Automated Capillary Rheometer Measurement and Analysis - ACS

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Downloaded by YORK UNIV on March 21, 2018 | https://pubs.acs.org Publication Date: September 24, 1982 | doi: 10.1021/bk-1982-0197.ch014

Automated Capillary Rheometer Measurement and Analysis T. P. DUNTON General Electric Company, Polymer Systems Technology, Plastics Business Operations, Pittsfield, MA 01201 S. D. FOSS General Electric Company, Materials Laboratory, Power Transformer Department, Pittsfield, MA 01201 In order to facilitate rapid melt viscosity measure ment and data analysis a modified Göttfert capillary rheometer has been interfaced to a Hewlett-Packard data acquisition system. All test parameters (temperature, barrel pressure, etc) are monitored automatically and the data is stored on magnetic tape. After testing is complete, raw data is entered into an analysis program used to compute tables and draw plots of shear stress, shear rate, and apparent viscosity. Examples of the applica tion of this system to commercial polymers are discussed. The c a p i l l a r y rheometer i s a v a l u a b l e t o o l f o r p r e d i c t i n g the p r o c e s s a b i l i t y of t h e r m o p l a s t i c r e s i n s . This i s done by measuring melt v i s c o s i t i e s at shear rates and temperatures commonly encountered i n e x t r u s i o n and i n j e c t i o n molding. This procedure i s d i f f i c u l t and time consuming due to the complex nature of r h e o l o g i c a l measurements and a n a l y s e s . An automated system f o r a c q u i s i t i o n and analyses of c a p i l l a r y rheometer data has been developed to speed up and s i m p l i f y t h i s important a n a l y t i c a l technique. Melt v i s c o s i t y measurements i n v o l v e monitoring the pressures produced when molten polymer i s forced through a c a p i l l a r y a t various shear r a t e s . V i s c o s i t i e s are c a l c u l a t e d from c a p i l l a r y rheometer data using the f o l l o w i n g equations Volumetric Flow Rate Q = S x A

where Q i s the volumetric flow r a t e , S i s the p i s t o n and A i s the p i s t o n c r o s s - s e c t i o n a l a r e a .

velocity

0097-6156/82/0197-0243$06.00/0 © 1982 American Chemical Society Provder; Computer Applications in Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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COMPUTER APPLICATIONS IN APPLIED POLYMER SCIENCE

Apparent Shear Rate 4 Q w where \ , i s the shear rate at the w a l l of the c a p i l l a r y and R i s the c a p i l l a r y r a d i u s . Shear Stress T


w

R P 2 L

where w i s the shear s t r e s s at the w a l l , P i s the melt pressure, and L i s the c a p i l l a r y l e n g t h . T

Apparent,Melt V i s c o s i t y shear s t r e s s shear r a t e A l l v a r i a b l e s i n these equations are f i x e d except f o r p i s t o n v e l o c i t y and melt pressure which are measured experimentally. An HP 3052A data a c q u i s i t i o n system i s used to a c q u i r e raw data from a G f l t t f e r t c a p i l l a r y rheometer and to c a l c u l a t e and p l o t v i s c o s i t y d a t a . EQUIPMENT CONSIDERATIONS To c a l c u l a t e shear s t r e s s , shear r a t e and melt v i s c o s i t y , the melt pressure and p i s t o n v e l o c i t y must be monitored. The former i s measured using a 0 to 20,000 psi pressure transducer mounted i n the rheometer b a r r e l j u s t above the c a p i l l a r y . A l i n e a r potentiometer i s used to monitor p i s t o n p o s i t i o n during the t e s t . U t i l i z i n g the p i s t o n displacement data and the HP real time c l o c k , the p i s t o n v e l o c i t y i s c a l c u l a t e d . The HP 3052A data a c q u i s i t i o n system c o n s i s t s of a 9825A c a l c u l a t o r w i t h a real time c l o c k , a multichannel scanner, a high speed d i g i t a l v o l t meter and a p r i n t e r / p l o t t e r f o r d i s p l a y purposes. The system software c o n s i s t s of a monitoring program and an a n a l y s i s program w r i t t e n i n HPL. DATA ACQUISITION ALGORITHM Operator i n t e r a c t i o n w i t h the data a c q u i s i t i o n system occurs i n the manner shown i n Figure 1. The user s p e c i f i e s a time i n t e r v a l , u s u a l l y several seconds, over which the computer w i l l c a l c u l a t e p i s t o n v e l o c i t y and melt pressure. With the system described above the time i n t e r v a l must be on the whole second s i n c e t h i s i s the r e s o l u t i o n of the real time c l o c k .

Provder; Computer Applications in Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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DUNTON AND FOSS

Automated

Capillary

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SYSTEM INPUTS OPERATOR SETS SPEED STARTS TEST

Figure 1.

Data acquisition algorithm.


