Recycling PET beverage bottles and improving properties

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1 Polymer International Polym Int 3:78 73 () DOI:./pi.389 Recycling PET beverage bottles and improving properties Abdulrasoul Oromiehie and Alireza Mamizadeh Iran Polymer and Petrochemical Institute, PO Box 8/8, Tehran, Iran Abstract: Recycling PET from bottles has been carried out by three different Under optimized processing conditions, a virgin poly(ethylene terephthalate (PET), recycled PET and a mixture of virgin and recycled PET, with and without the modifier polypropylene functionalized with maleic anhydride [PP-graft-MA]), were processed. Different methods were used to characterize the processed products. The results showed that the intrinsic viscosity and molecular weight decreased as the blend ratio of recycled PET was increased. This was due to thermal exposure as well as shear degradation of recycled PET. Thermal cycles of the processes used for recycling PET and its blending specimens with virgin PET show the importance of the thermal treatment in the improvement of mechanical strength and increased crystallinity. Nevertheless, the properties of the functionalized blends were improved. This behaviour is attributed to a series of chemical and physico-chemical interactions taking place between the two components. Society of Chemical Industry Keywords: recycling; polyethylene terephthalate; functionalized polypropylene; physical properties thermal properties; mechanical properties INTRODUCTION A major problem faced by the plastics industry is that of waste disposal. Recycling material would appear to offer a solution which is satisfactory in terms of preventing environmental pollution. Increasing interest has recently, been focused on the recycling of plastic wastes, especially poly(ethylene terephthalate) scrap. Poly(ethylene terephthalate) (PET) is already being recycled and a number of uses have been found for it, including: fibrefill, fabric, automotive parts, industrial strapping, sheet and film, new containers for both food and non-food products, containers for baked goods. From this wide variety of regenerated materials, numerous applications for recycled polyesters can be explored depending on the properties of the resin, as well as the additives used (impact modifiers, mineral fillers, glass fibres, etc) to upgrade the material. A common problem faced during processing of recycled PET is hydrolysis, which reduces its average molecular weight (MW). Thermal exposure, as well as shear degradation, will also lead to MW loss. These losses will result in plastic material with reduced melt viscosity, impact resistance and mechanical properties. Numerous methods of recycling disposable beverage bottles and other containers made of PET, or blends of PET with other materials have been reported. 3 7 These include reprocessing with virgin resin, blending and compatibilization, recycling through solutions and chemical reactions. 8,9 There are three main factors, which must be considered regarding the issue of recycling: first, the collection of the waste; second, the recycling process itself; and third whether or not there is a market for the end product of recycling. The third factor is of course the most important. Our aim in this project is the processing and modifying the mixture of virgin grade poly(ethyleneterephthalate) (V-PET) and recycled grade (R-PET), with and without modifier (functionalized PP [PP-graft- MA] ) by different extrusion methods, and then to characterize the physical, thermal and mechanical properties of the modified PET. EXPERIMENTAL Material and recycling runs Virgin grade PET (V-PET) (Eastlon CB-6), from Taiwan, PP-graft-MA (Exallor PO )was supplied by Exxon Chemical and the recycled beverage bottles (R-PET) were taken from the production line of Zamzam Co, Tehran. The waste materials (R-PET) were crushed to flakes by a Wieser laboratory grinder, washed with a detergent in water and rinsed with water. The crushed (R-PET) flakes and virgin bottlegrade (V-PET) were heated at C for h. of virgin resin (V-PET), extruded (E-PET) Correspondence to: Abdulrasoul Oromiehie, Iran Polymer and Petroleum Institute, PO Box 8/8, Tehran, Iran A.Oromiehie@ippi.ac.ir (Received March ; revised version received May 3; accepted 9 June 3) Published online April Society of Chemical Industry. Polym Int 99 83//$3. 78

