Use of coagents for adhesive and dynamic property improvements in peroxide-cured IR.

The rubber industry has utilized sulfur and organic accelerators for over 70 years as the primary method of curing elastomers. Other methods of crosslinking, such as peroxides and amines, have filled various niches in specialty polymers and high performance applications.

Until recently, peroxides and the commonly used coagents (acrylates, methacrylates, allyl cyanurates, low molecular weight polybutadienes and phenylene dimaleimides), have generally produced fairly crystalline vulcanizates with application-limiting original physical properties such as high modulus and hardness, combined with low tensile strength and elongation. Of course, their heat aging properties were superior to sulfur cure systems, making them very attractive in rigid specification compounds.

This article introduces peroxide-initiated metallic coagents to cure synthetic polyisoprene elastomers. The resulting physical properties are similar to the classic sulfur-accelerator cure systems. In addition, the flex properties obtained by using this system greatly surpass those of the sulfur system, while still maintaining the superior aging characteristics of a peroxide alone or peroxide-coagent system. Adhesion to various types of fiber and metal substrates can also be obtained by the use of this system. This unique combination of properties opens up a wide range of applications previously unavailable to peroxide-coagent curing of elastomers.

The purpose of this article is to:

* Highlight the optimization of a polyisoprene compound based on the use of a metallic methacrylate;

* provide performance property examples of both metallic acrylates and metallic methacrylates to educate the reader as to which coagent would be the best choice for use in a specific application:

* highlight adhesion data for both metallic acryates and methacrylates; and

* show how different peroxide types/cure temperatures can be used to overcome deficient scorch/cure times versus sulfur.

Peroxide-coagent cure mechanisms

Crosslinking with peroxide alone results in the formation of a covalent bond, as shown in figure 1. This carbon-carbon bond is quite rigid and stable (343.2 kJ bond energy), compared to the carbon-sulfur and sulfur-sulfur bonds in sulfur cured systems (C S, 276.2 kJ bond energy: S-S, 205.1 kJ bond energy). The excellent heat stability of this carbon carbon covalent bond explains the superior heat aging characteristics of peroxide cured systems.

[FIGURE 1 OMITTED]

In contrast, although polysulfide crosslinks formed in sulfur cures are thermally weaker and can slip/break along the hydrocarbon chain, they have the ability to reform. This mobility and breaking explains their superior tensile strength and tear properties.

The metallic coagent-peroxide crosslink bond is ionic as shown in figure 2. The technology and characteristics of this ionic bond have been detailed in the literature on commercial ionomers. These ionic bonds exhibit both excellent heat-aged stability and the ability to slip, break, and reform along the hydrocarbon chain. Thus, this unique system embodies the best characteristics of both the peroxide and sulfur cure systems: high tensile strength, high tear strength, high modulus and outstanding flex and heat aged properties.

[FIGURE 2 OMITTED]

Experimental

Materials

A masterbatch containing 100 phr Natsyn 2200, 50 phr N660 carbon black, five phr zinc oxide, one phr stearic acid and one phr Agerite resin D was used in all experiments related to polyisoprene (synthetic natural rubber). Natsyn 2200 was obtained from Goodyear Chemical. Luperox F, Luperox DC and Luperox 231 peroxides were obtained from Atofina Chemical. Sartomer supplied the metallic coagents used in this experiment.

Difficulties in mixing were initially encountered when the solid metallic coagent was used in its traditional powder form. The product is now being offered as a pelletized 75% active dispersion in ethylene propylene copolymer (EPM). These materials are marketed under the trade names Saret 75 EPM 2A (acrylate) and Saret 75 EPM 2M (methacrylate). Alternate copolymer binders are in development.

This form provides the improved mixing benefits of faster incorporation, more uniform dispersion and lower heat build up. This form is also dust-free, leading to a safer and healthier work place.

Formulations

Rubber compounds represented in table 1 were mixed in a prep mixer. Rubber compounds represented in tables 2-11 were mixed in a lab-scale mixer or two-roll mill. All samples (unless noted) were compression molded 21-23 minutes at 320[degrees]F, except for the sulfur samples, which were cured for 10 minutes. For DeMattia flex testing, cure times were 32 minutes for peroxide samples and 20 minutes for sulfur samples.

