Method 365A is used to determine the softness of flexible plastics materials by measuring the depth of penetration of a standard steel ball under a defined force into the surface of the material, in units of 0.01 mm. In this revision of method 307 A 'Softness number of flexible polyvinyl chloride extrusion compound' of BS 2782 : 1970 the main differences are that the diameter of the steel ball has been increased and a small increase has been made in the applied force to align the method with Method N of BS 903 : Part A26 : 1969, which is in technical agreement with ISO 48. The apparatus and procedures of Method 365A and Method N are now similar although the method of expressing the results is still different. I n Method N hardness is expressed in International Rubber Hardness Degrees (IRHD), high values of which correspond to low penetration of the steel ball and therefore low softness numbers.
The relationship between the two scales is given in table 3 of SS 903 : Part A26 : 1969. Because of the different characteristics of vulcanized rubbers and flexible plastics materials, the results obtained,on these materials should not be compared directly.
The results obtained by using this method are expected to be in experimental agreement with those obtained by using method 307 A of BS 2782 : 1970. Attention is also drawn to the microtest (Method M) of BS 903 : Part A26 : 1969, the results of which can also be similarly related to softness number.
This method describes a procedure for determining the softness of flexible plastics materials (usually flexible polyvinyl chloride compounds).
