additives in Wood-PHA composites. It is expected that commercialisation of Wood-PHA composites through the addition of additives, and industrial-scale process optimisation will further improve these mechanical properties [14]. However, current limiting factors in mechanical properties are an exhibited lower strain at failure, toughness and the resulting lower impact strength, as shown in Table 1. Nevertheless, these properties can be improved by incorporating existing solutions, such as the addition of plasticisers, rubber toughening particles, or the use of novel nano-reinforcements, such as cellulose nanofibers (CNF). Such property improvements within specific application contexts are key for expanding the use of Wood-PHA composites, allowing them to gain a significant market share.

Applications and Markets for Wood-PHA Composites
In 2016, the worldwide production capacity of bioplastics was reported to be 4.16 million tonnes/year [24], distributed across various markets but predominantly in packaging, consumer goods, and automotive sectors, as shown in Figure 3a. The main region for the production of biopolymers is Asia (43%), where production levels are almost as much as Europe and North America combined (at 50%) ( Figure 3b). From this 4.16 million tonnes/year market, the production of PHAs represents only 1.6% [3], and this reflects the PHA industrialisation infancy.
1 Tested according to ASTM D638 for Wood-PHA Composites and ISO 527-2 for commercial products. 2 Tested through standardised Charpy Test according to ISO 179-1/1eU.

Applications and Markets for Wood-PHA Composites
In 2016, the worldwide production capacity of bioplastics was reported to be 4.16 million tonnes/year [24], distributed across various markets but predominantly in packaging, consumer goods, and automotive sectors, as shown in Figure 3a. The main region for the production of biopolymers is Asia (43%), where production levels are almost as much as Europe and North America combined (at 50%) ( Figure 3b). From this 4.16 million tonnes/year market, the production of PHAs represents only 1.6% [3], and this reflects the PHA industrialisation infancy.  [25]; and (b) by region [3]. Data from European Bioplastics, Nova-Institute (2016).
The steadily growing WPC market is similar in volume to the global bioplastic market, and stood at 3.0 million tonnes/year in 2014 [4]. In Europe, the WPC market combined with the Natural Fibre Composites (NFC) market represents 15% of the total composite production (which includes glass and carbon synthetic fibres) [26]. This is a significant market share, and the growth of the WPC market al.one, forecasted at a compound annual growth rate (CAGR) of 10.5% [4], is less reliant on government mandated regulations and incentives compared to the bioplastics market in general. Applications for the WPC market differ noticeably compared to the bioplastics market. Decking (67%) and automotive interior parts (24%) are principal applications, as illustrated in Figure 4. However, technical applications, consumer goods, and furniture have been reported to exhibit the highest growth rates [27]. Considering attractive mechanical properties of early Wood-PHA Composites (Table 1), potential applications for these novel materials include automotive interior parts, consumer goods, furniture, and technical applications, which represent 28% of the total WPC market. Long-term performance in a range of environmental conditions is a focus of ongoing research, thus applications involving exposed outdoor environments (decking, siding, fencing, etc.) are not yet suitable to be included in the evaluation of the potential markets. Therefore, the potential market for Wood-PHA composites can be estimated to be around 840,000 tonnes/year (28% of the total WPC market). The market share will be more dependent on the price of PHA compared to PP and PE, if the use is to compete simply as a drop-in substitute. Therefore, in these cases, an important factor is the political framework, such as government legislation or incentives where the downstream benefits of biopolymers are also considered in the equation of cost. For bioplastics, the correlation between political framework and market success is very high. A positive framework can help to encourage initial market growth and investment, whereas a negative setting will put successful developments at stake. In 2011, Italy introduced a ban on plastic bags [28], and today it is the highest consumer of bioplastics in Europe [29]. France adopted the ban on plastic bags in 2016, and has now become the first country to ban plastic cups, plates and cutlery, by 2020 [30]. With these increasing trends to ban selected petroleum-derived products, conventional WPCs are likely to face similar application challenges in the coming decades. We postulate that the actual market share for The steadily growing WPC market is similar in volume to the global bioplastic market, and stood at 3.0 million tonnes/year in 2014 [4]. In Europe, the WPC market combined with the Natural Fibre Composites (NFC) market represents 15% of the total composite production (which includes glass and carbon synthetic fibres) [26]. This is a significant market share, and the