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Wednesday, January 14, 2015

Physical and Chemical properties of Wool Fibre

Wool fibre is a protein known as keratin. Keratin is made up of amino-acids joined by peptide linkages.

Wool Fibre

In addition to carbon, hydrogen, oxygen and nitrogen it also contains sulphur. The amino-acids present in wool are mainly bulky, while silk has chiefly small amino-acids. Due to this wool is relatively amorphous. The polymer system of wool is highly amorphous that is 75%.

Physical properties of wool

Wool is a complicated weak fibre. The low tensile strength is because of comparatively fewer hydrogen bonds. When it absorbs moisture, the water molecules steadily force sufficient polymers apart to cause a significant number of hydrogen bonds to break. The water molecules also hydrolyze several salt linkages in the amorphous regions of the strand. Breakage and hydrolysis of these inter-polymer forces of attraction are explicit as swelling of the fibre and result in loss of strength of the wet woolen material.
Elasticity and resiliency:
This is elastic and resilient. Covalent bonds can stretch, but they are strong. The disulphide bonds in the amorphous parts of the strand or fibre are able to stretch when the strand is extended. When the strand is released the disulphide bonds pull the protein molecules back into their original positions.
     If there are too few disulphide linkages as when the strand has been weakened by alkali or if the extension is great enough to break some of the covalent bonds, then some polypeptide chains will slide past one another. This causes a permanent extension of the wool. The natural crispness of the fibre also supports it to regain its real shape.
Hydroscopic nature:
It has the very absorbent nature because of the polarity of the peptide group, the salt linkages and the amorphous nature of the polymer system. The peptide groups and salt linkages easily attract water molecules which enter the amorphous polymer system of the fibre. In comparatively dry weather wool may develop static electricity. This is since these are hot enough. Water molecules in the polymer system support to distribute any static electricity which might develop.
It has a comparatively low density and therefore fibres are light with regard to their visible weight.
Conductivity of heat:
It has a low conductivity of heat and therefore makes it ideal for cold weather. The resiliency of the fibre is significant in the warmth properties of the fabric. Wool fibres do not pack well in yarns because of the crimp and scales, and this makes wool fabric process and capable of inserting much air. Air is one of the best insulators since it keeps body heat close to the body. The medulla of the wool fibre comprises air spaces that increase the insulating power of the fibre.
     This strand can take up moisture in vapor form. Absorbency is a factor also in the warmth of clothing. In winter, when people go from a dry indoor atmosphere into the damp outdoor air, the heat developed by the fibre in absorbing moisture keeps to protect their bodies from the impact of the cold atmosphere.
Dimensional stability:
It has poor dimensional stability and therefore shrinks easily. Felting or shrinkage results since under mechanical action, such as agitation, friction and pressure in the presence of heat and moisture, it tends to move root wards, and the edges of the scales interlock prohibiting the fibre from returning to its original position. This results in the fabric becoming thicker and smaller, that is it shrinks or felts.

Chemical properties of wool

Effect of acids:
Concentrated acids damage it since they hydrolyze the salt linkages and hydrogen bonds. Dilute acids do not affect it.
Effect of alkali:
It easily dissolves in alkaline solutions. Alkalis hydrolyze the disulphide bonds; hydrogen bonds and salt linkages of wool and cause the polymers to separate from each other, which is looked as dissolution of the fibres. Hydrolysis of the peptide bonds of wool polymers lead to polymer fragmentation and total destruction of the strand. Prolonged exposure to alkalis causes hydrolysis of the peptide bonds of wool polymers lead to polymer fragmentation and total destruction of the fibre.
Effect of bleach:
Chlorine bleach is ordinary harmful to the wool. KMnO4, Na2O2 are utilized for bleaching.
Effect of sunlight and weather:
Effect of sunlight’s ultra-violate radiation tends to yellow white or dull colored fabrics. The ultra-violate cause the peptide and disulphide bonds to sever. This leads to polymer degradation products on the surface of the fibre. As a consequence the strand not only absorbs more light but scatters the incident light to a greater extent. The prolonged exposure to sunlight weakens the fibres very much.
Like cotton wool is easy to dye. Acid dyes, chrome and mordant dyes are utilized to dye this. The dye molecules are attracted into the amorphous areas of wool.

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About the Author :

He is Md. Abu Sayed from Naogaon, Bangladesh. He is a Textile Engineer and working as a Testing Engineer at Bureau Veritas CPS (BD) Ltd. He also Likes article writing on Textile and Clothing Technology.

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