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
1999-12-22
2002-04-09
Wilson, D. R. (Department: 1713)
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...
C526S065000, C526S066000, C526S075000, C526S087000, C526S222000, C526S224000, C526S340000
Reexamination Certificate
active
06369158
ABSTRACT:
BACKGROUND OF THE INVENTION
Emulsion polymerization is used commercially to synthesize a wide variety of polymers. It is frequently desirable to reduce the molecular weight of such polymers. This is typically accomplished by conducting the emulsion polymerization in the presence of a chain transfer agent.
Mercaptans are normally used as chain transfer agents in emulsion polymerizations. The mercaptans used in commercial applications are typically complex mixtures of hundreds of similar compounds having boiling points within a narrow range. The chain transfer activity of different mercaptans with such mixtures can vary substantially. To further complicate the situation, the distribution of various mercaptans can also vary substantially between lots of material obtained from commercial sources. Thus, consistent molecular weight control is typically difficult to attain in free radical emulsion polymerizations. The use of mercaptans as chain transfer agents also results in odors that workers and customers often find very undesirable.
One of the most important products made by emulsion polymerization is styrene-butadiene rubber. A wide variety of rubber products are made with styrene-butadiene rubber (SBR). For instance, large quantities of SBR are utilized in manufacturing tires for automobiles, trucks, aircraft and other types of vehicles. SBR is commonly used in manufacturing tires because it generally improves traction characteristics.
SBR can be synthesized by utilizing either solution or emulsion polymerization techniques. SBR made by emulsion polymerization (emulsion SBR) generally exhibits better traction characteristics in tire tread compounds. However, SBR made by solution polymerization (solution SBR) typically exhibits much better rolling resistance and treadwear characteristics in tire treads.
In the synthesis of SBR by solution polymerization techniques, an organic solvent is used which is capable of dissolving the monomers (1,3-butadiene and styrene), SBR and the polymerization catalyst or initiator. As the polymerization proceeds, a solution of the SBR in the solvent is produced. This polymer solution is sometimes referred to as a “polymer cement.” The SBR is subsequently recovered from the polymer cement and can then be employed as a dry rubber in desired applications; such as, in formulating tire tread rubbers.
Typical emulsion systems employed in the synthesis of SBR contain water, an emulsifier (soap), a free radical generator, styrene monomer, 1,3-butadiene monomer, and optionally a chain transfer agent, such as a mercaptan. For example, in free radical emulsion polymerization systems, radicals can be generated by the decomposition of peroxides or peroxydisulfides.
Commonly employed initiators include t-butyl hydroperoxide, pinane hydroperoxide, para-menthane hydroperoxide, potassium peroxydisulfate (K
2
S
2
O
8
), benzoyl peroxide, cumene hydroperoxide and azobisisobutyronitrile (AIBN). These compounds are thermally unstable and decompose at a moderate rate to release free radicals. The combination of potassium peroxydisulfate with a mercaptan such as t-dodecyl mercaptan is commonly used to polymerize butadiene and SBR. In hot recipes, the mercaptan has the dual function of furnishing free radicals through reaction with the peroxydisulfate and also of limiting the molecular weight of polymer by reacting with one growing chain to terminate it and to initiate growth of another chain. This use of mercaptan as a chain transfer agent or modifier is of great commercial importance in the manufacture of SBR in emulsion since it allows control of the toughness of the rubber which otherwise may limit processibility in the factory.
A standard polymerization recipe agreed on for industrial use is known as the “mutual,” “standard,” “GR-S” or “hot” recipe. This standard polymerization recipe contains the following ingredients (based upon parts by weight): 75.0 parts of 1,3-butadiene, 25 parts of styrene, 0.5 parts of n-dodecyl mercaptan, 0.3 parts of potassium peroxydisulfate, 5.0 parts of soap flakes and 180.0 parts of water.
When this standard recipe is employed in conjunction with a polymerization temperature of 50° C., the rate of conversion to polymer occurs at 5-6 percent per hour. Polymerization is terminated at 70-75 percent conversion since high conversions led to polymers with inferior physical properties, presumably because of crosslinking in the latex particle to form microgel or highly branched structures. This termination is effected by the addition of a “shortstop” such as hydroquinone (about 0.1 part by weight) which reacts rapidly with radicals and oxidizing agents. Thus, the shortstop destroys any remaining initiator and also reacts with polymer-free radicals to prevent formation of new chains. The unreacted monomers are then removed; first, the butadiene by flash distillation at atmospheric pressure, followed by reduced pressure and then the styrene by steam-stripping in a column.
A dispersion of antioxidant is typically added (1.25 parts) to protect the SBR from oxidation. The latex can then be partially coagulated (creamed) by the addition of brine and then fully coagulated with dilute sulfuric acid or aluminum sulfate. The coagulated crumb is then washed, dried and baled for shipment. One of the first major improvements on the basic process was the adoption of continuous processing. In such a continuous process, the styrene, butadiene, soap, initiator and activator (an auxiliary initiating agent) are pumped continuously from storage tanks into and through a series of agitated reactors maintained at the proper temperature at a rate such that the desired degree of conversion is reached at the exit of the last reactor. Shortstop is then added, the latex is warmed by the addition of steam and the unreacted butadiene is flashed off. Excess styrene is then steam-stripped off and the latex is finished, often by blending with oil, creaming, coagulating, drying and bailing. For further details on SBR and the “standard recipe,” see The Vanderbilt Rubber Handbook, George G Winspear (Editor), R T Vanderbilt Company, Inc (1968) at pages 34-57.
U.S. Pat. No. 5,583,173 discloses a process for preparing a latex of styrene-butadiene rubber which comprises (1) charging water, a soap system, a free radical generator, 1,3-butadiene monomer and styrene monomer into a first polymerization zone; (2) allowing the 1,3-butadiene monomer and the styrene monomer to copolymerize in the first polymerization zone to a monomer conversion which is within the range of about 15 percent to about 40 percent to produce a low conversion polymerization medium; (3) charging the low conversion polymerization medium into a second polymerization zone; (4) charging an additional quantity of 1,3-butadiene monomer and an additional quantity of styrene monomer into the second polymerization zone; (5) allowing the copolymerization to continue until a monomer conversion of at least about 50 percent is attained to produce the latex of styrene-butadiene rubber. This process is sometimes referred to as the FIM (feed-injection-monomer) process.
By employing the technique disclosed in U.S. Pat. No. 5,583,173, the amount of soap required to produce styrene-butadiene rubber by emulsion polymerization can be reduced by greater than 30 percent. This is advantageous because it reduces costs and is environmentally attractive. U.S. Pat. No. 5,583,173 also reports that the styrene-butadiene rubber produced by the process described therein offers advantages in that it contains lower quantities of residual soap. This reduces fatty acid bloom characteristics in final products, such as tires, and makes plies easier to adhere together during tire building procedures.
SUMMARY OF THE INVENTION
This invention is based upon the unexpected discovery that dibenzyltrithiocarbonate will act as a pure compound to effectively and consistently control molecular weight in free radical emulsion polymerizations. The polydispersity (ratio of weight average molecular weight to number average molecular weight) of polymers made using d
Kulig Joseph John
Parker Dane Kenton
Senyek Michael Leslie
Egwim Kelechi
Rockhill Alvin T.
The Goodyear Tire & Rubber Company
Wilson D. R.
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