The Use of Metallocene Catalysed Polyethylene as a Blend Material in Blown Film Extrusion

Dev. Chem. Eng. Mineral Process.. 7(1/2), pp.201-218, 1999.

The Use of Metallocene Catalysed

Polyethylene as a Blend Material in Blown

Film Extrusion

C.M. Beagan, G.M. Mc Nally and W.R. Murphy

Polymer Processing Research Centre, The Queens University of

Belfast, Stranmillis Road, Belfast BT9 5AH, Northern Ireland

Metallocene catalysed polyethylenes are the latest addition to the polyethylene family and

are reported to have superior mechanical properties over conventional polyethylenes.

This paper sought to investigate rhe efect of blending these maferials with conventional

low density polyethylene for use in thin films, suitable as packaging materials and

produced by the blown film process. Two grades of Metallocene catalysed polyethylene

were used in the study. Films containing increasing percentages of these resins were

produced at various processing conditions and their mechanical properties considered.

These films were then characterised using Differential Scanning Calorimetry and

Dynamic Mechanical Thermal Analysis techniques. This work showed that increasing Ihe

percentage of metallocene catalysed polyethylene in a low-densi@ polyethylene film

improved the mechanical properties of the resultant film.

Introduction

By the year 2010 world polyethylene consumption will have almost doubled from its

1996 level I , to seventy million tonnes per annum. It is also predicted that by 2005

around 37% of low density polyethylene (LDPE) will be replaced by metallocene

catalysed resins’. Thus, there is great interest at present to determine best uses for

these materials.


Metallocene catalysts have been around for a number of years, but have only been

commercially exploited since the early 1990’s.’ Their benefit is due to the fact that all

their active sites are the same3, allowing, for the first time, polymer production which

is precisely controllable over a wide range of limits4. Metallocene catalysed

polyethylenes (mPEs) are reported to have superior properties over conventional

polyethylenes principally due to their narrow molecular weight distribution and more

uniform co-monomer distribution’. However these properties can also lead to

processing problems, in terms of higher viscosity’s being developed, and indeed,

many processors are finding difficulties getting maximum benefit from these grades2.

One method of reducing viscosity during processing is to blend the mPE with a

broader molecular weight distribution polymer, in this case low density polyethylene

(LDPE). The blending of materials for use in the blown film process has been utilised

for a number of years. Many studies have been carried on blends of low density

polyethylene (LDPE)/linear low-density polyethylene (LLDPE)6.7.8. It was found

that, in general increasing the amount of LLDPE results in a significant improvement

in the mechanical properties of the resultant film.

In the blown film process, polymer is melted in an extruder and forced through an

annular die. This results in the production of a molten tube which is drawn upwards

by the action of nip rollers and inflated with air through a port in the centre of the die.

The bubble is then collapsed by the nip rollers and forms a lay flat tube which can

then be wound and stored for subsequent use or slit and wound as two separate flat

sheets. The tube, or bubble, is cooled by an external air ring.

There are a number of variables in this process. One is the haul off speed, which is

defined as the rate at which the nip rollers pull the material from the die, in this case

is measured in m/min. The blow up ratio is defined as the ratio of the bubble diameter

to die diameter and is usually in the order of one to three.

The commercialisation of metallocene catalysed polymers has come at a time

when increasing pressure is being put on producers and users of packaging materials.

The December 1994 EC directive on packaging and packaging waste (94/62/EC)

introduced the principal of ‘polluter pays’ to packaging waste and in effect aims to

202

Use of metallocene catalysed polyethylene in blown film extrusion

Exxon I LDPE

reduce the amount of packaging waste going to landfill and set targets for recovery

and recycling of waste materialsg. Each link in the packaging chain, from raw

material manufacturer to product seller will have percentage obligation placed on

them. Thus, in terms of plastic packaging, any materials used that could lead to a

decrease in the amount of packaging produced will have a significant advantage over

conventional materials. These new mPEs, with their reported superior strength over

conventional polyethylenes, allow therefore the production of thinner films having

the same strength as the traditional, thicker polyethylene films. The aim of this paper

is to investigate the mechanical properties of blends of these mPEs with conventional

low density polyethylene to determine improvement in film properties, and then

characterise these films using DSC and DMTA techniques.

