Stabilizer system for the inhibition of ester-exchange reactions in

476

Ind. Eng. Chem. Res. 1989, 28, 476-481

Stabilizer System for the Inhibition of Ester-Exchange Reactions in Thermoplastic Polyester Melts M.-F. Cheung,* A. Golovoy, R. 0. C a r t e r , 111, and H . v a n Oene Research Staff, Ford Motor Company, Dearborn, Michigan 48121

Ester-exchange reactions have been reported to occur during extrusion of polyester blends. This paper describes the stabilization of polyarylate-polycarbonate-poly(ethy1ene terephthalate) ternary blends during extrusion using a three-ingredient stabilizer system. The criterion for stability is the retention of crystallinity of the poly(ethy1ene terephthalate) phase. Addition of organophosphite provides a blend that is stable up to an extrusion temperature of 280 "C. Extrusion a t 300 "C requires the addition of a polycarbodiimide t o prevent a change in the melting point of the poly(ethy1ene terephthalate). When the extrusion temperature is further raised to 325 " C , a hindered phenol is needed in order to retain a substantial fraction of the crystallinity of the poly(ethy1ene terephthalate). Differential scanning calorimetry was used to study the effectiveness of the stabilizers. The samples were further characterized by infrared spectroscopy. The spectroscopic data confirm the calorimetry results. Melted polyesters can undergo further esterification, hydrolysis, and ester-exchange reactions especially in the presence of moisture, residual catalyst from polymerization processes (Devaux et al., 1982a,b), and other impurities that may be present in the polymeric systems. Several papers have been published detailing with this subject (Kimura et al., 1983, 1984; Godard et al., 1986a,b; Mondragon and Nazabal, 1986). Ester-exchange reactions in polyester blends may lead to materials with poor mechanical properties such as reduced impact strength (Golovoy et al., 1987). When semicrystalline polyesters are used as one of the ingredients of a blend, the ester-interchanged products may not crystallize. As a result, the solvent resistance of the material can be reduced considerably (Kimura et al., 1984). Chemical strategies have been devised to retard the ester-exchange reactions. Organophosphorus compounds, such as phosphites (Devaux et al., 1982a,b), phosphonates (Smith et al., 1981), and phosphates (Bonun and Logeat, 1986), have been used to inhibit ester-exchange reactions especially in the molten state. However, the maximum temperature for effective stabilization was reported to be at or below 280 "C. This paper presents the use of a stabilizer system composed of phosphites, polycarbodiimides, and hindered phenols for the retardation of the ester-exchange reactions at temperatures above 280 "C. This temperature range is needed for blends which require higher processing temperatures and longer residence times in the processing equipment. The proposed stabilizer system retards ester-exchange reactions significantly in ternary blends consisting of polyarylate, polycarbonate, and poly(ethy1ene terephthalate). Polyarylate (PAr) and polycarbonate (PC) have a relatively high glass transition temperature and are amorphous. PAr has high tensile strength and impact strength. It is resistant to ultraviolet light and to fire (Maresca and Robeson, 1985). PC also has a good balance of mechanical properties, such as tensile and impact strengths, high elongation, and izod impact. Both PAr and PC have poor solvent resistance, which makes painting and other solvent-related processing difficult. Poly(ethy1ene terephthalate) (PET) is one of the most widely used polyester thermoplastics with a semicrystalline structure. Its crystalline structure enhances solvent resistance when blended with amorphous polymers. All three polymers have chemical groups in common, which may result in interactions between the similar chemical moieties. PAr is the condensation product of Bisphenol A and isophthalic/ 0888-5885/89 f 2628-0476$0l.50/0

