INFLUENCE OF AIR CONTENT ON THERMAL DEGRADATION OF POLY(ETHYLENE TEREPHTHALATE)

The aim of these research is to investigate the air content on aging of poly(ethylene terephthalate) (PET) preforms. Three air pressures were selected and in each pressure 5 samples were aged during 21 days in 80C. Three samples were selected to be cut for determination of density with the use of hydrostatic method. Sample mass, Young modulus and surface roughness were measured for each sample before and after aging and differences between those parameters were presented as results. The changes of parameters may lead to a conclusion that mechanism of polymeric chain oxidation is dominant during thermal aging of PET. However aging process is not the fastest in atmospheric pressure but in lower air contents. This effect may be caused by greater evaporation of small molecule degradation products and shifting of reaction equilibrium in the direction of further decomposition.


Introduction
The varied properties of polymers turned out to be extremely useful for industry, which resulted in large scale use. With time, however, irreversible and often negative changes in the properties of the material occur. Physical and chemical changes generated by usage are called aging. Natural aging proceeds under the influence of the surrounding natural environment during storage and use. Artificial aging of the material occurs under specially designed conditions and is used for research purposes. Several types of polymer aging can be observed based on physical conditions that cause structural change. This processes include: biological aging, electrical aging, mechanical aging, chemical aging and photochemical aging, thermal aging. Thermal degradation of polymers is called "molecular degradation resulting from overheating". In high temperatures molecules of the polymer are defragmenting (molecular defragmentation) (SOBKÓW, CZAJA 2009). Products of this process can react with each other and with remaining polymer chains resulting in change of materials properties. During thermal degradation the molecular mass of polymer chains is impacted the most. Physical and optical properties that the process influences are: strain at break, maximum stress, rigidity, color change.
In this work poly(ethylene terephthalate) (PET) products are studied. PET is one of most common polymers used in industry. It is used for production of bottles, sprockets, and garments. The reason for PET popular use is that it retains its shape memory, i.e. after deformation it returns to the shape given before crystallization. The properties of PET depend on its degree of crystallinity. With the normal proportion of the crystalline phase (approx. 40%), it is characterized by high dimensional stability, good chemical resistance, and good sliding and dielectric properties. PET is not resistant to factors such as phenols, concentrated acids and solutions, alkalis and long-term exposure to hot water (hydrolysis). Sterilization of PET products is carried out in an atmosphere of ethylene oxide or by irradiation. It is mainly processed by injection molding at 260-290℃ (injection shrinkage is 1.2-2%). With extrusion technique films, rods, plates and fibers are formed. Extrusion temperature is 260-280℃ (LÓPEZ-FONSECA et al. 2011). Researchers have studied the degradation of recycled PET during processing and concluded that chain scission can occur and that formation of grafted copolymers and crystallization can be facilitated (ITIM, PHILIP 2015) and others concluded that during repetitive extrusion, chain scission is a dominant process and no chain branching or cross-linking were observed which decreased capabilities and crystallinity (BADÍA et al. 2009), whereas others concluded that cross-linking and chain branching occur during extrusion (NAIT-ALI et al. 2011). Researchers currently focus on the influence of physical ageing on the mechanical properties of semicrystalline PET, using several methods to characterize both the change in morphology and physical properties of PET: Such as calorimetric analysis, FTIR, X-ray, NMR and other. These studies were carried out to enhance understanding of the structure-property relationships, which are important for materials that require stability and durability during their lifetime (ALJOUMAA, ABBOUDI 2016). During the exposure to sunlight, many reactions in PET molecular chains may occur: chains scission reaction due to thermal degradation of vinylic and carboxylic chain ends, which may recombine by trans-esterification reaction (EL-TOUFAILI 2006), photo-degradation of methylene groups which will cause an irreversible rupture of the polymeric chain, and change in the color of the bottle to yellow due to many substances used in the process of synthesis and fabrication (YANG et al. 2010). Moreover, at outdoor ageing, the exposure to light and air will cause a photooxidation of PET (photochemical ageing). It is known that PET absorbs at the extreme limit of the UV band (300 nm<  < 330 nm). This phenomenon is superficial, thin layer may degrade by this reaction and is limited by the O2 diffusion and superficial light absorption. This material is also susceptible to physical aging below the glass transition temperature caused by the slow change of quenched material at a thermodynamically nonequilibrium state to equilibrium. This is related to the relaxation processes with characteristic, different time constants (SATO, SPRENGEL 2012). It results in a reduction in entropy, enthalpy and specific volume with an increase in yield stress and tensile and flexural module. Hay investigated the effect of the crystalline phase on the behavior and properties of PET (KONG, HAY 2003). It turned out that the crystalline phase limits mobility of the chain segments, influencing the macroscopic properties of the material (PANOWICZ et al. 2021). The aim of this paper is to investigate changes in PET surface structure and mechanical properties under thermally accelerated aging conditions and variable air content.

