Chapter 7
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Flame-Retardant PET-PC Blends Compatibilized by Organomodified Montmorillonites 1
1,
B. Swoboda , E . Leroy1,2, F. Laoutid1,3, and J.-M. Lopez-Cuesta * 1CMGD, Ecole des Mines d'Alès, 6 av. de Clavières, 30100 Alès, France 2Current address: GEPEA (UMR CNRS 6144), 37 Bd de l'Université, B.P. 420, 44606 St-Nazaire Cedex, France 3Current address: Materia Nova-SMPC, University of Mons-Hainaut, Place de Parc 20, B-7000 Mons, Belgium *Corresponding author:
[email protected] Flame retardant PET nanocomposites and compatibilized PET/PC blends were obtained using a montmorillonite modified by a methyl-triphenoxyphosphonium salt. The organomodified M M T was shown to exfoliate into the PET matrix and to act as a compatibilizer between PET and PC in the blends. The combination of this nanofiller with triphenyl phosphite gives good mechanical properties and reaction to fire performance, including V-0 rating in the UL94 test. These results illustrate the advantage of combining various formulation strategies to obtain engineering polymeric materials, even from recycled plastics having intrinsically low performances (recycled PET and PC in this case).
© 2009 American Chemical Society
In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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Introduction The present main applications of reclaimed PET bottles after regeneration are as fibers, packaging, strips and films (/). Actually, the re-use of PET as an higher added value engineering plastic is limited due to several issues: when recycled PET is melt processed, various degradations mechanisms (thermal, mechanical, oxidative or hydrolytic) take place, generally promoted by remaining impurities, leading to a decrease of the average chain length, and thus to a decrease of rheological and mechanical properties (2). In a two recent papers (3,4), we studied the reaction to fire of polyethylene terephtalate/polycarbonate (PET/PC) blends made from recycled polymers, with the aim to increase the fire retardancy of recycled PET. Both the morphology and the compatibilization of the blends were shown to have a strong influence on final properties. For example, while the reaction to fire improved almost linearly for low percentages of added PC, blends containing more than 50% w./w. of PC reacted to fire like pure PC. This change of behavior was correlated with the formation of a continuous PC phase in the blend. Concurrently, the compatibilization of the blend by a transesterification reaction between P E T and PC (3), leading to the formation of copolymers at the interface, was shown to decrease the overall fire performances, despite a strong increase of charring. This was ascribed to the fact that the transesterification reaction, as a side effect, decreases the average PET chain length, which results in a strong decrease of the viscosity of the blends and of the temperature at which mass loss begins. The introduction of triphenylphosphite (TPP), after the compatibilization of the blends resulted in significantly improved reaction to fire, including V - 0 rating in the UL94 test (4). TPP actually has a triple role: acting as a flame retardant due to its phosphorus content (9% by weight), but also as a chain extender able to promote bridging reactions between PET chain ends, and as an inhibitor of the transesterification reaction between P E T and P C . The combination of these three mechanisms results in a strong increase of both the thermal stability and charring of the PET/PC blends (4). Recent research on PET/PC blends (5) shows the possibility to achieve blend compatibilization by adding specifically designed organomodified montmorillonites which locate at the interface between P E T and PC due to thermodynamic driving forces in the melt. In the present article, we will show that such compatibilization can be obtained using TPP derived phosphonium modified montmorillonites, and that the combination of such nanoparticles with pure TPP enables one to obtain recycled PET/PC blends with substantially improved mechanical and fire behavior.
In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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Experimental Materials and processing The recycled PET (post-consumer bottles flakes) was supplied by V A L O R P L A S T (France) (ηΡΕΤ = 0.76 dl/g, in 2-chlorophenol at 25°C, average molecular weight: 26300 g/mol. calculated using the Mark-Houwink equation (K = 3.8xl0 ;a=1.3)). Recycled PC (compact disc production waste, without metallic coating) was supplied by M P O (France) OlPC = 0.37 dl/g, in 2-chlorophenol at 25°C, average molecular weight calculated using the Mark-Houwink equation (K = 6.0x10 ; a = 1.22): 18000 g/mol). The C D ' s have been ground using a rotary cutter mill in order to obtain PC flakes of typically 8 mm. Purified Na Montmorillonite ( M M T - N a , Nanofil® E X M 757) was pur chased from Sud-Chemie. Triphenylphosphite (TPP, C A S : 101-02-0) and methyltriphenoxyphosphonium iodide salt (Figure 1, C A S : 17579-99-6) were purchased from Acros Organics.
