Presented before Divisions o f Industrial and Engineering Chemistrw and Paint, Varnish, and Plastics Chemistrg at the 112th Meeting of the American Chemical Societg, New York, AT. Y.
Introductory Remarks T. S. Carswell Commercial Solvents Corporation, Terre Haute, Ind.
I
T IS somewhat of a pioneering experiment to hold a symposium
is unique in t,hat the finished products are always mat,erials of extremely high viscosity and it' is in handling products which flow with such difficulty that most of t,he chemical engineering problems are met. Two of the papers in this symposium are specifically involved with t'his phase of the problem. Two other papers compare German developments with our own practices, and are particularly interesting in contrasting methods of handling which were developed abroad to those which we have doveloped. I t is to be hoped that. this symposium will lead to other and more exhaustive treatments of the subject. There is really very little information in the literature which the practical chemkal engineer can utilize in the design of equipment where flow of high viscosity plastics under pressure is concerned. The topic is a worth-while one for a more exhaustive study by some of our chemical engineering schools.
on chemical engineering techniques in the ,plastics industry. The plastics industry in this country is relatively new. The earliest synthetic plastics in the United States were the phenol aldehyde resins, which started with Baekeland's original experiments in 1909, but did not grow to a large volume of business until a number of years later. The manufacture of thermoplastics is much newer and the more important synthetic members of the latter group-styrene and vinyl chloride-began t o be important articles of manufacture in this country less than a decade ago. The organic and physical sides of the plastics industry have been thoroughly covered in recent years; there is much less information regarding the developments in chemical engineering. The processing of plastics is a branch of chemical engineering in so far as it involves handling chemical products under elevated temperatures and high pressures. I n fact, the plastics industry
Rheological Problems in the Processing of Plastics Rolf Buchdahl
H. IC. Nason
iMonsanto Chemical Company, Springfield, Mass.
Monsanto Chemical Company, Dayton, Ohio scribed. The problem of obtaining material constants which are independent of the instruments used is given particular attention. Because the viscosity coefficient of most high-polymeric systems is not only a function of temperature but also a function of time, shearing force (or rate of shear), and previous history, i t is important to obtain a complete evaluation of the flow properties, in order to establish a satisfactory correlation between them and the processing behavior. In the final section i t is shown how the flow- properties affect-and to some extent determine-the processing behavior of plastic systems. Because of the lack of extensive quantitative data it is possible to give only a qualitative discussion, The following processes are considered in some detail: extrusion, molding, calendering, and coating of surfaces,
]In the first part of this paper the various deformation mechanisms of high-polymeric substances are discussed in a general manner. Three different types of deformations are possible: ordinary elastic, highly elastic, and viscous deformation. The various constants which characterize these mechanisms are: a set of elastic moduli, a set or spectrum of relaxation times, and a viscosity coefficient. If a variable force is acting on the material, the dependence of the elastic moduli and of the viscosity as a function of the frequency must be taken into account. The relationship between the viscosity coefficient and molecular properties is considered with particular reference to Eyring's theory. I n the second part of the paper various methods of measuring the flow properties of plastic systems, covering a wide consistency range, are de-
642
E
VEEN a casual inspection of the various processing methods
used in the plastics industry-such as molding, extruding, casting, rolling, and calendering-will reveal their intimate relationship to the flow properties of highrpolymeric materials. The great increase in the use of plastic materials is in no small part due t o the fact that they possess rheological properties which over a range of temperatures and pressures will permit rapid and inexpensive processing and the development of desirable properties in the final product. It is not surprising, therefore, that the evaluation of the flow propertics of plastic systems has been of considerable interest to the industry as a whole, and during the past 20 to 30 years a large amount of work has been done in a n effort t o develop suitable plastometers or viscometers. Although one of the objectives of such work must have been the desire t o obtain quantitative data relating flow properties t o processing characteristics, little if anything can be found on this subject in the literature. The limited usefulness of many of the flow tests in use is typified by the statement (6) that ‘this method can be used t o determine batch-to-batch uniformity, but cannot be used with certainty t o determine whether different types of thermosetting materials are of the same mobility.’’ This inability to evaluate properly the flow properties of high polymers is the major cause for the lack of quantitative knowledge about the relationship between rheology and processing characteristics of plastics. The importance of this subject need not be labored. When it is realized that rheological phenomena play a n important role in the processing of plastics, it is clear that a thorough understanding of the flow properties would be of the greatest value not only in overcoming processing difficulties but also in developing still better methods of processing. PRINCIPLES O F DEFORMATION AND FLOW
Deformation under Constant Load. When one measures the deformation at constant loading as a function of time, one obtains, in general, a relationship as shown in Figure 1 which is characteristic of a great many different kinds of solid high-polymeric systems. A usually small instantaneous deformation is followed by a deformation process which changes with time in an exponential manner, plus a deformation which increases linearly with time. The analysis and interpretation of such deformation
RELATIVE LENGTH
Figure 2. Typical Stress-Strain Curve for Rubberlike Materials (27)
processes have been discussed by various investigators (3, 19, 69, 30, 31, 44, 46, 47). The instantaneous (or ordinary elastic) deformation is due t o a change in the equilibrium distance between chains and atoms. Because the deformation due t o this process is small compared to the other two mechanisms, it is neglected in this discussion. The second mechanism contributing t o the deformation-frequently referred t o as highly elastic or rubberlike deformation-is due t o chain orientation in the direction of applied stress. I n its simplest form this process can be characterized by two constants: one elastic modulus and one time constant, T. The final part of the curve shown in Figure 1is generally believed t o be due to the third mechanism-a viscous or irreversible deformation. A general mathematical expression for the complete deformation process can be written (3, 47):
where
t T
= time of deformation
= time constant
D&) = highly elastic deformation of the ithsegment as t---,D(or) = ordinary elastic deformation D(vi,,o.)= deformation due t o viscous flow. A word should be said about the time dependency of the total deformation a t constant stress. As Tuckett (47)has pointed out, the exponential time function for the highly elastic deformation
< lo-‘. I n such cases it is difficult t o distinguish clearly between highly elastic and viscous deformation unless one studies also the recovery as a function of time. On the other hand, it seems certain that in many cases the viscous deformation is not a linear function of time. It is important to keep these factors in mind when such data are used to calculate material constants or t o predict processing behavior. A single relaxation or orientation time and a single elastic modulus usually are not sufficient t o describe the highly elastic deformation of polymers (40). It is necessary t o introduce a discreet set of quantities or a continuous function goes over into a linear function, whenever t-
Figure 1. Deformation us. Time at Constant Load and Temperature (14) 643
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Vol. 40, No. 4
tion. The recovery increases strongly with the shearing force. This, too, is an indication of the presence of elastic deformation. The most extensive theoretical treatment of viscous deformations has been given by Eyring (85) and his collaborators. The rate of flow or shearing rate is interpreted in terms of a rate process involving the motion of molecules (or parts of them) from one equilibrium position to another which are separated by certain energy barriers. The net rate of shear in the direction of the applied shearing stress, assuming a symmetrical energy barrier, is given by the following expression:
2
= rate of shear =
L x K
h
x
2 sin h f e 2kT
(4)
where L , LI, A = constants of the molecule h = Plank’s constant f = shearing force A p t = free energy of activation
0
5
0
Figure 3.
