Properties of new silicone/acrylic rubber.

Properties of new silicone/acrylic rubber

It is well known that rubber composites provide improved properties which cannot be obtained by a single elastomer. Many combinations of rubbers are actually in use today, for example, natural rubber, various synthetic diene rubbers, SBR, EPDM, etc. In these cases the following two conditions should be satisfied.

* They can be cured by the same curing systems.

* Their compatibility is comparatively good.

Silicone rubber/organic rubber composites, which consist of two very different polymers, have been investigated for a long time. In patents, various rubbers were found to be used as the one component of the composite, for example, diene rubbers[1,2], EPDM[3-6], ACM[7], FKM[8,9] and so on. EPDM has been investigated intensively and only silicone rubber/EPDM composites are commercially produced.

Silicone rubber/EPDM composite show good heat resistance contributed from the silicone rubber component and good mechanical properties from the EPDM component, therefore, it can be considered a material with intermediate properties between silicone and EPDM[10,11].

Properties of JSR Jenix E (TEQ of Toshiba Silicone Co.), which began being produced in 1986, are shown in figure 1[11], as an example of these composites. This composite has good mechanical properties, heat-resistance, water-resistance and electrical properties, and it is used in plug caps, boots, electrical wires and various hoses.

The basic idea of the development of silicone rubber/acrylic rubber composite (referred to as QA) Typical upper and lower temperature limits for usage oil resistance and relative tensile strength of silicone rubber (referred to as Q) and saturated rubbers are given in figure 2[12,13]. EPDM can be considered as a high heat-resistant, good low-temperature-resistant and non-oil-resistant organic rubber. The properties of EPDM are comparatively close to those of Q.

If Q/organic rubber composites are investigated, organic rubbers which can improve Q's weak point, oil resistance, will be interesting materials for the one component of the composite. However, oil resistant rubbers have polar groups in their backbones, therefore, the compatibilities of those Q are worse than that of EPDM.

Moreover, only a few oil-resistant polymers can be cured by organic peroxides which are curing agents of Q. Due to the reasons described above, Q/oil-resistant organic rubber composites have not been investigated as intensively as Q/EPDM.

Acrylic rubber (ACM) has good heat resistance and oil resistance, however, the low temperature properties, especially brittleness temperature, are poor. Low temperature type ACM grades whose monomer compositions are adjusted to improve their low temperature properties are commercialized[14], however, their heat resistance and/or oil resistance are sacrificed. After all, the balance of heat resistance, low temperature properties and oil resistance cannot be found easily by adjusting their monomer compositions.

If a new ACM with the excellent heat resistance and low temperature properties of Q is developed without sacrificing oil resistance, the new polymer will be a very interesting and sensational material which has been awaited. We have developed materials which provide the advantageous properties of both Q and ACM.

In this article, the properties of QA developed recently will be compared to those of commercial Q and ACM, and a discussion concerning the properties will be described.

Compounding formulations and curing conditions

The compounding formulations of QA (no. 1) and comparative samples, Q (no. 2), ACM-1 (low temperature type no. 3), ACM-2 (low temperature type no. 4), ACM-3 (standard type no. 5) and ethylene-acrylic elastomer (AEM no. 6), are shown in table 1. All the conventional rubbers and ingredients used in this study are listed in table 2.

QA or Q and curing agents were mixed on an open roll mill at about 50 [degrees] C. ACM and AEM were mixed in a laboratory Banbury mixer without the curing agents. The curing agents were mixed on an open roll mill as in the case of QA.

The compoundings were press-cured at designated temperatures and the secondary cures were carried out in an oven. Curing conditions are also shown in table 1.

Test methods

Properties of compounds Mooney viscosity was measured at 100 [degrees] C according to Japan Industrial Standard (JIS)K6300 which is very similar to the ASTM procedure. The curing behavior of compounds was measured by a JSR Curelastometer III.

Properties of vulcanizates Tensile properties, tear strength and compression set were measured by JIS K6301. Heat resistance tests were carried out in a gear oven. The conditions of aging are listed in table 3. Low temperature properties were evaluated by Gehman torsion test, and brittleness temperature by JIS K6301.

Oil resistance tests were carried out by JIS No. 1, JIS No. 3 and a commercial engine oil (SF, 10W-40 Clean Excellent, made by Toyota). The conditions of the immersion tests are listed in table 3.

