Since the application of vacuum in the venting ports, the vaporized impurities and degradation products will be removed more effectively. Therefore, new negative reactions from the degradation prod- ucts and impurities will be blocked. In turn, viscosity of the polymer will be effected positively. Vacuum application can be employed one or two venting ports. All combinations should be examined in terms of vacuum application. Ethylene glycol (EG) may release from polymer due to degradation during the extrusion. While in the first venting port surface
humidity of the waste material is removed, in the second venting port removal of specially released EG will block to reverse the reaction. It is well known that, polymerization of PET is a reversible reaction. When excess EG is not removed, it will cause to reverse the reaction and macromolecule chains will start to breakdown.
Another important point is filtering in the stage of regaining of the waste fibers. Filtering will both catch degraded polymer parts and dust inside the material and help mixing of the material due to in- creased feed back inside the extruder.
Since a suitable dryer for the waste fiber could not be obtained, the drying steps were not applied in a required level. The level of drying was 300 ppm in the experiments while the required at least 50 ppm for staple fiber production, 30 ppm for filament fiber production. It is believed that, it is possible to extrude the material, which is dried up to 50 ppm level, with the help of venting port without hydrolytic degrada- tion. Therefore, the material should be transported directly to the extruder from the drier without having contact with air.
Nitrogen feeding should be used in order to eliminate oxidative degradation. It is thought that the nitrogen feeding from the extruder entrance towards the hopper side will be more beneficial. Therefore, the oxygen entrance into the extruder will be blocked and also some of the humidity on the material will be removed with the pressure of nitrogen. That means, the nitrogen feeding will both stop oxidative degra- dation and decrease hydrolytic degradation.
Some additives should be used in the extrusion of the waste material because the waste material is subjected into more degradation than that of a virgin material. In order to add controlled amount of additives a special device such as a dosage unit should be mounted to the entrance of the extruder. In addition to decrease the degradation during re- extrusion process, the use of stabilizator is recom- mended. It must also be remembered that recycled material contains some impurities such as spin fin- ishes and dyes besides-COOH groups and hydroper- oxides which start, increase and catalyze degradation reactions. Stabilizators containing phosphite and/or phenolic antioxidants are known beneficial for recy- cling process [12]. Stepek and Daoust [13] recom- mend to use stabilizators containing benzene rings and polymeric antioxidants due to decomposition of phenolic antioxidants at 100–150°C. Triphenyl phosphate, which is a common additive used in PET fiber production to prevent thermal degradation, was used in pre-tests without having dosage unit. The results from pre-tests showed that uncontrolled addition of triphenyl phosphate decreased color value b compared to waste material without additives. However, the amount of DEG significantly increased when triphenyl phosphate used. In turn, IV value of the product was lower than that of recycled material without additives.
The change of temperatures, even 1°C, affect the viscosity of the material. Therefore, temperature and heat control of the extruder is so important. From the extruder manufactured and the test made with PD derivative control system after 50°C, however, it was concluded that the control of the heat of resistance should be PID control system due to necessity of sensitive heat control of polymer which is affected with 1°C temperature change. In the experiments, type J heat element used, but the results indicated that PT 100 would be more beneficial.