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Study of thermal and mechanical properties of virgin and recycled poly(ethylene terephthalate) before and after injection molding
European Polymer Journal 36 (2000) 2075±2080
Study of thermal and mechanical properties of virgin and recycled poly(ethylene terephthalate) before and after injection molding N. Torres a, J.J. Robin a,*, B. Boutevin b b
a C.E.RE.MA.P. Route des Salins, B.P. 118, 34140 Meze, France UMR 5076 ± Laboratoire de Chimie Macromol eculaire, Ecole Nationale Sup erieure de Chimie de Montpellier 8, rue de l'Ecole Normale, 34296 Montpellier Cedex 5, France
Received 13 October 1999; accepted 9 December 1999
Abstract In this study, we compared the thermal properties (glass transition, melting point and crystallinity) and mechanical properties (YoungÕs modulus, elongation at break and impact strength) of post-consumer poly(ethylene terephthalate) (PET) bottles with those of the virgin resin. We studied two types of scraps of recycled PET: one arising from homogeneous deposits of bottles and the other of heterogeneous deposits soiled by contaminants such as PVC and adhesives. The presence of contaminants and residual moisture coming in the shape of scraps facilitate the crystallization of recycled PET compared to virgin PET and induces cleavages of chains during the melt processing. This leads to a reduction in intrinsic viscosity and consequently in molecular weight, and these decreases are more signi®cant when the recycled resin is soiled. Virgin PET exhibited a ductile behavior (>200% of elongation at break),whereas post-consumer PET bottles exhibited a brittle one (<10% of elongation at break). This is a consequence of the dierence in crystallinity, the presence of impurities in the recycled PET and the dierent thermal and mechanical history of the virgin and recycled materials. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Recycled poly(ethylene terephthalate); Intrinsic viscosity; Thermal properties; Mechanical properties; Injection molding
1. Introduction Poly(ethylene terephthalate) (PET) is a thermoplastic polyester used widely [1]. The main application of PET in Europe is the manufacture of bottles (8 ´ 105 tons), ®bers (2.7 ´ 105 tons), moldings (3 ´ 105 tons) and sheets (2 ´ 105 tons). This polymer is successful for these applications because of its chemical, physical and mechanical properties and its negligible permeability to CO2 [2]. PET recycling represents one of the most successful and widespread examples of polymer recycling. In 1998, 10.4 ´ 104 tons of PET were recycled in Europe com* Corresponding author. Tel.: +4-67-46-64-90; fax: +4-67-4371-81.
pared to 3.6 ´ 104 tons in 1995 and only 2.3 ´ 104 tons in 1993 [3]. The main driving force responsible for the increased in recycling of post-consumer PET is its widespread use, particularly, in the beverage industry which has made PET the main target for plastic recycling. The study is focused on the mechanical recycling of post-consumer PET bottles. We studied two types of scraps of PET bottles: the ®rst one arising from homogeneous deposits, and the second one coming from heterogeneous deposits soiled by PVC because a complementary sorting to separate colors and to reduce the rate of PVC would increase the cost of recycled PET. However, the presence of contaminants generates some problems [4±8] such as cleavage of chains, an increase in carboxylic end groups, a reduction in molecular weight, a decrease in intrinsic viscosity leading to a decrease in mechanical properties of material. So, we compared the
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N. Torres et al. / European Polymer Journal 36 (2000) 2075±2080
thermal properties (glass transition, melting point and crystallinity) and mechanical properties (YoungÕs modulus, elongation at break and Charpy impact strength) of two recycled PET before and after injection molding with those of virgin PET.
2. Experimental part 2.1. Materials Three types of materials were used in this study. Virgin PET (PETV) was supplied by Akzo (MO3-300) and was a bottle grade material with an intrinsic viscosity of 0.76 dl gÿ1 . Scraps of recycled PET (PETRb) came from homogeneous deposits of blue post-consumer bottles and contained less than 20 ppm of PVC. Scraps of recycled PET (PETRc) arised from heterogeneous deposits of various color post-consumer bottles and contained 6000 ppm of PVC. 2.2. Melt processing
2.3. Characterization A Perkin±Elmer DSC-4 calorimeter was used to obtain thermograms of virgin and recycled PET before and after injection molding. The temperature used was 50±280°C with a helium atmosphere and the samples (7±10 mg) were heated at 10°C/min. Glass transition temperature, Tg , crystallization temperature, Tc , melting temperature, Tm , enthalpy of crystallization, DHc , and enthalpy of melting, DHm , of samples were calculated. The percent of crystallinity (vc ) for PET was calculated from the Eq. (1) whenever a cold crystallization exotherm was present during a heating run. DHm ÿ jDHc j ; DHm0
3. Results and discussion 3.1. Thermal properties
Pellets of PETV were dried in a dehumidifying drier (5 h at 160°C). Scraps of recycled PET possessing a softening temperature inferior to these pellets because they were less crystalline, were dried for 2 h at 120°C and 4 h at 140°C. The pellets and scraps were injection molded in the shape of ISO 1 test bars with a BILLION 90 ton injection molding machine. The typical molding conditions are as follows: · barrel temperature: 250±280°C, · mold temperature: 8°C, · overall cycle: 30 s.
