ASTM D5422-17

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Designation: D5422 − 09 D5422 − 17
Standard Test Method for
Measurement of Properties of Thermoplastic Materials by
Screw-Extrusion Capillary Rheometer
This standard is issued under the fixed designation D5422; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
This test method uses capillary rheometry to measure the rheological properties of thermoplastics
and thermoplastic compounds. This test method utilizes a screw-extrusion-type capillary rheometer.
1. Scope*
1.1 This test method covers the use of a screw-extrusion-type capillary rheometer for the measurement of flow properties of
thermoplastics and thermoplastic compounds. The measured flow properties, which are obtained through laboratory investigation,
may help to describe the material behavior that occurs in factory processing.
1.2 Since a screw-type capillary rheometer imparts shear energy to the material during testing, the measurements will usually
differ from those obtained with a piston-type capillary rheometer (see Test Method D3835).
1.3 Capillary rheometer measurements for thermoplastics and thermoplastic compounds are described in Test Method D3835.
1.4 The values stated in SI units are to be regarded as standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
NOTE 1—There is no known ISO equivalent to this standard. The scope of this standard is mentioned in ISO 11443:2014, Plastics—Determination of
the fluidity of plastics using capillary and slit-die rheometers,” Annex C, “Uncertainties in the determination of shear viscosity by capillary extrusion.”
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D618 Practice for Conditioning Plastics for Testing
D883 Terminology Relating to Plastics
D1238 Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer
D3835 Test Method for Determination of Properties of Polymeric Materials by Means of a Capillary Rheometer
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
3. Terminology
3.1 Definitions: (See Terminology D883):
3.1.1 apparent shear rate (γ˙ ) —)—shear strain rate (or velocity gradient) of the thermoplastic or thermoplastic compound
a
extrudate as it passes through the capillary die.
This test method is under the jurisdiction of ASTM Committee D20 on Plastics and is the direct responsibility of Subcommittee D20.30 on Thermal Properties (Section
D20.30.08).
Current edition approved April 1, 2009Dec. 1, 2017. Published April 2009January 2018. Originally approved in 1993. Last previous edition approved in 20032009 as
D5422 – 03.D5422 – 09. DOI: 10.1520/D5422-09.10.1520/D5422-17.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D5422 − 17
3.1.1.1 Discussion—
This velocity gradient is not uniform through the cross-section of the capillary die. The shear rate is calculated for the region of
highest shear, which is at the wall of the capillary. By selecting a die diameter and controlling the volume flow per unit time
through the die, a specific level of apparent shear rate is achieved. Alternately, the shear stress (entrance pressure) is controlled,
and the apparent shear rate measured.
3.1.1.2 Discussion—
Mathematically, the apparent shear rate at the wall of the capillary for a Newtonian fluid at the capillary wall is given by the
following:
32·Q
˙γ 5 (1)
a 3
π·D
where:
−1
γ˙ = apparent shear rate, s ,
a
Q = quantity of fluid extruded per time, mm /s,
π = 3.142, and
D = diameter of the measuring capillary, mm.
3.1.2 apparent shear stress (τ ) —)—the measured resistance to the flow through a capillary die. It may be determined by
a
measuring the die entrance pressure for a specific die, then applying appropriate geometric factors.
3.1.2.1 Discussion—
Mathematically, apparent shear stress is given by the following:
P
τ 5 (2)
a
4· L/D
~ !
where:
τ = apparent shear stress, Pa,
a
P = pressure at the entrance of the measuring capillary, Pa,
L = length of the measuring capillary, mm, and
D = diameter of the measuring capillary, mm.
3.1.3 apparent viscosity (η ) —)—ratio of apparent shear stress to apparent shear rate, Pa·s.
a
3.1.3.1 Discussion—
For an extrusion capillary rheometer, the ratio is usually calculated at a given shear rate. At constant temperature, the apparent
viscosity of most polymers is not constant, but varies with shear rate. The viscosity is generally annotated with the shear rate at
which the measurement was made.
3.1.4 capillary rheometer—an instrument in which thermoplastics or thermoplastic compounds can be forced from a reservoir
through a capillary die. The temperature, pressure entering the die, and flow rate through the die can be controlled and accurately
measured.
