single site catalysts and dual reactor technology create more freedom in pe rotomolding resin and...

8/12/2019 Single Site Catalysts and Dual Reactor Technology Create More Freedom in Pe Rotomolding Resin and Product De

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SINGLE SITE CATALYSTS AND DUAL REACTOR TECHNOLOGY CREATE MORE

FREEDOM IN PE ROTOMOLDING RESIN AND PRODUCT DESIGNS

XiaoChuan (Alan) Wang, Mark Weber, Henry Hay, Marlee Cossar

NOVA Chemicals Corporation

Calgary, Alberta, Canada

Abstract

While polyethylene continues to be the resin of

choice for rotational molding, advancements in material

design are still required. From a structure-property

perspective, a thinner lamellar thickness of a polyethylene

resin may lead to a higher tie chain formation probability.

It is also believed that longer polymer chains with proper

comonomer incorporation enhance this probability. Single

site catalysts and octene comonomer usage enable the

design of rotomolding resins with superior propertiescompared with conventional Ziegler-Natta resins. This

paper combines our fundamental understanding of this

topic with experimental data.

Introduction

High performance octene polyethylene products

produced by single site catalysts (SSC) have been

discussed previously for high performance films [1], and

rotomolding products with enhanced processability [2].

With respect to rotomolding products, the objective of

this work is to provide more fundamental understandings

versus comparable Ziegler-Natta (ZN) hexenerotomolding resins.

It has been well known that for semi-crystalline

polyethylene, at least three phases must be considered to

describe its solid-state morphology, i.e., crystalline,

amorphous, and interfacial phases, as illustrated in Figure

1 [3,4,5,6,7]. Here, the interfacial phase is defined to be

that part of the non-crystalline material that represents a

transition zone between the surface of the crystal and that

point at which the presence of the crystal is no longer

significant in terms of affecting amorphous phase

molecular motion and conformation. This disordered

phase mainly includes loose chain loops, chain ends and

tie chains or molecules.It is widely believed that tie chains in

polyethylenes play key roles in affecting the end-use

physical properties such as toughness and environmental

stress crack resistance (ESCR). This is because tie chains

are the molecular chains connecting the lamellae together,

and their integrity is critical in order for the ductile-type

behavior to occur. Attempts have been made to infer the

relative amount of tie chains by experimental methods,

such as Fourier Transform Infrared Spectroscopy [8].

Calculation methods have also been suggested to infer the

information such as the tie chain formation probability

and relative concentration of tie chains, etc. [9, 10, 11].

However, the true quantity of tie chains has not been fully

confirmed by experimental results for a given resin and it

is expected to be very difficult from an experimental

perspective, although there have been some attempts to

correlate the experimental or calculated results with end-

use properties.

In this work, an experimental method has been

used to infer the relative amount of tie chains. From a

morphology point of view (Figure 1), the tie chains mustpass through the interfacial region. As a result, they may

be counted as part of the interfacial phase if measured by

an experimental method. With this fundamental

understanding in mind, for a similar polyethylene resin

category (e.g., roughly HDPE or MDPE), the measured

amount of the interfacial phase may be used as a relative

indication of the amount of tie chains, although not their

exact amount, since this phase includes at least loose

chain loops and chain ends as well. For this objective,

therefore, laser (514.5 nm) Raman spectroscopy internal

mode analysis of band intensities has been applied to

quantitatively characterize the polyethylene three-phase

structure, with respect to interfacial phase characterization

and hence the information about tie chains.

Experimental

Polymers and CharacterizationsThe solution octene HDPE and MDPE rotomolding resins

examined here were produced on a commercial plant

using NOVA Chemicals Advanced SCLAIRTECH

Technology and proprietary single site catalyst. The

bench mark hexene HDPE and MDPE rotomolding resins

were produced by a gas phase process and a Ziegler-Natta

catalyst. Analytical temperature rising elution

fractionation (A-TREF) fractions were obtained by ananalytical TREF system built in-house. Melt Index (MI or

I2) was measured in accordance with ASTM D-1238,

condition F (190oC/2.16kg). Density was obtained in

accordance with ASTM D-1505.

