Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-01-12
2003-06-03
Szekely, Peter (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S374000
Reexamination Certificate
active
06573339
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a relatively low cost process for the in situ synthesis of bis-benzothiazolesulfenamide vulcanization accelerators in a polymeric matrix, especially in an elastomer, which is advantageous in that no waste products are produced. The resulting combination can be used for the production of tires and other rubber products without the emission of nitrosamines into the workplace, thereby avoiding environmental and health issues.
2. Description of Related Art
Vulcanization may be defined as a reaction in the presence of heat wherein a chemical additive reacts with an elastomer to change it from a plastic, tacky solid to a thermoset, fixed solid with improved strength and elasticity and increased hardness. The vulcanization reaction is one in which the polymeric rubber molecules are cross-linked by the vulcanizing agent to form a network of macromolecules having less mobility and which have the desired physical properties of a usable rubber product. The type of cross-linking (or vulcanizing) agent will vary with the type of rubber used and the properties desired.
The most commonly used vulcanizing agent is sulfur as it enters into reactions with the majority of the unsaturated rubbers to produce vulcanizates. Sulfur, in the presence of heat, reacts with adjoining olefinic bonds in the polymeric backbone chains or in pendant chains of two elastomeric molecules to form cross-links between the molecular chains.
Vulcanization, as originally known, required long hours and elevated temperatures. Progress was made in speeding the process and improving the properties of the vulcanized product by using accelerators. Reduction in the time required for vulcanization is generally accomplished by changes in the amounts and types of accelerators used.
A type of accelerator used widely with a sulfur vulcanizate system are sulfenamides. Sulfenamides give fast vulcanization (approximately 30 minutes) while providing delayed curing action. Examples of sulfenamide accelerators include N-cyclohexyl-2-benzothiazole sulfenamide (CBS), N-t-butyl-2-benzothiazole sulfenamide (TBBS), N,N-dicyclohexyl-2-benzothiazole sulfenamide (DCBS), N,N-diisopropyl-2-benzothiazole sulfenamide (DIBS), 2-(4-morpholinylthio)-benzothiazole (MBS), 2-(4-morpholinyldithio)-benzothiazole (MBDS), and N-cyclohexyl-bis-benzothiazole sulfenamide (CBBS).
A sulfur acceleration system comprises a vulcanizing agent (e.g., sulfur), a primary accelerator (e.g., a sulfenamide) and, optionally, a secondary accelerator that activates the primary accelerator. Normally, the ratio of primary accelerator to sulfur ranges from 1:4 in a fast curing elastomer (e.g., natural rubber) to approximately 1:2 in a slower curing elastomer (e.g., EPDM).
A typical recipe using a sulfur acceleration system is:
SBR
100.00
ZINC OXIDE
3.00
STEARIC ACID
1.00
CARBON BLACK
50.00
MBT
1.00
SULFUR
1.75
The thiazoles, characterized by the mercaptobenzothiazoles and their derivatives, are an important and widely used class of accelerators. The discovery of this type of compound dates back to the 1920's, as shown by U.S. Pat. No. 1,544,687, which discloses 2-mercaptobenzothiazole (MBT). This discovery has led to a family of delayed-action accelerators in wide use today.
MBT is formed by reacting aniline with carbon disulfide and sulfur. The derivatives are built up chemically through the mercapto group. By oxidation, it may be changed into the disulfide form. The most important derivatives are the sulfenamides, which have long scorch delays coupled with good cure rates.
The sulfenamides are formed by oxidation of a mixture of MBT and an amine. Alternatively, N-chloroamine can be reacted with the sodium salt of MBT. The sulfenamides in commercial use are generally derived from secondary amines or from primary amines that are somewhat hindered.
When a sulfenamide accelerator is used in the rubber making process, spontaneous oxidation occurs via the reaction of the compound with the NO
x
present in ambient air. The formation of nitrosamines must be considered here. Their precursors are found in vulcanization accelerators and to a lesser degree in rubber fillers and additives.