2 Recycling PET beverage bottles and recycled (R-PET) and their blends with V-PET, as well as with blends of R-PET with V-PET, and of functionalized PP (PP-graft-MA) were prepared as shown in Table. All the extrusion runs were performed by different extrusion methods at the optimum conditions, including a single screw extruder (Haake Rheometer system 9) and a twin screw extruder (Collin, NR ) (co-rotating and counter rotating) as in Table. When comparing the variation in properties of the samples (Nos 8) in Table, the optimal process was used and the samples characterized under the same conditions. Virgin grade PET was fed first, extrudated as a strand cooled into a water bath, and then pelletized. The other samples were extruded with the above procedure. The physical, thermal and mechanical properties of the pelletized materials were characterized according to ASTM methods as follows. Tensile properties were measured on an Instron, model 6. Standard dumbbell-shaped specimens (ASTM D-638) were cut directly from the plaques. At least five specimens of each sample were tested under the same conditions, for processing conditions using a gauge length of mm for all samples. Izod impact tests of the samples were performed in a Zwick Impact tester model according to ASTM D-6 using a pendulum of joules. A Polymer Laboratory differential scanning calorimetry, PL-DSC system, was used to measure the thermal properties of the samples. The calorimeter was operated at a programmed rate of Cmin, with sensitivity of m Vcm, from to 8 C. The samples were held at 8 C for min and cooled to room temperature. The second melting thermograms were obtained using the same heating conditions as the first. The degree of crystallinity X c (%) was determined by the following equation: 8 X c (%) = H m H c /X Hf(PET) where H m and ( H f ),arethe enthalphy of melt or fusion, Hf(PET) = 3.calg Table. Composition of V-PET, R-PET and PP-graft-MA samples number type V-PET % 9% 7% % 7% 6% E-PET % R-PET % % % % % % PP-g-MA % % (by ASTM D-38), X = H f H c / H f, so that X = for % crystalinity =, the mass fraction of pure (%) PET. An Ubbelohde viscometer was used to determine the viscosity and molecular weight of the samples. The molecular weight (M w ) was determined using the following equation: [η] = 7. M.68 w RESULTS AND DISCUSSION Physical properties: viscosity and molecular weight The intrinsic viscosity ([η]), and the viscosity average molecular weight, M w, of the samples are given in Tables 3 and. The intrinsic viscosity decreased as the thermal process cycles and R-PET concentration were increased. This indicates that the molecular weight decreased as the blend ratio of R-PET was increased, because of thermal exposure as well as shear degradation of R-PET. 6 As a direct result this will produce a plastic material with reduced melt viscosity. This means that the molecular weight decreases by increasing the percentage of recycled PET. This result agrees with the fact that a decrease in molecular weight causes an increase in the power law index. These results show that R-PET is more sensitive to thermal and hydrolytic degradation than V-PET. This could be caused by the simultaneous presence of retained moisture coming from the specific surface of scraps being much greater than that from pellets and contaminants. Consequently, the traces of moisture and impurities may induce chain scission processes that lead to a reduction in the intrinsic viscosity and the average molecular weight of recycled PET.,3 Table 3. Intrinsic viscosity of the samples for different extrution methods number Intrinsic viscosity [η] rotating Nonextruded Corotating rotating Table. Optimum extrusion conditions Type of extruder Zone ( C) Zone ( C) Zone 3 ( C) Zone ( C) Zone ( C) Zone 6 ( C) Rotating speed -screw extruder (rev min ) Co-rotating twin-screw extruder rotating twin-screw extruder 7 Polym Int 3:78 73 () 79

3 AR Oromiehie, A Mamizadeh Table. Variation of molecular weight of the samples for different extrusion methods number Viscosity average molecular weight (M w ) rotating Nonextruded Corotating rotating When functionalized PP was added to the blends, their viscosity was increased. By increasing the ratio of PP-graft-MA in the blends, the intrinsic viscosity and the molecular weight of the blends were increased as well. The expected increase in molecular weight is due to the reaction of functional groups of PPgraft-MA in the melt with the terminal or the ester groups of PET. The reaction formed block or graft copolymers at the interface, thus providing adhesion and improving the mechanical properties.,6 Thermal properties Table shows the melting temperature, T m,crystallization temperature T c and degree of crystallinity % X of the samples defined in Table. Typical DSC thermograms of the above samples processed by different extrusion methods are shown in Figs. The results show that T m is decreased by increasing the ratio of R-PET in the blends. The T m values for the blended samples follow the same trends as the T m values of the components, E-PET and V-PET. The crystalline behaviour of the V-PET/R-PET blends with the decreasing trend of the T m values are also shown in Figs. Thermal cycles of the processed samples and changes of the T c values from V-PET, E-PET to R-PET are shown in the same figures. This indication of the higher temperature at which crystallinity begins to appear is related to the thermal cycles that tend to have a higher T c value, Heat Flow (mcal s ) Figure. DSC plots for the samples prepared by single-screw extruder. Heat Flow (mcal s ) Figure. DSC plots for the samples prepared by co-rotating twin-screw extruder. Heat Flow (mcal s ) Figure 3. DSC plots for the samples prepared by counter-rotating twin screw extruder. that is the crystallization of PET begins at higher temperature. The crystallinity increases slightly with increasing percentage of R-PET in the blends. These results are in accordance with the finding by Fann et al 6 that the thermal cycling process releases the Table. Thermal properties of the samples processed by different extrusion methods -screw extruder Co-rotating TSE -rotating TSE number T m ( C) T c ( C) %X T m ( C) T c ( C) %X T m ( C) T c ( C) %X TSE = twin-screw extruder. 73 Polym Int 3:78 73 ()