Materials and methods
Research material consisted of 26 preforms made from PET ( Fig. 1). Samples undergone the same heating cycle with variable air pressures. Samples from 1-5 were heated to 80 o C under atmospheric pressure and kept for 21 days. Samples 7-12 were kept in a 6.8 l vacuum chamber under 5.7 Pa of air pressure. Aging conditions for samples 13,14,15,17,18,19 were the same in terms of temperature and aging time but the vacuum chamber was depressurized to 2.8 Pa. The air pressure for aging process of samples 19, 21, 22, 23, 24 was 0.3 Pa. Mass and dimensions of samples were measured and presented in Table 1. Mass was determined with analytical scale RADWAG AS60/220.R2.
The samples roughness, Young modulus and mass were measured before and after aging process for all samples. Roughness was measured using SJ-210 Mitutoyo profilometer with elementary distance 0.8 mm and measuring distance 4 mm. Roughness was measured in 3 places inside of each sample (Tab. 2). Young modulus was determined using Impulse Excitation Technique (IET) with RFDA MF system by IMCE (Tab. 3).
Samples 6, 16 and 20 were chosen to measure non aged sample density by hydrostatic method with analytical scale RADWAG AS60/220.R2 equipped with density determination kit KIT-85 from Radwag. For density measurements distilled water was used as a submerging agent. With nondestructive tests (mass, roughness and Young modulus) difference between pre and post aging state were presented as results.

Results and discussion
The Hydroxyl groups generally lead to increase of crystalization rates, and thus rigidity, due to formation of hydrogen bonds (SANG et al. 2020). In lowest pressure creation of hydrogen bonds can be assumed as minimal, but cyclization processes that cause the reduction of degrees of crystalinity still occur (CHANG et al. 2015). There fore a lower rigidity of samples with regard to methane production. However the significant changes in PET samples can be atributed mainly to changes occuring because of oxidation reactions.
Samples density was measured for every type of samples. The results of measured density for aged and non aged samples is presented on Figure 5. Nonaged samples had density of 1.35 ±0.02 g/cm 3 . Samples aged at atmospheric pressure and at 0.3 Pa exhibited similar values of 1.35 ±0.02 g/cm 3 and 1.36 ±0.003 g/cm 3 respectively. PET preforms aged at 5.7 Pa and 2.8 Pa exhibited slight increase of desity values being 1.37 ±0.005 g/cm 3 and 1.39 ±0.005 g/cm 3 .
Density is usually correlated with rates of crystallinity. Higher crystallinity rate is associated with more polymer chains in a given space. Results of density change after aging are similar to mechanical properties results and show that gratest rates of crystalinity are to be expacted in samples aged under 5.7 Pa and 2.8 Pa of air pressure (XU et al. 2016)

Conlusions
The air content influences structural and mechanical properties of poly(ethylene terephtalate).
Lower air pressure can lead to higher degrees of degradation then atmospheric pressure due to faster vaporization of degradarion products. This effect can be used for faster degradation on PET products after reprocesing effects its utility properties to the point of non recyclibility.