- Email: [email protected]
bisphenol-a polycarbonate blends
J. Vol. 32, No. 9, pp. 1061-1066, 1996 Copyright 0 1996 Elsevier Science Ltd
Eur. Polym.
Pergamon PII: s0014-3057(%)ooo39-0
ELONGATIONAL RHEOMETRY TEREPHTHALATE/BISPHENOL-A BLENDS A. M. ROBINSON,’
B. HAWORTH**
Printed in Great Britain. All rights reserved 0014-3057/96
$15.00 + 0.00
OF POLYETHYLENE POLYCARBONATE and A. W. BIRLEY*
‘Advanced Railway Research Centre, Regent Court, Regent Street, Sheffield Sl 4DA, U.K. and *IPTME, Loughborough University of Technology, Loughborough LEl 1 3TU, U.K. (Received 20 July 1995; accepted in jinal form 20 September 1995)
Abstract-The elongational deformation properties of polyethylene terephthalate (PETP) and bisphenol-A polycarbonate (PC) blends were determined using a Rutherford elongational rheometer. The effects of temperature (in the thermoelastic region) and strain rate were studied on blends containing up to 50% PC. Addition of low levels of polycarbonate permits the thermoelastic processing of PETP over a wider temperature range. PETP is particularly sensitive to changes in temperature. Over the range studied, the effect of strain rate on elongational deformation is not very marked. However, the deformation temperature changes the strain levels at which strain stiffening occurs, an important observation in respect of processing and optimisation of physical properties of uniaxially oriented products. The effect of the addition of a phenoxy resin on the blend was studied, and although TEM analysis suggests that it did not compatibilise the blend, it did increase the available extension at higher temperatures. Copyright 0 1996 Elsevier Science Ltd
Poly(ethylene
terephthalate)
and bisphenol-rl-polycar-
bonate Thermoplastic polyesters, including bisphenol-Apolycarbonate and poly(ethylene terephthalate), represent a class of engineering plastics with excellent thermal resistance, chemical resistance and mechanical properties, but they are subject to hydrolysis [l] and subsequent property deterioration. They are extensively used for injection moulded articles, and oriented materials are used in synthetic fibres, films and in blow-moulded containers. The heat distortion temperature and toughness of moulded thermoplastic polyesters, such as PETP, may be improved by blending with amorphous polymers such as PC. The majority of the blends of polyesters with related condensation polymers exhibit at least partial miscibility [2-4]. Good mechanical properties observed for blends of PETP and PC are no doubt a reflection of the favourable interactions that occur between the two polymers at the molecular level, which make this system on the edge of complete miscibility [5]. These mechanical properties, together with the inherent chemical resistance PETP brings to blends with PC, justifies current interest in this system, as does the potential for increasing the temperature of use and shrinkage resistance of oriented PETP. The majority of previous research carried out on the PETP-PC blend system has been concentrated on quasi-isotropic (moulded or extruded) samples, or on materials prepared under idealised laboratory con*To whom all correspondence should be addressed.
ditions. In view of the importance of products produced from uniaxially-oriented or biaxially-oriented PETP (for which the physical properties are determined by the deformation history and subsequent strain-induced crystallinity), it is desirable to extend previous work by performing elongational deformation studies in the thermoelastic region (to simulate processing), and to determine if the PETP/PC blend can be processed in a way similar to PETP. If processing is feasible, oriented products might assume some of the improved properties associated with PC. Elongational
rheometry
A usual requirement of controlled elongational flow experiments on thermoplastics is that they are either carried out at constant stress, or at constant strain rate, whilst the other parameter is determined as it varies with time. When considering viscoelastic materials undergoing uniaxial extension to high strains at a constant rate of extension, the strain rate is a transient parameter. Therefore the instantaneous strain rate has to be defined, taking into account the current length L,; hence the instantaneous strain rate, &, is defined as: 1 dL
&=L,.x
where dL/dt is the end-separation specimen. Therefore:
velocity V of the
s,= Vt 7
This provides a convenient means of measuring instantaneous strain rate from measured test parameters. The constant strain rate mode of testing is 1061
A. M. Robinson er al
1062
Table I. Compounding conditions Die set temperatures (“C) Torque Die head pressure Nominal feed rate Screw speed
Barrel 271
21s 14.8 Nm (25%) -4 MPa 110 gjmin
Feed 265 (held constant)
265
260
(overall)
220 rpm
used by increasing the test velocity during the experiment, relative to the specimen deformation. In a viscous flow situation, the apparent elongational viscosity 1 can be defined as [6]:
where u is the true stress and i is the instantaneous strain rate. It is conventional to plot elongational viscosity 1 (sometimes referred to as the stress growth function; see equation (3)) against the experimental time t in these circumstances [6, 71, in order to observe the effects of changing strain rate and/or temperature for example. In other circumstances, however, particularly for rubber-like deformations observed for predominantly-amorphous thermoplastics at temperatures immediately above the glass transition, it is more appropriate to present the elongational deformation data as a plot of stress against a strain parameter. Axtell and Haworth [6, 71, working with PETP,
EXPERIMENTAL
found a significant deviation from the normal gradient of a true stress versus true strain graph at the onset of stress-induced crystallisation. This onset point could also be determined from stress growth function versus time graphs. This phenomenon is desirable in processing PETP in the thermoelastic region as products assume their maximum physical properties only when local stretching is sufficient to induce significant crystallinity [7]. Further study of the uniaxial deformation of PETP, taking into account the effects of blending, is clearly desirable. This paper will therefore examine the strain hardening behaviour of PETP/PC blends in uniaxial extension, noting the effects of blend composition, temperature and elongational strain rate. The Rutherford elongational rheometer was utilised to determine the thermoelastic behaviour of this system; the specification and method of operation of this equipment have been described previously [6-91. PROCEDURES
Materials
Polymer
Grade
Polyethylene terephthalate Bisphenol-A polycarbonate Phenoxy resin’
(PETP) (PC) (Ph)
B9OS 161
Code B X P
Intrinsic viscosity [IO] (dW 0.83 0.545
Supplier ICI GE Plastics Union Carbide
“This is a commercial material (a polyhydroxyether of bisphenol-A), available for blending thermoplastic polyesters. The repeat unit is given below; the grade used has a nominal density of 1.17 g/cm~, r, = 90°C and has a melt flow index (MFI) of 4.8dg/min. at 220°C.
