Easy-care and Durable Press Finishes of Cellulosics

Introduction 
Easy-care and durable press finishes are generally applied to cellulose and cellulose blends fabrics, but other fibers can benefit from these finishes. There are many words and phrases available to describe this application;
–Easy-Care

–Durable press
–Minimum care

–Easy to iron
–No-iron

–Wash and wear
–Crease resistant

–Permanent press
–Shrink proof

–Wrinkle resistant
–Wrinkle free
The most technically correct description would be “Cellulosic anti-swelling” or “Cellulosic cross linking“ finishes.
In addition to the dimensional stability properties, the sheen of calendered fabrics (permanent chintz) and the stand and hand of pile fabrics are generally improved by resin finishes.
The primary effects of easy-care finishes on cellulose are:
–Reduction in swelling and shrinkage
–Improved wet and dry wrinkle recovery (*CRA)
–Smoothness of appearance after drying.
–Retention of intentional creases and pleats.
* CRA (crease recovery angle), is the sum of the crease recovery angles of warp and the fill directions of the fabric, increases from about 150° to about 300°
An unavoidable side effect of the cellulosic crosslinking finishes is reduction in elasticity and flexibility of the cellulose fibers. This produces a considerable decrease in abrasion resistance, tear and tensile strength on cellulose.

Mechanisms of easy-care finishes

The primary cause of shrinkage of cellulosic fibers is the fact that these fibers can readily absorb moisture. This absorbed moisture facilitates internal polymer chain movements in the amorphous fiber areas by lubrication. It disrupts the internal hydrogen bonding between these polymer chains. When a moisture laden cellulosic fiber is stressed, the internal polymer chains of amorphous areas are free to move to relieve that stress. Hydrogen bonds can reform between the polymer chains in their sifted position. With no restoring forces available, a newly formed wrinkle or crease will remain until additional processes (ironing for example) apply adequate moisture and mechanical forces to overcome the internal forces.
The swelling of cellulosic fibers by moisture can be reduced by application of self crosslinking urea or melamine products as well as products that mainly crosslink with cellulose molecules.
Without such a crosslinking finish, cellulose fibers can take up more than 10% of their weight in water. As the fibers swell, the fabric must crease and shrink to relieve the internal stress caused by swelling.
Two different chemical approaches have been used commercially to produce non-swelling or durable press cellulose fabrics.
-The incorporation of a polymerized finish in the pores of the fibers, so that water molecules cannot easily penetrate the fiber.
–The reaction of multifunctional crosslinking agents with hydroxyl groups of adjacent cellulose molecules that hinder the swelling of cellulose fibers. Chemistry of easy-care finishes: 

1. Formaldehyde containing resins:

a. Urea formaldehyde resins (DMU):

i. Highly reactive. It has to be used within a few hours in the finishing bath
ii. Low stability to hydrolysis, low durability to laundering
iii. High Chlorine retention
iv. High content and release of formaldehyde
v. Very high elastic resilience
vi. End product: DMU (Dimetoxymethyl urea) 

b. Melamine-formaldehyde resins:

i. These products are mostly three to six reactive N-methylol groups connected to one melamine ring. This leads to a higher crosslinking and easy-care finish with better wash fastness.

ii.These products provide tri-to hexamethylol melamine (TMM, HMM) and their methyl ethers.
iii.TMM is preferred for easy-care finish, often only as a component of product mixture to give a better performance of effects. It is also used for permanent chintz of cellulose. These products produce firmer hands than HMM and are extensively used as hand builders.
iv. Properties of melamine-formaldehyde resins:
1) Better stability to hydrolysis and better washing durability.
2) Relatively high formaldehyde content and release
3) Better chlorine retention than DMU.
4) More dimensional stability and stiffness (also nylon and polyester) 

c. Glyoxal Resins:N,N-Dimethylol-4,5-dihydoxyethylene urea (DMDHEU):

i. This chemical is the basis for about 90% of easy-care and durable press finish products on the market. DMDHEU is synthesized from urea, glyoxal and formaldehyde.
ii. It is less reactive than DMU and TMM and therefore requires more active catalyst.
iii. It is more stable than finish baths with DMU and TMM.

d. Formaldehyde in Fabrics:

i. Free formaldehyde: Free formaldehyde is defined as the uncombined monomeric formaldehyde that exists in finish solution.
ii. Formaldehyde release: Formaldehyde release is the amount of formaldehyde that escapes from a fabric into the atmosphere. 

e. Linkages responsible for formaldehyde release:

i. Cellulose readily picks-up formaldehyde from the atmosphere. This will give positive reading during testing.
ii. Sources of gaseous formaldehyde are uncured resin or pendant N-methylol groups. It is difficult to cure 100% of the resin that is applied and make sure there are no pendant N-methylol groups left.

iii. The third source of released formaldehyde is the crosslink itself. The finish will decompose under certain test conditions and liberate formaldehyde.