Measurement

Cure characteristics, which include scorch time, cure rate and torque values, were measured over a 30 minute period at 160[degrees]C and 3[degrees] arc using an oscillating-disk rheometer according to ASTM method D-2084-95.

Mooney scorch data were obtained using a Mooney viscometer with a large rotor. All testing conformed to ASTM standard D-1646-94.

Compression set was determined by compressing samples at 25% deflection for 70 hours at 100[degrees]C. Each sample was then removed and the permanent set measured as a percentage of original height according to ASTM standard D-395B.

Durometer A hardness tests were determined for samples after molding using a hand-held durometer.

Original physical properties were measured using die D dumbbells tested at 20 in./min. All samples were tested according to ASTM standard D-412-97, method A; 2240-97.

Tear strength data were performed at 150[degrees]C. A die C specimen was used and testing conformed to ASTM standard D624-91.

Heat age testing was performed after 70 hours at 100[degrees]C. All testing conformed to ASTM standard D-573-88 (94).

Flexibility testing was measured according to DeMattia flexibility ASTM D-813-95. In this test, each specimen was pierced 0.08 or 0.10 inches and tested at 300 cycles per minute. Unless otherwise noted, cycles to failure were reported when the piercing grew to 0.5 inches.

Tan delta and complex modulus ratio (E* 120[degrees]C/E* 25[degrees]C) were measured through dynamic mechanical analysis (DMA). DMA testing followed ASTM standard D-2231-94.

Two types of adhesion testing were performed. Lap shear adhesion was determined by testing a rubber specimen cured between two cold rolled steel coupons. This 0.030 inch specimen was cured at 160[degrees]C for 40 minutes (unless noted) between 1 x 3 x 0.030 inch coupons over-lapped one inch. A pressure of 30,000 psi was applied during curing. This test followed ASTM standard D-816. T-peel adhesion testing was performed by curing a 1 x 3 rubber specimen between the appropriate substrate and measuring the average peel strength over 2-3 inches of rubber tear. This test followed ASTM standard D-413 B, with a crosshead speed of two inches/min.

Results and discussion

The initial study conducted in polyisoprene (IR) was a comparison of three cure systems: sulfur, peroxide and a peroxide-solid coagent (S-75 EPM 2M). Table 1 highlights the results. The high tensile data and the five-fold increase in flex life resulted in a much more extensive study of the new cure system in order to arrive at an optimum formulation. Polyisoprene was selected in this expanded study for two reasons:

* It is more 'pure' than natural rubber, i.e., less likely to interfere with the free radical mechanism (more consistent cures); and

* the large amount of historical flex data that is available for comparison with this polymer.

Goals were set in this study to minimize the tan delta and to have the ratio of the complex modulus (E* 120[degrees]C/E*25[degrees]C) approach 1.0 as closely as possible. The following fine-tuned study shows the effect on the above, plus original physicals, aged physical properties and flex data versus a sulfur-organic accelerator cured compound.

In table 2, the effects of varying peroxide levels while keeping coagent levels constant were studied. Once again, a conventional sulfur cure control was included. As shown in table 2, an increase in the level of peroxide has the expected effect of increasing hardness and modulus, while overall tensile strength remains relatively constant.

In table 3, the coagent level was varied with constant peroxide levels. Once again, a sulfur system was included. Surprisingly, there is little or no effect on the physical properties by increasing the coagent levels. In the past, trends have shown that by increasing the levels of standard acrylic coagents (TMPTMA, EGDMA, etc.), properties such as cured hardness and modulus increase dramatically.

Table 4 summarizes the dynamic mechanical and flex data for the compounds in tables 2 and 3. The E* ratio and tan delta data for the peroxide-coagent systems are comparable to the data for a sulfur-accelerator system. The DeMattia flex results are outstanding for a peroxide-cured compound, with results 3-6 times greater than the sulfur cured compound.

Another recent DeMattia flex study focused on the use of a more conventional sulfur-based cure system instead of the semi-efficient sulfur control (1.6 phr sulfur, 1.6 phr TBBS) evaluated previously. This conventional sulfur system used 2.25 phr sulfur and 0.70 phr TBBS. The DeMattia flex (under the same conditions as the samples in table 4) failed at 40,000 cycles.

