TA techniques, TMA, in developing and monitoring of cellular thermoset materials.

For the most part thermal analysis (TA) is used for material analysis and quality assurance, but is not widely accepted as a compound development tool. TA techniques can accurately measure mass, phase transitions, dynamic and dimensional changes in materials as temperature conditions are systematically scanned. Using the thermomechanical analysis (TMA) module of a TA system, dimensional changes of the sample are monitored as a function of temperature changes, or time at a constant temperature, with the sample being subjected to a constant force in either compression or tension.

Compared to other TA test methods, e.g., DSC & TG, very little has been published on the use of TMA in the study of thermoset elastomer systems. Dimensional analysis of the expansion characteristics of cellular expanded (sponge) compositions can accurately quantify effects of ingredient changes such as polymers, fillers, plasticizers, blowing agents, activators and cure systems. As mentioned, the effects of blowing agents can be rigorously evaluated and practical process concerns, such as compound bin stability, can be investigated. In this article, we will show equations and relationships on key compound parameters to allow design of sponge parts to exacting densities, using this rigorous technique.

Cellular rubber can be defined as material containing small hollow cells in which the cells are produced by design and under controlled conditions which result in uniform cell structure. This cellular rubber can be divided into two classes depending on cell structure. One type of cellular material is known as "open cell" sponge material. It is designed so that adjacent cells are interconnected. As a result of this open cell structure, this material exhibits low resilience, low compression set, low shock absorbancy and high fluid absorption.

The focus of this work is the other type of sponge material, called closed cell. It is designed so there is no opening between adjacent cells. With this type of unicellular structure, expanded rubber has higher resilience, higher compression set, better shock absorption and very low fluid absorption (tel. 1).

Methods used in traditional compounding of closed cell robber compounds appear often to have been based on past empirical data and trial and error. This article will attempt to shed new light on techniques to make this effort more scientific.

Experimental equipment

In this study, the Mooney viscometer, cone rheometer and the thermal analyzer were used in conjuction with an extruder/hot air curing unit, to evaluate the processing of our compounds. The cone rheometer isothermally measures torque development, i.e., cure rate state, and compound (gas) pressure development simultaneously. It has been used extensively to characterize thermoset sponge. Contrary to the cone rheometer, the TMA mode of the thermal analyzer allowed us to quantitatively measure sponge expansion during the cure and blow reactions and determine the blow/cure system gas activation temperature.

The instrument used in this work was a model 2940 thermomechanical analyzer manufactured by TA Instruments, Inc. The TMA is composed of a quartz sample holder (platform), a metal/quartz probe which is movable in the vertical axis and a linear vertical differential transformer. The probe contains an element which forms the core of the transformer. Small deflections are sensed by the transformer, with the data stored on computer disk to be plotted on an X-Y plotter later.

Both cone rheometer and TMA can point out compounds which blow out, i.e., the gas expands too quickly, or in such quantity that the developing compound modulus cannot confine it. At that point, the sponge cell walls are ruptured, allowing gas to escape, yielding poor skin surface and higher densities.

Experimentally the TMA can measure dimensional changes with a variety of probes or sample holders which act on test specimens in either compression or tension modes. In the compression mode. the penetration probe is designed to penetrate the sample. The expansion probe surface area of the foot is usually greater than the surface area of the sample. This avoids indentation and gives an accurate measure of sample growth. This probe was used in all of our studies. The dilatometer probe is used to measure volumetric changes in a sample without physically touching the specimen. The sample is immersed in a heat stable medium of known volume and the linear change in the height of the medium is measured. In tension the sample is usually a die cut ring of oval shape to allow good fit on the two hook members of the probe. An excellent review in basic TA equipment, techniques and elastomeric variations, which can influence results, has been published by Laird and Liolios (ref. 2).

Sample preparation

Closed cell sponge compounds are processed by internal or mill mixing, with the same techniques used for solid compounds to obtain good dispersion. Our compound mixing was done using a BR size internal mixer to prepare master-batches. Cure ingredients and blowing agents were added to the masterbatch on a 16" two roll mill. Since some blowing agents decompose at low temperatures (as low as 65.6[degrees]C), batches were kept cool during and after the blowing agent addition. After mill mixing, the compound was sheeted out and allowed to stabilize over night (24 hours if possible). This rest period produced more uniform expansions of the compound during ensuing analysis.

Preparation of the test specimen must be particularly exacting, since flat surfaces are required for intimate "seating" of the probe upon the sample (except for the dilatometer).