r
CHq
Coding sysrem Blend comuosition (PETPIPC w/w)
Blend code
9OjlO SO/20 SO/20 (5% Ph) 70/30 60/40 50/50
B9OX BSOX BSOXPb B7OX B6OX BSOX
‘Number refers to percentage PETP. b80/20 PEP/PC blend (95%) with 5% phenoxy resin (see also RESULTS AND DISCUSSION).
Blend preparation [IO]
A corotating
twin-screw
compounder
from
the APV range (MP 2030) was used (length/diameter ratio 15: I) to prepare
the blends. The screw configuration was made up as shown below: Die
Camel back discharge
/
,[is
1 :g
Polyethylene terephthalate/bisphenol-A All materials were dried thoroughly in a Conair-Churchill recirculating dehumidiier, prior to compound extrusion. An in-tine strip-die was specifically designed for this work, and was attached to the compounding unit; the amorphous product was drawn through a sizing die into the water bath by a variable speed haul-off. Blending was carried out at a constant torque level; compounding conditions are shown in Table 1. Experimental techniques The high thermal stability silicone oil bath in the Rutherford rheometer was preheated to the required test temperature. In set-up mode the carriage was returned to the
starting position and the specimen was placed on the specimen holders. It was then left to equilibrate for 4 min. The desired constant strain rate mode was selected and the initial specimen length and equivalent diameter were set on the control panel. (The “equivalent diameter” is required, since the specimen is a rectangular-section hoop (i.e. non-circular) the equivalent diameter is the diameter of a circle having the same cross-sectional area as the actual specimen.) The required strain rate is set on the control panel. The hoop specimen is immersed in the temperaturc-controlled oil bath which acts as an efficient heat transfer medium. On starting the test, the time, force and displacement signals were recorded. Several runs at each strain rate and temperature were carried out, and the average results are reported in the following sections. Elongational deformation-conditions All the PETP/PC blends were tested using the Rutherford rheometer to determine their elongational deformation behaviour in the thermoelastic region under the following conditions of temperature and strain rate:
polycarbonate blends
1063
(1) temperatures 80, 95 and 110°C; (2) strain rates 0.1, 0.5 and 0.95 set-‘. During the test, force, extension and time were recorded and the true stress and strain were calculated from these data. True stress versus strain plots were then produced so that the effects of the chosen variables could be demonstrated. This mode of presentation allows easier interpretation for polymers undergoing high levels of extension in the rubbery state. Morphology determination by staining and TEA4observations Thin sections of the bulk specimens (approximately 0.5 nrn thick) were obtained at room temperature using a Cambridge Huxley microtome equipped with a glass knife. A 0.5% stabilised aqueous solution of ruthenium tetroxide (RuO,) was obtained from Fluorochem Ltd in 10cm3 snapopen vials. The polymer sections, supported on mesh grids, were exposed for 30min to RuO, vapours in a desiccator containing a few millilitres of the solution. Stained sections were observed on a JEOL JEM 100 CX transmission electron microscope at 100 kV. Scanning mode TEM (STEM) was used, as this results in less beam damage to the sample and has better penetration of the relatively thick samples. STEM involves scanning a 8ne beam of electrons over the sample and detecting transmitted electrons with a scintillator photomultiplier arrangement. Viewing of the image is on a cathode ray tube. RESULTS AND DISCUSSION
Generally the shape of the plots obtained was similar to the representation in Fig. 1. It can be seen that the deformation behaviour can be divided into three types: elastic deformation, drawing, and strain-stiffening due to orientation and deformationinduced crystallisation. The last of these is a viscous response, associated with permanent deformation as crystallisation proceeds. The relative importance and positions of these deformation mechanisms will vary with material composition, temperature, etc. E$ect of temperature on the elongational deformation behaviour of PETPIPC blenak
Fig. 1. Ceneralised representation of a true stress vs strain curve for PETP and PETP/PC blends.
