>>4094077“Why does plastic become brittle with age?”
Plastics are made of polymers plus some additive materials (fillers, lubricants, plasticizers, colorants, stabilizers, ...). Performance of plastic products are determined and controlled either by their polymer(s) components or by the additives added for different purposes or both together. The service and storage environmental conditions also play significant roles on the plastics products performance, including their aging.
First, let’s look at the role of polymer itself in aging and changes of plastic products performance and properties along time. Polymers go through various kinds of degradation reactions during their storage period and service life in much shorter period of time than other two major classes of materials of ceramics and metals. Degradation can be caused or accelerated at elevated temperature (thermal degradation), under sunlight (photo degradation) and in the presence of air (oxidation) as well as exposure to biological agents and microorganisms (biodegradation) and contact with chemical materials (chemical degradation). Exposure of polymers simultaneously to a combinations of these degrading agents may synergistically affect the degradation process; for instance, sunlight and air together, causing photooxidation.
One of the mechanisms of degradation of polymers consists of formation of strong chemical-bonding crosslinks between macromolecular chains of polymer. As a result, the polymer losses its flexibility, becomes stiffer and brittle, while it was initially ductile and flexible. As a result, the macromolecular chains cannot move as easily as before. This is the outcome of the aging phenomenon of many polymers, therefore, their mechanical behavior changes along time due to aging. A good example is plastic (PE) films used as mulch in farms. Initially, the film is soft and flexible, but at the end of the season, it become brittle due to weathering process under sunlight, rain, and air.