*
0
0
.
7
For the case where f A L > 2kT one obtains:
Elastic Properties of Dilute Polymer Solutions (12)
which has been called the relaxation spectrum. The theory which has been developed by James and Guth (27) to account for the elastic modulus of a rubberlike molecule agrees well with experimental data obtained for soft rubber, as shown in Figure 2. The differential Young’s modulus is given by the following expression: =
KT(l
+ 2/Li)
and the logarithm of the rate of shear becomes a linear function of the shearing force. Flow measurements on some high-polymeric systems (15, 99, 4 1 ) seem to follow the relationship given in Equation 6; however, the qualitative agreement between theory and experiment in these cases might only be coincidental.
(2)
where E = elastic modulus Lz = extension in z direction T = absolute temperature K = constant characteristic of molecule The strong temperature dependence of the highly elastic deformations is due to the fact that the orientation times are an exponential function of the temperature. 71
=
Be
+ Ui RT
(3)
where B and R are constants and U equals activation eneigy of thc z t h segment. The molecular mechanism responsible for the appearance of thcse orientation times might be due to “the change from a considerably restricted to a relatively free rotation of the C-C bonds in the main polymer chain,” as has been suggested by Tuclcett ( 4 6 ) or it might be due to chain scission (or recombination), as has been suggested by Tobolsky (45)and others. As the temperature increases or the solvent and/or plasticizer content of the system is increased, the deformations due to viscous flow become more and more pronounced, although highly. elastic deformations can still be detected even in solutions of high polymers of very low concentration or in materials such as polystyrene a t a temperature of about 160’ C. The former case is illustrated in Figure 3, which is taken from a recent paper by Carver and Van Vazer ( 1 8 ) . These measurements were made in a rotational-type viscometer and show the change in stress for a given shearing rate as a function of time after the material has rested for various periods of t h e . The appearance of maxima very definitely indicates the presence of elastic forces if only for a very short period of time. Figure 4 shows the elastic recovery of polystyrene measured in rotational viscometer as a function of shearing stress after the material has received a constant deforma-
F- SHEARING FORCE (ARBITRARY
UNITS)
Figure 4. Recovery from Elastic Deformation of Polystyrene at High Temperatures, Measured with Rotational Viscometer ( 9 )
Studies by Flory (62) on long-chain polyesters have shown that the viscosity can be expressed in the following form: lnv
=
A R
+ RB
-
x
21/2
c + RT -
(7)
where A, B, and C = constants independent of temperature Z = average chain length of polymer molecule Equation 5 can be written:
Comparing Equations 7 and 7a shows that C is equivalent to the activation energy for the movement of the flow unit. Because
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1948
this quantity is independent of the chain length, it has been postulated that the flow unit of high polymers in the molten state is not the whole molecule but only a segment which consists usually of 20 to 30 carbon &toms. On the other hand, the entropy of activation is a function of the chain length:
AS$
Nh V
= Rln-
- A - BZ1ID
(7b)
This means that for viscous flow a certain coordination between the chain segments is nevertheless necessary. Because the FloryEyring equation has been used extensively during the past few years in theoretical and experimental investigations-such as second-order transition point, plasticizer-resin interaction, etc. (4,42)-it might be well t o point out the limitations of the Eyring theory at its present stage. 1. The net rate of shear (or the viscosity coefficient) is obtained by assuming that the motion of the molecules (or segments of the molecule) in the direction of the applied shearing stress is in equilibrium with the motion of the molecules in the opposite direction. The theory does not take into account transient or timedependent phenomena, as, for example, the progressive orientation of long-chain molecules during shear. 2. The potential field i n which the molecules move is assumed t o be a symmetrical one. Although this assumption seems t o be a reasonable one for simple liquids of low molecular weight, i t is doubtful whether it is justified for large and complex molecular systems such as plastic materials.
