Melt strength of polypropylene: its relevance to thermoforming.
In thermoforming, a thin extruded polymer sheet is heated to a desired temperature and is formed into a mold with the assistance of either pressure, vacuum, a mechanically operated plug or a combination of these to give the desired shape of the formed product. Polymers such as acrylonitrile-butadiene-styrene (ABS), polystyrene (PS), vinyl and acrylic polymers are widely used in this process.
For a crystalline polymer such as polypropylene (PP), the thermoforming process can only be performed over a very narrow range of temperature that is close to the melting temperature of the polymer. Throne (1) has reported that PP can be thermoformed in the temperature range of 143-166 [degrees] C. If the temperature is too low, the sheet will be softened but not fully melted and the formed product will not replicate accurately the detail of the mold. If the temperature is too high, the sheet loses its dimensional stability and flows downward under gravity to an excessive amount (known as sag). The result is that the thermoformed product will have uneven wall thickness and may even result in tearing of the sheet.
These processing difficulties in the melt phase thermoforming of PP arise from the fact that PP has a sharp melting point and poor melt strength (2). To overcome the sagging problem of PP in melt phase thermoforming, solid-phase forming has been developed (3). For amorphous polymers such as ABS and PS, a rubber-like elastic state exists over a wider temperature range compared to a semicrystalline polymer such as PP (4), hence these amorphous polymers can be thermoformed over a much wider temperature range than PP. A forming temperature range of 127182 [degrees] C has been reported for ABS (1). PP has good physical properties and is a cheaper raw material compared to ABS. Hence, there is a desire by industry to use PP for producing thermoformed products.
In recent years, the melt strength of a polymer has been recognised as one of the important processing parameters in melt processing operations where stretching or drawing is involved at one or more stages of the process. Typical industrial processes where stretching occurs along the process streamline are melt spinning, blow molding, extrusion coating, film extrusion, fibre extrusion and thermoforming. The melt strength of a polymer is a measure of its resistance to extensional deformation. The poor melt strength of PP influences the thermoforming behavior of this polymer, especially the tendency of the PP sheet to sagging. Recently, high melt strength grades of PP have been developed to improve the processing performance in thermoforming (2, 5, 6).
In the past, the melt strength of low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and LLDPE/LDPE blends in relation to the film blowing process has been studied by researchers such as Ghijsels et al. (7) and Micic et al. (8). Relatively few melt strength measurements have been performed on PP, possibly due to the difficulties of performing the experiment (9). Since sagging involves draw down of a polymer sheet under gravity, the sagging problem with PP can be studied by measuring its melt strength. The present study focuses on the measurement of the melt strength of PP grades that can be used for thermoforming and discusses the consequence of the results on the sagging resistance of PP during thermoforming.
In this work, the melt strength of fare different grades of PP (PP-1, PP-2, PP-3, PP-4, and PP-5) were measured to study the sagging resistance for thermoforming applications. Grades PP-1 to PP-4 and PP-5 were supplied respectively by ICI Australia Plastics and Montell, Australia. PP-1 and PP-4 were homopolymers. PP-2 and PP-3 were block copolymers containing 6% of ethylene comonomer by weight. PP-1, PP-2, PP-3 and PP-4 can be used for thermoforming applications. PP-5 was a high melt strength (HMS) grade of PP. It contains long chain branching and is believed to be prepared by radiation treatment. Since ABS can be easily processed during thermoforming, a thermoforming grade of ABS (ABS 2041) supplied by the Huntsman Chemical Company of Australia was used as a reference material and its melt strength was also measured in this work.
In order to study the effect of MFI on the melt strength of PP, three homopolymer grades of PP (PP-6, PP-7 and PP-8) with higher MFI were also used. The melt strength data of these three PPs were also measured at our Rheology and Materials Processing Centre.
The MFIs of PP and ABS were measured according to the ASTM standard D1238. A 10 kg load and a temperature of 220 [degrees] C were used to measure the MFI of ABS. The ABS sample was dried in an oven at 70 [degrees] C for two hours to evaporate the absorbed moisture before testing. The polymer samples used in this study are listed in Table 1.
2.2 Melt Strength Measurement
The melt strength of PP was measured by using a Gottfert "Rheotens" Melt Strength Tester as shown diagramatically in Fig. 1. It consists of a pair of rollers rotating in opposite directions that are mounted on a balance beam. A polymer melt strand extruded vertically downwards from a capillary die is drawn by the rotating rollers whose velocity increases at a constant acceleration rate. The polymer melt being stretched undergoes uniaxial extension. The tensile force in the strand measured by the balance beam can be plotted as a function of time or velocity of the rollers. The force at which the polymer melt breaks is called the "melt strength".