terephthalic acids. PC contains the Bisphenol A and carbonate moieties and poly(ethy1eneterephthalate) is the ester derived from terephthalic acid and ethylene glycol. It is known that processing of PAr, such as melt extrusion, is difficult and may require a processing temperature of 340-390 "C due to its high Tgand melt viscosity. On the other hand, the viscosity of PET decreases drastically after melting ( T , = 255 "C). The melt behavior of polycarbonate is in between that of PAr and PET. The blended combination of PAr/PC/PET gives a workable consistency at temperatures ranging from 280 to 325 "C, depending on the molecular weight of the constituents. At the higher blending temperatures, efficient stabilization is required in order to maintain the physical properties. The effectiveness of the stabilizers singly and in combination was gauged with thermal cycling using differential scanning calorimetry (DSC) by following the extent, temperature, and sharpness of the crystal-amorphous phase transition of poly(ethy1ene terephthalate) in the blend. Experimental Section The polymeric ingredients used are all high-performance engineering thermoplastics. Polyarylate (Ardel D-100) was obtained from Amoco Performance Products, Inc., and polycarbonate (Calibre 300-15) was acquired from the Dow Chemical Co. Poly(ethy1ene terephthalate) (Cleartuf 1006B) was supplied by Goodyear Chemicals Co. Ultranox 624 was obtained from Borg-Warner Chemicals, Inc., Stabaxol P-100 and Stabaxol KE 7646 (15% Stabaxol P-100 in PET) were supplied by Mobay Corporation, and Ethanox 330 was received from Ethyl Corp. Reactive polystyrene is an experimental resin, XUS40056.01, from Dow Chemicals Co., which has an oxazoline functionality built into the polystyrene backbone. All polymers and stabilizers were used as received except that the polymers were dried under vacuum at 115 "C for at least 16 h prior to compounding. Stabilizer concentrations used were kept constant throughout: 0.5 phr (parts per hundred part of resin) of Ultranox 624 (phosphite); 0.25 phr of Stabaxol P-100 (polycarbodiimide); 0.2 phr of Ethanox 330 (hindered phenol). All the stabilizers were compounded into concentrates and dry blended with the rest of the polymer. Dry pellets were premixed and then charged to the hopper of a Haake Rheomex 254 single-screw laboratory extruder. Temperature control was divided into three zones, with the first zone about 10 deg lower than the middle zone, which was 1989 American Chemical Society

Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 477 60

osc RATE 5 ' C I M i n

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Figure 1. DSC thermograms of PET, PC, and PAr.

the highest, and about 5 deg lower at the third zone. The die section was disassembled in order to obtain a larger strand for further evaluation. The die adaptor portion of the extruder, in this case, was wrapped with heating tape in order to prevent premature crystallization. Differential scanning calorimetry was conducted on a Mettler Thermal Analysis Systems TA3000 using the TClOA processor to monitor and collect data. Two experimental schemes were designed as outlined below: A.

DSC CYCLES

Figure 2. Heat of fusion versus DSC scan cycles: (A)PAr/PET, (0) PC/PET; (m) PC/PET, commercial blend; (0) PET. Numerical values is the melting point in "C. RATE 5 'C/Min

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In the cycling experiments, repeated scanning of the same cell was conducted. The cooling time from the peak temperature back to the starting temperature was about 3 min. The heat of fusion (AHf), melting point (T,,J, and glass transition temperature ( Tg)were determined by the TClOA processor. Infrared spectra were obtained on a Mattson Sirius 100 FTIR spectrometer equipped with a broad-band MCT, nitrogen-cooled detector. Samples were prepared by cryogrinding the tested samples from the DSC in liquid nitrogen. The powdered polymer was mixed with potassium bromide and pressed into a pellet.

Results and Discussion Figure 1 shows the DSC thermograms of the three ingredients; all the polymers were annealed about 5 "C below their Tgin order to enhance the transition. The glass transition temperatures of PET, PC, and PAr are seen at 82, 150, and 190 "C, respectively. The thermogram of the PET also features a cold crystallization peak with a peak temperature a t about 120 "C and a melting peak with a T,,, a t 255 "C. Before the behavior of ternary blends of these polymers was studied, a preliminary investigation was conducted to determine the relative stability of PET in binary blends. Figure 2 shows plots of the heat of fusion (AHf)of PET versus the number of cycles in a DSC cycling test. The data are for neat PET and for binary blends of PET (40% by weight) with PC and with PAr. The numbers next to the data points are the T, values. No additives were added to the extruded binary blends. In the neat PET, AHf increases with the thermal cycles, probably

Figure 3. Typical DSC thermograms of PAr/PC/PET (50/20/30) blend without stabilizer: (1) second step after extrusion a t 280 "C; (2) after annealing a t 190 "C for 1 h; (3) after 10 min at 280 "C.

