Peroxide curing of millable polyurethane.

Polyurethane rubber is a specialty rubber that finds use in many common robber articles such as skate wheels, conveyor belts, rubber covered rolls and other applications where urethane is used because of its properties. Urethane rubber compounds possess a unique combination of excellent abrasion resistance, excellent solvent and oil resistance, high tensile and tear properties, good resistance to ozone and oxygen, and good low temperature properties.

Millable urethanes are a form of urethanes that are processable on conventional rubber machinery and compounded much in the same way as other commonly used rubbers. Sulfur curing is the most widely used form of vulcanization for urethanes because of the cure flexibility and the better properties achieved in some areas. Peroxide curing is also fairly common because of the benefits to set and aging properties.

This article studies peroxide curing of a polyester type of millable urethane, evaluates peroxide levels and coagent types and levels, and compares the results to a typical sulfur cure of the same base compound.

Experimental

A polyester urethane (ASTM designation AU, Mooney viscosity ML (1+4)/100 [degrees] C of 40), identified as PU-1, was chosen for this study. The base formulas used are shown in table 1. Dicumyl peroxide was chosen for the peroxide study because it's one of the most commonly used in rubber vulcanization. The identification of the ingredients used is shown in table 2.

Table 1 - standard formulations used

                        Peroxide     Sulfur

PU-1                     100.00      100.00
N330 black                25.00       25.00
DBEEF (plasticizer)        5.00        5.00
Stearic acid               0.25        0.25
PA-1                       1.00        1.00
Zinc stearate                          0.50
Coagent               As indicated      ---
DCP40                 As indicated      ---
MBTS                                   4.00
MBT                                    2.00
C-4                                    1.00
Sulfur                                 1.50
Table 2 - ingredients used

Ingredient
used         Description                           Trade name

ACo          Acrylic coagent, scorch retarding     Saret 517
C-4          Zinc chloride-MBTS complex            Caytur 4
DBEEF        Di(butoxy-ethoxy-ethyl) formal        TP90B
DCP40        Dicumyl peroxide, 40%                 Dicup 40C
MACo         Metallic diacrylic coagent            Saret 633
MPDM         N,N'-m-phenylenedimaleimide           HVA-2
PA-1         Process aid                           Struktol WB-212
PU-1         Polyester urethane rubber             Vibrathane 5008
SBC          Styrene butadiene random copolymer    Ricon 100
TMPTMA       Trimethylol propane trimethacrylate   SR-350

Ingredient
used         Supplier

ACo          Sartomer Co.
C-4          Uniroyal Chemical
DBEEF        Morton International
DCP40        Hercules Inc.
MACo         Sartomer Co.
MPDM         DuPont Dow Elast.
PA-1         Struktol Co.
PU-1         Uniroyal Chemical
SBC          Ricon Resins, Inc.
TMPTMA       Sartomer Co.

Compounds were mixed in a laboratory internal mixer, with the following mix procedure:

* Rotor speed - 75 rpm, water temperature - 20 [degrees] C;

* Load polymer and processing aids, ram down, clock to 0';

* At 1', add black, plasticizer and coagents, ram down;

* At 2.5', sweep;

* At 3.0', dump (dump temperature 90 [degrees] -110 [degrees] C);

* Curatives were added to stock on a cool mill.

Compounds were cured and tested to ASTM procedures. Compounds with sulfur cure systems were press cured 10'/ 160 [degrees] C, peroxide cure systems were cured 20'/160 [degrees] C. Samples for Bashore resilience, 12.7 mm in thickness, were cured for 2x the standard cure time.

Abrasion resistance was tested by three different methods:

* NBS abrasion (ASTM D1630) - this test determines abrasion resistance by measuring the number of revolutions needed to abrade 2.5 mm from the compounds from a rotating drum covered with an abrasive paper and comparing the results to that of an IR compound standard. The test is commonly used for footwear.

* Pico abrasion (ASTM D2228) - this test determines abrasion resistance by measuring the volume loss of samples abraded by tungsten carbide knives compared to reference compounds.