0.93 I 2

Experimental Details

Dow Basf

Materials Selection

The table below describes the materials used in this study.

I Manufacturer 1 Type I Density I MFI 1

mPE 0.921 0.85 mPE 0.887 1.4

Both metallocene polyethylene grades used in this study are experimental grades.

Sample Preparation

All films were manufactured using a Killion blown film coextrusion line consisting of

one 38mm extruder and two 25mm extruders, fitted with a 75mm diameter annular

die, die gap of 2.0mm. Barrel temperature profiles were maintained from 160°C at the

feed section to 220°C at the die. Films were produced at a constant blow up ratio of

2.0 and a constant screw speed, of 27rpm,set to produce a film thickness of 50pm at a

haul off speed of 3mlmin.

Blends of LDPE and mPE polyethylene were tumble mixed prior to extrusion in

the following wlw ratios: 80120, 60140, 40160, 20180. Previous experience has shown

203

C.M. Beagan, G.M. Mc Nally and W.R. Murphr

that films manufactured Erom certain mPEs are susceptible to excessive blocking,

therefore an anti-block agent (5% w/w) was incorporated into the blend mixtures

containing the BASF experimental grade prior to extrusion. It was not necessary to

use an anti-blocking agent for the Dow blends.

Tensile Testing

All films were tested to BS 2782: Part 3 using an Instron 441 1 universal tensile tester.

Crosshead speed was set at 500mmlmin with a gauge length of 50mm. For each film

a minimum of ten samples were prepared and tested in both machine and transverse

direction. From the resultant data, values for break strength, % elongation and

Youngs modulus were obtained.

DSC Analysis

Crystallinity of various films were determined using Differential Scanning

Calorimetry. This was carried out using a Perkin Elmer DSC 6, over the temperature

range 20°C to 150°C with a scan rate of 10°C/min. Thennograms from first heating

were used to determine crystallinity developed during processing. A value of

289.9J/gl0 was used to represent a 100% crystalline sample.

Dynamic Mechanical Thermal Anaosk (DMTA)

Changes in film transitions, were studied using a MK I1 Polymer Laboratories'

DMTA in the tensile mode, suitable for thin film analysis. Films were tested from

-15OoC to 100°C at a heating rate of 4'C/min using a frequency of 10Hz.

Results and Discussion In blown film extrusion a film will undergo biaxial orientation as it is being drawn

from the die. Machine direction orientation (parallel to the nip rollers) is mainly

controlled by the haul off speed and transverse direction orientation is governed

primarily by the BUR.

204


Figures 1 and 2 show the effect of mPE content on the break strength of films in

the machine and transverse directions. Clearly, increasing the percentage of mPE will

lead to an increase in break strength. At 40% mPE content both films are around 30%

stronger in the machine direction than a conventional LDPE film of the same

thickness. Note also that at over 40% mPE content, the Dow film shows a more

considerable increase in machine direction break strength, being 17% stronger than

the BASF film at 60% mPE content and over 30% stronger at 100% mPE content.

The results also show that maximum break strength occurs at the 80% mPE content

for the BASF film while break strength reaches a maximum at 100% mPE for the

Dow films. The 100% Dow film has the highest recorded break strength, being over

55% stronger than the 100% LDPE film in the machine direction.

45 - 4 0 ~

E 5 35 . M

30.

$ 2 5 .

15 I 0 20 40 60 80 100

YO mPE

Figure 1. The Eflect of mPE content on Break Strength of Films in MD.

205

C.M. Beagan, G.M. M c Nally and W.R. Murphy

33 31

$ 29 27

'D I 25 23

ZJ 21 % 19 g 17

15 0 20 40 60 80 100

YO mPE

Figure 2. The Efect of mPE content on Break Strength of Films in TD.