vc wt:% 100
capillaries in a mixture of phenol and 1,1,2,2-tetrachloroethane (60:40 by volume) at 25°C. The intrinsic viscosity, [g], was determined by extrapolation using the Huggins equation. The average molecular weight, M w , was determined from the Mark Houwink relation, wa . Constants were K 7:44 10ÿ4 dl gÿ1 and g K M a 0:648 at 25°C [10]. Prior to testing injection molded specimens, they were conditioned at 20°C at the laboratory atmosphere for a minimum of three days. The reported values for all properties are the average of at least 10 determinations. Tensile tests were performed according to ISO 527, using a Zwick tensile tester model 5101. YoungÕs modulus measurements were made at a crosshead speed of 1 mm minÿ1 , whereas elongation measurements were made at 50 mm minÿ1 . Impact tests were performed according to Charpy ISO 179, notched specimens, using a Zwick pendulum impact tester model 5102 (2 J).
1
where DHm0 is the heat of fusion of 100% crystalline PET (DHm0 135.8 J gÿ1 ) [9]. Solution viscosity measurements were carried out in an automatic viscosimeter equipped with Ubbelhode
We studied by dierential scanning calorimetry (DSC) the thermal properties of a virgin PET and two recycled PET before and after injection molding at a high temperature. 3.1.1. Virgin and recycled PET During the ®rst heating run, it appears on the DSC thermograms of virgin and recycled PET, an endotherm peak associated with the fusion of the crystalline fraction about 245°C (Table 1). The pellets of virgin PET are opaque and exhibit 46% crystallinity, whereas the scraps of recycled PET are transparent and possess 31% crystallinity. Unlike pellets, scraps are totally transparent because the organization of crystals at the microscopic scale is very dierent: one presents an anisotropic character (scraps) and the other presents an isotropic character (pellets). During the second heating run of virgin and recycled PET samples after quenching from the melt, DSC thermograms show (Fig. 1): · the glass transition temperature, Tg , about 81°C · an exothermic peak known as ``cold crystallization'' [11] arising from the crystallization of the amorphous phase. This phenomenon is typical of polymers such as PET, PPS or PEEK. This arises from the weak mobility of planar benzenic nuclei (sp2 hybridization of carbon) and can easily be explained: chains being frozen, the heating involves a critical mobility leading to the reorganization of the structure. The peak of crystallization of recycled PET appears at a lower temperature. The onset and minimum crystallization temperatures of virgin and recycled PET can be clas-
N. Torres et al. / European Polymer Journal 36 (2000) 2075±2080
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Table 1 Data given by DSC thermograms of virgin and recycled PET Tg (°C)
Tc;onset (°C)
Tc;min (°C)
jDHc j (J gÿ1 )
Tm;onset (°C)
Tm;max (°C)
DHm (J gÿ1 )
vc (wt.%)
PETV First heating run Second heating run
± 81
± 141
± 160
± 15
238 229
245 243
63 22
46 5
PETRb First heating run Second heating run
± 81
± 139
± 151
± 32
234 228
248 245
42 35
31 2
PETRc First heating run Second heating run
± 82
± 123
± 127
± 23
234 228
247 246
42 33
31 7
Fig. 1. DSC thermogram of virgin PET recorded during the second heating run after quenching from the melt.