Polyethylene Three-Phase CharacterizationsRaman spectra were collected from a 514.5 nm laser

Raman Spectroscope at the Research Laboratory

Canmet Energy Technology Centre, Devon, Alberta,

Natural Resources Canada. The principle for using Raman

ANTEC 2005 /973


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technique to characterize the interfacial phase and hence

to infer the relative amount of tie chains is as follows.

Since the total integrated intensity Iref of the CH2twisting band at 1298 cm

-1is irrelevant to the

conformations of molecular chains, it can be used as an

internal intensity reference. The curve-fitting technique

was performed using Grams AI (Version 7.00). The totalintegrated intensity Iref at 1298 cm-1

was measured using

the integrate program in the Grams AI where the integrate

area from 1352 cm-1 to 1253 cm-1 was used. The band at

1416 cm-1

arises from crystalline regions. A curve fitting

program was used to obtain I1416 in which three peaks at

1461, 1442 and 1416 cm-1

were fitted in the area between

1550 cm-1

to 1400 cm-1

region using a 100% Gaussian

distribution and iterated until convergence. From the

integrated intensity of this band (I1416), the fraction of CH2units in the crystalline orthorhombic phase, or the

crystallinity of the sample can be calculated:

I

I

refc

46.0

1416=

(1)

where the constant 0.46 is measured from the spectrum of

fully crystalline polyethylene using 514.5 nm laser Raman

spectroscopy [12,13].

In the CH2 twisting vibration region, i.e., the

region of 1400 and 1250 cm-1

, a mixed 60% Gaussian and

40% Lorentzian curve-fitting program was used to obtain

I1303 in which the two peaks at 1303 cm-1

and 1298 cm-1

were fitted, the position of 1303 cm-1

was limited between

1303 cm-1

and 1305 cm-1

region and iterated until being

converged. The fraction of amorphous phase can then be

calculated:

I

I

ref

a

1303=

(2)

where the constant 1.0 is measured from the spectrum of

fully amorphous polyethylene using 514.5 nm laser

Raman spectroscope [12,13]. Finally, the mass fraction of

the interfacial region is given by:

acb = 1 (3)

Rotational Molding Trials and TestingThe resins were molded with a Ferry Industries

Inc. RS-160 rotational molding machine using

30.530.535.6 cm3

cast aluminum molds, under the

conditions of 5600F oven temperature and forced air

cooling. The oven time was changed to optimize the state

of cure and hence the physical properties. The impact

strength at 400C of the rotomolded part was evaluated

using the method recommended by the Association of

Rotational Molders (ARM) International. For the purpose

of this paper, only the maximum impact strength

achievable with 100% ductility (prior to the point where

the effects of possible thermal and oxidative

degradation/crosslinking are evident) is reported here and

will be explained below. The environmental stress crack

resistance of compression-molded plaques at constant

tensile load (abbreviated as CTL-ESCR) recommended byARM was evaluated at 300C, a stress level of 4.5 MPa,

20% notch depth, and 10% Igepal concentration.

Results and Discussion

Polymer Properties Table 1 shows the molecular

characteristics of the SSC and ZN rotomolding resins

examined in this paper. Two groups can be defined

according to the catalyst and comonomer type, i.e.,

SSC/octene (SSC-A and SSC-C) vs. ZN/hexene (ZN-B

and ZN-D). Note that within the same group (SSC-A vs.

SSC-C, and ZN-B vs. ZN-D), the change in weight

average molecular weight (Mw) is similar, in the range of

about 22000, and the molecular weight distribution

(MWD) remains relatively unchanged. Also note that the

SSC/octene resins have narrower molecular weight

distributions than their ZN/hexene counterparts (SSC-A

vs. ZN-B in HDPE resin category, and SSC-C vs. ZN-D

in MDPE resin category). It is well known that a

fundamental difference between a SSC and ZN resin is in

their comonomer distribution uniformity. A SSC resin has

a uniform comonomer distribution while that of a ZN

resin is heterogeneous. This is reflected by the

comonomer distribution breadth index (CDBI) measured

by A-TREF in Table 1. A higher value of CDBI is

indicative of a more homogeneous comonomerdistribution. Other molecular parameters being similar, it

is believed that a more uniform comonomer distribution

helps reduce the lamellar thickness and hence increases

the probability to form tie chains.