The presence of small particles-fillers or pigments-has a marked effect on viscous flow and the problem has been dealt with extensively from the theoretical point of view (37). Rased on the early work of Einstein, who investigated the effect of spherical particles at low concentrations, others have extended these calculations to consider various geometrical forms and the effect of higher concentrations. The major conclusions which can be drawn from these investigations are: The viscosity depends strongly on the shape of the particles and not on the size, and the viscosity increases with increasing concentration. Model experiments (16) have confirmed the essential predictions of the theoretical calculations. Behavior under Dynamic Conditions. So far we have considered only the various deformation mechanisms under a constant static load. However, the behavior of high-polymeric systems under dynamic conditions is in many cases of the greatest importance. Because of the existence of a wide range of relaxation times, the response of high polymers to dynamic stresses is a function of the frequency of these stresses. The problem is analogous to the problem of anomalous dispersion of dielectrics. The elastic modulus (which is the mechanical analog of the dielectric constant) and the elastic losses (which are comparable to the reciprocal Q value in the electrical case) are a function of temperature and frequency.
645
Figure 5 shows the temperature dependence of the elastic moduli and elastic losses of plasticized polyvinyl chloride (data from a recent paper by Sack et al., 38). At low temperatures the elastic losses frequently can be represented by a frequency-independent part and a part which is linearly dependent on the frequency. This latter term has been described as a viscous term, the proportionality constant having the dimension of a viscosity coefficient. However, such a viscosity coefficient is not necessarily identical with the previously discussed viscosity coefficient, as the former is associated primarily with a recoverable deformation, whereas the latter is related to a permanent and, therefore, nonrecoverable deformation. It appears likely that a close relationship should exist between the two quantities, because the response of a highpolymer molecule at a certain frequency depends not only on the structure of the molecule itself but also on the viscosity of its surrounding medium. MEASUREMENT OF FLOW PROPERTIES
We turn now to a discussion of the methods that have been employed to measure the rheological properties of high polymers. The ultimate aim of any rheological investigation is to obtain material constants which describe completely the viscous deformation process and which are independent of the instrument constants. Although it is possible t o obserGe flow under a multitude of experimental conditions, there exist only relatively few cases where the measured quantities can be expressed in terms that are independent of the particular instrument used. This is due to the fact that the hydrodynamic equation for viscous flow can be solved only when the boundary conditions are fairly simple. The importance of obtaining instrument-independent data is not only of academic interest but is of great practical significance to the subject under discussion. Results which depend as much on the instrument as on the characteristic properties of the material obviously are not very useful when applied to processing equipment of different design than the instrument used. If a close similarity exists between the two, the results might be of some value-particularly from a comparative point of view-although the usefulness of such data is very limited.
-
T STRESS
II
1.0
Figure 6.
General Shearing Stress Rate of Shear Diagrams ( I )
9
.? 6
5 4 .3 .2 .I
$5
-10
0
K ) 2 0 3 0 4 0 5 o W 7 0 TEMPERATURE %.
Figure 5. Dynamic Elastic Moduli and Loss Factors of Polyvinyl Chloride Compounds (38) A 20 %, B 38%, C 40% plasticizer
,
The various methods which are being used a t the present time to evaluate the flow properties of thermosetting resins are all of this type. The irreversible change of the flow properties of thermosetting material during the application of heat introduces tremendous experimental difficulties which so far have prevented a real rheological study of these materials. The simplest type of viscous flow is one which can be described completely by one constant, the viscosity coefficient (see curve 1, Figure 6 ) . This means that the rate of shear, at a given temperature, is a linear function of the shearing stress. Systems which obey this relationship are called Newtonian, whereas those systems which exhibit a more complicated relationship between rate
I
646
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 7.
Parallel Plate Type of Plastometer (14)
of shear and shearing stress are in general described as non-Newtonian. For example, curve 2 of Figure 6 is the flow curve which follows from Eyring’s theory (see Equation 6), and curve 3 is generally described as a Bingham-type flow curve. Only in the case of Newtonian systems are the flow properties given by a single measurement of rate of shear as a function of shearing stress (or vice versa) ; for all other systems it is necessary to determine the one quantity as a function of the other at various points along the flow curve in order to obtain a reasonably complete picture of the rheological properties. The complete evaluation of the flow properties is further complicated by the fact that a t constant shearing force the rate of shear can be a function of the time. A complete flow diagram should, therefore, be drawn in three dimensions; shearing force-time-rate of shear, as was first suggested by Umstatter (48). This property is usually described as thixotropy or rheopexp, following a terminology introduced by Freundlich ( S 4 ) . I n the former the rate of shear decreases, in the latter it increases with time. -4lthough these phenomena have been studied extensively for various colloidal systems-such as paints, printing inks, and paper coating compositions-and their importance in the processing of these materials has long been recognized, this has not been the case in the rheological study of plastics. This is due to the fact that most of the methods used to measure rheological properties of plastics do not lend themselves to an evaluation of time-dependent flow properties. The following techniques have been employed t o measure the rheological properties of high-polymeric systems:
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of the most widely used instruments in the plastics industry. Figure 7 shows an instrument of this type (which has been dcscribed by Dienes, 14). The deformation (or rate of deformation) is observed under a given load as a function of time. Figure 8 shows the results of a typical experimental run with this instrument. If one assumes that the linear portion of this curve is due to viscous flow, it is possible to calculate a viscosity coefficient independent of the instrument constants, provided the total deformation is small compared to the height of the sample. (The importance of carefully separating elastic deformation from viscous deformation has already been mentioned.) The calculation of rate of shear and shearing stress is considerably more involved and is a t best only approximate (28). Because the rate of deformation is slow, only a small part of the flow diagram can be investigated in experiments of this type. Time-dependent flow properties cannot be evaluated properly by this technique, although they might affect the experimental results. The capillary type of viscometer has been widely used in the rubber industry (15); however, its application in the plastics industry has been rather limited so far. The Rossi-Peakes instrument is of this type and procedure for its use with thermoplastics has been standardized by the A.S.T.M. (26). Although this instrument originally was designed for use with thermosetting