growth of the WPC market al.one, forecasted at a compound annual growth rate (CAGR) of 10.5% [4], is less reliant on government mandated regulations and incentives compared to the bioplastics market in general. Applications for the WPC market differ noticeably compared to the bioplastics market. Decking (67%) and automotive interior parts (24%) are principal applications, as illustrated in Figure 4. However, technical applications, consumer goods, and furniture have been reported to exhibit the highest growth rates [27]. Considering attractive mechanical properties of early Wood-PHA Composites (Table 1), potential applications for these novel materials include automotive interior parts, consumer goods, furniture, and technical applications, which represent 28% of the total WPC market. Long-term performance in a range of environmental conditions is a focus of ongoing research, thus applications involving exposed outdoor environments (decking, siding, fencing, etc.) are not yet suitable to be included in the evaluation of the potential markets. Therefore, the potential market for Wood-PHA composites can be estimated to be around 840,000 tonnes/year (28% of the total WPC market). The market share will be more dependent on the price of PHA compared to PP and PE, if the use is to compete simply as a drop-in substitute. Therefore, in these cases, an important factor is the political framework, such as government legislation or incentives where the downstream benefits of biopolymers are also considered in the equation of cost. For bioplastics, the correlation between political framework and market success is very high. A positive framework can help to encourage initial market growth and investment, whereas a negative setting will put successful developments at stake. In 2011, Italy introduced a ban on plastic bags [28], and today it is the highest consumer of bioplastics in Europe [29]. France adopted the ban on plastic bags in 2016, and has now become the first country to ban plastic cups, plates and cutlery, by 2020 [30]. With these increasing trends to ban selected petroleum-derived products, conventional WPCs are likely to face similar application challenges in the coming decades. We postulate that the actual market share for wood-PHA composites will strongly depend on government legislation in addition to technological advances in PHA production.
Polymers 2018, 10, x FOR PEER REVIEW 7 of 15 wood-PHA composites will strongly depend on government legislation in addition to technological advances in PHA production. The current top three applications for biodegradable polymer in the EU are plastic bags, rigid packaging, and disposable tableware [29]. The last two applications are particularly suitable for Wood-PHA composites, which exhibit a high stiffness. In addition, wood-PHA composites have a unique advantage of being 100% renewable and truly biodegradable WPCs. This offers an attractive solution for bio-based products, particularly for large companies, as it can act to promote sustainability. IKEA (Älmhult, Sweden) is one example of a large multinational company to have highlighted a commitment for their plastic materials to be 100% renewable and/or recycled by 2020 (UN Climate Summit, 2016). As part of this commitment, IKEA have recently signed a £10 billion agreement with a PHA producer (Newlight Technologies, Irvine, CA, USA), with an offtake promise to purchase 50% of the AirCarbon TM material from Newlight's 23,000 tonnes/year plant in the USA [31]. The use of PHAs in the IKEA product line will start with their home furnishing products, which represent approximately 40% of the total plastic volume used across IKEA [31]. In terms of WPCs, IKEA has already produced injection moulded WPC products, such as chairs [32], which represents a natural opportunity for wood-PHA composites materials. An additional partnership between Newlight Technologies and The Body Shop (London, United Kingdom), a large international cosmetics company, will aim at introducing PHA in The Body Shop's packaging range [33]. Other key applications, where advanced niche products have been showcased, include a lamp by the Italian designer company FLOS (Merano, Italy), made from Minerv ® PHA, developed by Bio-on in Bologna, Italy [34]. Bio-on is further developing and licensing the production of their PHA grade for a wide range of applications, including novel milk carton packaging from renewable origin and fully biodegradable [35]. Toys are also a relevant target for WPCs. In this sector, Lego (Billund, Denmark), the largest toy manufacturer in the world, announced in 2016 an investment of US$150 million, for a bio-based polymer solution to replace their current petroleum-based acrylonitrile butadiene styrene (ABS) plastic [36]. This series of announcements strongly suggests that there is coherent interest towards a goal to use bio-polymers like PHA and move away from petroleum-based plastics. We consider that the involvement of large international companies and supportive government legislation are two key factors that can facilitate market entry of PHAs, increasing the potential for growth in production capacity and consequently allowing for the development of wood-PHA composites.

