Wrinkle free finish/ Crease Resist Finish
The wrinkle free finish ( also known as "Easy Care", Durable Press", "Wrinkle-Resistant", "Wash and Wear", "No-Iron" etc. ) is obtained by cross-linking cotton. Permanent press finishes function by forming cross-links between adjacent cellulose polymer chains, these give cotton some elastic and resiliency properties. Such cross-linked cotton can recover from deformation stresses and thus wrinkles will not form.
Even today, DMDHEU is the main cross-linking agent.
With magnesium chloride as the acid catalyst to initiate a reaction, it forms cross links in the amorphous regions of the fibre. The structure and reaction of DMDHEU is shown in the figure 1. Through the reaction(>NCH2=>NH+HCHO) free formaldehyde is released. Inadequate curing also leads to the liberation of formaldehyde at high temperature. A number of approaches have been developed to limit the amount of formaldehyde released, such as after washing of cured fabrics; the addition of formaldehyde scavengers like carbohydrazide to the bath; use of urea in the pad-bath or application through a spray; modification of DMDHEU to etherized, glycolated or methylated DMDHEU
Alternatives to DMDHEU are also being researched. Main alternatives are polycarboxlicacids, typically BTCA(1,2,3,4 butane tetra carboxylic acid) or citric acid. However, BTCA is expensive to use and citric acid causes yellowing. Another approach has been to use polymers of maleic acid to form ester cross-links, and yet another to fix a quaternary group through an epoxidation reaction to the cellulose chain to form cross-links. Research on all these alternatives continues. Commercially, it is the modified DMDHEU (glycolated or methylated) that is most used today. The product is pre-buffered to prevent premature curing and also pre-blended with a catalyst. Magnesium based catalysts are the most popular in use today. Citric acid or aluminum chloride can also be blended to increase catalysis when curing times are short. A typical formula for a pre-cure or post-cure finish is as follows: DMDHEU (45%), 2-20% of weight of DMDHEU but not less than 1% owb; wetting agent. 0.1% owb; and softeners, 2-8% owb.
The wetting agent allows the fibre to be internally penetrated, so cross linking can take place throughout the fibre cross-section. A high density polyethylene restores some of the lost tear strength and abrasion resistance by providing lubrication. It also has a synergetic effect with silicone or fatty-based softeners to produce a very pleasing hand. Among the silicone softeners, reactive polysiloxanies with silanol functional end-groups ( Fig 2 ) act as elastomeric finishes imparting higher crease resistance, good dimensional stability (smooth drying properties) and excellent soft handle with good sewability. They can also reduce free formaldehyde release by replacing part of the resin. Several types are available today: Pretavyl VP 1601A by Boehme Filatex Inc., and siltouch RS by Yorkshire Chemicals.
Types of Technology for achieving "wrinkle free" finish
The major application methods currently used are based on the following processes: pre-cure; post-cure; garment-dip; spray (metered) application; and Vapour phase.
In pre-cure, the fabric is treated with resin, dried and cured in flat open-width form. This fabric can be used to produce garments that resist wrinkling during wearing and have a smooth appearance after washing and tumble drying. However, it will not be possible to introduce sharp durable creases as the cross-linked fabric will resist any attempt to press in creases. Such fabrics usually find application in the womenswear market for some skirts, casual trousers and shirting where smoothness is the main criterion.
A post-cure process gives an option to produce a garment with smooth drying and wrinkle resistant properties along with sharp creases that are durable for the life of the garment. In this process the resin is padded onto the fabric and dried at low temperature ( as in the Koretron process ). The fabric is then cut, garment constructed and creases pressed into the garment. A high temperature cure in this configuration is given to cross-link the resin. This process, though giving excellent results, has not been too successful with garment manufacturers owing to obvious limitations of colors, styles and fabric weight, and the need for a direct interface between mills, garment manufacturers and retailers.
In an improvement to this process, a company in Japan gave a post-cure finish to fabric that was mercerised in liquid ammonia, giving exceptional easy care properties together with the soft handle of non-cross-linked cotton. Liquid ammonia mercerization is a treatment given at ultra-low temperatures and it causes deconvolution of cotton; smoothing of the surface; swelling of the fibre to a circular cross section; improved absorbency, strength and lustre; and a very soft touch.
