Mailbag.

Dear editor:

I would like to address the problem of improper and proper flow testing of rubbers. By "improper" I mean incorrect and misleading. By "proper" I mean testing that it is informative for the purpose for which it is carried out.

Regretfully I have to take the stance that the flow testing of rubbers since Mooney reported his viscometer in 1934 (ref. 1) are improper. This is not surprising with our present knowledge of the non-Newtonian behavior of all rubbers and compounded stocks. The Mooney viscometer operates at around one second, at a temperature of 100 [degrees] C while modern internal mixers operate around 160 [degrees] C with shear rates averaging several hundred seconds (ref. 2).

Despite no dispute on the above paragraph, the Mooney viscometer is ubiquitously chosen as the standard test instrument by which others are calibrated.

This is not the mark of a technically vigorous industry that it still uses a 1934 standard test instrument. It indicates that the industry, its suppliers and industrial and academic laboratories have failed in an obvious line of research and development.

Inadequacy of flow testing also means that "processing" and "processability" have inadequate meaning.

Finally in this diatribe, all rubber technologists deep down know that it is "elasticity" which is the prime property of rubbers and that this is being totally ignored.

How have we arrived at this state of affairs? Mooney had no model for the material he was studying. Also, he was narrowly focusing on a tester for compression molding, where the temperature and shear rate, at the time of his viscometer, were in the right range. He was interested in giving the factory a test it could readily perform. However, it is nevertheless inexcusable that he made his viscometer according to figure 1. He had, since 1928, adopted a Couette viscometer design according to figure 2.

[ILLUSTRATIONS OMITTED]

Where did Mooney go off the track? He unwittingly explains where in his Goodyear Medal Address of 1962 (ref. 3), where he recounts that he was unable to design a Couette viscometer for easily inserting a sample and particularly for clamping it up manually. On the other hand, he was "inordinately proud" to make a double-toggle clamping for the generally flat viscometer of figure 1. So, the case is not that of a wrong technical choice, it is a matter of emotion.

It was obvious to Mooney that his "viscometer" measured a mix of elements of different true viscosity. He unconvincingly tried to explain this away by some dubious mathematics, which seems to have settled the considerable doubts that were initially expressed on taking up his viscometer. Indeed, but for the viscometer being adopted by the wartime general rubber program, it may well not have been accepted.

The Monsanto rheometer of the 1960s started a plethora of vibrating instruments. They were fine for measuring cure-time, but users stretched it to beyond its capabilities in measuring flow.

Why this is so is again simple just by taking an objective look. Oscillation of a few degrees measures the stretch of rubber of a percent or so, which has no causal relation to the stretch in a rubber processing machine.

Also, the attempt to use science to give "phase angle" or "tan delta" would apply to the spring and dashpot model, but cannot be used for the viscoelastic model of a rubber where the viscosity is due to the need to do work to move neighboring elements held attracted by Van der Waal's forces and elasticity is the entropy effect of stretched chains returning to a random configuration.

The melt flow tester is the standard instrument for measuring both viscosity and "elongational flow" (equivalent to my "elasticity"), with the elaborate corrections now verified, for thermoplastics. It does so "properly."

It has recently been applied to rubbers, but objections to it are formidable. The corrections are probably greater than the effect for most rubbers. It is a once-off measurement as the sample is extruded from the instrument. Elasticity measurement depends crucially on the extent of die swell and rubbers notoriously do not extrude as exact cylinders. Also, the shear rates and strains of rubber in rubber machinery are at rates difficult to achieve with this material, of so much higher poise than a thermoplastic.

I will mention a biconical rotor since it seems generally accepted that this shape gives a uniform flow rate, ideally by the simple geometry of congruent triangles to the enclosed material (ref. 4). Unfortunately, Piper and Scott (ref. 4) did not realize a fatal flaw in their analysis, namely that rubber is non-Newtonian through its varying depth in the triangular profile.

So we come back to the Couette viscometer, the tester that Mooney had rejected by 1934! It gives tree viscosity and elasticity measurements.

Also for rubbers, it is important that the rubber sample can be irregular lumps cut off a bale or sheet. Transfer molding can nicely fill the annular space. Sensors can readily pick up the torque and normal force. So we have:

Torque = viscosity measure + elasticity measure.

My article in Rubber World, February 2001 edition, describes such a machine (ref. 5), illustrated in figure 3.

[ILLUSTRATION OMITTED]

So, with the Couette viscometer (figure 3), we finally arrive at a proper and convenient machine for measuring the elasticity and viscosity of raw rubber and compounded stock.

What can the consequences be?

* First of all, bales of rubber can be measured for their changes of elasticity and viscosity from their raw state through compounding. This can establish a measure of consistency and processability (ref. 5).

* From this there can be a quantitative value for raw rubbers with the manufacturer's specification and so ultimately, consistency of vulcanized product properties (ref. 5).

* Processability can be precisely defined and applied (ref. 5).

* Scientists and technologists can measure the non-Newtonian elasticity and viscosity values of rubber for a variety of purposes, including for establishing a "proper" standard test method.

I hope I will be excused for telling my fellow technologists how to suck eggs. Perhaps my start in the industry in 1949 is an excuse.

My observation is that education and training today fail to deal with the basics. For example, is George Bloomfield's paper on the constitution of rubber in latex consulted? Is Marshall Pike's and my paper on cold and hot mastication analyzed? Is Graham Moore's chemistry of vulcanization? Is Leonard Mullin's swelling modulus and crosslink density? Is L.R.G. Treloar's theory and experiment of the elasticity of vulcanized rubber? Is Adolph Schallamach's theory of wear? (They are all treated in The Chemistry and Physics of Rubber-Like Substances [ref. 6] of 1963).

Meanwhile, rubber technologists in the U.S. should pay attention to some very interesting papers emanating from elsewhere, e.g., the Proceedings of the International Rubber Study Group For-um (ref. 7) held at Antwerp in September 2000, and Dr. R. Suchida of Thailand's paper (ref. 8) on his approach to eventually a robber supply of consistent processability and product reproducibility.

As corporate bodies, the professional and standards bodies could look back to activists in the "golden days" (not in business nor politics but only in technical meaning) when the U.K. Institution of the Rubber Industry in 1952 cooperated in producing "The History of the Rubber Industry" (ref. 9) and the Big Four and others such as Du Pont and the carbon black companies did so much fine research. They might well look forward to emulate the old boys.

Finally, my purpose is a disinterested stirring of the pot. If it has for somebody boiled over, well and good!

References

(1.) M. Mooney, Industrial Engineering Chemistry, 6, p. 147 (1934).

(2.) W.F. Watson, Rubber Industry Journal 8, p. 106 (1974).

(3.) M. Mooney, Rubber Chemistry and Technology, 35, p. xxvii (1962).

(4.) G.H. Piper and J.R. Scott, Journal of Scientific Instruments, 22, p. 26 (1945).

(5.) W.F. Watson, Rubber World, 223, no. 5, p. 23 (2001).

(6.) "The chemistry and physics of rubber-like substances," Ed. L. Bateman, Wiley Press (1963).

(7.) Proceedings of September 2000 Forum, International Rubber Study Group, Heron House, Wembley, London.

(8.) R. Suchida, 8th International Seminar in Elastomers, May 2001, Le Mans University.

(9.) The History of the Rubber Industry (1952), Ed. P. Schidowitz and T.R. Dawson, Heffer Press (1952).