The sample prep starts with a sheeting process that is very important and should be controlled closely. On odd speed mills. a smoother surface was obtained if the material was banded to the faster rotating roll. A small rolling bank of material worked best when milling material. This allowed the milling process to work out any trapped air in the compound (large mill banks do the opposite).

To provide greater specimen uniformity an additional step was developed. Cold or warm slab pressing techniques were used to prepare 1.905 mm slabs which were smooth and contained no trapped air. We have since found that mixed stock sheeted from the mill can be cooled and cut to 0.762 to 2.032 mm thickness with parallel surfaces using a standard industrial splitter. By using this method there is virtually no chance of premature heating of the stock sample.

Test descriptions

The TMA expansion test was used for determination of activation temperatures and % expansion of material. Using a carefully milled fresh sheet of stock, a sample is cold pressed into a uniform 1.905 mm slab. A 9,525 mm diameter pellet is died out from the slab and placed in the heating chamber of the TMA module. A flat tipped, expansion probe is adjusted to seat on the 1.905 mm slab and a programmed heating rate of 20[degrees]C/min. is started (a typical temperature scan is from room temperature to 220[degrees]C). As the uncured slab expands (from thermal expansion, curing or sponging caused by blowing agents), the probe rides on the sample surface and a read-out of the expansion is graphed on the TA recorder. Key data points are: Activation temperature (the temperature at which the blowing agents start to produce expansion) and % expansion (the recorded sample growth rs. the original uncured slab thickness) at a specific scan temperature of 100[degrees]C.

The Monsanto cone rheometer is used for determination of torque development of the curing compound, similar to an ODR, and pressure development (composite of internal stock and gas pressure) during the cure/blow reaction. A sample of uncured stock is placed in a cone shaped rotor/stator chamber. Isothermally, at 191 [degrees]C for our study, a three min.-run is made in which the curing and blowing reactions are completed. As one recorder plots torque development, a second recorder plots pressure, sensed by a pressure transducer in the rotor/stator chamber.

Both pressure and temperature recording have defined scales or ranges similar to a conventional ODR. Key data to observe are: the shape of the torque curve, parameters such as [T.sub.2] and Tc90 times, Mc90 torque values and the shape and values of the pressure curve, watching for good high pressure development or the occurrence of rapid pressure loss indicating blow out.

Discussion

Polvmer selection For the purposes of our study, two common grades of EPDM were focused upon, i.e., Nordel 1470 (amorphous type) and 2744 (crystalline type), because they cure at roughly equivalent rates. Rheological differences, such as viscosity, melt tension strength, crystallinity, molecular weight distribution (broad vs. narrow) were markedly different allowing us to change the sponge expansion, surface feel and compression load characteristics by varying the polymer ratios without affecting the blowing, loading system or cure rate

Curing system

From previous laboratory work, one main acceleration system was identified to be studied. The fairly conventional sulfur, zinc dibutyldithiocarbamate (ZDBC), tellurium diethyldithiocarbamate (TDEC), and 2-mercaptobenzothiaole (MBT) system contained one unusual additive - thiocarbaniiide. This earlier work had described thiocarbanilide's contribution to rapid viscosity development and subsequent cure rate, which are vital to entrapment of blowing agent gases during the sponge expansion process. The uniqueness of thiocarbanilide centers on its characteristic of not lowering blowing agent activation temperatures while increasing cure activation dramatically, i.e., reducing MS at 121[degrees]C by nearly 50%.

Cure/blow systems

The difficult part of compounding closed cell sponge for continuous extrusion/expansion processes is matching material modulus or viscosity development (a composite for thermal softening and curing) against the activation temperature and amount of internal pressure developing by the gases being generated by the blowing agents and activators.

Early work on which this study was based, used azodicarbonamide 130 alone or in combination with pentaerythfitol. As our work proceeded, other blowing agents, activators and combination systems were evaluated. The system of azodicarbonamide 130, 42% dinitrosopentamethylenetetramine and pentaerythritol proved to be a unique combination which optimized our compound's cure/blow characteristics. It consistently gave good cell structure, surface and uniform expansion. By changing the total amounts of blowing agents, but keeping the 2:1 ratio of azodicarbonamide 130:42% dinitrosopentamethylenetetramine with pentaerythritol at a constant 2 phr level we have developed sponge densities from 240 kg/[m.sup.3]-480 kg/[m.sup.3] (.48 - .24 gr/cc) with no other compound or cure adjustments.

