An improved method for measuring filler dispersion of uncured rubber.

The mixing process of a rubber compound can simply be described as combining a number of individual ingredients into a single rubber compound. The actual process, however, is not simple and both the quantification of the mixing process and the control of the process are the source of a vast quantity of research. In particular, the effect of mixing on filler dispersion has been shown to have dramatic effects on material properties. Properties relating to processing and performance characteristics have all been shown to relate to the degree of mixing (ref. 1).

Mix cycles are commonly controlled by mixing to a specific time, temperature, work input or a combination of these (refs. 2-5). For example, rubber may be broken down, masticated, for a given time before the filler is introduced into the mixer. The mix may then continue until either a specific temperature or work input is reached indicating that the batch should be "dumped."

Rubber compounds are commonly mixed as either a single or multiple pass mix (ref. 2). A single pass indicates that all ingredients are added to the compound in a single cycle of the mixer. The advantage of a single pass mix is generally that mixer output is increased. For some compounds, however, the time required to incorporate and disperse all ingredients, including the fillers, may result in temperatures that would cause the batch to scorch in the mixer. In these cases, the rubber, fillers and selected other ingredients may be first mixed together in a masterbatch (ref. 6). The purpose of the masterbatch is to produce a uniformly blended compound in preparation for the second or final pass. It is in the final pass where the accelerators and curing agents are added. The final is generally mixed a shorter length of time and to a cooler temperature, thereby avoiding premature scorch.

Quality control of rubber compounds often centers on final mixes in order to assure that all ingredients have been incorporated into the compound in the proper proportions, are of the proper quality and will produce a final product meeting the quality requirements of the end user. Perhaps the most common quality control test of a final mix is the curemeter. Original studies by Jones (ref. 7) showed the oscillating disk curemeter to be superior to durometer, specific gravity and stress strain properties in determining compound variations. These tests typically are made on compounds that have had accelerators and curing agents added and are ready to go to the end process. The purposes of these tests are:

* To segregate batches that are out of specification before they go into final processing; and

* to make adjustments to the process to assure correct future batches.

These tests are generally designed to fingerprint the batch. If the specific batch matches the fingerprint, it is assumed that it will make an acceptable final product.

Processability tests, as opposed to the previously mentioned tests, are designed to determine the ability of the material to go through the forming processes (ref. 8) and are not specifically geared toward assuring that the final, cured rubber products meet end-use specifications. Processability tests indicate a compound's ability to be extruded, injection molded or otherwise formed into its final shape. In general, the test of processability ends here. Processability tests include, among others, the Mooney viscosity test, the Mooney stress relaxation test and DMRT instrument tests (refs. 8-10). These tests characterize the uncured compound at temperatures that simulate the formation stage of the process, not the curing stage. Other tests then indicate how the cured product will behave.

When considering both of the above types of quality control tests, the test results indicate a cumulative effect of mixing, weighing and raw material variations. This is well needed and is aimed at assuring quality end products. According to Boonstra, however, the mixing process "introduces more variance in its mechanical properties than any other step on the road to the final vulcanizate" (ref. 11). The above tests do not specifically address the mixing process and even more specifically do not address the dispersion of the filler.

Many authors have discussed the need for a method of measuring dispersion. A detailed review of filler dispersion is given by Hess (ref. 12). Here he describes the effects of poor dispersion as: "Reduced product life or poor performance during service; poor product appearance; poor processing and manufacturing uniformity; waste of raw materials; excessive energy usage."

The importance of filler dispersion has led to the development of various methods to quantify the level of dispersion. These methods can be broadly categorized into electrical conductivity, mechanical measurements, microscopic and optical analysis (ref. 13). Sample preparation for these methods includes sulfur hardening and polishing a sample surface (ref. 12), freezing and microtoming the specimen (ref. 14), and pressing an unvulcanized specimen between heated platens (ref. 1). Cured specimens are cut or torn for optical analysis (ref. 14).

Quality and process control tests in a rubber mixing operation generally require a measurement that is fast enough to be used concurrently with the production process. A masterbatch, for example, may be mixed every two to six minutes. If every batch were to be sampled, the test would need to be performed at approximately the same rate. Although techniques do exist for measuring filler dispersion of unvulcanized rubber, most require a sample preparation and testing procedure that make the test too complex and time consuming for use in quality control.

