In recent years, in an attempt to substitute the conventional synthetic sound absorption material, natural fibers and their sound absorption properties have been increasingly studied. This is due to the fact that conventional synthetic fiber has potential health risks for human beings and significant environmental impact. In this review, existing and newly emerging natural fiber sound absorbers are summarized and highlighted in three categories: raw material, fiber assembly and composite. The sound absorption mechanism, several widely used prediction models and the popular acoustic characterization methods are presented. The comparison of sound absorption properties between some natural sound absorbers and glass fiber is conducted in two groups, i.e., thin material and thick material. It is found that many natural fibers have comparable sound absorption performance, some of them can be the ideal alternatives to glass fiber, such as kapok fiber, pineapple-leaf fiber and hemp fiber. Last, the conclusion part of this review gives an outlook regarding the promotion of the commercial use of natural fiber by means of theoretical study, efficient and environmentally friendly pretreatment and Life Cycle Assessment.

In this work, a new approach to apply silica aerogel into textiles by laser treatment and laminating technique was used to fabricate aerogel-encapsulated textiles. Thermal insulation performance of the prepared multilayer aerogel-encapsulated laminated fabrics was characterized in terms of thermal conductivity, thermal resistance and thermal diffusivity by Alambeta instrument. Infrared thermography data were collected by FLIR ThermaCAM TVS300 thermal camera. The ORIENTEC STA-1225 Universal Testing Machine was employed for testing the compression properties like compression resistance, compression resilience and thickness loss. Results showed that the aerogel-encapsulated structure has significant effect on thermal insulation enhancement. A temperature gap of 1℃ to 1.5℃ was observed between aerogel-filled samples and regular ones. It is also found that nonwoven fabrics with aerogel-filled voids generally have lower compression resistance than regular nonwovens, samples with air-filled voids and aerogel-filled voids are observed to recover compression to a lower extent than regular samples. Nonwoven based aerogel-encapsulated composites exhibit lower thickness loss in comparison with untreated composites. The findings in this study can be used for further research in aerogel-encapsulated composites.

Polyester nonwoven fibrous material is widely used in construction and automobile industries for thermal insulation purposes. It is worthy and meaningful to understand the effect of structural parameters on the thermal property. Fiber orientation, as one of the most vital parameters, has a significant effect on thermal property. However, there has been little quantitative analysis focusing on this aspect. This paper theoretically and experimentally analyzes the thermal conductivity of samples with varying fiber orientation. Existing models were selected to predict the thermal conductivity of polyester nonwoven samples. Two different apparatus were applied to carry out the experimental measurements. The relative differences between the predicted and measured results were compared. One commonly used model was modified for accurate prediction. It was shown that some existing models under- or overestimate the thermal conductivity compared to the measured values. The results indicate that the modified model can accurately predict the thermal conductivity of polyester nonwoven materials within a 0.2% relative difference.

In this work, three types of multi-layered fibrous materials with different middle layer structures, including Struto nonwoven, Struto nonwoven with air pockets and Struto nonwoven with air pockets filled by aerogel particles, were prepared based on laser engraving technique and laminating method. A custom-built new device was fabricated to evaluate convective thermal behaviour of the multi-layered materials in cross flow. It was found that in cross flow the heat transfer coefficient of the multi-layered material with encapsulated aerogels is directly proportional to Reynolds number. There are considerable variances in heat transfer rates of the three structures at low airflow velocity (less than 10 m/s), but the values are very close at high airflow velocity. It is concluded that the air pockets and aerogels present in the multi-layered fibrous material have significant effect on convective thermal behaviour of the overall structure in cross flow. The finding is a new contribution to the field of aerogel-based fibrous materials as thermal insulators in building and industrial facilities.