Abstract

The present work deals with the photodegradation of cotton fabric in Egypt produced by monthly and continuous one year exposure to environmental conditions at industrial areas e.g. Helwan city and urban areas e.g. Alexandria city. Thus cotton fabric was exposed to environmental conditions at Helwan and Alexandria cities in May 2002 at first and till the end of April 2003. The produced deterioration was assessed by different methods. Thus, physical and mechanical changes in tensile strength, elongation as well as whiteness, yellowness and brightness were studied and evaluated. The variation of acidity of cotton fabric was also illustrated.

Introduction:

The deterioration of textiles is largely chemical in nature. Both natural and atiificial light sources can cause photochemical degradation. The effect is cumulative and irreversible. The level of illumination and the duration of exposure determine the rate of deterioration. Limiting both factors will reduce damage. The exposure of high polymers to the action of sunlight is known to result in progressive break down of molecular chain at the exposed surface ^{(I, 2)}.

Two kinds of reactions, photolysis and photosensitization, may be involved when cellulose is degraded by light, and a third reaction is heat, which involves light but is not photochemical, since no light is absorbed⁽¹⁾.

Photolysis is a true photochemical reaction. When sufficient light of proper wavelength is absorbed by cellulose itself to cause disruption of a chemical bond, the reaction is referred to as photolysis. Pure cellulose does not absorb light which exceeds that of extremely low wavelength far below the limiting 2700 of solar ultraviolet reaching the earth. Therefore, direct degradation of cellulose by light on earth takes place only in the laboratory under carefully controlled conditions of light source ^{(2, 3)}.Furthermore, in photosensitization which is a secondary photochemical reaction, cotton fabric itself does not absorb near the ultraviolet or visible light. The primary action (which is photochemical) is between the light and foreign matter present in the cellulose. An added substance either as an impurity or added treatment, can absorb light or shift the effective light range limits. This second substance, the sensitizer, carries the absorbed energy to the reacting molecules of the fibers. Sensitizer absorbs the light and causes a reaction changing water or oxygen present to either hydrogen peroxide or ozone. Hence, in turn react with the cellulose in a straight chemical reaction to degrade it. Milligan⁽⁴⁾, suggested that possibly the cotton substrate acts as an activator and oxygen is converted to ozone ( photosensitization of the oxygen ) which then degrades the cellulose, or the ultraviolet radiation activates without degrading the cellulose which is thereafter susceptible to be attacked by gaseous oxygen. The effects of photosensitization will vary tremendously, depending on the amount of moisture and oxygen present ^{(5, 6)}

Light and oxygen convert the cellulose to oxycellulose which is then degraded further to strongly colored low molecular weight compounds⁽⁷⁾.

Heat has a destructive effect on cellulose, and degrades it by increasing the rate of chemical reaction ⁽⁸⁾.

Higher templtratures accelerate the rate of chemical reactions, speeding up the degradation of fibers, dyes; and contaminants ⁽⁹⁾.

Many studies have recommended that the effect of temperature and relative humidity on the photodegradation of fibers is best studied together, because a change in temperature almost invariably cusses a change in moisture content ^(10-15). At dawn the temperature is lowest, the relative humidity of the air is greatest, and therefore the moisture content is in the maximum, during the day the temperature rises, the humidity of the air consequently falls, and the moisture content of the pattern also falls reaching a minimum value. It is clear that these two factors work in opposition for when the temperature is highest; the moisture content is the lowest, and vice versa. ^(16.17)