Polymers as Rheology Modifiers - American Chemical Society

Chapter 19

Melt Rheology and Strain-Induced Crystallization of Polypropylene and Its High-Density Polyethylene Blends Michael S. Chuu and Boh C. Tsai

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Downloaded by UNIV OF GUELPH LIBRARY on August 6, 2012 | http://pubs.acs.org Publication Date: May 13, 1991 | doi: 10.1021/bk-1991-0462.ch019

American National Can Company, Barrington Technical Center, 433 North Northwest Highway, Barrington, IL 60010

Melt elasticity and strain-induced crystallization (SIC) conducted under oscillatory shear for two different polypropylene resins: i.e. control -rheology PP and its virgin PP, having similar melt flow rates and polypropy lene-high density polyethylene blends were investigated. It was found that the melt elasticity of the virgin PP was only slightly greater than that of the control-rheology PP at the low frequency side, but the induced crystallization rate of the virgin PP was much faster than that of the control-rheology PP. The high molecular weight tails of PP were concluded to play a critical role in determining the SIC rates. The rate was also dependent on the percent of strain imposed within the range of 1 and 10% studied. It increased with increasing strain. The addition of high-density polyethylene increased the melt elasticity of PP, and also slightly increased the quiescent crystallization temperature of PP. However, the SIC rate decreased with increasing HDPE content in the PP/HDPE blends. The e f f e c t of molecular weight and molecular weight d i s t r i b u t i o n on processing and the consequent 1

Current address: Amoco Chemical Company, Naperville, IL 60566

0097-6156/91/0462-0312$06.00/0 © 1991 American Chemical Society

In Polymers as Rheology Modifiers; Schulz, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


19.

CHUU&TSAI

313

PP and PP-HDPE Blends

p r o p e r t i e s of f i n i s h e d products has been reported and discussed quite extensively. The f o c a l point has t r a d i t i o n a l l y been on the e f f e c t of high molecular weight components on v i s c o e l a s t i c behavior. However, i t i s f e l t that the importance of the high molecular weight component i s not only i n i t s r e s u l t a n t v i s c o e l a s t i c behavior but also i n i t s e f f e c t on s t r a i n induced c r y s t a l l i z a t i o n . T h i s paper emphasizes that the contribution of the high molecular weight component should be, i n many a p p l i c a t i o n s , assigned to i t s s t r a i n - i n d u c e d c y r s t a l l i z a t i o n behavior instead of its viscoelastic behavior, especially for semic r y s t a l l i n e polymers. Many papers have been published i n the f i e l d of s t r a i n - or s t r e s s - i n d u c e d c r y s t a l l i z a t i o n . Polyethy lene with high molecular weight was r e p o r t e d ! to crystallize faster at a given r o t a t i o n a l shear and temperature. A l s o , i t was found that the a d d i t i o n of a nucleating agent did not a f f e c t the rate of s t r a i n - i n d u c e d c r y s t a l l i z a t i o n of polypropylene random copolymer at shear rates above 5 sec" . Unique mechanical properties of polypropylene under flow-induced c r y s t a l l i z a t i o n were also i n v e s t i g a t e d i n film and stretched ribbon . Of the papers p u b l i s h e d , the experimental techniques used to induce crystallization were mostly by r o t a t i o n a l shear , drawing from melt or solid state and extrusion . A patent has also been issued for using SIC to produce enhanced mechanical p r o p e r t i e s of polybutene-1. It i s recognized that induced c r y s t a l l i z a t i o n of polymers under u n i a x i a l shear is expected to take place at a higher temperature than that under dynamic shear. However, both rotational and oscillatory techniques can be used to induce c r y s t a l l i z a t i o n of semi-crystalline or c r y s t a l l i n e polymers. In this paper, the o s c i l l a t o r y shear was a p p l i e d to study s t r a i n - i n d u c e d c r y s t a l l i z a t i o n (SIC) of polypropylene and the e f f e c t of blending high e l a s t i c i t y HDPE on the SIC of PP. 1

1

2

5

1

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6

5

7

Experimental

Sample P r e p a r a t i o n s . Two isotactic polypropylene resins used were the virgin PP, PP-I, and the control-rheology of the virgin PP, PP-II. Their c h a r a c t e r i s t i c s are l i s t e d i n Table I. The i s o t a c t i c PP used i n the studies of PP/HDPE blends i s a commercial grade, PP I I I , which has a melt flow rate of 1.60 g/10 mins. HDPE used i s also a commercial grade and has a melt index (measured at 190°C and 2.16 kg) of 0.45 g/10 mins. PP-III/HDPE blends were mixed i n a Brabender Mixer at 60 rpm and 220°C f o r 3 minutes under continuous nitrogen purge.


314

POLYMERS AS R H E O L O G Y MODIFIERS

Table

I.