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4
4
+
Ο
Figure 1. Chemical structure of methyltriphenoxyphosphonium iodide salt.
+
Organic modification of MMT-Na
and properties of MMT-P
The ion exchange reaction was performed in aqueous medium at low temperature (typically between 273 and 277 K ) : 50 g of M M T - N a were dispersed with stirring in 3 liters of distilled water for 24h. Concurrently the phosphonium salt was dissolved with stirring for 12 hours in an 80/20 w./w. mixture of distilled water and acetone with a concentration of 25g of salt per 3 liters of solvent. The two solutions containing the M M T - N a and the phosphonium salt, respectively, were then mixed with stirring for 16 hours. The +
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In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
86 resulting organomodified M M T which will be called M M T - P in the following, was then filtered, washed by centrifugation using a mixture of water and acetone, and finally lyophilized in order to obtain a fine powder. The modified montmorilonite M M T - P has the following characteristics: •
According to X-ray diffraction analysis, the gap between clay layer increased from 1.24 nm ( M M T - N a ) to 1.60 nm due to the intercalation of phosphonium ions.
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+
•
Infrared spectroscopic analysis of the M M T - P powder showed the presence of absorption bands at 1586 cm* , 1483 cm" , and 686 cm" , which are typical of the initial phosphonium salt (Figure 1). 1
1
1
•
The weight percentage of organic phase present in M M T - P after washing and lyophilization was 9.5 wt.%. This value was determined by thermogravimetric analysis (TGA, 5°C/min, 25-700°C, samples of typically 25 mg, using a Netzsch S T A 409 device). Another important characteristic of M M T - P revealed by T G A is that the onset of the mass loss due to the phosphonium ion takes place around 300°C, while pure TPP starts to evaporate around 200°C (Figure 2). This increase of thermal stability is of particular interest since the melt mixing with PET will be done at 270°C.
•
Finally, the electrophoretical mobility (Zeta Potential) of M M T - N a and M M T - P were measured using a Malvern Nanosizer N A N O Z S : in water, it decreases from 40.9 mV to 22.9 mV; while in tetrahydrofuran (THF) it increases from 11.2 mV to 44.5 mV. This indicates that M M T - P particles are strongly organophilic, as opposed to initial hydrophilic M M T - N a .
+
+
The different formulations were compression molded at 270 °C at a pressure of 100 bars for 30 seconds, directly after blending, in order to obtain 100 χ 100 χ 4 mm sheets, which were then cut to the requisite size for fire and mechanical testing. 3
Polymer compounding Due to the high sensitivity of PET to moisture, the following protocol was used before melt mixing: PET and PC flakes were dried separately in vacuum at 120 °C for 16h. The choice of these conditions was the result of a detailed study of PET drying kinetics and sensitivity to moisture during melt processing (5). Melt blending was performed using a Haake internal mixer (Rheomix) at 270 °C and 60 rpm for 10 minutes (except for formulation 9, see Table I). Samarium acetylacetonate catalyst was used for the compatibilization of the formulation # 10 during melt mixing. Catalyst was directly dispersed in PETr/PCr pellets in order to obtain catalyst weight contents of 0.05%.
In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
Figure L I.R. Spectra of phosphonium ion, pristine and modified montmorillonite
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In Fire and Polymers V; Wilkie, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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Figure 2. Thermogravimetric analysis ofphosphonium ion, pristine and modified montmorillonite (5°C/min, in air)
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Table I. Formulations prepared (w./w. fractions). In the case of formulation #9, T P P is added to the melt mixture after 10 minutes and the melt mixing is maintained during 5 minutes Sample
PET
PC
MMTNa
MMT-P
TPP
Catalyst
1
100
0
0
0
0
0
2
80
20
0
0
0
0
3
50
50
0
0
0
0
4
95
0
0
5
0
0
5
76
19
0
5
0
0
0
0
6
47.5
47.5
0
5
7
76
19
5
0
0
0 0
8
76.8
19.2
0
0
4
9
72.8
18.2
0
5
4
0
10
80
20
0
0
0
5.10"
4
Characterization techniques The dispersion of M M T - P particles in the PET (and PET/PC blends) matrix was studied by X-ray diffraction and dynamic rheological tests at low shear rate at 260 °C using an A R E S Rheometrics Scientific apparatus in plate/plate geometry ( 0 25 mm, 1mm gap). In order to detect the glass transitions of the different polymeric phases, samples of dimensions 4xlO>