Observation of morphology of QA QA vulcanizate was frozen in liquid nitrogen, and then sliced by microtome. The morphology of the sample was observed by TEM.

Morphology of QA

Figure 3 shows the morphology of the QA vulcanizate. In this figure, the light part is ACM and the dark part is Q. The small black spots are filler particles used. It was clearly seen that Q dispersed in ACM homogeneously. The diameters of Q particles were smaller than 1 [micro] m. From the evaluation of the phase structure of QA, it was found that Q domain dispersed in ACM matrix.

General properties of QA

Selection of curing system of QA In this study, an organic peroxide, a co-agent and a retarder were used for the curing system of QA. The results of the investigation to optimize the curing system is partially shown in figure 4.

The figures show the contour graphs of tensile strength and compression set with a fixed retarder level, 0, 0.3 and 0.6 parts per 100 QA composite (phc), and with variable organic peroxide and co-agent amounts from 0.5 to 2.5 phc.

The tensile strength increased with the increase in the retarder level. This result indicated that the retarder was necessary to obtain higher tensile strength. On the other hand, the compression set increased with the increase in the retarder level.

The compression set was minimum when the amounts of the organic peroxide and the co-agent were both added about 1.5 phc. Therefore, the curing system of QA shown in table 1 was chosen.

Table 4 shows the general properties of QA cured by this curing system. The cure curves of QA and comparative samples are shown in figure 5.

Mechanical properties The tensile strength and the tear strength of QA were lower than those of ACM and higher than those of Q. The compression set of QA was slightly larger than that of ACM and Q. The reason for this has not been clarified yet.

Heat resistance of QA The results of the heat resistance tests on QA and comparative samples are shown in table 5 (aged at 175 [degrees] C) and table 6 (aged at 200 [degrees] C). The elongation of Q did not change with the increase in aging time from 70 to 1,000 hours. On the other hand, the elongations of ACM-1 and ACM-2 decreased continuously, and after aging of 1,000 hours, they reached the minimum level. The elongation of ACM-3 decreased slower than that of the other ACMs. However, after 1,000 hours aging it showed no elongation, similar to the other ACMs. AEM, which is regarded as a highly heat-resistant acrylic rubber, showed characteristics similar to those of the other ACMs.

The elongation of QA changed by aging. However, the speeds of the change were slower than those of ACM and AEM. After 1,000 hours aging, the change in elongation was about -50%. In the case of QA, more than 100% elongation remained.

The speeds of the elongation change of QA and ACM-3 aged at 200 [degrees] C were slower than those of ACM-1, ACM-2 and AEM. However, after 100 hours aging, all samples lost most of their elongation. The only rubber which can be used at 200 [degrees] C is Q.

The hardness of Q did not change after 1,000 hours aging, similar to the elongation. On the other hand, the hardness changes of ACM-1 and ACM-2 were over 10 points after only 70 hours aging. The hardness of ACM-3 and AEM were over 95 after 1,000 hours aging, and the samples lost their elastic properties.

The hardness change of QA was larger than that of Q. However, after 1,000 hours aging, it was only 7 points. It can be said that QA is superior to the other ACMs.

After aging at 200 [degrees] C for 168 hours, the hardness of QA was 83 and different from that of the other ACMs, whose hardnesses were all near 100.

From the results described above, it can be said that the heat resistance of QA is superior to that of ACM and AEM, especially superior in the elongation after a long time aging at 175 [degrees] C and in the hardness change.

Finally, the effects of blending Q into ACM for improving its heat resistance appeared in the hardness change rather than in the elongation change.

Low temperature properties of QA The results of the Gehman torsion test and brittleness temperature test are shown in table 7. Q showed good low temperature properties, both low temperature flexibility (indicated as T10) and brittleness temperature (Tb).

T10 of QA was found between those of ACM-1, 2 and ACM-3. The effect of Q blending was not so large.

On the other hand, brittleness temperature, which is another index for low temperature properties, was different. Those of the three ACMs and the AEM were related almost linearly, and the low T10 grade showed lower Tb. T10 of QA was higher than that of ACM-1 and ACM-2. However, the actual value of Tb, -43[degree]C, was superior to that of ACM and AEM. The effect of improving ACM by Q blending appeared in brittleness temperature rather than in low temperature flexibility. One of the disadvantages of ACM was its higher brittleness temperature, therefore Q blending was very effective for improving ACM.