plastics, its application to such materials has not been standardized as yet. Attempts have been made t o utilize this instrument for obtaining basic rheological data, but the results have not been very satisfactory and the instrument does not appear suitable for measurements of this type. At present it is regarded in the industry as an empirical tool. Figure 9 shows a n instrument developed by one of the authors (36) which has been used to study the flow properties of thermoplastics, and Figure 10 gives some typical results obtained with this instrument. Here, as in other capillary instruments, the efflux rate is measured as a function of the applied pressure. Provided the diameter of the capillary is very small compared t o its length, shearing stress and rate of shear can be calculated (1, 2) from the measured quantities and the instrument constants. I n many extrusion-type plastometers this condition is not fulfilled and the calculation of material constants then is not justified. Furthermore, for systems which are capable of highly elastic deformation, the measurements obtained in an extrusion-type plastometer are not necessarily due to viscous flow alone but might well be a mixture of both types of deformations. The evaluation of thixotropic properties cannot be carried out satisfactorily with this type of instrument. The falling ball viscometer has been used extensively in the plastics industry, particularly for the measurement of viscous high-
1. Parallel plate plastometers 2. Capillary viscometers 3. Falling ball viscometers 4. Rotational viscometers 5. Creep measurements (torsion or tension) 6. Penetrometer type viscometers,, 7. Molding tests under “practical conditions The techniques listed under 1, 5, and 6 can be applied only t o materials having viscosity coefficients not lower than about lo4 to 106 poises; on the other hand, capillary and rotational viscometers are not practicable for systems having viscosities higher than about lo’ poises and the falling ball type of viscometer covers a range from about 10-1 to 106 poises. The parallel plate type of plastometer has been, until now, one
Figure 8. Deformation-Time Curve for Polyethylene at 115” C. (14) , Parallel plate plastometer
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1948
EOURDON GAGE TERMOMETER or THfRYOCOUPLE
.
NTROL VALVE *PRESSURE
SYSTEM
HERMOREOULATOR
IMMERSION HEATER
Figure 9.
.
Monsanto Rheometer (36)
polymer solutions. From a rheological point of view this type of instrument is, however, particularly unsatisfactory because it does not permit the evaluation of relationships between shearing stress and rate of shear. The rotational viscometer, particularly the type employing concentric cylinders where the clearance between the cylinders is small compared to their length, is probably the most useful instrument for the study of rheological properties. The calculation of the rate of shear and shearing stress from the measured quantities-shearing torque and radial velocity of the moving cylinderis straightforward for Newtonian and non-Newtonian liquids. I t s usefulness goes beyond the capillary type of instruments because time-dependent flow properties can be studied conveniently with this method. Rotational viscometers of the Stormer type have been used by various investigators (19, 10) to study the flow properties of concentrated high-polymer solutions. Several years ago Mooney ($4) described a n instrument of this type for measuring the flow properties of rubber (see Figure 11). However, not much further work was done with this instrument, probably because “it is much too slow for factory control work.” For this reason Mooney developed a simpler type of rotational instrument, the shearing disk viscometer (S2), which has been used extensively during the last few years in the development of natural and synthetic rubber compounds. Nevertheless, from a rheological point of view the former instrument is a much more valuable tool. Creep measurements and penetrometer-type viscometers have so far found little application in the plastics industry. Wiley (60) used creep data to determine the flow properties of polystyrene, and extensive measurements on plasticized cellulose derivatives were made by Eley using this method. A penetrometer-type instrument has recently been used by Fox and Flory (25) to study the “melt” viscosities of polystyrenes of various molecular weights. For both types of measurements it is possible, under proper experimental ’ conditions, to calculate the shearing stress and the rate of shear, whereas it is difficult to evaluate time-dependent flow properties. “Practical” molding tests have been employed extensively, particularly in the control testing of thermosetting plastics. Burns (10)describes a test of this type in which some elements of the capillary-type viscometer are also incorporated. His method yields a “plasticity index’’ which, though empirical, has been found of value in controlling the molding properties of phenoplasts. The test is not capable ofrresolving the rheological variables mentioned previously in this discussion, however. Debing and Silberkraus ( I S ) describe other molding tests used to evaluate flow of phenoplasts but likewise incapable of resolving rheological characteristics. A detailed discussion of the rheological data of high-polymer systems goes beyond the scope of this paper, but it seems justified to emphasize the wide range of properties which these systems exhibit. Very dilute solutions of high polymers usually behave like Newtonian liquids. Depending on the size and shape of
647
the high-polymer molecules, orientation at high rates of shear leads to a decrease in the viscosity coefficient. Those systems which are susceptible to gelation even at low concentration frequently will exhibit some time-dependent flow properties and transient highly elastic deformation. As the concentration increases, the over-all viscosity coefficient increases (see, for example, the recent paper by Spencer and Williams, .@, on the concentration, temperature, and molecular weight dependence of viscosity for concentrated solutions of polystyrene). At the same time the systems exhibit more and more non-Newtonian and thixotropic flow properties and the use of a single viscosity coefficient loses its significance. At still higher concentrations the systems resemble the solid polymer ($1) where a large part of the deformation is due to rubberlike elasticity which is superimposed on the already complex flow behavior. RELATION OF FLOW PROPERTIES TO PROCESSING CHARACTERISTICS
The preceding sections have dealt with the various deformation mechanisms of plastics and methods which measure primarily the irreversible or viscous flow, in order to bring the subject; under discussion into proper focus. Any attempt to correlate processing behavior and rheological properties is bound to fail unless one recognizes the complex character of plastics during most processing operations and the importance of obtaining rheological data which are characteristic of the system and do not depend on the particular instrument used. At various times investigators have tried to overcome the inherent difficulties of this problem by developing instruments t h a t subject the system to deformations which resemble very closely the ones it encounters in the actual processing operation. This approach is subject to several disadvantages: I n many cases it does not yield more information than can be obtained from a careful examination of the process itself. Any correlation t h a t might be obtained is usually applicable only to the particular processing equipment, and its usefulness is, therefore, limited, although it might be very helpful in controlling one specific process. A more reasonable approach to the problem would be t o study the flow and deformation properties in a very thorough manner and to measure important processing qualities and establish empirical relationships between the two sets of quantities.