Table 1 deals with one of our earliest investigations in selection of blowing agents. It is a comparison of three different blowing agents: azodicarbonamide 130, p,p'-Oxybis(benzesulfonyl hydrazide), 42% dinitrosopentamethylenetetramine. In this gross comparison, the TMA was found to be a better, more simplified analysis technique, as compared to traditional technology, i.e., the cone rheometer. For example, comparing compounds 208, 212, 213 with TMA (figure 1), the activation temperature and % expansion values recorded present a very concise picture of what has happened. In contrast, the cone rheometer data are much more difficult to interpret (figure 2). In comparing the compounds, the activation temperature value differences of nearly 40[degrees]C are remarkably apparent using the TMA. Compound % expansion numbers show similar clarity as opposed to the traditional cone rheometer data. One can immediately see how TMA can be a radically useful tool for one's compound development. We will continue this theme with several more examples.

Besides using the TMA to screen various types or classes of blowing agents, a compounder could look at more subtle issues such as different particle sizes on similar blowing agent materials. Compounds 268, 270 and 272 are identical formulations with the only variation being particle sizes of azodicarbonamide. Analysis with the TMA allows us to look at similarities (activation temperature) but also differences (% expansion). The partical size of the blowing agents shows no difference in activation temperatures (approximately 185[degrees]C), but does show differences in % expansion (nearly 35%) (table 2). Besides the possible separate effect of blowing agent particle size, one can begin to speculate that the TMA expansion data on azodiocarbonamide 60 is directly demonstrating some critical process characteristics, i.e., the faster cure or Mooney scorch imparted by this blowing agent. These two pieces of information, so critical in sponge development, i.e., activation temperature and % expansion, appear to be readily available using the TMA (figure 3). Again, figure 4 demonstrates the more complex cone rheometer curves for these compounds, as shown by compound 270.

Expanding further into compound development, the next study looked at blends of blowing agents (table 3). Compounds 326, 327, 328, and 330 compared blends of blowing agents at a total level of 6 phr. The TMA isolated the differences in the various formulations, with the blends of blowing agents showing faster scorch times (Ts2) and lower expansion than straight azodicarbonamide 130. It is interesting to note the dramatic change in compound 328 when an activator, pentaerythritol, was added at the 2 part level, i.e., compound 330. The TMA showed a dramatic change in activation, temperature and % expansion, without showing any signs of blow out (figure 5). As would be expected, the density of compound 330 was much less than that of compound 328. Thus the addition of pentaerythritol, a cure retarder but blow activator, increased expansion and optimized the cure rate to expansion/blowing action quite effectively. Figure 6 shows how much more complicated traditional cone rheometer analysis would be for the same series of blowing agent changes. The clarity and quantitative type of information from TMA studies would be very useful if one were trying to modify the density of an existing compound.

If screening blowing agents and activators were the only uses for TMA in sponge development, it would be of limited value. In compounds 280, 281 and 284 (table 4), we left the blowing agent content and varied the formulation viscosity, through oil and polymer changes. In formulation 280 vs. 281, we had similar polymer blends, and a 15 phr oil difference. In the 284, the oil level of 281 was maintained, but the polymer blend ratio between the amorphous and crystalline grades was changed. In this example we saw that the traditionally known facts, that compound viscosity can be altered by changes in plasticizer level or polymer type, and thereby affect sponge product densities. But, expansion and density differences from the TMA demonstrate that not all viscosity modifications produce equal or expected effects. The effect of oil reduction is predictable, but the move to a 100% amorphous grade gives the opposite effect, i.e., higher viscosity, but also higher expansion. The cone rheometer offers little evidence that these ingredient and level changes are of great importance. In the next set of experiments, the TMA was used to evaluate similar changes with insightful results.

We looked at effects of different structure carbon blacks using a blend of blowing agents as shown (formulation 348 vs. 349). The higher structure, higher surface area black N-660 when compared to the N-774 should increase compound viscosity and modulus along with decreasing scorch time. The MS @ 121[degrees]C values along with the TM-100 maximum values, substantiate these expectations. But the quantitative % expansion numbers shown by the TMA values, 96% for the N-774 formulation vs. 80% for the N-660 formulation, could be of much greater use when developing compound modifications for exact expansion needs.

Similarly so, the TMA quantitatively identifies the expansion increase of compound 351 vs. 348 when a lower viscosity polymer is used. The substitution of a 30 ML point lower, but equally amorphous grade, decreases the compound viscosity and yields a 10% increase in expansion.