Figure 3 shows the effect of mPE content on % elongation of the films in the

machine direction. Again it is evident that increasing the percentage mPE content will

lead to an increase elongation. Both sets of films show approximately similar

elongation up to 60% mPE content, where there is around a 35% increase in

elongation over the 100% LDPE film. At 80% mPE content, elongation in the BASF

film is slightly higher than the Dow film, however at 100% mPE content, the Dow

film exhibits the greater elongation, with the BASF film elongation falling to below

the 60% mPE content level.

1000 - - 900 .

1.i: 0 800 .

I p 700 - 1 600 .

1

e:

I

s 1 500'

I 0 20 40 60 80 100 ,

400 ,

! YO m PE I

I 1 Figure 3. The Eflect of mPE content on the % Elongation of Films in MD.

206


Figures 4 and 5 deal with the effect of increasing mPE content on Youngs

modulus in the machine and transverse directions. For the BASF blends increasing

the % mPE will lead to an almost linear decrease in Youngs modulus. The Dow

blended films exhibit varying behaviour, showing an initial decrease in Youngs

modulus, and then an increase after 20% in MD and 40% TD. Maximum values are

recorded for these films at 60% mPE content in both machine and transverse

directions. The Dow blends are, therefore, stiffer than the corresponding Basf blends,

tending to indicate that the crystallinity of the Dow mPE was greater than that of the

Basf.