Optimization of peroxide/coagent levels

Having studied a grid of peroxide levels from 2-4 phr and coagent levels from 5-9 phr, it seemed necessary to try to optimize the new coagent system (keeping in mind the relative cost of the cure system versus the sulfur cure system). Tables 2 and 3 suggested that lower peroxide and median coagent levels would be optimum. Table 5 is the result of that optimization. From that table, it is concluded that compounds 7 and 8 exhibit the optimum cure levels, although each application will require its own optimization.

S-75 EPM 2A

Although the S-75 EPM 2M was chosen for the above study, the S-75 EPM 2A also deserves special attention. Table 6 provides a general summary of the performance differences between the S-75 EPM 2A and 2M.

Both the S-75 EPM 2A and 2M are evaluated in the remaining topics below.

Low temperature DeMattia flex response

In many critical applications, there is a need for a high flex response under low temperature conditions. Table 7 summarizes DeMattia test data taken at -40[degrees]C (after each sample was conditioned 10 minutes at -40[degrees]C). This portion of the study included the S-75 EPM 2A acrylate, as well as three examples of sulfur controls (conventional, semi-EV and EV).

The peroxide-coagent systems still provide excellent flexibility even at low temperatures, whereas all three sulfur-accelerator combinations are brittle at the temperatures and ultimately cracked during testing. The use of metallic coagents also shows an improved low temperature flex response versus peroxide alone.

Adhesion

In recent years, a great deal of research has been put into evaluating metallic coagents in robber compounds for increased adhesion to steel, brass, zinc, aluminum, polyester, rayon and other substrates. A large percentage of previous adhesion work was performed using EPDM elastomers. This work was recently expanded to include the use of poly-isoprene for adhesion to both fabric and metallic substrates.

Adhesion to cold rolled steel was performed with both S-75 EPM 2M and 2A. Figure 3 highlights these results.

[FIGURE 3 OMITTED]

As expected, using the metallic acrylate results in a higher bond to steel. The acrylate does become rather rigid at higher loadings, so one would expect the adhesion to level off as indicated in figure 3. Although the acrylate produces considerably higher adhesion levels to steel compared to the S-75 EPM 2M, the methacrylate is also a suitable choice for potential applications where bonding is desired, along with high flex demands.

These coagents also can increase bonding to fabric substrates. Figure 4 shows t-peel adhesion data for bonding to nylon fabric. In this case, a common RFL (resorcinol formaldehyde latex) treatment was applied to the fabric prior to bonding to the elastomer. It should be noted that these coagents typically offer greater adhesion to both treated and untreated fabrics.

[FIGURE 4 OMITTED]

Techniques for optimization of cure rate

One of the downfalls of this peroxide-coagent system is deficient scorch and cute times versus a sulfur-accelerator system. Tables 8 and 9 summarize rheology data from tables 2, 3 and 5.

As seen in the tables, increasing the coagent level does little to help the cure time, and has only minimal effects on improving scorch times. Increasing the peroxide level results in both a scorchier cure and minimal change in overall cure time.

Cure times can be improved by the manipulation of two variables, curing temperature and peroxide type. Up until this point of the study, dicumyl peroxide was the only peroxide evaluated, and 320[degrees]F was the standard curing temperature. Table 10 highlights cure and physical property data at 320[degrees]F, 338[degrees]F and 356[degrees]F cure temperatures.

These data indicate that by curing at higher temperatures, physical properties are not severely affected, even though the scorch time (Ts2) does decrease (as expected). The acrylate (S-75 EPM 2A) appears to withstand the temperature changes better than the 2M, but both coagents maintain their cured elongation at all three curing temperatures. By using higher curing temperatures and shorter cycle times, this system can now be used to compete with a sulfur-accelerator based system. Table 11 shows an example of this scenario.

Varying the peroxide type can also have a significant effect on cure rate and cured properties. Studies using bis peroxides, dicumyl peroxides and peroxyketals reveal a wide variety of curing scenarios. Figure 5 gives cure times at various temperatures. 6.7 phr S-75 EPM 2M was included in these formulations. Peroxide levels were chosen to have equal active oxygen content.

