Plastic aging means plastics' physical, chemical, and mechanical properties degrade irreversibly during processing, storage, and use. External factors such as light, heat, oxygen, water, chemicals, and biology change the molecular structure, leading to function loss and value reduction.
Color: Plastics may discolor or fade. For example, white ones can turn yellow, and colored plastics lose their brightness as molecular changes affect light interaction.
Surface: The surface gets rough, cracks, wrinkles, blisters, or powders. Outdoor plastic pipes often crack due to stress and internal structure changes, and polymer surfaces decompose into powder, losing luster.
Strength: Aging reduces plastics' tensile and bending strength. Aging plastic supports may break under less weight.
Toughness: Plastic becomes brittle as its ability to absorb impact energy decreases. Aging plastic toys break easily on collision because of molecular chain breakage.
Elasticity: Plastics lose elasticity and become stiff, like an aging rubber sealing ring losing its sealing function.
Solubility: Aging can change plastic solubility in solvents. Some plastics may swell or partially dissolve in solvents they were once resistant to, like aging plastic containers exposed to certain chemicals.
Reactivity: Aging plastics form more active molecular groups, increasing reactivity. This can speed up aging or cause problems such as surface stains from reactions with air pollutants.
Ultraviolet (UV) light in sunlight breaks plastic molecular bonds, causing chain breakage or cross - linking. PVC plastic, for instance, becomes hard, brittle, and discolored under long - term UV exposure. Outdoor plastic products like doors, windows, and billboards are highly affected.
In high - temperature environments, plastic molecular chains decompose or cross - link when the temperature exceeds the plastic's heat resistance limit. PP plastic may decompose thermally, losing weight and softening. Plastic parts in a car's engine compartment, exposed to high temperatures, age quickly.
Oxygen in air oxidizes plastics, forming peroxides that trigger chain reactions, leading to chain breakage or cross - linking. PE plastic in an aerobic environment gradually loses tensile strength and surface integrity. Plastic films and bags age due to oxygen even at room temperature.
For hydrophilic plastics or those in a humid environment, water penetration swells the plastic and breaks intermolecular forces. Metal ions in water can catalyze hydrolysis reactions. Nylon plastic swells and degrades in a humid environment, affecting underwater and bathroom plastic products.
Exposure to acids, alkalis, and organic solvents causes chemical reactions. Acids and alkalis can erode plastic chains, and solvents can dissolve plastic components. PET plastic hydrolyzes in an alkaline environment, and ABS plastic swells or dissolves in certain solvents. Plastic appliances in chemical - related settings are at risk.
Microorganisms like bacteria and fungi can grow on plastic surfaces and secrete enzymes to decompose plastic molecules. PLA plastic in the natural environment can be decomposed by enzymes, losing strength and integrity. This is common in biodegradable plastics and plastic waste in landfills.
Antioxidants: They capture free radicals during oxidation, halting chain reactions. Hindered phenols and phosphite esters are common types. Adding hindered phenol antioxidant to polyethylene production inhibits oxygen aging.
Light Stabilizers: These absorb or reflect UV light. Ultraviolet absorbers (e.g., benzotriazole compounds), light shielding agents (e.g., carbon black, titanium dioxide), and hindered amine light stabilizers are used. Adding light stabilizers to polypropylene for outdoor products improves light - aging resistance.
Temperature Control: Different plastics have suitable processing temperature ranges. For PS, it's 180 - 240 °C. Precise temperature control during processing reduces thermal aging risks.
Shorter Processing Time: Reducing the plastic's residence time under high - temperature and high - shear conditions, like in injection molding, lessens molecular chain degradation.
No Stress Concentration: Design plastic products with rounded corners to avoid stress - concentrating parts. Stress concentration accelerates aging.
Uniform Wall Thickness: Keep plastic product wall thickness uniform to prevent residual stress, which promotes aging, as in plastic pipe design.
Light Avoidance: Keep light - aging - prone plastics away from direct sunlight. Indoor plastic furniture and electrical housings should be placed away from strong sunlight.
Temperature and Humidity Control: Store plastics in proper temperature and humidity conditions. Precision plastic instruments need stable environments, and warehouses should manage temperature and humidity.
Coating: Coat plastics with anti - aging coatings like acrylic or polyurethane. The coating offers physical and chemical protection. Outdoor plastic fences with polyurethane coating have better weather resistance.
Electroplating: For plastics with special needs, electroplating creates a metal protective film. Chromium - plated plastic decorative parts are more attractive and have better aging resistance.