2. The main properties of DMDHEU based products: 

a. Low to very low reactivity (when ether modified)
b. Excellent durability and laundering.
c. Low chlorine retention
d. Medium to very low formaldehyde release.
e. The most commonly used durable press products.

3. Non-formaldehyde containing products(DMeDHEU): 

a. DMeDHEU doesn’t contain formaldehyde. It is synthesized from the relatively expensive N,N-dimethyl urea and glyoxal.
b. Like DMDHEU, it can be modified by reaction with alcohols such as methanol, diethylene glycol or 1,6-hexanediol to ether derivatives.
c. These products are less reactive than DMDHEU types because of their hydroxyl groups. Stronger catalysts or harsher reaction conditions are needed for successful crosslinking.
d. DMeDHEU costs about twice as much as DMDHEU products.
e. In order to achieve comparable easy-care and durable press effects to DMDHEU, nearly twice the amount of DMeDHEU is needed. However, a1:1 mixture of DMDHEU and DMeDHEU is popular because of its reduced formaldehyde levels with slightly inferior physical properties at an acceptable cost. 

4. The main properties of DMeDHEU products: 

a. Formaldehyde free
b. Very low reactivity
c. Very low chlorine retention.
d. Limited durability to laundering
e. Yellowing effect when not ether modified
f. Development of unpleasant odors, depending on the product formulation
5. Catalysts for easy-care and durable press finishes
a. The reaction of DMDHEU with cellulose requires an acid catalyst for acceptable yields under conditions suitable for textile processing.
b. The most common catalysts are Lewis acids such as magnesium chloride and zinc nitrate that generate acid conditions during curing process, thus providing neutral liquors and good finish bath stability.
c. Sulfuric and hydrochloric acids and their ammonium salts serve as excellent catalysts but also lead to undesirable fiber degradation.
d. Often citric acid is combined with Lewis acid to provide additional boost to reactions especially for short shock condensation.
e. Under acidic conditions, DMDHEU products react via oxygen protonation.

Fabric properties:

1. DP performance Vs Add-on 
a. There is a sharp increase in wrinkle recovery with increasing resin level. As bath concentration approaches 7% DMDHEU (pure), wrinkle recovery and DP rating begin to levels-off. Above this level, the rate of improvement is less rapid and only modest gains are obtained with massive amounts of resin.
2. Tensile strength and abrasion resistancea. Losses in tensile and abrasion resistance in 100% cotton are directly related to the number of cross-links.3. Crease recovery versus temperature
a. Avoid flash curing (Curing in a very short time)4. Points to considera. Losses in physical properties due to rigidification of fiber are unavoidable. Losses in strength due to cross-links can be recovered by mild acid strip. Boiling 1 hour in 1% phosphoric acid buffered with urea will remove almost all cross-links and restore about 70% of lost strength.
b. Catalyst damage also lead to losses in physical properties. That portion of the lost not recovered by acid strip. The best to avoid this problem is to use mild catalyst and avoid over curing.
5. Chlorine resistancea. The term chlorine resistance encompasses two problems
b. Yellowing of fabric by the bleach
c. Tendering (strength loss)
i. Residual–NH groups are responsible for this problem (because of the formation of chloramides)6. Fabric odora. Finished fabrics are beset by two types of odors, fish odor and formaldehyde odor. Some over cured fabrics develop an unpleasant burnt or fish odor. Fish odor is trimethyl amine which is produced by reaction of free formaldehyde with ammonia.

Application of easy-care finishes:

1. The traditional pad-dry-cure method is a dry curing process. All of the water has been removed from fabric prior to the actual crosslinking reaction.
2. It is also possible to crosslink cellulose in a wet process. Fabric padded at 80% wet pick-up with the finishing chemicals is wrapped in plastic film and batched at room temperature for about 24 hours before washing and drying. The water content of the cellulose fibers during the crosslinking step greatly affects the final fabric properties.
3. Normally a high dry crease recovery angle is preferred because the appearance of the dry textile is more important than the wet one. Between these two extremes is the ’’moist cure’’: a 5-20 hours room temperature reaction with 6% above normal moisture regain. The fabric properties of moist cure are a good compromise between the extremes of ‘’dry’’ and ‘’wet’’ process. The moist cure preferred when high tear strength of finished fabric is required.
4. A main difficulty of moist cure is humidity control (6-10%).