I I I

200 ,

$ 150 1~ z = 100 ~

& 50 . I

- z e

0 ,

I 0 20 40 60 80 100 I I

YO mPE

~~~

Figure 4. The Efect of mPE content on Youngs Modulus of Films in MD.

207


J

;I” 0 ,

6 50 - I s + Baf

0 20 40 60 80 I00

% mPE I

Figure 5. The Egect of mPE content on Youngs modulus in TD.

Figures 6- 10 are DSC traces of selected films, these traces show that the blended

films have curves similar to that of their components. Note that the Basf films show

t he broadest melting peak, with the onset of melting occurring at about 20°C below

the melt temperature. The Dow films have an onset temperature approximately 10°C

below melt temperature and the LDPE has an onset temperature around 7°C below

melt temperature.

Figure 6. DSC Truce for 100% LDPEfilm.

208


Figure 7. DSC Trace for 100% Basf mPE.film.

'925 .Peak = 124 1M'C

/ P e a k H ~ = 5 4 7 1 1 m W

ii Onset = 115.161 'C A i

5 -

! i

i ' - r- i

/ t

i

Figure 8. DSC Trace of 100% Dow rnPEfilm.

0 8 -

5

Ol?d = 111 5 ° C

i i

! i I

9. DSC Trace of Blended Film comprising 60% Dow mPE, 40% LDPE.

209


P U - 1axm -c

An. - 375.rOZm.l D.Y. H - 6. SnJip

k

2 8 -

a3.

24 .

P U - 1axm -c

3J

An. - 375 T0Zm.l 28

D.Y. H - 6. SnJip

a3

24

c

Figure 11 shows the effect of mPE content on the melting point of films as

recorded by DSC. 100% LDPE film melts at approximately 110°C. This graph

clearly highlights the difference in the two mPE grades, with the 100% Basf film

meiting at 97°C and the 100% Dow film melting at 124°C. The melting point of the

blends falls within the range of the 100% values as would be expected.

I 140 7 1

0 4 0 20 40 60 80 100

'30 mPE

Figure 11. The Effect of mPE content on Melting Point of Biended Films.

Figure 12 shows the effect of mPE content on crystallinity of the films. Generally

it is noted that increasing the percentage mPE in a film will lead to a decrease in the

210


overall crystallinity. The 50pm, 100% LDPE film exhibits around 36% crystallinity,

100% Basf film has a crystallinity of 28% and the Dow film 33% crystallinity. The

degree of crystallinity in a polyethylene is an indicator as to the degree of branching it

contains. As the amount of branching increases, crystallinity will tend to decrease

since branching will disrupt the overall crystalline microstructure. From this data, the

Basf mPE appears to contain the greatest degree of branching as it exhibits the lowest

crystallinity values. I

35

2. 30 ‘1 .- 25

20 15 10

- c s 5

0 1 I 0 20 40 60 80 100

Yo mPE I

Figure 12. The Efect of mPE content on the Ctystallinity of the Blended Films.

Dynamic mechanical thermal analysis (DMTA) is a powerful technique which can

be used to study changes in glass transition temperature of a polymer. The glass

transition temperature is defined as being the temperature at which a polymer obtains

sufficient thermal energy to enable its chains to move freely enough to behave like a

viscous liquid”, that is, to behave as a flexible material rather than a hard rigid solid.

For polyethylene this transition occurs at very low temperatures. The DMTA plots

in this paper are those of tan 6 vs. temperature. Tan 6 can be described as being the

ratio of the dynamic storage modulus to the dynamic loss modulus, and gives

information on the relative contributions of the viscous and elastic components of a

material‘’.

Prior to melting polyethylene in general undergoes three transitions, the a, p and y.

The Q peak generally occurs from around 20 to 70°C and attributed to the crystalline

211

C.M. Beagan, G.M. Me Nally and W.R. Murphy

phase. The p relaxation usually occurs between -5 and -3S0CI3 and is related to the

branching of polymers. The y relaxation appears between -110 and -130°C. It has been noted that the intensity of this peak tends to decrease with increasing density,

indicating the involvement of mostly an amorphous phaseI3. Some debate still exists

as to which peak (y or p) contains the glass transition of the polymer. This paper will

deal with y and p peaks in term of discussing shifts in these peaks with changing

blend compositions.

Figure 13 shows a tan 6 plot of the Basf blends in the machine direction. Note the

broad y transition curve, this exists for all the blends. This figure also includes 100%

LDPE film. This exhibits the lowest tan 6 value. Khanna et al. noted that as tan 6

decreases, the density increases. This was shown to be the case as the Basf material

ehb i t s the lowest density. Figure 14 shows the dynamic mechanical response for

the Basf blends in the transverse direction. The results shown are similar to those in

Figure 16, with the 80% blend showing the highest tan 6 values.

20% 40%

A 60%

-150 ,100 -50 Temperature 0 50

Figure 13. Tan SPlot ofBasf Blended Films in MD.

212


, \ n I " I

-150 -100 Temp8ature 0 50

figure 14. Tan SPlot of Basf Blended Films in TD.

Figures 15 and 16 show peaks for the Dow blended films. Note that these show a

more flattened l3 transition, although they do not exhibit as high a value as the Bad

