Upgrading Recycled Polypropylene from Textile Wastes in Wood Plastic Composites with Short Hemp Fiber

1. Introduction

Cannabis Sativa

plant family, an annual plant that grows in temperate regions like China. The main properties of hemp are good mechanical strength and Young modulus; cellulose content is between 55% and 72%, and lignin content is around 2–5% [

New materials research over the last 40 years has led to an increase in the number of wood plastic composites (WPCs). Currently, they can be used in many applications such as decking (indoor or garden furniture), packaging, or consumer goods [ 1 2 ]. The automotive industry is one of the areas where these composites have a great incursion due to their excellent weight-strength ratio. The use of natural fibers in polymeric matrices could reduce about 20% of the overall weight and 30% of the cost. Weight reduction in cars leads to fuel saving and as a result a reduction in greenhouse gases [ 3 ]. Natural fibers, such as hemp, flax, and jute, among others, are used as a reinforcement in thermoplastics due to their good mechanical properties and their ease of acquisition. This factor facilitates the replacement of synthetic fibers such as aramid, carbon, and glass currently used because the production of synthetic fibers is more expensive, and they have a negative effect on the environment [ 4 5 ]. Moreover, the use of bio-based materials with the aim to reduce the dependency on petroleum-based materials is an actual trend [ 6 ]. This trend is related to the principles proposed by the circular economy, which seeks to reuse or recycle wastes [ 7 ]. In this context, wool fibers obtained from industrial wastes are gaining considerable interest as plastics-reinforcing fillers [ 8 9 ]. Hemp fibers come from theplant family, an annual plant that grows in temperate regions like China. The main properties of hemp are good mechanical strength and Young modulus; cellulose content is between 55% and 72%, and lignin content is around 2–5% [ 10 11 ]. Hemp fibers are mainly used for textile applications. However, a bundle of short hemp fibers (SHFs) is usually excluded from textile use as a by-product mainly because of their poor resistance [ 12 ]. Therefore, WPCs based on SHF have gained considerable interest in several industrial sectors, such as automotive, building, and furnishing industries.

Annually, between 25 and 30 million tons of plastic waste are generated in Europe (in 2018, 29.1 million tons were generated [ 13 ]), and only about 30% can finally be recycled [ 13 14 ]. Polyolefins (polyethylene and polypropylene) are the most demanded plastic resins by industrial converters. Particularly, PP is highly demanded for several industrial sectors. Thus, one of the main components from the generated waste is polypropylene (PP) since it is employed in many applications because of its interesting properties such as low density, good processability, and a competitive price. Some of the applications can be injection molded parts for the automobile industry or household appliances, as well as single-use plastic products [ 15 16 ]. According to Gu et al. [ 17 ], recycling plastics could save 20–50% of the cost compared to using virgin materials. Other benefits of recycling thermoplastics are the reduction of pollution produced when they are incinerated and the reduction of the volume of waste in the landfill [ 15 ], as well as the reduction of petrochemical sources consumption for the production of virgin PP.

Nowadays, synthetic fibers account for about two-thirds of the fiber production every year, which consumes about 14.5% of the global plastic production. For this reason, part of this plastic waste comes from the textile industry where the production of synthetic fiber increased by 15% between 2017 and 2018 [ 18 ]. Following the recommendations proposed by the European Parliament in the Waste Framework Directive, if the textile cannot be used, it should be recycled as proposed in the 2008/98/EC directive [ 19 ]. There are some applications where recycled polypropylene can be employed. For instance, Yin et al. [ 20 ] proposed the introduction of recycled fibers as a reinforcement of concrete; they compared the difference between virgin fibers, an actual alternative to steel meshes in concrete. Similar results were obtained for both kinds of fibers, therefore they can replace the steel meshes.

