Characterization of Waste Poly(Ethylene-Terephthalate) after Alkali Treatment

U radu je proveden postupak obrade otpadnog PET-a prije samog recikliranja, koji osim sakupljanja i mljevenja uključuje i alkalno pranje PET-a. Postupak pranja PET-a proveden je pri sljedećim uvjetima: temperaturama od 70 i 75 °C te u različitom trajanju (15, 18, 21, 25 i 30 minuta). Uzorci otpadnog PET-a okarakterizirani su prije i nakon alkalnog pranja infracrvenom spektroskopijom (FTIR), diferencijalnom pretražnom kalorimetrijom (DSC) i metodom kontaktnog kuta. FTIR-spektroskopijom potvrđeno je nastajanje karboksilnih i hidroksilnih skupina na ṽ =3428 cm-1 tijekom alkalnog pranja. Smanjenje intenziteta karakterističnih vibracijskih vrpci (CO, COO i CH2) također ukazuje na depolimerizaciju PET-a odnosno nastajanje kraćih polimernih lanaca. DSC-om su određene temperature kristalizacije i taljenja te se iz dobivenih rezultata može zaključiti da je tijekom pranja došlo do djelomične depolimerizacije PET-a. Metodom kontaktnog kuta određena je površinska energija, a povećanje polarne komponente površinske energije ukazuje na postojanje skupina -OH i -COOH na površini PET-a. Općenito se može zaključiti da tijekom postupka alkalnog pranja dolazi do djelomične promjene u strukturi PET-a zbog utjecaja alkalnog medija koji uzrokuje hidrolizu tj. djelomičnu depolimerizaciju, ali je PET na kraju postupka pranja ipak prikladan za recikliranje i dobivanje recikliranog materijala zadovoljavajuće kvalitete.Poly(ethylene terephthalate), PET, recycling represents one of the most successful and widespread examples of polymer recycling. This material is fully recyclable and may be used for manufacturing new products in many industrial areas. Nevertheless, the excellent properties of PET needed for its many applications are also responsible for the difficult degradation of PET and an accumulation of polymer waste, which in turn creates serious environmental problems connected to littering and illegal landfilling or incineration. The main goal of this study was to examine the effect of alkali pretreatment on the properties of PET flakes. PET flakes were washed at two temperatures, 70 °C and 75 °C and in various time intervals of 15, 18, 21, 25, and 30 min. All samples were characterized by FTIR spectroscopy, differential scanning calorimetry and by contact angle measurements. The results showed that during the alkali treatment the partial depolymerization of PET was obtained, which resulted in the formation of various types of oligomers with hydroxyl and carboxyl end groups, which were the result of loss of high molecular structure. Decrease of intensity of characteristic vibrational bands (CO at 1717, COO at 1265 and CH2 at 722 cm-1) with extended time was observed (Figs. 1 and 2). Further on, the formation of hydroxyl groups at ṽ = 3428 cm-1 was also observed as a result of PET depolimerization during the alkali treatment, which behaviour was better visible for samples washed at 75 °C and with extended washing time (Fig 2b). During the DSC thermal analysis, multiple melting peaks were observed in some studied samples which could be linked to partial melting and re-crystallization of PET or to the occurrence of new polymer fractions of lower molecular mass (Figs. 3 and 4). It is evident that the contact angle of PET samples (Fig. 5) decreases in comparison to the PET 0, which points to the changes on the PET surface during the alkali treatment. Decrease in contact angle (which is measured with water) indicates an increase in surface hydrophilicity and increase in the number of present polar -OH and -COOH groups formed during the partial degradation. Also, the values of total surface energies and their polar and dispersive components indicate that during the alkali treatment the surface characteristics of PET flakes were slightly changed due to depolymerization (Table 3). Generally, it can be concluded that partial depolymerization of PET flakes occurs during the alkali treatment but the material retains its good properties and it is appropriate for the further recycling process