si®ed as indicated: (Tc;onset ; Tc;min PETRc < Tc;onset ; Tc;min PETRb < Tc;onset ; Tc;min PETV. This suggests that the spherulitic crystallization of PETRc is facilitated with regard to these PETRb and PETV as it occurs at a lower temperature (Table 1). The presence of impurities in the recycled PET samples may play the role of nucleating agents, facilitating crystallization. · an endothermic peak associated with the fusion of the crystalline fraction about 245°C as for the ®rst heating run. The onset melting temperature (Tm;onset ) of virgin and recycled PET is shifted at lower temperatures because the small chains which have crystallized during the second heating run, melt ®rst. The samples
possess an enthalpy of crystallization jDHc j of 15 J gÿ1 (PETV), 32 J gÿ1 (PETRb) and 23 J gÿ1 (PETRc) (Table 1). If the samples were completely amorphous after quenching, all the crystals formed during the heating should melt and the enthalpy of crystallization should be strictly equal to the enthalpy of melting. This is not observed (DHm 22 J gÿ1 (PETV); 35 J gÿ1 (PETRb) and 33 J gÿ1 (PETRc)). The degree of crystallinity of samples after quenching (less than 10%) shows that it is very dicult to obtain a completely amorphous sample. This study shows that semi-crystalline samples possessing a dierent thermal and mechanical history keep the rates of crystallization close (less than 10%) after heating run following quenching. 3.1.2. Injection molding of virgin and recycled PET By using DSC, we studied the thermal properties of injection molded PET during the ®rst heating run (Table 2). This permits one to give some indications on the microstructure of PET test bars. These indications can be directly related to the mechanical properties of material. The samples were taken from the core of the test bars. The following appear on the thermograms: (1) The glass transition temperature, Tg , about 80°C. (2) An exothermic peak arising from the crystallization of the amorphous phase. The chains being oriented
Table 2 Data given by DSC thermograms, recorded during a ®rst heating run of injection molded virgin and recycled PET
PETV PETRb PETRc
Tg (°C)
Tc;onset (°C)
Tc;min (°C)
jDHc j (J gÿ1 )
Tm;onset (°C)
Tm;max (°C)
DHm (J gÿ1 )
vc (wt.%)
80 81 80
131 128 126
137 133 134
23 23 20
233 234 235
247 249 251
36 41 42
10 13 16
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N. Torres et al. / European Polymer Journal 36 (2000) 2075±2080
during the injection, they crystallize more easily. All the samples possess a crystallinity not exceeding 16% arising from the used parameters during the injection molding (time of cycle, mold temperature, etc.). In our case, PET in the molten state is quickly quenched by contact with the cold surface of the mold, preventing the development of the spherulitic crystallization. Inversely, Mathew et al. [12] have shown that the molding at 150±200°C reduces the cooling rate and allows the polymer to stay longer at about 165±175°C (interval of temperature, where the rate of crystallization exhibits a maximum), facilitating the spherulitic crystallization of PET. The onset and the minimum crystallization temperature of recycled PET samples appear at a lower temperature (Table 2). The crystallization of recycled PET is facilitated with regard to those of virgin PET. This is caused by the presence of contaminants such as PVC, nucleating agents and adhesives in the PET. (3) An endothermic peak whose maximum of peak Tm;max is shifted of 2°C (PETRb) or 4°C (PETRc), compared to Tm;max of PETV. The increase in the melting temperature indicates that the crystalline entities in the recycled PET samples, particularly in the PETRc are higher than those in the PETV. The melting peak of PETRc is broader than those of PETRb or PETV. This indicates that the distribution of crystalline entities is larger in the recycled PET, particularly PETRc. The test bars of PETRc possess the higher rate of crystallinity (Table 2). These results are con®rmed by the visual observance of test bars. Those made up in virgin PET are transparent, whereas those in recycled PET (PETRc) are opaque. These dierences cannot arise from processing conditions because the same protocol has been used for injection molded, PET but could arise from the quality of PETRc (bottles of dierent colors and varied origin and then made from dierent resins). Inversely, the scraps of PETRb have the same color and arise from homogeneous deposits. This study shows that the crystallization of PETRc is facilitated compared to those of PETRb and PETV. It is necessary to determine the average molecular weight of virgin and recycled resins before and after injection molding because the increase of the rate of crystallization can arise from a decrease in average molecular weight after processing. Lin [13] studied the rate of crystallization of PET by DSC and reported that the
Table 3 Intrinsic viscosity [g] and average molecular weight M w of virgin and recycled PET before and after injection molding Pellets of PETV Injection molded PETV Scraps of PETRb Injection molded PETRb Scraps of PETRc Injection molded PETRc
[g] (dl gÿ1 )
M w (g molÿ1 )
0.76 0.74 0.77 0.69 0.80 0.61
44 000 42 200 44 900 37 900 47 600 31 300
rate of crystallization becomes higher as the molecular weight decreases. 3.2. Viscosimetry Table 3 gives the intrinsic viscosity, [g], and the average molecular weight, M w , of virgin PET and two recycled PET before and after injection molding at 280°C. Before processing at a high temperature, virgin PET possesses a value of [g] close to those of recycled PET because PET used by the manufacturers of bottles has [g] included between 0.74 and 0.8 dl gÿ1 . The decrease in [g] and M w of virgin PET after injection molding is weak, whereas [g] and M w of recycled PET after injection molding reduced strongly (from 44 900 to 37 900 g molÿ1 for PETRb and from 47 600 to 31 300 g molÿ1 for PETRc). These results show that recycled PET are more sensitive to thermal and hydrolytic degradation than is virgin PET. This could be caused by the simultaneous presence of retained moisture coming from the speci®c surface of scraps being much greater than that of pellets and contaminants such as PVC and adhesives. These contaminants generate acid compounds [14,15] (hydrochloric acid and acetic or abietic acid, respectively) during processing which catalyze the hydrolytic cleavage [16±19] of the ester bond to yield carboxylic acid end group and hydroxyl±ester end group (Fig. 2). Consequently, the traces of moisture and the impurities induce chain scission processes that lead to a reduction in the intrinsic viscosity and the average molecular weight of recycled resins [20]. These decreases are greater in the presence of the soiled recycled resin such as PETRc. These results (Table 3) underscore the importance of
Fig. 2. Hydrolysis reaction responsible for the reduction of molecular weight during the melt processing of PET.
N. Torres et al. / European Polymer Journal 36 (2000) 2075±2080
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Table 4 Mechanical properties of injection molded virgin and recycled PET
PETV PETRb PETRc
M w (g molÿ1 )
YoungÕs modulus (N mmÿ2 )
Elongation at break (%)
Charpy impact strength (notched, 20°C, kJ mÿ2 )
Aspect of test bars
42 200 37 900 31 300
2140 (206) 2170 (184) 1996 (210)
270 (57) 5.4 (0.6) 3.0 (0.4)
3.0 (0.2) 2.4 (0.5) 1.8 (0.3)
Transparent Opaque Opaque
purity in maintaining the intrinsic viscosity of PET during the melt processing. 3.3. Mechanical properties The percent crystallinity, the size of spherulites and the molecular weight of semi-crystalline polymers, such as PET, aect the mechanicals properties of materials. Usually, the crystallization produces a drastic mobility restriction that renders the material brittle. Giannotta [6,7] has shown that PVC contamination can also increase the level of undesirable cyclic and linear oligomers formed in PET during melt processing. These oligomers can aect mechanical properties of the material [21]. By injection molding at 8°C, we molded virgin and recycled PET to compare their mechanical properties (Table 4). The test bars of virgin PET are transparent, whereas those of recycled PET are opaque. The presence of spherulitic crystallization in the recycled PET test bars does not aect YoungÕs modulus (2000 N mmÿ2 ) but strongly reduces the elongation at break from 270% to 5% and leads to a decrease in the impact strength from 3 to 2 kJ mÿ2 . Our results are in accordance with those given by Akkapeddi [22] who has shown that amorphous PET possesses an elongation at break greater than 100%, whereas a crystalline PET possesses an elongation at break less than 10%.
4. Conclusions This study shows that the recycled PET suers a thermomechanical degradation during injection molding. The virgin PET possesses a ductile behavior, whereas the recycled PET exhibits a brittle one. This result is a consequence of the dierence in crystallinity between the materials, although they were molded under the same conditions. The crystallization of recycled PET can be favored by: · the presence of impurities and the increased content of cyclic or linear oligomers that act as nucleating agents [6], · the decrease in intrinsic viscosity and average molecular weight,
· the dierent thermal and mechanical histories (the scraps coming from bottles were crystallized by mechanical stretching, whereas the pellets were crystallized by heating), · the presence of residual moisture coming in the shape of scraps, · the molecular orientation of material during the injection molding. These results also show that the degree of purity of recycled resins is an important parameter for recycling. PETRc being more soiled than PETRb, is more sensitive to the thermal and hydrolytic degradation than PETRb. This leads to a decrease in intrinsic viscosity and average molecular weight facilitating the spherulitic crystallization which strongly reduces the elongation at break and the impact strength of PETRc. To develop new applications of recycled PET arising from post-consumer bottles other than that of the ®bers which require a low intrinsic viscosity, it should interesting to modify chemically recycled PET by adding a coupling agent to increase its intrinsic viscosity. This method will be examined later in a forthcoming article.
References [1] Gossot J. Caoutchoucs et Plastiques 1995;738:128. [2] Kodapak PET-Thermoplastic Polyester. Eastman Chemical Products Inc, Kingsport, USA, 1982. [3] Meyer JM. Usine Nouvelle 1998;2659:38. [4] Buxbaum L. Angew Chem Int Ed Engng 1978;7:182. [5] Ravens DAS, Ward IM. Trans Faraday Soc 1961;57:150. [6] Giannotta G, Po R, Cardi N, Occhiello E, Garbassi F. Characterization of reprocessed polyethylene terephthalate bottle scraps. Proc Int Recyc Congress, Geneva, Switzerland, 1993. p. 225. [7] Giannotta G, Locatelli L, Po R, Cardi N, Occhiello E, Garbassi F. The eect of drying cycles on the processing of recycled PET from bottles scraps. Ninth Annual Meeting of the Polym Processing Soc (PPS-9), Manchester, UK, 1993. p. 419. [8] Cardi N, Po R, Giannotta G, Occhiello E, Garbassi F, Messina G. J Appl Polym Sci 1993;50:1501. [9] Starkweather HW, Zoller P, Jones GA. J Polym Sci, Polym Phys Ed 1983;21:295. [10] Berkowitz S. J Polym Sci 1984;29:4353.
2080 [11] [12] [13] [14]
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Cobbs WH, Burton RL. J Polym Sci 1953;10:275. Mathew BA, Nunn RE, Orroth SA, AntecÕ89 1989. p. 333. Lin CC. Polym Engng Sci 1983;23:113. Milgrom J. In: Ehrig RJ, editor. Plastic recycling. Munich: Hanser, 1992. [15] McNeill LC. In: Eastmond GC, Ledwith A, Russo S, Sigwalt P, editors. Comprehensive polymer science, vol. 6. Oxford: Pergamon Press, 1989. p. 474. [16] Shah TH, Gamlen GM, Dollimore D, Bhatty JI. J Macromol Sci Chem 1985;A22:1545. [17] Seo KS, Cloyd JD. J Appl Polym Sci 1991;42:845.
[18] Zimmermann H. In: Grassie N, editor. Developments in polymer degradation, vol. 5. Barking (England): Applied Science, 1984. p. 112. [19] Campanelli JR, Kamal MR, Cooper DG. J Appl Polym Sci 1993;48:443. [20] M uller AJ, Feijoo JL, Alvarez ME, Febles AC. Polym Engng Sci 1987;27:796. [21] Dulio V, Po R, Borrelli R, Guarini A, Santini C. Die Angew Makromol Chem 1995;225:109. [22] Akkapeddi MK, Van Buskirk B. Polym Mater Sci Engng 1992;67:31.
Study of thermal and mechanical properties of virgin and recycled poly(ethylene terephthalate) before and after injection molding N. Torres a, J.J. Robin a,*, B. Boutevin b b
a C.E.RE.MA.P. Route des Salins, B.P. 118, 34140 Meze, France UMR 5076 ± Laboratoire de Chimie Macromol eculaire, Ecole Nationale Sup erieure de Chimie de Montpellier 8, rue de l'Ecole Normale, 34296 Montpellier Cedex 5, France
Received 13 October 1999; accepted 9 December 1999
Abstract In this study, we compared the thermal properties (glass transition, melting point and crystallinity) and mechanical properties (YoungÕs modulus, elongation at break and impact strength) of post-consumer poly(ethylene terephthalate) (PET) bottles with those of the virgin resin. We studied two types of scraps of recycled PET: one arising from homogeneous deposits of bottles and the other of heterogeneous deposits soiled by contaminants such as PVC and adhesives. The presence of contaminants and residual moisture coming in the shape of scraps facilitate the crystallization of recycled PET compared to virgin PET and induces cleavages of chains during the melt processing. This leads to a reduction in intrinsic viscosity and consequently in molecular weight, and these decreases are more signi®cant when the recycled resin is soiled. Virgin PET exhibited a ductile behavior (>200% of elongation at break),whereas post-consumer PET bottles exhibited a brittle one (<10% of elongation at break). This is a consequence of the dierence in crystallinity, the presence of impurities in the recycled PET and the dierent thermal and mechanical history of the virgin and recycled materials. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Recycled poly(ethylene terephthalate); Intrinsic viscosity; Thermal properties; Mechanical properties; Injection molding
1. Introduction Poly(ethylene terephthalate) (PET) is a thermoplastic polyester used widely [1]. The main application of PET in Europe is the manufacture of bottles (8 ´ 105 tons), ®bers (2.7 ´ 105 tons), moldings (3 ´ 105 tons) and sheets (2 ´ 105 tons). This polymer is successful for these applications because of its chemical, physical and mechanical properties and its negligible permeability to CO2 [2]. PET recycling represents one of the most successful and widespread examples of polymer recycling. In 1998, 10.4 ´ 104 tons of PET were recycled in Europe com* Corresponding author. Tel.: +4-67-46-64-90; fax: +4-67-4371-81.
pared to 3.6 ´ 104 tons in 1995 and only 2.3 ´ 104 tons in 1993 [3]. The main driving force responsible for the increased in recycling of post-consumer PET is its widespread use, particularly, in the beverage industry which has made PET the main target for plastic recycling. The study is focused on the mechanical recycling of post-consumer PET bottles. We studied two types of scraps of PET bottles: the ®rst one arising from homogeneous deposits, and the second one coming from heterogeneous deposits soiled by PVC because a complementary sorting to separate colors and to reduce the rate of PVC would increase the cost of recycled PET. However, the presence of contaminants generates some problems [4±8] such as cleavage of chains, an increase in carboxylic end groups, a reduction in molecular weight, a decrease in intrinsic viscosity leading to a decrease in mechanical properties of material. So, we compared the
0014-3057/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 3 0 1 - 8
2076
N. Torres et al. / European Polymer Journal 36 (2000) 2075±2080
thermal properties (glass transition, melting point and crystallinity) and mechanical properties (YoungÕs modulus, elongation at break and Charpy impact strength) of two recycled PET before and after injection molding with those of virgin PET.
2. Experimental part 2.1. Materials Three types of materials were used in this study. Virgin PET (PETV) was supplied by Akzo (MO3-300) and was a bottle grade material with an intrinsic viscosity of 0.76 dl gÿ1 . Scraps of recycled PET (PETRb) came from homogeneous deposits of blue post-consumer bottles and contained less than 20 ppm of PVC. Scraps of recycled PET (PETRc) arised from heterogeneous deposits of various color post-consumer bottles and contained 6000 ppm of PVC. 2.2. Melt processing
2.3. Characterization A Perkin±Elmer DSC-4 calorimeter was used to obtain thermograms of virgin and recycled PET before and after injection molding. The temperature used was 50±280°C with a helium atmosphere and the samples (7±10 mg) were heated at 10°C/min. Glass transition temperature, Tg , crystallization temperature, Tc , melting temperature, Tm , enthalpy of crystallization, DHc , and enthalpy of melting, DHm , of samples were calculated. The percent of crystallinity (vc ) for PET was calculated from the Eq. (1) whenever a cold crystallization exotherm was present during a heating run. DHm ÿ jDHc j ; DHm0
3. Results and discussion 3.1. Thermal properties
Pellets of PETV were dried in a dehumidifying drier (5 h at 160°C). Scraps of recycled PET possessing a softening temperature inferior to these pellets because they were less crystalline, were dried for 2 h at 120°C and 4 h at 140°C. The pellets and scraps were injection molded in the shape of ISO 1 test bars with a BILLION 90 ton injection molding machine. The typical molding conditions are as follows: · barrel temperature: 250±280°C, · mold temperature: 8°C, · overall cycle: 30 s.
vc wt:% 100
capillaries in a mixture of phenol and 1,1,2,2-tetrachloroethane (60:40 by volume) at 25°C. The intrinsic viscosity, [g], was determined by extrapolation using the Huggins equation. The average molecular weight, M w , was determined from the Mark Houwink relation, wa . Constants were K 7:44 10ÿ4 dl gÿ1 and g K M a 0:648 at 25°C [10]. Prior to testing injection molded specimens, they were conditioned at 20°C at the laboratory atmosphere for a minimum of three days. The reported values for all properties are the average of at least 10 determinations. Tensile tests were performed according to ISO 527, using a Zwick tensile tester model 5101. YoungÕs modulus measurements were made at a crosshead speed of 1 mm minÿ1 , whereas elongation measurements were made at 50 mm minÿ1 . Impact tests were performed according to Charpy ISO 179, notched specimens, using a Zwick pendulum impact tester model 5102 (2 J).
1
where DHm0 is the heat of fusion of 100% crystalline PET (DHm0 135.8 J gÿ1 ) [9]. Solution viscosity measurements were carried out in an automatic viscosimeter equipped with Ubbelhode
We studied by dierential scanning calorimetry (DSC) the thermal properties of a virgin PET and two recycled PET before and after injection molding at a high temperature. 3.1.1. Virgin and recycled PET During the ®rst heating run, it appears on the DSC thermograms of virgin and recycled PET, an endotherm peak associated with the fusion of the crystalline fraction about 245°C (Table 1). The pellets of virgin PET are opaque and exhibit 46% crystallinity, whereas the scraps of recycled PET are transparent and possess 31% crystallinity. Unlike pellets, scraps are totally transparent because the organization of crystals at the microscopic scale is very dierent: one presents an anisotropic character (scraps) and the other presents an isotropic character (pellets). During the second heating run of virgin and recycled PET samples after quenching from the melt, DSC thermograms show (Fig. 1): · the glass transition temperature, Tg , about 81°C · an exothermic peak known as ``cold crystallization'' [11] arising from the crystallization of the amorphous phase. This phenomenon is typical of polymers such as PET, PPS or PEEK. This arises from the weak mobility of planar benzenic nuclei (sp2 hybridization of carbon) and can easily be explained: chains being frozen, the heating involves a critical mobility leading to the reorganization of the structure. The peak of crystallization of recycled PET appears at a lower temperature. The onset and minimum crystallization temperatures of virgin and recycled PET can be clas-
N. Torres et al. / European Polymer Journal 36 (2000) 2075±2080
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Table 1 Data given by DSC thermograms of virgin and recycled PET Tg (°C)
Tc;onset (°C)
Tc;min (°C)
jDHc j (J gÿ1 )
Tm;onset (°C)
Tm;max (°C)
DHm (J gÿ1 )
vc (wt.%)
PETV First heating run Second heating run
± 81
± 141
± 160
± 15
238 229
245 243
63 22
46 5
PETRb First heating run Second heating run
± 81
± 139
± 151
± 32
234 228
248 245
42 35
31 2
PETRc First heating run Second heating run
± 82
± 123
± 127
± 23
234 228
247 246
42 33
31 7
Fig. 1. DSC thermogram of virgin PET recorded during the second heating run after quenching from the melt.
si®ed as indicated: (Tc;onset ; Tc;min PETRc < Tc;onset ; Tc;min PETRb < Tc;onset ; Tc;min PETV. This suggests that the spherulitic crystallization of PETRc is facilitated with regard to these PETRb and PETV as it occurs at a lower temperature (Table 1). The presence of impurities in the recycled PET samples may play the role of nucleating agents, facilitating crystallization. · an endothermic peak associated with the fusion of the crystalline fraction about 245°C as for the ®rst heating run. The onset melting temperature (Tm;onset ) of virgin and recycled PET is shifted at lower temperatures because the small chains which have crystallized during the second heating run, melt ®rst. The samples
possess an enthalpy of crystallization jDHc j of 15 J gÿ1 (PETV), 32 J gÿ1 (PETRb) and 23 J gÿ1 (PETRc) (Table 1). If the samples were completely amorphous after quenching, all the crystals formed during the heating should melt and the enthalpy of crystallization should be strictly equal to the enthalpy of melting. This is not observed (DHm 22 J gÿ1 (PETV); 35 J gÿ1 (PETRb) and 33 J gÿ1 (PETRc)). The degree of crystallinity of samples after quenching (less than 10%) shows that it is very dicult to obtain a completely amorphous sample. This study shows that semi-crystalline samples possessing a dierent thermal and mechanical history keep the rates of crystallization close (less than 10%) after heating run following quenching. 3.1.2. Injection molding of virgin and recycled PET By using DSC, we studied the thermal properties of injection molded PET during the ®rst heating run (Table 2). This permits one to give some indications on the microstructure of PET test bars. These indications can be directly related to the mechanical properties of material. The samples were taken from the core of the test bars. The following appear on the thermograms: (1) The glass transition temperature, Tg , about 80°C. (2) An exothermic peak arising from the crystallization of the amorphous phase. The chains being oriented
Table 2 Data given by DSC thermograms, recorded during a ®rst heating run of injection molded virgin and recycled PET
PETV PETRb PETRc
Tg (°C)
Tc;onset (°C)
Tc;min (°C)
jDHc j (J gÿ1 )
Tm;onset (°C)
Tm;max (°C)
DHm (J gÿ1 )
vc (wt.%)
80 81 80
131 128 126
137 133 134
23 23 20
233 234 235
247 249 251
36 41 42
10 13 16
2078
N. Torres et al. / European Polymer Journal 36 (2000) 2075±2080
during the injection, they crystallize more easily. All the samples possess a crystallinity not exceeding 16% arising from the used parameters during the injection molding (time of cycle, mold temperature, etc.). In our case, PET in the molten state is quickly quenched by contact with the cold surface of the mold, preventing the development of the spherulitic crystallization. Inversely, Mathew et al. [12] have shown that the molding at 150±200°C reduces the cooling rate and allows the polymer to stay longer at about 165±175°C (interval of temperature, where the rate of crystallization exhibits a maximum), facilitating the spherulitic crystallization of PET. The onset and the minimum crystallization temperature of recycled PET samples appear at a lower temperature (Table 2). The crystallization of recycled PET is facilitated with regard to those of virgin PET. This is caused by the presence of contaminants such as PVC, nucleating agents and adhesives in the PET. (3) An endothermic peak whose maximum of peak Tm;max is shifted of 2°C (PETRb) or 4°C (PETRc), compared to Tm;max of PETV. The increase in the melting temperature indicates that the crystalline entities in the recycled PET samples, particularly in the PETRc are higher than those in the PETV. The melting peak of PETRc is broader than those of PETRb or PETV. This indicates that the distribution of crystalline entities is larger in the recycled PET, particularly PETRc. The test bars of PETRc possess the higher rate of crystallinity (Table 2). These results are con®rmed by the visual observance of test bars. Those made up in virgin PET are transparent, whereas those in recycled PET (PETRc) are opaque. These dierences cannot arise from processing conditions because the same protocol has been used for injection molded, PET but could arise from the quality of PETRc (bottles of dierent colors and varied origin and then made from dierent resins). Inversely, the scraps of PETRb have the same color and arise from homogeneous deposits. This study shows that the crystallization of PETRc is facilitated compared to those of PETRb and PETV. It is necessary to determine the average molecular weight of virgin and recycled resins before and after injection molding because the increase of the rate of crystallization can arise from a decrease in average molecular weight after processing. Lin [13] studied the rate of crystallization of PET by DSC and reported that the
Table 3 Intrinsic viscosity [g] and average molecular weight M w of virgin and recycled PET before and after injection molding Pellets of PETV Injection molded PETV Scraps of PETRb Injection molded PETRb Scraps of PETRc Injection molded PETRc
[g] (dl gÿ1 )
M w (g molÿ1 )
0.76 0.74 0.77 0.69 0.80 0.61
44 000 42 200 44 900 37 900 47 600 31 300
