Textile Finishing Processes
The whole cycle of finishing consists of mechanical and chemical processes, which are used depending on the kinds and end uses of the fabric. Mechanical processes include drying, calendaring, schreinering, embossing, sueding, raisingetc and chemical processes include in the application of special substances on the fabric, impregnation with size, starch, dextrin and other polymeric substances.
A fabric steamer uses steam rather than heat to remove wrinkles. The steam, and slight pressure of the steamer’s surface, relaxes the fibers rather than flattening them. Because of this process, using a fabric steamer is gentler on clothing, faster than using an iron, and eliminates scorching.
The fabric steamer is ideal for use on napped fabric, such as velvets and velveteen. A traditional iron will crush the nap, unless used with a needle board, but the fabric steamer doesn’t exert pressure, preserving the luxurious look and feel of any material. Even very delicate materials, such as satins and silks, benefit from the gentle care of a fabric steamer.
It is a process whereby the fabric is run through a sanforizer; a machine that has drums filled with hot steam. This process is done to control the shrinkage of the fabric.
It is the mechanical straightening and dying of the fabric. Tenter fames hold the fabric with special pins. The chain is spread apart to the desired width of the fabric. The fabric is moved through dying units. Later the fabric is rolled on cylinders.
Fabric calendaring is effected in special machines I.e. calendars, the main working organ of which is rolls with smooth surface for normal calendaring engraved surface for emboss calendaring and engraved finer lines for schreinering calendaring or for getting crepe effect. The calendar may be 3 bowl or five bowl and the contacting one bowl is plain steel roller and the other may be covered with rubber otherwise the fabric at nip point will break if both bowls are hard.
Glazing or rolling calendar: This method is not particularly important for nonwoven fabrics, with occasional exceptions. The smooth surface can be obtained usually by selecting an appropriate form of bonding and, especially, for drying a wet-bonded web. Calendaring has not met with much success since it is often accompanied by undesirable compression. The only time a rolling calendar is used is when two steel rollers are paired to break the so-called ‘blotches’ in spun-bonded fabrics.
Moiré or goffering calender: The calenders are common in nonwoven finishing and are used in the compacting of the webs made of natural and synthetic fibers. This type of calendering can be considered to be both a bonding and finishing process. Webs composed of longitudinally oriented cotton or viscose fibers with a GSM of about 10-30 g/m2 can be stiffened and compacted sufficiently by passing them through a goffering calender when slightly damp. Hot embossing of synthetic fiber webs, even when the fibers are longitudinally oriented, produces a product remarkably strong due to the fibers melting at the embossed areas. The patterns can be of grid, webbed or point type. The temperature of the heated rollers is generally 20-30°C above the melting point of the fibers and the nip roll pressure 20-50dN/cm, depending on the volume of the web and the proportion of synthetic fibers it contains. If the web is cross-laid, point embossing results in maximum strength. If the fibers are arranged lengthwise, webbed embossing is employed.
The embossing effect is used to obtain special effects such as leather graining, simulated weave, plaster, brush strokes, cord and mock tiling. Another area in which heated calenders used is in the manufacture of laminates. Here thermoplastic fibers, layers of thread or film are placed between two layers of non-plastic web and are fused together by heat and pressure. Such laminates are used as tablecloths, seat and cushion covers. Calenders are also used in the transfer printing of the bonded webs.
Crabbing is a preliminary treatment for both un-dyed and dyed woven fabrics with differing objectives. In the case of un-dyed woven material the crabbing process serves to fix the fabric so as to avoid too intensive creasing and felting at the subsequent dyeing stage. After being dyed the woven fabric is smoothed and leveled by crabbing. Silicone blankets are used in this process.
Perforating and Slitting
The nonwoven bonded fabrics produced are too stiff and are, therefore, unsuitable for clothing. This is because the individual fibers are not free to move in relation to one another, as are threads in woven or knitted fabrics. Perforating and slitting are two methods practiced to improve the fall or drape of nonwoven bonded fabrics.
The Artos method is a method of perforating in which the web, which has been bonded by using chemicals, is perforated with hot needles. This process not only punches holes but also reinforces as a result of cross-linking and condensation of the bonding agent. The Hungarian firm Temaforg uses a similar method to perforate webs made of synthetic fibers to produce nonwoven bonded fabrics which are strong and yet supple enough for use as building and insulation materials.
Slitting originally developed to improve the softness and drape of films was used by the Breveteam Company for interlinings, in particular for adhesive
fixable interlinings. The optimum cut length and distance between the slits to get maximum softness and fall without serious reduction of strength can
be calculated. The effect of slitting allows greatest flexibility at right angles to the direction of the slit.
The slitting is accomplished by a roller with small blades mounted on it, for example, in an off-set arrangement 1.7 mm apart, making slits of a
maximum length of 6.5mm. Rotary knives with spreaders can be fitted to the roller, thus making an interrupted cutting edge. Polyethylene or polyamide
film shaped by splitting or embossing and stretching by the Xironet and Smith-Nephew methods make good air permeable bonding layers.
