Polymer Degradation and Stability 28 (1990) 67-75

Influence of Injection Rate and Melt Temperature on Polypropylene during Injection Moulding without

Packing*

J. Billiani & E. F l e i s c h m a n n

Institut fiir Chemische und Physikalische Technologie der Kunststoffe, Montanuniversitfit Leoben, Franz-Josef-Strage 18, 8700 Leoben, Austria

(Received 11 May 1989; accepted 27 May 1989)

ABSTRACT

The molecular degradation of isotactic polypropylene, which occurs during injection moulding without packing, has been studied. Particular attention was given to the effects of injection rate and melt temperature on the molecular weight distribution of the polypropylene. Selected layers of the plates produced were analyzed to determine changes in the molecular weight averages by gel permeation chromatography. Molecular weight distributions were obtained for surface layer and core. Serious degradation ( - 3 0 % in weight average) is caused mainly by high melt temperatures. The effect of high shear rates and high shear stresses was found to be less. The result of thermal and mechanical measurements are correlated with the polymer degradation especially as measured by changes in the weight average molecular weight.

I N T R O D U C T I O N

There have been few publications on the subject of the thermomechanical degradat ion of polypropylene during injection moulding using the conditions typically encountered in industrial processing, x'2 Generally, correlations between polymer degradation and mechanical properties are reported. Other publications have been concerned with degradation phenomena due to repeated injection moulding 3 and repeated extrusion, 4- 6 or with environmental degradation. 6'7

* Dedicated to Professor J. Schurz on the occasion of his 65th birthday. 67

Polymer Degradation and Stability 0141-3910/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

68 J. Billiani, E. Fleischmann

In this investigation, results are presented on degradation during injection moulding without packing, using industrial processing conditions. In this processing method, especially high injection rates and injection pressures are used. Thus, significant degradation effects are to be expected. These are investigated in various layers of moulded plates by means of gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and tensile impact tests.

EXPERIMENTAL

Materials and sample preparation

The material used was a commercial isotactic polypropylene, DAPLEN KS 10 (melt flow index (230°C/21.19N) = 8.0 g/10 min, M w = 290 000 g/mol, M w / M , = 5.7) produced by Petrochemie Danubia, Austria. Rectangular plates (230 × 70 x 2mm 3) with film gate were moulded without packing using an injection moulding machine, model ES 250 manufactured by ENGEL, Austria. The flow front velocities, the injection pressures, and the processing temperatures used to prepare the plates are detailed in Table 1. Furthermore, calculations were made to estimate the magnitude of wall shear stress, Zw, and shear rate, 7, in the cavity. The values of Zw and calculated at the end of the filling phase are given in Table 1. By the use of a three-plates tool the mould cavity was mechanically sealed in the gate. In principle, the skin-core structure characteristic of injection moulded polypropylene articles consists of two layers; namely, a highly oriented surface layer, and a typically spherulitic core. a Using a polarizing microscope (Leitz, West Germany) the thicknesses of both these layers were

TABLE 1 Injection Moulding Conditions for the Plates of PP-KS 10 (TM Melt Temperature, TW Wall Temperature, VF Flow Front Velocity, Pim.x Maximum Pressure in the Cavity; 7m.x Maximum Shear Rate, rw Wall Shear

Stress, cf Text)

Sample no. TM/TW VF P/max Ymax ~'w [°C/°C] [mm/s] [bar] [ l /s] IN/mini

1 100 1 559 400 198000 2 200/20 800 1 627 3 800 197 000 3 2 200 1 478 11 900 209 000 4 100 1 544 400 112 000 5 280/50 800 1 464 3 500 123 000 6 2200 1472 9400 141000

Influence of injection rate and melt temperature on polypropylene 69

determined at a distance of 100mm from the gate. Microtome sections (25 mg) of both the layers were removed from the plates at a distance of 100 mm from the gate, and these specimens were used for GPC- and DSC- measurements.

Gel permeation chromatography (GPC)

Molecular weight distribution and molecular weight averages were determined using a high-temperature GPC system consisting of solvent delivery pump (Model 560, Waters Associates, USA), sample injector and column oven (Knauer, FRG) and an IR detector (DuPont Company, USA). Three columns connected in ser ies--GMH6--HT 30cm (Toyo Soda, Japan), PS40000 and PS4 25 cm (Merck, FRG)--were employed with 1,2,4- Trichlorobenzene (TCB) at 135°C as the mobile phase. The flow rate was 0.59ml/min. Polymer concentration was measured by IR using the absorption of the CH 2 - groups at 3.41/~m.

