Compression plastimeters

The principle of the compression plastimeter is very simple - the test piece is compressed between parallel plates under a constant force and the compressed thickness measured. This simplicity accounts for the early adoption of this type of instrument and its subsequent continued popularity. The work of Williams16 led to the first widely used parallel plate instrument and eventually to various modified forms all working on the same principle. Apart from simplicity, the compression principle has no real inherent advantages but a number of disadvantages ... 

Nevertheless, compression plastimeters have been found very useful for routine testing, particularly of uncompounded rubber, where only basically similar materials are compared. 

There are basically two forms of parallel plate compression plastimeter (a) with both compression plates much larger than the test piece (Figure 6.2(a), plate test) and (b) with one or both plates of approximately the same diameter as the test piece (Figure 6.2(b), disc test). 

In the past, parallel plate compression plastimeters have been quite widely used for measuring rate of cure and methods have been standardised. The test pieces are heated for various times and then tested in the plastimeter. The change in plasticity or recovery or some combination of these, is then plotted against time of heating to give a scorch curve. An even more time consuming procedure was to measure tensile properties as a function of cure time. 

Compression Plastimeters Plate Test and Disc Test... 

ISO 200722 specifies a rapid plastimeter procedure using an instrument with one platen either 7.3, 10 or 14 mm diameter and the other platen of larger diameter than the first (i.e. disc type method). The size of the first platen is chosen such that the measured plasticity is between 20 and 85. The test piece is cut with a punch which will give a constant volume of 0.40 0.04 cm, the thickness being approximately 3 mm and the diameter approximately 13 mm. The test piece is pre-compressed to a thickness of 1 0.01 mm within 2 sec and heated for 15 sec. The test load of 100N is then applied for 15 sec when the test piece thickness is measured. The usual temperature of test is 100°C and the result is expressed as the thickness of the test piece at the end of the test in units of 0.01 mm and called the rapid plasticity number. The Wallace rapid plastimeter, and presumably other commercial instruments, conform to this specification but it would be sensible to check with the manufacturers. A technically identical method is given in BS 903 Part A5923. 

ISO 732324 specifies a parallel plate test based on the Williams plastimeter with plates 4 cm in diameter. The test piece is 2.00 0.02 cm3 in volume and can conveniently be a cylinder 16 mm diameter and 10 mm thick. As discussed above, a close tolerance on volume is necessary for this type of plastimeter. The test piece is preheated for 15 min (the temperature of test is usually 70°C or 100°C) and compressed under a force of 49N. The thickness of the compressed test piece is measured in mm and this value multiplied by 100 quoted as the plasticity number. The preferred time of application of the force is 3 min. The correction to the standard in 2003 was to change the tolerance on the force from 0.05N to 0.5N. 

ASTM D92625 gives similar methods to ISO 7323 but has two recovery procedures. In procedure A the test piece is removed from the plastimeter and allowed to recover. In procedure B the test piece is compressed, not... 

The main advantage of rotation plastimeters over compression instruments is that shearing at constant rate can be continued for as long as required so that thixotropic or structure effects can be studied. Rather higher shear rates are possible, although the Mooney operates at only about 1 sec 1 (the shear rate varies across the diameter of the rotor). A practical difficulty is to avoid slippage of the rubber over the metal parts and this is why the Mooney operates with a positive hydrostatic pressure and has grooves cut in the metal surfaces. 

Many workers have studied and published correlations between various types of plastimeter, often to show that they do not agree and to illustrate the superiority of the instrument which supposedly agrees best with processing behaviour. Several comparisons are included in the literature already noted and other examples are shown in Figure 6.6. Figure 6.6a shows the relatively close correlation obtained between two compression instruments, the Wallace rapid and Williams plastimeters, for materials of similar flow characteristics (plasticised natural rubber). When such rubbers are compared on two basically different instruments (compression and extrusion) the... 

Figure 6-6. Correlations between readings of various plastimeters. Rapid = Wallace rapid, compression, disk type Williams = compression, plate type Extrusion = Griffiths (1926) ...

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