PRESSURE-PSI.
Figure 10. Typical Flow Curve Obtained with Capillary Viscometer (36)
INDUSTRIAL AND ENGINEERING CHEMISTRY
648
Vol. 40, No. 4
long as mixing or dispersing is accomplished by mechanical means it seems reasonable to assume that the effectiveness of the operation is related to the work done on the material. However, the power consumed is directly proportional to tho viscosity of the material, provided the major deformation process in rolling is due to flow. The question then arises: What are the Bow properties of the material as it passes between the rolls? In order to answer this question one must know the shearing stress or the rates of shear between the rolls. Making a number of simplifying assumptions-no slippage, deformation considered as a homogenous compression and entirely due to viscous and not elastic deformation-Eley ( 1 7 ) has calculat,ed the roll pressure which is equivalent t o the vertical compression stress and is given by the following equation: (i
(8)
= 4v4
where 17 = viscosity coefficient and 4 = rate of compression of the plastic a t any point on t,he arc of contact. 4 in turn is given by the following equation: Figure 11.
4
Rotational Viscometer ( 3 4 )
2V tan ____h2
y
where T h a t such an approach is possible was recently shown in a very interesting paper by Mooney (SS),in which he discussed the relationship between certain rheological properties and processing defects of raw rubber, such as shrinkage and surface roughness of calendered or extruded materials. I n the plastics industry this problem has not been given the attention it rightly deserves and the following discussion must, therefore, by necessity be almost entirely of a qualitative nature. At best it can do no more than stimulate more active research in this field. In the following is discussed the relationship between flow properties and processing behavior in the rolling, calendering, extrusion, and molding operations. The mass polymerization process is not incldded in this discussion. The fact that the over-all viscosity increases continuously during polymerization is important as it affects the uniformity of the product but the specific rheological propert.ies of the systems are of no particular significance here. I n the casting process the detailed rheological properties are not important, although a careful control of the over-all consistency is essential. The primary functions of the rolling (milling) operation are to adjust the composition of the mass as desired; to obtain as homogeneous a mass as possible, because in most subsequent production operations there cxists no possibility to affect further homogenization; and to impart to the material certain deformation characteristics which will prevent or minimize such processing defects in the product as shrinkage, strain marks, etc. As
l2,OOO
Hd 6
4,000
0
tI
EXIT PLANE
Figure 12.
‘I
2
3
Y,CM.
4
tI
ENTRANCE PLANE
Curve of Vertical Roll Pressure over the Arc of Contact (Z7)
y =
V
angular coordinate on the arc of contact
= volume rate of flow of plastic per unit width of roll
h = height of plastic a t any point
According to Eley and Pepper (f8),the viscous flow of plasticized cellulose derivatives at high stress can be expressed by the following equation: 17
A(f - f a )
(10)
where A is a function of temperature, f is the applied compressive stress, and fo is a constant which according to the authors can be neglected. Using a value of 3.2 X lo7 poises for the viscosity, they obt,ained the results shown in Figure 12 for the roll pressure between the entrance and exit plane of the rolls. The average calculated value is 5200 pounds per square inch. An experimental determination gave values of 2300 and 4300 pounds per square inch. The discrepancies between calculated and measured quantities are not bad considering the various approximations made in the calculation. Considerably better agreement might be obtained if one considers the possibi1it.y of thixotropic breakdown during working, which might easily lower the viscosity coefficient to be used in Equation 7 by a factor of 2 or more. The power consumption during rolling (or ot’her processing operations) usually is not too important in itself but, because a large part of the power is always converted into heat and, therefore, increases t,he temperature of the material, it is important as it might lead to local ‘or over-all degradation of the polymer molecule. For viscous deformation processes the power consumption is directly proportional to t,he effective viscojity, and this has been confirmed by Vila (4.9) for the power consumption in t,he exbrusion of rubber. The same should hold for rolling of plastics, The viscosity range of rubber during ext,rusion is probably between lo4 and 106 poises. The condition of the material a t the end of the rolling prbcess is of considerable importance to the subsequent processing operations (baking, sheeting, and seasoning), particularly if the variables (such as temperat,ure, pressure, and time) in these operations are held constant. If the stock is too “soft,” excessive amounts of materials might be lost during the baking operation, sheeting might be difficult, and excessive shrinkage during seasoning might take place. If the stock is too “hard,” lLweldlines” might be formed during baking, delicate colom might be damaged, and air pockets might remain. It is clear that if the deformation were entirely due to viscous flow-whether Newtonian or otherwise-shrinkage would disappear entirely.
