Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-02-10
2001-01-30
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S493000, C524S492000, C524S262000, C523S313000, C423S332000, C423S331000, C423S333000
Reexamination Certificate
active
06180710
ABSTRACT:
FIELD OF INVENTION
The physical properties of a precipitated silica filled vulcanizate can be modified by using a precipitated silica with above 1 wt. % residual inorganic salt such as sodium sulfate. The vulcanizate rubber can be natural rubber, EPDM rubber or a synthetic rubber having at least 30 wt. % repeat units from a conjugated diene having 4 to 8 carbon atoms. The tan delta value at 50° C. can be reduced to form a pneumatic tire composition which would have reduced rolling resistance.
BACKGROUND OF THE INVENTION
The physical properties of rubber vulcanizates have traditionally been modified with particulate fillers to change their stiffness and hardness for specific applications such as tires. Conventional fillers include carbon blacks, precipitated silicas, clays, and talc. Carbon black has been highly preferred for tire applications as it contributes not only to the stiffness of the cured vulcanizate but also imparts added strength and toughness. The availability of a multitude of carbon blacks with different surface areas, particle sizes and structure has allowed researchers to quantify the variables that determine rubber reinforcement. Particulate fillers with high surface areas per gram of material, good surface interaction with rubber, and high structure contribute more to rubber reinforcement than particulate fillers with lower amounts of these three variables. Carbon black tends to have better surface interaction with rubber than does silica making it a preferred filler. The manufacturing technologies for carbon black and silica allow easy control over the surface area per gram and the structure of both types of fillers. Silicas include precipitated silica, fumed silica, natural deposits high in SiO
2
and manufacturing byproducts high is SiO
2
. Fumed silica is highest in price and more often is used in silicone rubbers than diene based elastomers. This is predominantly due to its higher cost per pound. Precipitated silicas are used in significant quantities in diene based elastomers.
Precipitated silicas are prepared by neutralizing a soluble silicate such as sodium silicate under controlled conditions such as temperature, silicate concentration, pH, electrolyte concentration, and controlled particle nucleation during particle growth cycles. Neutralizing agents for sodium silicate include hydrochloric acid, sulfuric acid, phosphoric acid, and carbonic acid. After a precipitated silica of the desired characteristics is formed the silica is generally separated from the excess water by filtration and washed to remove most of the nonsilica byproducts, e.g. Na
2
SO
4
, NaCl, NaHCO
3
, Na
2
HPO
4
and NaH
2
PO
4
or Na
2
CO
3
. Inorganic salts are not conventionally added to rubber compositions and silica manufacturers expend effort to reduce the inorganic salt content to less than 0.5 wt. %. Besides certifying particle size surface area, and pH most silica manufacturers certify less than 0.5 wt. % inorganic salt. The silica is generally dried to less than 10 wt. % total water and the particle size adjusted by milling and/or air classification as desired. For increased reinforcing effects silica can be treated with silane coupling agents to increase the interaction of the silica surface with the rubber thereby modifying the physical properties of the vulcanized rubber. The silane coupling agents which are most useful typically have or generate silanol groups which can react and bond to the silanol groups on the silica surface. The silane coupling agent can have mercaptan groups, polysulfide groups, or vinyl groups which can react with and crosslink to the diene based elastomers.
SUMMARY OF THE INVENTION
Conventional precipitated silicas are washed during manufacture to free them of adherent inorganic salt byproducts from the precipitation. The removal of the 20 to 30 wt. % of inorganic salt based on the dry silica component weight reduces the weight of the yield from the process and the inorganic salt removed must be discarded or recycled. The inorganic salt unexpectedly has been found to modify a compounded rubber's properties by improving filler dispersion, improving processability and desirably decreasing the tan delta at 50° C., reduce the 50% strain modulus, and increasing the 300% strain modulus of cured vulcanizates when it is not removed or only partially removed from precipitated silicas. When these silicas with higher salt content are used in vulcanizates they can optionally include from about 0.1 or 0.5 to about 18 or 20 parts by weight per 100 parts of silica of a silane or a shielding agent or combinations thereof applied to the silica. The most common adherent inorganic salts on silicas are the sodium or potassium salts of chloride, sulfates, phosphates or carbonates. Combinations of the salts may also be present. Desirably the vulcanizates are cured with sulfur and/or sulfur based accelerators. A preferred use of the vulcanizates is as a component in a pneumatic tire.
DETAILED DESCRIPTION OF THE INVENTION
It has been found that adherent inorganic salts (often a byproduct of silica precipitation) need not be predominantly removed from precipitated silica before the silica is used in some rubber vulcanizates. The presence of the inorganic salt can modify a compounded rubber's properties by improving filler dispersion, improving processability and desirably decreasing the tan delta value at 50° C. of a vulcanizate, decreasing the modulus at 50% strain, and increasing the modulus at 300% strain. The salts are desirably added from an aqueous solution if they are not byproducts of the silica precipitation.
Any conventional precipitated silica can be used in the process or product. Usually sodium or potassium silicate is one starting material and the other is a strong acid such as sulfuric acid, hydrochloric acid, phosphoric acid, or carbon dioxide or combinations thereof. Processes for producing precipitated silicas are described in U.S. Pat. Nos. 2,657,149; 2,940,830; and 4,681,750 which are herein incorporated by reference. The amount of the precipitated silica component is desirably from about 4 to about 90 phr, more desirably from about 8 to about 70 phr and preferably from about 10 to about 50 phr. The physical properties of the vulcanizates will depend on the amounts and type of rubber(s) used, the amount and characteristics of silica used, the amount and type of curatives, and the amount and type of other additives.
The preferred silicas for rubber vulcanizates will have BET surface areas of from about 60 to about 550 m
2
/g more desirably from about 100 to about 450 m
2
/g and preferably from about 100 to about 300 m
2
/g. BET refers to a Brunauer, Emmett, and Teller method according to ASTM 819-77 using nitrogen as the adsorbate but modified by outgassing the system and the sample for 1 hour at 180° C. Desirably the silica will have a pH from about 5 to about 11.5 and more desirably from about 6 to about 10.5 most desirably from about 6 to about 7. Desirably the percent SiO
2
will be above 50 wt. % and more desirably from about 60, 70 or 75 wt. % to about 80, 90, 95 or 98 wt. % of the silica component. The percent water as determined by drying 2 hours at 105° C. will desirably be up to about 20 wt. % and more desirably from about 2 to about 10 or 15 wt. %. Additional more tightly bound water may be lost on drying at 1000° C. for 2 hours. The silica desirably can absorb from about 1 to about 5 times its weight in dibutyl phthalate, more desirably from about 1.5 to about 4 times its weight. The dibutyl phthalate absorption values indicate higher structure with higher absorption. If it is a milled product the silica will desirably have a weight average agglomerate size (diameter) of from about 1 or 2 &mgr;m to several hundred &mgr;m. If it is a spray dried product the particles may be larger, but will break down to the smaller sizes when mixed with the rubber. The aggregate size can be controlled by milling, micronizing and/or classification (e.g. air classification).
The adherent inorganic salt can be present from about 1 or 2 to about 2
Cole William M.
Hergenrother William L.
Oziomek James
Arndt Barbara E.
Asinovsky Olga
Bridgestone Corporation
Hall Daniel N.
Hornickel John H.
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