Coir is a lignocellulosic natural fiber derived from the coconut's husk, an abundantly found fruit or nut worldwide. This fiber has some unique characteristics, such as its resistance to seawater, microbial attack, high impact, etc. But its low thermal conductivity or high thermal insulating property makes it suitable for being used as insulators in civil engineering sites. On the other hand, the sustainability of a material depends heavily on its environmental impact of the material. For making sustainable materials like biocomposite, there are no options other than using polymers derived from natural renewable sources. Polylactic acid(PLA) is an example of those types of material. And these materials are often being reinforced by fibers like coir for various reasons including improving mechanical properties, reducing the cost of the material, and improving the material's sustainability. Many coir-reinforced sustainable biopolymer composites have already been produced in many pieces of research, which will be discussed in this paper, along with the chemical and physical structure of coir fiber. In addition, this paper will try to focus on the insulating properties of coir and coir-reinforced composites while will also compare some properties of the composites with some commonly used materials based on different parameters to show the suitability of using the coir fiber in heat-insulating applications and to produce sustainable biocomposite materials.

Fully biodegradable flax/polylactic acid (PLA) thermoplastic composites were fabricated by using random (nonwoven mat) and aligned (unidirectional yarn) flax fiber as reinforcements (39% flax by volume) and Polylactic acid (PLA) as matrix. Results revealed that the aligned flax fibers have a greater reinforcing effect due to the uniform distribution of load axially along the fiber length in the composite. The aligned flax/PLA and random flax/PLA showed the tensile strength of (83.0 ± 5.0) and (151.0 ± 7.0) MPa respectively and flexural strength of (130.0 ± 5.0) and (215.0 ± 7.2) MPa respectively. Young’s modulus of (9.3 ± 1.5) and (18.5 ± 2.0) GPa and flexural modulus of (9.9 ± 1.0) and (18.8 ± 1.0) GPa was attained for the random and unidirectional fiber composites, respectively. It was also found that both composite constituents, fiber and matrix, were degradable if buried in compost soil (ready soil after composting process), which is a distinctive advantage of the new composite structures. Remarkably, the biodegradation property of aligned flax fiber composites was significantly lower than random mat composites, possibly due to the less water swelling behavior of the aligned fiber composites. After 120 days burial test, the aligned flax/PLA composite displayed the reduction of 19% mass, residual flexural strength and modulus decreased by 57 and 50% respectively, while the random mat composites exhibited the loss of 27% mass, residual flexural strength and modulus declined by 80% at the same period.