Three-Phase Measurements by Laser Raman As an

example, Figure 2 shows the de-convoluted crystalline

phase at the band of 1416 cm-1 in the crystalline

orthorhombic phase, and the de-convoluted amorphous

phase between the region of 1400 and 1250 cm-1

for ZN-

B. Table 2 summarizes the measured amounts of the

crystalline, amorphous and interfacial phases, and other

physical properties. It can be seen from Tables 1 and 2that for the comparable resins in the same resin category

(MDPE or HDPE), a SSC/octene resin has a comparable

or higher amount of the interfacial phase than its

ZN/hexene counterpart, even under the condition of

increased density or stiffness. This is significant, as one

would normally expect a smaller interfacial region for

resins with higher crystallinity, or density.

For the above two groups, comonomer distribution

uniformity and comonomer type have impacts on the

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amount of the interfacial phase. Recall that within the

same group (SSC-A vs. SSC-C, and ZN-B vs. ZN-D), the

change in Mw is similar, and MWD remains relatively

unchanged. It can been seen that, within the SSC/octene

resin group, an increase in octene content of 0.8 mol.%

results in a decrease in crystallinity (c) of 8.75 wt.%.

However, this trend is not so significant within theZN/hexene resin group, mainly due to its heterogeneous

comonomer distribution and hence reduced ability to

decrease crystallization. For example, within the

ZN/hexene group, an increase in a similar amount of

comonomer (0.9 mol.%, but it is hexene comonomer)

leads to only 5.0 wt.% reduction in crystallinity. Within

SSC/octene resin group, the amorphous (a) and

interfacial (b) amounts increase by 6.04 wt.% and 2.71

wt.% respectively, while for ZN/hexene resin group, the

increase in amorphous phase amount is similar (5.95

wt.%), however the interfacial amount seems to increase

by only 0.96 wt.%. Therefore, with the similar change in

comonomer content of about 0.8 mol.%, the change in the

amount of the interfacial phase within SSC/octene resin

group is about 2.8 times that within the ZN/hexene resin

group for the resins examined here. This is attributed to a

combination of the comonomer distribution uniformity of

a SSC resin, certain molecular species produced by the

multiple reactor process technology, and possibly the

longer chain length of octene comonomer.

Within either the HDPE or MDPE resin category in Table

2, why are higher amorphous contents (a) for these ZN

rotomolding resins observed? This is probably because

conventional ZN catalyst and its process technology

create more short polymer chains with many short chain

comonomer branches. A good example is in Figure 3 thatshows the analytical temperature rising fractionation

profiles for SSC-A and ZN-B. The weight fractions

between 25 to 900C elution temperatures are shown in

Table 2. It can be seen that ZN-B has a much higher

amount of chains with many branches that typically have

lower molecular weight. It is known that polyethylene

chains having comonomer branches normally do not enter

the tightly packed lamellar lattice. Depending on their

comonomer content and distribution, molecular weight

and crystallization conditions, these chains may enter

either amorphous or interfacial regions. For the

ZN/hexene group, the lower molecular weight molecules

with many branches seem to be concentrated mainly inthe amorphous phase, and this has be detected by laser

Raman spectroscopy.

Contrary to the ZN/hexene resin group, the use of single

site catalysts reduces the formation of those chains having

low molecular weight and many branches. As a result,

their amorphous region amount seems to be less if

compared with those of their ZN/hexene counterpart

examined here. Nevertheless, the higher molecular weight

of some species helps to increase the tie chain formation

probability. As a result, even with higher density or

stiffness, their interfacial phase amount and hence

probable amount of tie chains seem to be equivalent or

higher than their ZN/hexene counterparts. This increase in

tie molecules is anticipated to improve the inter-lamellar

failure resistance and be favorable to many physical

properties such as toughness at low temperature.

Meanwhile, it should be noted that the interfacial phase

also contains other species such as loose chain loops and

chain ends, in addition to tie chains. This has been hinted

by the data in Table 2. Therefore, the use of the interfacial

amount as a relative indication of tie chains should be

done with caution, preferably within the similar resin

category (e.g., HDPE or MDPE as an example here).

Toughness at low temperatures For a rotomolded

product, one of the key performance indicators is impact

strength at low temperatures. Table 2 contains the

maximum ARM impact strength (-400C) achievable with

100% ductility for the four resins examined. The rationalefor using this value is as follows. Assuming that the resins

are properly stabilized and the powders are fully sintered,

one may postulate that this value reflects more about the

intrinsic resin properties and hence the morphology.

