Analytical Specialties . : : . ANALYZER THEORY . : : .

Plastometers are utilized to study melt flow behavior (plastic behavior) of plastic materials or the molecular weight distribution of polymers.

A number of materials do not begin to flow unless the stress has exceeded their yield value. Such materials behave as elastic bodies with shears below the yield value, but will yield increasingly with time under greater stresses. This phenomenon is called plastic behavior. Bingham assumed that for some materials the rate of flow was zero for stresses below a critical value and was linear with stresses above this value. Such a body is called a Bingham solid, or ideal plastic. Extremely few solids show this behavior. In general, the yield value is not sharp and shows nonlinear behavior with a delayed and imperfect recovery. The plastic behavior differs for different types of plastic solids and at different temperatures.

For this reason, the plastometer gives a relative indication of polymer behavior, thus characterizing a plastic material. Since the shear stress/shear rate relation is based on the molecular behavior of materials, the plasticity can be related to the molecular weight of a material or to its distribution. The plasticity of plastic materials is expressed in arbitrary units such as ASTM melt index (MI), or Canadian Industries Limited (CIL) flow index, or Mooney points, or percent of full scale.

In general, the plastometer consists of a constant temperature sample body heating chamber, a mechanism to apply high shear force, and a device to measure torque or flow rate of material in detecting the plasticity of the sample.

It can be used in a laboratory to study polymeric behavior with time under high shear stress or may be used on-line to control the process manually or automatically. This type of rheometer is an invaluable tool in manufacturing plastics and synthetic rubbers.

Cone-and-Plate Type

This plastometer incorporates the features of the Mooney plastometer and is designed to meet the requirements of ASTM standard test method D-1646. Basically, the working principle is the same as the one discussed in connection with cone-and-plate viscometers. It is designed to eliminate polymer 'ball-up" and slippage tendencies by confining the sample in a disc-shaped cavity. The cone-and-plate plastometer is successfully applied to conduct tests for the evaluation of crude rubber, rubber compounds or reclaims, control of mill breakdown of polymer molecules, determination of time to scorch, calculation of optimum cure time and the evaluation of the processing characteristics of plastics.

In operation, the sample is placed in the cylindrical test chamber and is allowed to reach the predetermined test temperature (up to 400°F, or 204°C) by the use of integral heaters. Machined serrations on all of the die and rotor surfaces prevent slippage. After a warm-up period, the rotor is driven at a constant speed (normally at 2 RPM) or at various speeds from 0.05 to 2 RPM with a continuously variable speed drive to test for relative molecular weight or to study plastic behavior. The shearing action which takes place between the rotor and the die cavity is measured by the deflection of a calibrated U-spring attached to the torque sensing rotor. The deflection of the U-spring is read from a dial indicator and is directly proportional to the shearing torque of the specimen being tested. An electronic strain gauge may be attached to the U-spring to transmit the signal for continuous recording.

This type of plastometer is equally suitable for both laboratory work and on-line indication or control of highly viscous flowing materials. The chief advantage of this instrument is that it measures plastic behavior or melt viscosity of plastic materials under very similar conditions to those prevailing in processing equipment. This instrument is widely used for plastics and rubber, food, pigment, cement, paint, cosmetics, and for coating products. There are many different shapes of measuring heads available to accommodate wide ranges of viscosities.

In operation, the sample is kept at a constant temperature (up to 570°F, or 299°C) by the jacketed heater. The kneader is driven by a dynamometer (available with variable speed as well as fixed speed drives). Resistance encountered by the mixing blades in the material under test is transmitted to the dynamometer housing which tends to rotate in a direction opposite to that of the driving shaft. The torque is transmitted on a direct reading balance system through levers and is recorded. The pen movement can be transmitted to a controller for automatic control of processes involving medium viscosity, free flowing fluids.

The reading and recording are on a 1000-unit division scale, which is arbitrary for indicating shearing torque of the specimen being tested. Repeatability is + 1% of full scale. By the application of different weights on the scale system, the measuring range can be varied.

This is designed for use in polymer manufacturing plants and is based on the capillary-tube viscometer principle. The instrument is calibrated to record either ASTM melt index (flow rate of polyethylene through an open ended capillary at 190°C [374°F) and 43.2 P510 [298 kPa] pressure) or CIL flow index (190°C and 1500 P510 [10 Mpa] for polypropylene) or it can record both by alternating from one to the other. These two measurements together can be interpreted as a molecular weight parameter and as a molecular weight distribution parameter for a particular polymer. The capillary-type plastometer is not as versatile as those previously covered.

The advantage of this instrument is that it can be used as an automatic on-line process control device for both viscous fluids and plastic solids. Especially for the solution polymerization processes, the polymer solution can be directly analyzed in the plastometer from the processing reactors through an auto-sampling device which flashes the solvent and the unreacted monomers, and melts the polymer before feeding it to the plastometer. For plastic solids, the die unit (capillary) can be mounted to the pelletizing extruder of the process stream for measurement. The capillary type plastometer is ideally suited for study of plastic behavior of those materials that are processed through injection molding or finishing operations. In feeding the capillary type plastometer, the polymer melt is first conditioned to be at some specific temperature and pressure. It is then extruded through a capillary of suitable dimensions. The rate of flow of the polymer through the capillary is measured (the output of DC generator tachometer on the metering pump) and recorded in units appropriate for the test.

The operating range of this plastometer is 0 to 200 MI and 0 to 100 CIL units with repeatability within +2% of full scale. The unit is designed to operate at up to 570CF (299°C) and up to 5000 P510 (35 MPa) in pressure. Erratic results may arise from the same causes that have been pointed out in connection with the capillary-type viscometers.