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Correlation between intrinsic viscosity and flow rate of thermoplastics
Polymer Testing 5 (1985) 309-313
Short Communication Correlation Between Intrinsic Viscosity and Flow Rate of Thermoplastics
Rheological evaluations of polymer products serve as useful tools in characterising such materials for the purpose of process quality control. Two parameters, essentially for thermoplastic polymers, are of considerable importance: dilute solution viscosity and flow rate. T h e solution viscosity is a function of the root-mean-square size of macromolecules and hence indicates the degree of polymerisation achieved by a polymer process. T h e flow rate indicates flow behaviour of molten polymer and thereby provides useful data in characterising the material for further processes such as extrusion. Acrylic moulding and extrusion pellets of four grades are presently manufactured and m a r k e t e d by Polymers Corporation of Gujarat Ltd under the trade name of 'Acrypol-P'. T h e pellets are, in general, copolymers of methyl methacrylate and methyl acrylate produced by suspension polymerisation. As is evident from Table 1, pellets are classified with reference to their heat distortion temperature and flow rate values to provide guidance on processing suitability. The high heat resistant grade is designated as 1001 HH, the extrusion grade as 932 H R , the general-purpose grade as 876 G and the easy-to-flow grade as 8015 FG. T h e process quality control is done by evaluating intrinsic viscosity (limiting viscosity number) and flow rate (the so-called melt flow index) of all the grades. T h e classical m e t h o d of evaluating intrinsic viscosity is based u p o n evaluating and plotting either reduced or inherent viscosity against various respective concentrations in order to get the viscosity value at zero concentration which represents the intrinsic viscosity. 1 However, the mathematical m e t h o d of Billmeyer 2 provides a reasonable approximation of intrinsic viscosity. For quality-control purposes, the intrinsic viscosity is estimated from data 309
Polymer Testing 0142-9418/85/$03.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Northern Ireland
310
A. M. Dave T~dlILE 1
Characteristic Properties of Acrylic Pellets
Property
Specific gravity Heat distortion temperature (°C) Flow temperature (°C) Flow rate (g/10 min) Tensile strength (kg/cm2) Elongation at rupture (%) Modulus of elasticity (kg/cm2) Compressive strength of yield (kg/cm2)
ASTM test method
Pellet grade 1001 H H
932 FIR
876 G
8015 G F
D792
1.19
1.19
1.19
1.19
D648 D569 D1238
100 172 1-4
93 163 2-2
87 152 6
80 142 15
D638
730
700
650
650
D638
3
3
3
3
D638
3 X 10 4
3 X 10 4
3 x 104
3 x 104
D695
1100
1100
1000
1000
o b t a i n e d at a single concentration b y the following equation [~1] = , t_,I,~, 1/z-s_ 1) 2.5 C
(1)
w h e r e 71,e~is the relative viscosity (the ratio of solution efltux time to solvent efttux time corrected for respective kinetic energy errors) and C is the concentration of the solution (g/litre). In. general, a weighed quantity of pellets is dissolved in chloroform in o r d e r to have a concentration of nearly 10 g/litre and subsequently efltux times are m e a s u r e d with an Ostwald viscometer at 25 °C. T h e flow rate is m e a s u r e d 3 by a typical extrusion p l a s t o m e t e r called the Melt Indexer (Type L200, T a k a r a K o g y o C o m p a n y Ltd, Japan). This instrument is basically a dead-weight piston p l a s t o m e t e r comprising a steel cylinder with a die at the b o t t o m e n d and a weighed piston moving vertically d o w n w a r d s in the cylinder. T h e cylinder is thermostatically h e a t e d to 230 °C and timed flow rate m e a s u r e m e n t s are m a d e manually employing 3 - 1 0 kg dead weights. All m e a s u r e m e n t s are converted to weight p e r time as Flow rate = w i t
(2)
Intrinsic viscosity a n d ]tow rate o f thermoplastics
I001 HH
II|Z HA
071l G
311
|01S FG
I,, tt6Zl 11"$f
6.0
S~
. . . . . . MI I0 14 zo )~ ~kO 0,0 Fig. 1. Relationship between intrinsic viscosity (Iv) and flow rate (Mi) of various acrylic pellets.
$.o
tO
I.$
1.0 i.e
t.o
where w is the weight (g) of extruded mass during time t. T h e time period is usually 10 rain. Thus this m e t h o d is useful for determining the rate of extrusion of molten polymer through the die analogous to an extruder under specified conditions of load, temperature, piston position and time. This approach facilitates quality control of thermoplastic materials with relatively low melt viscosities. Correlation between intrinsic viscosity (Iv) and flow rate (Mi) is investigated. A simple plot of the two properties yields a linear relationship in all four cases (Fig. 1). T h e slope being negative, in general, intrinsic viscosity is inversely proportional to flow rate I v oc 1 / M i
(3)
T h e quantitative relationship may be expressed by the general mathematical formula y = c - mx (4) where y is the intrinsic viscosity and x is the flow rate. Constants c and m are characteristic functions of the material under consideration represented as the intercept and the slope of the straight line. Typical values of these constants are given in Table 2.