Production Capacities for PHA and Wood-PHA Composites
The commercial production capacity of PHAs saw a steep increase following the rise in oil prices in 2003. Since then new plants opened in China, US, Italy and Brazil [5]. The current worldwide PHA production capacity (2016) is estimated at 66,000 tonnes/year (representing 1.6% of the global 4.16 million tonnes/year of bioplastics production) [3,24]. As a frame of reference, the global production capacity of bio PE (I'm green TM , from Braskem, São Paulo, Brazil) is estimated at The current top three applications for biodegradable polymer in the EU are plastic bags, rigid packaging, and disposable tableware [29]. The last two applications are particularly suitable for Wood-PHA composites, which exhibit a high stiffness. In addition, wood-PHA composites have a unique advantage of being 100% renewable and truly biodegradable WPCs. This offers an attractive solution for bio-based products, particularly for large companies, as it can act to promote sustainability. IKEA (Älmhult, Sweden) is one example of a large multinational company to have highlighted a commitment for their plastic materials to be 100% renewable and/or recycled by 2020 (UN Climate Summit, 2016). As part of this commitment, IKEA have recently signed a £10 billion agreement with a PHA producer (Newlight Technologies, Irvine, CA, USA), with an offtake promise to purchase 50% of the AirCarbon TM material from Newlight's 23,000 tonnes/year plant in the USA [31]. The use of PHAs in the IKEA product line will start with their home furnishing products, which represent approximately 40% of the total plastic volume used across IKEA [31]. In terms of WPCs, IKEA has already produced injection moulded WPC products, such as chairs [32], which represents a natural opportunity for wood-PHA composites materials. An additional partnership between Newlight Technologies and The Body Shop (London, United Kingdom), a large international cosmetics company, will aim at introducing PHA in The Body Shop's packaging range [33]. Other key applications, where advanced niche products have been showcased, include a lamp by the Italian designer company FLOS (Merano, Italy), made from Minerv ® PHA, developed by Bio-on in Bologna, Italy [34]. Bio-on is further developing and licensing the production of their PHA grade for a wide range of applications, including novel milk carton packaging from renewable origin and fully biodegradable [35]. Toys are also a relevant target for WPCs. In this sector, Lego (Billund, Denmark), the largest toy manufacturer in the world, announced in 2016 an investment of US$150 million, for a bio-based polymer solution to replace their current petroleum-based acrylonitrile butadiene styrene (ABS) plastic [36]. This series of announcements strongly suggests that there is coherent interest towards a goal to use bio-polymers like PHA and move away from petroleum-based plastics. We consider that the involvement of large international companies and supportive government legislation are two key factors that can facilitate market entry of PHAs, increasing the potential for growth in production capacity and consequently allowing for the development of wood-PHA composites.