The system was calibrated with a series of narrow polystyrene standards (Polymer Laboratories Inc., USA) in the range M = 1250 to 7.7 x 107 g/mol and a broad polyethylene standard (NBS 1475 HDPE, 9 National Bureau of Standards, USA) using the integral calibration method.l° Calibration data were converted to polypropylene by universal calibration using the following Mark-Houwink constants, K = 1.9 x 10-4dl/g, 1.9 x 10-4dl/g, and a = 0"690, 0.725 for polystyrene ~ and polypropylene 12, respectively.

It may be difficult to obtain molecular disperse solutions of polyolefins. The thermal stability of these solutions and the reproducibility of data remain a subject of research. 13'a4 Polypropylene solutions were heated under nitrogen at 170°C for 90min and stirred occasionally. The preparation of polystyrene calibration samples involved dissolution at ambient temperature overnight in TCB and short heating periods (10 min, 140°C) in a nitrogen atmosphere. Sample concentrations were in the range 0.2~.4mg/ml and 0.5mg/ml of stabilizer (Irganox 1010, Ciba-Geigy, Switzerland) was added to the solvent before dissolution of the sample to prevent oxidation. The stabilizer peak was used as an internal standard for flow rate monitoring. The sample solutions (loop volume = 0.305 ml) were injected without filtration.

During the sample run on the GPC system, the analog data from the concentration detector and the system pressure were digitized and collected by means of a personal computer equipped with a commercially available analog/digital converter device of 5/~V resolution (model DASCON, Metrabyte, USA). Software for acquisition and processing of GPC data was developed in our laboratory. Due to the broad distribution of the polypropylene samples no axial dispersion correction was applied to the

70 J. Billiani, E. Fleischmann

chromatograms. 15 A critical examination and further development of the GPC interpretation used are in progress. ~6

Differential Scanning Calorimetry (DSC)

A DSCo7 apparatus (Perkin Elmer, USA) was used to determine the melting point. Samples (3-5 mg) were sealed in a standard aluminium pan and analyzed in a nitrogen atmosphere at 10 K/min scanning rate. Temperature and heat of fusion calibrations were carried out by using indium and zinc as references. The peak temperatures were taken as the melting temperatures T m.

Mechanical measurements

A degradation effect is especially expressed in the impact load properties. For this reason tensile impact tests according to DIN 53448 were carried out at ambient temperature. The specimens were taken from the plates in the direction of flow at a distance of 100 mm from the gate.

Results and Discussion

The investigation of small changes in the molecular weight distribution (MWD) of polymers requires careful interpretation of the GPC results. In some cases shear degradation has been reported to occur within the chromatographic columns. 17 - 19 Therefore, the raw material was chromato- graphed several times to establish the precision of the GPC separation system. Variations of flow rate were derived from the retention volume of the stabilizer peak and proved a standard deviation of _+0.12%. In most cases, in-column degradation is correlated with a perceptible pressure rise during the passage of the polymer solution through the columns. 19 In our measurements, no pressure rise was observed during the GPC runs.

The individual chromatograms were normalized with respect to the area and averaged (Fig. 1). The area of the standard deviation curve was found to be less than 6% of the whole chromatogram area. This averaged chromatogram was used to calculate the molecular weight distribution of the raw material. The average molecular weights were determined as weight average M w = 323 000 g/mol and number average Mn = 46 000 g/mol. The standard deviation from eight runs was _+4% for Mw and -+8% for Mn.

The results for the surface zone and core zone are given in Table 2. For samples 1-3 (melt temperature 200°C) the variation of the flow front velocity from 100 mm/s to 2200 mm/s yields practically the same average molecular weights, for both the core zone and the surface layer. It may be noted,

Influence o f injection rate and melt temperature on polypropylene 71

40.0

i i

iO.O

0 . 0 ' 14 .0 18 .0 tB.O 2 0 . 0

Volume [roll

Fig. 1. Reproducibility of GPC measurements by averaging eight independent runs. ( averaged

M

2 0 . 0 >

m

22.0

) normalized chromatogram, ( . . . . . ) s tandard deviation, ( . . . . . ) s tandard

deviation/chromatogram height (relative error).

however, that the weight average decreases significantly compared with the raw material.