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When the t e s t i s s t a r t e d and computer monitoring i n i t i a t e d , p i s t o n v e l o c i t y and melt pressure are c o n t i n u o u s l y c a l c u l a t e d f o r each time i n t e r v a l using the f o l l o w i n g e q u a t i o n s : Piston Velocity S = ((Ri

- R

Q

) x

F )/T p

where S i s the p i s t o n v e l o c i t y (cm/sec), R i s the potentiom e t e r reading (ohms) at the s t a r t of the i n t e r v a l , R] i s the potentiometer reading at the end of the i n t e r v a l , Fp i s a c a l i b r a t i o n f a c t o r (cm/ohm) f o r the potentiometer, and T i s the time i n t e r v a l .


0

Melt Pressure P = V x

F

t

where P i s the melt pressure (PSI)-, V i s the transducer s i g n a l ( v o l t s ) and F^ i s the transducer c a l i b r a t i o n f a c t o r ( P S I / v o l t ) . Only data f o r the three most recent time i n t e r v a l s are stored i n working memory. The computer s i g n a l s the operator when data have been taken f o r three i n t e r v a l s . At t h i s time i f the melt pressure has l e v e l e d o f f s a t i s f a c t o r i l y , the operator stops the p i s t o n movement. The pressure drop s i g n a l s the computer to stop t a k i n g d a t a . The p i s t o n v e l o c i t y and melt pressure data f o r the i n t e r v a l , (of the three most r e c e n t ) , w i t h the highest melt pressure reading are taken to be the raw data f o r t h a t t e s t . The operator can continue t e s t i n g or e x i t as the e x p e r i ment d i c t a t e s . This a l l o w s f o r maximum f l e x i b i l i t y . MELT VISCOSITY ANALYSIS AND DISPLAY ALGORITHM The a l g o r i t h m shown i n Figure 2 demonstrates the f l e x i b i l i t y of t h i s program. Data i s entered e i t h e r manually or from magnetic tape. P l o t s c a l i n g i s done a u t o m a t i c a l l y or i s operator designated. C o r r e c t i o n s to the data are o p t i o n a l . RESULTS AND DISCUSSION S i g n i f i c a n t savings i n data a c q u i s i t i o n and a n a l y s i s time have r e s u l t e d from the automation of our rheometer. Piston v e l o c i t i e s are now determined r a p i d l y and more a c c u r a t e l y than p r e v i o u s l y had been p o s s i b l e . P r e l i m i n a r y data i s q u i c k l y and e a s i l y p l o t t e d thus a i d i n g the d e c i s i o n s on f u r t h e r experimen tation. V i s c o s i t y c a l c u l a t i o n s are done i n seconds and c o r r e c t i o n s such as the Rabinowitsch c o r r e c t i o n shown below are convenient l y used.


DUNTON AND FOSS


14.

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RABINOWITSCH CORRECTION

PLOT AND TABULATE DATA Figure 2.

Melt viscosity analysis and display algorithm.

American Chemical Society Library 1155 16th St., N.W. Provder; Computer Applications in Applied Polymer Science Washington, O.C.Society: 20036Washington, DC, 1982. ACS Symposium Series; American Chemical

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with n =

{3r)

+

A

V

4n

;

d log T d log y

where n i s the power law index f o r r a t e of shear at the w a l l i n non-Newtonian f l u i d s . An example of the a p p l i c a t i o n of t h i s c o r r e c t i o n i s shown i n Figure 3 . The v i s c o s i t y data f o r a commercial polycarbonate r e s i n i s shown p l o t t e d w i t h and without the Rabinowitsch c o r r e c t i o n . As can be seen from the f i g u r e , there i s a s i g n i f i c a n t s h i f t i n v i s c o s i t y values when the c o r r e c t i o n i s a p p l i e d . A very important advantage gained by the r h e o l o g i s t using t h i s system i s the a b i l i t y to r a p i d l y compare up to s i x v i s c o s i t y curves on one p l o t to a i d i n data i n t e r p r e t a t i o n . The v i s c o s i t y data f o r several d i f f e r e n t molecular weight grades of commercial polycarbonate r e s i n are shown i n Figure 4. From t h i s p l o t the r e l a t i v e p r o c e s s a b i l i t y of these m a t e r i a l s can e a s i l y be determined by comparing t h e i r v i s c o s i t i e s at the shear rates used i n p r o c e s s i n g . The ease of operation of t h i s system makes i t p o s s i b l e f o r even inexperienced personnel to q u i c k l y l e a r n to use i t s u c c e s s f u l l y . Thus r h e o l o g i c a l data w i t h s o p h i s t i c a t e d analyses can be e a s i l y obtained w i t h a minimum of operator training. Through automation we now have a f l e x i b l e , e f f i c i e n t , easy to operate system f o r o b t a i n i n g r h e o l o g i c a l d a t a .


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Figure 3. Viscosity data for a commercial polycarbonate resin, LEX AN 135-111, at 250°C. Key: %, apparent viscosity; |, Rabinowitsch corrected viscosity.



250


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Automated Capillary Rheometer Measurement and Analysis - ACS

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