The advantages of using filler dispersion of unvulcanized rubber and particularly of the compound masterbatch for quality control are:

* The results directly indicate the effectiveness of the mix;

* as an indicator of the mix, the dispersion rating of the masterbatch can then be used as a process control measurement to indicate changes in the mixing process;

* out of specification dispersion ratings of the masterbatch allow the batch to be held and/or adjusted before additional processing.

To develop a quality/process control method, this article concentrates on the reflected light method of dispersion measurement. For this to be an effective and timely test of an unvulcanized sample the following must be accomplished:

* Preparing a specimen free from entrapped air;

* cutting the specimen for examination;

* measuring dispersion using the reflected light method.

The reflected light method used for this study is fully described by Lovgren and Persson (ref. 13). The method is dependent on the elasticity of the specimen and the action of large agglomerates as the specimen is cut. This is shown in figure 1. As a razor blade is advanced through the specimen, it comes into contact with agglomerates. The agglomerates are moved leaving either an indentation or a bump in the cut surface. This surface roughness is a measure of the filler dispersion. The principle of the method is shown in figure 2. When the light is reflected from the smooth surface, the camera, sensor, sees nothing and the image appears black. When the light is reflected from either a bump or indentation, the light is reflected into the sensor and appears as white. In the method being shown, the surface roughness of the specimen is then compared to a standard set of images, indicating dispersion rating on a scale of 1 to 10 where 10 is the best. The instrument automatically determines the specimen's dispersion rating based on this scale. The method relies on a sample cut that is free from cut marks and entrapped air. A method for the sample preparation has been described by Coran and Donnet (ref. 1).

[FIGURES 1-2 OMITTED]

The objective of this work is to describe a modified method of sample preparation and dispersion testing of unvulcanized rubber that would be suitable for process and quality control testing.

Experimental

Description of apparatus and instrumentation

The DisperGrader Model 1000NT was used for the dispersion measurement (ref. 13). This model has several scales available for comparison. The scale that was selected for these tests was the RCB scale. This scale is typically used for measurement of compounds filled with reinforcing carbon black. Ten reference pictures are used for determining the dispersion rating (DR). These reference pictures represent continuous improvements in dispersion from the poorest being 1 to the best being 10. An algorithm has been derived using these reference pictures and is then applied to an unknown sample. The DisperGrader then analyzes an unknown sample and automatically assigns a DR to the unknown sample. The analysis allows dispersion ratings to be less than 1 and greater than 10. Higher DR represents better dispersion. Visual comparison is seen on a computer monitor. The unknown specimen is shown on one half of the screen and the reference picture is displayed simultaneously adjacent to it. The numerical value of DR is shown on the screen and output to a separate computer for further analysis.

A special sample press was designed. This is a manually operated press holding six samples. The intended use is to place the sample press at the batch-off mill and press the samples while they are hot. This proved effective in preparing a sample without entrapped air. The presser foot is approximately 2.5 x 2.5 cm. When fully clamped, the thickness of the sample is approximately 0.65 cm.

The difficulty of testing uncured samples has not only been dependent on a sample free from entrapped air, but also the requirement that the cut specimen does not exhibit cut marks and spearing (ref. 1). Examples of several cuts are shown in figure 3. In some cuts, the action of the razor blade can be seen on the cut surface. This is shown in figure 3A. In other cuts, the razor blade may smooth over the bumps and indentations, causing the "smeared" surface to appear smooth. This is shown in figure 3B and gives an unrealistically high value for the dispersion rating. An example of a good cut is shown in figure 3C. Here the bumps and indentations are shown as white areas and the smooth surface is black.

[FIGURE 3 OMITTED]

A new uncured sample cutter was made to help assure good cuts of an uncured compound. This fixture starts to pull the specimen just before the sample cutter reaches the specimen surface. A 1.4 cm diameter air cylinder actuates the pull and a flow regulator adjusts the speed. Since the rubber is continually being moved during the cut, smearing of the cut surface is reduced or eliminated. The air pressure was set at 5.5 bar.