C h a r a c t e r i s t i c s of

Polypropylene Resins PP-II

PP-I Mn Mw MWD (Mw/Mn) MFR *(g/10 min)


* ASTM D-1238;

92,500 427,000 4.62 1.46

94,500 460,000 4.87 1.47

Measured at 230°C and 2.16kg

R h e o l o g i c a l § Strain-Induced C r y s t a l l i z a t i o n Measure ments. The dynamic mechanical spectrometer used was Rheometrics Mechanical Spectrometer RMS-7200 with a 10,000 gram-cm torque transducer and two 25mm p a r a l l e l plates. Measurements of the storage moduli, G , and the loss moduli, G", f o r PP and PP/HDPE blends were conducted under n i t r o g e n purge. The SIC experiment was conducted by heating the PP to 200°C under nitrogen purge i n the p a r a l l e l p l a t e s for 5 minutes, then i t was cooled down f r e e l y to the desired temperature at an estimated rate of 15°C/min. The o s c i l l a t o r y shear at a frequency of 1 rad/sec and 1mm gap was a p p l i e d while the temperature was lowered and the storage modulus, G , l o s s modulus, G , and complex viscosity, T L * , were recorded. Measurements were stopped when G reached about lxlO dynes/cm due to slippage. The induction time was taken from the i n f l e c t i o n point of G v s . time p l o t when the temperature was at isotherm (as shown i n Figure 1). 1

1

! t

f

7

2

1

1

Quiescent C r y s t a l l i z a t i o n . Perkin-Elmer s Differen t i a l Scanning Calorimetry DSC-7 was used to measure the s t a t i c c r y s t a l l i z a t i o n of PP and PP/HDPE b l e n d s . Results

and

Discussion

Polypropylene. Quiescent c r y s t a l l i z a t i o n s of PP-I and PP-II were measured at 2 0 ° C / m i n heating and followed by immediate c o o l i n g and reheating at the temperature range between 4 0 ° and 2 1 0 ° C . Table II l i s t s the t r a n s i t i o n temperatures and heat of t r a n s i t i o n s f o r PP-I and P P - I I . The on-set crystallization and reraelt temperatures were only about 1 - 2 ° C greater f o r the v i r g i n PP, P P - I , than for the c o n t r o l - r h e o l o g y PP, PP-II. It i s also noted that PP has a very wide range of supercooling, temperature difference between melting and crystallization. A small strain introduced could e a s i l y induce c r y s t a l l i z a t i o n i n t h i s


19.

CHUU&TSAI

Table

II.

PP and PP-HDPE Blends

T r a n s i t i o n Temperatures § Heat of Transitions for PP-I and PP-II

PP-I Heating Cooling Reheating


315

PP-II Heating Cooling Reheating

Peak ( ° C ) 165.1 109.4 162.0

On Set ( ° C )

Γ5ΤΓ8

166.2 107.0 163.1

114.4 154.2

-92.8 90.9

151.7 112.6 153.2

80.1 -89.1 85.0

The melt e l a s t i c i t i e s vs. frequencies of the PP-I and PP-II were measured at 160°, 200°, and 240°C. They were then converted to form the master curves as shown i n F i g u r e 2. A d i f f e r e n c e i n e l a s t i c moduli of PP-I vs. PP-II was found i n the low frequency s i d e . The G values of PP-I were the same as that of PP-II at high frequencies but were greater than that of PP-II at low frequencies. The greater G at low frequencies was due to the c o n t r i b u t i o n of melt e l a s t i c i t y from the high MW t a i l s of P P - I . Figure 3 shows the p l o t s of G vs. time at 130°C and 10% s t r a i n for PP-I and P P - I I . It was found that the G of the c o n t r o l - r h e o l o g y PP-II increased under shear at a much l a t e r time than i t s v i r g i n r e s i n , PP-I. No quiescent c r y s t a l l i z a t i o n peak was found when PP-I was subjected to the same thermal h i s t o r y with DSC and then held at 130°C or 134°C f o r 20 minutes. T h e r e f o r e , the increase of G isothermally at 130°C or higher was a t t r i b u t e d to the straininduced c r y s t a l l i z a t i o n (SIC). The high molecular weight t a i l s retained i n the v i r g i n PP, P P - I , played a c r i t i c a l r o l e i n determining the SIC r a t e . The high MW t a i l s c r y s t a l l i z e d under shear f i r s t and could act as a nucleating site for the subsequent c r y s t a l l i zation. The induced c r y s t a l l i z a t i o n of PP-I and PP-II under shear was also studied at 1 3 2 ° and 134°C at 10% strain. Table III summarizes the induction time, ti, from the isothermal p l o t s of G vs. time, for PP-I and PP-II at these temperatures s t u d i e d . It was found the induction time increased with i n c r e a s i n g temperature as expected f o r both PP-I and PP-II. In all cases, PP-I has a much shorter i n d u c t i o n time than P P - I I . The e l i m i n a t i o n of high MW t a i l s prolonged the induced c r y s t a l l i z a t i o n process of meta-stable s t a t e . f

1

1

!

f

f



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POLYMERS AS R H E O L O G Y MODIFIERS

τ t

Time

Figure 1.

i 1

Schematic P l o t of Induction Time, t j ,

Log G vs. Measurement

Time

For

E7 3* •>

>

1

PP-I

i 4

c PP-II

r/ftnDD

10OPΡ

!!

ί\

•

—i 1ο

• •

• •

1 3

0.1

1.0

10.0

Frequency

Figure 4.

100.0

(rad/sec)

1

G v s . Frequency f o r 100/0, 80/20, and 0/100 PP-III/HDPE Blends at 240°C

• 100 PP ο 80PP/20HDPE • 30 PP/70 HOPE ι3 100 HOPE

30/70

ο

Polymers as Rheology Modifiers - American Chemical Society

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