Compressive Strength of Floor Tile Composites from Recycled PET Reinforced with Natural Fibers

ABSTRACT The quantity of bottled water and other household packaging materials increases enormously in developing countries like Ethiopia. Most of these materials are packed with polyethylene terephthalate (PET) plastics which results in the mounting of wastes rapidly. Due to their non-biodegradability, PET wastes cause challenges such as landfills, health problems, and a reduction in productivity. PET, a thermoplastic polymer, can be remelted and molded into different products. In this research, the recycled PET is reinforced with natural fibers such as false banana and sisal fibers to produce floor tile composites. The processing steps include a collection of waste PET and natural fibers, cleaning, shredding, treating fibers, composite manufacturing, and characterizing. The use of natural fibers in recycled plastics resulted in the production of cheaper, effective floor tile composites, and a reduction in environmental pollution. The FTIR analysis shows the removal of certain functional groups in chemically treated natural fibers and the occurrence of chemical and mechanical degradations during the recycling process. Natural fibers reinforced composite samples showed about 36% lightness compared to glass fibers reinforced composites and resulted in maximum compression strength of 3.1 MPa. Further study needs to be undertaken to enhance the mechanical properties of such composites in the future.


Introduction
There is a continuous increase in the production of plastics in the world today and it is predicted to persist. The progressively growing use of plastics generates enormous amount of non-biodegradable wastes in the environment . As these wastes stay hundreds of years without degradation, they need to be reprocessed or removed before being dumped into landfills . treatments help to loosen the hydroxyl groups and reduces the hydrophilic behavior of the natural fibers, which in turn results in improved mechanical strength and dimensional stability of NFPCs .
Characteristics and performance of NFPCs are affected by several factors. The hydrophilic nature of the natural fiber (Shalwan and Yousif 2013) and the fiber loading have great impacts on the composite properties (Norul Izani et al. 2013). Several studies reveal that, good properties of NFPCs are attained by high natural fiber loading (Mohammed et al. 2015;Shesan et al. 2019;Tawakkal, Cran, and Bigger 2014). The composition of natural fibers the percentage of cellulose, hemicellulose, lignin, and waxes also affect the properties of resulted composite . The chosen process techniques and parameters also highly influence and determine the characteristics of produced composites (Ku et al. 2011). The suitability, competitiveness, and capabilities of embedding natural fibers in the polymeric matrices were examined by several researchers (Arrakhiz et al. 2013;N. Kumar, Bedi, and Singh 2021;Neto et al. 2021;Pereira et al. 2020). Fiber surface modifications effect on manufacturing processes and improving fiber/polymer compatibility were also vastly studied (Latif et al. 2019;Mohammed et al. 2015;Zwawi 2021). The suitability of jute/plastic composites to the eco-design components of automotive industry is investigated and properties such as thermal stability, fiber modification, crystallinity, weathering resistance, and durability were analyzed (Al-Oqla and Sapuan 2014).
The mechanical performance of the composites is affected by number of aspects such as orientation of fiber (Şanal and Özyurt Zihnioǧlu 2013), length of fibers (Himanshu et al. 2015;Prakash et al. 2021;Sajin et al. 2022), physical properties of fibers (Bénézet et al. 2012), volume fraction (Das et al. 2018;Prakash et al. 2021), interfacial adhesion property of fibers Lee, Khalina, and Hua Lee (2021) and many more. The chemical, physical, or biological approaches of natural fibers surface functionalization could be used to enhance their compatibility with polymeric matrix to produce advanced NFPCs. Most commonly used chemical treatments for plant based-fibers includes alkalization, silanization, acetylation, acrylation, benzoylation, peroxide treatment, maleated coupling treatment, potassium permanganate treatment, acrylonitrile grafting, stearic acid treatment, isocyanate treatment, triazine, sodium chlorite, fungal and enzyme treatment (Tavares et al. 2020). As it was mentioned above, the main purpose of surface treatments of natural fibers is to enhance fibre/matrix interfacial bonding and stress transferability of the composites. Taking one of the surface treatments of natural fibers as an example, the alkaline treatment helps to remove the amorphous compounds and increases the crystallinity index of fiber bundles (Venkateshwaran, Elaya Perumal, and Arunsundaranayagam 2013;Yiga, Lubwama, and Wilberforce Olupot 2021;Zin et al. 2018). The appropriate concentration of alkali should be used based on the experiments to have better properties of treated fiber reinforced composites. Low alkali concentration may not fully remove the amorphous components, whereas high alkali concentration may damage the surface of fibers which in turn results in a decrease of mechanical properties. Several researchers have successfully developed polymeric composites reinforced with natural fibers. However, the authors of this study did not find enough studies done on natural fibers reinforced with recycled polymeric composites (NFRPCs) from online literature. Therefore, the possibility of effective NFRPCs production and effects of recyclability on the properties of the resulted composites need further study. Thus, this study is designed to study and evaluate the properties of composite materials developed from natural fibers reinforced recycled PET.