blended films.

h c I

d 9 e

-150 - 100 -50 0

Temperature

Figure 15. Tan 6 Plot of Dow Blended Films in MD.

213

C.M. Beagan, C.M. Mc Nally and W.R. Murphy

-150 -100 -50 0 50 Temperature oC

Figure 16. Tan GPlot of Dow Blended Films in TD.

Figures 17 and 18 show the changes in tan 6 maximum intensity with mPE

content for the p transition in machine and transverse directions. Both graphs show

that the Basf films exhibit the greater tan 6 values, with values increasing as the

percentage mPE increased in the film. This would tend to indicate an increase in

branching, and show that the Basf material is more branched than the LDPE. With the

Dow blended materials however, increasing the percentage of mPE will lead to a

decrease in tan 6, indicating that the Dow material contains less branching than the

LDPE. These results are in agreement with the DSC data, which also suggested that

the Basf material contained the greatest degree of branching. Sha et al. suggest that p transition intensity seems to be inversely related to polymer crystallinity, with

samples of lower crystallinity showing more intense peaks14. Indeed this was found

to be the case in this study, with the lower crystallinity Basf blended films showing

the more intense peaks.

2 14


F 0.06 0.04 0.02

- - / + D O ~ ' - - - -

Figure 1% Comparison of Tan ha for the p Transition on MD.

= 0.06

0.02 p 0.04

Figure 18. Comparison of Tan ha for the p transition in TD.

-- --

-- '+&Sf,

0 20 40 60 80

Figure 19 shows how the maximum p transition temperature changes with

increasing mPE content in the transverse direction. Clearly, as mPE content is

increased, the p maximum temperature decreases. Khanna et al. noted that the p

215


transition temperature for LDPE was higher than expected and thought this was due

to bulky side groups in the LDPE. Figure 20 shows the shift in the tan 6 , for y transition with changing blend

composition again in the transverse direction. This clearly shows that increasing the

percentage of mPE will cause an increase in the tan 6 maximum value.

Figure 19. Changes in Tbtm with changing blend composition.

216


0.065

0.06 m = 0.055

e 0.05

0.045

X

8 c

I

+ Basf _ _

0.04 I 0 20 40 60 80

YO mPE

Figure 20. Comparison of Tan Sm, for y Transition in TD.

Conclusions

From the data presented above it is possible to draw several conclusions. Firstly,

incorporation of mPE into an LDPE film will lead to a significant increase in tensile

properties. At 40% mPE content the resultant film will be approximately 30%

stronger than the conventional LDPE film. The results showed that the Dow blended

films exhibited the greater break strengths and increased stiffness, in terms of a higher

Youngs modulus value, over the Basf blended films. This work has shown that there

is a potential for downgauging films using mPEs, thus reducing the overall amount of

waste packaging produced.

The Basf films have a lower melting point than the Dow films, thus providing an

advantage for heat sealing applications, allowing for a lower seal initiation

temperature. The Basf films also exhibit lower crystallinity values than the Dow

blends. Addition of 20% Basf mPE to an LDPE film will result in an approximate

30% decrease in crystallinity. The 100% LDPE film contains the greatest degree of

crystallinity.

217

C.M. Beagan. G.M. Mc Nally and W.R. Murphy

The DMTA technique was used to determine changes in transitions with varying

mPE content. It was found that for the p transition the Basf films showed the greater

tan 6 intensity, indicating that this material contained the highest degree of branching,

which was in agreement with the DSC data. Increasing the percentage of mPE in a

film led to a decrease in p transition temperature.

Acknowledgements

The Authors would like to thank Jordan Plastics Ltd, Portadown, for providing all

materials used in this research.

References

1. http://www.dsm.nl/po/press~engels/business~achtergrond.htmI 2. http://www.modplas.com/month0597/film05.htm 3. Chemistry in Britain, February 1998,45. 4. Kaminsky, Macromol. Chem. Phys. 197,3907 - 3945 (1996). 5. UK Freezer film Symposium, Birmingham, November 12, (1996). 6. Mc Nally, G.M., Bermingham, C., Murphy, W.R., Trans IChemE, Vol. 71, Part A,

7 . Norton, D.R., Keller, A., J. of Mat. Sci., 19,447-256 (1984). 8. Hill, M.J., Barham, P.J., Keller, A., Polymer, 33(12), 2530-2541 (1992). 9. Donaghy, T., The Engineers Journal, Vol. 52 (4), 4 1, (1998). 10. Ramesh, N.S., Malwitz, N., Antec 1954- 196 1 (1 997.) 1 I. Polymers: Chemistry and Physics of Modern Materials. J.M.G. Cowie, 2”d Edn. 12. Stark, p., Em. Polym. J. Vol. 33, No. 3, pp. 339-348 (1997). 13. Khanna, Y.P., Turi, E.A., Taylor, T.J, Macromolecules, 18, 1302-1309 (1985). 14. Sha, H., Zhang, X., Harrison, I.R, Thermochimica Acta, 192,233-242 (1991).

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The Use of Metallocene Catalysed Polyethylene as a Blend Material in Blown Film Extrusion