Page 17 - Curriculum Visions Dynamic Book

char. Polyester, the most commonly used artificial fibre, is one of them.
At this high temperature (called the tensile glass temperature) the fibres can be changed permanently from a rigid, glasslike form to a plastic one. This is the temperature at which permanent-press features can be added to a fabric. These features will remain in the fabric no matter how many times it is washed and ironed.
Another approach is a chemical treatment, often called durable press. Many fibres are partly held together by special bonds called hydrogen bonds, which link the chains of molecules in a polymer (a process called cross linking). This makes the fibres very stable and resistant to change. That is why it is difficult to wrinkle a dry fabric.
However, hydrogen bonds are the same bonds that
hold water molecules together, so in damp conditions (for example, when wearing sweaty clothes) water molecules can also be bonded by the hydrogen bonds. Water then begins to get trapped and bonded between the chains. This breaks up the tightly cross-linked structure of the dry material. In fact, technically, water acts as a plasticiser, making it easier for the chains to take on a new shape. If the fabric now becomes creased, such as happens when wearing clothes or washing them, there are fewer hydrogen bonds to keep the chains held tightly in shape.
If a fabric like this is to resist creasing, then new bonds have to be provided that are not affected by water. This
is the basis of easy-care and permanent-press treatments
of fabrics. It is usually achieved by dipping the fabric in formaldehyde. Formaldehyde is not soluble in water. The formaldehyde produces its own bonds between chains.
If the crease-resistant process is to be used, then any desired pleats have to be put in place before the treatment.
Using formaldehyde can make the fabric wear out more quickly. To compensate for this problem, fibres are blended. Some of the fibres then take on the crease-resisting chemical, while others in the blend do not, so keeping their strength. For this reason polyester fibres are often blended with cotton or rayon. The polyester fibres provide the easy-care finish, and the rayon or cotton add strength or wear-resistance.
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