Figure 2 shows the effect of temperature on the elongational flow behaviour of 90/10 PETP/PC blend at 0.5 set-’ strain rate. The figure shows that for PETP/PC blends the effect of temperature is very important, as the extension for a given stress level increases with temperature, and strain-stiffening is delayed correspondingly. This type of behaviour is also observed for PETP alone [6, 7,9]. It can be seen that at 80°C the temperature is sufliciently low to
Temperalum ‘C 4,000
110 95
Sbaln 4.000 I
t
ml.3 Is
0.1 t 0.5
--c 0.95
-c 0.1 -8 0.5 -(r
0.95
++
=0
1
2
3
4
5
6
7
shin
Fig. 2. Effect of temperature on the elongational deformation of PETP/PC B9OX (strain rate 0.5 set-I).
Fig. 3. Effect of strain rate on the elongational deformation of PETP/PC B80X (temperatures 80 and 110°C).
A. M. Robinson et al.
1064
PCrmlent(%) 4.c.m -
PC
content (%)
60 -
60 40
3.000 I
II
/
r'/r
30 +
&
t
g
1
20
*
2.Ow
-910 -60
20 10
2 l.OW
0
0
1
2
3
4
6
6
7
Fig. 4. Effect of PC content on PETP/PC elongational deformation (95°C. strain rate 0.1 m-l). PC content (%)
-
so 40
t 30 NDt M -E10 -40 +
7
Strain
Fig. 5. Effect of PC content on PETP/PC elongational deformation (SYC, strain rate 0.5 see-I). PC canlent (%) 4.!ml ,
I
50
smln
Fig. 6. Effect of PC content on PETP/PC elongational deformation (95’C, strain rate 0.95 xc’).
the PETP extending signScantly before strain-stiffening occurs. In practice, this is a desirable effect, so long as sufficient deforming force can be achieved during processing. This result would suggest that thermoelastic processing of PETP/PC blends should be carried out at lower temperatures (N 8o”C), in order to achieve high orientation at modest strain levels. prevent
E$ect of strain rate on the elongational deformation behaviour of PETPIPC blena!s Figure 3 shows that the elongational deformation and flow behaviour of PETP/PC 80/20 blends is relatively unaffected by changes in strain rate in the range 0.1495 set-‘. The effect of temperature is again demonstrated; as the temperature increases from 80 to 110°C the extension increases (for a given stress) and strain-sti&ning is delayed. Although
Fig. 7. Effect of PC content on PETP/PC elongational deformation (8O”C, strain rate 0.95 w-l).
temperature is dominant in our experiments, strain rate effects are not insignificant in commercial processes, where very high rates of deformation are often used [7]. Effect of PC content on the elongational deformation behaviour of PETPIPC blenak Figures 4-6 show that (at SY’C), as the PC content increases, the material becomes increasingly rigid, resulting in earlier strain-stiffening. It has been demonstrated [6,7] that the onset of an abrupt change in gradient corresponds to the onset of strain-induced crystallisation in PETP. From the results presented here, it would appear that the presence (and amount) of the less mobile polycarbonate-rich phase is contributing signiflcantly to this effect. Earlier onset of strain-stiffening is generally a desirable property, as the improved mechanical properties associated with strain-induced crystallisation can be achieved at lower strains, as long as the force required to continue to deform the blend can be generated. This is usually the case with processes such as fibre drawing, tape making, etc. The addition of PC to PETP, resulting in earlier strain-stiffening, appears therefore to be a desirable effect. This occurs for all strain rates (0.1,O.S and 0.95 set-‘) in Figs 4-6, respectively, at 95°C. When the temperature is reduced to 80°C (Fig. 7) strain-stiffening occurs in all blends at very low strain levels (< 100% strain). This suggests that, so long as the required force levels can be generated, the thermoelastic processing of PETP/PC blends could be carried out at 80°C. At 110°C (Fig. 8) for the PETP/PC blends the systematic increase in rigidity with PC content can be PC
lxlntent (%) -
Y
60
40 -c 30 t 20 -E10 + 0 -!U-
Fig. 8. Effect of PC content on PETP/PC elongational deformation (1 lO”C, strain rate 0.95 w-l).
Polyethylene terephthalate/bisphenol-A
polycarbonate blends
1065
Fig. 9. PETP/PC blend stained with Ru04 (magnification x 10 K). (a) 50% PC, (b) 40% PC, PC, (d) 10% PC.
clearly seen. It is interesting too that PETP at 110°C also appears to be a very rigid material, indicating that the B90S has crystallised (thermally) during the
preheat period of the test. This precrystallisation means that chain stiffening can occur at lower strains (at 110°C in pure PETP), in comparison to the blends, where the PC component restricts the (thermally-induced) crystallinity level achieved for a given preheat time. In uniaxially-oriented PETP tapes and fibres, the mechanical properties are derived primarily from the
4.cal
BBOX 880X
!