due to the increase of crystal perfection with thermal cycling (Fontaine et al., 1982). The PC/PET blend is stable with respect to AHf and T,. This is also true for a commercial blend of PC/PET (Mobay's Makroblend) shown as a comparison. Nassar et al. (1979) reported similar results. The small change in T, with the number of cycles in the PC/PET blends is within experimental error. The PET in the PAr/PET blend, on the other hand, is not stable. It is evident that both AHf and T, decrease rapidly, especially after the third cycle. Similar findings have been reported by Kimura et al. (1984). All the ternary blends discussed here have the composition of 50/20/30 (PAr/PC/PET) by weight. A typical DSC thermogram of the ternary blend immediately after extrusion is shown in Figure 3, curve 1. In this case, no stabilizers were used and the extrusion temperature was 280 "C. Note that the PET features are well preserved, i.e., a Tgat about 83 "C, a cold crystallization exotherm a t about 145 "C, and a melting endotherm a t about 250 "C. The glass transition temperatures of PC and PAr are not seen because of the PET'S cold crystallization exotherm (Hughes and Sheldon, 1964). If the cold crystallization peak is removed by annealation of the blend, then the Tg's of the amorphous polymers become apparent. Figure 3, curve 2 is the DSC scan of the same sample after annealing at 190 "C for 1 h. The transition regions (indicated by arrows) at about 150 and 180 "C correspond to the glass transition temperature of PC and PAr, respectively. The appearance of the transition is less apparent

478

Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989

Table I. Results of Isothermal DSG Tests extrusion stabilizers time a t sample temp, "C A" B" C" 300 "C, min A 280 x 0 30 B 300 x 0 30 C 300 x x 0 30 D 300 x R-PstyO 0 30 E 325 x x 0 10 30 F 325 x x x 0 30 60

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"C 249 245.7 253.7 247 249.7 249.7 254 251 251.8 251.1 244.4 252 248 246

"A, Ultranox 624; B, Stabaxol P-100; C, Ethanox 330; R-Psty, reactive polystyrene.

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than in the pure polymers because of the small size of the PC domains. Moreover, in the case of PAr, the detection of T gis compounded further since the specific heat difference between the glassy and rubbery states (AC,) of PAr is relatively low (Robeson, 1985). The decrease in the Tg of the PAr phase to 180 "C is probably the result of some transesterification which enhances its miscibility with the other constituents (Kimura et al., 1984, Golovoy et al., 1987). When the ternary blend was held at 280 "C for 10 min, the originally opaque sample turned transparent, a characteristic of amorphous materials. The thermogram of that sample (Figure 3, curve 3) verifies its noncrystalline nature. What is observed is a broad glass transition temperature at about 125 "C, indicating that a mixture of miscible copolymers was formed. Thus, within 10 min a t 280 "C, the exchange reaction in the ternary blend has proceeded to the point that the resulting material is noncrystalline, since no crystal-related transition is observable (Kimura et al., 1984). The results above clearly show that in order to maintain the crystalline nature of PET in the ternary blend it is necessary to inhibit the ester-exchange reactions. For that purpose, up to three stabilizers were incorporated into the ternary blend. The results of the study on stabilization are summarized in Table I. The criterion of stability is again the retention of the crystalline melting peak of PET (last two columns in Table I) during isothermal testing at 300 "C. An important factor in the stability of polyester blends is the processing temperature. To cover temperatures that may be encountered in practical situations, the ternary blends were extruded at 280,300, and 325 "C, and the results are discussed in relation to these temperatures. When phosphite (Ultranox 624) was incorporated into the blend system and the system extruded at 280 "C, the blend was found to be fairly stable. After 30 min at 300 "C, T, decreased by only 3 "C and the heat of fusion by 2 J/g (Table I, sample A). When the extrusion temperature is raised to 300 "C (sample B), a larger decrease in AHf and T, is observed. The combination of Ultranox 624 and Stabaxol P-100 improved the blend stability for extrusion a t 300 "C (sample C): both T , and AHf remain constant after testing for 30 min at 300 "C. Reactive polystyrene (sample D), which has an oxazoline functionality built in the polystyrene backbone, produces the same effect as the carbodiimide, but the resulting heat of fusion seems to indicate a reduced stability compared to that produced by the carbodiimide. When the extrusion temperature is raised to 325 "C, the stabilizer composition used in sample C is not as effective.