Production Capacities for PHA and Wood-PHA Composites
The commercial production capacity of PHAs saw a steep increase following the rise in oil prices in 2003. Since then new plants opened in China, US, Italy and Brazil [5]. The current worldwide PHA production capacity (2016) is estimated at 66,000 tonnes/year (representing 1.6% of the global 4.16 million tonnes/year of bioplastics production) [3,24]. As a frame of reference, the global production  Figure 5. Production of wood-PHA composites is currently non-existent. A hypothetical usage of the global PHA production capacity for WPC manufacturing (at an average 50 wt % wood content) would lead to 132,000 tonnes/year of wood-PHA composites. This production volume represents only 4.4% of the global WPC market, which is currently growing at a rate of 10.5% [4]. Therefore, the current WPC market already has an ample volume and growth rate to absorb a significant investment in PHA production given meaningful drivers and applications for the biopolymer as the WPC matrix.
Polymers 2018, 10, x FOR PEER REVIEW  8 of 15 200,000 tonnes/year, and other bioplastics production capacities are shown in Figure 5. Production of wood-PHA composites is currently non-existent. A hypothetical usage of the global PHA production capacity for WPC manufacturing (at an average 50 wt % wood content) would lead to 132,000 tonnes/year of wood-PHA composites. This production volume represents only 4.4% of the global WPC market, which is currently growing at a rate of 10.5% [4]. Therefore, the current WPC market already has an ample volume and growth rate to absorb a significant investment in PHA production given meaningful drivers and applications for the biopolymer as the WPC matrix. Figure 5. Global production capacity of bioplastics in 2016 [24]. Source: European Bioplastics, Nova-Institute (2016).
Due to its fledgling market presence, the total production of PHA biopolymer is estimated in terms of capacity. Precisely assessing the actual production that is genuinely delivered to end-users is difficult to estimate with certainty, since most plants only disclose capacity and not production. The list of companies contributing to the estimated total capacity of 66,000 tonnes/year is summarised in Table 2. Production is conducted today by industrial pure culture fermentation; however, the processes vary in terms of substrate choice, bacterial strain, type of PHA, and method of integration. The scientific literature is rich with intent to establish production methods at reduced costs, including but not limited to the use of wastes and industrial residues as feedstocks, the use of mixed culture bioprocess, and the integration of PHA production with wastewater treatment services [37][38][39][40].
Having secured an agreement with IKEA to purchase 50% of their 23,000 tonnes/year production, Newlight Technologies can become a central player to stimulate the growth of the PHA market. A further report indicates a long-term intention to expand production capacity to 453,000 tonnes/year [31]. In addition, Newlight Technologies have recently signed a 15-year production license agreement with Paques Holdings (Balk, The Netherlands), allowing Paques to manufacture, process and sell PHA at a rate of up to 1.3 million metric tons per year [41].
Danimer Scientific (previously Meredian Holdings Group Inc. MHG, Bainbridge, GA, USA), purchased the full set of patents from the P&G company (Cincinnati, OH, USA) to develop a branched PHB polymer, which disrupts crystal formation, allowing for a more flexible and tougher material. Their product Nodax TM is available as foams, fibres or nonwovens, films and latex among others [42]. Their pilot plant in Bainbridge produces 13,600 tonnes/year of PHA, with construction beginning at the site for a plant that will produce 91,000 tonnes/year of PHA [43].
Bio-on (Bologna, Italy) is an intellectual property company, developing PHB polymers from sugar beet and granting licenses worldwide. In 2015, an agreement was signed with Cristal Union, a sugar production company in Aube, France, to open a 5000 tonnes/year production site (expandable to 10,000 tonnes/year) operational by 2018 [44]. More recently (December 2016), an additional agreement worth €55 million with a large multinational client was announced, which will see Bio-on construct a series of plants in Europe and Asia for an overall output of 100,000 tonnes/year [45]. Due to its fledgling market presence, the total production of PHA biopolymer is estimated in terms of capacity. Precisely assessing the actual production that is genuinely delivered to end-users is difficult to estimate with certainty, since most plants only disclose capacity and not production. The list of companies contributing to the estimated total capacity of 66,000 tonnes/year is summarised in Table 2. Production is conducted today by industrial pure culture fermentation; however, the processes vary in terms of substrate choice, bacterial strain, type of PHA, and method of integration. The scientific literature is rich with intent to establish production methods at reduced costs, including but not limited to the use of wastes and industrial residues as feedstocks, the use of mixed culture bioprocess, and the integration of PHA production with wastewater treatment services [37][38][39][40].