April 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
Similarly, a viscous deformation mechanism where the viscosity coefficient increases with increasing shearing or compression stress would make i t easier t o remove weld lines without giving excessive flow. However, it is well known that the deformation of rolled stock is to a very large degree of a highly elastic nature, and the best t h a t can be accomplished is to reduce the contribution of the highly elastic deformation t o the total deformation process. No quantitative data are as yet available to show ho; the rolling operation determines the rheological properties of the rolled stock. The few data available indicate t h a t cellulose acetate and nitrate plastics should have a viscosity in the range from 108 t o 108 poises at the operating temperature for satisfactory baking in the "block" or "sheeter" process. Some of the most troublesome processing difficulties in extrusion and calendering operations are concerned with shape variations during or after the forming process has been completed-e.g., gage variation and shrinkage. This phenomenon is closely related to the recovery process following a highly elastic deformation (8). Figure 13 shows the correlation obtained in one experiment between calender shrinkage and the elastic recovery as measured in a disk viscometer (data taken from Mooney (88). The amount of elastic recovery, however, depends on a great many factors and a fixed type of recovery experiment will not always give as good a correlation a8 shown in Figure 13. For example, the rate of deformation has a marked effect on the elastic recovery. Preliminary experiments with polystyrene in a rotational viscometer have shown t h a t for the same total deformation the elastic recovery increases strongly with the rate of deformation (see Figure 4). Furthermore, temperature changes and changes in composition during the processing operation have a marked effect. If the system during processing is capable of viscous flow, orientation and subsequent elastic recovery will be reduced. Experiments by Mueller (86)on styroflex films show this very clearly. At 100" C. (slightly above the heat distortion point) a strain of 20y0 is necessary t o produce a certain double refraction (which is a measure of chain orientation), whereas at 150" C. i t is necessary to strain the material 70% to obtain the samp results. At the higher temperature a large part of the deformation is due t o irreversible flow; equal strains would therefore produce a smaller elastic recovery at 150" C. than at 120" C. The dependence of the elastic recovery on temperature is very well demonstrated in some other experiments carried out by Mueller o n styroflex. Figure 14 shows the logarithkic decrease of relative double refraction as a function of time for various temperatures. The striking differences in the change of double refraction below and above the second-order transition point are a direct consequence of the strong changes in the rheological properties at these temperatures. The nonlinear decay of the double refraction has been interpreted by Mueller t o be due t o the nonNewtonian flow behavior of polystyrene. The rate of extrusion is another very important processing quantity. I n the plastics industry a great variety of extruders a r e in use which, from a rheological point of view, can be separated into two distinctly different groups: (1) The material is forced through an orifice only, which might have a circular or rectangular shape or might be a long and narrow slit; (2) the material is first carried past a "torpedo" or a spreader, as in the injection molding process, and then passes through a suitable orifice. In the first case one might raise the question: T o what extent is the rate of extrusion through a short orifice due to viscous deformation? The rate of extrusion in most industrial machines is fairly high and the material uridergoes deformation during a very short period of time. Even at fairly high temperatures most polymers retain some highly elastic deformability which might play a n important role in the extrusion through orifices. The authors have found that the application of a small stress on the extruded strand increases considerably the rate of extrusion of polystyrene at 160' C. through a circular orifice;
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II 12 13 RECOVERY AT I MIN., ARBITRARY
10
Figure 13.
14
I
15
UNITS
Calender Shrinkage US. Elastic Recovery in Disk Viscometer (33)
it also produces a very noticeable double refraction in the extruded strand. Furthermore, the fact t h a t the extruded strand usually has a considerably wider diameter than the orificeunless i t is held under tension-indicates t h a t a large part of the flow in this type of extrusion is due to elastic deformation. The second method of extrusion is part of all modern injection molding machines and has, therefore, become of great interest to the plastics industry within recent years. Here the material passes between two concentric cylinders and the clearance between the cylinders is small compared t o length. It seems clear that in this case the deformation near the orifice is predominantly viscous, particularly in those regions where the temperature of the material is close t o the full operating temperature of the injection molding machine. Because of the non-Newtonian flow behavior of most thermoplastic materials, the viscosity coefficient is a function of the rate of dkformation. Accordingly, one should expect that the rate of extrusion as observed in this process would not correlate with a viscosity coefficient determination obtained from a single point measurement but t h a t it must be related t o t h e differential viscosity coefficient determined from the slope of the D-T curve (see Figure 10) in the appropriate range.
10
20
--IN.
0
30 '
10
40
15
2OMlN
b
Figure 14.
