Effect of Light on Cotton Textiles

P. James Fynn, James D. Dean.

To the chemist, light is something more than just the absence of darkness. It is a form of energy that can change the physical and chemical nature of substances.

Light consists of a group of electromagnetic radiations capable of stimulating the eye. It is a small portion of a wide range of radiations that include radio and radar waves, diathermy waves, ultraviolet rays, and X-rays.

When the radiations are arranged in a progressive order of their wavelengths, the array is referred to as a spectrum. The visible spectrum begins at the long-wavelength end with red and passes through the rainbow colors to violet at the short-wavelength end, each color having' a progressively shorter wavelength. When all of the visible wavelengths stimulate the eye at once, the effect is perceived as white light. We use the word light to include also invisible radiations immediately adjoining the visible spectrum, the infrared at one end and the ultraviolet at the other.

Radiation absorbed by substances increases the energy content of their molecules and so may cause changes, the nature of which depends upon the kind of radiation absorbed. Thus, long-wavelength radiation, when absorbed, increases the average speed of molecular movements and so raises the temperature of the absorbing material. Short-wavelength radiation may cause displacement of electrons in the atoms composing each molecule of the absorbing substance and activate it so that it becomes chemically unstable. A chemical change resulting from this absorption of radiation is known as a photochemical reaction.

VARIOUS WORKERS who have studied the photochemical reaction of cotton have shown that the most important effect of light is an activation of the cotton cellulose. It accelerates enormously the reaction of cotton cellulose with oxygen of the atmosphere.

We do not know when the discovery was made that light has a harmful effect on cotton textiles. About the middle of the nineteenth century, manufacturers of curtain and casement cloth noticed that the materials lost more strength in the parts directly exposed to light than in the shaded folds. Probably the systematic investigation of photochemical action on cotton was fostered in the latter part of that century by Georges Witz, vice president of the Societe Industrielle de Rouen, who published results of some original studies on oxidized cellulose. That in itself would be of academic interest only, were the oxidation of cellulose not accompanied by changes in the strength and appearance of the cotton. Because of the importance of such characteristics to the users of cotton textiles, it is of practical interest. During the First World War the harmful effect of light on textiles became a military problem. The Second World War stimulated further research on the mechanism of deterioration and the development of protective treatments.

Many scientists now are engaged in research to evaluate and solve the problem of photochemical degradation of cotton. Basic to this research is an understanding of the chemical nature of cellulose, of which cotton is almost entirely composed.

Cellulose is a polymer substance : Its chemical structure consists of a simple molecular pattern that is repeated many times. The patterns are joined like links of a chain into long, threadlike molecules. The chainlike structure gives cotton its fibrous qualities. Light attacks the cellulose at the points of linkage and thus shortens the average chain length. Although photochemical degradation, until it is well advanced, does not alter the feel or appearance of the cellulose, it reduces the strength and finally causes the loss of all fibrous properties.

To approach logically a study of the destruction of cotton by light, one has to know something of the quality and quantity of the light.

Because the quality of light depends on wavelength, an evaluation of the effect on cotton samples requires selection and control of the wavelengths. That is accomplished in several ways:

(1) By selecting a light source that radiates its energy in narrow bands of wavelengths, as does the mercury arc;

(2) by using conjunctively a wavelength selector, such as a spectrometer, and a light source that radiates all wavelengths of the light spectrum (known as a continuous source) ; (3) by using a continuous source and shutting out all but the desired portion of the spectrum with colored-glass filtering screens.

None of the methods is perfect. In the first, the kind of light available is limited to the particular wavelengths characteristic of the source. The second gives the greatest selection and purity of radiation, but it is limited because of the very small area illuminated by the selected radiation and because of the feebleness of the beam obtained with even the most powerful sources. The third is limited by the nature of glass color filters, which either transmit efficiently broad and often overlapping spectral bands or transmit inefficiently the required narrow and exclusive spectral bands.

The amount of light falling on the cotton can be measured accurately by several devices. Bolometers, thermopiles, pyrheliometers, and radiometers operate on the principle of the conversion into heat of all radiation falling on an especially prepared surface. The resulting thermal effects can be measured accurately by a change in electric current or potential, the expansion of mercury in a glass tube, or the increase in velocity or torsion of a suspended, rotating vane. Actinometers depend on measuring the chemical change in a system that reacts in proportion to the total amount of light it receives. Photoelectric cells of various kinds either generate or conduct an electric current proportional to the amount of light falling upon the sensitive surface.

After one has subjected samples of cotton to the action of known quantities of light of selected quality, he can measure the photochemical effects by several means. The most direct method, but not the most sensitive, is to compare the tensile strength of the cotton before and after exposure, as determined by a machine that registers the force necessary to break the material by pulling. Because of the inherent variability of the mechanical properties of cotton, even the most carefully determined strength measurements cannot be precisely duplicated. Because of its directness, speed, and simplicity, however, the breaking-strength method is widely used.

A more sensitive index of photochemical damage is in the measurement of the viscosity of a solution of the exposed cotton. Cuprammonium hydroxide solution, a solvent made from ammonia, water, and copper, will convert cotton into a clear solution. Solutions thus made with undegraded cotton are thick, viscous liquids, because the long, threadlike molecules of cellulose move with difficulty past one another when the liquid flows. Badly degraded cotton, whose chainlike molecules are broken and fragmented by photochemical reaction, however, produces a thin, watery solution.

Cotton in various stages of degradation yields solutions with all gradations of viscosity between that of cotton with a high degree of polymerization and that of seriously damaged cotton with a low degree of polymerization. For example, data obtained by exposing cotton to the action of sunlight for progressive periods up to 3 months gave viscosity indices of 0.07, 0.04, and 0.03 poise (unexposed control, 0.37 poise), which indicates an average molecular chain length in each instance of 1000, 600, and 430 polymer units (unexposed control, 2250). By way of comparison, the results were approximated by 30 hours of exposure to radiation from either the carbon-arc or mercury-arc lamp, two sources of artificial light.

OTHER MEASURES of the photochemical damage to cotton by exposure to light depend on detecting and following the chemical changes produced by irradiation. One of these, the copper-number method, measures the ability of the cotton to reduce the copper in a copper salt solution from the high- to the low-valence form. The reducing power of the cotton cellulose depends on the reaction of the terminal group of atoms at the end of each chainlike cellulose molecule. It is obviously greater when many short-chain molecules, rather than few long-chain molecules, compose a given sample.

Another method of following the chemical change produced in cotton by radiation is the methylene-blue test, which depends on the affinity of the basic dyestuff methylene blue for oxidized cellulose, the cotton absorbing more of the dye as the oxidation proceeds.

Still another method requires a closed system, so that analyses can be made of the sample and its surrounding atmosphere before and after irradiation to detect evidence of oxidative reaction.

These methods, developed by many investigators, have contributed to a growing body of information on the problem of the deterioration.

According to the accepted theory of photochemistry that higher energies are associated with the shorter wavelengths of light, the ultraviolet light should be expected to be the most damaging to cotton. That has been shown by the rapid degradation of cellulose exposed to light rich in ultraviolet, such as the light from mercury- and carbon-arc lamps. Although the proportionately greater effect of ultraviolet light made it customary to disregard the effects of other parts of the spectrum, it was our thought that light from other parts of the spectrum might not be insignificant. This prompted us to investigate the spectral distribution of the cotton-degrading radiation in sunlight.