Performance of a novel activator for azodicarbonamide for sponge and cellular rubber.

In the process of investigation into the activation of blowing agents based on azodicarbonamide (AC) a new zinc compound was isolated. Results from x-ray crystallography and IR/Raman spectroscopy have identified the new chemical as diamine bis (cyanato-N) zinc with the chemical structure: [Mathematical Expression Omitted]

Technical DBZ is a white solid with a diffuse melting and decomposition point around 140-170 [degrees] C.

This compound exhibits strong activation of AC-based blowing agents and initial screening studies established DBZ as a secondary accelerator in sulfur vulcanization of polymers such as EPDM, SBR, NBR and natural rubber. Its cure activation property was readily demonstrated by the fact that optimum rate and state of cure could be obtained in the absence of stearic acid. The presence of stearic acid can have a detrimental effect as it may react with DBZ and reduce its availability for both activation and acceleration.

The effect of DBZ on blowing agents

It is well known that zinc oxide is an activator for lowering the decomposition temperature of AC and AC-based chemical blowing agents. Since zinc oxide is almost always present in rubber formulations then it is inevitable that the decomposition temperature of AC, if present, will be lowered.

Studies have been carried out to investigate whether DBZ, being a zinc based chemical, has any additional activation effects on AC-based blowing agents both in the presence and absence of zinc oxide.

The effect of DBZ on AC decomposition using differential scanning calorimetry (DSC)

Compositions of the blends which have been investigated are shown in table 1 and given reference letters which are used throughout the text.

Table 1 - compositions of blowing agent/DBZ
blends
               A    B    C    D    E    F
Ingredients:
AC            100  100  100  100  100  100
DBZ            -    1   2.5   5   10   20

Figure 1 demonstrates a typical effect of DBZ on the temperature at which the peak exotherm of azodicarbonamide decomposition occurs as measured by DSC. The figure shows the activation effect of five parts of DBZ on 100 parts AC, i.e., blend D. In this case, the temperature of AC decomposition has been reduced from 213 [degrees] C to 167 [degrees] C.

The peak exotherm was also measured by DSC for blends B, C, E and F. These are shown in a plot in figure 2 of temperature of peak exotherm against DBZ ratio to 100 parts AC.

As can be seen from the graph, the addition of one part of DBZ has a pronounced effect on decomposition temperature and shows further dramatic reduction up to 10 parts. Thereafter the effect of further addition becomes limiting.

If DBZ were the only zinc containing material present in a rubber compound, its effect on the decomposition of AC could be gauged from this graph.

However, as has been stated above, most rubber formulations will contain zinc oxide as an integral part of the vulcanization system, which itself will have an effect on AC decomposition temperature. This effect, in the presence and absence of DBZ, has been studied.

Effect of zinc oxide and DBZ o AC using DSC

Table 2 summarizes the composition of the blends tested and the results of measurement of the temperature of peak exotherm of AC decomposition by DSC.

Table 2 - composition of AC/ZnO blends and
their peak exotherm temperatures
                             A     G     H     I
Ingredients:
AC                          100   100   100   100
ZnO                          -     30    40    30
DBZ                          -     -     -     10
Peak exotherm
temperature ([degrees] C)   213   165   163   146

As can be seen in this table, the addition of 30 parts of zinc oxide has a significant effect on the decomposition temperature of AC reducing it by 48 [degrees] C. The addition of a further 10 parts of zinc oxide has very little effect on further reducing this decomposition temperature.

However, the further addition of 10 parts of DBZ produces a further significant reduction in decomposition temperature down to 146 [degrees] C. This clearly demonstrates the powerful activating effect of DBZ compared to zinc oxide. This is also demonstrated by the observation that only five parts addition of DBZ reduces the decomposition temperature of AC to 167 [degrees] C (figure 1).

The effect of DBZ on sulfonhydrazide blowing agents

Sulfonhydrazides are commonly used in producing cellular rubber products and the possible effect of DBZ on this class of blowing agent has also been considered.

Since zinc oxide is not considered to be an activator for sulfonhydrazides, it is not expected that DBZ would produce the same dramatic effects as can be seen when in combination with azo compounds.