For getting anti crease effect usually melamine formaldehyde, urea formaldehyde and dimethylol dihydroxy ethylene urea (DMDHEU), butane tetra carboxylic acid (BTCA) etc. can be used. At very high temperature, they react with cellulose and give permanent anti crease effect. The following reactions take place between the cellulose macromolecule and DMDHEU
The usual method is Padding with DMDHEU and Catalyst -> Drying at (90-100) degree C for 5 minutes -> Curing at (140-150) degree C 5-3 minutes
The following recipe can be used:
Stabitex FRD/Fixapret CPN (DMDHEU) = 75 g/L
Ploy Vinyl acetate = 20 g/L
Ammonium sulphate = 10 g/L Or Magnesium Cholride = 10 g/L
Sodium perborate = 0.3 g/L
The fabric is padded with the above solution and then dried at 100 degree C following curing at 160 degree C for 3 minutes. Curing can be carried out
in the stentering machine or curing chamber.
Static electricity tends to build up in nonwovens made of synthetic fibers due to their lack of moisture regain and conductivity. This can cause problems such as clinging and dragging during processing, apparel that clings and crackles, dangerous discharge of static electricity in explosive atmospheres and tendency to attract airborne dirt and soil in processing and use. The antistats work in three basic ways. They improve the conductivity of the fibers, coat the fiber with a thin layer of material that will attract a thin layer of moisture, and finish the fabric such that it holds a charge opposite to that normally accumulated on the fiber to neutralize the static charge. Antistats can be either durable or non-durable. Examples of durable antistats include vapor deposited metals, conductive carbon or metallic particles applied by binders, polyamines, polyethoxylated amine and ammonium salts and carboxylic salts. Non-durable antistats usually consist of inorganic or organic salts or hygroscopic organic materials. Examples are quaternary ammonium salts, imidazoles and fatty amides which are cationic. Anionic antistats include phosphates, phosphate esters, sulfonates, sulfates and phosphonates. Examples of nonionic antistats include glycols, ethoxylated fatty acids, ethoxylated fatty alcohols and sorbitan fatty acid esters.
These are used to control populations of bacteria, fungi, algae and viruses on the substrate. The treatment usually prevents the biological degradation of the product or prevents the growth of undesirable organisms. Broadly classed, the antimicrobials are either fixed or leachable. The fixed treatments are durable, but the leachable treatments may transfer to the surrounding environment through migration, solubility or abrasion. A generic list of the treatments include alcohols such as isopropanol or propylene glycol, halogens such as chlorine, hypochlorite, iodine, N-chloramine and hexachlorophene, metals such as silver nitrate, mercuric chloride and tin chloride, various peroxides, phenols quaternary ammonium compounds, pine oil derivatives, aldehydes and phosphoric acid esters. Care should be taken in the application of these compounds to prevent inactivation, loss of durability or masking of the active ingredient with other finishes.
Lubricants or slip agents are generally applied as processing aids to help in stretching or to improve the process ability of nonwovens. They are also applied to aid in sewing, quilting, tufting or other processes where needles penetrate the fabric. Lubricants impart the same properties as softeners but specifically reduce fiber friction. Common chemicals include sulphonated oils, oil emulsions, silicones, esters, polyethylene dispersions and fatty acid soaps. Many surfactants may also be used. Care should be taken to avoid excessive strength loss.
UV absorbers and polymer stabilizers
Ultraviolet light can do great damage to the polymers causing photo-degradation, yellowing, loss in strength and fading of the colors. The damage is generally due to the formation of destructive free radicals in the polymer. The finish can protect the fabric by shielding the fiber or absorbing the light or by chemically quenching the free radicals. The three main classes of products used are, substituted benzotriazoles, benzophenones which are UV absorbers, and hindered amines which are free radical reactants. They are applied from a bath or added to the polymer.
Thermoplastic binders, resins and emulsion polymers
Binders and resins are widely used in the finishing of nonwovens to add strength, control stiffness, add mold ability or pleat ability, provide durable flame retardants, color, reduce linting and control shrinkage. They soften when exposed to heat and return to their original state when cooled and, hence, can be set. Emulsion polymers are also called latexes. The common binders, resins and polymers include acrylics, PVC, poly-acrylic acid, urethanes, starch, vinyl acetate etc.
Thermosetting resins and crosslinking agents
These are used to produce wrinkle resistant or permanent-press textiles. They are used to crosslink cellulose for wrinkle resistance, crosslink binders for wash durability and solvent resistance. The technology is based on the ability of formaldehyde to react with cellulose and nitrogen containing resins. The important resin types are melamine-formaldehyde, urea formaldehyde and dimethyloethylene urea. The reaction is usually catalyzed by acids, such as Lowry-Bronsted or Lewis acids. Problems encountered include formaldehyde generation, tensile loss, discoloration and amine odor.
To impart softness, smoothness and flexibility it is necessary to apply a softening agent. According to ionic nature softener can be classified into:
- Anionic softener
- Cationic softener
- Amphoteric softener
- Non ionic softener
Among them, cationic softeners are mostly used because most of the textile is anionic in nature. Therefore cationic softeners have a god affinity towards textile fibers.
The following recipes can be used:
Basosoft 8 kg
Glycerine 1 kg
Water as required
Stentering speed 45-60 m/min
Temperature in different chambers of the stenter m/c
To impart hard or stiff handle it is necessary to apply a softening agent .For stiffening treatment the following chamber chemicals can be used.
- Starch or modified starch
- Polyvinyl acetate(PVA)
- Polyethylene Emulsion
|Perapet PE 40(polyethylene)
|Fabric speed in stenter
|Pressure in Padder