As there is a huge amount of recycled thermoplastics, they can be a promising raw source for WPCs, especially because of the low cost of these kinds of materials [ 21 ]. However, wood fibers traditionally reported hydrophilic nature promoted by the hydroxyl groups in cellulose, hemicellulose, and lignin, which represents one of the main drawbacks of natural fibers as reinforcing fillers of thermoplastic polymers [ 22 ]. This behavior results in a lack of compatibility between the polymeric matrix and the fibers. Poor adhesion between the fibers and the polymeric matrix could promote water absorption and also a decrease in the mechanical properties [ 23 24 ], consequently leading to the deterioration of the properties during the aging of the WPC product. Thus, the high hydrophilic nature of wood fibers limits the WPC outdoor applications, such as windows, frames, furniture, decking, and construction materials [ 25 ]. Another important disadvantage of natural fibers for WPC production is their thermal sensitiveness since the fibers can be degraded either during composite thermal processing and/or during composite material service [ 26 ]. To this effect, there are several methods to improve the polymer/ natural fiber interaction, among which are the incorporation of nanoparticles or a silane treatment to the fibers, another possibility is to introduce compatibilizers or coupling agents based on maleic anhydride (MA) that promote the hydrogen and covalent bonds with hydroxyl groups of the cellulose [ 27 28 ]. Although PP has been widely reinforced with natural fibers compatibilized with PP-g-MA, recycled PP (rPP) WPCs have been less studied. In this context, Islam et al. reinforced rPP with kenaf fibers using PP-g-MA as compatibilizer and observed a significant reduction of the water absorption of rPP/kenaf fiber due to the PP-g-MA addition [ 29 ]. Srebrenkoska et al. prepared PP reinforced with kenaf fiber composites compatibilized with PP-g-MA and further simulated the recycling process by reprocessing the composites; they observed that recycled composites showed some improvement on the interfacial adhesion [ 30 ]. Kord et al. reinforced virgin PP with 50 wt% of hemp fibers (100 mm in length) compatibilized with PP-g-MA and compared it with rPP-based composites obtaining the rPP from the waste spindle during the processing of virgin PP. The rPP-based composites showed a higher swelling effect than virgin PP-based composites due to the high hydrophilicity of hemp fiber and required a fourth component such as a clay (montmorillonite) to reduce the hydrophilicity of the composites [ 31 ]. On the other hand, some authors reported that the introduction of maleinized linseed oil (MLO) enhanced the compatibility of different WPCs by the formation of bonds between the carboxylic ester of MLO and the hydroxyls groups of cellulose [ 32 33 ]. Nevertheless, MLO compatibilized SHFs as the reinforcing phase for PP and/or rPP has not been studied yet.

With the main objective of mitigating the environmental problems associated with polypropylene textile wastes, in this work, the reuse of polypropylene fiber industrial wastes in the manufacture of wood plastic composites was evaluated. The rPP was obtained from non-woven polypropylene fabrics, and composites were prepared by an injection molding process to simulate the most typical WPC industrial processing conditions. The influence of short hemp fibers (SHFs), another textile waste, as reinforcement fiber for rPP, was studied by loading rPP with 30 wt% of SHFs (with length less than 3 mm). Since the main drawback of natural fibers is the high hydrophilicity which leads to low compatibility with PP matrix, two types of compatibilizing agents were used, polypropylene grafting maleic copolymer (PP-g-MA) and maleinized linseed oil (MLO), to improve the interaction between the reinforcement fibers and the polymeric matrix. The wood plastic composites were evaluated in terms of mechanical, thermal, and thermo-mechanical properties. The interaction between the matrix and the reinforcing fiber was analyzed by FTIR and FESEM. Finally, since these composites are intended for industrial applications where the hydrophobicity represents a handicap (i.e., automotive, construction materials, outdoor applications, etc.), the effect of the addition of compatibilized short hemp fibers in different percentages (10, 20, and 30 wt%) on the water absorption properties of the rPP-based composites was studied. Finally, the composites’ appearance was evaluated, and the color properties were assayed to get insights regarding the industrial applicability of the PP textile wastes recyclability as WPC materials.