[FIGURE 5 OMITTED]

Even though peroxides can be used in conjunction with a coagent to change cure times, their level of efficiency varies. In figure 6, the peroxyketal shows the lowest crosslinking response (as expected), but has a fairly stable cure profile from 275-330[degrees]F. The bis peroxide also shows a fairly stable cure profile from approximately 329[degrees]F to 383[degrees]E The dicumyl peroxide, although highly effective, is most dramatically affected by curing at high temperatures. But, looking again at figure 5 and tables 10 and 11, cure times and physical properties can be optimized at around 350[degrees]F using this cure system.

[FIGURE 6 OMITTED]

In using these techniques, care must be taken during the compounding phase. As expected, using different peroxides will result in different time and temperature limits before scorch will begin to occur. The same concept is true when a coagent is used in conjunction with peroxide. Figure 7 provides a "mixing guideline" when using different peroxides with S-75 EPM 2M.

[FIGURE 7 OMITTED]

Summary

Previously, sulfur-accelerator and peroxide-coagent cure systems had their own domains of applications. By using a metallic coagent-peroxide system, the two can be used interchangeably with regard to original physical properties. Although this work was completed using synthetic natural rubber (polyisoprene), many of the concepts presented have been previously tested in EPDM elastomers. This indicates that these concepts could also apply to a variety of other elastomers.

In addition to the synergy between the two systems, the peroxide-coagent system offers much improved aging characteristics, greatly enhanced flex properties, excellent compression set, and much improved adhesion to various metals and fabrics when compared to conventional, semi-efficient, and efficient sulfur-cured systems.

Table 1 - historical comparison

                                 Peroxide   Sulfur   Peroxide-S

Polyisoprene                          100      100          100
Carbon black                           50       50           50
Zinc oxide                              5        5            5
Stearic acid                            1        1            1
Agerite Resin D                         1        1            1
Dicumyl peroxide                        4       --            2
  (40% active)
S-75 EPM 2M                            --       --            4
  (active phr)                         --      1.6           --
TBBS                                   --      1.6           --
Sulfur                                 --      1.6           --
Tensile strength (psi)              2,825    2,840        3,165
100% modulus (psi)                    385      430          465
Elongation, %                         375      355          380
Dynamic mechanical
  analysis (DMA)
  ASTM D-2231-94
E*120[degrees]C/E*25[degrees]C      1.017    1.112        0.899
Tan delta @ 25[degrees]C             0.22     0.18         0.19
DeMattia flexibility,
ASTM D-813-95
Cycles to failure                  33,000   30,000      156,000
Compression set %                      16       54           36

Table 2

                             1       2       3     Sulfur
                                                   control

Polyisoprene MB              157     157     157       157
Dicumyl peroxide               2       3       4        --
  (40% active)
S-75 EPM 2M                    5       5       5        --
  (active phr)
TBBS                          --      --      --       1.6
Sulfur                        --      --      --       1.6
Rheometer data
Max torque (in.-lbs.)       56.7    69.4    76.5      71.8
Scorch time                  1.8     1.6     1.4       3.3
  (Ts2, min.)
Cure Time                   17.0    17.5    17.1       6.1
  (Tc90, min.)
Original physicals
  ASTM D-412-97,
  Method A; 2240-97
Tensile strength (psi)     3,380   3,450   3,340     2,840
300% modulus (psi)         1,810   2,260   2,450     1,720
Elongation, %                520     450     430       440
Hardness durometer A          58      60      61        63
Heat aged physicals
ASTM D-573-88(94)
Tensile strength; % chg.    -8.3    -2.9    -3.9     -20.4
300% modulus; % chg.          -5    -1.8    -2.9      16.3
Elongation, % chg.          -3.8    -6.7    -4.7     -25.0
Hardness, chg. pts.           +1      +2      +1        +3

Table 3
                             4       5       2    Sulfur
                                                 control

Polyisoprene MB            157     157     157       157
Dicumyl peroxide             3       3       4        --
  (40% active)
S-75 EPM 2M                  9       7       5        --
  (active phr)
TBBS                        --      --      --       1.6
Sulfur                      --      --      --       1.6

Rheometer Data;
  ASTM D-2084-95
Max torque (in.-lbs.)     64.2    71.8    69.4      71.8
Scorch time                2.0
(Ts2, min.)                        1.6     1.6       3.3
Cure time                 15.4    16.1    17.5       6.1
(Tc90, min.)

