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Polypropylene BCF yarns are first blended into yarns from two different shrinkage fibers, and then the yarn is treated in steam or hot air or boiling water. At this time, the fibers with high shrinkage produce large shrinkage and are located at the center of the yarn. The low-shrinkage fibers which are mixed together are squeezed on the surface of the yarn to form a loop shape due to the small shrinkage, thereby obtaining a bulky, full, elastic and bulky yarn.
The grey fabric woven from polypropylene BCF yarn is generally finished after dyeing, napping (single or double sided), and the fabric is required to be soft, fluffy, elastic and evenly dyed. Therefore, the bulking degree of the bulked yarn, the specification of the raw material, the uniformity of mixing, and the like are the main performance indexes.
The degree of expansion is the degree of expansion of the original yarn after wet heat treatment. It is the main factor determining the bulkiness, softness and elasticity of the fabric. Generally, the degree of bulkiness of the yarn is above 22%. In the actual production, the shrinkage rate of the yarn generally used as the measured index for testing the bulking degree is generally between 18% and 19.5%. The following issues should be noted regarding the measurement of shrinkage:
(1) The original yarn should be pre-loaded with a tension of 20g before measuring the length;
(2) steaming the measured raw materials in steam for 5 minutes;
(3) The lengthened yarn is also added after adding 20 g of tension to the bulked yarn.
Too much expansion will result in tight, fluffy softness and inelasticity of the grey fabric; too small expansion will result in loose fabric and poor plush feel.
The uniformity of the mixing of raw materials is an important reason for the dyeing effect. Uneven mixing will result in uneven dyeing of the cloth and also affect the properties of the expansion.
The process of polypropylene BCF yarn is somewhat complicated, and the production cost is higher than other finished yarns. It is hoped that the production technology will be greatly improved in the near future, opening the way for polypropylene expanded yarn.