Md. Jasimuddin Mandal Govt. College of Engineering and Textile Technology, Serampore, India. Email ID: jasimmandal@gmail.com
Weighting of Silk: The weighting process is carried out to increase the silk weight, providing fuller hand, more luster and bulk, and making the fiber suitable for the manufacturing of fabrics to be used, for example, for ties. We can say simply, the process of increasing the weight of the silk material is known as weighting of silk. The weight increase is expressed as percentage weighting above or below the parity. Parity weighting means that the fiber regains the original weight it had before the degumming process:
Percentage weighting = (weight after weighting – raw weight) x 100/ raw weight.
There are many types of weighting; till some years ago, a few mills still carried out mineral weighting, but now this process has been abandoned definitively. Today, the most frequently applied type of weighting is synthetic weighting (or chemical linking).
As a result of the loss of sericin during degumming, silk loses up to 25% of its weight. It is often commercially desirable to make up the weight loss. Traditionally, weighting was achieved with the addition of metal salts, notably tin salts, which possessed the added advantage of providing flame resistance. However, the use of these salts is nowadays considered ecologically undesirable, and indeed silk fibers are rendered weaker because of their increased susceptibility to hydrolysis and oxidation. Weighting with salts has now been superseded by grafting methacrylic acid or methyl methacrylate to the fibers and initiating polymerisation. This approach does not compromise the strength of the fibers.
Objectives of Silk Weighting:
After the processing of silk material, it loses about 25% of its weight particularly after degumming.
This loss in weight leads to a great loss of money since they are very expensive.
To compensate the loss, some weight is artificially added to the material by chemical means.
Owing to the expensive nature of silk, it is necessary to compensate the weight loss.
Other Objects:
Weighting is also done to reduce limpness.
To impart a bulky effect.
To control the scroopy effect.
To give body to the fabric.
To give a greater filling capacity.
Silk Weighting Process: Tin salts are widely used for silk weighting. A variety of methods has been used over the years to weight silk; these can be classified into three categories:
Vegetable weighting
Mineral weighting
Polymer weighting
1. Vegetable weighting: The use of tanning agents is a simple operation causing improvements in hydrophobicity; however, due to their inherent color, they cannot be applied on light-colored dyed fabric or white fabrics. The method is now rarely used.
2. Mineral weighting: Tin phosphate/silicate is the most popular weighting process. The application is made in several stages and careful monitoring ensures minimum undesirable side effects. The silk is soaked in stannic chloride at room temperature for about 1.5 hr, hydro-extracted, and washed in cold water. Washing removes any unfixed stannic chloride and hydrolyzes fixed stannic chloride into metastannic acid. The silk is further treated with dilute disodium phosphate for 1 hr at 60–70°C, washed, and acidified to give an insoluble tin phosphate compound. The whole process is repeated until the required increase in weight is obtained. Finally, an insoluble tin silicophosphate is formed by treating silk with dilute sodium silicate at 70°C for 1 hr.
Deposition of the metallic salt takes place in the amorphous regions of the fiber, with little alteration of fabric appearance.
Mineral weighted fabric can be identified by a burning test. Untreated silk melts partially and forms little char, whereas mineral weighted silk burns to leave an ash. The silk weighting process includes the application of 30–300% solutions of inorganic salts of aluminum, iron, lead, tin, or zinc to the fabric in order to increase the body and weight.
3. Polymer weighting: Organic weighting by grafting has recently been introduced to decrease cost and to avoid deleterious effects of metallic weighting. This method of weighting is based on a treatment of methyacrylamide and ammonium persulfate. It is reported that polymer weighting of up to 50% can be achieved without impairing the fabric handle. The disadvantage of this method is that it alters the dyeing properties of the fiber; however, it is preferred as it is less harmful to the environment than mineral weighting.
In other ways of silk weighting, There are three methods which are followed:
1st method: The silk is soaked with stannic chloride solution followed by fixation with sodium carbonate followed by soaping.
Marginal weight increase is observed but the strength is also adversely affected in this method.
2nd method:
The silk is soaked in stannic chloride and the fixed with sodium phosphate.
It is then washed and treated with little amount of sulphuric acid.
It is then soured, washed and taken out.
Even though increase in weight is considerable, the strength loss is still high in this method.
3rd method:
In this method the fixation is done with sodium silicate.
This brings out the required increase in weight without affecting the strength much.
Normal Method:
In the normal practice silk is soaked in stannic chloride solution called Picking.
Later it is treated with sodium phosphate called Phosphating.
The picking and Phosphating is carried out alternatively till the sufficient weight is achieved and the sequence is:
Picking washing – Phosphating – acidifying
Finally after sufficient loading it is treated with Sodium Silicate.
Chemistry:
Stannic chloride + sodium phosphate Tin phosphate
Tin phosphate + sodium silicate Tri silicate of tin
Synthetic Weighting: Chemical principle for weighting with methacrylamide: The monomer used for synthetic weighting is often derived from acrylic or methacrylic acid. The silk weighting with acrylonitrile and methymethacrylate has been studied and described thoroughly; in this process, starters are formed by a redox system based on iron salts (Fe++) and hydrogen peroxide, persulphates and other substances.
References:
Advances in Silk Science and Technology Edited by Arindam Basu
Reference Book of Finishing By Pietro Bellini, Ferruccio Bonetti, Ester Franzetti, giuseppe Rosace, Sergio Vago
The Chemistry of Textile Fibres by R. H. Wardman and R. R. Mather
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