Original physicals
  ASTM D-412-97,
  Method A; 2240-97
Tensile strength (psi)   3,220   3,590   3,350     2,840
300% modulus (psi)       2,180   2,360   2,260     1,720
Elongation, %              440     460     450       440
Hardness, durometer A       61      62      60        63

Heat aged physicals
  ASTM D-573-88(94)
Tensile strength          -6.3    -8.6    -2.9     -20.4
% chg.
300% modulus              -4.6    -6.4    -1.8      16.3
% chg.
Elongation, % chg.         0.0    -4.3    -6.7     -25.0
Hardness, chg. pts.         +2      +1      +2        +3

Table 4

                                 Peroxide         4         5
                                  control

Polyisoprene MB                       157       157       157
Dicumyl peroxide                        2         3         3
  (40% active)
S-75 EPM 2M                            --         9         7
  (active phr)
TBBS                                   --       --         --
Sulfur                                 --       --         --

Dynamic mechanical
  analysis (DMA)
  ASTM D-2231-94
E*120[degrees]C/E*25[degress]C       1.02      0.92      0.96
Tan delta @ 25[degrees]C             0.22      0.20      0.14

DeMattia flexibility,
ASTM D-813-95
Cycles to failure                  33,000   140,000   110,000

                                       1         2        3    Sulfur
                                                              control

Polyisoprene MB                      157       157      157       157
Dicumyl peroxide                       2         3        4        --
  (40% active)
S-75 EPM 2M                            5         5        5        --
  (active phr)
TBBS                                  --        --       --       1.6
Sulfur                                --        --       --       1.6

Dynamic mechanical
  analysis (DMA)
  ASTM D-2231-94
E*120[degrees]C/E*25[degress]C      0.93      0.96     0.97      1.04
Tan delta @ 25[degrees]C            0.17      0.16     0.15      0.15

DeMattia flexibility,
ASTM D-813-95
Cycles to failure                240,000   140,000   80,000    30,000

Table 5

                            6         7         8      Sulfur
                                                      control

Polyisoprene MB             157       157       157       157
Dicumyl peroxide              2         2         2        --
  (40% active)                                             --
S-75 EPM 2M                  --         3         5
  (active PHR)
TBBS                         --        --        --       1.6
Sulfur                       --        --        --       1.6
Rheometer data;
  ASTM D-2084-95
Max torque (in.-lbs.)      39.4      47.8      51.7      75.1
Scorch time                 1.8       1.8       2.0       2.8
  (Ts2, min.)
Cure time                  17.5      16.6      16.6       5.1
  (Tc90, min.)
Original physicals
  ASTM D 412-97,
  Method A; 2240-97
Tensile strength (psi)    2,320     3,120     3,230     3,270
300% modulus (psi)        1,010     1,530     1,480     1,610
Elongation, %               550       530       540       530
Hardness, durometer A        49        56        57        64
Heat aged physicals
  ASTM D 573-88(94)
Tensile strength (psi)    1,720     2,530     2,440     2,250
300% modulus (psi)          680     1,460     1,520     2,230
Elongation, %               540       470       440       300
Hardness, durometer A        45        54        56        66
Compression set, %           15        34        36        56
DeMattia flex            50,000   130,000   190,000    30,000

Table 6

S-75 EPM 2M                 S-75 EPM 2A

Slower cure                 Faster curing, higher initial
                              modulus
Greater flexibility         Higher initial modulus
Less potential for scorch   Higher potential for scorch
Moderate adhesion           Significant adhesion
  promoter                    promoter
Improved compression set    Compression set slightly
  versus sulfur               improved over S-75 EPM 2M

Both coagents have excellent flex, tan delta, dynamic
modulus and heat stability

Table 7

                          1          2          3
System                2 phr      2 phr      2 phr
Crack growth        dicumyl   peroxide   peroxide
(in.; avg. of 3)   peroxide      5 phr      5 phr
                                  S-75       S-75
                                EPM 2A     EPM 2M