~, !
(5% Ph) -
1WC _!
Strain
Fig. 10. Effect of the phenoxy compatibiliser on PETPjPC elongational deformation (BSOX, 0.5 set-I).
(c) 20%
orientation and strain-induced crystallinity, which depends upon temperature, strain rate and the addition of PC. A high stress developed at low strain would appear to represent a definite potential advantage for the use of PETP/PC blends in thermoelastic processes. Morphology of the PETP/PC
blends
Figure 9 shows the morphologies of the PETPJPC compositions from 50 to 90% PETP, observed using STEM. In Figs 9(a) and 9(b) the two-phase blends have a rough appearance, which is probably due in part to the difficulty in microtoming a toughened blend containing 40-50% PC [lo]. Figure 9(c) shows a very interesting morphology; the PC nodules are very small, evenly distributed and circular. Figure 9(d) shows that even when the PC content is as little as lo%, the nodules are still distinct, showing that PETP and PC are immiscible, and the morphology remains a two-phase structure. Effect of phenoxy (compatibiliser) on the elongational deformation behauiour of PETPjPC blendr
A phenoxy resin [phenol 4,4-( I-methylethylidene) bis-, polymer with (chloromethyl) oxirane (PH)] was
1066
A. M. Robinson et al.
Fi 11 shows the effect of phenoxy on the mosphdooy of the PETPjPC. It can clearly be seen that the addition of phenoxy results in larger PC nodules being formed in the immiaeible blend. There was little or no evidence from the !3TEM study to support any claims that phenoxy increases the small-scale compatibility between PETP and PC. However, it does appear to increase the available extension in the blend at 1lO”C, which could be due to the phenoxy interacting with the larger PC nodules and allowing the resultant continuous phase of PEI’P to extend relatively freely. In the unmodified blend with finer, well-dispersed particles, it may be more di5eult for the PETP to be continuous and therefore extension is limited. CONCLUSIONS
(1) Addition of PC at low levels (lO-20%) permits
Fig. 11. Effect of phenoxy on the morphology of the PETP/PC blend. (a) B80X, (b) B80X + 5% phenoxy. supplied by Union Carbide as a compatibiliser for the PETP/PC blends. In the composition which had added phenoxy, the PETP/PC weight ratio was kept as stated and the phenoxy was then added as a percentage of the whole compound. In the PETP 8O/PC 20 blend, 5% phenoxy was added: i.e. PETPjPC made up 95% of the blend and phenoxy the remaining 5%. Figure 10 shows that phenoxy appears to have an effect on the thermoelastic behaviour of the PETP/PC blend only at llO”C, if added at the 5% level. The phenoxy appears to delay the strain-hardening behaviour, i.e. the available strain (extension) is increased and the stress required to achieve a specific strain is reduced. This effect is not a desired one for improvement of mechanical properties in stretchformed products; however, it may be of use if deformation is high, and products are required to be thin, and also where mechanical properties are a secondary consideration,
the thermoelastic processing of PETP over a wider temperature range, and suggests that uniaxial orientation, and sigr&ant strain-induced crystallisation in the PETP-phase, can be achieved at lower strain levels. (2) Over the range studied, the effect of strain rate on elongational deformation is not very marked, in comparison to tire influence of temperature (80-l WC). (3) Addition of phenoxy to PETP/PC blends has only a minor effect, delays the onset of strain-hardening only at 1lO”C, and contributes to a more coarse, two-phase blend morphology. (4) There is little evidence for miscibility in the PETP/PC blend; a two-phase system was identified over the composition range 5&90% PETP. REFERENCES 1. V. M. Nadkami, V. L. Shingankuli and J. P. Jog. Polym. Eng. Sci. Zs, 1326 (1988). 2. X. Y. Chen and A. W. Birley. Br. Polym. J. 17, 341 (1985). 3. A. W. Birley and X. Y. Chen. Br. Polym. J. 16, 17 (1984). 4. A. W. Birley and X. Y. Chen. Br. Polym. J. 17, 297 (1985). 5. S. R. Murff, J. W. Barlow and D. R. Paul. J. Appl. Polym. Sci. 29, 3231 (1984).
6. F. H. Axtell and B. Haworth. Polym. Test. 9,53 (1990). I. F. H. Axtell and B. Haworth. Plast. Rd. Comp. Proc. Appl. 22, 127 (1994).
8. B. Haworth, L. Chua and N. L. Thomas. PVC 93, The Future. IOM Conference, Brighton, U.K., April, p. 210 (1993). 9. F. H. Axtell. A study of the flow properties
and processability of thermoplastic polyesters. Ph.D. Thesis, Loughborough Univer&y of T&hnology (1987). 10. A. M. Robinson. Blends of nolvtethvlene tereohthlate) with bisphenol-A polycarbo&~.‘Ph.~D. Thesis: Lough: borough University of Technology (1991).
