Plastified novolaks as additive to rubber mixtures

Details Plastified novolaks as additive to rubber mixtures Plastified novolaks as additive to rubber mixtures

2002-12-12

2004-12-07

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S504000, C525S508000, C525S480000, C525S539000, C524S270000, C524S271000, C524S272000

Reexamination Certificate

active

06828391

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method of use of plastified novolaks as additive to rubber mixtures, leading to higher tear propagation resistance and also to lower compression set in the vulcanizates produced therefrom.
BACKGROUND OF THE INVENTION
For a higher service life or durability of rubber items made from natural or synthetic rubber it is desirable to maximize tear propagation resistance, and also to have low compression set, alongside the required mechanical properties such as Shore hardness, elongation at break, ultimate tensile strength, and moduli.
Examples of rubber vulcanizates are vehicle tires (e.g. for cars or trucks), and also industrial rubber goods, such as V-belts, tubes, toothed belts, membranes, conveyor belts, and substrate matrices for abrasives and frictional linings, etc.
Some attempts to achieve an ideally balanced combination of the abovementioned properties for a particular application of rubber vulcanizates are known from the prior art. Examples of systems described are: use of short fibers as reinforcing agent (Short Polyester Fiber Reinforced Natural Rubber Composites; Senapati, A. K.; Kutty, S. K. N.; Pradhan, B.; Nando, G. B. in Int. J. Polym. Mater. 12(3), pp. 203-224, 1989); rubber-modified polyethylene foam for sports shoe soles (WO-A 88/08860); preparation of mixtures made from natural rubber and lignins (Compounding of Natural Rubber with Lignins; Kumaran, M. G. and De, S. K. in Kautschuk+Gummi Kunststoffe, Volume 30, No. 12, pp. 911-915); Comparison of Rubber Reinforcement Using Various Surface-Modified Precipitated Silicas; Thammathadamikul, V. O.; O'Haver; Harwell, J. H.; Osuwan, S.; Na-Ranong, N. and Waddell, W. H., J. Appl. Polym. Sci., 59(11), 1741-1750 (1996); A New Binary Acceleration System for Rubber Vulcanisation, Kuriakose, A. P.; Mathew, G., Indian J. Technol., 26(7), 344-347 (1988).
When short fibers are used, very marked anisotropy effects arise. For example, the tear propagation resistance and the compression set parallel to the preferred orientation of the fibers is higher than in the direction perpendicular thereto. In addition, a very complicated mixing technique is needed to prepare these rubber vulcanizates comprising short fibers. The literature also discloses that the folding of, and passage of, the rubber mill bands through the narrow gap between the rolls always has to take place in the same direction, and using an ideal number, which has to be predetermined, of passages through the rolls. These complex mixing procedures require complicated monitoring when converting from laboratory to production scale. However, the rubber industry often uses internal mixers which are not particularly likely to give the fibers any desired orientation. As fiber concentration rises, furthermore, the incorporation time inevitably rises, the consequence being a drastic lowering of mixing capacity. In addition, the “scorch time” becomes markedly shorter, and this is in turn equivalent to an increase in risk associated with the processing of the mixtures.
Ultimate tensile strength and elongation at break of fiber-filled rubber vulcanizates are known to vary in opposite directions. For example, when comparison is made with the initial mixture (with no fiber content) comparable ultimate tensile strengths parallel to the orientation of the fibers are achieved only at fiber contents of 20 phr (parts per hundred resin; 20 g for 100 g of the rubber) and above, whereas acceptable elongations at break of from 100 to 300% are found here only for the transverse (perpendicular) direction. For example, the elongation at break falls from 650 to 30% in parallel orientation on addition of 10 phr of fibers. Although the tear propagation resistance can be increased by various levels of addition (from 10 to 40 phr) of the fibers from 23 N/mm to 116 N/mm for parallel orientation and from 25 N/mm to 97 N/mm for transverse orientation, this is achieved at the cost of a drastic reverse, as described above, in elongation at break, from 650 to 30%. In addition, a no less drastic rise in initial hardness occurs, from 41 to 92 Shore A.
The mechanical properties of short-fiber-reinforced natural rubber mixtures are thus dependent on the fiber orientation, the fiber concentration, and the ratio of length and diameter in the fibers used.
In WO-A 88/08860, Kozma et al. describes a rubber-modified polyethylene foam for sports shoe soles. The polyethylene used in the foam is EPDM-modified and crosslinked with peroxides. When comparison is made with an unmodified fine EPDM foam material, the result of the modification is, inter alia, an improvement in compression set.
Other studies (Kumaran and De) concern the incorporation of lignin, a by-product of papermaking, into rubber mixtures, in particular natural rubber mixtures. The rubber mixtures or the vulcanizate prepared therefrom exhibit a drastic drop in tear propagation resistance and an increase in compression set.
In the abovementioned article by O'Haver et al. (Comparison of Rubber Reinforcement Using Various Surface-Modified Precipitated Silicas), the authors study the effect of modified silicas on vulcanization curve, and also on the mechanical properties of a natural rubber mixture of the vulcanizates prepared therefrom. The modification described of the silicas by means of in-situ polymerization with styrene/isoprene or styrene/butadiene as comonomers is very complicated, however. For example, the in-situ polymerization comprises four reaction steps:
Adsorption of the surface-active substance (hexadecyltrimethylammonium bromide) onto the silica particles, adsorption of the monomers via a solution procedure, polymerization of the monomers, and removal of the surface-active substance by bleaching. This time-consuming and expensive procedure is balanced by improved tear propagation resistances and compression sets—with lowered elongation at break.
The authors Mathew and Kuriakose succeeded in improving ultimate tensile strength and tear resistance by using a two-component accelerator system [1-phenyl-2,4-dithiobiuret/TMTD (tetramethylthiuram disulfide)]. However, mechanical tests also showed that rebound resilience and compression set remain unchanged when comparison is made with the standard vulcanization conditions.
In EP-A 0 362 727, modified novolaks are disclosed comprising terpenes and unsaturated carboxylic acids, and/or derivatives of these compounds, the mass ratio between the terpenes and the unsaturated carboxylic acids being from 98.2:2.5 to 2.5:98.5, and the ratio of the mass of the phenolic component and the total mass of the modifiers being from 95:5 to 5:95, preferably from 10:90 to 90:10. Addition of these modified novolaks to other mixtures brings about a rise in hardness and moduli. Nothing is said concerning the effect on tear propagation resistance and compression set.
Surprisingly, it has now been found that the desired high tear propagation resistance can be obtained together with low compression set in vulcanizable rubber mixtures or vulcanizates prepared therefrom by incorporating certain plastified novolaks, and also the usual curing agents for curing the same, e.g. hexamethylenetetramine (HMT) or hexamethoxymethylmelamine (HMMM).
The present invention therefore provides a method of use, as modifier resin for rubber mixtures, of novolaks ABC plastified using terpene-type olefinically unsaturated natural substances C, such as rosin or rosin derivatives, comprising admixing the said components ABC to rubber mixtures the ratio between the amount of substance of the phenol or phenol derivative B and the amount of substance of the natural substance C in the modified novolak being from 1:6 to 6:1, and the melting point of the modified novolak being above 90° C.
The preparation of the plastified novolaks is based on the known methods, for example as described in DE-C 22 54 379.
For preparing the plastified novolaks ABC according to the invention, it is in principle possible to use any of the phenolic compounds which have at least one reactive hydroge

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