* DIN abrasion (ASTM D5963) - this test determines abrasion resistance by measuring the volume loss of samples sliding across the surface of an abrasive sheet attached to a rotating dram, comparing the results to that of a standard rubber. Samples were tested using a rotating test specimen.

Results and discussion

Varying peroxide level

In the polyester urethane, peroxide levels were varied from 1 to 5 phr of 40% dicumyl peroxide (0.4 - 2.0 active ingredient). The data showed the following trends and comparisons to a sulfur cure:

* Hardness increased linearly as peroxide level increased, with the sulfur cure giving hardness similar to the 4 phr DCP40 level (figure 1). Modulus also increased linearly, with about a 2.3 phr level of DCP40 giving similar modulus to the sulfur cure (figure 2). The elongations decreased with increasing peroxide level, with about a 1.4 phr level giving similar elongation to the sulfur cure. The sulfur cure gave higher tensile strength than the peroxide cures. The tensile strength for the peroxide cures went through a maximum at the 2.0 peroxide level.

[Figures 1-2 ILLUSTRATION OMITTED]

* The tear strength showed a similar trend as the tensile strength, peaking at the 2 phr peroxide level. Tear also was superior with the sulfur cure (figure 3).

[Figure 3 ILLUSTRATION OMITTED]

* Bashore resilience values were similar for the varying peroxide levels (15-21%), with no apparent trend. The sulfur cure gave a lower resilience (11%) than all the peroxide cures.

* Compression set is one property that really shows the benefits of peroxide curing. Figure 4 shows the compression sets plotted for the three conditions tested and for all three conditions, even the lowest peroxide levels showed superior set compared to the sulfur cure. The set did, as expected, decrease with increasing peroxide level up to 4 phr, with the 5 phr level showing minimal additional benefit.

[Figure 4 ILLUSTRATION OMITTED]

* The NBS abrasion resistance increased as the level of peroxide increased, peaking at the 4 phr level of DCP40 (figure 5). The DIN abrasion seemed to show a maximum, but at the lower level of 2 phr DCP40 (figure 6). The Pico abrasion increased linearly with increasing peroxide level, with the 5 phr level showing the best abrasion. For all three abrasion tests, the sulfur cure was matched or beaten by the best of the peroxide cures. The data also illustrate the need to determine which abrasion test best simulates the type of abrasion seen in service before determining the best balance of compound variables, including curative type and levels.

[Figures 5-6 ILLUSTRATION OMITTED]

* In regard to heat aging properties, the peroxides showed much better retention of properties than the sulfur cure system, especially in aged elongation retention. Figure 7 shows the change in elongation and hardness after aging 70 hr./ 100 [degrees] C. The sulfur cure system lost 30% of its elongation, while the peroxide cures had maximum elongation losses of 10%. The higher levels of peroxide showed generally better retention of elongation than the lower peroxide levels. The change in hardness after aging also shows the sulfur cure to be somewhat deficient to the peroxide cures. The elongation and hardness changes after aging 70 hr./125 [degrees] C, in figure 8, show the same trends as the lower temperature aging, but the percent changes in elongation are more linear as peroxide level increases.

[Figures 7-8 ILLUSTRATION OMITTED]

Varying TMPTMA and dicumyl peroxide

Another part of the study was to vary the levels of the coagent TMPTMA and dicumyl peroxide in an experimental design to determine the optimum levels of each for the best properties, also comparing them to the sulfur cured control. The figures referenced show contour plots of the properties with the value seen for the sulfur cured compound noted for reference.

* The scorch, t5 at 125 [degrees] C, decreases as both the peroxide and TMPTMA increase. Generally, DCP40 + TMPTMA totals over 4 phr yielded scorchier compounds than the sulfur cure, while totals less than 4 phr were less scorchy than the sulfur cured control. The rate of cure, indicated by t90 at 150 [degrees] C, improved by increasing both ingredients, but even the highest levels evaluated didn't match the t90 of the sulfur cure, 11.8'.

