**2. Materials**

**1. Introduction**

218 Composites from Renewable and Sustainable Materials

material's solution [1].

quencies [14–19].

whiskers.

regenerated wood fibres [12] and canola straw [13].

Composites are more and more popular products present in our lives. An extensive use of composites results from the diversity of their functional properties, what is connected with the combination of different components, diversed in terms of materials and forms, and with diversity of the final structures. Recently, the composites have been produced from renewable and sustainable materials. Renewable materials are natural, environmentally friendly and usually cheap. The composites produced from such materials are called 'green composites', but this concept is much broader and should concern their production and usage. Now the filling components in composites should be waste and the matrix material should be recyclable such as, for example, thermoplastics. Sustainability concerns three aspects: environmental, economic and social. The use of biomass, paper, fibres, wood as a waste filling material and bio-based thermoplastic polymers as a matrix can be the optimal

The production of environmentally friendly composites on the basis of materials obtained from renewable resources is important for the economy and the environment. Recently, new materials on the basis of different plants are more and more popular and used in many industrial and life fields. Materials on the basis of straw, reeds, cattails and bent grass stalks are used in the ecological building sector [2]. The natural fibres can be used as a reinforcement or as a filling in composites, but also can give some new functions to them. Moreover, there is observed a growing trend towards replacing high-modulus reinforcing fibres with natural fibres [3–5]. Many technologies of composites based on different plants, natural fibres and even fibres isolated from plants are developed. The cellulose micro- or nanofibres, or micro-fibrils can be obtained through the chemical, mechanical, ultrasonic and enzymatic treatment of plants, such as jute [6, 7], soya bean source [8], wheat straw [9], soy hulls [10], rice straw [11],

Due to the great variety of plants, the properties of composites made of natural components are much diversified what favors different applications. Natural fibre-reinforced composites are used not only for construction but also for new uses, e.g., attenuation of sounds. Soundabsorbing composites can show a high degree of sound absorption, especially at high fre-

The commercially available porous sound-absorbing materials are usually fibrous. The fibres used are mostly synthetic, but recently, natural fibres are more and more popular as a raw material of the sound-absorbing products. Natural fibres are biodegradable and safer for human health than most mineral or polymer synthetic fibres. Introduction of the cellulose ultra-short/ultra-fine fibres prepared from different kinds of biomasses into functional composite structures causes increase in their sound absorption. Conversion of biomass to ultrashort/ultra-fine fibres perfectly fits into the current trends of cellulose nanostructures receiving by a top-down method. Depending on the size of the structures the obtained cellulose is suitably named, for example: microcrystalline cellulose, nanocrystalline cellulose or nanoIn order to thermally connect the composite constituents, one of them, forming the composite matrix, should be thermoplastic. The other ones are reinforcing/filling constituents.

To have a broader view of the acoustic properties of the composites the different raw materials and different combinations of reinforcing materials were used.

As a matrix material, the polylactide (PLA) fibres (6.7 dtex/64 mm) with a melting point in the range of 165–170°C have been selected. These fibres, Ingeo Fiber type SLN2660D, finished with polylactide resin without any hazardous substances, were delivered by Far Eastern Textile Ltd., Taipei, Taiwan.

As a reinforcement the following materials were used:

(a) waste natural fibres, **Figure 1**:


