polycarbonate blends

polycarbonate blends

Polymer Degradation and Stability 90 (2005) 250e255 www.elsevier.com/locate/polydegstab Recycling of poly(ethylene terephthalate)/polycarbonate blend...

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Polymer Degradation and Stability 90 (2005) 250e255 www.elsevier.com/locate/polydegstab

Recycling of poly(ethylene terephthalate)/polycarbonate blends F. Fraı¨ sse a, V. Verney a,*, S. Commereuc a, M. Obadal b a

Laboratoire de Photochimie Mole´culaire et Macromole´culaire, CNRS/UMR 6505 Universite´ Blaise Pascal, Avenue des Landais, 63177 Aubiere Cedex, France b Department of Polymer Materials and Technology, Tomas Bata University in Zlı´n, Faculty of Technology, TGM 275, 76272 ZLI´N, Czech Republic Received 22 October 2004; received in revised form 26 January 2005; accepted 6 February 2005 Available online 11 July 2005

Abstract Weathering and recycling of poly(ethylene terephthalate) (PET) causes degradation of the polymer backbone and results in the loss of physical properties. To enhance the properties, PET waste was blended with polycarbonate (PC) which shows a higher glass transition temperature as compared with PET. Three PET/PC blends were studied: 80/20, 70/30 and 50/50 wt.% PET/PC, respectively. Different grades of recovered materials (pellets, plates, and injection-moulded specimens) were tested by differential scanning calorimetry (DSC) and rheological measurements. The results indicate that the blends have better properties than neat PET. Moreover, a comparison of mechanical properties of injection-moulded specimens prepared from pellets and from flakes demonstrates that twin-screw extrusion is an essential operation in the recycling process as it permits a small amount of transesterification reaction between both polymer phases during blend preparation. This is a key factor to improve the polymer compatibility. Thus, it becomes possible to process new articles from 100% PET/PC waste blends with suitable thermo-mechanical properties. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Recycling; Polymer blends; Chemical modification; Degradation; Mechanical properties; Photo-ageing

1. Introduction During last decades, thermoplastics have become routinely applied in building construction. Actually, poly(vinyl chloride) (PVC) is the most commonly used polymer [1]. Nevertheless, PVC recycling brings a lot of environmental problems because it is very sensitive to thermo-mechanical stresses during recycling leading to degradation [2]. Nowadays, the mechanical recycling of PVC is an important approach as old PVC formulations contain heavy metals. An alternative way is to find a substitute for PVC in building applications. The idea

* Corresponding author. Tel.: C33 4 73 40 71 82; fax: C33 4 73 40 77 00. E-mail address: [email protected] (V. Verney). 0141-3910/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2005.02.019

of our work is to use recycled poly(ethylene terephthalate) (PET) as an alternative to PVC, and therefore this paper focuses on a detailed study of PET recycling. However, PET recycling is a complicated process because of its chemical and mechanical degradation during reprocessing [3], and therefore this process has to be carried out with utmost caution. The first step is to determine the level of degradation of PET wastes caused by service-life ageing [4] and its impact on the mechanical properties. These properties can be improved even more by the addition of polycarbonate (PC) [5]. Thus, our work is aimed to use wastes of these polymers (PC and PET) for blend preparation. First of all, the characterization of both PET and PC wastes was carried out. PET and PC blends were subsequently prepared in various compositions (80/20, 70/30 and 50/50 wt.%). Then, the properties of the

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blends were studied. The results based on this work are discussed in order to find a suitable way of recycling PET and PC which should enable production of their blends with mechanical properties comparable or better than those of PVC [6]. Moreover, as building applications include often outdoor applications, we have also focused on the photo-ageing of the final materials.

FTIR was performed by a Nicolet spectrometer. A Rheometrics ARES mechanical spectrometer was used to measure dynamic melt viscosity at 260  C with parallel plates (diameter of 25 mm, gap of 1 mm) in frequency sweep test from 0.01 rad/s to 100 rad/s. For this rheological test a sinusoidal strain is applied. The strain amplitude was fixed to maintain the measurements within the linear viscoelastic domain. To determine mechanical properties, a Zwick 5102 Charpy impact tester and a Zwick 1456 tensile tester were used.

2. Experimental 2.1. Materials

3. Results and discussion

PET wastes were obtained from post-consumer bottle disposal. They were ground in order to prepare PET flakes. These flakes were washed and dried to eliminate stickers and labels. PC wastes were used either from degraded water bottles or from synthetic glasses. PC bottles were ground into flakes with a diameter approx. 4 mm. Flakes from PC glasses were obtained with a diameter approx. 8 mm.

3.1. Waste properties

To prepare polymer films by compression moulding, a manual press was used and set to 260  C. Moulding time was about 1 min. In this case, the thermal degradation of the polymer was limited. Pellets were prepared using a Clextral twin-screw extruder at 270  C. The residence time of the material in the extruder was about 2 min. Overall material recovery is summarized in Scheme 1. Accelerated photo-ageing was carried out in an SEPAP 12e24 irradiation device [7]. Thermal properties were measured by a Mettler e Toledo 822e Differential Scanning Calorimeter. The DSC method used included three steps. The first step was heating from 30  C to 300  C at 10  C/min in order to eliminate the thermal history of the sample. The second step was cooling at 10  C/min to 30  C. Finally, the samples were re-heated according to the first step. PET or PC wastes Crushing Flakes Extrusion and granulation Pellets Drying and injection moulding

Extrusion and compression moulding Plates pressed Drying and compression moulding

Specimens

Scheme 1. Schematic representation of transformations from wastes.

0,6

Delta absorbance at 3270 cm-1(a.u.)

2.2. Techniques

Firstly, ageing of PET wastes was studied. FTIR measurement of both PET wastes and virgin PET (see Fig. 1) shows that their rate and extents of degradation are virtually similar. In the same kind of result, Fig. 2 shows the Carboxyl Index changes for a PET film during accelerated photoageing. Carboxyl Index, defined as the relative content of carboxyl end-groups, is a useful parameter to quantify the polymer degradation [8]. It was measured from FTIR data by the ratio of carboxyl end-groups absorption (peak at 3290 cmÿ1) and a reference peak (centred at 2970 cmÿ1). Fig. 2 shows an increase in the Carboxyl Index with exposure time. This result is consistent with the proposed chemical mechanism of PET photodegradation in the literature [9] and is in agreement with the results of Fechine et al. [8]. Polycarbonate also shows chemical modifications during UV exposure [10,11]. PC photo-degradation leads to the formation of acidic and alcohol photoproducts. Identification of acidic products can be carried out according to absorption bands at 1713 cmÿ1 (see

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Fig. 1. Absorbance at 3270 cmÿ1 (hydroxyl region) during photoageing in SEPAP 12e24 for PET bottle and virgin PET.

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∆ absorbance in the hydroxyl region (a.u.)

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Irradiation time in sepap 12-24 (h)

Irradiation time (UV hours) Fig. 2. Carboxyl Index as a function of photo-ageing time in SEPAP 12e24 for PET film.

Fig. 4. Absorbance in the hydroxyl region for PC samples as a function of irradiation time.

Fig. 3) while alcohol groups are detectable by absorption bands at 3490 and 3527 cmÿ1 (see Fig. 4). The initial samples possess significant differences in their properties because of their different manufacturing processes (moulding conditions, orientation in the mould, etc.) or application (using with gaseous liquid or non-gaseous liquid) [12]. We have checked that a statistical blend of pellets in a big bag allows us to obtain reproducible results with several batches.

and PC wastes was also detected in the molten state by DSC measurements carried out at 260  C. Indeed, Fig. 5 shows that after approx. 3 h, thermograms of PET/PC blends exhibit a single glass transition temperature, whereas for shorter times two glass transitions are recorded: a glass transition was observed at about 80  C for the PET-rich phase and a glass transition was perceived at about 140  C for the PC-rich phase. Without catalysts and compatibiliser, the transesterification time is too long for a total reaction during manufacturing. However, it is important to point out that this transesterification occurs in the molten state with PET and PC wastes, and should facilitate the miscibility of the polymers to enhance the properties after thermal and mechanical treatments [15,16]. Extrusion of blends with small flakes prepared from PC bottle was easier than extrusion of PC glass flakes. The size of the flakes seems to be a very important factor for the quality of extrusion and resulting pellets. In addition, to assess sample properties, compression moulding was used. Blends with compositions of 50/50 and 70/30 (PET/ PC glass wt.%) were compression moulded in order to obtain sheets from melt material. Furthermore, specimens were manufactured by injection moulding directly from the flakes to compare mechanical properties with the other specimens prepared from the pellets.

3.2. Blend preparation Pellets of blends were obtained using twin-screw extruder from flakes dried for about 12 h at 130  C. As shown by Ignatov et al. [13], transesterification between virgin PET and PC can occur in molten state [14,15]. In our case, the transesterification between PET 1,5 PC bottle PC synthetic glass

absorbance at 1713 cm-1

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3.3. Thermal properties

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Irradiation time in sepap 12-24 (h) Fig. 3. Absorbance at 1713 cmÿ1 (carboxylic acid group) for PC samples as a function of irradiation time.

DSC measurements show that the glass transition temperature associated with the PET phase in all blends is higher than in the case of neat PET. In blends the glass transition temperature is near to 85  C, while Tg of neat PET is around 80  C (see Fig. 6). This proves that the transesterification reaction occurs in blends prepared from waste materials. This result is in good agreement

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Tg (°C)

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time (mn) Fig. 5. Glass transition temperature change of a PET/PC blend (70/30) with reaction time at 260  C under nitrogen atmosphere.

with literature data [15]. Based on NMR and DSC experiments, Zhang et al. [15] have shown that an increase in Tg would correspond to an amount of ester exchange less than 5%. Such amount of reaction is shown to be sufficient for an improvement of polymer compatibility and homogenisation of the blend [15]. The glass transition temperature of the PC-rich phase of processed samples cannot be determined directly during the first heating step with the used method (see Section 2.2) as cold crystallization of PET takes place in the same temperature range. Thus, the Tg values of the PC phase were determined during the second heating step. However, the most important information is to know that the glass transition temperature of the PET-rich phase, which is the lowest, increases as a result of transesterification. In this case, mechanical properties

may be enhanced in comparison with the totally incompatible unreacted blend or neat PET as the solid-state properties may be linked to molecular mobility. 3.4. Rheological properties G# and G$ moduli of both blends and homopolymers are represented as a function of the frequency in Figs. 7 and 8, respectively. As expected the viscoelastic mechanical moduli of the blends lie between PET and PC moduli. At low frequency, the storage modulus G# of the blends is close to the storage modulus of PC. The loss modulus values (G$) for the blends are in the same range as PET loss modulus at high frequency and range between PET and PC below 1 rad/s. 3.5. Mechanical properties Tensile, flexural and impact characteristics of the PET/PC blends have been measured and are listed in Table 1. For building specific applications PET and PC blends exhibit suitable tensile and flexural properties. Mechanical modulus E in tensile test is higher for PET/ PC blends than for neat PET (measured by Torres et al.

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Table 2 Mechanical properties of the specimens prepared from flakes

PC PET 50/50 70/30 80/20

Test

Properties

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Modulus (MPa) Strain for the conventional deflection of 6 mm (MPa) Yield stress (MPa) Deflection at yield stress (mm)

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at about 2100 N/mm2) [17]. Young’s modulus values are above 3100 N/mm2 for specimens from blend pellets. These results concerning tensile properties, which are close to PVC properties [6], confirm that PET/PC blends can be a suitable candidate for various applications, e.g. profile applications in the building sector. A comparison of the results in Table 1 with those obtained for specimens prepared from the flakes (Table 2) shows that the twin-screw extrusion is a necessary operation to obtain improved blend properties. The best results have been obtained for tensile and flexural tests. However, impact strength remains rather low in all studied blend compositions for their use in building application. Nevertheless, the maximal impact strength value obtained for a 50/50 blend in Table 1 could be improved by its specifically controlled crystallization. This can open an alternative way for the PVC profile substitution.

Properties

Tensile E-modulus (Young) (MPa) Yield stress (MPa) Elongation at yield stress (%) Elongation at break (%)

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Flexural Modulus (MPa) 2120e2150 2225 2240 Strain for a conventional 70e71 71 71 deflection of 6 mm (MPa) Yield stress (MPa) 77e81 82 85 Deflection at yield 8.3e9.1 9.4 10.1 stress (mm) Impact Impact strength (kJ/m2)

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2.2

Acknowledgments The authors acknowledge ADEME, CEREMAP, Lyce´e Val de Dore in Thiers, Freeglass, St. Gobain and Chateaud’Eau companies for their helpful participation.

Table 1 Mechanical properties of the specimens prepared from pellets Test

Degradation caused by ageing is limited in PET and PC wastes obtained from post-consumer bottles disposal. Thus, they can be used in a new process to obtain a recycled material. In the case of PET recycling, the addition of PC to PET makes the overall recycling process easier and brings even better properties as compared to the neat PET [17]. Consequently, it seems that blending of PET and PC limits the degradation consequences caused by mechanical and thermal processing during recycling. PET and PC blends show suitable mechanical properties for building applications, near to the properties of PVC. The glass transition temperatures of blends show that transesterification between PET and PC wastes occurs during blending in the molten state. In the recycling process, extrusion seems to be a necessary step to obtain the required properties. Future study has to be directed towards enhancement of the impact properties for use of PET/PC blends in building applications.

3.8

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