What is Solvent Scouring? Solvent scouring is the treatments of fabrics in organic solvent medium to remove impurities such as lubricating oils and spin finishes. Certain organic solvents will readily dissolve oils fats and waxes and these solvents can be used to purify textiles. Removal of impurities by dissolution is called Extraction. Basically solvent scouring is a refinement of dry-cleaning and is carried out either batchwise or continuously.
Solvent scouring appears to be alternative to aqueous scouring and particularly suitable for polyester and woollen goods. This process not only permits scouring, but also simultaneous scouring and desizing or even partial bleaching in one step. Solvent scouring is also useful for the removal of lubricating oils from knitted polyester, oligomer from polyester and bulking of knitted goods. Wool felts less in solvent media and is particularly useful in wool milling and application of shrink-resist resins to wool. Solvent processing has been established due to reduced water pollution, reduced energy cost and consumption apart from effective removal of impurities. Solvent preparation gives excellent results in terms of uniformity, reproducibility and high absorbancy.
Suitable Solvents: It is found that waxes are removed by solvents like chloroform, benzene, carbon tetra chloride etc. But those are not used industries because of their high cost and toxicity. Now a days widely used trichloro ethylene, perchloro ethylene etc.
Properties of Solvents Used in Solvent Scouring:
Boiling Point: Temperature at which solvent is converted from liquid to a gas.
Specific Heat: The amount of energy needed to raise one gram of solvent one degree centigrade (Calories/gram/°C).
Latent Heat of Evaporation: The amount of energy needed to vaporize one gram of solvent (Calories/gram).
Table: Properties of solvents used in solvent scouring
Elements
B.P.°C
Latent heat of evaporation (Cal/g)
Specific heat (Cal/g/°C)
Surface Tension (Dyenes.cm)
Water
100
545.1
1
72
Trichloroethylene
87
57.3
22
32
1,1,1 Trichloroethane
74
58.5
0.25
26.4
Perchloro ethylene
121.2
50.1
0.21
32.3
Trichlorofluoroethane
47.6
35.1
0.21
17.3
Listed above are some of the more common solvents used commercially. These are among the safest as they are generally non-flammable. However, they must the handled with care because the chlorinated ones are on the suspect carcinogen list of regulated chemicals.
Working Process of Solvent Scouring: There are commercial processes where textiles are cleaned with organic solvents. Fabrics processed this way are said to be “Dry Cleaned”. Although not widely used as a fabric preparation step, it is an important way of removing certain difficult to remove impurities, where a small amount of residuals can cause downstream problems. Garment dry-cleaning is more prevalent.
For fabrics that do not have to be desized, solvent scouring is an effective way of removing fiber producer finishes, coning and knitting oils. Knitted fabrics made from nylon, polyester, acetate and acrylics, are particularly amenable to this method of preparation. Wool grease is effectively removed by solvent scouring.
Extractions are particularly useful in the laboratory for determining the amount of processing oils added to man-made fibers and the residual amounts of oils and waxes left by aqueous scouring. Properly controlled, fabrics can be produced with very little residual matter.
Fig: Schematic diagram of a Solvent extraction unit
Above figure shows a schematic of a continuous, solvent scouring range. The entire range is enclosed so the vapors are contained and not allowed to escape into the atmosphere. Recovery units are installed on the range to insure that. none of the solvent is allowed to vent to the environment. Usually carbon adsorption towers are use for this. Also a solvent distillation unit is needed to reconstitute the pure solvent and separate the removed contaminants.
The term solvent scouring is also used to describe processes where amounts of organic solvents are added to aqueous scouring formulations to assist in the removal of oils and waxes. This technique is widely used and a more in-depth discussion will be found elsewhere. Organic solvents have a number of advantages that make them particularly useful for wax removal.
One important factor for solvent processing is the stability of solvents to recovery by distillation or by adsorption process. The recovery of stabilizer in the solvents is also equally important.
Advantages and Disadvantages of Solvent Scouring: Solvents dissolve almost all oils and waxes. They have low liquid surface tensions and quickly and easily wet out and penetrate fabrics with waxes. They are much easier to evaporate than water, requiring less time and energy. On the negative side hydrocarbon solvents are flammable and present explosion hazards.
Most chlorinated solvents are proven or suspect carcinogens and some are known to contribute to atmospheric ozone depletion. Chlorinated hydrocarbons thermally decompose to form phosgene and hydrochloric acids. These decomposition by-products are corrosive to metals and also damage cellulosic fibers. Solvents are expensive so they must be recovered and purified by distillation requiring special equipment. The distillation residue becomes a solid waste disposal problem. Solvents do not aid in the removal of motes, metal ions, starch and other solvent.
There are several major advantages of solvent scouring for wool. Felting and entanglement associated with aqueous scouring are largely eliminated, wool grease recovery is much increased, suint recovery may be designed into the process, and aqueous effluent problems are avoided. At first sight then, there are several clear cut advantages to be gained from solvent scouring, with only minor disadvantages – the necessity for solvent recovery, possible toxicity and fire hazards to be set against it.
The main difficulties of solvent scouring are the need for systems to recover the solvent from fabrics after processing. Apart from this, solvent can remove binders from bowl fillings and adhesives from laminated bowl covers and rapidly attacks~ conventional rubber coverings with expensive effect. At high temperatures adsorption of solvent into the fiber web increases and the decreasing action in subsequent steaming operation can have embarassing side effects. However, the growing problems associated with the increasing demand for raw water in some countries and disposal of effluent in general have given an impetus to the solvent scouring system.
References:
Chemical Technology in the Pre-Treatment Processes of Textiles by S. R. Karmakar
Wool: Science and Technology Edited by W S Simpson and G H Crawshaw
Dyeing and Chemical Technology of Textile Fibres by E. Trotman
Pretreatment of Textile Substrates by Mathews Kolanjikombil
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