The next logical step in our work was to look at analysis of cure changes. Of all of our TMA studies, our work with cure systems proved to be the most difficult. When looking at this TMA data, we saw that one must be careful to include in the analysis the TM-100 ODR data. Thus, a case can be made for understanding the % expansion values, but only after looking at minimum compound viscosity values, along with the T2, T15, T90 and maximum values for each compound. A compound's initial viscosity (how fast it builds modulus) and its maximum torque value all play a roll in the final % expansion. We did find that thermal softening is more dramatic for high levels of our high viscosity, amorphous grade in the blends, and that extremely slow cure systems (peroxide/tetraethylthiuram monosulfide) produced low expansion because of blow out.

In this area, more TMA analysis must be done to develop a base for better understanding of cure system interactions with activation temperatures. But even with these current limits, TMA analysis is easier to understand than with the cone rheometer, and expansion characteristics can be quantified for further refinement or investigation.

In one of our last experiments, compounds 348, 364, 365, 367 and 368 investigated straightforward blowing agent changes in an optimized formulation. The cured sponge expansion/density results again followed what was being shown with the TMA, i.e., more expansion/lower density as the blowing agent level was progressively increased. With compound 368 the TMA also showed "blow out" (curve peaked and then expansion dropped). The blow out that occurred because of excessively high blowing agent levels indicates that other methods of adjusting expansion would be needed to meet 240kg/[m.sup.3] or lower densities.

The cone rheometer was difficult to interpret, but did show the blow out of compound 368. In the end, the TMA approach allowed us to study compounds in a manner that presented us with data that were much easier to interpret and understand. As will soon be demonstrated, increasing levels of azodicarbonamide 1301dinitrosopentamethylene-tetramine at 2:1 ratio increased expansion in a predictable mathematical manner.

Theoretical calculations method

Not all sponge development work must be done in laboratory experiments. Earlier it was stated that mathematical methods had been developed to predict sponge densities from published data, provided that compounding changes were straightforward and that the types of cure/blow agents remained the same. Some of the most useful, published information about raw materials to help in compound development is on blowing agents. One approach we have found helpful, is to take density test data on a well characterized, known compound and calculate predicted densities for that compound, based on blowing agent level changes using gas evolution values (GEV).

The following describes the methods used and the key technical points which must be considered. Major factors controlling closed cell sponge density and expansion are:

Cure characteristics - viscosity and modulus development provides the resistance to expansion and the strength for entrapment of gases. These are very dependent on the original compound viscosity, green strength or modulus development, reflected by the MS or ODR. The higher the viscosity or the quicker modulus develops, the greater the resistance present, to inhibit expansion.

Blowing agent level and type for the extruded, "closed cell" (ECC) sponge formulation we are recommending a blend of blowing agents and an activator are used. Published values are available for the theoretical volume of gases each of these blowing agents can liberate, if they are completely reacted. Increasing or decreasing the level of these materials will change the volume of gases that potentially are available to cause sponge expansion. Since these two factors can be measured or mathematically calculated, a method to relate the two to theoretical sponge expansion should be possible. In fact, we feel that the actual expansion or density can be predicted from cure measurements and blowing agent calculation if compounding changes are straight forward and the types and ratios of blowing agents are not altered. Only one fully characterized (actual) sponge must be available to provide a reference formulation and density. From that, a blowing agent efficiency factor can be calculated which is used to relate theoretical expansion to actual density.

The calculation for this blowing agent efficiency factor (BAEF) is shown in table 5. The information required is total formula weight, specific gravity of compound, characterized (reference) sponge density, phr of each blowing agent, and GEV for each agent. The BAEF can then be calculated.

This number is then used in the calculations in table 6. It is here that the compound density relationship to level of blow agents is determined. Again this calculation can be put on computer for ease of use. The key to this calculation is remembering that when your are working with blends of blowing agents, the calculation requires the ratio of the two materials.

Table 7 shows various density calculations using the BAEF method for our test formula 348. In looking at our TMA work, we would predict blow out for our 240-256 kg/m3 compound. These calculations should hold true, providing compound viscosity and scorch time of the basic compound are not altered. For compound 348 the calculated BAEF = 65%. Figure 7 is a reference scale which relates the compound variables of viscosity (minimum viscosity from MS @ 121[degrees]C ) to the BAEF. Based on our work, the axis of the chart that approximates a log scale. Once a BAEF has been calculated for a compound (65% for compound 348), the effects of changes in viscosity and scorch on density of sponge can be predicted, if blowing agent levels, types or ratios are not changed.

