Sustainable Lightweight Insulation Materials from Textile-Based Waste for the Automobile Industry

1. Introduction

In this modern era of environmental awareness, manufacturers and consumers are mainly focusing on the production and consumption of nature-based biodegradable, biocompatible and recyclable products [ 1 4 ]. The automobile industry is not exempt from this and seeks to fabricate various automobile parts derived from natural and renewable raw materials rather than traditional synthetic fibres (glass, carbon, aramid fibres etc.) and petroleum-based oils for polyurethane foams [ 5 8 ]. Besides, it has been found that every year, in the European Union, end-of-life vehicles (ELV) generate around 8–9 million tonnes of waste [ 9 ]. Moreover, as of 1 January 2015, the Directive 2000/53/EC on end-of-life vehicles set detailed quantified targets, which state that 85% of the vehicle must qualify for Reuse–Recycle and 95% for Reuse–Recover purpose [ 9 ]. This has pushed car manufacturers to consider the use of sustainable manufacturing materials such as natural fibres especially for the noise and heat insulation. The ability to isolate the mechanical noise, vibration, heat, and thermally insulate the interior of the vehicle leads to comfortable driving, and significant energy savings from the air-conditioning usage perspective. These insulation materials can be used for floor coverings, under the seat cover, door trim, pack panels, engine compartment, and on the ceiling [ 6 10 ]. Nonwovens, which can be easily made into different densities, thickness, and forms, have been applied in the car interior owing to their light weight, simple processing, flexibility, porosity, and sound insulation properties [ 11 ]. Needle-punching, a large-scale nonwoven manufacturing method using mechanical interlocking to bond, shows its advantages in simple processing and low cost rather than the other nonwoven web bonding methods. Fibres are interlocked during the needle-punching process, which substantially increases the resistance to the sound wave [ 12 ].

Natural fibres (flax, bamboo, kenaf, and jute) are increasingly being considered by the car manufacturers to offset the limitations of synthetic fibres [ 13 14 ]. However, their uses are limited to inner composite panels due to the harsh handle, poor aesthetic appearance, and limited insulating efficiency. The range of natural fibres, which can comprehensively satisfy the multiple functionalities (insulation, aesthetics, and comfort) of a car interior is therefore limited, and new opportunities have not been fully explored. Wool, as an animal protein fibre, is inherently flame retardant, odour resistant, aesthetically pleasing, luxuriously soft, and thermally and acoustically insulating [ 3 15 ]. This fibre possesses a very complex chemical and physical structure, which plays an important role in several functional properties and applications [ 3 16 ]. The surface of wool fibre is covered with overlapping cuticle cells (scales) separated by a cell membrane complex (CMC). The outermost external thin membrane named as the epicuticle consists of proteinaceous materials containing a high level of amino acid cysteine [ 17 18 ]. Due to its chemistry, wool can absorb water up to 30% of its weight without feeling wet, which may help in regulating the humidity of the car interior [ 3 19 ].

Although only a few reports have been published on the insulating properties of the wool fibres for their application in the building construction [ 20 24 ], the literature lacks information regarding both the acoustic and thermal characteristics of the nonwoven developed from wool as an automotive insulator. Ballagh et al. [ 25 ] studied the influence of wool fibre diameter, density, and flow resistivity on wool material acoustical properties. He found that the sound absorption coefficient of the wool material increased with the increasing density when the sample thickness was 25 mm. However, in the case of the thicker sample (100 mm thickness), the sound absorption coefficient increased with the density up to 47 kg/mand decreased with the further increment in the density [ 25 ]. Additionally, the sound absorption coefficient decreased with the increment in fibre diameter, but the wool acoustical property did not strongly rely on the flow resistivity. In another work, Broda and Bączek [ 26 ] investigated the influence of a multilayer structure on wool nonwovens on the sound absorption and found that the multilayer structure improved their sound insulating properties at a low number of layers.

There is limited information available on the performance of the commercially widely used insulating materials and their role in altering the insulating properties for the automotive interior. There are also very few, if any, reports on the application of only wool fibres (100%) as a candidate automotive insulator, although wool-based car interiors have been realised by some limousines for a long time. Therefore, this study focuses on two parts: First, it aims to understand the performance of current commercial automotive insulators associated with their construction and insulating properties. These results are important for estimating the required properties and insulation for designing an individualised car insulator from 100% natural wool fibres. Second, this study aims to utilise wool to address the combined limitations of synthetic and other natural fibres as an insulator. The fibre size and limitation of natural fibres was overcome by the use of a mixture of waste wool and virgin wool. A variety of densities and thicknesses of wool nonwoven material (WNM) was fabricated using the needle-punching method and systematically compared with the commercially available materials. The influence of fibre diameter, surface, layered structure, thickness, and area density on the acoustic properties were studied. Additionally, the thermal insulating properties, antibacterial and antifungal properties, and biodegradability of all the samples were investigated to better understand the suitability of the wool nonwoven materials (WNM) as renewable sound and thermal insulating materials to be used in different parts such as door trim, pack panels, engine compartment, and on the ceiling.