Plastics undergo a number of changes in their chemical composition and structure during processing, storage, and use as a result of the combined effects of internal and external factors like light, heat, oxygen, water, high-energy radiation, chemical media, and biological erosion. These changes also affect the physical characteristics of the plastics. The aging of plastics is a phenomena that corresponds to degradation, such as hardness, stickiness, brittleness, discoloration, loss of strength, etc.

The fundamental aspect of plastic aging is a change in its physical structure or chemical composition, which manifests as a progressive deterioration in the material's performance and the loss of its usefulness.

Causes of polypropylene's aging

The macroscopic aging of polypropylene can be determined by the material's decreased intrinsic viscosity or increased melt flow rate. The major way modified polypropylene delays aging is by including different antioxidants. Under normal circumstances, it primarily serves to stop free radical and thermal oxygen aging. When thermal oxygen aging occurs, peroxides are formed. These peroxides target regular polypropylene molecules and cause a fresh cycle of thermal oxygen aging.

The modified plastic polypropylene particles' molecular main chain contains tertiary carbon atoms, which make them resistant to aging in low-temperature rooms.

The appearance of carbonyl peaks in the infrared absorption spectrum, followed by the generation of peroxides, and the formation of free radicals after cleavage, however, indicate that chemical changes are very easy to occur under certain circumstances, such as heat, oxygen, ultraviolet rays, copper damage, and other external factors. These additional free radical reactions result in the complete macromolecular chain cracking, branching, and crosslinking, which causes polypropylene to lose its performance and the properties of polymer materials.

Aging Polypropylene Effects

The macroscopic aging of polypropylene can be determined by the material's decreased intrinsic viscosity or increased melt flow rate. The inherent viscosity or melt flow rate of polypropylene decreases, which results in a reduction in the molecular weight of the material. For instance, if polypropylene with a molecular weight of 271,000 is extruded three times at 310°C, the molecular weight will be decreased to 52,300, which will result in a considerable rise in the melt index.

The melt index rise brought on by this aging is distinct from the melt index rise brought on by utilizing a cooled masterbatch, so keep that in mind. This aging is disorganized and unstoppable, and it is accompanied by a significant loss of mechanical strength.

Polypropylene woven bags are an extreme example of polypropylene aging. It makes no difference whether you keep them inside for a while. They will pulverize and lose their usefulness after 90 days if they are left outside.

Certain metals, such as copper damage, can accelerate the aging of polypropylene molecules, causing the polypropylene cable insulating layer to quickly lose its mechanical qualities. One such instance is copper damage to polypropylene cable material, which is often fixed by tinning and adding certain additives to polypropylene.

Free radicals are produced in high quantities as a result of the polypropylene molecule's primary chain breaking. On the one hand, it will keep attacking the main chain's carbon atoms, causing fresh degradation processes. Additionally, coupling or cross-linking between free radicals will occur concurrently. Although the pace of deterioration may slow down, the material will macroscopically harden and become brittle. The sensitivity to light-induced degradation will be further increased by oxidation structures produced during the degradation process (such as carbonyl groups, peroxides, etc.).

Ways to delay the aging of polypropylene

The major way modified polypropylene delays aging is by including different antioxidants. Under normal circumstances, it primarily serves to stop free radical and thermal oxygen aging. When thermal oxygen aging occurs, peroxides are created and damage the regular polypropylene molecular chains, beginning a new round of thermal oxygen aging.

Free radicals are tiny, active molecules that interact with polypropylene polymer chains to cause chain breaking and aging as well as to gently dissociate the material over time.

Therefore, we should also start from these two elements and add various antioxidants in order to stop the aging of modified polypropylene. Antioxidants can be split into two categories based on their mode of action: primary antioxidants (free radical terminators) and auxiliary antioxidants (peroxide decomposers). Antioxidants come in a wide variety and are readily available nowadays. Price, compatibility with polypropylene, and anti-oxidation properties all play a role in the selecting process.

The compound of phenol 1010 and phosphite 168, also known as B215 or B225, is now acknowledged as one of the most effective antioxidants in terms of technology and cost. The ratio of the former 1010 to 168 is 1:2, while the latter 1010 and phosphite 168 ratio is 1:1. However, the aforesaid compound cannot be employed under water immersion settings because to antioxidant 168's weak hydrolysis resistance.

Polymer materials' aging characteristics

Polymer materials age differently and exhibit diverse aging processes and properties due to various polymer kinds and usage scenarios.

For instance, agricultural plastic films that have been exposed to sunlight and rain become discolored, brittle, and less transparent. Aviation plexiglass that has been used for a long time develops silver streaks and loses transparency. Rubber products that have been used for a long time lose their elasticity and harden, crack, or become soft and sticky. The following four alterations serve as a summary of the aging phenomena.

(1) Aesthetic changes such stains, spots, cracks, silver streaks, icing, powdering, stickiness, warping, fish eyes, wrinkling, shrinkage, scorching, optical distortion, surface darkening, discoloration, and other hues a change.

(2) Modifications to physical characteristics include adjustments to solubility, swelling, rheology, refractive index, and heat, cold, water, and air permeability.

(3) Modifications to mechanical qualities can affect hardness, elongation at break, tensile strength, bending strength, shear strength, impact strength, fatigue strength, and elastic modulus.

Changes in the dielectric constant, electrical breakdown strength, surface resistance, and volume resistance are examples of changes in electrical characteristics.