The DSC trace obtained from heating a mixture of 100 parts OBSH and 10 parts DBZ shows a broad peak exotherm at only 3 [degrees] C lower than that of OBSH alone, indicating that little, if any, activation of OBSH occurs. Complexation of DBZ with OBSH followed by decomposition of OBSH around its usual decomposition temperature may be occurring.

The effect of DBZ and zinc oxide on the AC decomposition by gas evolution tests

The work described above has shown that, as measured using a DSC technique, DBZ has a dramatic effect on the AC decomposition temperature.

However, the use of DSC has its limitations as well as its advantages. In particular the small sample size used for measurement can lead to potential problems and give misleading results. For example, obtaining a representative sample can be difficult, particularly where powder blends are being tested. Also, in the case of blowing agents, decomposition is usually accompanied by significant movement of the solids in the sample pan and this can lead to some variations from run to run and on occasions, the appearance of spurious peaks.

Because of this, it is necessary and prudent to measure the events shown in the DSC experiment using another technique.

In the case of chemical blowing agents, it is customary to subject products and blends to gas evolution techniques from which not only the gas yield can be obtained but also the decomposition temperature can be observed.

The method chosen for this work is an in-house adaptation of a draft method described in ASTM D1715. This method is primarily designed to measure gas yield. However, it is possible for the operator to make the observation of when the highest rate of gas yield is being obtained and this is interpreted as the temperature of decomposition.

The results of these tests are shown in table 3 together with comparative data from the DSC experiments.

Table 3 - effect of DBZ on decomposition
temperature and gas yield
                                           A     G     I
Ingredients:
AC                                        100   100   100
ZnO                                        -     30    30
DBZ                                        -     -     10
Peak exotherm temperature ([degrees] C)   213    165   146
Decomposition temperature ([degrees] C)   210    173   155
Gas yield (ml/g AC)                       200    212   208

In the case of AC on its own the correlation between this technique and DSC is as good as would be expected, from the use of a pure substance with a sharp, observable decomposition point. The differences observed where blends are under test might also be expected, since the decomposition point is now less distinct than with a pure substance and is reliant on operator observation. Also in this method the heating rate is not precisely controlled as with DSC. It is known from experience that different heating rates can alter the decomposition mechanism and pathways leading to different decomposition temperatures. However, as can be seen in table 3 the same trend in reduction in decomposition temperature can be demonstrated.

It is also seen that, although the decomposition temperature of AC has been further reduced by the presence of DBZ, the gas yield per gram of AC has not changed.

An alternative decomposition method using a gas evolution technique in which a small sample of blowing agent was heated at 6 [degrees] C/minute in a closed system gave the decomposition temperatures shown in table 4.

Table 4 - effect of DBZ on decomposition
temperature
                                          A    G    I
Ingredients:
AC                                       100  100  100
ZnO                                       -    30   30
DBZ                                       -    -    10
Decomposition temperature ([degrees] C)  210  165  140

As can be seen, very similar results and trends are also demonstrated by this method.

The application of DBZ in cellular polymers

As stated, DBZ has been found to take a very active part in the sulfur vulcanization of both natural and synthetic rubbers. When added at 1 to 1.5 phr to existing formulations of sulfur-cured polymers, the result is almost always a faster curing compound with enhanced rate and state of cure. DBZ appears to:

* Act as a synergist with thiazole accelerators;

* allow the more efficient use of elemental sulfur.

In sulfur-vulcanized cellular polymer systems, DBZ serves as both an accelerator synergist and an activator of azodicarbonamide. Because of this dual action on the curing and expansion processes it is necessary to obtain the right balance between the rate of cure of the compound and the temperature and rate of decomposition of the blowing agent.

The effect of DBZ on cellular EPDM

The powerful activation of AC by DBZ in the presence of zinc oxide demonstrated above is thought to proceed through an intermediate metal complex where at least a part of DBZ is used up in the activation mechanism. It is therefore necessary to incorporate a sufficient amount of DBZ in the polymer matrix if it is to act also as an accelerator.

Extensive work has been done with DBZ in EPDM formulations. Results from this work also suggest that for EPDM sponge, a better balance between the rate of cure and the decomposition temperature is obtained by using a combination of AC with OBSH or p-toluene sulfonhydrazide (TSH) together with the usual level of 1 to 1.5 phr of DBZ. The sulfonhydrazides appear to restore the rate and state of cure by taking part in the cross-linking mechanism and partially compensating for the amount of DBZ consumed in the activation of AC decomposition.