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The AHf of the sample extruded a t 325 "C has already decreased to 7.7 J/g, as compared to 12.2 J / g for the sample extruded at 300 "C (sample C). When sample E is kept at 300 "C for 10 min, the melting point and the heat of fusion are fairly constant. But after 30 min at 300 "C, a large decrease in both AHf and T , is observed. The addition of Ethanox 330 gives a blend (samples F) which is stable at 300 "C for 60 min. Thus, when a combination of three stabilizers is used in the ternary blend, the thermal characteristics of PET are preserved quite well over a broad temperature range of .extrusion. In addition to the isothermal procedure, a cycling test was used. The cyclic test gives a finer look at the effectiveness of the stabilizers. When blends with different combinations of stabilizers were subjected to cycles of DSC scanning (up to 280 "C), they also showed progressive differences in the effectiveness of the stabilizer system. All the samples were prepared under the same conditions (except for the extrusion temperature) and had the same thermal history (annealed at 210 "C for 3 h prior to the cyclic testing) and were sized to be as similar as possible. Figure 4 shows thermograms of the second and third cycles of the blend without stabilizers. After the third cycle, it is obvious that the exchange reactions have decreased T , and the heat of fusion. With the addition of Ultranox 624, the blend, extruded at 280 "C, is more stable, as suggested by the fact that the thermograms of the second and the eighth cycles (Figure 5) are very similar. But when the extrusion temperature is raised to 300 "C, the phosphite stabilizer alone is not as effective. Figure 6 shows the importance of the extrusion temperature. At the second DSC cycle, the sample extruded at 280 "C still exhibits the original thermogram characteristics, but the

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Ind. Eng. Chem. Res., Vol. 28, No. 4, 1989 479 RATE 5 T / M i n

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Figure 6. DSC second scan thermograms of PAr/PC/PET with Ultranox 624 extruded at (-) 280 O C and at (-- -) 300 "C. 15 681mg

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one extruded a t 300 "C shows a marked decrease of the cold crystallization exotherm and the heat of fusion. A comparison of the second and the eighth cycles with the combination of phosphite and carbodiimide as stabilizers, extruded a t 300 "C, is shown in Figure 7. Again, the system appears to be well stabilized. Figure 8 shows the second cycle thermograms of the same blend, i.e., stabilized with phosphite and carbodiimide, extruded at 300 and 325 "C. It is evident that the stability has decreased at the higher extrusion temperature. Following the addition of a third stabilizer, a hindered phenol (Ethanox 330), and extrusion at 325 "C, the system is quite stable, as indicated

by the similarity of the second and the eighth cycles (Figure 9). Infrared spectra of some of the samples from the DSC study were recorded in order to observe the transformations taking place in the blend samples on a molecular level. Infrared data of ester-exchange reactions in polymer blends have been reported but have been limited to the binary systems. The application of this method to the ternary blends is feasible only if the earlier reports are used as reference points. In the work of Godard et al. (1986a,b), the PC/PET binary blend transesterification and loss of carbonate COzwas observed. The transesterification was characterized by the growth of 1770-, 1740-, and 1070-cm-' bands. The 1740- and 1070-cm-' bands are attributed to a doubly aromatic ester moiety. The 1770-cm-l shoulder was attributed to a carbonate of mixed aromatic and aliphatic type. However, the relatively extensive loss of 1780-1770-cm-' intensity compared to the growth of the doubly aromatic ester was attributed to the loss of C 0 2 from carbonate groups. The situation is somewhat more complex in the analysis of the infrared spectra of the ternary blend system under study in this report, since the PAr contains just such a doubly aromatic ester group. This can be clearly seen in Figure 10 where the 2000-800-cm-' spectra of the starting polymers are reproduced. Thus, the changes will be somewhat more subtle, and some of the exchange products will not be distinguishable from the reactants. Figure 11 shows the infrared spectra of the unstabilized blend as extruded (curve l),the unstabilized blend after isothermal treatment at 300 "C for 30 min

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