Change of Double Refraction with Time at Various Temperatures Polystyrene filmrr (35)
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not only possible changes in the composition of the adhesive during processing but also the fact t'hat the forces necessary to effect separation depend markedly on thc velocity or rate of deformation. For Kewt,onian system this force is directly proportional to the velocity. However, most. adhesive materials are non-Newt,onian and t,he shearing forces, therefore, vary in a more. complicated manner with the rate of shear. Usually, the rate of shear is high because of the small injt8ial distance bct,ween the shearing forces, even though t,he velocity of deformation might be small. The coating of various types of solids, such as paper, textiles, metals, etc., is almost entirely a rheological process. The cornpounds used in these operations might Gary over a very wide range in over-all consistency. The different methods of application (brush, roller, dip, or spray applicaTIME IN MINUTES tion) require that the system have certain Figure 15. Time-Dependent Flow Properties of Polystyrene ( 9 ) characteristic rheological properties. For example, the highly thixotropic and non-NewMeasurements made by the standard A.S.T.M. met,hod (6) t,onian properties of a paper coating composition make it possible to evaluate the moldability of thermoplastics frequently give t'o use a roller application a t very high speeds, although the same completely erroneous results, whereas measurements such as system could hardly be applied with a brush. On the other have been obtained by one of the authors (36) usually give a fair hand, the brushability of paints is very closely related t'o the thixotropic properties of pigmented syst'ems a t very low rat,es of corrclat,ion between rate of extrusion and the position of the flow curve in the D-T diagram. Similarly, Rlooney (33) has shear or shearing forces. found a reasonable correlation between rate of extrusion for Obviously, elast,ic and viscous deformation and flow are invarious rubber stocks and the relative stress increment measured volved in many operations commonly employed for the fabricaa t 8 and 2 r.p.m. with the shearing disk viscometer. For tion of plastic materials-e.g., bloring, swagging, and forming. No quantitative data are available for such procedures, ho\T-evcr. certain materials, however, it, might be necessary to take also the time-dependent flow properties into account.. By using The processing of plastics is a field in which chemical enginccra rotational viscometer, the authors have found that the rate of ing operations are of paramount importance but t'o which the shear increases vcry strongly with time, as shown, for example, in science has not yet devoted its full repertory of talents. Since rheological changes are involved in practically every processing Figure 15. :knother factor nhich has to be carefully considered is the operation, and most usually under conditions of temperature, pressure, and shear differing markedly from those usually ern-. temperature variation of the stock and its effect on the rheoployed for laboratory studies, there are both a need and an logical properties. As the stock moves through the extruder part of t,he injection molding machine, it,s temperature is conopportunity for the development' of' basic rheological dat'a and. stantly increasing and the viscosity, therefore, decreases expofor the use of this in the improvement of t'he present operationsand in the devising o h e w and better ones. nentially. The. available experiment,al data on t,he temperature dependence of the viscosity coefficient'show very marked differI n all these processes, some progress has been made during. the past few years in correlat'ing the processing characterist'ics ences between different high polymers. The addition of side and difficulties with the rheological propert'ies. I t is obvious, groups, for example, seems to increase the activation energy of viscous flow from a value of about 8 to 10 k.-cal. to possibly 50 however, that a great' deal more work will have t o be done before, we have a clear understanding of these problems.in terms of' to 60 k.-cal. Furthermore, bhis energy also depends in some cases on the rate of shear or shearing force. At molding temmeasurable quantities which clearly characterize the system. under investigation. The benefits which can be derived from peratures polystyrene has a viscosity value of between lo4 and such a knowledge should be well worth the effort necessary t'o 106 poises. When the stock is forced int'o the mold the pressure increases achieve this goal. rapidly and the stock temperat'ure decreases. The deformation LITERATURE CITED characteristics undergo a rapid change accordingly. Whereas a large part of the flow in the mold is cert'ainly of a viscous nature, (1) scad. Sei, Amsterdam, corn. for Study of viscosity, "First the last steps might frequently be due to highly elastic deformaReport on Viscosity and Plasticity," 2nd ed., New York,. Nordemann Publishing Co., 1939. tion. The extent to which the latter deformations are incorpo(2) Ibid*gSecond Report* rated in a molded article greatly affects its dimensional stability a t (3) blexandrov, A. P., and Lasurkin, J. S., Acta Ph~/sicochim.. various temperatures and its ultimatq mechanical properties. U.R.S.i3., 12, 647 (1940). .4s more and more synthetic resins are being used in adhesives (4) Alfrey, T.,J . Chem. Phys., 12,374 (1944). (5) Am. Soc. Testing Materials, Designation D469-44T. and surface coatings, the processing problems connected with 'pee. D731-44T* Am* these operations become of increasing interest to the plastics (7) Bikerman, J. J., J . Colloid Sci., 2, 163 (1947). industry. It is we11 recognized now, for example, t h a t the (8) B ~L., T ~ ~Faraday ~ ~ ~sot., ~ 41, ~ ~81 ,(1945). . tackiness of adhesives is closely related to their rheological proper(9) Buchdahl, R., Monsanto Chemical Co., unpublished data. (10) Burns R., Proc. Am. SOC.Testing iMateriaZs, 40, 1283-8 (1940). ties ( 7 ) . I n most cases tackiness is still being evaluated by sub(11) Busse, W. F., Lambert, J. M., and Verdery, R. B., J. Applied! jective tests, although several attempts have been made to measPhus., 17, 376 (1946). ure t'his quant'ity (11, 26). In order to correlate the performance (12) Carver, E.K., and \Tan Waser, J. R., J.Phys. Colloid Chem., 51,. of adhesives n-ith rheological measurements, one must consider 751 (1947).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
(13) Debing, L. M., and Silberkraus, S. H., IND.ENQ. CHEM.,33, 972-5 (1941). (14) Dienes, J., and Klemm, H. F., J . Applied Phys., 17,458 (1946). (15) Dillon, J. H., and Johnston, N., Physics, 4, 225 (1933). (16) Eirich, F., Bunze, M., and Margaretta, H., Kolloid-Z., 74, 376 (1936). (17) Eley, D. D., J . Polymer Sci., 1, 529 (1946). (18) Eley, D. D., and Pepper, D. C., Nature, 154,52 (1944). (19) Ferry, J. D., J. Am. Chem. SOC.,64,1323 (1942). (20) Ibid., p. 1330. (21) Ferry, J. D., “Mechanical Properties of Concentrated Solutions (22) (23) (24) (25) ‘(26) (27)
of High Polymers,” Advancing Fronts in Chemistry, Vol. I, p. 153, New York, Reinhold Publishing Corp., 1945. Flory, P. J., J . Am. Chem. SOC.,62, 3032 (1940). Fox, T. G., Jr., and Flory, P. J., Division of Rubber Chemistry, 111th Meeting of AM.CHEM.SOC., Atlantic City, N. J. Freundlich, H., Actualitls sci. ind., No. 267 (1935). Glasstone, S.,Laidler, K. J., and Eyring, H., “Theory of Rate Processes,” New York, McGraw-Hill Book Co., 1941. Green, IND. ENG.CHEM.,ANAL.ED., 13,632 (1941). Guth, E., James, H. M., and Mark, H., “Advances in Colloid Science,” Vol. 11, p. 253, New York, Interscience Publishers,
1946. (28) Houwink, R., “Physikalische Eigenschaffen und Feinbau von
Natur und Kunstharzen,” Leipzig, Akademische Verlagsgesellschaft, 1934. (29) Kuhn, W., Helv. Chim. Acta, 30,307 (1947).