As used throughout this specification, the term “nitrosamines” refers, for example, to N-nitroso-dimethylamine, N-nitroso-diethylamine, N-nitroso-dibutylamine, N-nitroso morpholine, N-nitroso-diisopropylamine, and the like, either collectively or individually.
The N-nitroso compounds are formed by the reaction of a substance containing secondary amino groups and a nitrosating agent derived from the oxides of nitrogen (NO
x
) or nitrite salts.
Government agencies, such as OSHA and NIOSH in the United States, have been concerned about worker level of exposure to nitrosamines in many industries, including rubber manufacture. The nitrosamines produced by certain sulfenamide accelerators are an undesirable byproduct and there is a desire both by government agencies and the rubber industry to eliminate them.
Another important concern of the rubber vulcanization process is scorch, which may be defined as premature vulcanization. It is considered to be extremely important in defining processability limits (as stated, for example, in Rubber Technology, 3rd Edition, Morton, 1987) and is an additional aspect of the current invention.
Continuous measurement of viscosity at processing temperatures will indicate the time available for further processing. A good stock will have a scorch time slightly longer than the equivalent of the maximum heat it may accumulate during processing.
Of the previously mentioned sulfenamide accelerators in current use, N-cyclohexyl-2-benzothiazole-sulfenamide (CBS) and N-t-butyl-2-benzothiazole-sulfenamide (TBBS) have poor scorch safety. 2-(4-morpholinothio)benzothiazole (MBS) is known to exhibit a level of scorch safety that is very desirable in many rubber compounds. The problem of nitrosamine generation is present, however, using MBS.
WO 92/05218 discloses the use of certain N-alkyl, N-benzyl, N-dibenzyl, or N-cycloalkyl substituted bis(2-benzothiazolesulfen)amides as curing accelerators for rubbery thermosettable polymers in a process of manufacturing rubber articles without generation of N-nitrosamine compounds in the manufacturing environment or article. The articles include tires, belts, hose, and other rubber articles.
It is known from the literature that in the reactions of monosulfenamides with carboxylic anhydrides, bis-sulfenamides are formed (Ignatov et al.,
Zhurnal Obshchei Khimii
47(5):1096-1103 (1977)).
U.S. Pat. No. 3,875,177 discloses a process for the preparation of bis(benzothiazylsulphene)amides of the general formula:
in which the symbol R represents a hydrocarbon radical, by reaction of a benzothiazylsulpheneamide of the general formula:
with an organic acid anhydride having a structural formula that includes at least one ring.
In the anhydrides of cyclic formula, a ring can be bonded to the acyl group, for example in aromatic carboxylic acid anhydrides such as benzoic anhydride. The ring can also originate from two carboxyl groups in an intramolecular anhydride, such as anhydrides of aliphatic dicarboxylic acids containing four to six carbon atoms (for example maleic, succinic or glutaric anhydrides). Condensed polycyclic anhydrides can also be used. These are intramolecular anhydrides of aromatic carboxylic acids such as phthalic anhydride and pyromellitic anhydride.
It is preferred that R represent a linear or branched alkyl radical of 1 to 12 carbon atoms or a cycloalkyl radical with 5 to 6 ring carbon atoms such as cyclopentyl or cyclohexyl.
The bis(benzothiazylsulphene)amides obtained by the process of this patent are said to show a high storage stability, make possible high vulcanization speeds with is great safety in use, and impart advantageous mechanical properties to vulcanized products.
Other processes for producing bis(sulfen)amides involve the reaction of a monosulfenamide with HCl gas in a solvent; thus, producing the bis(sulfen)amide and an amine hydroch
Cornell Robert J.
Hajdasz David
Otis Jeffrey
Stieber Joseph F.
Wefer John M.
Grandinetti Paul
Reitenbach Daniel
Szekely Peter
Uniroyal Chemical Company, Inc.
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