Significance of the study
The use of renewable resources such as natural fibers in composite materials has been significantly increased due to their economic and environmental advantages. Utilizing these natural fibers as a reinforcement for recycled plastic will have huge benefits in terms of sustainable development. A large number of studies have utilized plastic waste as aggregate or fiber to improve the mechanical properties of composite for building materials such as concrete. To the best knowledge of authors, a little is known about composite materials produced through melting of recycled plastic reinforced with natural fibers. Tile composites produced from manually melted recycled PET reinforced with natural fibers has not been investigated yet and this study will fill the gap within the area.

Materials and equipment's
To undertake this study the following materials and equipment were used. Natural fibers such as false banana, sisal fibers, shredded PET waste bottles, gypsum, stainless steel melting pot, four partitioned compacting mold, sodium hydroxide (NaOH), blowing fan, charcoal, water, and plastic barrel are used.
The following testing device were used: universal testing machine (WAW-600D) was used to test the compressive strength of the composite and PerkinElmer FTIR instrument was used to test the functional groups.

Methods
The following main procedures are followed in this research:

Fiber extraction
Sisal fibers are extracted from the raw sisal plant mechanically by researchers whereas the false banana fibers are procured from local farmers. After cleaning the impurities, the long fibers are cut into short fibers of about 3-4 cm length by using scissors to imitate the aspect ratio of synthetic fibers like glass fibers as shown in Figure 2.

Treatment of natural fibers
To illustrate the effect of surface treatment in composites, false banana fibers are treated with NaOH at different concentrations. The surface treatments follow previous researcher steps (Kabir et al. 2012;Shalwan and Yousif 2013): fibers are pre-cleaned to remove impurities which increase the fiber surface roughness and preventing the ions absorption that remove the OH groups from the fibers surface as shown in equation below.

Collection and cleaning of waste PET
Waste PET bottles are collected from Bahir Dar city, Ethiopia. Then, the bottle covers and tags are removed and cleaned to remove the impurities and any contaminations.

Shredding
Cleaned bottles are shredded by a local shredding machine at Aychew and friends a small-scale enterprise as shown in Figure 3.

Composite manufacturing
Shredded PET wastes are melted in stainless steel melting pot by continuously blowing the charcoal using electric fan. Charcoal was used in this study due to the absence of alternative methods like a heavy-duty gas or electrical stove and the use of such energy sources is recommended in future studies. After PET completely melts, the gypsum of 22 wt% is added slowly and stirred gently until it is homogenously distributed without agglomerates formation and finally fibers of 3 wt% is introduced in a speedy and gentle mixing until it is uniformly distributed in melted PET matrix. The proportion of gypsum and fibers selected for this study is based on the researcher preliminary study and also referring other's researcher findings (Turlapati and Vineel 2020). The processing parameters such as temperature and time have significant impact on the properties of the produced tiles. High temperature beyond melting temperature and prolonged stay at the melted state lead to low strength tile products due to thermal degradation of PET. Thus, researchers stop blowing the charcoal as soon as the PET completely melts and added fillers and fibers within two minutes to avoid the degradation of both natural fibers and PET. Finally, the melted solution is immediately poured to a four partitioned compacting metal mold of each 20 mm x 20 mm x 30 mm size and pressing it with a high pressure until it solidified for about 5-7 minutes to form floor tile of uniform and flat surface. Figure 4 shows the summary of manufacturing process of NFRPCs.

