Polymer Testing 7 (1987) 151-152

Editorial

Whereas several years back relatively few laboratories formally cali- brated more than a small number of instrument scales, today most are very conscious of the quality need to calibrate all instruments for all parameters on a formal and traceable basis. A special type of, or a near relative to, calibration is to employ standard reference materials, and they have been in widespread use in the polymer industry for a long time to aid the rationalisation or normalisation of test results.

A reference material is only satisfactory if it is reliable as regards consistency with time and to achieve this it must be closely specified, produced under controlled conditions, and preferably come from a single source. Consistent production is not particularly easy, especially over long periods of time and clearly both specification and supply require considerable effort.

The problems vary somewhat with the particular type of reference material. Standard rubber hardness blocks are relatively easy to produce from reliable and stable formulations but the variability from repeat mixes is such that each batch, and normally each block, has to be individually calibrated. Apart from the effort involved this poses no great difficulty because both the specification and dead load instruments capable of precise calibration exist.

Considering that even a basic property like hardness will vary to some extent even under good conditions one should perhaps be warned that standard rubbers for other properties are likely to be difficult. Abrasion standards are known to have given problems because even with a strict formulation and production schedule serious variability can occur. In certain instances a major contributing factor may have been an unfortunate choice of standard formulation which was sensitive to processing variables. Abrasion standards would seem a prime case for a single source of supply but traditionally it has been the practice for many laboratories to produce their own. The problem has not in the past been properly recognised in standards which have not always specified closely how the standard rubber should perform. This somewhat lax approach to standard rubbers is by no means restricted to abrasion.

151 Polymer Testing (7) (1987)----O Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Northern Ireland

152 Editorial

ASTM oils are the most renowned and widely used standard reference materials in the polymer industry. Their specification was not particularly comprehensive but the theory was presumably that good consistency could be obtained by careful control from a single source. However, these standard oils have caused considerable difficulty when the particular source runs out and the specification is not foolproof. It is understood that great testing efforts have been made in the USA to authenticate replacement oils. In fact, some of the changes in the source and specification of ASTM oils which have and may occur are not too clear to most of us. Perhaps an informed American reader would care to provide a small article?

A few years ago considerable effort was expended in ISO TC 61 to produce specifications for plastics as molecular weight standards and also as light dosage standards for weathering tests. The work included defining the whole standardising process including interlaboratory tests to determine mean levels of the parameters and requirements for ensuring continuity of supply. Unfortunately, most of this effort seems to have been wasted in that the standards have not been published as intended. Also much of the thinking would have been, and still is, applicable to other polymeric reference materials. If reliable standard materials are to be needed in the future then further consideration will have to be given to their specification and production.

R. Brown