rate of crystallization becomes higher as the molecular weight decreases. 3.2. Viscosimetry Table 3 gives the intrinsic viscosity, [g], and the average molecular weight, M w , of virgin PET and two recycled PET before and after injection molding at 280°C. Before processing at a high temperature, virgin PET possesses a value of [g] close to those of recycled PET because PET used by the manufacturers of bottles has [g] included between 0.74 and 0.8 dl gÿ1 . The decrease in [g] and M w of virgin PET after injection molding is weak, whereas [g] and M w of recycled PET after injection molding reduced strongly (from 44 900 to 37 900 g molÿ1 for PETRb and from 47 600 to 31 300 g molÿ1 for PETRc). These results show that recycled PET are more sensitive to thermal and hydrolytic degradation than is virgin PET. This could be caused by the simultaneous presence of retained moisture coming from the speci®c surface of scraps being much greater than that of pellets and contaminants such as PVC and adhesives. These contaminants generate acid compounds [14,15] (hydrochloric acid and acetic or abietic acid, respectively) during processing which catalyze the hydrolytic cleavage [16±19] of the ester bond to yield carboxylic acid end group and hydroxyl±ester end group (Fig. 2). Consequently, the traces of moisture and the impurities induce chain scission processes that lead to a reduction in the intrinsic viscosity and the average molecular weight of recycled resins [20]. These decreases are greater in the presence of the soiled recycled resin such as PETRc. These results (Table 3) underscore the importance of
Fig. 2. Hydrolysis reaction responsible for the reduction of molecular weight during the melt processing of PET.
N. Torres et al. / European Polymer Journal 36 (2000) 2075±2080
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Table 4 Mechanical properties of injection molded virgin and recycled PET
PETV PETRb PETRc
M w (g molÿ1 )
YoungÕs modulus (N mmÿ2 )
Elongation at break (%)
Charpy impact strength (notched, 20°C, kJ mÿ2 )
Aspect of test bars
42 200 37 900 31 300
2140 (206) 2170 (184) 1996 (210)
270 (57) 5.4 (0.6) 3.0 (0.4)
3.0 (0.2) 2.4 (0.5) 1.8 (0.3)
Transparent Opaque Opaque
purity in maintaining the intrinsic viscosity of PET during the melt processing. 3.3. Mechanical properties The percent crystallinity, the size of spherulites and the molecular weight of semi-crystalline polymers, such as PET, aect the mechanicals properties of materials. Usually, the crystallization produces a drastic mobility restriction that renders the material brittle. Giannotta [6,7] has shown that PVC contamination can also increase the level of undesirable cyclic and linear oligomers formed in PET during melt processing. These oligomers can aect mechanical properties of the material [21]. By injection molding at 8°C, we molded virgin and recycled PET to compare their mechanical properties (Table 4). The test bars of virgin PET are transparent, whereas those of recycled PET are opaque. The presence of spherulitic crystallization in the recycled PET test bars does not aect YoungÕs modulus (2000 N mmÿ2 ) but strongly reduces the elongation at break from 270% to 5% and leads to a decrease in the impact strength from 3 to 2 kJ mÿ2 . Our results are in accordance with those given by Akkapeddi [22] who has shown that amorphous PET possesses an elongation at break greater than 100%, whereas a crystalline PET possesses an elongation at break less than 10%.
4. Conclusions This study shows that the recycled PET suers a thermomechanical degradation during injection molding. The virgin PET possesses a ductile behavior, whereas the recycled PET exhibits a brittle one. This result is a consequence of the dierence in crystallinity between the materials, although they were molded under the same conditions. The crystallization of recycled PET can be favored by: · the presence of impurities and the increased content of cyclic or linear oligomers that act as nucleating agents [6], · the decrease in intrinsic viscosity and average molecular weight,
· the dierent thermal and mechanical histories (the scraps coming from bottles were crystallized by mechanical stretching, whereas the pellets were crystallized by heating), · the presence of residual moisture coming in the shape of scraps, · the molecular orientation of material during the injection molding. These results also show that the degree of purity of recycled resins is an important parameter for recycling. PETRc being more soiled than PETRb, is more sensitive to the thermal and hydrolytic degradation than PETRb. This leads to a decrease in intrinsic viscosity and average molecular weight facilitating the spherulitic crystallization which strongly reduces the elongation at break and the impact strength of PETRc. To develop new applications of recycled PET arising from post-consumer bottles other than that of the ®bers which require a low intrinsic viscosity, it should interesting to modify chemically recycled PET by adding a coupling agent to increase its intrinsic viscosity. This method will be examined later in a forthcoming article.
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