Eur. Polym.
Pergamon PII: s0014-3057(%)ooo39-0
ELONGATIONAL RHEOMETRY TEREPHTHALATE/BISPHENOL-A BLENDS A. M. ROBINSON,’
B. HAWORTH**
Printed in Great Britain. All rights reserved 0014-3057/96
$15.00 + 0.00
OF POLYETHYLENE POLYCARBONATE and A. W. BIRLEY*
‘Advanced Railway Research Centre, Regent Court, Regent Street, Sheffield Sl 4DA, U.K. and *IPTME, Loughborough University of Technology, Loughborough LEl 1 3TU, U.K. (Received 20 July 1995; accepted in jinal form 20 September 1995)
Abstract-The elongational deformation properties of polyethylene terephthalate (PETP) and bisphenol-A polycarbonate (PC) blends were determined using a Rutherford elongational rheometer. The effects of temperature (in the thermoelastic region) and strain rate were studied on blends containing up to 50% PC. Addition of low levels of polycarbonate permits the thermoelastic processing of PETP over a wider temperature range. PETP is particularly sensitive to changes in temperature. Over the range studied, the effect of strain rate on elongational deformation is not very marked. However, the deformation temperature changes the strain levels at which strain stiffening occurs, an important observation in respect of processing and optimisation of physical properties of uniaxially oriented products. The effect of the addition of a phenoxy resin on the blend was studied, and although TEM analysis suggests that it did not compatibilise the blend, it did increase the available extension at higher temperatures. Copyright 0 1996 Elsevier Science Ltd
Poly(ethylene
terephthalate)
and bisphenol-rl-polycar-
bonate Thermoplastic polyesters, including bisphenol-Apolycarbonate and poly(ethylene terephthalate), represent a class of engineering plastics with excellent thermal resistance, chemical resistance and mechanical properties, but they are subject to hydrolysis [l] and subsequent property deterioration. They are extensively used for injection moulded articles, and oriented materials are used in synthetic fibres, films and in blow-moulded containers. The heat distortion temperature and toughness of moulded thermoplastic polyesters, such as PETP, may be improved by blending with amorphous polymers such as PC. The majority of the blends of polyesters with related condensation polymers exhibit at least partial miscibility [2-4]. Good mechanical properties observed for blends of PETP and PC are no doubt a reflection of the favourable interactions that occur between the two polymers at the molecular level, which make this system on the edge of complete miscibility [5]. These mechanical properties, together with the inherent chemical resistance PETP brings to blends with PC, justifies current interest in this system, as does the potential for increasing the temperature of use and shrinkage resistance of oriented PETP. The majority of previous research carried out on the PETP-PC blend system has been concentrated on quasi-isotropic (moulded or extruded) samples, or on materials prepared under idealised laboratory con*To whom all correspondence should be addressed.
ditions. In view of the importance of products produced from uniaxially-oriented or biaxially-oriented PETP (for which the physical properties are determined by the deformation history and subsequent strain-induced crystallinity), it is desirable to extend previous work by performing elongational deformation studies in the thermoelastic region (to simulate processing), and to determine if the PETP/PC blend can be processed in a way similar to PETP. If processing is feasible, oriented products might assume some of the improved properties associated with PC. Elongational
rheometry
A usual requirement of controlled elongational flow experiments on thermoplastics is that they are either carried out at constant stress, or at constant strain rate, whilst the other parameter is determined as it varies with time. When considering viscoelastic materials undergoing uniaxial extension to high strains at a constant rate of extension, the strain rate is a transient parameter. Therefore the instantaneous strain rate has to be defined, taking into account the current length L,; hence the instantaneous strain rate, &, is defined as: 1 dL
&=L,.x
where dL/dt is the end-separation specimen. Therefore:
velocity V of the
s,= Vt 7
This provides a convenient means of measuring instantaneous strain rate from measured test parameters. The constant strain rate mode of testing is 1061
A. M. Robinson er al
1062
Table I. Compounding conditions Die set temperatures (“C) Torque Die head pressure Nominal feed rate Screw speed
Barrel 271
21s 14.8 Nm (25%) -4 MPa 110 gjmin
Feed 265 (held constant)
265
260
(overall)
220 rpm
used by increasing the test velocity during the experiment, relative to the specimen deformation. In a viscous flow situation, the apparent elongational viscosity 1 can be defined as [6]:
where u is the true stress and i is the instantaneous strain rate. It is conventional to plot elongational viscosity 1 (sometimes referred to as the stress growth function; see equation (3)) against the experimental time t in these circumstances [6, 71, in order to observe the effects of changing strain rate and/or temperature for example. In other circumstances, however, particularly for rubber-like deformations observed for predominantly-amorphous thermoplastics at temperatures immediately above the glass transition, it is more appropriate to present the elongational deformation data as a plot of stress against a strain parameter. Axtell and Haworth [6, 71, working with PETP,
EXPERIMENTAL
found a significant deviation from the normal gradient of a true stress versus true strain graph at the onset of stress-induced crystallisation. This onset point could also be determined from stress growth function versus time graphs. This phenomenon is desirable in processing PETP in the thermoelastic region as products assume their maximum physical properties only when local stretching is sufficient to induce significant crystallinity [7]. Further study of the uniaxial deformation of PETP, taking into account the effects of blending, is clearly desirable. This paper will therefore examine the strain hardening behaviour of PETP/PC blends in uniaxial extension, noting the effects of blend composition, temperature and elongational strain rate. The Rutherford elongational rheometer was utilised to determine the thermoelastic behaviour of this system; the specification and method of operation of this equipment have been described previously [6-91. PROCEDURES
Materials
Polymer
Grade
Polyethylene terephthalate Bisphenol-A polycarbonate Phenoxy resin’
(PETP) (PC) (Ph)
B9OS 161
Code B X P
Intrinsic viscosity [IO] (dW 0.83 0.545
Supplier ICI GE Plastics Union Carbide
“This is a commercial material (a polyhydroxyether of bisphenol-A), available for blending thermoplastic polyesters. The repeat unit is given below; the grade used has a nominal density of 1.17 g/cm~, r, = 90°C and has a melt flow index (MFI) of 4.8dg/min. at 220°C.