Having secured an agreement with IKEA to purchase 50% of their 23,000 tonnes/year production, Newlight Technologies can become a central player to stimulate the growth of the PHA market. A further report indicates a long-term intention to expand production capacity to 453,000 tonnes/year [31]. In addition, Newlight Technologies have recently signed a 15-year production license agreement with Paques Holdings (Balk, The Netherlands), allowing Paques to manufacture, process and sell PHA at a rate of up to 1.3 million metric tons per year [41].
Danimer Scientific (previously Meredian Holdings Group Inc. MHG, Bainbridge, GA, USA), purchased the full set of patents from the P&G company (Cincinnati, OH, USA) to develop a branched PHB polymer, which disrupts crystal formation, allowing for a more flexible and tougher material. Their product Nodax TM is available as foams, fibres or nonwovens, films and latex among others [42]. Their pilot plant in Bainbridge produces 13,600 tonnes/year of PHA, with construction beginning at the site for a plant that will produce 91,000 tonnes/year of PHA [43].
Bio-on (Bologna, Italy) is an intellectual property company, developing PHB polymers from sugar beet and granting licenses worldwide. In 2015, an agreement was signed with Cristal Union, a sugar production company in Aube, France, to open a 5000 tonnes/year production site (expandable to 10,000 tonnes/year) operational by 2018 [44]. More recently (December 2016), an additional agreement worth €55 million with a large multinational client was announced, which will see Bio-on construct a series of plants in Europe and Asia for an overall output of 100,000 tonnes/year [45].
Other companies, including Tepha Inc. (Lexington, MA, USA), PolyFerm Canada (Kingston, ON, Canada), and Terra Verdae Bioworks (Edmonton, AB, Canada) have focused on commercialisation of PHA for high-value biomedical applications. Their products range from absorbable sutures to heart valves, scaffolds, and controlled drug release [5,49], and consequently represent smaller, but high value, volumes. The use of PHAs for biomedical applications and their cytotoxicity, noncarcinogenicity and biocompatibility has recently been reviewed in [50].
Yield10 Bioscience, previously known as Metabolix (Woburn, MA, USA), made a significant investment in the production of PHA from corn sugars, and in 2009 announced the opening of the largest PHA plant with a production capacity 50,000 tonnes/year. However, in 2016, a decision was made to close their biopolymer activities, and their intellectual property and assets were sold in 2016 to an affiliate of CJ CheilJedang Corporation (Seoul, South Korea) [51]. Overall, production of PHA is found to be distributed across the USA, China, and Europe. With regards to actual production, and based on the limited commercial availability of PHA-based products, it appears that most companies are producing below capacity, meaning that overall production levels can be considered to be conservatively overestimated. PHA producers are being cautious about expanding their production capacities, to ensure that investment matches demand and consequently returns. During the initial stages of biopolymer developments, small capacities and volumes are typically associated with high costs, creating stronger incentives for large-scale production when this jump becomes feasible within the market. This is a general issue and is valid for any new material or product that has become large in the market in the past. In the case of PHA, we have previously mentioned that the main two commercialisation barriers are: (1) higher production cost of PHA compared to petroleum-based polymers (indirectly to oil prices), and (2) uncertain market adoption (which can be bolstered by government incentives). Initial risk for these materials can be due to the attempt to compete directly as a drop-in with established products and markets with mature and reliable supply chains. Disruptive technologies can gain market entry and initial growth through providing products and services that, of their nature, offer a unique benefit without being in direct economic competition [12].