An example of an EPDM sponge compound in which DBZ and a combination of AC and OBSH has been used is shown in table 5. In this case, an existing continuous vulcanization cure system which contains nitrosamine forming accelerators has been reformulated with DBZ and other accelerators known not to generate volatile nitrosamines. The results obtained are shown in table 6.

Table 5 - cellular EPDM formulation with DBZ
Formulation                  Control    EP71-8
EPDM base(a)                   278.0     278.0
Stearic acid                    1.0       0.5
TDEC                            1.5        -
DPTTS                           1.0        -
CBS                             1.5        -
ZDBC                            2.0        -
DTDM                            2.0        -
Sulfur                          1.3       1.5
Ficel OB(b)                     2.0       2.0
Ficel AC(c)                     3.0       3.0
ZBEC                             -        1.5
MBTS                             -       0.75
Rhenogran Geniplex-80(d)         -        1.4
(a) 100, high ENB EPDM; 80, carbon black; 20, Ca[CO.sub.3]; 60,
oil; 10 ZnO; 4, process aid; 4, CaO;
(b) Oxybisbenzenesulfonydrazide
(c) Azodicarbonamide
(d) 80% polymer bound form of DBZ
Table 6 - ODR results [at] 180 [degrees] C, arc [+ or -] 3 [degrees]
                             Control      EP71-8
[t.sub.2] (min)               0.68        0.75
[t.sub.90] (min)              2.93        3.01
[M.sub.L] (dN.m)              7.00        7.00
[M.sub.H]-[M.sub.L] (dN.m)   69.04       77.07

From the rheometer data shown in table 6 it can be seen that DBZ has been successfully used to reformulate an existing accelerator system without significant change in the cure characteristics. In addition it is known from practical experience of use of this compound cured by UHF that foam with a lower density and improved compression set can be obtained.

The lower density is attributed to the activation effect of the DBZ on the AC which gives expansion at the optimum time with respect to state of cure.

The improved compression set is a result of the more efficient use of the sulfur and this could not have been achieved without the use of DBZ.

The effect of DBZ on cellular polychloroprene (CR)

In other elastomers, where the cure mechanism is not based on sulfur cross-linking, DBZ is likely to act only as the blowing agent activator. However, in doing so, the exotherm which will now occur at a lower temperature (table 3), will have an influence on the cure characteristics. The cure rate is faster at this lower temperature than would otherwise be expected.

These effects have been demonstrated in press-cured foamed polychloroprene rubber (CR) using both non-sulfur modified and sulfur modified types.

Examples of typical formulations are shown in tables 7 and 8. [TABULAR DATA 7 and 8 OMITTED]

In non-sulfur modified CR the results show that in the absence of the blowing agent, DBZ has had little effect on the cure characteristics. If anything the cure has been slowed down. In the presence of the blowing agent it can be seen that curing at 155 [degrees] C also produces maximum expansion since no further density reduction can be achieved in a second stage oven cure. This is in stark contrast with the system containing blowing agent but no DBZ.

In sulfur modified CR the same dramatic effect of DBZ on density reduction in presence of AC is apparent even at 155 [degrees] C. In this type of rubber an effect on the cure characteristics can be expected and is shown by the slight rise in the state of cure (table 8).

These results clearly demonstrate the very attractive possibility of using a one-stage curing/foaming process for polychloroprene in certain applications, for example, in the manufacture of wetsuit material.

Conclusion

Mixtures of AC, ZnO and DBZ have a decomposition temperature in the region of 140-150 [degrees] C without reduction in the gas yield for a given weight of AC.

This, coupled with the positive cure characteristics of DBZ, can be used beneficially in the manufacture of both EPDM and CR foams.

In particular the significant effect of DBZ is well illustrated in the case of CR where favorable results can be obtained using a one stage foaming process.

Acknowledgements

"Review of antioxidants" is based on a paper presented at the May, 1993 meeting of the ACS Rubber Division.

"Performance of a novel activator for azodicarbonamide for sponge and cellular rubber" is based on a paper presented at the May, 1992 meeting of the ACS Rubber Division.

"Expanding rubber through the years" is based on a paper presented at the May, 1992 meeting of the ACS Rubber Division.