At the melt temperature of 280°C (samples 4-6) the weight average decreases rapidly with rising flow front velocity whereas no significant changes in the number average molecular weight and the melting point are noted. Again it is observed that there are no differences in the results for the inner and surface zones.

TABLE 2 Influence of the Processing Conditions on Mw, M., and Tm of

Surface Layer (a) and Core Zone (b)

Sample no. M w [g/mol] M. [g/mol] T,, [° C] × 10 -3 × 10 -3

a b a b a b

1 287 290 37 42 164 163 2 283 282 37 34 164 163 3 289 270 36 40 164 164 4 270 263 45 43 164 164 5 257 260 39 39 164 164 6 242 240 40 40 163 163

72 J. Billiani, E. Fleischmann

7 0 . 0

mlo -~

6 0 . 0

5 0 . 0

4 0 . 0 z

• 3 0 . 0

2 0 . 0

t 0 . 0

0 . 0 3 . 0 4 . 0 5 . 0 5 . 0

log 14

(a)

7 . 0

5 0 . 0

g

e-i

w t ° ' s ~ i

25.0

• • • • • . . . . . . . .

: ~--.~''~..----- "-, " . . ./~.. o o . . . . . . . . . . .

- 2 5 ° ( " ' . . . . . " " ' ' " " ' . . . . . " " " " ~ ~ " - - - ' ~

- 5 0 . ¢ i i I i i i i I i i i

3 . 0 4 .0 5 . 0 6 . 0 7 . 0

log 14

(b) Fig. 2. (a) Molecular weight distribution of the raw material ( ), sample 3 ( - - - - ) , and sample 6 (----). (b) Differences in the molecular weight distribution between the raw material

and samples 3 ( - - - - ) and 6 ( ) compared with the error band ( .... ).

Influence of injection rate and melt temperature on polypropylene 73

At a constant flow front velocity, the rise of the melt temperature from 200°C to 280°C leads to a significant decrease in the weight average. On the other hand, the influence of the flow front velocity on Mw is exhibited only at the high melt temperature. Hence the temperature seems to have a greater effect on degradation processes than the flow front velocity. 6 Obviously, the two effects cannot be differentiated from each other. Comparison of the results concerning the core and the surface layer indicates that a separate discussion on each is not necessary. Therefore the results for the surface layer and core are averaged for the purposes of the following discussion.

From the results for various processing conditions (cf Table 1), a significant change in the molecular weight distribution of the specimens was observed. Figure 2(a) shows the effect of processing on the distribution function of the virgin granulate. The high molecular weight tail of the distribution is reduced in all cases. To quantify the changes, the reference distribution of the virgin granulate was subtracted from the average distributions of the individual specimens (Fig. 2(b)). In the range of M = 5 x 105 to M = 5 x 106 a weight loss of 2"5-4"5% was observed. The lost mass appears in the medium molecular weight range of the distribution. Whereas the extent of the degradation process is rather low in this case in comparison to other approaches, the effects are significant in comparison with the precision of the measurement. The changes of the MWD behave as predicted from a midpoint scission mechanism of degradation.

Tensile impact strength tests were carried out to obtain correlations between degradation and mechanical properties. Table 3 shows the impact strengths compared with averaged values of weight average, number average and melting point. These values are also given for the virgin granulate.

TABLE 3 The Effect of Processing Parameters on Weight Average and Number Average Molecular Weight, Melting Point T,, and Tensile Impact Strength az, (DIN

53448, Five Samples for Each Measurement; the Scattering is + 11%)

Sample no. Mw [g/mol] M. [g/moll T,. az. X 10 -3 × 10 -3 ['~C] [kpcm/cm z]

granulate 323 46 163.5 - - 1 288.5 39'5 163.5 271 2 282.5 35'5 163.5 - - 3 279-5 38 164 196 4 266'5 44 164 177 5 258.5 39 163.5 - - 6 241 40 163 149

74 J. Billiani, E. Fleischmann

The impact strength decreases with decreasing Mw. But it must be taken into consideration that different processing conditions cause different layered structures and, consequently, different mechanical behavior. Thus molecular orientation usually increases impact strength if the impacting force is parallel to the chain orientation. 2° The orientation decreases as melt temperature and/or flow front velocity increase. 21 The samples involved in this work have already been the subject of optical and morphological investigations. 22 The effects of polymer structure and degradation on the impact strength cannot be clearly differentiated.

