Relaxation of knitted fabrics is a prerequisite to restoration of fabric properties and performance. The Hong Kong Research Institute of Textiles and Apparel (HKRITA) has developed a new low pressure relaxation (LPR) process for sweater manufacturing.

Conventional washing and tumble drying (WTD) is used for relaxation in sweater manufacturing and is achieved by mechanical agitation. However, it is time consuming and involves a lot of water and energy consumption. It also adversely affects the surface texture of the product.

Unlike the conventional WTD process, the LPR process developed by HKRITA applies micro-agitation to achieve sweater relaxation.

The process is based on the principle that when air pressure drops, water evaporates and its volume expands. The force field created by water vapour expansion in a garment acts on its fibres and yarns, resulting in an agitation action occurring, which has a relaxation effect on the garment. A LPR chamber is built to provide a suitable environment.

More sustainable with less water and electricity

The LPR process is potentially beneficial to sweater production, according to HKRITA. Compared to WTD, the new process is far more sustainable since the LPR process uses less water and electricity with shorter processing time, while restoring more effectively the sweater properties of dimensional stability, fabric appearance and hand feel at the same time.

It is estimated that the LPR process takes only 3-5 minutes (depends on the type of fibre) to complete while WTD takes 2-3 hours. Each sweater undergoing a LPR process only consumes 0.2 litres of water and 0.132kWh of energy.

In addition, the LPR offers a cleaner fabric appearance, higher pilling test grade and brighter colours especially for darker shades such as black and navy blue.