The other three options are for finishing the fabric once it has been constructed into a garment. In the garment-dip method, garments are constructed from non-resinated fabric, then impregnated with a resin formula similar to that used in the post-cure process, extracted to about 65% wet pick-up and then tumble dried to 8-10% moisture content, a critical factor that is determined using a moisture meter.
In the spray method, the resin is applied by spraying it onto the garment during tumbling in an enclosed rotational device. A microprocessor is used to meter the exact amount of chemicals and to control the rotation time, desired wet pick-up, spray rate and process time. The garments are then pressed and cured as in the case of the post-cure process. The process is increasingly used for both menswear and womenswear with the market moving towards washed-down looks and softer handles.
In the vapour phase process, the fabric is dyed and finished at the mill, cut sewn and pressed into garment form before cross linking. Gaseous formaldehyde is then applied together with an acid catalyst in a special chamber oven. The garments are later steamed to induce cross-linking. Excess moisture is then exhausted. The formaldehyde itself forms the cross-links ( conventional resin will always have unreacted N-methylol groups that can hydrolyze to release formaldehyde ). The process is being used today by manufacturers of shirts and other light weight garments. However, it is reportedly difficult to control, potentially resulting in uneven treatments and higher strength losses.
Choosing fabric for "wrinkle free" finish
The garment finisher usually does not manufacture his own fabric and he may be faced with unacceptable losses in tensile strength, tear strength and abrasion resistance in the fabric when cross-linking. Because of the added value, garments rejects due to low strength may prove to be more expensive than fabric rejects.
For producing an acceptable wrinkle-free garment, several precautions must be taken in fabric selection :
(a) The base fabric must have sufficient strength to withstand 40-60% loss in tensile and tear strength and still maintain sufficient strength to provide a garment of acceptable wear life and durability
(b) It must also have excellent absorbency to allow resin to penetrate into the very interior of the fibres and form crosslinks. Surface adhering resins do not serve any useful purpose and are inefficient and wasteful
(c) If the fabric is dyed the dye must be fast to acid catalysis and high temperatures. Sulphur dyes, which are known to generate acid upon storage, are to be strictly avoided
(d) Residual extractable on the fabric (like starch from size) can react with resin and lower its effectiveness, a high degree of size removal is thus essential
(e) fabric pH should be between 6.5 to 7.0 with an alkalinity of less than 0.05% ( expressed as sodium hydroxide
Selection of equipment
Manufacturers and researchers are constantly developing equipment to cater to this specialty segment. But whether or not these new machines - particularly presses and curing ovens - are worth the investment is still an issue. Many aspects must be considered when choosing the technology; type or product (shirt or trouser), WR process, time, temperature, vacuum, and chemicals.
For high quality pressing of 100% cotton, permanent press garments, the two most important criterion are temperature and control of cycle. Presses should be equipped with high-heat, cast aluminum heads or with supplementary electrical heaters to give a processing temperature of approximately 150 degree centigrade. This temperature is crucial as it starts the curing process and controls the overall fabric smoothness. Typically, presses working in garment units which operate within the range of 116-125 degree centigrade will not give a proper finish.
In the latest series of pressing machines from companies such as Hoffman and Vapour-press International, both the temperature control and pressure applied is accurately controlled by a microprocessor. A head-hesitation feature slows down the head lifting so that wrinkles are not formed on the garment -- such wrinkles can permanently set while curing. As an extra precaution, a head positioning system allows the head to lift by an inch (after pressure application) allowing a vacuum to completely dry the garment. Premature interruption by machine operators is a major problem in garment units. An operator penalty system that adds up penalty points every time an operator tries to interrupt the cycle takes care of this problem.
Curing is usually carried out in hot-air-box or tunnel ovens. A tunnel oven is faster but requires a certain amount of garment preparation and a material handling system to and from the tunnel. Air flow pattern in these ovens controls temperature fluctuations, movement of the garment and crease relaxation, and is therefore the most important selection criteria. Pladrest Heating Ltd., pioneers in this field, have come up with a range of ovens to permanently press garments. The air velocity is finally controlled, in as much that temperature fluctuation has been brought down to a remarkable 3 degree Centigrade over a single garment. Developments are also underway to perfect microwave curing technology. As against normal convection curing, the temperature rise in microwave ovens is expected to be quicker, uniform throughout the fibre cross section, and to give minimum damage to the fibre.