Within the similar HDPE or MDPE resin category, it can

be seen that SSC-A and SSC-C have higher resin

densities and hence higher stiffness than ZN-B and ZN-D

respectively. For conventional commercial Ziegler-Natta

rotomolding products, typically the increase in stiffness

may lead to the subsequent decrease in toughness.

However, even with higher stiffness in the HDPE resin

category, the SSC/octene resin SSC-A appears to have

statistically (test error not shown here) equivalentmaximum impact strength compared to that of the

ZN/hexene resin ZN-B having lower stiffness. It is

believed that this is due to the increased amount of its

interfacial phase (b) and hence probably higher amount

of tie chains. This trend is further confirmed in the MDPE

resin category with the data of SSC-C. It has a much

higher density and hence stiffness than ZN-D.

Nevertheless, its higher interfacial amount or more tie

chains imparts a higher toughness than that for ZN-D, as

seen in Table 2.Environmental Stress Cracking Resistance Another

key performance indicator is environmental stress crackresistance (ESCR). Generally, environmental stress crack

resistance (ESCR) of polyethylene may be evaluated at

constant-strain tests (such as the bent-strip tests in ASTM

D 1693); and constant load testing under the tensile mode.

For the bent-strip test, there has long been a concern

about its ability to isolate the yield-stress (it actually

ought to be flexural modulus in the opinions of the

authors in this paper) property as a parameter independent

of the polymers stress-crack properties [14], since

different moduli of samples may result in different initial

ANTEC 2005 /975


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and persistant stress levels when subjected to such a test.

This can be understood from the test procedure. In the

bent strip test, samples are stressed by bending them into

a fixed-radius U-shape and holding them in place. Initial

stress levels are dependent upon the stiffness of the resin,

and long-term stress levels depend upon the creep

characteristics of the resin. For constant-tensile-load testswith an automatic failure-detection mechanism, specimen

stiffness also has a somewhat confounding effect, since

different stiffness may lead to a different deformation

under the constant load.

Although there has been some ambiguity with the

interpretation of ESCR results, it was surmised that

environmental stress cracking in polyethylene takes place

because of inter-lamellar failure, which in turn is caused

by relaxation of tie molecules [14]. It was further deduced

that the resistance to this failure mode can be best

reflected through constant-tensile-load testing, while

taking into account the differences in yield point among

various polyethylenes [14].

In this work (Table 2), it is found that the relationship of

the interfacial amount with either CTL-ESCR or

toughness at low temperature indeed seem to be in line

with the concept of an inter-lamellar failure mechanism,

when compared within the similar resin category.

Evidently, the higher interfacial amount and hence

possibly the increased number of tie chains for

SSC/octene resins seems to improve the ductile-brittle

transition in either low temperature impact testing or

constant-tensile-load ESCR testing, even under the

conditions of higher stiffness.

Conclusions

The amounts of the crystalline, amorphous and interfacial

phases for comparable single site/octene and Ziegler-

Natta/hexene rotomolding resins have been measured by

514.5 nm laser Raman spectroscopy. The results indicate

that the resins produced by single site catalyst and

multiple reactor process technology, in conjunction with

octene comonomer, seem to have higher interfacial

amount and hence possibly more tie chains even under the

conditions of higher density or stiffness, when compared

within the similar resin category. This possible increase in

the amount of tie molecules seems to improve the inter-lamellar failure resistance and is supported by the

experimental data, such as toughness at low temperature

and ESCR measured under a constant tensile load.

Compared with Ziegler-Natta/hexene counterparts, the

SSC/octene resins seem to have a better balance of higher

stiffness and equivalent or even higher toughness, while

meeting the practical needs in many rotomolding

applications.

Acknowledgements

The authors acknowledge NOVA Chemicals Corporation

for permission to publish this work.

Disclaimer

This information is furnished in good faith, without

warranty, representation, inducement or license of any

kind. All implied warranties and conditions (including

warranties and conditions of quality and fitness for a

particular purpose), are specifically excluded. No

freedom from infringement of a patent owned by NOVA

Chemicals or others is to be inferred.