In figure 8 the compound viscosity is changed from 12.5 to 15 with no change in blow agent or cure system. The scale predicts that the original BAEF, 65%, should be multiplied by .7 to give a new BAEF of 46%. Thus a viscosity change from 12.5 to 15 would increase density from .36 gr/cc to .455 gr/cc. Figure 9 shows change in scorch time from 4.0 to 5.0 minutes. The BAEF changes from 65% to 94% (65% x 1.45) with predicted density values of .26 gr/cc from .36 gr/cc. It should be remembered that Mooney scorch measurement is somewhat variable so test values must be considered as guides and would be more accurate within one set of experiments. This predictive method is based on limited lab work, but may show the magnitude of differences in sponge densities caused by viscosity and cure changes. The well known extreme sensitivity of final part density and size control to small viscosity and scorch changes are most likely the reason extruded closed cell sponge manufacturers at times experience abnormal product variation.

Bin stability When extruding sponge compounds, dimensional control has been identified as one key to producing a quality product. As part of this study, we were eager to see if TMA, using the expansion or dilatometer probe, could give manufacturers a reliable test for determining percent expansion for materials that exhibit shelf-life variability. The material used in this study was a standard sulfur modified G type neoprene closed cell sponge compound. The material studied was mixed using our standard mix procedures in a OOC internal mixer with the cure and blowing agents being added on the mill after a 24 hour rest period for the masterbatch.

In trying to simulate factory inventory and storage conditions the material was not kept in cool storage but left in normal room temperature storage for the months of August-October. Samples were tested on the TMA at 20, 30, 40, 48, 62, 80 and 100 days using all sample preparation procedures described earlier. A typical TMA expansion plot from materiaI aged 30 days is shown in figure 10. The plot shows the onset of blowing agent activation and the calculated % of expansion. Figure 11 shows the plot of these values over the August-October storage time period.

In running this type of test we felt that manufacturers of expanded products could keep the quality of their products high, by better knowing what types of expansion rates they could expect from their compound after extended storage. There is a dramatic decrease in expansion rate at around 30-40 days. By running viscosity measurements on the compound, we have been able to use the BAEF factor calculation to closely predict percent expansion within a 30-40 day period. We must state that this was a much longer time than we had anticipated. Although some positive change was noted, our laboratory procedures routinely have expanded rubber stocks discarded and remixed if over one week old.

It should be noted that this initial work has been done in a laboratory environment, where line speeds are slower and equipment scale is smaller than would be seen in a production situation. But we believe the premise is still a valid one. It is our intent to build on this line of study regarding material aging since we feel it has such importance in product manufacturing. With new test equipment in place, such as the dilatometer probe, it is felt that this type of testing and analysis provides meaningful data that can be taken from the laboratory to the manufacturing floor.

Conclusion

In the examples we have given, we have shown how material differences commonly encountered in closed cell sponge compounding can be quantified using the TMA, and through analysis, steps can be taken to speed compound development or correct potential production problems. TMA can be used in compound development and it is our belief that correlation of material aging with processing could be established with more assurance, using the TMA to define the aging process. As with any methodology, experimentation with different ways of running the equipment, along with careful data analysis, will be required to firmly establish valid procedures and material relationships.

Thus in conclusion, we feel that using the thermomechanical module of the thermal analyzer system, dimensional analysis of expansion characteristics of cellular sponge compositions can accurately quantify effects of ingredient changes such as polymers, fillers, plasticizers, blowing agents, activators and cure systems; the effects of the changes of types and/or levels of blowing agents can be rigorously evaluated; equations and relationships on key compound parameters can be established to allow design of parts to exacting densities using this rigorous technique; finally, the application of the TMA can highlight practical cellular material changes caused by compound stability considerations.

It is hoped that through the examples in this article, we have demonstrated the utility of TMA technique in material development and process parameter monitoring.

[TABULAR DATA OMITTED]

References

1. Encyclopedia of polymer science & engineering, 2nd Ed., Vol. 3, Wiley-Interscience, 1985.

2. "TA techniques for the rubber laboratory, "J.L. Laird & G. Liolios, Du Pont, ACS Rubber Division Meeting, Detroit. MI, October 17-20, 1989.

3. "Activators for chemical blowing agents," Donald G. Rowland, ACS Rubber Division Meeting, Montreal. Quebec, May 26-29, 1987.

4. Thermal analysis, 3rd Ed, Wesley Wm. Wendlandt, chapter IL Wiley-Interscience, 1986.