      0 cycles        0.080      0.080      0.080
    100 cycles        0.118      0.110      0.090
  1,000 cycles        0.140      0.113      0.090
  3,000 cycles        0.160      0.123      0.103
  5,000 cycles        0.160      0.127      0.103
 10,000 cycles        0.160      0.133      0.107
 20,000 cycles        0.160      0.133      0.107
 30,000 cycles        0.160      0.143      0.110
 40,000 cycles        0.160      0.150      0.115
 50,000 cycles        0.160      0.150      0.115
 60,000 cycles        0.160      0.150      0.115
 70,000 cycles        0.160      0.150      0.115
 80,000 cycles        0.203      0.150      0.115
 90,000 cycles        0.203      0.150      0.115
100,000 cycles        0.203      0.150      0.115

                           4           5                6
System               1.6 phr     0.5 phr   2.5 phr sulfur
Crack growth          sulfur      sulfur     0.8 phr TBBS
(in.; avg. of 3)     1.6 phr     2.4 phr   (conventional)
                        TBBS    TBBS(EV)
                   (semi-EV)

      0 cycles         0.080       0.080            0.080
    100 cycles         0.107       0.697          samples
  1,000 cycles         0.107   (failure)            broke
                                 samples
  3,000 cycles         0.120       broke               --
  5,000 cycles         0.127          --               --
 10,000 cycles         0.137          --               --
 20,000 cycles         0.147          --               --
 30,000 cycles         0.170          --               --
 40,000 cycles         0.180          --               --
 50,000 cycles         0.190          --               --
 60,000 cycles         0.217          --               --
 70,000 cycles         0.233          --               --
 80,000 cycles       samples          --               --
                       broke          --
 90,000 cycles            --                           --
100,000 cycles            --          --               --

Table 8--effects of varying coagent levels on
cure rate

S-75 EPM 2M       40% dicumyl     Scorch time    Cure time
(active phr)     peroxide (phr)   (Ts2, min.)   (TC90, min.)

0                      2              1.8           17.5
3                      2              1.8           16.6
5                      2              2.0           16.6
7                      2              2.0           17.1
9                      2              2.3           16.7
Sulfur control         --             3.3            6.1

Table 9--effects of varying peroxide levels on cure rate

S-75 EPM 2M     40% dicumyl     Scorch time    Cure time
(active phr)   peroxide (phr)   (Ts2, min.)   (TC90, min.)

     5               2              2.0           16.6
     5               3              1.6           17.5
     5               4              1.4           17.1

Table 10

                             1        2       3        4        5
System                    3.0 phr    40%     DCP    3.0 phr    40%
                                                    6.7 phr   S-EPM

Cure temp. ([degrees]F)     320      338     356      320      338
Tc 90 (min.)               32.2     14.8    6.83     28.0     12.0
Ts 2 (min.)                2.34     1.75    1.29     2.50     1.96
Mold cycle time (min.)      40       20      10       40       20
Tensile strength (psi)     3,050    3,100   2,830    3,150    3,100
Modulus 100 (psi)           180      190     175      380      350
Total % elongation          650      650     620      470      470

                            6        7        8       9
System                     DCP    3.0 phr    40%     DCP
                           2A     6.7 phr   S-EPM    2M

Cure temp. ([degrees]F)    356      320      338     356
Tc 90 (min.)              5.41     30.2     13.5    6.49
Ts 2 (min.)               1.41     2.49     1.84    1.34
Mold cycle time (min.)     10       40       20      10
Tensile strength (psi)    3,100    3,340    3,100   3,000
Modulus 100 (psi)          350      340      280     270
Total % elongation         480      530      530     520

Table 11--cured physical properties after ten minute cure cycle

                          Conventional   SEV sulfur
                             sulfur      (1.6 phr     3 phr 40% DCP
System                     (2.25 phr      sulfur,        6.7 phr
                          sulfur, 0.7     1.6 phr       S-EPM 2A
                           phr TBBS)       TBBS)

Cure temp. ([degrees]F)       320           320            350
Tensile strength (psi)       3,620         3,760          3,400
Modulus 100 (psi)             315           333            315
Modulus 300 (psi)            1,120         1,215          1,480
Tear strength (pli)           325           335            310