Learner's Digest to Textile Finishing

Image source: effeendustri.com

This post is contributed by Arnab Sen.

A learner recently asked me,
“When I stepped into the textile industry for my internship, I was asked how to detect a good fabric from a bad one. What would be the correct answer?” 
I said, “Nothing in textiles can be classified as good or bad. Even the roughest of fabric can be chosen for rugs, and the weakest of them as a filter cloth. So, it entirely depends on the end-use.”

Truly, a lot in textiles depend on the end usage. And the variety of end uses is so wide that literally nothing is wasted in textiles; everything gets consumed in some way or the other. But to prepare a particular fabric for a targeted end use, one has to choose a process or a series of processes to impart the required properties in it. And here comes the role of textile finishing.

Textile finishing the heart of fabric preparation.

We all know the phrase- “carding is the heart of spinning”, in the context of yarn preparation. Many have the wrong notion that weaving is the backbone of fabric preparation. While it is so, the heart of the entire process lies somewhere else- it is textile finishing, which we also refer to as fabric processing.
In order to have an overview of textile processing, let us try to classify them for our ease of understanding. But the classification is again not limited to only one way; there are multiple ways of doing so.

Classifying textile processing

Textile processing is often termed as chemical processing of textiles, since in most cases, there are certain chemicals involved. But it is not that only chemicals have to be relied upon for processing; there are other or combined ways as well.

1. Based on process sequence

But to start with, let us simplify the classification in some other way- based on the sequence or stage of the fabric when it is being processed. We can divide processing in this way into:-

  (i) Pre-treatment.
  (ii) After-treatment.

In pre-treatment, the processes include those required by a fabric before it is dyed. Thus, scouring, bleaching and mercerisation come under this section. These are aimed at improving the properties and absorption of the fabrics during dying, and also remove the impurities.

In after-treatment, the processes that follow fabric dying are included. They are mostly chemical finishes, while sometimes some other finishes are also included.

#2. The way to achieve a finish

Further, we can subdivide the processing of fabrics on the basis of the way finishing is done; that means whether they are finished chemically or mechanically. When we say “fabrics are finished”, we are not referring to any destructive method; we just mean to say we have come to the last part or the finishing part of the fabric manufacturing stages.

So, fabrics can be “finished” in two ways, which are:-

  • Mechanical finishes.
  • Chemical finishes.

Mechanical finishes include value addition to the fabric by use of rubbing or abrasion, like in emery, diamond and peach finished fabrics. The intensity of rubbing may be varied, thereby increasing the varieties. Also, the effect can be brought in by abrading only the face side or both sides of a fabric. These processes that depend on abrasion are done as a pre-treatment, i.e., before dying. The shades developed during dying can cover up the variations, if any, in the intensity of abrasion to a certain extent.

Another kind of mechanical finishing process exists, and it uses compressed air to pass through the fabrics at a high speed. This is called blasting, in which the softness in texture similar to mechanical abrasion based processes can be achieved in fabrics but with less negative effect on their durability. Sometimes, sand particles mixed in air are also used blast the fabrics. These processes based on blasting are however done as after-treatments.

In chemical finishes, a lot of processes come in. All of those that utilises chemicals for achieving certain special properties, like oil resistant, soil release, water repellent and other such finishes fall under this category.

#3. End use becomes important

In the third category, we can divide fabric processing on the basis of end usage, or the properties achieved. Thus, we have:-

  • Aesthetic finishes.
  • Functional finishes.

Aesthetic finishes are those that add aesthetic values to the fabrics in the form of either a better look or an enhanced feel and texture, or both. The mechanical finishes mostly fall under this category where they improve the texture of fabrics, although at the cost of their strength and durability. Some chemical finishes also qualify under this category like liquor ammonia durable press finish that makes a fabric crease resistant, easy to iron finish, etc.

Functional finishes are the ones that add functional value or improve one or more properties of the fabrics. Most of the chemical finishes fall under these categories like oil, soil and water repellent finishes, flame retardant finishes, etc.

#4. Durability of finishes

Any kind of finishing effect on the fabrics gets diminished with time and eventually vanishes. Thus, every finish has a certain bit of longevity. How to measure it? In textiles labs, it is done using the number of wash cycles that a finish can withstand. A wash cycle comprises of washing a fabric under standard conditions of temperature, time, pH and other parameters and using certain chemicals as specified by the chosen test method, followed by drying the fabric and keeping it under standard temperature and relative humidity for at least 8 hours.

On the basis of longevity, finishes are categorised as:-

  • Temporary finish- lasts only for 1-2 wash cycles.
  • Semi-durable finish- lasts 5-6 wash cycles.
  • Permanent or durable finish- lasts 10 wash cycles or more.

Most of the finishes, both chemical and mechanical, last for 1-2 wash cycles. After repeated and extensive research work, some of them nowadays show longevity till 5-6 wash cycles; but having a textile finishing effect that is permanent? It is still some way to go!

