**2.1 Polypropylene**

Polypropylene (PP), as a representative of modified plastics, is widely used in textiles, construction, and other industries, and can also be used to make fiber products, automotive plastic parts, woven bags, etc. Of these, PP is an attractive material for packaging due to its low cost, higher tensile strength, glossy and versatility [10].

However, since isotactic polypropylene is a spiral crystal and its unique molecular structure, its molecular chain is more susceptible to light, heat, oxidative degradation, and poor product cold resistance, resulting in its application range, especially as engineering materials and outdoor products are greatly restricted [11].

Gallo and co-workers have studied the natural weathering of both thin and thick polypropylene film samples [12–14]. The authors concluded that the degradation behavior of these PP films is different in its oxidation products and crystallinity. The structural rearrangement or chemical modification occurs mainly in a region nearer to the surface of the material during weathering. Hence, the thinner PP films were degraded easily than the thicker PP films or plates.

Rajakumar et al. [15] used mathematical models to predict attempted life time of PP. Their experimental and simulation results showed that the carbonyl growth is more affected by ultraviolet (UV) and cumulative total solar radiation for PP weathered during summer. The loss in tensile strength of PP weathered during summer is more dependent on the average temperature and the UV portion of the total solar radiation whereas, intensity of UV radiation has profound effect on the tensile strength of PP weathered during winter.

Meanwhile, the UV-induced degradation of PP has been investigated by many authors and reported in a number of reviews and research articles [16–18].

Ni et al. [19] have studied the effects of indoor temperature difference aging and ultraviolet light aging on the aging degree of PP samples by means of mechanical properties, capillary rheology and scanning electron microscopy.

Their experimental results show that the retention of elongation at break was 58.7% at 20°C and 6.7% at 100°C, respectively. And the greater the temperature difference, the faster the brittle fracture speed of PP and the greater the degree of fracture. The torque at a temperature difference of 100°C was reduced from 0.188 N·m to 0.099 N·m, the sample is degraded to a large extent and loses its value in use. The elongation at break under ultraviolet irradiation of 340 nm for 20 days was 15.3%, while under ultraviolet irradiation of 313 nm, which was 3.1%. The torque of PP samples aged under ultraviolet irradiation of 340 nm decreased from 0.188 N·m to 0.112 N·m for 20 days, while for samples under ultraviolet irradiation of 313 nm, the torque decreased to 0.084 N·m, respectively. At this time, the sample completely loses its use value. They concluded that different aging methods make different aging degrees for PP and the ultraviolet irradiation of 313 nm makes more serious aging for PP samples because of higher energy.

## **2.2 Polyethylene**

According to the difference in production technology and physical and chemical properties, polyethylene molecules can be divided into low-density polyethylene (LDPE), high-density polyethylene (HDPE), medium-density polyethylene (MDPE) and linear low-density polyethylene (LLDPE) [20].

Among them, LDPE molecules are generated under high temperature and highpressure conditions, and the reaction conditions are violent. The generated molecular chain has numerous branches, loose molecular arrangement, and relatively low crystallinity (that is, the percentage of crystalline regions in the polymer molecular chain), which is manifested as material density Small, poor strength, but with good flexibility, light transmission, and relatively easy to degrade; HDPE is produced by polymerization under lower temperature and pressure conditions through the action of a catalyst, with a few branches, dense molecular chain arrangement, and high molecular crystallinity. The material has high density, high mechanical strength, and the slowest degradation [21–23].

Polyethylene has a long molecular chain, large relative molecular mass, high crystallinity of the chain segment, strong hydrophobicity, and it is difficult to contact with biological or chemical substances or enter the microbial body to be catabolized; in addition, inside the polyethylene structural unit (−[CH2-CH2]n-), the physical and chemical properties of the CC and CH bonds of [CH2-CH2]n are stable, and higher energy or force is required to break the molecular bonds. The relative molecular weight is large, the molecular chain is long, the chain segment is high in crystallinity, and the hydrophobicity is strong [24–27]; these determine that the polyethylene molecule is difficult to degrade, so the degradation process under natural conditions is very slow [24].

Albertsson et al. [28] tracked and monitored the 14CO2 release of polyethylene film under soil landfill conditions through 14C labeling of polyethylene materials. The study found that the degradation rate of polyethylene film was only 0.2%~0.5% when buried in soil for 10 years. Ohtake et al. [29–31] further analyzed the surface molecular structure and molecular weight of polyethylene film and plastic bottles that have been buried in the soil for 32 years and calculated that the complete degradation of the polyethylene film with a thickness of 60 μm under the conditions of field soil landfilling is probably required 300 years.

The non-biological oxidative degradation of polyethylene refers to the oxidative cleavage of covalent bonds when the molecular chain of polyethylene is subjected to non-biological factors such as light, heat, and mechanical force higher than the energy of the covalent bond between the molecules. Under aerobic conditions, the process of rapidly reacting to generate unstable intermediate products such as peroxy or hydroperoxy, and further reacting to generate small molecules or low molecular weight substances such as aldehydes, ketones, acids, esters, carbon monoxide, etc. [32].

Chiellini et al. [33] analyzed the material molecular weight and surface properties and other indicators and confirmed that the decomposition rate of polyethylene film increased significantly with the increase of temperature (55, 70°C).

