DESIZING OF SYNTHETIC SIZES

A. RONCOLI

G. Bozzetto S.p.A., ITALY

INTRODUCTION 

The desizing operation is essential for textile finishing.

This handbook is our practical contribution to operators in the sector.

As we all know, ,insufficient, non-uniform removal of size, which protected the warp during weaving, produces a fabric with a “ hard papery” handle, or causes haloes and stains compromising its appearance and subsequent use.

Knowledge of the chemical  / physical behaviour as sizing products is certainly a key factor in fully eliminating them. As work requirements often make it necessary to treat fabrics with different types of size on the same machine and in the same bath, we think an overall assessment is useful. We shall then include indications on eliminating individual sizing products in the specific technical instructions.

 A Reaction of sizes to different agents

 Action of surfactants The specific action of surfactants is to endow baths with wetting, emulsifying and detergent capabilities.  They therefore have a favourable effect on removing both size and finishing oils. All surfactants lower surface tension of water and thus accelerate bath-into-fibre penetration time (this is very important especially for desizing in continuous systems).

 Some surfactants have also a certain ''solubilising'' power on resins used for sizing. It is, therefore, essential that, when selecting which surfactant to use, one should bear in mind all factors that help to improve and speed up desizing.

 In particular, the surfactant's stability with respect to pH and to the electrolytes in the desizing bath must be  evaluated, as well as its suitability in connection with the characteristics of the system , in which it is to be used, in order to prevent unwanted foam formation. 

The products we suggest case by case, and which can be identified among various brands  (Biomegapal-DS-Fimbil-ldrosolvan-Laif-Ottin), satisfy all specific requirements. In most cases, these products are non-ionic composites or preparations containing anionic and non-ionic detergents. Cationic surfactants are not taken into consideration because they would produce incompatibility effects on all resins containing an anionic charge. 

Use of detergent containing solvents is particularly recommended for very dirty or highly ''charged'' fabrics in ensimages that are difficult to emulsify.

 Action of sequestering agents In hard water, some sizes tend to become insoluble and thus precipitate into the bath. Although using purified water, a gradual increase in hardness can be noted on continuous lines due to the fabric (fabrics mixed with cellulose fibres) releasing calcareous salts. As a result a specific sequestering agent must be added to the recipe, which also has a strong dispersing and anti-flocculating power. Of the various possibilities offered on the market, we think that phosphonates are the family most closely satisfying the requirements of the desizing process. Bozzetto sequestering agents are easily identified by their commercial name: Sequion. 

Action of alkali Alkali can influence the desizing process either positively or negatively, according

to the chemical composition of the size in the warp.

 To give an example, acrylic sizes (ammonia salts) necessarily require caustic alkali to ensure they dissolve completely, whereas sizes with a soluble polyester base become insoluble and precipitate under the same conditions. 

The most commonly used alkali are Solvay soda (sodium carbonate) and caustic soda.

We shall therefore provide -case by case -precise indications on the type and suitable concentration of alkali to use according to type of size, fibre and desizing system. 

Action of acids Similarly to alkali, behaviour varies according to sizes in the case of acids too. Some of them are not dissolved, others are easily eliminated in dyeing pH conditions.

 Action of chlorinated solvents No kind of size is soluble in trichloroethylene, perchloroethylene or fluorochlorine derivatives.

 Action of oxidants The use of moderate quantities of hydrogen peroxide and sodium hypochlorite in desizing baths does not prevent acrylic and polyester resins from becoming soluble, but neither does it contribute to accelerate their dissolution time. The use of these substances is, in any event, recommended if there are mould stains on the fabric. The oxidant most suitable for direct use in the desizing bath is hydrogen peroxide, since it acts at high temperatures and, therefore, enables both mould to degrade and sizes to rapidly solubilize. In case of very large, widespread mould stains, long treatment time is essential and, as a result, we recommend discontinuous desizing processes (flow-jigger-etc.).