4 Recycling PET beverage bottles Heat Flow (mcal s ) Figure. DSC plots for the samples prepared by single-screw extruder. entanglement and increases the crystallinity. With this mild thermal degradation, R-PET is able to crystallize and the less amorphous segment is retained. 6 When portions of PP-graft-MA were added to the V-PET/R- PET blends, the crystallinity of the blends decreased; this may be due to adhesion and the reaction of PET end groups with the carbonyl groups of MA which protect the PET from further degradation and increase the toughness of PET.,7 Mechanical properties The results of mechanical testing are shown in Figs. The tensile strength and impact strength, except the modulus properties, decrease by increasing the amount of R-PET from to %, but in different ways as in: single screw > co-rotating > counter-rotating (twin-screw extruder). The change in the mechanical properties is in agreement with the results of the physical and Tensile Str (MPa) Figure. Tensile strength at break of the samples prepared by different Tensile Str (MPa) Figure 6. Tensile strength at break of the modified samples prepared by different Modulus (MPa) 3 Figure 7. Modulus of the samples prepared by different Modulus (Mpa) 7 8 Figure 8. Modulus of the modified samples prepared by different Impact Str (Jm ) Figure 9. Impact strength of the samples prepared by different Impact Str (Jm ) Figure. Impact strength of the modified samples prepared by different thermal properties as discussed above. Torres showed that the percentage of crystallinity, the size of spherulites and the molecular weight of a semi-crystalline polymers such as PET, affect the mechanical properties of the materials. The presence of spherulitic crystallization in recycling PET leads to a decrease in the impact strength. The mechanical properties showed positive blending effects by addition of functionalized PP-graft-MA. Polym Int 3:78 73 () 73

5 AR Oromiehie, A Mamizadeh This improvement may be due to the crystallinity of PET, which implies that the toughness is inferior to that of pure PET.,7 The impact strength was found to increase with increasing concentration of functionalized PP-graft-MA. This may be due to the plasticizing effect of PPgraft-MA which increases the toughness of the material. 8,9 CONCLUSION We have examined three extrusion techniques for their effect on the properties of PET resins, preforms and bottles. The results show that recycled PET is more sensitive to thermal and hydrolytic degradation than virgin PET. Recycling PET gives rise to a decrease in the melt viscosity, average molecular weight, thermal and mechanical properties of the material because of the hydrolytic chain scission and thermomechanical degradation undergone during processing. The variation of intrinsic viscosity was in accordance with the proportion of the two components in the blended samples (R-PET/V-PET). This means that the molecular weight decreases when the percentage of recycled PET is increased, but these properties were improved by adding functionalized PP-graft-MA to the blends. The DSC crystallization study indicated that recycled PET is as good as any engineering grade, if not better, for some applications, because of ease of crystallization and processing. The mechanical properties are correlated to the composition as the percentage of R-PET increases from to %. Further improvement of impact strength was accomplished by blending with functionalized PPgraft-MA, but resulted in lower modulus and tensile strength. The compatibilization effect between PET and PP-graft-MA is probably based on the reaction between OH groups of PET and MA groups in PPgraft-MA. REFERENCES Ehrig RJ, Plastics recycling, Carl Hanser Publishing, Munich (99). Meyer JPh, Leblance D, and Trinchero F, Recycle 9, March 8, Switzerland (99). 3 US Patent (993). US Patent (998). La Mantia FP and Vinci M, Polym Degrad Stab : (99). 6 Fann DM, Huang SK and Lee JY, J Appl Polym Sci 6:6 (996). 7 Abu-isa I, Jaynes CB and Ogara JF, J Appl Polym Sci 9:97 (996). 8 Scheirs J, Polymer recycling, John Wiley, New York, Chapter (998). 9 Adak S, in Proceedings of National Seminar on Recycling and Plastics Waste Management, India, 6 Sept, (997). p 7. Oromiehie AR, Hashemi SA, Meldrum IG and Waters DN, Plast, Rubber Compos Process Appl :9 (998). Solomon OF and Ciuta IZ, J Appl Polym Sci 6:683 (96). Torres N, Eur Polym J 36:7 (). 3 Muller AJ, Alvarez ME and Febles AC, Polym Eng Sci 7:796 (987). Tanrattanakul V, Hiltner A, Baer E, Perkins WG, Massey FL and Moet A, Polymer 38:7 (997). Ken Cheung M and Chan D, Polym Int 3:8 (997). 6 Han CD, Multiphase flow in polymer processing, Acadimic Press, New York, Chapter (98). 7 Oromiehie AR and Meldrum IG, Iranian Polym J 8:93 (999). 8 Lintell DT and Smith S, Progr Rubber Plast Technol : (996). 9 Tsiourvas D, Tsartolia E, Stassinopoulos S, Barrell M and Bontemps J, Adv Polym Tech :7 (99). 73 Polym Int 3:78 73 ()