References

1. N. Aubee, C. Dobbin, S. Marshall, T. Swabey,

TAPPI 2004

2. H. Hay, M. Weber, R. Donaldson, I. Gibbons, C.

Bellehumeur, ANTEC 2004

3. L. Mandelkern, J. Phys. Chem., 75, 3909 (1971)

4. L. Mandelkern, Faraday Discuss. Chem. Soc., 68,

310 (1979)

5. L.E. Alexander, X-Ray Diffraction Methods in

Polymer Science, Krieger Publishing Co., New

York, 1979

6. L. Mandelkern, M. Glotin and R.A. Benson,

Macromolecules, 14, 22(1981)

7. M. Glotin and L. Mandelkern, 14, 1394(1981)8. A. Lustiger, and N. Ishikawa, J. Polym. Sci., Part B,

Polym. Phys., 29, 1047(1991)

9. Y.L. Huang, and N. Brown, J. Polym. Sci., Part B,

Polym. Phys., 29, 129(1991)

10. R.M. Patel, K. Sehanobish, P. Jain, S.P. Chum, and

G.W. Knight, J. Appl. Polym. Sci., 60, 749 (1996)

11. R. Popli, and D. Roylance, Polym. Eng. Sci., 25(13),

828 (1985)

12. M. Failla, R.G. Alamo and L. Mandelkern, Polymer

Testing, 11, 151(1992)

13. R. P. Paradkar, S.S. Sakhalkar, X. He, and M.S.

Ellison, J. Appl. Polym. Sci., 88, 545 (2003)

14. A. Lustiger, Understanding Environmental Stress

Cracking in Polyethylene, Medical Plastics andBiomaterials, July/August, 12(1996)

Keywords: tie chain, interfacial, amorphous,crystalline, Raman, single site, Ziegler-Natta, rotational

molding

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Table 1: Polymer Properties.

Table 2: Laser Raman Measurement Results, CTL-ESCR and ARM Maximum Impact Strength.

Figure 1: Illustration of Three-Phase Morphology of Polyethylene [3,4,5,6,7].

Figure 2: An Example of the Raman Spectra in the Crystalline and Amorphous Regions (ZN-B).

Figure 3: Comparison of Analytical Temperature-Rising Elution Fractionation Profiles of SSC-A and ZN-B.Resins.

0 . 0 0

0 . 0 5

0 . 1 0

0 . 1 5

0 . 2 0

0 . 2 5

2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0 1 1 0

T e ( C )

Absorbance

S S C - A , S S C / O c t e n e

Z N - B , Z N / H e x e n e

Catalyst

SSC-A HDPE SSC Octene 87500 2.19 0.3 90.9 0.944 1.7 129.4 187.3

ZN-B HDPE ZN Hexene 87000 2.90 0.8 51 0.942 1.8 128.6 174.5

SSC-C MDPE SSC Octene 64500 2.26 1.1 80.5 0.939 5.2 126.3 172

ZN-D MDPE ZN Hexene 66200 3.5 1.7 30 0.935 5 125.8 157.1

FTIR

Conomomer

content

(mol.%)

Resin

Category

H m(J/g)Sample Mw M w/M n CDBIComonomer DSC Tm

(oC)

Nominal

Density

(g/cm3)

M I

(dg/min.)

SSC-A HDPE SSC Octene 7.1 71.59 13.64 14.77 240 235 990 23.5

ZN-B HDPE ZN Hexene 28.7 68.4 18.74 12.86 280 254 920 21.1

SSC-C MDPE SSC Octene 41.5 62.84 19.68 17.48 110 202 750 19.9

ZN-D MDPE ZN Hexene 43.3 63.41 24.69 11.9 87 160 620 17.4

Resin

Category

Catalyst CTL-

ESCR

(hrs)

Max

ARM

Impact

(J)

Sample Comonomer A-

TREF

(< 900C

fraction

a

(wt.%)

c

(wt.%)

b

(wt.%)

Tensile

Yield

Stress

(MPa)

1%

Flexural

Modulus

(MPa)

Z N - B

C r y s t a l l in e

P h a s e

A m o r p h o u s

P h a s e

ANTEC 2005 /977

single site catalysts and dual reactor technology create more freedom in pe rotomolding resin and...

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