Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-05-12
2003-06-03
Wu, David W. (Department: 1713)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S606000, C556S482000, C528S392000, C528S396000
Reexamination Certificate
active
06573412
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to polymers employed in silica-filled vulcanizable elastomeric compositions. More particularly, the invention relates to stabilization of the viscosity of the polymers when exposed to moisture during desolventization and ambient storage conditions.
BACKGROUND OF THE INVENTION
When producing polymers for use in rubber articles, such as tires, power belts, and the like, it is desirable that these polymers are easily processable during compounding and have a high molecular weight with a controlled molecular weight distribution, glass transition temperature (T
g
) and vinyl content. It is also desirable that reinforcing fillers, such as silica and/or carbon black, be well dispersed throughout the rubber in order to improve various physical properties. Good dispersion of carbon black may be achieved, for example, by end-capping polydienes by reacting a living end of the polymer with an end-capping agent, or by utilizing functionalized anionic polymerization initiators such as lithium-based amine or amide initiators that incorporate a functional group onto one or both ends of the polymer chain. Rubber articles produced from vulcanized elastomers exhibiting these characteristics, will have reduced hysteresis resulting in an increase in rebound, a decrease in rolling resistance and less heat build-up when mechanical stresses are applied. These properties, in turn, result in lowered fuel consumption for vehicles using such tires.
With the increasing use of silica as a reinforcing filler for rubber, filler dispersion in rubber stocks has become a major concern. Because polar silanol groups on the surface of silica particles tend to self-associate, reagglomeration of silica particles occurs after compounding, leading to poor silica dispersion, a high compound viscosity and a shorter scorch time. One approach to achieving better dispersion of silica during compounding, disclosed in our co-pending, co-owned U.S. patent application, Ser. No. 08/891,569, involves termination of elastomeric polymers, such as diene rubbers, with a tin-containing coupling agent, such as tin tetrachloride, an organo-tin halide, a dialkyldioxastannylane compound, and the like, resulting in an increase in the Mooney viscosity of the gum polymer, which is desirable for better initial processability of the polymer. During compounding of the tin-functionalized polymers, the polymer carbon-tin bonds are cleaved, resulting in lower molecular weight fragments and, concomitantly, a lowered viscosity which allows better dispersability of the silica filler during compounding.
Another approach to improving dispersion of silica filler, involves modification of polymer chains with functional end groups that interact with or shield the surface hydroxyl groups on the silica filler. In particular, siloxane-terminated polymers are well known and their preparation is disclosed in U.S. Pat. Nos. 3,244,664 and 4,185,042. Siloxane-terminated polymers have a carbon-silicon bond and at least one terminal O—R group that reacts with the silica surface, forming an Si—O—Si linkage.
Although siloxane-terminated polymers have provided adequate dispersion of reinforcing fillers during compounding, there has been a problem with stabilizing the Mooney viscosity of the gum polymer prior to compounding. In particular, polymers produced by solution polymerization in inert organic solvents, such as hexane, require a desolventization step after polymerization. Although desolventization may be achieved by drum-drying, in commercial practice desolventization is achieved through the use of either steam or heated water. When the polymer chains are terminated by siloxane compounds, hydrolysis of pendant —Si—OR end groups during the desolventization step, leads to coupling of the polymer chains via formation of —Si—O—Si— bonds between two or more siloxane end groups, resulting in a large increase in the polymer molecular weight and, concomitantly, a large increase in the Mooney viscosity of the gum polymer. Moreover, during storage of siloxane-terminated polymers for a period of time prior to compounding, humid environmental conditions and residual water from desolventization can lead to further hydrolysis of end groups and polymer chain coupling, and a further increase in viscosity.
Several approaches have been taken to overcome this hydrolysis and coupling problem. For example, U.S. Pat. No. 5,659,056 discloses the use of acids such as C
1
to C
12
aliphatic and C
6
to C
12
cycloaliphatic and aromatic carboxylic acids including acetic acid, propionic acid, butyric acid, decanoic acid, cyclohexanoic acid, benzoic acid, and the like, as well as acyl halides thereof, as viscosity stabilizing agents to treat the polymer prior to desolventization. These viscosity stabilizing agents do not react with the siloxane terminal end groups of the polymer, but rather serve to neutralize the by-product lithium compounds in admixture with the polymer, thereby slowing the rate of formation of low boiling alcohols during desolventization, slowing the rate of the hydrolysis reaction and, therefore, slowing the rate of coupling of the polymer. In this manner, the increase in Mooney viscosity of the gum polymer during the steam or heated water desolventization step is substantially reduced.
Other approaches to controlling the Mooney viscosity of such siloxane-terminated polymers have included the use of alkyl alkoxysilanes, such as n-octyl triethoxysilane, as viscosity stabilizing agents. As disclosed in our co-owned, copending U.S. patent application Ser. No. 09/360,551, these viscosity stabilizing agents are also added prior to desolventization but, unlike the acids described above, they react with the siloxane-terminated polymers. Moreover, viscosity stabilizing agents such as alkyl alkoxysilanes eliminate, rather than just slow down, any increase in the Mooney viscosity for a period of time. The successful use of these viscosity stabilizing agents, however, is concentration dependent. That is, the number of —SiOR groups available from the addition of the viscosity stabilizing agent must be such that the majority of the Si—O—Si bonds formed are between the hydrolyzable siloxane-terminated polymer and the viscosity stabilizing agent, not between the polymers themselves. Moreover, alkyl alkoxysilanes are relatively expensive compared to many other materials.
In another approach, our co-owned, copending U.S. patent application Ser. No. 09/449,303 discloses a method for stabilizing the Mooney viscosity of siloxane-terminated polymers, having at least one hydrolyzable constituent, by exchanging the alcohol of the siloxane terminal group for a long chain alcohol, such as an aliphatic, cycloaliphatic, or aromatic alcohol having at least 6 carbon atoms, or with a fatty acid ester of a hydrogenated or non-hydrogenated C
5
or C
6
sugar. The presence of the long chain alcohol or fatty acid ester sterically inhibits the availability of the hydrolyzable bond(s) to moisture. This approach results in slowing of the rate of coupling of the polymer which, in turn, slows the foreseen increase in Mooney viscosity of the siloxane-terminated polymer.
Although the above approaches to control of the Mooney viscosity of polymers having a hydrolyzable substituent have been shown to be successful, the need continues for alternative methods for controlling the rate of increase of Mooney viscosity of polymers prior to compounding, and also to provide a desirable lower viscosity during and after compounding for processability and adequate dispersion of the reinforcing fillers, especially silica.
SUMMARY OF THE INVENTION
The invention provides chain-coupled polymeric alkoxide compounds for use as high molecular weight polymers in vulcanizable elastomeric compositions comprising silica, carbon black, or mixtures of silica and carbon black as reinforcing fillers. The polymeric compounds of the invention are especially useful because of their processability when used in rubber making, i.e., they have an initial high molecular weight (high visc
Hall James E.
Hergenrother William L.
Hogan Terrence E.
Arndt Barbara
Bridgestone Corporation
Lee Rip A
Palmer Meredith E.
Wu David W.
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