Bio-based Nonwovens Made from Viscose Fibres for Sound Absorption

Bio-based nonwovens are becoming increasingly important in acoustic interior design. The WHO classifies noise pollution as a major environmental problem in Europe, which is driving demand for effective sound absorbers. While polyester-based moulded parts currently dominate the market, fully bio-based alternatives are increasingly coming into focus – in particular systems based on viscose fibres, which combine acoustic effectiveness with sustainability in material sourcing.

This development is based on Galaxy viscose fibres with a trilobal cross-section. This special fibre geometry creates an increased surface-to-volume ratio, which enlarges the contact area for sound waves. The acoustic effect is based on the principle of friction: sound waves are converted into heat by friction on the fibre surfaces. PLA bicomponent fibres with a core-sheath structure are used as binding fibres. The low-melting sheath enables the fibre structure to be thermally bonded, while the higher-melting core ensures mechanical stability during and after the moulding process.

The nonwoven fabric is produced using roller carding machines and needle punching. The carding process produces parallel-aligned fibre nonwovens, which are processed into multi-layer nonwovens by a counter-rotating double belt layer. By varying the layer angle and the number of layers, basis weights between 100 and 1,070 g/m² can be achieved, with a preferred fibre orientation in the transverse direction. Consolidation is carried out with a needle board of 4,500 needles/m² at penetration densities of 60 to 300 penetrations per cm².

The moulded part is produced in two stages: first, the nonwovens are heated above the melting point of the binder fibres, then cooled and consolidated in a cooled press. Core temperatures of 160°C, heating temperatures of 200°C and press gaps of 5 to 9 mm have proven to be optimal. The resulting semi-finished products have a basis weight of between 1,000 and 2,000 g/m².

Influence of process parameters on acoustic and mechanical properties

The core temperature during the pressing process has a decisive influence on the quality of the moulded parts. Only complete melting of the binding fibre coating ensures uniform bonding of the viscose fibres. However, excessive heat input above 200°C leads to thermal decomposition and discolouration. Interestingly, in the temperature range from 120 to 180°C, there is a negligible influence on sound absorption, while the mechanical properties benefit significantly from higher core temperatures.

The weight per unit area has a significant influence on acoustic behaviour. In all test series, the absorption coefficient increases with frequency. Up to 2,500 Hz, the frequency-dependent absorption coefficient increases with increasing weight per unit area. At 4,000 Hz, this trend reverses – absorption increases as the weight per unit area decreases. As expected, mechanical properties such as the modulus of elasticity and strength improve with higher grammage. A grammage of 1,350 g/m² has established itself as a balanced compromise between material usage and sound absorption.

The puncture density significantly influences the formability during the compression moulding process and must be adapted to the intended component geometry. Pinch densities above 120 pinches/cm² lead to a significant deterioration in mechanical properties due to reorientation and breakage of the fibres. Due to the overlapping effects of pinch density and basis weight, it is not possible to make clear, isolated statements about the influence of pinch density on the absorption coefficient.

Industrial implementation and application potential

For industrial moulded part production, the nonwovens are heated to 180 to 195°C. The heating element on the visible side operates at 180°C to prevent thermal discolouration, while the opposite element operates at 195°C to ensure sufficient consolidation and dimensional stability. Higher weights per unit area extend the cycle time and impair economic efficiency. Punch density and fibre length are kept to a minimum within the limits, as increasing them in combination with the tool geometry can lead to local damage – including nonwoven fabric tears, delamination (unintended separation or detachment of individual layers in a multi-layer composite material) or wrinkling.

The development of bio-based nonwovens made from viscose fibres with an average wall spacing of 47 mm represents an alternative to petrochemical acoustic panels for applications in offices and conference rooms. The combination of renewable resources, compostability and uniform surface quality eliminates the limitations of previous solutions. While polyester-based systems are recyclable, they cause microplastics during and after their useful life.

Source: Trade journal ‘Technische Textilien’

Photo: ArtificialHorizons