Pretreatment of silk
To prepare a silk yarn for dyeing and silk fabrics for dyeing and printing, it is necessary to partially or completely remove sericin, as well as natural oils and organic impurities. Depending on the percentage of sericin removed during scouring (sericin is present in raw silk in a ratio between 20 % to 25 %), the end-product is defined as unscoured (used only for shirts and suits), `souple' or degummed.
Scouring can be carried out either on the yarn or on the fabric. It is easier to remove sericin more evenly by performing the operation on the yarn itself. Nevertheless, the operation is generally carried out on the fabric, in order to exploit the protective action of this natural size against possible damage during weaving.
The scouring treatment can be carried out in a neutral, acid or alkaline solution, depending on the desired results. At the industrial level, treatment in alkaline conditions is by far the most common. It is extremely important to control the temperature.
Scouring baths present a high total organic charge; the concentration of nitrogen organic components in particular is high.
The process to produce unscoured silk consists in removing from raw silk fabrics all residual substances from previous operations, with a minimal elimination of sericin (1 % - 2 %), so that the fabric keeps the characteristic stiff-handle. The operation is carried out in a slightly alkaline soap bath at low temperature.
The process is carried out on weft yarns under acid conditions. The loss of weight is approximately 10 %.
This treatment is carried out on both yarns and fabrics and ensures a complete elimination of sericin, as well as substances added in previous operations, without modifying the fibroin.
Hydrolytic degradation of the sericin protein macromolecule can be obtained by simply using soaps, by using synthetic surfactants or mixtures of soaps and synthetic surfactants, by means of an enzymatic treatment, or by treating the silk in water at high temperature and under pressure.
In the degumming process with soap, yarn and fabric are plunged into two baths (degumming baths). Each bath contains green soap at different concentrations. The treatment is followed by washing with ammonia and rinsing. The process temperature varies between 95 °C and 98 °C. The soap concentration in the degumming bath varies between 10 g/l and 15 g/l. On average the entire treatment lasts for 2 hours. It is possible to re-use exhausted baths after appropriate addition of soap.
Degumming with synthetic detergents implies the partial or total replacement of soap with synthetic non-ionic surfactants (e.g. ethoxylate fatty alcohol). It is also possible to combine a degumming treatment with an oxidizing or reducing bleaching and, in some cases, even with dyeing, thus improving water and energy saving. Generally, alkali and detergent mixtures are used at temperatures around 95 °C - 98 °C. Such a treatment is suited to continuous processing.
Degumming under pressure at high temperature is a specific treatment essentially used to degum yarns. It is necessary to prepare an aqueous bath without surfactants and the temperature should be between 110 °C and 140 °C. A post-treatment washing is required to eliminate substances used in previous processes.
The weighting operation is carried out mostly on yarns to promote recovery of the weight loss after the removal of the sericin. The treatment consists in the deposition of tin salts or in grafting polymer chains to the functional groups of the fibroin protein chain.
Weighting is defined as «equal» if the final substrate weight is the same as it was before degumming, and as higher if the weight is higher. Weighted silk is different in touch and in draping. The most frequently applied procedure is a «mix weighting» process.
Weighing with minerals
The silk yarn undergoes a treatment using tin tetrachloride in baths at different concentrations (strong or weak baths) in an acid medium. In a strong bath, silk can increase its weight by about 10 % simply by absorbing the salt. The procedure is followed by accurate washing cycles to eliminate unfixed salt and to hydrolyse the salt present on the fibre. Such operations can be repeated to further increase the silk weight. To ensure fixation of tin salts, weighting is completed with a treatment in a sodium phosphate bi-basic solution, followed by a second treatment in sodium silicate.
Disadvantages of this procedure are the long processing time and the high water and energy consumption