Brittleness sunlight exposure

Properties Thermoplastic, white powder or clear film odorless tasteless. Chlorine content 29-30.5%. Soluble in aromatic hydrocarbons. Softens at 110-120C. Films are highly resistant to moisture, oils, acids, and alkalies but tend to become brittle on exposure to sunlight. The life of such films is greatly extended by the incorporation of suitable stabilizers and plasticizers. Nonflammable, nontoxic. 

Exposure to solar irradiation (sunlight) may alter their chemical integrity and in due course some physical properties such as mechanical strength (e.g. brittleness) or color (e.g. fading) as well. Additives like industrial ultra violet (UV)-absorbers, hindered amine light stabilizers, radical scavengers and antioxidants are incorporated or applied in these products to effectively reduce or delay such radiation induced deteriorations. 

Becomes pliable at 25-30°, plastic at 60°. mp 100° (partial dec). On exposure to air and sunlight, it absorbs oxygen and becomes brittle. Insol in water partially sol in hot alcohol 90% or more dissolves in chloroform, carbon disulfide, petr ether, oil of turpentine. Keep under water and protected from light. 

Exposure of the molded HDPE articles to sunlight and air also attacks the polymer over time, especially at wavelengths less than 400 nm. Photooxidation resembles thermooxidation in that it is a complex chain of radical transformations. Such exterior aging of the polymer results in development of surface cracks, brittleness, changes in color, and a deterioration of mechanical and dielectrical properties. Photooxidation degradation is prevented by small amounts of light stabilizers, such as 2-4% carbon black or, for colorless articles, esters of salicylic acid or derivatives of benzotriazole or benzophone and others in the 0.1-0.5% range. 

Conventional spar or marine varnishes produce a film-forming finish and are not generally recommended for exterior use on wood. Shellac or lacquers should never be used outdoors because they are brittle and very sensitive to water. Exposure to sunlight causes varnish coatings to become brittle and to develop severe cracking and peeling, often in less than 2 years. 

Polystyrene (PS) is prepared by the polymerization of styrene (CeHs—CH=CH2), also known as vinylbenzene. Commercial PS is mostly of the atactic variety and is therefore amorphous. The polymer, on decomposition, unzips and forms the monomer with some benzene and toluene. Its major defects are poor stability to weather exposure, turning yellow and crazing in sunlight. In spite of these drawbacks and its brittleness it has found wide use as molded containers, Uds, bottles, electronic cabinets. As a foamed plastic it is used in packaging and insulation. The thermal conductivity of the expanded PS foam is about 0.03 Wm K The foam can absorb aromatic hydrocarbons usually found in the exhaust of automobiles and buses, causing the foam to disintegrate after long periods of normal exposure to a polluted environment. 

Unstabilised PE very rapidly becomes brittle when exposed (prolonged exposure) to sunlight, caused mainly by UV radiation. Degradation is accompanied by marked deterioration in mechanical properties, gloss loss, and cracking. 

There are good prospects for lead in a range of other new or potential uses. These include the use of lead compounds as anti-oxidant stabilisers in asphalt which has a tendency to become brittle after prolonged exposure to heat and sunlight. Road trials continue in the United States, Canada and Australia, and commercialisation appears imminent. This woik has indicated that the most effective of these compotmds can double the life of asphalt, with clear cost and material economies. 

---