312
A. M. Dave TABLE 2
Typical Values of Constants m and c of Eqn. (4) Pellet grade
m ( x 10 -3)
c
1001 HH 932 HR 876 G 8015 FG
-8.77 -3.35 -3.08 -1-04
0.0667 0-0685 0.0742 0.0700
A logical justification m a y b e given on the basis of macromolecular structures. A s the copolymers are synthesised b y free radical polymerisation they are comprised of so-called linear chain structures. T h e chains are held together b y entanglements u n d e r the influence of van d e r Waals and dispersion forces. These forces are r e d u c e d in the presence of solvent molecules rendering macromolecular chains free to move. O n the o t h e r hand, vibrations caused by thermal energy m a y o v e r c o m e these forces rendering the chains free to move. Thus, in b o t h cases, characteristic flows are o b s e r v e d specified by viscosity. Thus solution viscosity and flow rate are comparable. T h e inverse relationship b e t w e e n the two properties is obviously d u e to the fact that flow rate is m e a s u r e d in terms of p o l y m e r mass collected p e r unit time. It is important to mention that a satisfactory correlation b e t w e e n intrinsic viscosity and flow rate is possible when copolymers are p r o d u c e d from the same process which ultimately yields a similar configuration of macromolecules. M o r e o v e r , consistency in intrinsic viscosity data d e p e n d s essentially u p o n consistency in p o l y m e r additives, solvent, viscometer and t e m p e r a t u r e of measurements. Thus it m a y b e concluded that a linear correlation b e t w e e n intrinsic viscosity and flow rate is possible for the p u r p o s e of quality control b y identical tests on thermoplastic p o l y m e r products manufactured b y identical processes. T h e calibration curve m a y b e useful in deriving a p a r a m e t e r in e m e r g e n c y s h u t - d o w n of either test instrument or in counterchecking o n e or o t h e r parameter. Such calibration curves m a y also provide a quick reference for data concerning the processor.
Intrinsic viscosity and flow rate of thermoplastics
313
REFERENCES 1. ASTM D 1243, Part 35, p. 489. 2. Billmeyer, F. W., Jr. (1943). J. Polym. Sci. 4, 183. 3. ASTM D 1238, Part 35, p. 467. A . M. D a v e Polymers Corporation of Gu]arat Ltd, P O Petrofils Baroda 391 347 India
Short Communication Correlation Between Intrinsic Viscosity and Flow Rate of Thermoplastics
Rheological evaluations of polymer products serve as useful tools in characterising such materials for the purpose of process quality control. Two parameters, essentially for thermoplastic polymers, are of considerable importance: dilute solution viscosity and flow rate. T h e solution viscosity is a function of the root-mean-square size of macromolecules and hence indicates the degree of polymerisation achieved by a polymer process. T h e flow rate indicates flow behaviour of molten polymer and thereby provides useful data in characterising the material for further processes such as extrusion. Acrylic moulding and extrusion pellets of four grades are presently manufactured and m a r k e t e d by Polymers Corporation of Gujarat Ltd under the trade name of 'Acrypol-P'. T h e pellets are, in general, copolymers of methyl methacrylate and methyl acrylate produced by suspension polymerisation. As is evident from Table 1, pellets are classified with reference to their heat distortion temperature and flow rate values to provide guidance on processing suitability. The high heat resistant grade is designated as 1001 HH, the extrusion grade as 932 H R , the general-purpose grade as 876 G and the easy-to-flow grade as 8015 FG. T h e process quality control is done by evaluating intrinsic viscosity (limiting viscosity number) and flow rate (the so-called melt flow index) of all the grades. T h e classical m e t h o d of evaluating intrinsic viscosity is based u p o n evaluating and plotting either reduced or inherent viscosity against various respective concentrations in order to get the viscosity value at zero concentration which represents the intrinsic viscosity. 1 However, the mathematical m e t h o d of Billmeyer 2 provides a reasonable approximation of intrinsic viscosity. For quality-control purposes, the intrinsic viscosity is estimated from data 309
Polymer Testing 0142-9418/85/$03.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Northern Ireland
310
A. M. Dave T~dlILE 1
Characteristic Properties of Acrylic Pellets
Property
Specific gravity Heat distortion temperature (°C) Flow temperature (°C) Flow rate (g/10 min) Tensile strength (kg/cm2) Elongation at rupture (%) Modulus of elasticity (kg/cm2) Compressive strength of yield (kg/cm2)
ASTM test method
Pellet grade 1001 H H
932 FIR
876 G
8015 G F
D792
1.19
1.19
1.19
1.19
D648 D569 D1238
100 172 1-4
93 163 2-2
87 152 6
80 142 15
D638
730
700
650
650
D638
3
3