Compression strength tests
The compressive strengths of floor tile composites produced from fibers reinforced recycled PET are measured by using automatic compression tester machine model 50-C46P02, Italy CONTROLS S.P. A. according to ASTM C469 standard test method as shown in Figure 5.

Functional group analysis
The functional groups found in composite were analyzed using the PerkinElmer FTIR instrument. FTIR spectrum outputs were obtained in the range of 4000-500 cm −1 using five scans and recorded in the absorbance mode as a function of wavenumber.

Results and discussion
Recycling of plastics have benefits such as decrease of greenhouse gas discharges, energy savings, and nonrenewable sources like oil and gas savings. Furthermore, recycling offers an income for many people and families in emerging countries, both in the form of formal employment or informal economic activities. The previous research work of other researcher shows that recycled PET gives an improved sets of properties when it is reinforced with synthetic fibers than natural fibers. The glass fibers introduced in the polymeric matrix result in better and more consistent performance than other fillers and reinforcements in recycled PET composites (A. Kumar, Bedi, and Singh 2021). This is due to the high aspect ratio of the fibres (the diameter is in the range of 10-20 μm and final length of 3 cm), which translates in an excellent reinforcing ability. Accordingly, this study was undertaken to investigate the performance of natural fibers by mimicking the aspect ratio of glass fibers through cutting it to appropriate dimension. Also, the glass fibers reinforced recycled PET composites is produced to compare its properties with that of natural fibers reinforced composites in this study.
The floor tile composites, shown in Figure 4(f), produced from recycled PET filled with gypsum of about 22 wt% and reinforced from untreated and treated natural fibers of about 3 wt% show low weight compared to that of glass fibers reinforced composites of similar fibers content. Thus, the comparative advantages of using the lignocellulosic fibers are their lower density and costs as well as renewability, biodegradability, recyclability and neutrality with respect to CO 2 emission which is responsible for global warming. Floor tile composites produced from natural fibers resulted in lower costs of 15.8% compared to that of glass fibers reinforced tiles of similar fibers ratio. Figure 6 shows the mean weight of chemical treated as well as untreated false banana fibers and sisal fibers reinforced composites have almost similar weight of about 1.4 kg whereas floor tiles weighing about 1.9 kg is produced with glass fibers reinforcement. This result mainly due to the lightweight properties of natural fibers and certain degradation occurring during the composite processing at high temperatures.
The lignocellulosic fibers have limited application in high-temperature engineering composites due to their restricted thermal resistance. As limited works are available to analyze the feasibility of lignocellulosic fibers at high temperature, this study is devoted to consider the effectiveness of false banana fibers and sisal fibers reinforcement in the melted recycled PET. A thermogravimetric analysis of sisal fibers studied by Dantas et al (Dantas et al. 2019) shows a first mass loss event around 100°C and a second main degradation event of the fibers between 200°C and 400°C. As it was shown by this study, the major thermal loss of sisal fibers occurred mainly above 300°C. This implies that sisal fibers and false banana fibers which have almost similar properties could be successfully used to reinforce polyethylene terephthalate at its melting temperature of about 260°C in this study.
In order to improve the adhesion and compatibility of false banana fibers to the PET matrix, raw fibers were chemically treated with sodium hydroxide. The reaction of cellulose in false banana fibers is confirmed by FTIR spectrum analysis of the raw and treated false banana fibers reinforced composite as shown in Figures 7 and 8, respectively. The FTIR spectrum shows that there is a difference between the non-treated and treated false banana fibers reinforced composite. Some of the functional groups have been removed from the treated false banana fibers. This could be due to the reduction in molecular weight or reduction of the chain length due to the reduction of wax, hemicellulose, pectin, lignin, and other water-soluble components during the treatment of false banana fibers and thermal degradation. The FTIR spectrum of the nontreated false banana fibers-based composite shows the absorption band in the region of 3435 cm −1 , 1733 cm −1 and 2901 cm −1 due to O-H stretching vibration, C=O stretching vibration and C-H stretching vibration respectively. These absorption bands are due to the hydroxyl group in cellulose, carbonyl group of acetyl ester in hemicellulose and carbonyl aldehyde in lignin . On the other hand, the characteristic peak at 1733 cm −1 and other peaks i.e., 1428 cm −1 and 1248 cm −1 were completely disappeared upon chemical treatment with sodium hydroxide.
The FTIR spectrum of the sisal fibers reinforced recycled PET composite is shown in Figure 9. Throughout the recycling process, there is both chemical and mechanical degradations. This affects the mechanical properties and chemical resistance as well as melt viscosity of recycled PET which is less than that of virgin PET. The factors that cause a reduction in the physical, mechanical, chemical, and rheological properties of recycled PET will also affect the brittleness of the recycled polymer. These factors also make recycled PET lose its melting elasticity behavior. Even though the floor tiles composite manufactured from natural fibers prevails in lightness, the compression test result shows relatively poor compressive strength compared to that of the glass fibers reinforced composites. Figure 10 shows the average value of compression strength of sodium hydroxide treated and untreated false banana fibers, sisal fibers, and glass fibers reinforced tile composites. As it can be seen from the graph, the surface modification of false banana fibers by sodium hydroxide improved the compression strength compared to that of untreated false banana fibers.
The compression strength of NaOH treated false banana fibers reinforced floor tile composites value was improved to about 2.9 MPa from 2.7 MPa of untreated false banana fibers. Relative to false banana fibers, sisal fibers reinforced recycled PET composites shows relatively higher compression strength of 3.1 MPa which is about 15% more than the results obtained by untreated false banana fibers. Generally, this study reveals that the compression strength of glass fibers reinforced recycled PET composites is almost thrice of the sisal fibers and false banana fibers reinforced recycled PET composites. Thus, the decrease in compression strength of natural fibers reinforced recycled PET composites could be due to relatively low mechanical strength of natural fibers and possible thermal deterioration of the natural fibers at the melting temperature of recycled PET. However, the floor tiles produced from natural fibers reinforced recycled PET composites fulfill the compression strength criteria and can be used as potential candidate materials due to its lightweight, economic, and conserve environment. Comparison of compressive strength of floor tiles composites produced in this study and other researchers is demonstrated in Table 1. As seen from Table 1, the floor tiles produced from natural fibers reinforced recycled PET composites have comparative compressive strength and/or better strengths than the reference composites in certain cases.