Table 1. Polymer Samples Used. Melt Flow Index (MFI) Polymer Sample Type of Polymer (g/10min) PP-1 Homopolymer 2.0 PP-2 Copolymer 1.5 PP-3 Copolymer 0.8 PP-4 Homopolymer 0.8 PP-5 High melt strength 2.5 PP-6 Homopolymer 3.7 PP-7 Homopolymer 8 PP-8 Homopolymer 40 ABS - 2.7
The melt strength parameter does not give a well-defined rheological property because neither the strain nor the temperature is uniform in the polymer melt being stretched (10). However, the test is useful in obtaining meaningful comparisons of the drawing behavior of different polymers. The melt strength of a polymer is affected by several parameters such as melt temperature, extrusion rate, ambient temperature and the distance between the capillary die and melt strength tester. The effect of these parameters on the melt strength of LDPE has been studied by Wolff (11).
The melt strength measurements were performed over a range of temperature approaching the thermoforming region. The ABS sample was dried in an oven at 70 [degrees] C for two hours prior to extrusion. The extrusion and drawing conditions used in this study are shown in Table 2. Prior to the melt strength measurements, the polymer melt was extruded at the required extrusion rate for 20 min to ensure an equilibrium flow condition was achieved in the extruder.
The melt strength measurement could not be performed at an extrusion rate of 5.2 x [10.sup.-3] kg/min and temperatures of 180 [degrees] C for PP and 190 [degrees] C for ABS, because melt fracture was observed on the polymer melt strand.
At the beginning of each measurement, the roller velocity was adjusted so that the polymer melt was not under tension. This prevented any prestretching of the polymer melt before the actual measurement started. Initially, the melt strength of PP was difficult to measure because the melt tended to stick and wind-up on the fluted rollers supplied with the tester, effectively ending the measurement. The sticking and winding-up problem was reduced when a pair of nonfluted matt steel rollers was used and the drawn strand was carefully pulled away manually from the rollers in a horizontal direction to minimize any effects on the measured force. A diagram of fluted and nonfluted matt steel rollers is shown in Fig. 2. The sticking problem could also be minimized by running the polymer melt in contact with the front of the closed rollers, as suggested by Ghijsels and De Clippeleir (9).
A typical "Rheotens" test result for a conventional PP sample is shown in Fig. 3. The measured force increases as the roller velocity is increased and then flattens out. Near the end of the measurement (i.e. at high roller velocity), a draw resonance instability was sometimes observed that gave rise to an oscillation in strand diameter and in the measured force. Reproducible results were obtained when the average force value in the draw resonance region was taken as the melt strength. This is indicated by the dashed line in Fig. 3.
Table 2. Test Conditions for Melt Strength Measurement. extrusion temperature(*) 180 - 230 [degrees] C extrusion rate 3.2 X [10.sup.-3] and 5.2 X [10.sup.-3] kg/min, [+ or -] 2 X [10.sup.-4] kg/min residence time(**) at extrusion rate = 3.2 x [10.sup.-3] kg/min, residence time = 24 min at extrusion rate = 5.2 x [10.sup.-3] kg/min, residence time = 15 min capillary die dimensions D = 2 x [10.sup.-3] m, L/D = 20, entrance angle = 90 [degrees] distance between die exit and rollers 0.21 m acceleration of rollers 1.2 x [10.sup.-2] m/[s.sup.2] cooling conditions cooled by ambient air roller type non-fluted matt steel rollers * The actual melt temperature measured at the exit of the die is 8 - 10 [degrees] C higher than the extrusion temperature possibly because of shear heating in the extruder. ** Residence time is the time required for the polymer melt to travel from the feed zone of the extruder to the exit of the die.
For the melt strength experiments, each sample was tested at least five times and the average result taken. The maximum percentage error of the measurements was less than 12%.
3.1 Temperature Dependence of Melt Strength
Figures 4 and 5 show the melt strength of different polymers as a function of extrusion temperature at an extrusion rate of 3.2 x [10.sup.-3] and 5.2 x [10.sup.-3] kg/min, respectively. These graphs show that the melt strength of the polymers increases as the extrusion temperature is decreased. ABS has the highest melt strength in the low extrusion temperature region where thermoforming is more likely to take place. This indicates that ABS has minimal sagging problem during thermoforming. PP has a higher melt strength at a lower extrusion temperature region. This supports the thermoforming of PP at low temperature in order to maximize the melt strength and so improve the sagging resistance of the material. Figures 4 and 5 also show the improved melt strength of the HMS PP compared to conventional PP at most extrusion temperatures, suggesting that it has an improved sagging resistance. However, further experimental studies are necessary to confirm whether the HMS PP has desirable properties for thermoforming applications.