The melting points show no changes due to degradation. So it can be assumed that the temperature ranges in which crystallization occurs are not shifted by the decrease in Mw. 2

CONCLUSIONS

In this study, various layers of polypropylene plates injection moulded without packing were investigated for the first time by GPC measurements. The experiments were performed on plates moulded with very high injection rates and pressures of about 1500 bar. As expected, the degradation effect due to processing is significantly expressed by the weight average molecular weight. However, the number average molecular weight does not change within the precision of the measurements. Thus injection moulding without packing causes narrowing of the molecular weight distribution. Whereas the shear rates for samples 3 and 6 are of the same order, only sample 6 is significantly degraded. Hence degradation is caused by oxidative chain scission processes due to the high melt temperatures.

The impact strength strongly decreases both with increasing melt temperature and flow front velocity. The surface layer and the spherulitic core cannot be differentiated with respect to molecular weight distribution and melting temperature. Therefore it seems likely that the degradation processes take place in the sprue and plasticizing system.

ACKNOWLEDGEMENTS

The authors are very much indebted to the Austrian 'Fonds zur F6rderung der wissenschaftlichen Forschung' for sponsoring this work in the course of a national research program on injection moulded articles (projects S 3305 and S 3307). Thanks are due to Maschinenfabrik Engel KG for assistance in the preparation of the plates and to PCD Ges.m.b.H. for supplying the test material. Last, but not least, the cooperation of M. Sabernik is very much appreciated.

Influence of injection rate and melt temperature on polypropylene 75

R E F E R E N C E S

1. Schleede, D. & Schfilde, F., Plastverarbeiter, 11 (1960), 161. 2. Ries, H. & Menges, G., Kunststoffe, 78 (1988), 636. 3. Knappe, W. & Kress, G., Kunststoffe, 53 (1963) 346. 4. Schott, H. & Kaghan, W. S., S.P.E. Trans., 3 (1963) 145. 5. Hornsby, P. R. & Sothern, G. R., Plastics and Rubber Processing and

Applications, 4 (1984) 165. 6. Zarmorsky, Z. & Muras, J., Poly. Deg. and Stab., 14 (1986) 41. 7. Severini, F., Gallo, R. & Ipsale, S., Poly. Deg. and Stab. 22 (1988) 185. 8. Kantz, M. R., Newman, H. D., Jr. & Stigale, F. H., J. Appl. Polym. Sci., 16 (1972)

1249. 9. Nat. Bur. Stand. (U.S.), Spec. Publ. 26442 (Sep. 1972) Eds. H. L. Wagner, P. H.

Verdier. 10. Yau, W. W., Kirkland, J. J. & Bly, D. D., Modern Size Exclusion Liquid

Chromatography, Wiley & Sons, Inc., 1979, p. 294. 11. Barlow, A., Wild, L. & Ranganath, R., J. Appl. Polym. Sci., 21 (1977) 3319. 12. Scholte, Th. G., Meijerink, N. L., Schoffeleers, H. M. & Brands, A. M. G., J. Appl.

Polym. Sci., 29 (1984) 3763. 13. Grinshpun, V. & Rudin, A., J. Appl. Polym. Sci., 30 (1985) 2413. 14. Utracki, L. A. & Dumoulin, M. M., ACS Symp. Series, 245, ed. Th. Provder,

1984, p.97. 15. Billiani, J., Rois, G. & Lederer, K., Chromatographia, 26 (1988) 372. 16. Billiani, J. & Lederer, K., Proceedings of the 8th International Symposium on

Chromatography, 1989, Bratislava, CSSR. J. Liqu. Chromatog. (in press). 17. Barth, H. G. & Carlin, F. J., J. Liqu. Chromatog., 7 (1984), 1717. 18. Lederer, K. & Billiani, J., Spectra 2000, 122 (1987) 39. 19. Rooney, G. & Verstrate, G., Polymer Preprints, 21 (1980) 196. 20. Nunes, R. W., Martin, J. R. & Johnson, J. F., Polym. Eng. Sci., 22 (1982) 205. 21. Fleischmann, E. & Koppelmann, J., Kunststoffe, 77 (1987) 405. 22. Fleischmann, E. & Koppelmann, J., Kunststoffe, 78 (1988) 453.