Ariful Haque Ashik
Assistant Professor, Dept. of Textile Machinery Design and Maintenance
Bangladesh University of Textiles (BUTEX)
To many people, Spun sizing is an art and that chemistry is a minor and annoying part of that. But what we will discuss today will hopefully lead us to believe that sizing is both an Art and a Science. We will also try to make the Science or Chemistry part of the discussion understandable and enjoyable.
Now as we have seen on the first and second part, about using starch and PVA as film former, in this last part of the article, we will discuss about the other parts of the sizing chemical recipe.
It has been believed for years that lubricants are required for spun sizing. The reasons for their usage range from: 1) reduces the sticking of the size film, especially PVA, to the dry cans; 2) Eases the break at the bust rods; 3) reduces the yarn/metal friction forces that the yarns experience during weaving.
There are others who argue that waxes actual hurt the spun sizing operation. They say that: 1) waxes degrade the size film resulting in a decrease in film strength and a decrease in flexibility; 2) waxes are hard to remove in finishing and contribute significantly to the quality problems experienced by the finishing plant.
For the purpose of this article, we will not enter this argument. We will simply state the different types of lubricants used in the textile market today. It is of note that there are significant differences in lubricant type usage between the US spun textile market and the rest of the world.
In the US domestic market, tallow based wax is the norm. Depending upon styling, the wax level can range from 4-12% with the majority of mills running around 7-8%. Just as starch and PVA had different grades, so does wax.
The basis of all tallow waxes is Hydrogenated Tallow Glycerides (HTG) and bleached tallow. In fact, the combination of these two is the #1 utilized wax in the domestic textile market. Other additives that are added to the wax include:
Anti-sticking agents (Lecithin) to stop sticking to dry cans.
Emulsifiers to assist the finishing mill with desizeability
Defoamers to assist in reducing the foaming of a size solution.
Paraffin or Marine Glycerides to harden the wax and better lubricate the yarns.
(These harder waxes could pose a problem in the finishing area).
In the US domestic textile market, they also use a small amount of synthetic lubricants, such as Polyethylene Glycols (PEGs); however, their use has been minimized due to the higher price of these types of waxes.
Internationally, however, the exact opposite of the above picture is true. Synthetic lubricants are the norm and tallow waxes are used to a much smaller extent. The reason appears to be due to the finishing concerns, especially in Europe. Many mills had experienced quality issues related to the removability of tallow waxes in Finishing. While US mills will use caustic to “saponify” the wax to assist in its removability, many International Finishing mills expect the wax to come off in the desize step, with water or enzymes only.
Humectants are no more than products that will hold a certain amount of moisture in the size product, even under extremely low relative humidity situations.The most common types of humectants are: urea, sugar, and glycerin.
By holding moisture in a size film, the size film can be made to be more flexible and less brittle.This is especially important for formulas with high levels of starch where film dryness can cause increased shedding.
Humectants are added to a size blend at levels between 2-10%, based on the amount of starch used, the grade of starch, and the weave room relative humidity conditions. You don’t want to be adding too much Humectant so as to make the warp sticky!
Binders are a whole seminar by themselves.They are used to impart properties to the size that the principal film formers can not offer.For example, for a hard, plastic like film, a Polyacrylate would be added to the size blend to impart this property.Maybe a high viscosity is required, in that case we may add a CMC or Guar Gum binder. Other types of binders available are Polyesters, Polyacrylamides, etc.
Binders are usually added at less than 20% of the size blend.They are usually added in a liquid form, but can be incorporated into one-piece pre-packaged products.Binders have some limitations.The first is related to their cost.They are usually more costly than PVA when considered on a dry pound basis. For example, if PVA were to cost $1.00/lb in a bag and the liquid binder (25% solids) were to cost $0.50/lb. in a drum, that does not mean that the liquid binder is cheaper.Quite the opposite.Remember to take the solids level into consideration.In this case the binder would cost $0.50/0.25 or $2.00/lb dry or double the price of PVA !
Putting it all Together:
Now that we know what size chemicals we have available to us, how do we decide on the exact formulation? This now gets into the Art and economics of sizing. Again, it is important to remember that the sizing department is NOT a profit center and they will want to watch their costs closely.The one cost they can control is their size chemical cost. However, it may not always be in the best interest of the company to reduce this cost. It must be remember why we are sizing: To weave the yarns at a high efficiency and quality.If substituting sizes for a lower priced or inferior product occurs, what is the outcome in the weave room? Does the efficiency go down and the warp stops up?
One number that all weave room managers should have at their fingertips is what does 1 warp stop or 1 fill stop per CMPX (hundred thousand picks) cost this weave room in real dollars over a one month or year period? If that number is compared to the savings experienced by changing sizes, I think you will see that in over 80% of the cases, the efficiency number far exceeds the cost of sizing!
What this is saying is that slashing departments should not just begin with price in mind, but performance!
The ability to put together sizes for textile mills relies on knowledge of how various ratios of the different starches react with PVA, waxes and humectants. It requires the ability to understand the settings on the slasher and how to manipulate those settings to reach the optimum size composition. This is not something that is taught in schools, but something that is learned in actual; practice in the mill.
I recommend that each textile mill make use of their size suppliers to assist them with formulating the size solutions for their individual mill. Remember that these size companies have years of formulating experience and have probably formulated products for styles and conditions very similar to those that you might be looking at. Believe me when I say that I am sure that any size company will be happy to assist you with your formulating questions.