3
3
D638
3 X 10 4
3 X 10 4
3 x 104
3 x 104
D695
1100
1100
1000
1000
o b t a i n e d at a single concentration b y the following equation [~1] = , t_,I,~, 1/z-s_ 1) 2.5 C
(1)
w h e r e 71,e~is the relative viscosity (the ratio of solution efltux time to solvent efttux time corrected for respective kinetic energy errors) and C is the concentration of the solution (g/litre). In. general, a weighed quantity of pellets is dissolved in chloroform in o r d e r to have a concentration of nearly 10 g/litre and subsequently efltux times are m e a s u r e d with an Ostwald viscometer at 25 °C. T h e flow rate is m e a s u r e d 3 by a typical extrusion p l a s t o m e t e r called the Melt Indexer (Type L200, T a k a r a K o g y o C o m p a n y Ltd, Japan). This instrument is basically a dead-weight piston p l a s t o m e t e r comprising a steel cylinder with a die at the b o t t o m e n d and a weighed piston moving vertically d o w n w a r d s in the cylinder. T h e cylinder is thermostatically h e a t e d to 230 °C and timed flow rate m e a s u r e m e n t s are m a d e manually employing 3 - 1 0 kg dead weights. All m e a s u r e m e n t s are converted to weight p e r time as Flow rate = w i t
(2)
Intrinsic viscosity a n d ]tow rate o f thermoplastics
I001 HH
II|Z HA
071l G
311
|01S FG
I,, tt6Zl 11"$f
6.0
S~
. . . . . . MI I0 14 zo )~ ~kO 0,0 Fig. 1. Relationship between intrinsic viscosity (Iv) and flow rate (Mi) of various acrylic pellets.
$.o
tO
I.$
1.0 i.e
t.o
where w is the weight (g) of extruded mass during time t. T h e time period is usually 10 rain. Thus this m e t h o d is useful for determining the rate of extrusion of molten polymer through the die analogous to an extruder under specified conditions of load, temperature, piston position and time. This approach facilitates quality control of thermoplastic materials with relatively low melt viscosities. Correlation between intrinsic viscosity (Iv) and flow rate (Mi) is investigated. A simple plot of the two properties yields a linear relationship in all four cases (Fig. 1). T h e slope being negative, in general, intrinsic viscosity is inversely proportional to flow rate I v oc 1 / M i
(3)
T h e quantitative relationship may be expressed by the general mathematical formula y = c - mx (4) where y is the intrinsic viscosity and x is the flow rate. Constants c and m are characteristic functions of the material under consideration represented as the intercept and the slope of the straight line. Typical values of these constants are given in Table 2.
312
A. M. Dave TABLE 2
Typical Values of Constants m and c of Eqn. (4) Pellet grade
m ( x 10 -3)
c
1001 HH 932 HR 876 G 8015 FG
-8.77 -3.35 -3.08 -1-04
0.0667 0-0685 0.0742 0.0700
A logical justification m a y b e given on the basis of macromolecular structures. A s the copolymers are synthesised b y free radical polymerisation they are comprised of so-called linear chain structures. T h e chains are held together b y entanglements u n d e r the influence of van d e r Waals and dispersion forces. These forces are r e d u c e d in the presence of solvent molecules rendering macromolecular chains free to move. O n the o t h e r hand, vibrations caused by thermal energy m a y o v e r c o m e these forces rendering the chains free to move. Thus, in b o t h cases, characteristic flows are o b s e r v e d specified by viscosity. Thus solution viscosity and flow rate are comparable. T h e inverse relationship b e t w e e n the two properties is obviously d u e to the fact that flow rate is m e a s u r e d in terms of p o l y m e r mass collected p e r unit time. It is important to mention that a satisfactory correlation b e t w e e n intrinsic viscosity and flow rate is possible when copolymers are p r o d u c e d from the same process which ultimately yields a similar configuration of macromolecules. M o r e o v e r , consistency in intrinsic viscosity data d e p e n d s essentially u p o n consistency in p o l y m e r additives, solvent, viscometer and t e m p e r a t u r e of measurements. Thus it m a y b e concluded that a linear correlation b e t w e e n intrinsic viscosity and flow rate is possible for the p u r p o s e of quality control b y identical tests on thermoplastic p o l y m e r products manufactured b y identical processes. T h e calibration curve m a y b e useful in deriving a p a r a m e t e r in e m e r g e n c y s h u t - d o w n of either test instrument or in counterchecking o n e or o t h e r parameter. Such calibration curves m a y also provide a quick reference for data concerning the processor.
Intrinsic viscosity and flow rate of thermoplastics
313
REFERENCES 1. ASTM D 1243, Part 35, p. 489. 2. Billmeyer, F. W., Jr. (1943). J. Polym. Sci. 4, 183. 3. ASTM D 1238, Part 35, p. 467. A . M. D a v e Polymers Corporation of Gu]arat Ltd, P O Petrofils Baroda 391 347 India