Mooney viscosity and Mooney relaxation tests were run according to ASTM D1646 (ref. 15).

Experimental overview

A natural rubber formulation shown in table 1 was chosen for evaluation. The experiment was designed to mix a masterbatch at various conditions of rubber breakdown and filler incorporation times as shown in table 2. Dispersion was measured on the masterbatch and the results compared to various properties of the final compound and vulcanizate.

Procedures

The test formula, TPI, was mixed in a laboratory internal mixer, size B. The nominal capacity of the mixer is 1.5 liters. Rotor speed was set at 77 rpm for both the masterbatch and final passes. The mixer was equipped with temperature and power recording devices. The final mix was intentionally undersized in order to minimize increased dispersion of the final. The batch weights of the masterbatch and final are given in table 3.

Nineteen batches of the formula TPI were mixed according to the mix cycle variations given in table 2. The basic mix procedures for the masterbatch and final are shown in table 4. After each pass, the compound was taken directly from the internal mixer to a 30.5 x 15.25 cm two-roll mill. The mill nip was set at approximately 0.7 cm. The compound was passed through the mill, without banding, four times for sheeting purposes.

The procedure for the second/final pass varied slightly. Since the batch was undersized, 200 grams of the masterbatch were milled to approximately 0.3 cm. The cure system was then placed in the middle of this milled sheet and the sheet folded to create an accelerator pocket. This helped assure that the entire cure package was mixed into the final compound.

Immediately after sheeting from the mill, a strip of the compound approximately 12 x 2 cm was cut and placed in the sample press. The sample was still hot at this time. When the samples were cool, they were cut using the uncured sample cutter and tested for dispersion. Any sample that appeared to be poorly cut, as reflected in figure 3, was discarded and a new sample cut and tested.

Compression set buttons were cured for use in the dispersion rating measurements of the cured specimens. The cure was 15 minutes at 154 [degrees] C. These samples were cut using the standard sample cutter supplied with the DisperGrader.

Mooney viscosity and stress relaxation tests were run at 100 [degrees] C using the large rotor. Viscosity measurements were take at four minutes and stress relaxation measurements were run for an additional three minutes. ML1+4 was taken for the viscosity measurement and t80 for the Mooney stress relaxation measurement. The t80 result is the time that it takes the viscosity to drop 80% after the rotor stops.

The dispersion ratings were made using the DisperGrader, 100x magnification. The scale used for comparison was the RCB scale. Three measurements were taken on each cut and the average reported for the result. The sample was cooled in about 3-4 minutes. The cutting and measurement operation took approximately one minute.

Results and discussion

The dispersion rating results, mixing data Mooney viscosity and Mooney relaxation data are given in table 5. In analyzing these data the following questions can be asked:

* How does the dispersion rating of the masterbatch relate to work input of the masterbatch?

* How does the dispersion rating of the masterbatch relate to the Mooney viscosity and stress relaxation?

* How does the dispersion rating of the masterbatch relate to the dispersion rating of vulcanizate?

Comparison of masterbatch dispersion rating to work input of the MB

Since work input can be considered a primary measure of mixing, the relationship of the dispersion rating of the masterbatch can be compared to the work input of the masterbatch (ref. 5). For example, a typical factory mix would keep the rubber breakdown constant and the majority of mixing variation would occur after filler and other ingredients are added. Compounds 1 to 7 in this study keep the rubber breakdown the same at one minute, but dramatically vary the filler mix time. The comparison of the dispersion rating of the masterbatch and mixing energy is shown in figure 4. As would be expected, these results indicate a good correlation between dispersion rating and work input of the masterbatch. It is expected that the DR would reached a maximum level at some work input level. A log linear regression analysis was therefore used, rather than a linear analysis.

[FIGURE 4 OMITTED]

Batches 8 through 17 vary not only the filler mix time, but also the natural rubber breakdown time. The dispersion rating of all masterbatches is compared to the work input in figure 5. Even here, where both the rubber breakdown and mixing time of the filler are varied, there is a good correlation between dispersion rating and work input in the masterbatch. The correlation reinforces the concept that dispersion is not only related to the mixing lime of the fillers, but also other factors, including the wetting of the filler by the polymer (ref. 5).