For the conventional PP grades, a sharp increase in the melt strength is observed at low extrusion temperatures approaching the melting point of PP, which is about 165 [degrees] C. Such an effect has been observed previously for PP by Ghijsels and De Clippeleir (9), but has not occurred with polyethylene. This phenomenon is thought to be caused by flow-induced crystallization (9, 12, 13). For certain polymer melts such as PP, a crystalline structure can be formed in the melt under high stress, particularly when combined with high pressure and low temperature. The formation of the melt structure is mainly due to stretching of the polymer rather than shear flow. In the melt strength measurement, flow-induced crystallization can occur in the entrance of the capillary die where converging flow and stretching of polymer melt is taking place. Further crystallization can occur when the polymer melt leaves the capillary die and is stretched by the nip rollers of the melt strength tester. The sharp increase in melt strength in the low extrusion temperature region is not observed for the HMS PP, which shows a linear increase in melt strength with decreasing temperature over all the temperature range. This effect could be due to the different molecular structure in the modified PP.
The temperature dependence of the melt strength can be further studied by using the Arrhenius type equation as shown in Eq 1 [ILLUSTRATION FOR FIGURE 6 OMITTED]:
Log (melt strength) = E/RT + Log C (1)
In Eq 1, C is a constant, E is the activation energy of melt strength (J/mol), R is the molar gas constant (8.314 J/mol.K) and T is the absolute temperature (K).
[TABULAR DATA FOR TABLE 3 OMITTED]
The activation energy of the melt strength for PP and ABS at an extrusion rate of 3.2 x [10.sup.-3] kg/min can be calculated from the slope of the high extrusion temperature region (above 200 [degrees] C) of Fig. 6. Similar calculations can be done for an extrusion rate of 5.2 x [10.sup.-3] kg/min. Activation energy results are shown in Table 3. Values between 27 x [10.sup.3] and 40 x [10.sup.3] J/mol were obtained for PP. Values of 31 x [10.sup.3] to 35 x [10.sup.3] J/mol have been reported by Ghijsels and De Clippeleir (9). In general, there was no significant change in activation energy of the melt strength as the extrusion rate was increased. The difference between the activation energy for different conventional PP grades and HMS PP was also small. This indicates that in the high extrusion temperature region, the temperature dependence of melt strength is similar for all types of PP. The activation energy calculated from the melt strength measurements is also comparable to those obtained from the shear flow measurement. Cassagnau et al. (14) has reported the activation energy for flow of PP as 36 x [10.sup.3] J/mol. According to Eyring's theory (15), activation energy of a polymer is the required energy for individual molecules to jump from one equilibrium position to another. It was found that below a certain level of molecular weight (i.e. 30 carbon units for paraffins), the activation energy increased as the molecular weight increased for a low molecular weight polymer, but approached a constant value as the molecular weight increased above this critical level of molecular weight. This suggests that for a long chain molecule, only a short segment of the molecular chain can move at a time. Therefore it is expected a certain class of polymer with high molecular weight has a constant activation energy value.
ABS has an activation energy of melt strength of 64.6 x [10.sup.3] J/mol at the extrusion rate of 3.2 x [10.sup.-3] kg/min and a value of 72.7 x [10.sup.3] J/mol at the extrusion rate of 5.2 x [10.sup.-3] kg/min. These values are relatively high compared to the activation energy of PP. Schott (16) has reported that branches or bulky side groups on the backbone chain of a polyhydrocarbon can increase the activation energy of the polymer. ABS contains styrene side group on the backbone chain, the rotation of this styrene group around the backbone chain is difficult. It can reduce the molecular chain flexibility and this rigid chain can only move as an entire unit. Therefore the size of the flow unit is increased and as a result the activation energy of ABS is expected to be higher than for PP.
