Rapid plastimeters

An instrument for measuring the plasticity of an elastomer, e.g., Wallace Rapid Plastimeter Plastisol... 

Fig. 16a-

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. 

High shear testing machines such as the Wallace rapid plastimeter, the Monsanto processability tester and extrusion viscometers may better differentiate between natural rubber and polyisoprene, and even between polyisoprenes from different suppliers. None of these machines alone, however, will accurately predict the behaviour of the polymer or batch for all processing operations. Thus Mooney viscosity plus practical experience remain the principal factors in categorising factory processing. 

Wallace rapid plastimeter, Wallace Instruments Ltd, Croydon, UK. Wingstay The Goodyear Tire Rubber Co., Akron, Ohio, USA. ZD ,C, zinc dimethyldithiocarbamate. 

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) ...

BS 903 Part A59.1, 1997. Methods using plastimeters - Determination of the rapid plasticity number. 

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