Çapraz bağlanabilen polietilenin geri kazanılması

Polyethylene (PE) has excellent electrical insulating properties, stable mechanical properties, good fatigue resistance and resistance to chemical attacks. Because of these properties, polyethylene is one of effective and important insulating material in the cable production. PE is suitable till 75°C working temperature. Moreover, many power cables require 90°C for working temperature. Therefore, thermoplastic PE is not suitable for this kind of cables, which require 90°C working temperature. PE has molecular structure, which has linear chains of independent polyethylene molecules loosely held together by weak molecular bonds. This molecular structure of PE can be converted in to more thermal stable thermosetting compound by the process of crosslinking. Perpendicular chemical bonds are formed between parallel chains of the polyethylene molecules by crosslinking process. The parallel loose two-dimensional molecular structure is converted into a cellular three-dimensional polymeric structure. Crosslinked polyethylene (XLPE) shows superior insulation properties and high thermal stability over conventional thermoplastic insulating materials. The desirable properties of polymers are achieved through proper molecular weigth and molecular weigth distribution. Any change to the original molecular structure will change these properties and usually is considered degradation. Antioxidants are very important additives whose role it is to maintain the chemical and physical properties of polymer during transportation, storage, processing, and serving conditions. Antioxidants are chemical substances, which are added to polymers in small amounts with generally 1-2% and are capable in the course of autoxidation of trapping emerging free radicals or unstable intermediate products such as hydroperoxides and to transform them into stable end products. Primary antioxidants act by trapping or deactivating radicals after they are formed. Secondary antioxidants prevent the formation of free radicals through non-radical decomposition of hydroperoxides. In general, primary antioxidants can be used alone more or less successfully, whereas secondary are used to enhance the performance of the primary antioxidant and are not used alone. Molecular network of XLPE gives high thermal stability but makes recycling nearly impossible. Thus, the waste of XLPE is mainly burnt as a fuel or disposed in landfills. Some recycling attempts are also known, such as grinding in small particuls and put into road fillers. Some works are known in which the XLPE is tried to plasticize by various techniques. One of them applies alcoholysis of the silane crosslinked polymer in supercritical reactor. The others apply suitable heating and shearing process for partially decomposition of polymers. Commercially crosslinkable polyethylene compounds (CPEC) which contains crosslinking agent inside are ready to crosslinking. They can crosslinked easily when they faced proper condition such as high temperature. One of the main applications of CPEC is cable insulation. Some amount of CPEC needs to be bleeded from the dies of the extruders due to the providing of required process condition before starting the process. Bleeded material becomes scrap because of presence of crosslinking agent inside which limits reuse of material. Terminate crosslinkability of bleeded material will be it is reusable and prevents it to be a scrap. In this study, for recovery of commercial crosslinkable polyethylene compound (CPEC) was studied. CPEC has crosslinking agent inside and is ready to crosslinking when appling to appropriate conditions such as high temperature. Phenolic antioxidant (AO1), blend type of antioxidant (AO2), which has primary hindered phenolic and secondary phosphite, and amine class antioxidant (AO3) be on the point of being three different types of commercial antioxidants were used. Each type of antioxidant was added 1%, 3% and 5% into CPEC of separately. Another sample was prepared without adding any antioxidant into CPEC as blank sample. The samples with 1.5% AO1 + 1.5% AO2 and 2.5% AO1 + 2.5% AO2 were prepared to check the synergetic effects of AO1 and AO2. Laboratory plasticorder was used for mixing. All samples were mixed at 110°C with 50 rpm. The effect of antioxidants on crosslinking characteristics of samples were investigated structurally, mechanically, and thermally. Crosslinking characteristics minimum torque (ML), maximum torque (MH) of the compounds were obtained by using rubber process analyser (RPA) at 185 °C. While MH values decreasing, while the ML values were about stable when the types and the amounts of antioxidants were changed. The difference between MH and ML (MH-ML) can be expressed cure extend which gives information about degree of crosslinking of the compounds due to torque values are close to each other. The smallest crosslinking degrees, which was 0.36 and 0.37, obtained when CPEC mixed with 5% AO1 and 2.5% AO1 + 2.5% AO2 respectively. Mechanical property analysis of the samples showed that amount of antioxidants had a pronounced effect on the tensile properties of samples. The maximum tensile strengths decreased approximately 30% when all type antioxidants were used with 5%. Maximum decrease in elongation at break was observed when 5% of blend type of antioxidant AO2. Minimum change at tensile strength and elongation break values observed, when phenolic antioxidant used as 1%. Differential thermal calorimetry was used to measure the melting temperatures and enthalpy of fusion of the samples. There were not significant changes in melting temperatures (Tm), when any type and amount of antioxidants were added. Results of Tm values of all samples were obtained between 107 °C to 109°C. Although addition of AO1 and AO2 increased enthalpy of fusion of the samples, while those of AO3 decreased. The highest value was 117.7 J/g when 1% of AO2 was added into CPEC. On the other hand, the lowest value of enthalpy was 79.7 J/g with 1% of AO3. Hot-set test is special test for cable sector that represents mechanical resistance of XLPE by controlling its crosslinking degree. Positive result in hot-set test means polyethylene chains crosslinked each other under specified conditions. On the other hand, crosslinking is not satisfactory when negative results are obtained. Hot-set test was performed to check that CPEC can crosslinkable after antioxidant addition. According to hot-set results CPEC, CPEC with 1% blend antioxidant (AO2) and CPEC with 1% amine class (AO3) antioxidant passed the test. The other combinations failed in hot-set test. It means that, the addition of antioxidants to CPEC prevented the crosslinkability of the other samples.