Use of composites as engineering material has opened up a new horizon of materials science. The widespread use of composites in structural and semi-structural applications has played an instrumental part in shaping the modern developments. However, due to these tremendous advantages, composite consumption has skyrocketed in recent years. According to a survey, municipal solid waste (MSW) generation per capita was of 500 g/day in 2011, which was relatively high in urban India. Enormous waste generation leading to a rise in the greenhouse gas emissions has also been reported, which results in threats like global warming, climate change and poor air quality index [1]. These environmental concerns, although necessary, are limiting the composite utilisation and raising concerns related to sustainable development. The recycling of many composite materials is still hindered by techno-economic reasons [2]. Thus, renewable and sustainable eco-friendly composites have become the need of the hour. The use of biocomposites in various applications like automotive, packaging and medical field has become a subject of great interest to the scientific society [3–5]. The use of bio-based polymers with natural fibres as reinforcement represents a sustainable solution to recycling and solid waste generation. However, there is a limited number of bio-based polymers used as the matrix in composite materials. Polylactic acid (PLA) is one of the bio-based polymers that fulfils the mechanical, engineering and economic requirements for large scale implementation.
Polylactic acid (PLA), a biodegradable bio-based thermoplastic high-strength, and high-modulus aliphatic polyester is produced from natural renewable sources like corn sugar, sugar cane, potato, etc. [6]. Among the biopolymers, PLA has been reported to be the most successful in developing alternatives and replacing the materials based on non-biodegradable fossil polymers [7]. However, applications of PLA are still limited due to its low flexibility. To improve the toughness of PLA, many fillers and plasticisers have been used [8, 9]. Natural fibres are one of the potential candidates in improving the toughness of PLA. Besides improving the toughness, natural fibres offer advantages like environmental friendliness, biodegradable nature and a lower density than the synthetic fibres, like glass, carbon etc., commonly used in the composites. Siakeng et al. [10] found that incorporating coir/pineapple fibre at 30 wt.% in PLA improved its toughness by 30%. Similarly, Ovlaque et al. [11] reinforced PLA with milkweed floss fibres and found that the impact strength of PLA was improved by 12%. Jayamani et al.[5] reported that the impact strength of PLA improved after incorporation of sisal fibre to about 6 kJ.m− 2 from 4 kJ.m− 2. Aydemira and Gardner [12] used cellulose nanofibrils (CNFs) to reinforce PLA and polyhydroxybutyrate blend to improve the ductility of PLA. They reported that CNFs improve the mechanical and thermal properties of PLA at 1 wt.% loading.
In general, it has been claimed that natural fibres can pave the way for sustainable composites by offering advantages such as weight reduction, low cost, improved mechanical properties and low carbon footprint compared to synthetic fibres. However, tensile properties of natural fibres are inferior to those of their synthetic counterparts, though the former's specific tensile strength and stiffness are comparable with glass. For example, ramie fibre has higher stiffness than glass, and the former is well accepted as a reinforcement for polymeric composites. Nevertheless, the hydrophilic nature of natural fibres results in an inferior interface and thereby causes poor mechanical properties of composites to reinforce hydrophobic matrices [13–16]. Thus, to improve the interfacial adhesion, chemical or physical modifications of natural fibres are required. Among chemical treatments, alkaline treatment is extensively used for the modification of cellulosic fibres [17, 18]. This treatment exposes cellulose microfibrils and promotes the hydroxyl group's ionisation present on the fibre surface to the alkoxide groups [19]. True et al. [18] studied the effect of alkaline and/or silane treatment of the sisal fibres on the mechanical properties of sisal/PLA composites and found that the strength of sisal/PLA composite improved significantly as compared to that of untreated sisal/PLA composite. Yang et al. [20] used cyclic loading after alkaline treatment to improve ramie/PLA composites' mechanical properties. This study confirmed that the alkaline treatment and cyclic loading act synergistically to enhance ramie/PLA composites' overall mechanical properties. Another well-established way to enhance the interface of natural fibres in polymeric matrices is the use of coupling agents. Silane-based coupling agents promote covalent bonding between natural fibres and polymeric matrices [21–26]. The mechanism of surface modification using coupling agents involves the reaction of alkoxysilane group with hydroxy groups present on the fibre surface and another active group with a polymer matrix [27]. Song et al. [28] treated hemp fibres with silane before manufacturing its composites with PLA. They found that silane treatment helped uniform distribution and better adhesion of the hemp fibre into PLA, which resulted in improved mechanical and thermal properties of hemp/PLA composites.
Similarly, Jandas et al. [29] and Li et al. [30] used various silane coupling agents for the surface treatment of natural fibres. They found that these composites' mechanical properties improved significantly compared to that of untreated natural fibre composites. Aphichartsuphapkhajorn et al. [24] used alkaline and silane simultaneously on flax fibre followed by composite fabrication with biobased resin furan. The study revealed that the combination of alkali and silane yielded better mechanical properties than that obtained with individual alkali or individual silane treatment.
Ramie is a very strong natural fibre and available in Korea, China, Japan, and India's north-eastern parts. With its good mechanical performance, low specific mass, lustre, absorbance and resistance to bacteria, ramie fibre is a promising reinforcement among cellulosic fibres [31, 32]. The search of extant literature shows that development of green composites using ramie fabric has not been explored much [33–36].
The present study explores the effect of one bath alkaline/silane treatment on the properties of ramie/PLA composites. In the first step, ramie fabric was treated with silane and alkali (i.e. sodium hydroxide and ammonium hydroxide), followed by investigating the effect of treatment on the mechanical, morphological properties of ramie fabric. In the second part, PLA composites with untreated and treated ramie fabrics were prepared using compression moulding technique followed by evaluation of mechanical performance and thermomechanical analysis.