Patent Application
20070249787
In the following, the invention is described in detail without, however, being limited to these examples.
In the following Tables 1 to 3, comparison and inventive rubber mixtures are shown which can be utilized for the elastomeric products. Table 1 shows rubber mixtures based on EPDM and Table 2 shows rubber mixtures based on EPM. These mixtures can, for example, be utilized for the ribs of V-ribbed belts. Table 3 shows fiber-filled mixtures based on HNBR which can, for example, be utilized for toothed belts. In all mixture examples contained in the tables, the quantity units are parts by weight based on 100 parts by weight of the total rubber (phr). The comparison mixtures are designated “V” while the mixtures of the invention are designated “E”. The mixtures in the tables vary with respect to the amounts of sorbic acid, zinc salt-formers and zinc salts used.
The zinc sorbate used in Table 1 was produced by the following method:
224 g sorbic acid were added to 1,000 ml ethanol. While stirring, 117 g basic zinc carbonate was added incrementally at room temperature. The mixture formed a readily stirrable suspension. At room temperature, stirring was continued and, after a short delay, the formation of CO2 started which showed that the reaction was occurring. After 24 hours, enough zinc sorbate had formed so that a firm mass had formed. In a rotary evaporator, the ethanol was drawn off at 60° under vacuum. A white mass of zinc disorbate was recovered and ground in a mill.
Production of the mixture was conducted under conventional conditions. The conversion times to achieve the relative cross-linking stages of 10% (t10) and 90% (t90) as well as the difference between end thrust force Fe and start thrust force Fa (as a measure of the degree of cross-linking) were determined with a moving disc rheometer (MDR) at 180° C. according to DIN 53 529. From all mixtures, test specimens were produced by vulcanization under pressure at 180° C. (heating times: 10 minutes for Tables 1 and 2, 20 minutes for Table 3) and, with these test specimens, material characteristics typical for the rubber industry are determined which are listed in the tables. For the tests on test specimens, the following test methods were used:
aα,α′-di-tert.-butyl-peroxy-di-isopropyl-benzene, 40 weight-% on an inorganic carrier
bactive zinc oxide, “Zinkoxid aktiv” from the LANXESS Company, Germany
The addition of different cross-linking activators was investigated with the mixtures which are listed in Table 1. The mixture of column 1 contains no sorbic acid, zinc salt-former or zinc salt. The cross-linking is achieved exclusively with the peroxide. A degree of cross-linking of 34 dNm is not sufficient for use in such abrasion resistant products as drive belts.
The mixture of column 2 is cross-linked using 15 phr zinc methylacrylate as is known in the art, for example, from European patent publication 1 205 515 A1. The compression set increases, that is, the mixture tends to creep. This leads, for example, in V-ribbed belts or toothed belts, to a situation wherein the surface geometry of the belts changes and therefore leads to a poorer wear resistance. The mixture of column 4 contains zinc disorbate, which provides little benefit with respect to degree of cross-linking and compression set. A degree of cross-linking of 36.28 dNm is attained which is not significantly different from mixture 1 which contains no metal salts of carboxylic acids. Compared to the latter, only a slight increase in hardness occurs which suggests that the sorbate acted as an inactive fill material.
If now free sorbic acid is used in combination with zinc oxide in a similar mixture (mixture of row 3), then, surprisingly, a very high degree of cross-linking is attained, and that mixture is well adapted for the production of wear-resistant elastomeric products such as drive belts. This is particularly surprising since it appears that the reaction mechanism could not have run via intermediary formation of zinc disorbate. In that case, the attainable results should not have been any better than with the mixture of column 4. The cross-linking mechanism of mixture 4 appears to involve, in substantial degree, binding of sorbic acid to the EPM-molecule ahead of salt-bridge formation. There are different possibilities for this. An EN-type reaction is possible via hydrogen atoms with the chains of the EPM, in the case of a hydrogen abstraction of EPM-molecules, the sorbic acid is also capable of very fast Diels-Alder reactions. The compression set is markedly reduced and thus improves the service life of, for example, belts, because creep and compression set of the mixtures is reduced.
A comparison experiment with free methylacrylic acid and zinc oxide had to be abandoned because the mixture was extremely lacrimatory because of unconverted methylacrylic acid residues and had to be ended abruptly for workplace hygienic reasons. Commercial production of marketable belts in this way is not practical for environmental reasons.
aα,α′-di-tert.-butyl-peroxy-di-isopropyl-benzene, 40 weight-% on inorganic carrier
bactive zinc oxide, “Zinkoxid aktiv” from the LANXESS Company, Germany
cZinc white HARZSIEGEL ®, Norzinco GmbH Harzer Zinkoxide
Table 2 shows experiments with different zinc salt-formers in cooperation with sorbic acid in EPDM-mixtures. The degree of cross-linking increases with increasing amounts of sorbic acid. With active zinc oxide (mixtures 5 and 6) the highest degrees of cross-linking are attained, which results in especially low abrasion. With respect to strength and stress value at elongation, these two mixtures show the highest values. Table 2 also indicates that abrasion resistance runs parallel to the degree of cross-linking.
aα,α′-di-tert.-butyl-peroxy-di-isopropyl-benzene, 40 weight-% on inorganic carrier
bactive zinc oxide, “Zinkoxid aktiv” from the LANXESS Company, Germany
d34% acrylonitrile content, 4% remaining double bonding
ep-aramid fibers having an average length of 3 mm, Twaron ®-fibers
The mixtures of Table 3 are fiber reinforced mixtures based on HNBR as they are conventionally used for toothed belts. Because of the presence of fibers aligned because of the calandering process, the data obtained from tension testing differs in the longitudinal and transverse directions.
In the transition from mixture 11 to mixture 12, the zinc dimethacrylate was replaced on an equimolar basis for sorbic acid and zinc oxide. Mixture 2 is distinguished by an increased degree of cross-linking.
It is furthermore advantageous that the time to 10% cross-linking (t10) is lengthened and the time to 90% cross-linking (t90) is shortened. That means that in production processes the mixture has an improved scorch safety while, at the same time, having a shortened total vulcanization time. With respect to product characteristics, the mixture 12 further shows a clear improvement in strength and clearly increased tensile values, and both of these characteristics are also manifested at increased temperatures. These characteristics are especially important in a toothed belt because they counter shearing off of teeth and shortened service life.
It is understood that the foregoing description is that of preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 018 717.2 | Apr 2006 | DE | national |