The costs of producing PHA remain a main barrier to commercialisation within the traditional polymer application space. The price of PHA is dependent on the scale, choice of substrate, bacterial strain, and the type of PHA. For instance, an economic evaluation of PHB production by fermentation with the recovery method of surfactant-hypochlorite digestion was reported to result in a final price of 5.58 US$/kg for an annual production of 2850 tonnes/year. As the production scale increased to 1 million tonnes/year, the price of PHB dropped to 4.75 US$/kg [58]. The cost of substrate was also reported to significantly affect the overall economics of PHA production [59]. In the case of PHBV copolymer, a study, specific to a type of bacteria, revealed that the production cost of P(3HB-co-3HV) increased linearly with the increase in HV fraction, from 4.3 to 5.7 US$/kg, for 3HV fractions from 5 to 50 mol % [60]. This is to emphasize the difficulty of comparing PHA prices, due to the complexity of specific PHA polymer types and purities within the PHA family. The costs requirements for a successful market entry of PHA cannot be strictly defined, since these will depend on the market and volumes, juxtaposed the context of the application and the relationships in the value chain. For biomedical applications, where biocompatibility and a high purity are critical, a market price above 10 US$/kg is acceptable. For properties similar to commodity polymers, a cost comparable to PP or PE would be expected, which is of the order of 1.6 US$/kg. Currently, the cost of PHBV, with mechanical properties similar to PP, is about double that of PP and PE, though the emerging producers, including Newlight Technologies, claim PHA can be produced at a price lower than petroleum-based plastics. This would suggest that PHAs have the potential to compete with petroleum-based plastic on price, and consequently gain significant share of their market. Notwithstanding the value of the material in the context of the application will most likely provide for a more stable business model. The cost of the material can also be made by compounding, wherein the fraction of PHA in the composite is reduced but it is still vital to the target of the material properties. Combining PHA with wood reinforcements for the production of WPCs would help in this way to reduce the cost by at least 40%, and provide a unique selling point as a biodegradable WPC. A preliminary economic assessment is essential to evaluate the advantages of this strategic approach.

Economic Assessment of Wood-PHA Composites
An economic assessment was conducted to investigate the cost benefits of producing wood-PHA composites, according to two different scenarios: (1) a wood-PHA composite with 50 wt % wood content, based on existing mechanical properties, and (2) a wood-PHA composite based on expected properties with a 60 wt % wood content. Both scenarios are compared to the production of a neat PHA product. Petroleum-based WPCs, which have benefited from several decades of development, are currently commercially available with wood contents typically around 50 wt % and 60 wt % [17], and can be as high as 70 wt % [18]. Various reports and claims on the cost of PHA lead towards a speculative price per kg in the range of approximately 2-8 US$/kg. In 2009, Metabolix was selling PHA pellets at 5 US$/kg [5,10]. Today, PHA pellets are being sold by TianAn Biopolymers at approximately 6.8 US$/kg, and Newlight Technologies claims to be able to produce them in the range of 2-3 US$/kg. For the purpose of this economic assessment, the cost of PHA is assumed to be 7 US$/kg (a conservative, yet realistic value). In comparison, the cost of PP and PE-HD (high density) is reported to be around 1.6 US$/kg, and 1.7 US$/kg respectively [61]. The cost of wood fibre reinforcement is estimated at 0.4 US$/kg. Thus wood-PHA composites with 50 wt % and 60 wt % wood contents, as shown in Table 3, would result in a raw material cost of 3.7 US$/kg and 3.0 US$/kg, respectively.