3.1.5 corrected shear rate (γ˙ ) —)—the actual shear rate at the wall of the capillary die determined by applying the
w
−1
Rabinowitsch correction for non-Newtonian materials, s .
3.1.5.1 Discussion—
The Rabinowitsch correction mathematically adjusts the shear-rate values to compensate for non-Newtonian behavior of the
polymer. To obtain corrected shear rate, at least two measurements of apparent shear stress and apparent shear rate are made. This
is generally accomplished by increasing the rate of extrusion (Q) while using the same measuring capillary.
3.1.5.2 Discussion—
D5422 − 17
As a first step, the Bagley correction (as stated in 3.1.6) is made to the shear-stress values. Then, either by algebraic means (if only
two measurements are made), or by a regression method (for a greater number of points), the equation in 3.1.11 is solved for n,
using the corrected shear stress (τ ).
w
3.1.5.3 Discussion—
The corrected shear rate (γ˙ ) is determined by the following:
w
3 11
n
˙γ 5 ·˙γ (3)
F G
w a
n
For most thermoplastics and thermoplastic compounds, the magnitude of shear sensitivity (n) will vary, depending on ma-
terial composition.
3.1.6 corrected shear stress (τ ) —)—the actual shear stress at the wall of the capillary die, Pa. The corrected shear stress is
W
obtained by applying the Bagley Correction Factor (E) to the apparent shear stress (see 3.1.7.1 and 3.1.7.2). The Bagley correction
compensates for energy losses at the entrance and exit of the die.
3.1.6.1 Discussion—
This correction is often applied as though it were an additional length of capillary. The correction is often termed “end effect.”
Capillary entrance angle and geometry have great influence on the magnitude of this correction.
3.1.6.2 Discussion—
The Bagley correction will also remove the influence of any static pressure in the system that does not vary with die length.
3.1.6.3 Discussion—
Since the magnitude of correction is a function of shear rate, data for this correction are obtained by using two or more dies of
different length, but of the same diameter (and thus the same apparent shear rate, as calculated in 3.1.4.2). If the data from these
additional dies are compared, either graphically or mathematically, a linear relationship of extrusion pressure with die geometry
is usually obtained in the following form:
L
P 5 c· 1E (4)
F G
D
where:
E = the Bagley Correction Factor. (This term is often called “end effect.” It is expressed as the equivalent length of capillary
necessary to extrapolate the pressure-line value on the P versus L/D plot to zero, mm.)
c = slope of the line.
The Bagley Correction Factor (E) and the slope (c) are functions of the thermoplastic compound and the shear rate. Corrected
shear stress is therefore as follows:
P
τ 5 (5)
w
4·@~L/D!1E#
3.1.6.4 Discussion—
Each value of corrected shear stress must be annotated with the shear rate with which it is measured.
3.1.7 corrected viscosity (η)—the ratio of corrected shear stress to corrected shear rate, Pa·s.
3.1.7.1 Discussion—
Since both the material properties and the correction equations are functions of shear rate, it is very important to state the particular
value of shear rate at which each measurement is made.
3.1.7.2 Discussion—
D5422 − 17
Other corrections to measured values are often made in rheological research studies to compensate for the effects of pressure,
viscous heating, compressibility, time effects, etc. The terms “true shear stress,” “true shear rate,” and “true viscosity” are often
used for the results of such exhaustive calculations. This test method addresses only the two most important corrections, Bagley
and Rabinowitsch.
3.1.8 die entrance pressure (P)—the pressure in the reservoir at the die entrance.
3.1.9 newtonian fluid—a material for which the measurement of viscosity is not changed by changing the shear rate. Simple
liquids, such as water, are considered Newtonian whereas most polymeric materials are not.
3.1.10 power-law fluid—a material for which the viscosity varies with the shear rate in accordance with the following known
relationship:
n
τ 5 K· ˙γ (6)
~ !
a a
where:
τ = apparent shear stress, Pa,
a
−1
γ˙ = apparent shear rate, s ,
a
K = a material constant, often called “consistency index,” and
n = shear sensitivity, dimensionless.