3.2 The Influence of Extrusion Rate on Melt Strength
Figure 7 shows that the melt strength of PP-1 generally increases as the extrusion rate is increased from 3.2 x [10.sup.-3] kg/min to 5.2 x [10.sup.-3] kg/min. Han (17) has shown that PP has a converging flow pattern in the entrance region of a capillary die. The converging flow pattern indicates that there is an acceleration of melt flow and this implies the presence of uniaxial stretching (10). As the polymer melt is being stretched, the molecular chains can be aligned in the direction of stretching where regular structure of the melt can be formed and enhance the elasticity of the melt. It was found that polymer melt also had the similar converging flow pattern in a rectangular slit die (17). It was observed that the flow birefingence in the entrance of the rectangular slit channel increased as the flow rate of the polymer melt was increased. This suggests that by increasing the flow rate of a polymer melt with a converging flow pattern in a die, higher stress can be developed in the melt. In this work, higher stress is believed to be developed in the polymer melt in the flow stream of the extruder's die at a higher extrusion rate and as a result, higher melt strength is observed. In the low extrusion temperature region for the conventional PP, the melt strength increases more rapidly at the higher extrusion rate. Figure 8 shows that a similar phenomenon is not observed for the HMS PP.
Figure 9 shows the relationship between the melt strength of conventional PP and extrusion rate at 200 [degrees] C. The solid lines have been drawn to show the trend. It can be seen that the melt strength of conventional PP tends to increase linearly with increasing extrusion rate at a constant temperature.
3.3 Effect of MFI on Melt Strength
Figure 10 shows the melt strength of PP as a function of MFI. The melt strength of conventional PP decreases linearly as the MFI of PP increases. Lower MFI PP tends to have longer molecular chains and these long chain molecules can form more entanglements in the polymer structure. Polymers with higher degree of entanglements will have higher resistance to extensional deformation and as a result, lower MFI polymers tend to have higher melt strength. Therefore, PP with low MFI should be used to minimize the sagging problem in thermoforming. This finding is supported by the experimental results obtained by Hylton (18) on the sagging test of PP with different MFI values. HMS PP shows a higher melt strength than conventional PP with a similar MFI value. The same kind of behavior was observed by Ghijsels and De Clippeleir (9) for PP, which has a wider molecular weight distribution.
The melt strength results for homopolymer PP grades (PP-1 and PP-4) and those for the copolymer PP grades (PP-2 and PP-3) with similar MFI value indicate very little difference in the melt strength for the two types of PP. A wider range of grades would need to be tested to establish whether any melt strength differences exist between homopolymer and block copolymer PPs.
3.4 Comparison of Strain Rate Between Melt Strength Measurement and Thermoforming Process
The strain rate of the melt strength measurements can be calculated from the following equations derived by Laun (19)
[Mathematical Expression Omitted] (2)
[Lambda] = [v.sub.1]/[v.sub.0] (3)
where [Mathematical Expression Omitted] is the tensile strain rate ([s.sup.-1]), [v.sub.0] is the strand velocity at the die exit (m/s), H is the draw distance (m), [Lambda] is the stretch ratio and [v.sub.1] is the velocity at the rollers (m/s). [S.sub.2] ([Lambda]) is an extrapolated swelling factor which can be neglected for high elongation ratio. In this study, the term [S.sub.2] ([Lambda]) is neglected and the strain rate region in which the melt strength of the polymer samples is evaluated has the values of 0.56 [s.sup.-1] to 6.49 [s.sup.-1]. Throne (1) has reported that the thermoforming process has a strain rate of 0.1 to 10 [s.sup.-1]. Therefore the strain rates obtained in the melt strength test are comparable to those in the thermoforming process.
The melt strength of PP and ABS was measured using a Gottfert "Rheotens" melt strength tester. It was found that the melt strength of the polymers increased with decreasing temperature and increasing extrusion rate. ABS had the highest melt strength in the low extrusion temperature region, indicating that it has good sagging resistance during thermoforming. HMS PP had a higher melt strength compared to conventional PP but further experimental studies are necessary to confirm if the HMS PP has desirable thermoforming properties. The strain rate obtained in the melt strength measurements is also comparable to those in the thermoforming processes.
For conventional PP, there was little difference in the melt strength between the homopolymer and copolymer grades studied. To minimize sagging for conventional PP, the results indicate that the forming temperature should be close to the melting temperature of the PP and a low MFI grade should be used.
The authors wish to acknowledge the assistance of ICI Australia Plastics for supplying polymer samples and for helpful discussion and Australian Research Council for providing an Australian Postgraduate Award (Industry) to one of the authors (HCL).
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|Author:||Lau, H.C.; Bhattacharya, S.N.; Field, G.J.|
|Publication:||Polymer Engineering and Science|
|Date:||Nov 1, 1998|
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