[FIGURE 5 OMITTED]

Comparison of dispersion rating to processing characteristics Seeing that the dispersion rating correlates to the work input of the mix, the next step would be to see how the dispersion rating relates to the processability of the final compound. As above, batches 1 to 7 were used for comparison, since only the filler mix time was varied. For this comparison, Mooney viscosity tests and Mooney stress relaxation tests were run. It has been shown that the Mooney stress relaxation values are a good indicator of processability even when Mooney viscosity values are similar (refs. 9 and 10). Figure 6 shows the correlation of the dispersion rating to Mooney viscosity. Figure 7 shows the relationship of the dispersion rating to Mooney stress relaxation. Both Mooney viscosity and Mooney stress relaxation relate well to the dispersion, but it is interesting to note that the stress relaxation data exhibit a better correlation.

[FIGURES 6-7 OMITTED]

The correlation of the dispersion rating to Mooney data is important in two respects:

* The DR correlated well with not only the Mooney viscosity, but also the work input. The work of Van Buskirk, Turetzsky and Gunberg established the relationship of Mooney viscosity to the work input of the mix (ref. 16). This suggests that this method of dispersion rating may be used as a quantification of the mix cycle.

* The improved correlation of the dispersion rating to Mooney stress relaxation suggests that, since t80 is a measure of the compound's elasticity, DR is a better indicator of processability than just the Mooney viscosity (refs. 9 and 10).

Comparison of the dispersion rating of the masterbatch vs. final vulcanizate

Now that the dispersion rating of the masterbatch has been shown to correlate to the mixing process and to the processability indicators of Mooney viscosity and Mooney relaxation, it will be compared to the dispersion rating of the cured product. The dispersion rating of the masterbatch and the vulcanizate is shown in figure 8 and shows good correlation.

[FIGURE 8 OMITTED]

The literature has explicitly shown that dispersion directly relates to the physical properties of the cured products. For example, Coran and Donnet show dispersion to correlate to tensile strength, hystersis and fatigue (ref. 1). These are cured properties. The results obtained from dispersion rating analysis of these 19 batches indicate that the dispersion rating of the masterbatch does correlate with that of the vulcanizate. In this case then, a process control parameter Of dispersion rating could be assigned to the masterbatch which would indicated the properties of the vulcanizate.

Example of dispersion rating used for process control

The effect of the dispersion on physical properties of the vulcanizate varies, depending on the polymer, filler and other compounding ingredients. In general, it has been seen that longer mixing and therefore better dispersion lowers the Payne (ref. 17) effect, lowers E', E" (ref. 18), hardness (ref. 11), and in some cases lowers modulus (ref. 17). Longer mixing, in general, increases tensile strength (refs. 17 and 18), tear resistance (ref. 18), fatigue resistance (ref. 1), improves cut growth resistance (ref. 11), but increases bound rubber (ref. 17). Other vulcanizate properties such as hysterisis, heat build up and permanent set are properties that relate to tan [delta]. These are of great interest to both the properties of processability, i.e., die swell, and the finished product. Studies have shown these tan [delta] related properties to improve with mixing (ref. 18) in some compounds, to degrade in other compounds (ref. 20), and in some to actually reverse their effect during the mix (ref. 1). These general relationships are shown in figure 9. It is clear that to achieve the required vulcanizate properties, the mix cycle must be optimized. Once the optimum mix is established, a dispersion rating can be assigned and limits attached. As related to the process, the lower limit is the minimum DR allowed to achieve the required properties. The upper limit brackets the required properties and limits the mixing time to maximize profits.

[FIGURE 9 OMITTED]

The results of this study indicate that the dispersion rating of the masterbatch can be used to predict the dispersion rating of the vulcanizate even before the final compound is mixed. That is, the dispersion rating of the masterbatch indicates the dispersion rating of the final vulcanizate, and therefore its physical properties. Other factors not related to filler dispersion may also further influence the vulcanizates' properties, and this would be the reason to continue to fingerprint the final mix with a test such as the curemeter. With dispersion testing of the masterbatch in place then, the curemeter fingerprint would indicate what it does best, the curing characteristics.