* Both hardness and 100% modulus increased with increasing levels of DCP40 and TMPTMA. Each part of DCP40 increased the hardness, on the average, 2 points, while each part of TMPTMA increased the hardness about 1.5 points. Levels of DCP40 or TMPTMA over 3.5 phr or combined levels of the two over about 4.5 phr gave higher hardnesses than the sulfur control. The modulus showed a similar trend, with each part of DCP40 increasing the 100% modulus about twice as much as a part of TMPTMA. Higher modulus related to lower elongation, with all but the lowest levels of peroxide and coagent giving higher modulus and lower elongation values than the sulfur cure.

* The tensile strength data, when analyzed, didn't show a high degree of correlation, but there did seem to be a maximum between 2 and 3 phr of DCP40 and little effect of increasing TMPTMA. All peroxide cures gave lower tensile strength (maximum of 29.9 MPA with 2 DCP40, 0 TMPTMA) than the sulfur cure (33.5 MPa).

* Tear strength seemed to go through a "sweet spot," with the highest values in the 1.5-3.0 phr DCP40 and 1-6 phr TMPTMA area. Tear is another area where the sulfur cure shines, giving significantly higher tear strength (58 kN/m) than the highest peroxide/coagent cure (42 kN/m).

* The polyester urethane used is known for its excellent damping (low rebound) properties. The analysis of the Bashore rebound data didn't show a high degree of correlation, but the data did suggest that slightly higher rebound is achieved with lower levels of TMPTMA. The level of peroxide did not seem to be a significant factor.

* The Pico abrasion showed fair correlation with the experimental data, showing higher abrasion resistance with higher levels of both peroxide and methacrylate coagent. The NBS abrasion test showed good correlation with DCP40 level, increasing abrasion resistance as the level of DCP40 increased, with the moderate to high levels of DCP40 surpassing the NBS abrasion index of the sulfur cure. The DIN abrasion test results showed poor correlation.

* Data from compression sets tested under all three conditions (22 hr./70 [degrees] C, 22 hr./100 [degrees] C and 70 hr./100 [degrees]) showed similar trends. The set was mostly influenced by the peroxide level, improving (decreasing in set) dramatically as the level of DCP40 increases, although the data suggest an optimum level of peroxide at 4 phr. The level of TMPTMA was less a factor, but it did suggest that, as levels of TMPTMA increased, lower set was seen at low peroxide levels but higher set at high peroxide levels. As expected, the sulfur cures were much poorer for set under all conditions, giving roughly 3-4 times the set values as the peroxide cures.

* The heat aging data, after oven aging 70 hr./125 [degrees] C, showed the trend of less elongation change as the peroxide level increased, however the absolute elongation values were, of course, lower at the higher levels of peroxide than the lower levels. The sulfur cure showed up fairly well after these aging conditions, having 380% elongation, only a loss of 34% of its original elongation.

Varying coagent type

Several types of coagents were evaluated for their effectiveness in improving various properties. These coagents were evaluated in a compound with 2.25 phr DCP40 peroxide. The coagents evaluated are shown in table 3. The key properties affected are:

* Scorch and cure rate: TMPTMA and MPDM had the biggest effect on scorch, both reducing the compound scorch from 26' to 11'-14' (figure 9). The scorch retarded methacrylate, ACo, increased the scorch time to 38'. All three acrylate-types and the MPDM showed a decrease in t90 (at 150 [degrees] C), with the MPDM showing the largest effect. The styrene butadiene copolymer (SBC) showed longer t90 values than the control.

[Figure 9 ILLUSTRATION OMITTED]

* Hardness: The TMPTMA and ACo, at the four part level, increased the hardness four and six points, respectively (figure 10). The MPDM had a similar effect, albeit at the one and two part levels.

[Figure 10 ILLUSTRATION OMITTED]

* Modulus and tensile strength: TMPTMA and MPDM do the best job of increasing modulus (figure 11), with SBC actually lowering the modulus at the higher, 4 phr, level. Tensile strength was somewhat lower with all the coagents evaluated vs. the control, with ACo giving the lowest tensile strength.