Oil resistance of QA The results of the immersion tests in JIS No 1 and JIS No. 3 oil for 70 hours at 150 [degrees] C are shown in table 8. It is generally said that volume swell of vulcanizates is related to aniline points of immersing oils[15]. The relations between the volume swell of the evaluated samples and aniline points of the oils used in this study are shown.

The volume swell of AEM was the largest in those of the samples and that of Q was the second largest. The volume swell of ACM-3 was the smallest. It is presumed that the monomer compositions of low temperature type of ACMs (ACM-1 and ACM-2) were chosen to improve their low temperature properties, even though the oil resistance was slightly sacrificed. In this study, ACM-1 and ACM-2 showed larger volume swell than ACM-3. The volume swell of QA was close to that of ACM-1 and ACM-2.

The results of the immersion tests in the engine oil which contains oil additives are shown in table 9. The volume swell of all samples except Q reached the maximum level after immersing for about 100 hours. The behavior of the swollen volume of QA was different from that in JIS test oils. The volume swell of QA was larger than that of ACM-1 and ACM-2 and also larger than the volume expected from the aniline point of the engine oil. This difference may be due to the oil additives.

Oil resistance of QA is superior to that of Q. However, it is necessary to check the resistance toward each oil before use, because different types of oil additives are used in the oils to improve their properties.

Various properties of QA

The various properties of QA were discussed above. Figure 6 illustrates the properties of QA compared to those of Q and ACM-1. QA has intermediate properties between those of Q and ACM except for the compression set.

In figure 7, the relations between the hardness change after 70 hours aging at 200[degrees]C and the brittleness temperature are illustrated to make the difference from ACM clear. Q showed good heat resistance and low temperature properties. On the other hand, ACM-3, which has higher heat resistance, showed poor brittleness temperature. ACM-1 and ACM-2, with better brittleness temperature, showed lower heat resistance. The heat resistance and the brittleness temperature of ACM are in antinomic relation.

AEM has a better balance of properties than ACM. However, it has poor oil resistance. Its application is limited because of the poor oil resistance.

QA developed this time, has a better balance of properties than AEM. Moreover, its oil resistance is at the same level as that of ACM-1 and ACM-2.

Thus, it can be said that QA has a unique property balance, which no existing material has, and it is the material expected to be developed at the beginning of this research.

Conclusions

The properties of the new silicone rubber/acrylic rubber composite (QA) were compared with those of commercial silicone rubber (Q), acrylic rubber (ACM) and ethylene-acrylic rubber (AEM). Especially, the difference between the property balances of QA and existing rubbers was made clear concerning heat resistance, low temperature properties and oil resistance.

At the same time, the properties of ACM, improved by blending with Q, were observed. Moreover, the effects of Q blending were clearly observed in the following properties:

* Heat resistance; suppress the hardness change.

* Low temperature resistance; improve brittleness temperature.

From its properties described above, we believe that QA is very useful for oil seals, orings, gaskets and various hoses. [Figures 1 to 7 Omitted] [Tables 1 to 9 Omitted]

References [1]General Electric Co. U.S. 3,021,292. [2]General Electric Co. U.S. 3,288,879. [3]General Electric Co. U.S. 3,277,777. [4]Shin-Etsu Chemical Co. U.S. 4,150,010. [5]Toray Silicone Co. U.S. 4,234,702. [6]Toshiba Silicone Co. Japanese patent application No. 47,865/87. [7]Toray Silicone Co. Japanese patent application (Laid-open) No. 7,814/80. [8]Minnesota Mining & Mfg. Co. WO 81/00573. [9]Chicago Rawhide Mfg. Co. Japanese patent application (Laid-open) No. 96,154/34. [10]J.M. Mitchell, SAE Technical Paper, 870195, Feb. 23-27 (1987). [11]I. Umeda, The Rubber Industries, 23 (12), 20 (1987). [12]J.W. Horvath et. al., Rubber World, 12, 21 (1987). [13]JSR data. [14]R.D. Demarco, Rubber Chemical Technol., 52, 1273 (1979). [15]H. Saeki, Junkatsu, 25,599 (1980).