Typical hand builder chemistry

Hand building finishes that lose their effect after one or two launderings are considered non-durable. The two major product types of non-durable hand builders are starch derivatives and polyvinyl alcohol. Natural starch is produced commercially from potatoes, com, wheat, rice and tapioca. Its two main components are polymers of a-glucose, amylopectin and amylose, shown in Fig. 4.1. The thickening effect of starch is mainly due to the long and branched amylopectin chains. 

Polyvinyl alcohol (PVA) is a water-soluble polymer with a glass transition temperature, T, of 80 °C. Although its name implies a homopolymer stmcture, the structure of commercial PVA is a copolymer of vinyl alcohol and vinyl acetate as seen in Fig. 4.2. PVA is made by the incomplete hydrolysis of polyvinyl acetate 

Hand building finishes that retain their stiffening and fullness effects after repeated launderings are considered to be durable. These products are usually aqueous emulsions of polymers that form water-insoluble films on the fibre surface when dried. The three main types of products are vinyl acetate-containing polymers, acrylic copolymers and thermosetting polymers. 

Typically, polymers of these acrylic and methacryUc esters are produced as copolymers with other acrylic and vinyl monomers. For example, acrylonitrile is often added to impart additional water and solvent resistance. Other features that can be improved include abrasion resistance, adhesion, elasticity, flexibility and film hardness. Enhanced durability to laundering can be achieved by incorporating reactive, especially crosslinking, monomers such as A -methylol acrylamide, hydroxyethyl acrylate, acrylamide, acrylic and methacrylic acid. Optimisation of polymer properties with the large variety of available monomers leads to near endless combinations of copolymers that are limited only by the imagination of the chemist and by the reality of the cost-efficiency ratio. 

Another category of durable hand builders are formaldehyde-containing thermosetting polymers. These products are usually supplied as dispersions or aqueous solutions of precondensates of urea or melamine with formaldehyde, for example di-methylol urea or di- to hexa-methylol melamine and their methyl ethers. The thermosetting polymers are comparatively inexpensive and provide fabrics with stiffness and resilience. However, they have a tendency to reduce abrasion resistance, yellow after exposure to heat, and release formaldehyde. Melamine-based hand builders are more highly crosslinked than urea-based products and are accordingly more durable. Butylated urea condensates are especially useful for rayon fabrics (see also Chapter 5 Easy-care and durable press finishes of cellulosics). 

---