To go back one step further in the process, consider using the dispersion rating directly after mixing the masterbatch for SPC control. An example of this type of individuals chart is shown in figure 10. Here +/- 3 sigma may be assigned according to standard SPC practices, along with the specification limits described. Trends and out of control situations may be detected and used to correct and adjust the mixing process. The advantages of control applied to the masterbatch are two-fold:

[FIGURE 10 OMITTED]

* If the DR is out of specification on the low side, then the batch needs to be held or mixed further to achieve the proper final dispersion level.

* If the DR is out of control on either the low or high side, or out of specification on the high side, then the mixing process needs to be reviewed.

SPC control of the masterbatch dispersion rating provides a tool for evaluating the mixing cycle in order to achieve maximum economy of the mixing process and consistent vulcanizate quality.

Summary

Process and quality control tests are essential parts of manufacturing. Process controls help assure proper procedures and improve product quality. In typical two pass mixes, testing of the masterbatch must be easy, fast, and must relate to the mixing process, the forming process and the final product. The described method of rating the dispersion of the masterbatch was shown to:

* Correlate with the energy consumed during the mix of the masterbatch;

* correlate to the processability as indicated by the Mooney viscosity and Mooney stress relaxation of the final compound;

* correlate to the dispersion rating of the final vulcanizate:

* provide a dispersion rating in 4-5 minutes, including time to cool, cut and test the sample;

Use of the dispersion rating of the masterbatch is ideally suited to supply an intermediate set of data that can be used to monitor the mixing process, allowing savings in direct energy consumption, mixer overhead, and. ultimately, improved final product quality.

Table 1 - formulation for TPI

Ingredient                  PHR

SMR 5                     100.0
N330 black                 45.0
6 PPD                       1.5
TMQ                         1.5
Stearic acid                2.0
Zinc oxide                  3.0
Sulfur                      1.5
MBS                         1.5
TMTM                        0.2
Table 2 - mixing variations

Batch    Rubber break     Filler mix    Total mix
number     down, min.     time, min.    time, min.

  1            1               5             6
  2            1               6             7
  3            1               7             8
  4            1               8             9
  5            1               9            10
  6            1              11            12
  7            1              13            14
  8            0               6             6
  9            0              11            11
  10           2               7             9
  11           2               9            11
  12           3               5             8
  13           3               8            11
  14           3              13            16
  15           5               7            12
  16           5               9            14
  17           7               6            13
  18           7               8            15
  19           7              11            18
Table 3 - batch weights

Ingredient               PHR      Batch weight
Masterbatch

SMR 5                  100.0          840.0
N330 black              45.0          378.0
6 PPD                    1.5           12.6
TMQ                      1.5           12.6
Stearic acid             2.0           16.8
Zinc oxide               3.0           25.2
Masterbatch total      153.0        1,285.2

Final
Masterbatch            153.0          900.0
Sulfur                   1.5            8.82
MBS                      1.5            8.82
TMTM                     0.2            1.18
Final total            156.2          918.82
Table 4 -  mixing procedure

Mixing event              Occurs at:
Masterbatch:

Rubber breakdown          Time variable according to table 2
Filler and misc.          Time variable according to table 2
ingredient mix time
Sweep at:                 2.5, 3.75, 5.5 and 8 minutes where
                          applicable
Drop batch at:            Total time according to table 2

Final:
Breakdown MB              1 minute
Add cure system
Drop batch at:            2.5 minutes
Batch     Filler   Rubber   Total     Dispersion     Work
number     mix      mix      mix     MB     Cured   input
          time,    time,    time,    DR      DR     MB, MJ
           min.     min.    min.