Briassoulis et al. [34] used artificial heating and ultraviolet radiation to speed up the aging of the residual polyethylene mulch film and backfilled it into the soil to observe the decomposition of the mulch film in its natural state. The results show that the polyethylene film without artificial accelerated aging treatment is not degraded significantly after being buried in the soil for 8.5 years, but after high temperature (50°C treatment for 800 h) and ultraviolet radiation (under 35~45 W·m-2 ultraviolet radiation, the lamp irradiated at a distance of 25 cm for 800 h). The polyethylene film was backfilled into the soil for 8.5 years and then

*Accelerate the Aging of Polymer as Energy-Saving Method Prior to the Pyrolysis Process DOI: http://dx.doi.org/10.5772/intechopen.99995*

completely decomposed into plastic micro-particles with a diameter of less than 1 mm, and the degradation process continued. The above results show that environmental factors such as light and heat can significantly promote the degradation reaction of polyethylene film.

Prakash Bhuyar et al. [35] selected plastic packaging sheet as LDPE sample (the low-density polyethylene) and garbage plastic sheet as HDPE sample (high-density polyethylene) for the degradation test under ultraviolet light. Such samples were placed in the Laminar Air Flow (LAF) under UV light of wavelength of 253.7 nm up to 30 days.

They found that the LDPE sheet after 30 days of UV-treated is more fragile and completely broken up into small parts compared to the LDPE sheet after 15 days of UV-treated. The longer the UV treatment applied to the LDPE sheet, the more crack and fragile the plastic sheet. The percentage weight loss for UV-treated LDPE was 87.5% for 30 days.

By comparison, they found HDPE sheet after 30 days of UV-treated is more crack and tear up into small parts compared to the HDPE sheet after 15 days of UV treated. The longer the UV treatment applied to the HDPE sheet, the more crack the HDPE sheet will be. The percentage weight loss for UV-treated HDPE was 21.6% for 30 days. This shows that the percentage weight loss for UV-treated LDPE is higher than the weight loss for UV-treated HDPE.

LU Lin et al. [36] have conducted the exposure test of medium density polyethylene (MDPE) for different time periods up to one year in Xisha (Paracel) Islands.

Their experimental results showed that during the aging period, the color abbreviation and hardness increased, while the gloss, the retention of tensile strength and that of elongation at break decreased. The aging evaluation showed a stepwise increasing tendency of aging rate with the extending of exposure time.

### **2.3 Polystyrene**

Polystyrene (PS) is one of the most commonly used plastics at present. In recent decades, PS has been widely used through different ways of modification. Therefore, it is of important significance to study the aging behavior of PS. Polystyrene (PS) is a multipurpose polymer that is used in varied applications in rigid and foamed form. Polystyrene is manufactured by the addition polymerization of the styrene monomer unit. At room temperature, polystyrene is normally a solid thermoplastic, but can be melted at higher temperature for molding or extrusion, then resolidified. Styrene is an aromatic monomer, and polystyrene is an aromatic polymer [37].

PS is a widely used as thermoplastic. Its hardness, hydrophobic nature and chemical composition cause it to persist in nature without any decomposition for long period of time thus cause environmental pollution [38].

Polystyrene degrades very slowly in nature and the expanded polystyrene is not easily recyclable because of its lightweight and low scrap value. It is generally not accepted in curbside programs. Expanded polystyrene foam takes 900 years to decompose in the environment and has been documented to cause starvation in birds and other marine wildlife [39].

Shah et al. [39] have observed that the UV-irradiation having wavelength range of 365 nm has a profound effect on polystyrene sample. The reduction of the molecular weight of polystyrene shows the degradation of polystyrene macromolecules by irradiation. The increase in number of chain sessions per polymer with the increase irradiation time indicates in the increase in rate of degradation of polystyrene with irradiation time.

Zhang et al. [40] have conducted several UV accelerated weathering tests on PS samples in 4 different ways. They used UV 340 nm, irradiance 0.89 W/m2 . Among them, one way, the sample is unaged. The test method A, the sample was irradiated for 12 hrs at 60°C. The test method B, the sample was irradiated for 8 hrs at 60°C, then 4 hrs condensation at 50°C. The test method C, the sample was irradiated for 8 hrs at 60°C, then 3.75 hrs condensation at 50°C, 0.25 hrs water spray at ambient temperture (without irradiation) as the last step.

Zhang et al. [40] have investigated the thermogravimetric curve of PS samples which can reflect the aging degree of the sample because when the molecular weight decreases under the action of ultraviolet light, resulting in a corresponding decrease in the initial thermal decomposition temperature. The result is shown in the **Figure 1**. In the **Figure 1**, the PS sample aged by Method A has the lowest initial decomposition temperature, followed by B, and then C compared with unaged sample. It shows that the degree of influence of the three test conditions on the aging of the sample is method A>method B>method C. Their results show that UV radiation is the main factor for the chemical aging of PS, the temperature promotes the aging behavior, but humidity has little effect on the aging behavior.

### **2.4 Polystyrene foam**

Polystyrene foam (EPS) is prepared by adding polystyrene (PS) resin as the main body and adding foaming agents and other additives. When EPS is exposed to the natural environment, under the influence of light, heat, oxygen, water, and other factors, the appearance will appear yellowing or even cracks, which will deteriorate its performance and shorten its service life…The aging behavior of EPS is the fundamental basis to determine service life and service environment.

Yao et al. [41] have found: with increasing time of UV accelerated aging, on the molecular chain of EPS some coloring groups that turned the color of the sample surface yellow were produced. With increasing aging time, cushioning efficiency decreased. When aging time was more than 100 h, there was a tendency to lose cushioning property. With increasing aging time, molecular weight of EPS

**Figure 1.** *TG curves of PS after UV aging test (regenerated figure from reference [40]).*

decreased, the stability of the molecular structure of EPS was destroyed and photooxidative degradation occurs. The chemical degradation, resulting in molecular chain breaks and recombination phenomena, causing in the decline of mechanical properties.