Most non-Newtonian fluids follow this relationship for at least short ranges of the shear rate variable. The power-law equation
is generally used in its logarithmic form as follows:
log τ 5 log K 1n log ˙γ (7)
~ !
~ ! ~ ~ !!
a a
3.1.11 shear sensitivity (n)—a dimensionless material parameter, also called the “power-law index,” that represents the
magnitude of the shear sensitivity of a polymer. It is equal to 1.00 for Newtonian fluids, and generally less than 0.8 for
non-Newtonian fluids.
3.1.11.1 Discussion—
Mathematically, the shear sensitivity is given by the following:
dlog τ
~ !
w
n 5 (8)
dlog~˙γ !
a
where:
d log(τ ) = the change in log corrected shear stress over varying extrusion rates, and
w
d = the change in log apparent shear rate over varying extrusion rates.
log(γ˙ )
a
4. Summary of Test Method
4.1 The thermoplastic material is fed into a laboratory extruder, the barrel of which is equipped with a temperature control. The
output end of the extruder is equipped with a capillary die containing an insert of specified dimensions. Temperatures of the
extruder barrel and capillary die are normally kept constant. (It may be necessary to alter the die-set temperature only to
compensate for shear heating of the material at different extrusion rates.)
4.2 A suitable pressure transducer and temperature-measuring device, such as a thermocouple, are positioned in the die just
before the entrance to the insert.
4.3 The rate of material extrusion, or mass throughput (Q) is determined by collecting extrudate over a timed interval and then
weighing it. The extrusion rate is controlled by adjusting the drive speed.
4.4 In order to calculate the flow properties of the material, extrusion is performed at a minimum of two different drive speeds
through an insert of specified dimensions (Die A). Then, extrusion is performed again, at the same drive speeds, through at least
one additional die insert of different specified dimensions (Die B or Die C).
4.5 This procedure allows for the determination of apparent shear rate, apparent shear stress, apparent viscosity, corrected shear
stress, corrected shear rate, corrected viscosity, shear sensitivity, and entrance/exit effects.
5. Significance and Use
5.1 This test method is useful for the characterization of thermoplastics and thermoplastic compounds, in terms of viscosity, or
resistance to flow.
D5422 − 17
FIG. 1 Schematic of Screw-Extrusion-Type Rheometer Cross-Section
5.2 The data produced by this test method has been found useful in both quality-control testing and compound development.
However, direct correlation with factory conditions is not implied.
5.3 Flow-performance data permits quality control of incoming thermoplastics and thermoplastic compounds because the flow
parameters are sensitive to molecular weight and molecular-weight distribution. Therefore, this test method may distinguish
differences between lots.
5.4 The shear viscosity or flow viscosity of thermoplastics and thermoplastic compounds will not only be sensitive to the
raw-polymer molecular properties, but will also be affected by the type and amount of filler, additive, plasticizer, or stabilizer, by
the type of copolymer blend, and by the addition of other compounding materials. This test method can serve as a quality-control
tool for either incoming materials or for in-house quality-assurance checks on production mixing. This test method is useful to the
research and development of new products in that the rheological behavior of a yet uncharacterized thermoplastic or thermoplastic
compound can be measured and considered for comparative analysis.
6. Interferences and Precautions
6.1 Since the flow properties of non-Newtonian materials are not linear, capillary rheometers should be operated at conditions
of flow (temperature, pressure, and rate) similar to those of selected commercial processes. These processes include mixing,
calendering, molding, and extrusion of thermoplastics and thermoplastic compounds.
6.2 Screw-extrusion-type rheometers impart significant amounts of energy to the thermoplastic or thermoplastic compound
before the measurement is made. Interpretation of the data for factory operations such as production extrusion, calendering, or
injection molding is therefore more straightforward than for compression-molding operations, where factory-work input is quite
small.
6.3 Increasing the rate of extrusion will induce shear heating, and therefore may alter the temperature of the material flowing
through the capillary die. It is essential to maintain a constant melt temperature in the die in order to perform accurate viscosity
measurements. It may be necessary to compensate for shear heating by manually adjusting the die-heater set temperature.
6.4 Extruder residence time and shearing actions at a particular melt temperature will often affect a material’s viscosity. It is
recommended that con
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