 B Factors influencing desizing 

Many factors influence desizing -we mention the most important ones below:

composition of baths: pH/ concentration and chemical[ nature of assistants 1 presence of electrolytes (salts -hardness)

 We shall provide, in the following chapters, specific indications on selecting assistants aııd on the most suitable conditions to adopt, depending on type of size and fibre used. 

The presence of calcium and magnesium salts, or of any highly electrolytic substance can cause serious insolubilisation problems to various types of size:

this is why we recommend always using purified water (with hardness below 1 o dH) for both 1 desizing and washing (at Ieast for the first rinse). 

Desizing temperature The best results are obtained by maintaining desizing bath temperature in the range of  85°C to 95°C, except when treating acetate rayon and mixed wool fabrics, which only in certain cases support these temperatures (see chapters III and V.7). 

From our experience, we consider that treatments at higher temperatures (under pressure) do not help in btaining better results in terms of size removal.

 Operating at temperatures below 85°C size film dissolution times quickly increase and, therefore, treatment time must be prolonged to maintain the same desizing efficiency. 

To clarify matters further, the following table shows the relationship between temperature and the dissolution time of a film of size in baths prepared either with or without a specific surfactant /emulsifier.

Remember that in order to obtain uniform desizing on continuous lines, temperature fluctuations must be avoided. 

Contact time between desizing bath and sized fabric Time of contact between bath and fibre is essential to establish whether a certain desizing treatment is possible on a system and prepare the best recipes according to the effective characteristics noted.

 An ''ideal'' minimum contact time between bath and fibre that can be used as a reference cannot be defined, due to the diverse mechanical concepts of different systems. However, we can provide some indications acquired through experience.

 a) Desizing on continuous lines: we recommend a minimum presence time of 60'' for the fabric in the desizing bath, divided into at least two distinct units, and a further 60'' washing time divided into two or more units.

b) Desizing in jigger: 4 passages with desizing bath, drain bath, 3 washing passages with water at 85-90°C, drain bath, cold washing by continuous drainage from overflow.

c) Desizing in Overflow-Jet-Beam: 30' with desizing bath, drain bath, 1 0-15' washing at 85-90oC, drain bath , final cold washing.

It is clear that the more accurate and prolonged the desizing and washing treatments, the better the final result. For very dense or ''special'' articles (microfilaments-textured), a double desizing  treatment may be necessary, especially in systems with Iimited mechanical action or with a very Iimited liquor ratio. 

Mechanical action of the system In general, the stronger the mechanical action, the more efficient the size removal. The mechanical action is provided by squeezing, bath injectors, bath circulation speed in the bowls and fabric transit speed.

 Weave of fabric Highly compact synthetic fabrics hinder bath penetration to a greater extent, due to the considerable swelling of the fibres (especially at temperatures above 1 OO°C), which causes further compacting of the structure.

Sizing products reduce bath penetration speed and thus desizing. It is very important to reach in rinsing the same degree of swelling obtained during desizing. This is to allow the bath to remove resin residues which had penetrated deep into the fibre (substantial swelling of the fibre actually allows sizing products to be more easily eliminated, but also enables them to penetrate inside the fibre to a greater degree). 

Goods-liquor ratio In any type of system, and especially in discontinuous systems, size elimination is encouraged by a high liquor ratio. In view,of this, desizing in jigger is certainly less efficient and sure, partly due to the system's limited mechanical action. 

Final washing All too often the importance of washing after desizing is underestimated. Remember that size residues not fully removed from the fabric Iead -after thermal fixing -to insoluble precipitates which, after dyeing, appear as haloes and stains. Furthermore, the fabric handle becomes more or less stiff, to a degree depending on the quantity of size remaining.

 In addition to totally removing the different sizing products, the final washing must ensure that the pH values of the fabric are uniform and as close as possible to neutral. Washing temperature must be maintained at 80 to 90°C, at least for the first rinses.

 C Type of process 

Discontinuous The usual systems are used, such as:

Open width treatment = jigger -beam. Rope treatment = overflow -jet -open winch

 Semi-continuous We advise against work cycles involving padding and cold storage (pad-batch),

as Iow temperature has no appreciable effect on the synthetic sizes.

