Regulatory and Environmental Consulting
All About Starch
by Nathan Schiff, Ph.D.
Associate Editor, Institutional, FABRI CARE
In the May/June issue of Fabricare Canada, there is a description of the first time the industry had a major problem with starch. Newly-introduced men's dress shirts withattached soft collars were giving shirt laundries fits. Customers wanted the new comfortable collars to be starched in order to provide the same protection from soiling that they enjoyed with detachable collars. But the construction of the collars made itimpossible for the fabric to absorb a large amount of starch.
Chemists working for industry suppliers found a way to handle this problem, and starch problems disappeared until the advent of blended fabric in the 1970's. Once again many customers demanded the stiffness and protection from soil that starch can give. And, once again, shirt launderers were not able to give it to them. Back to the drawing board!
Chemists found an ingenious way to starch the cotton portion of a 65/35 cotton/poly blend of fabric and then add stiffness to the polyester portion with another additive. I'll tell you how it was done. But first let me give you some basic information about starch.
Starch in Nature
We've all heard of starch in food. That same starch can be extracted for use to stiffen fabrics. The most common sources are rice, corn and wheat. It can be extracted from plants, dried into granules and used to make commercial starch. Before starch can be used in a laundry it has to be released from its granules. This is done by cooking it in a pressure vessel at 130º C. Cooked starches penetrate natural fibres more effectively than uncooked starches, and also cling better to the outer surfaces.
When you cook starch the tiny granules absorb water and swell to many times their original size. At a certain point, they break apart, releasing their starch contents (amylose and amylopectin), forming a very viscous solution. When fully cooked, the starch molecules become suspended in the water, resulting in a thick, smooth, creamy mixture. If you have starch which is a sticky, and non-uniform, viscous mass, you haven't cooked it enough. Such undercooked starch is the result of a partial breakdown of the granules, which results in uneven absorption on the surface of the shirt.
Starch is applied at an acidic pH during the last wash cycle. The solution is then drained, the fabric extracted and finished. The problem of starch build-up on a shirt press is usually the result of incorrectly-prepared starch.
The Best type of Laundry Starch
The best starch is derived from rice. This starch imparts a high quality finish to natural fibres. In the liquid or suspended form, it does not readily congeal after cooking and gives a satin-like finish to the fabric. For the customers with 100% cotton shirts who like heavy starch, a larger concentration of starch will give the stiffness they like. Other starches such as those made from wheat or corn are less expensive but do not impart the same high quality finish as does rice starch.
Chemically Modified Starches
To increase the uptake and penetration, natural starches are modified, by chemically reacting them with surfactants to form cationic starches. The effect of this reaction is to impart positive charges to the starch molecule, similar to that of a fabric softener. Modified starches attach themselves to negatively charged areas on natural fibres. They are more firmly bonded to these fibres than is possible with the simple absorption of unmodified starch. Also a greater amount of starch can be deposited, resulting in greater stiffness.
Starching Synthetic Garments
Starch attaches itself best to garments composed of natural fibres such as cotton or linen. It does not bind well to synthetic fibres. You can stiffen synthetic fibres with sizing made of polyvinyl acetate (PVA). This forms a film on the outer surface of the fibres but is not absorbed as startch is, into the natural fibres.
Only the cotton portion in a 65/35 blend of cotton/polyester absorbs starch. The higher the concentration of starch, the greater the amount deposited into and onto the cotton. The remaining 35% of the garment, made up of the synthetic fibres, remains relatively limp. For this reason using starch alone does not give a starched effect in a blended fabric. On the other hand, PVA attaches itself mainly to the synthetic fibres in proportion to the outer surface area of the fibres. It is interesting to note that a garment made of micro fibres offers a greater surface area to PVA than does one composed of more conventional fibres. Therefore, the micro fibers can be sized to a higher degree of stiffness.
Answer for Blended Fabrics
The chemists who had to find a way to impart stiffness to a cotton/polyester shirt came up with an ingenious solution. Here's how it works. A modified rice starch is added to the last wash cycle, and allowed to contact the shirts for several minutes. This maximizes the amount of starch which can be absorbed by the cotton portion. Then PVA is added to the water. This binds quickly to the synthetic fibres. Since the cotton fibres have already been saturated with starch, there is little competition for the PVA between the cotton and synthetic fibres.
This gives you the maximum amount of stiffness which a cotton / polyester blend can achieve. And it brings our story up to date. You can be sure that whatever challenges the textile and apparel manufacturers throw at us in the future, the industry chemists will be ready to meet them.
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