Most common finishes in textiles

Here, we will have a closer look at some of the textile finishing processes, although we will omit those in pre-treatment. We will focus mainly on the after-treatments that add value and are imparted after fabric dying. Also, we will take up only those that are different than normal in some way or the other, and are hence worth mentioning. Lastly, we will try to go in a lucid way so that it is not a technical paper and general users can enrich from the knowledge. So, here we go:-

#1. Peach, Emery and Diamond Finishes

We take up these mechanical finishes at first since they add some features that are unique and highly appreciated if properly imparted. That is why despite an important disadvantage that these finishes come up, they are extremely popular in the industry.

In these finishes, the fabrics are passed under tension over a set of rollers, one or two of which are covered with a rough surface, much like an emery or sand paper; hence the name. The rollers may be heated or cold and the tension may also vary. Even the number of grains of the rough surface, or in other words, the extent of roughness of the surface, may vary. These variations will impart different levels of effect, often referred to as suede effect, on the fabric.

What happens here is this- the rough surface abrades the surface fibres of the yarns that the fabrics are made up of. These fibres thus get partially ruptured and compressed under the resultant pressure of the tension, much like it happens with the bristles of a tooth brush after a certain time of usage. This compression gives rise to fluffing of the partially ruptured fibres together in small pools all over the fabric, thereby imparting a unique soft feel to it. This can happen on the face side or on both sides of a fabric.

The variation in tension, temperature of the rolls, speed of the fabric over these rollers (and hence the time of contact) and extent of roughness of the surface of the rollers bring about the variations in the level of suede effect in the fabric. Whether it is single sided or double sided can further add to the variations.

The major disadvantage is that the strength and durability of the treated fabrics go down, which is quite obvious since a portion of the fibres in the yarn are permanently damaged by the process. Yet, the suede effect is aesthetically so appealing that manufacturers keep on producing these special types of fabrics and clients also serve a sustainable market for them. Among the properties that nosedive because of suede effect, the most important is tearing strength. So a suede fabric must be checked for the average tearing strength that it offers.

#2. All kinds of repellent finishes

Most of the repellent finishes that have to be imparted to a fabric has to be done using chemical. While some of them block the functional groups present inside fibres to prevent an undesired attachment with certain types of molecules; some others reduce the surface tension between the fibres and foreign molecules. Some others again form a polymer coating over the surface of the fabric as a layer of protection.

In all cases, there has to be certain chemicals that are to be applied as accessories to the main chemicals acting on the fibres. These accessories help in many things, but the most important of them is binding the main chemicals to the fibres. The chemicals that help in achieving this are generally termed as binders.

Thus, oil and soil repellent fabrics are coated with chemicals that reduce the surface tension between the fibres on one hand and the oil and soil particles on the other to such an extent that they cannot bind together. The same is the course of action for achieving water repellent properties in fabrics. In case of flame retardant fabrics, a thin layer of polymers, often termed as a cross linked film, is laid on the surface of the fabrics on the face side that helps to prevent the fabric from catching fire by increasing the time of ignition; it also enhances the time of propagation of flame across a fabric.

But the binder and the film that it develops on a fabric reduces the fabric hand, or in other words, the feel of the fabric. Thus, these fabrics do not have a soft feel or texture; rather, they are crispy to touch. They also become less flexible, i.e., the drape of the fabric is reduced. Also, breathability goes down and it is difficult to wear them as apparels comfortably. We remember many a times wearing the rain coats during school days and starting to perspire very soon, thereby feeling disgusted at these, in our school days!

Besides these, there are two more major drawbacks of these finishes. One is that they are only semi-durable or temporary finishes, lasting up to 5-6 wash cycles only. It has not yet been possible to impart any of these finishes a durability beyond these numbers of wash cycles till date.

The second drawback is definitely the loss of strength. More number of times the fibres go for wet treatment, their strength keeps dipping down because of a lowering of the degree of polymerisation. These are chemical finishes, and with the formation of cross linking polymers, the strength goes down further. It is thus important to test the fabrics for tensile and tear strength after manufacturing.

#3. Blast Finishes

The effect achieved here is very close to peach, emery and diamond finishes, with advantages that (a) the effect is generally better; and (b) the effect is on both side. Besides, the lowering of strength and durability is also much less.

In this process, compressed air is passed through the fabrics in free flowing condition for some time. The high speed of compressed air together with the release of the pressure imparts a soft feel. This is achieved once again by disturbing the alienation of the surface fibres of the yarns, giving rise to small groups of tufts as colonies all over the fabric surfaces. But since any mechanical abrasion is not involved, the longevity of the fabrics is retained to a better extent.

Sometimes sand particles are mixed with compressed air to enhance the effect further, and the process is termed as sand blasting instead of air blasting. However, sand blasting has deep hazardous effects on the workers and this has been a major topic of debate in the textiles industry, especially in the context of denim fabric manufacturing, for long.

About the author

Arnab Sen is an Assistant Professor, Textile Design, National Institute of Fashion Technology (NIFT), Bhopal, India. He is a B.Sc.Tech. in Textile Technology from the University of Calcutta and an M.Tech. in Fibre Science and Technology from the Indian Institute of Technology Delhi. He has served the textiles industry of India as a teacher and a professional since 2001.

Post a Comment

Previous Post Next Post


Contact Form