Chemical and Physical Structure of the Fiber

Chemical Composition. In polyamide 6 (PA 6, polymerization product of e-caprolactam) and polyamide 66 (PA 66, adipic acid polymerized with hexamethylene-diamine) one chain end consists of an amino group, which can be present in the free state or in the acylated form. Amino groups are of special importance for dyeing because they form ammonium groups in an acidic dyebath by addition of protons. The lower dye uptake in comparison to wool is caused by the comparatively low number of amino groups. The depth of color achieved on PA 6 is somewhat less than that on PA 66. 

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The ability of a textile fiber to absorb water from the atmosphere is important in many applications. In addition to the chemical and physical structures of the fiber, the temperature and water content of the atmosphere strongly influence the amount of water that is absorbed by the fiber. In this respect, a standard atmosphere is defined as air maintained at 20 °C and 65% relative humidity. Special humidity and temperature conditions are sometimes prescribed for the testing of textiles for specific purposes. The test specimens are often brought to moisture equilibrium in the standard atmosphere before tests are carried out. 

The main influencing factors are the chemical and physical structure of the fiber material and the design of the textile (for example, capillary effects). Also, finishing processes can play a part. 

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