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(80) Kuhn, W., 2. physik. ehem., B42;l (1939). (31) Mark, H., “Physical Chemistry of High Polymers,” New York, Interscience Publishers, 1941. (32) Mooney, M., IND.ENQ.CHEM.,ANAL.ED., 6, 147 (1934). (33) Mooney, M., J . Colloid Sci., 2, 69 (1947). (34) Mooney, M., Physics, 7, 73 (1936). (35) Mueller, F. H . , Wiss. Verofentl. Siemens-Werken, 19,110 (1940) (36) Nason, H. K., J . Applied Phys., 16,338 (1945). (37) Phillippoff, W., “Viskositat der Kolloide,” Ann Arbor, Mich., Edwards Bros:, 1944. (38) Sack, H. S.,Motz, J., Raub, H. L., and Work, R. N., J . Applied Phys., 18,450 (1947). (39) Sheppard, S . E., Carver, E. K., and Sweet, S. S., IND.ENG. CHEM.,18, 76 (1926). (40) Simha, R., J . Phgs. Chem., 47,348 (1943). (41) Smallwood, H. S., J . Applied Phys., 8,505 (1937). (42) Spencer, R. S., and Boyer, R. F., Ibid., 16, 594 (1945). (43) Spencer, R. S., and Williams, J. L., J . Colloid Sci., 2, 117 (1947). (44) Taylor, N. W., J . Applied Phys., 12, 753 (1941). (46) Tobolsky, A. V., and Andrews, R. D., J. Chem. Phys., 13, 3 (1945). (46) Tuckett, R. F., Trans. Faraday SOC.,38, 310 (1942). (47) Ibid., 39, 158 (1943). (48) Umstiltter, H., Kolloid-Z., 70, 285 (1935). (49) Vila, G. R., IND. ENQ.CHEM.,36, 1113 (1944). (50) Wiley, F. E., Ibid., 33. 1377 (1941). RECEIVED December 29, 1947.
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Stimulation of Plastics Technology by German Disclosures John M. DeBell and Henry M. Richardson DeBell and Richardson, Inc., SpringJield, Mass. T h e paper reviews the significant contributions to plastics technology that have come from Germany and discusses adaptations and improvements that have been made in the United States.
T
HE Quartermaster consultants (W. E. Gloor, W. C. Goggin, and J. M. DBBell) who investigated the German plastics industry in 1945 found a number of practices which seemed t o represent substantial advances in technology (18). This paper reviews such American adoptions and improvements of these practices as have come to attention. The techniques and products in question often resulted from earlier fundamental work of British and American scientists; they were frequently dictated by the war economy of the Germans; and this brief catalog can cover only a small fraction of the extensive Ameripan developments which have been stimulated by the disclosures. All in all, the investigators felt t h a t the total accomplishments of the American plastics industry during the war outweighed those of any other country; but credit must nevertheless be paid to the very significant contributions made in the same period in Germany. TECHNOLOGICAL
I n the cellulosic field, all the German manufacturers of cellulose acetate used, for the regeneration of their acetic anhydride, the of vapor phase cracking of glacial aceticacid Wackerprocess (28,36) t o ketene in the presence of triethyl phosphate catalyst. American rights to this process for some time have been in the hands of the Tennessee Eastman Corporation, a large acetate producer. The demonstrated economy of the process: however, has led to serious consideration of licensing by other large manufacturers of cellulose acetate, whether for rayon or plastic. Carboxymethylcellulose, whose German manufacture was observed by one of the authors in Germany in 1934, has blossomed
broadly as a water-soluble thickener, largely displacing its predecessor, methylcellulose. Originally, the sodium salt was used for textile size and in special small cases where highly viscous, aqueous solutions were desired. The Germans had made widespread application as a n aid to soaps and detergents, permitting substantial reductions in soap consumption-an important consideration when fats were scarce. At least three American companiesDow Chemical Company, E. I. du Pont de Nemours & Company, and Hercules Powder Company-have undertaken large-scale production of this product, which has found principal use in improving the characteristics of drilling muds in oil wells under fresh water and salt water systems. Small amounts have been in use in this country in formulations of both fatless and ordinary soaps, and wider application as an aid to sulfonated detergents is under concentrated study. It is also used in the thickening of foods, in textile printing paskes, in paper coating for better crease resistance and wet strength, as a textile finish, in waste paper de-inking, as a primer t o reduce absorption of coating wax, as a deflocculating agent for pulp slurries, as a stabilizer for various emulsions including latex, and as a thickener for emulsion types of adhesives and printing inks. The customary vigor of these companies has resulted in improvement in the product, both in clarity and smoothness of solution, over the German material. Continuous downflow bulk polymerization of styrene has been studied by American producers, but it does not appear substantially to have affected American methods, which include plate polymerization and the suspension polymer of the Koppers Co., which recently appeared on the market. The later vinyl chloride plants in Germany utilized continuous emulsion polymerization. This has been applied in the United States, and additional full-scale units are now building. The use of polyvinyl chloride pastes-Le., a viscous, flowable suspension