Wood fibres being hollow, their apparent density is highly dependent on their state of collapse, and can vary from approximately 0.4 to 1.1 g/cm 3 . In this assessment, radiata pine thermomechanical pulp fibres are considered to be almost fully collapsed after the extrusion process, and exhibit a density of 1.0 g/cm 3 (density of the fibre wall is 1.15 g/cm 3 ). The density of PHA biopolymer is 1.25 g/cm 3 . Consequently, the final densities of a 50 wt % and a 60 wt % wood-PHA composite are equal to 1.11 g/cm 3 and 1.08 g/cm 3 , respectively (Table 3). Thus, at equivalent weight, a 50 wt % wood content wood-PHA composite is capable of exhibiting a significantly higher stiffness (tensile modulus), as per Table 1, when compared to neat PHA. In product design, specific mechanical properties are often used to evaluate the amount of the material required in order to achieve a given performance. At equal performance, a high-specific-strength material may result in significant cost savings even though the raw material cost per kg is higher, since less material is required. Table 4 lists the specific properties of a 50 wt % wood content wood-PHA composite in comparison to neat PHA. As shown in Table 4, a 50 wt % wood content wood-PHA composite exhibits identical specific strength compared to neat PHA material, and a 250% increase in specific stiffness. However, an 85% reduction in strain at failure is observed. The economic assessment for a final product (excluding manufacturing costs) can be approached using two considerations. A first case is one in which wood-PHA material is introduced as a drop-in solution. Existing processing moulds will be used, and the final product will have an identical geometry and volume to the original product, with matching properties. The second case involves the design of a new product to meet specific performance stiffness requirements. This case is only valid for applications where stiffness is the driving factor for the product's performance, such as rigid packaging, disposable tableware or biodegradable plant pots. Considering both scenarios of product design, a further reduction in final product cost can be achieved, as shown in Table 3. A wood-PHA product of equal volume to a 7 US$ neat PHA product would cost 3.3 US$ and 2.6 US$ for a 50 wt % and 60 wt % wood content, respectively. This product would have a similar tensile strength, higher stiffness and lower strain at failure, compared to a neat PHA counterpart (as per Table 1). However, in applications where stiffness is the sole driving property, a wood-PHA composite product with identical stiffness to a 7 US$ neat PHA would cost as low as 1.5 US$, which represents only 21% of the neat PHA product cost, and is comparable with the cost of PP (1.6 US$/kg) and PE (1.7 US$/kg) [61].
Given the overall higher specific properties of wood-PHA composites, the addition of 50 wt % or 60 wt % wood fibres can consequently reduce the final cost of a product beyond its raw material costs. Further cost savings could be achieved for a 70 wt % wood content composite; however, this has not been considered in this assessment. Apart from the gain in certain specific properties, the investment risk of developing wood-PHA composites compared to a neat PHA product is in essence reduced since 50 wt % or more of the product is composed of a 'filler'.

Conclusions
Wood-PHA composites are considered to be a promising opportunity for PHA biopolymers, which could result in a reduced product entry cost equal to 21% of neat PHA cost, and benefit from a rapidly growing WPC market. However, the current drawback of wood-PHA composites is a low strain at failure, which results in poor impact strength and this narrows scope for applications. Although the strain at failure of commercially available WPCs has seen improvements over the last decade, these developments are yet to be made for wood-PHA composites. An added challenge is that even if wood-PHA composites bring a 21% cost reduction compared to similar neat PHA products, they still exhibit a higher cost compared to petroleum-based WPCs. Therefore, the motivation for the use of the biobased composites require benefits that are exclusive to these biodegradable materials in their service or in their after-service management.
The next steps for accelerating the commercial development of wood-PHA composite supply and value chains would involve support from government legislation to enforce environmentally friendly products. Upfront price competition does not necessarily consider downstream burdens and so a more expensive drop-in material cannot expect to succeed without a unique selling attribute of customer benefit, or commonly accepted societal need. Improving strain at failure of wood-PHA composites is identified as a principal composite property in order to broaden the application spectrum for these novel materials. Ultimately, we believe the final stages for a successful implementation of wood-PHA composites would require a genuine commitment to produce wood-PHA pellets on a large scale, coupled with a purchase agreement from a long-term end-user.