r
CHq
Coding sysrem Blend comuosition (PETPIPC w/w)
Blend code
9OjlO SO/20 SO/20 (5% Ph) 70/30 60/40 50/50
B9OX BSOX BSOXPb B7OX B6OX BSOX
‘Number refers to percentage PETP. b80/20 PEP/PC blend (95%) with 5% phenoxy resin (see also RESULTS AND DISCUSSION).
Blend preparation [IO]
A corotating
twin-screw
compounder
from
the APV range (MP 2030) was used (length/diameter ratio 15: I) to prepare
the blends. The screw configuration was made up as shown below: Die
Camel back discharge
/
,[is
1 :g
Polyethylene terephthalate/bisphenol-A All materials were dried thoroughly in a Conair-Churchill recirculating dehumidiier, prior to compound extrusion. An in-tine strip-die was specifically designed for this work, and was attached to the compounding unit; the amorphous product was drawn through a sizing die into the water bath by a variable speed haul-off. Blending was carried out at a constant torque level; compounding conditions are shown in Table 1. Experimental techniques The high thermal stability silicone oil bath in the Rutherford rheometer was preheated to the required test temperature. In set-up mode the carriage was returned to the
starting position and the specimen was placed on the specimen holders. It was then left to equilibrate for 4 min. The desired constant strain rate mode was selected and the initial specimen length and equivalent diameter were set on the control panel. (The “equivalent diameter” is required, since the specimen is a rectangular-section hoop (i.e. non-circular) the equivalent diameter is the diameter of a circle having the same cross-sectional area as the actual specimen.) The required strain rate is set on the control panel. The hoop specimen is immersed in the temperaturc-controlled oil bath which acts as an efficient heat transfer medium. On starting the test, the time, force and displacement signals were recorded. Several runs at each strain rate and temperature were carried out, and the average results are reported in the following sections. Elongational deformation-conditions All the PETP/PC blends were tested using the Rutherford rheometer to determine their elongational deformation behaviour in the thermoelastic region under the following conditions of temperature and strain rate:
polycarbonate blends
1063
(1) temperatures 80, 95 and 110°C; (2) strain rates 0.1, 0.5 and 0.95 set-‘. During the test, force, extension and time were recorded and the true stress and strain were calculated from these data. True stress versus strain plots were then produced so that the effects of the chosen variables could be demonstrated. This mode of presentation allows easier interpretation for polymers undergoing high levels of extension in the rubbery state. Morphology determination by staining and TEA4observations Thin sections of the bulk specimens (approximately 0.5 nrn thick) were obtained at room temperature using a Cambridge Huxley microtome equipped with a glass knife. A 0.5% stabilised aqueous solution of ruthenium tetroxide (RuO,) was obtained from Fluorochem Ltd in 10cm3 snapopen vials. The polymer sections, supported on mesh grids, were exposed for 30min to RuO, vapours in a desiccator containing a few millilitres of the solution. Stained sections were observed on a JEOL JEM 100 CX transmission electron microscope at 100 kV. Scanning mode TEM (STEM) was used, as this results in less beam damage to the sample and has better penetration of the relatively thick samples. STEM involves scanning a 8ne beam of electrons over the sample and detecting transmitted electrons with a scintillator photomultiplier arrangement. Viewing of the image is on a cathode ray tube. RESULTS AND DISCUSSION
Generally the shape of the plots obtained was similar to the representation in Fig. 1. It can be seen that the deformation behaviour can be divided into three types: elastic deformation, drawing, and strain-stiffening due to orientation and deformationinduced crystallisation. The last of these is a viscous response, associated with permanent deformation as crystallisation proceeds. The relative importance and positions of these deformation mechanisms will vary with material composition, temperature, etc. E$ect of temperature on the elongational deformation behaviour of PETPIPC blenak
Fig. 1. Ceneralised representation of a true stress vs strain curve for PETP and PETP/PC blends.