Conclusion
Composite materials used for floor tiles were developed successfully using sisal fibers and false banana fibers of 3 wt% as reinforcement in melted recycled PET matrix where 22 wt% of gypsum added as fillers improved shrinkage on molding. The statistical results of floor tile composites produced from natural fibers showed lightweight and low compressive strength compared to synthetic fibers reinforced composites of the same composition. Chemical treatment of natural fibers resulted in the removal of certain functional groups due to the reduction of non-cellulosic components such as hemicellulose, wax, pectin and lignin. The FTIR analysis demonstrated the occurrence of chemical and mechanical degradations during the recycling process. Natural fibers developed floor tile composites showed about 36% lightness compared to glass fibers composites, whereas sisal fibers reinforced recycled PET composites had recorded maximum compressive strength of about 3.1 MPa. The comparison of compressive strength between this study and reference studies undertaken on the floor tile composites demonstrated it was within the given ranges. The results obtained in this study confirm that the natural fibers such as false banana and sisal fibers could be used to produce lightweight and relatively low compressive strength composites with melted PET wastes. Therefore, reinforcing the recycled plastics with natural fibers at relatively low temperature make them suitable for infrastructure applications like floor tile composites. However, further research needs to be undertaken to enhance the mechanical performance of the floor tile composites. Authors would like to recommend to use a heavy-duty gas or electrical stove energy source with certain automation to enhance the results obtained in this study and conserve the environment further.

Highlights
• As polyethylene terephthalate prolongs in the environment for hundred years without degradation its recycling will contribute to the conservation of the environment. • Natural fibers are viable for the reinforcement of recycled PET composites due to their abundance, renewability, degradability, low density and low cost. • Floor tile composites were successfully produced by reinforcing the recycled PET with false banana and sisal fibers and using gypsum as filler. • The produced floor tile composites resulted in 36% lightness compared to the glass fibers composites with the same proportion, and a compressive strength of up to 3.1 MPa.

Disclosure statement
No potential conflict of interest was reported by the author(s).