In some cases, storage at 80 -90°C (pad-roll), with specific detergents (I DROSOLVAN) helps the avivage oils to emulsify -the oils are more easily removed when the fabric is subsequently washed. Storing mixed fabrics consisting of chemical and cellulose fibres is not recommended due to possible interaction of sizes with ''the electrolytes'' in the cellulose fibres, which give rise to the resin insolubilisation process thus increasing desizing difficulty.

Therefore the advisability of pad-batch desizing, during cold bleaching with peroxide 4>f fabrics mixed

with cellulose, needs assessing case by case. 

Continuous Continuous lines are based on a different mechanical concept, and also differ

considerably in terms of capacity, bath volumes, batch exchanges, etc.

In general, system management is particularly important, because operational uniformity is essential

in order to prevent, by continuous bath changes, size from accumulating.

Of course saving on water and energy must not be overlooked, but this must not be to the detriment

of the objective: good desizing results during the whole working day. 

D Notes on the tables 

Two tables appear at the beginning of each chapter, showing the behaviour of size films in different treatment baths and the indicative quantities of the films according to the different sized threads.

a) Size features To evaluate these features, the following are examined: different temperatures,

1 pH values in both acid and alkaline fields, and water hardness.

In greater detail:

-in purified water at 20°C. -50°C. -90°C.

-in an acid environment for acetic acid 10 ml/l (pH 3), at 90°C.

-in an alkaline environment for caustic soda 36°Be 5 ml/l (pH 12), at 90°C.

-in water at 30° French degrees of hardness (pH 7), at 90 °C.

Size behaviour is defined as follows:

l = indicates good dissolution of size present in the specific treatment bath, at sufficiently rapid times, ensuring good size removal when treating a sized fabric;

¥ = indicates complete solubility of the film if some specific conditions are observed -to be mentioned time by time.

¢ = indicates difficult or partial solubility of the film.

As one may not always know what type of size is present in the warp of a fabric, we advise you to run preliminary tests to define quality, following the procedure described in detail in chapter VI.2. 

b) Guide-lines on size quantity We shall indicate types of size and their percentages according to the characteristics of the threads and their use on dry or water-jet Iooms. It's easy to understand the importance of these quantitative elements in defining the best composition of desizing baths. 

CHAPTER 1 

Desizing polyester fibre fabrics 

I.1 Size used / I.2 Capability of sizes to be removed / I.3 Desizing fabrics containing sizes with acrylic copolymers (ammonium salts) / I.4 Desizing fabrics containing solube polyester sizes / I.5 Desizing fabrics containing vinyl rein sizes / I.6 Single – bath desizing of acrylic – polyester – vinyl sizes / I.7 Recovery of faulty pieces ( due to hard – papery handle or with stains caused by poor desizing) / I.8 Oligomers 

I.1 Size used 

Solube polyester resins : Wisester N 530 – Wisester N 102

Modified polyester resins : PS 15207

Acrylic copolymers – Ammonium salts : Wisacril PNS – Wisacril VLS 

 Notes The indicated percentages refer to warp only.

Size quantities are indicative and could be modified to type of thread an loom used.

 I.2 Capability of sizes to be removed

 KEY

m  : Good film dissolution

¥  : Reasonable film dissolution ( however, we advise you not to go below pH 4 to avoid reducing film dissolution speed)

l : Difficult / patrial film solubility

I.3 Desizing fabrics containing sizes with acrylic copolymers (ammonium salts) 

Presence of caustic alkali is recommended, as these types of size are virtually insoluble in a neutral environment or, worse still, in an acid environment. The ideal quantity of caustic soda depends .especially in continuous systems .on the time the fabric is present in the treatment bath. Using a fairly high concentration of NaOH is an advantage, providing a good final washing is possible on the machine. 

Specimen recipes

I.3.1 - On discontinuous machines

a)      Jigger

 or