Figure 2 shows the effect of temperature on the elongational flow behaviour of 90/10 PETP/PC blend at 0.5 set-’ strain rate. The figure shows that for PETP/PC blends the effect of temperature is very important, as the extension for a given stress level increases with temperature, and strain-stiffening is delayed correspondingly. This type of behaviour is also observed for PETP alone [6, 7,9]. It can be seen that at 80°C the temperature is sufliciently low to
Temperalum ‘C 4,000
110 95
Sbaln 4.000 I
t
ml.3 Is
0.1 t 0.5
--c 0.95
-c 0.1 -8 0.5 -(r
0.95
++
=0
1
2
3
4
5
6
7
shin
Fig. 2. Effect of temperature on the elongational deformation of PETP/PC B9OX (strain rate 0.5 set-I).
Fig. 3. Effect of strain rate on the elongational deformation of PETP/PC B80X (temperatures 80 and 110°C).
A. M. Robinson et al.
1064
PCrmlent(%) 4.c.m -
PC
content (%)
60 -
60 40
3.000 I
II
/
r'/r
30 +
&
t
g
1
20
*
2.Ow
-910 -60
20 10
2 l.OW
0
0
1
2
3
4
6
6
7
Fig. 4. Effect of PC content on PETP/PC elongational deformation (95°C. strain rate 0.1 m-l). PC content (%)
-
so 40
t 30 NDt M -E10 -40 +
7
Strain
Fig. 5. Effect of PC content on PETP/PC elongational deformation (SYC, strain rate 0.5 see-I). PC canlent (%) 4.!ml ,
I
50
smln
Fig. 6. Effect of PC content on PETP/PC elongational deformation (95’C, strain rate 0.95 xc’).
the PETP extending signScantly before strain-stiffening occurs. In practice, this is a desirable effect, so long as sufficient deforming force can be achieved during processing. This result would suggest that thermoelastic processing of PETP/PC blends should be carried out at lower temperatures (N 8o”C), in order to achieve high orientation at modest strain levels. prevent
E$ect of strain rate on the elongational deformation behaviour of PETPIPC blena!s Figure 3 shows that the elongational deformation and flow behaviour of PETP/PC 80/20 blends is relatively unaffected by changes in strain rate in the range 0.1495 set-‘. The effect of temperature is again demonstrated; as the temperature increases from 80 to 110°C the extension increases (for a given stress) and strain-sti&ning is delayed. Although
Fig. 7. Effect of PC content on PETP/PC elongational deformation (8O”C, strain rate 0.95 w-l).
temperature is dominant in our experiments, strain rate effects are not insignificant in commercial processes, where very high rates of deformation are often used [7]. Effect of PC content on the elongational deformation behaviour of PETPIPC blenak Figures 4-6 show that (at SY’C), as the PC content increases, the material becomes increasingly rigid, resulting in earlier strain-stiffening. It has been demonstrated [6,7] that the onset of an abrupt change in gradient corresponds to the onset of strain-induced crystallisation in PETP. From the results presented here, it would appear that the presence (and amount) of the less mobile polycarbonate-rich phase is contributing signiflcantly to this effect. Earlier onset of strain-stiffening is generally a desirable property, as the improved mechanical properties associated with strain-induced crystallisation can be achieved at lower strains, as long as the force required to continue to deform the blend can be generated. This is usually the case with processes such as fibre drawing, tape making, etc. The addition of PC to PETP, resulting in earlier strain-stiffening, appears therefore to be a desirable effect. This occurs for all strain rates (0.1,O.S and 0.95 set-‘) in Figs 4-6, respectively, at 95°C. When the temperature is reduced to 80°C (Fig. 7) strain-stiffening occurs in all blends at very low strain levels (< 100% strain). This suggests that, so long as the required force levels can be generated, the thermoelastic processing of PETP/PC blends could be carried out at 80°C. At 110°C (Fig. 8) for the PETP/PC blends the systematic increase in rigidity with PC content can be PC
lxlntent (%) -
Y
60
40 -c 30 t 20 -E10 + 0 -!U-
Fig. 8. Effect of PC content on PETP/PC elongational deformation (1 lO”C, strain rate 0.95 w-l).
Polyethylene terephthalate/bisphenol-A
polycarbonate blends
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Fig. 9. PETP/PC blend stained with Ru04 (magnification x 10 K). (a) 50% PC, (b) 40% PC, PC, (d) 10% PC.
clearly seen. It is interesting too that PETP at 110°C also appears to be a very rigid material, indicating that the B90S has crystallised (thermally) during the
preheat period of the test. This precrystallisation means that chain stiffening can occur at lower strains (at 110°C in pure PETP), in comparison to the blends, where the PC component restricts the (thermally-induced) crystallinity level achieved for a given preheat time. In uniaxially-oriented PETP tapes and fibres, the mechanical properties are derived primarily from the
4.cal
BBOX 880X
!