[Figure 11 ILLUSTRATION OMITTED]

* Tear strength: The ACo, MACo and MPDM compounds showed them to give a slightly lower tear strength than the control, with the other coagents giving comparable tear strength (figure 12).

[Figure 12 ILLUSTRATION OMITTED]

* Resiliency: Bashore rebound data showed no significant differences between the coagent compounds and the control.

* Compression set: The data showed that the MACo gave higher (poorer) set values at all set conditions than the other samples (figure 13). The TMPTMA and the MPDM compounds showed slightly lower set when tested at the 22 hrs. and 70 hrs. at 100 [degrees] C conditions.

[Figure 13 ILLUSTRATION OMITTED]

* Abrasion resistance: NBS abrasion testing showed the 2 phr MPDM compound to have significantly higher abrasion resistance than the control, with the MACo, ACo and SBC compound showing lower abrasion than the control (figure 27). Pico abrasion tests also showed the MPDM compounds to be the best, with the 2 phr level giving the highest abrasion resistance values. The 1 phr MPDM and TMPTMA and ACo compound also had higher abrasion resistance than the coagent-less control. The DIN abrasion tests showed all of the coagents to give slightly lower abrasion resistance than the control.

* Heat aging properties: The ACo, TMPTMA and SBC (2 phr) compounds showed an increase in tensile after aging 70 hr./125 [degrees] C, along with minimal loss of elongation. The SBC at 4 phr and 1 and 2 phr levels of MPDM all gave somewhat higher losses of elongation after aging than the coagent-less control.

Table 3 - coagents evaluated

                                              Levels
Coagent   Description                         evaluated

TMPTMA    Trimethylos propane
           trimethycrylate                    4
ACo       Acrylic coagent, scorch retarding   4
MACo      Metallic diacrylic coagent          4
MPDM      N,N'-m-phenylenedimaleimide         1,2
SBC       Styrene-butadiene random
           copolymer                          2,4

Conclusions

Peroxide cures generally give, compared to sulfur cures:

* Lower tensile and tear strength;

* better (lower) compression set;

* somewhat comparable abrasion resistance properties, but this property is very dependent on the type of abrasion test conducted and the level of peroxide and/or coagent used;

* better resilience (rebound);

* better heat aging properties.

For each property tested, the following are suggestions to optimize that property:

* Tensile strength: Sulfur gives the highest tensile strength. Of the peroxide cures, the 2 phr level of DCP40 without coagent gave the highest tensile.

* Tear strength: Sulfur gave the highest tear strength; 2 phr of DCP40, with or without 1-5 phr TMPTMA, gave the highest tear strength among the peroxide cures.

* Resilience: Peroxide cures gave slightly higher Bashore rebound than the sulfur cure, with little effect of peroxide level or coagent.

* Compression set: Peroxide cures easily outperformed the sulfur cure in this area. The best (lowest) compression set was seen with the highest levels of peroxide (4-5 phr of DCP40). Of the coagents, which were evaluated with a lower peroxide level (2.25 phr), MPDM gave the best set properties. A high level of peroxide combined with MPDM may give even lower compression set values than those seen in this study.

* Abrasion resistance: For NBS abrasion, peroxide cures with 2 phr MPDM gave the highest abrasion, with compounds containing 3-5 phr DCP40, with or without TMPTMA, also giving very high values, higher than the sulfur cure. For Pico abrasion, compounds with MPDM gave the highest abrasion resistance. For DIN abrasion, both the 2 phr DCP40 level (no coagent) and the sulfur cure gave comparable high values.

* Heat aging resistance: Peroxide cures were better than sulfur cures for heat aging as measured by change in properties after the aging exposure. The property changes decreased as the level of peroxide increased.

Overall, both peroxide cures and sulfur cures can be used very effectively to vulcanize solid urethane rubber compounds. Rubber compounding almost always involves some compromise, and decisions on what properties are most important are necessary to make educated decisions on ingredient types and amounts to use. Hopefully, this article provides data that will help in those decisions.3