   1        5        1        6     -3.94    6.99    4.384
   2        6        1        7     -1.62    7.77    5.368
   3        7        1        8      2.20    9.09    5.976
   4        8        1        9      2.61    9.18    6.588
   5        9        1       10      3.89    9.31    7.455
   6        11       1       12      6.26    9.48    9.338
   7        13       1       14      7.92    9.62   10.797
   8        6        0        6     -3.02    7.55    3.883
   9        11       0       11      3.90    9.27    8.775
  10        7        2        9      4.50    9.34    6.973
  11        9        2       11      5.42    9.47    8.168
  12        5        3        8      1.67    8.23    5.891
  13        8        3       11      6.98    9.50    8.168
  14        13       3       16      7.91    9.88   11.180
  15        7        5       12      5.38    9.00    8.509
  16        9        5       14      7.18    9.90    9.912
  17        6        7       13      6.58    9.49    8.511
  18        8        7       15      7.05    9.74   10.078
  19        11       7       18      9.15   10.12   12.442

Batch     Final
number    Mooney data
          ML1+4 t80, min.

   1      103.0   0.79
   2      101.1   0.64
   3       99.9   0.54
   4       93.6   0.42
   5       89.3   0.32
   6       80.3   0.24
   7       73.3   0.21
   8      104.6   1.05
   9       75.9   0.22
  10       92.9   0.38
  11       91.0   0.36
  12       99.9   0.53
  13       90.9   0.34
  14       79.2   0.23
  15       88.4   0.33
  16       79.6   0.26
  17       86.7   0.32
  18       81.5   0.26
  19       74.9   0.22

References

(1.) A.Y. Coran and J.B. Donnet, Rubber Chem. and Technology, 65, pp. 973-997 (1992).

(2.) P.S. Johnson, in "Basic compounding and processing of rubber," Harry Long, Ed., Rubber Division, American Chemical Society, 1997, p.52.

(3.) F.S. Myers and S.W. Newell, Paper no. 38, 111th Technical Meeting, Rubber Division, American Chemical Society, Chicago, IL, May 1977

(4.) G.E. O'Connor and J.B. Putman, Rubber Chem. Technology, 51, 799 (1978).

(5.) P.S. Johnson, in "Basic elastomer technology," K.C. Baranwal, H.L. Stephens, Eds., The Rubber Division, American Chemical Society. 2001, pp. 118-120.

(6.) ASTM D1566-00b, "Standard terminology relating," American Society for Testing and Materials, West Conshohocken, PA, p. 302.

(7.) R.W. Jones, Rubber Age, Sept. 1968, pp. 52-57.

(8.) A.B. Sullivan and R.W. Wise, in "Rubber Technology," M. Morton, Ed., 3nd Ed., Van Nostrand Reinhold. NY, 1987, pp. 105-108.

(9.) W. Cousins and J. Dick, Rubber World, Jan. 1998, pp. 28-35.

(10.) H. Burhin, W. Spreutels and J. Sezna, Paper no. 74, Technical Meeting, Rubber Division, American Chemical Society, Detroit, MI, October 1989.

(11.) B.B. Boonstra, in "Rubber Technology," M. Morton, Ed., 2nd ed., Van Nostrand Reinhold, NY, 1973, p. 70.

(12.) W.M. Hess, Rubber Chem, Technology, 64, p. 387, (1991).

(13.) P. Lovgren and S. Persson, Tire Technology International, 1994.

(14.) ASTM D2663-95a, "Standard test methods for carbon black-dispersion," 09.01, American Society for Testing and Materials, West Conshohocken, PA, pp. 401-411 (2001).

(15.) ASTM D1646-00, "Standard test methods for rubber-viscosity, stress relaxation and pre-vulcanization characteristics (Mooney viscometer)," 09.01, American Society for Testing and Materials, West Conshohocken, PA, pp. 318-329 (2001).

(16.) P.R. van Buskirk, S.B. Turtzsky and P.F. Gunberg, Rubber Chem. and Technology, 48, p. 590 (1975).

(17.) A.R. Payne, Rubber Chem. and Technology, 39, pp. 365-374 (1966).

(18.) B.R. Richmond, Paper no. 68, Rubber Division, American Chemical Society meeting, Denver; CO, May 1993.

(19.) J. Brennan and D.H. Lambert, Rubber Chem. and Technology, 45, p. 105 (1972).

(20.) A.Y. Coran and J.B. Donnet, Rubber Chem. and Technology, 65, pp. 1,025 (1992).

John B. Putman and Tech Pro. and Matthew C. Putman Robert Samples, Akron Rubber Development Laboratory