~, !
(5% Ph) -
1WC _!
Strain
Fig. 10. Effect of the phenoxy compatibiliser on PETPjPC elongational deformation (BSOX, 0.5 set-I).
(c) 20%
orientation and strain-induced crystallinity, which depends upon temperature, strain rate and the addition of PC. A high stress developed at low strain would appear to represent a definite potential advantage for the use of PETP/PC blends in thermoelastic processes. Morphology of the PETP/PC
blends
Figure 9 shows the morphologies of the PETPJPC compositions from 50 to 90% PETP, observed using STEM. In Figs 9(a) and 9(b) the two-phase blends have a rough appearance, which is probably due in part to the difficulty in microtoming a toughened blend containing 40-50% PC [lo]. Figure 9(c) shows a very interesting morphology; the PC nodules are very small, evenly distributed and circular. Figure 9(d) shows that even when the PC content is as little as lo%, the nodules are still distinct, showing that PETP and PC are immiscible, and the morphology remains a two-phase structure. Effect of phenoxy (compatibiliser) on the elongational deformation behauiour of PETPjPC blendr
A phenoxy resin [phenol 4,4-( I-methylethylidene) bis-, polymer with (chloromethyl) oxirane (PH)] was
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A. M. Robinson et al.
Fi 11 shows the effect of phenoxy on the mosphdooy of the PETPjPC. It can clearly be seen that the addition of phenoxy results in larger PC nodules being formed in the immiaeible blend. There was little or no evidence from the !3TEM study to support any claims that phenoxy increases the small-scale compatibility between PETP and PC. However, it does appear to increase the available extension in the blend at 1lO”C, which could be due to the phenoxy interacting with the larger PC nodules and allowing the resultant continuous phase of PEI’P to extend relatively freely. In the unmodified blend with finer, well-dispersed particles, it may be more di5eult for the PETP to be continuous and therefore extension is limited. CONCLUSIONS
(1) Addition of PC at low levels (lO-20%) permits
Fig. 11. Effect of phenoxy on the morphology of the PETP/PC blend. (a) B80X, (b) B80X + 5% phenoxy. supplied by Union Carbide as a compatibiliser for the PETP/PC blends. In the composition which had added phenoxy, the PETP/PC weight ratio was kept as stated and the phenoxy was then added as a percentage of the whole compound. In the PETP 8O/PC 20 blend, 5% phenoxy was added: i.e. PETPjPC made up 95% of the blend and phenoxy the remaining 5%. Figure 10 shows that phenoxy appears to have an effect on the thermoelastic behaviour of the PETP/PC blend only at llO”C, if added at the 5% level. The phenoxy appears to delay the strain-hardening behaviour, i.e. the available strain (extension) is increased and the stress required to achieve a specific strain is reduced. This effect is not a desired one for improvement of mechanical properties in stretchformed products; however, it may be of use if deformation is high, and products are required to be thin, and also where mechanical properties are a secondary consideration,
the thermoelastic processing of PETP over a wider temperature range, and suggests that uniaxial orientation, and sigr&ant strain-induced crystallisation in the PETP-phase, can be achieved at lower strain levels. (2) Over the range studied, the effect of strain rate on elongational deformation is not very marked, in comparison to tire influence of temperature (80-l WC). (3) Addition of phenoxy to PETP/PC blends has only a minor effect, delays the onset of strain-hardening only at 1lO”C, and contributes to a more coarse, two-phase blend morphology. (4) There is little evidence for miscibility in the PETP/PC blend; a two-phase system was identified over the composition range 5&90% PETP. REFERENCES 1. V. M. Nadkami, V. L. Shingankuli and J. P. Jog. Polym. Eng. Sci. Zs, 1326 (1988). 2. X. Y. Chen and A. W. Birley. Br. Polym. J. 17, 341 (1985). 3. A. W. Birley and X. Y. Chen. Br. Polym. J. 16, 17 (1984). 4. A. W. Birley and X. Y. Chen. Br. Polym. J. 17, 297 (1985). 5. S. R. Murff, J. W. Barlow and D. R. Paul. J. Appl. Polym. Sci. 29, 3231 (1984).
6. F. H. Axtell and B. Haworth. Polym. Test. 9,53 (1990). I. F. H. Axtell and B. Haworth. Plast. Rd. Comp. Proc. Appl. 22, 127 (1994).
8. B. Haworth, L. Chua and N. L. Thomas. PVC 93, The Future. IOM Conference, Brighton, U.K., April, p. 210 (1993). 9. F. H. Axtell. A study of the flow properties
and processability of thermoplastic polyesters. Ph.D. Thesis, Loughborough Univer&y of T&hnology (1987). 10. A. M. Robinson. Blends of nolvtethvlene tereohthlate) with bisphenol-A polycarbo&~.‘Ph.~D. Thesis: Lough: borough University of Technology (1991).