New research on wax.

New research on wax

In spite of new polymers, additives and test procedures, protection of rubber from attack from oxygen and ozone still challenges many compounders. While many of the mechanisms of ozone attack are understood, there are still many questions regarding the mechanism by which additives achieve the protection they do.

One manufacturer of wax, Crystal Inc., recently commissioned a well-known independent research organization to conduct a complete experimentation and research program on waxes.

The company noted that many of their customers had been asking many penetrating questions about the significance of various factors in evaluating waxes for use as a protectant against ozone cracking. Since many of the experts at Crystal were also concerned about the same questions, and since little data exists, the research project was initiated.

The following dialog is the result of an interview between Rubber World Magazine and Michael J. Reynolds, CQE, manager of marketing, sales and quality service for Crystal Inc. in Lansdale, PA. The purpose of the interview was to explore the area of Crystal's research and review the current status in wax technology.

RW: How and why did you decide to do your research project?

Reynolds: Following one very strong plea by a large customer of ours for some answers to the vital questions concerning their tire compounders, we interviewed nine tire companies and six industrial products companies regarding their needs for wax parameters that can specify and possibly predict actual product ozone resistance performance. We found a very high frequency of the same concerns. Also, there was a nearly universal lament that the different wax suppliers made different claims, none of which seemed to be supported by objective technical data. We decided to help to clear the air on these issues.

RW: What are the significant questions asked by the customers whose needs have prompted this work?

Reynolds: Four needs reoccured:

* To understand how varied physical properties (GLC distribution, melt point, refractive index, etc.) relate to the performance characteristics (ozone resistance) of waxes.

* To understand the role of microcrystalline wax in wax performance.

* To study the surface phenomena of the wax at the rubber/wax interface.

* To study these criteria under both static and dynamic (surface stress disturbed) conditions.

Other questions related to the need for a single analytical (characteristic) test that might predict performance; and the likelihood of GLC being this unique method.

RW: Who is the independent research organization? How are they especially suited to do this work?

Reynolds: The Akron Rubber Development Lab in Akron. We are working with Tom Knowles and his staff at ARD Lab with our project as essentially an extension of his MS chemistry work that is about to be published by the University of Akron. Tom's work especially focuses on the critical stress aspect of the ozone attack of the rubber surface and the onset of the cracking.

RW: What is "critical strain?"

(Answered by Tom Knowles of Akron Rubber Development Labs): Critical strain is the strain (usually elongation, also compression) at which the onset of ozone attack crack growth occurs at the strained rubber surface; no attack (crack growth) occurs at strain less than the critical strain under a given set of conditions.

RW: How do you handle the "problem" of running the ozone exposure at "critical" strain conditions?

Reynolds: We use the ICI Annulus Method which provides a continuous strain gradient from 0 to 100% across the specimen.

RW: Describe the ICI Annulus method.

(Answer from Tom Knowles at ARDL): An annulus is a donut-like ring cut from a standard tensile sheet. The ring design of the annulus test piece (inside diameter exactly 1/2 the outside diameter) allows a continuous range of strain observations (0 to 100%) when the test piece is mounted in the ozone test chamber apparatus. (Refer to figure 1). Advantages include:

* Rapid method for specimen comparison,

* Sure calculation of critical stress,

* Avoids overlooking cracking that occurs at higher strain levels

RW: You've talked about complete experimentation, under carefully controlled conditions. What do you mean by this?

Reynolds: We have designed our experiments essentially along the lines suggested by the Rubber World Tech Service column (Waxes role in rubber p. 14-15) in the February 1989 issue.

RW: How did you choose your independent variables?

Reynolds: A careful reading of the prominent publications about wax as an ozone resistor shows a disparity of advice of the operations limits that waxes should work within. The performance limit of top concern is temperature, most specifically the temperature that the rubber product will experience in its service lifetime. We have used a deductive approach to build an experiment to answer the following questions:

* What is the temperature significance relative to ozone resistance?

* What is the wax composition significance relative to ozone resistance?

* What is the wax concentration significance relative to ozone resistance?

* What is the interrelationships (temperature, composition, concentration) significance relative to ozone resistance?

Other workers have studied one or two of these variables at a time but to our knowledge, we are the first to build a model to look at all of these variables simultaneously.

RW: Do you feel that a single test can be used to predict performance?

Reynolds: Yes, if it is a product service performance test. No, if it's a laboratory test. We are using an ozone box to establish parity between physically "different" waxes which are performing identically in service. After parity, we hope to optimize, and then go outdoors and into tires, belts, hose, etc. to establish performance.

RW: You mentioned the significance of the breadth of the testing relative to the breadth of performance. What is this significance?

Reynolds: Let's talk first about temperature. One prominent wax researcher promotes the claim that ozone functions to crack rubber in the narrow temperature range of 0 [degrees] C to 45 [degrees] C. We have clear results that ozone is still very active at 65 [degrees] C. Furthermore, one needs only to look at inside sidewall winter cracking on the tires of vehicles stored outdoors in Minnesota and the Dakotas to know that a 0 [degrees] C minimum is

too constricting.

Our ozone test chamber work is varied from -20 [degrees] C to +65 [degrees] C, to represent a breadth of temperature more representative of actual performance conditions. Our plan is to establish parity in the laboratory (the ozone box) and then move outdoors where the true performance laboratory really is. Both composition and concentration, similarly, need to be broad enough to allow for controlled minimum blooming, yet be enduring until the end of the service life of the rubber product.

RW: How is your performance model constructed?

Reynolds: Our DOX data will allow us to configure and (computer) generate a three dimensional representation of resistance to cracking (critical stress point of onset) vs. temperature vs. concentration vs. composition (several performance =, different waxes)

RW: Is your model expected to show differences between waxes, to maybe even show the performance superiority of one wax over another?

Reynolds: We do not know what crack resistance characterizations that we will see, but subsequent testings of designed compositions should lead us to an optimal, perhaps even superior wax.

RW: What other "performance" concerns are answered by your work?

Reynolds: The four needs of our customers ask us to explain what is happening at the rubber/wax interface. We know that the wax is there, of course, but so is the antiozonant, along with fragments of the cure system, plasticizers, etc. Yet, no one knows exactly how much of these things are there, and even more importantly, exactly where each is in the composite that is lying on the rubber surface. We are coming to the answers to these questions with our research.

RW: What do you plan to do with the research results?

Reynolds: These results belong, first, to our customers. We have made a preliminary disclosure of generalized trends but were careful to avoid drawing premature conclusions for two reasons. The first is that these early results were a bit premature because of limited statistical confidence of our data. Secondly, we are also being very careful to live with the constraints of data disclosure of the Rubber Division of the American Chemical Society, since a portion of Tom's (Knowles) research will be published at the Detroit meeting in October. Subsequently, we will publish fully by direct presentation to our customers.

Ultimately, we expect to use the results to build a model that can help us design an optimum wax for the demands of minimum wax on the tire surface, while providing maximum long term, long mileage sidewall protection.