Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
1999-11-12
2002-03-26
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C528S032000
Reexamination Certificate
active
06362299
ABSTRACT:
TECHNICAL FIELD
The invention relates to crosslinkable organopolysiloxane compositions and to a process for the preparation thereof.
BACKGROUND ART
Only 2,4-dichlorobenzoyl peroxide (U.S. Pat. No. 4,260,536) and para-methylbenzyl peroxide are currently suitable for crosslinking of silicone rubber extrudates under normal pressure. Both peroxides have disadvantages. Thus, 2,4-dichlorobenzoyl peroxide forms cleavage products which are in some cases toxic, and para-methylbenzyl peroxide leads to a severe odor nuisance, the rate of vulcanization is slow, and transparent vulcanization products can have a slight yellow coloration.
DISCLOSURE OF INVENTION
The object of the invention is to overcome the disadvantages of the prior art, in particular to achieve a rapid crosslinking without an odor nuisance and yellow coloration. The object is achieved by the invention, which employs specific mixtures of crosslinking agents.
BEST MODE FOR CARRYING OUT THE INVENTION
The invention relates to organopolysiloxane compositions which can be prepared from an organopolysiloxane and a crosslinking agent comprising bis-4-methylbenzoyl peroxide (“PMBP”) and at least one compound from the group consisting of benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (“DHBP”) and di-tert-butyl peroxide.
Organopolysiloxanes which can be used are all the organopolysiloxanes which can be crosslinked peroxidically, preferably organopolysiloxanes (A) of units of the general formula
R
r
SiO
4
-
r
2
(
I
)
in which
R can be identical or different and is an optionally substituted hydrocarbon radical and
r is 0, 1, 2 or 3 and has an average numerical value of 1.9 to 2.1.
Examples of hydrocarbon radicals R are alkyl radicals such as the methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl-, tert-butyl, n-pentyl, isopentyl, neopentyl and tert-pentyl radicals, hexyl radicals such as the n-hexyl radical, heptyl radicals such as the n-heptyl radical, octyl radicals such as the n-octyl radical and iso-octyl radicals, such as the 2,2,4-trimethylpentyl radical, nonyl radicals such as the n-nonyl radical, decyl radicals such as the n-decyl radical, dodecyl radicals such as the n-dodecyl radical, and octadecyl radicals such as the n-octadecyl radical; cycloalkyl radicals such as cyclopentyl, cyclohexyl and cycloheptyl radicals and methylcyclohexyl radicals; aryl radicals such as the phenyl, biphenyl, naphthyl and anthryl and phenanthryl radicals; alkaryl radicals, such as o-, m- and p-tolyl radicals, xylyl radicals and ethylphenyl radicals; and aralkyl radicals, such as the benzyl radical and the &agr;- and the &bgr;-phenylethyl radical.
Examples of substituted hydrocarbon radicals R are halogenated alkyl radicals, such as the 3-chloropropyl, the 3,3,3-trifluoropropyl and the perfluorohexylethyl radical, and halogenated aryl radicals, such as the p-chlorophenyl and the p-chlorobenzyl radicals.
The radical R is preferably a hydrogen atom or a hydrocarbon radical having 1 to 8 carbon atoms, particularly preferably the methyl radical.
Further examples of radicals R are the vinyl, allyl, methallyl, 1-propenyl, 1-butenyl, 1-pentenyl, 5-hexenyl, butadienyl, hexadienyl, cyclopentenyl, cyclopentadienyl, cyclohexenyl, ethynyl, propargyl and 1-propynyl radicals. Preferred unsaturated radicals R are alkenyl radicals having 2 to 8 carbon atoms, particularly preferably the vinyl radical.
In the case of optionally substituted hydrocarbon radicals having 1 to 8 carbon atoms, the methyl, vinyl, phenyl and 3,3,3-trifluoropropyl radicals are particularly preferred.
Preferably, alkyl radicals, in particular methyl radicals, are bonded to at least 70 mol % of the Si atoms contained in the organopolysiloxane (A) of units of the formula (I). If the organopolysiloxanes also contain Si-bonded vinyl and/or phenyl radicals, in addition to Si-bonded methyl and/or 3,3,3-trifluoropropyl radicals, the total amount of vinyl and/or phenyl radicals is preferably 0.001-30 mol %.
The organopolysiloxanes (A) preferably predominantly comprise diorganosiloxane units. The end groups of the organopolysiloxanes can be trialkylsiloxy groups, in particular the trimethylsiloxy radical or the dimethylvinylsiloxy radical; however, one or more of these terminal alkyl groups can also be replaced by hydroxyl groups, or by alkoxy groups such as methoxy or ethoxy radicals.
The organopolysiloxanes (A) can be liquids or highly viscous substances. The organopolysiloxanes (A) preferably have a viscosity at 25° C. of between 10
3
and 10
8
mm
2
/s.
The crosslinking agent used is preferably a mixture of bis-4-methylbenzoyl peroxide (PMBP) and at least one compound from the group consisting of benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (DHBP), and di-tert-butyl peroxide in a weight ratio of 1:0.3 to 1:1, preferably in a ratio of about 1:0.8. If at least two compounds from the group consisting of benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (DHBP) and di-tert-butyl peroxide are used, these can be used in any desired mixing ratios in the context of the weight ratio first mentioned. In this case, however, mixtures of equal parts by weight of the compounds from the group consisting of benzoyl peroxide, dicumyl peroxide, 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (DHBP) and di-tert-butyl peroxide are preferred, mixtures of benzoyl peroxide and 2,5-dimethyl-hexane-2,5-di-tert-butyl peroxide (DHBP) in a weight ratio of 1:1 being particularly preferred.
The organopolysiloxanes (A) according to the invention furthermore preferably comprise reinforcing and/or non-reinforcing fillers. Examples of reinforcing fillers are pyrogenic or precipitated silicic acid having BET surface areas of at least 50 m
2
/g.
Silicic acid fillers can have a hydrophilic character or may be hydrophobicized by known processes. In this context, reference is made, for example to DE-38 39 900 A (Wacker-Chemie GmbH; filed on Nov. 25, 1988) and the corresponding U.S. Pat. No. 5,057,151. In general, the hydrophobization is then carried out with 1 to 20% by weight of hexamethyldisilazane and/or divinyltetramethyldisilizane and 0.5 to 5% by weight of water, in each case based on the total weight of the organopolysiloxane composition, these reagents advantageously being added to the organopolysiloxane (A) which have already been initially introduced into a suitable mixing device, such as, for example, a kneader or internal mixer, before the hydrophilic silicic acid is incorporated subsequently into the composition.
Examples of non-reinforcing fillers are quartz flour, diatomaceous earth, calcium silicate, zirconium silicate, zeolites, metal oxide powders, such as aluminum, titanium , iron or zinc oxide, barium silicate, barium sulfate, calcium carbonate, gypsum and powders of plastics, such as polyacrylonitrile powder or polytetrafluoroethylene powder. Fibrous components, such as glass fibers and synthetic fibers, can furthermore be employed as fillers. The BET surface area of these fillers is preferably 50 m
2
/g.
The organopolysiloxane compositions according to the invention, which can be crosslinked to elastomers, comprise filler (B) in amounts of preferably 1 to 200 parts by weight, particularly preferably 30 to 100 parts by weight, in each case based on 100 parts by weight of organopolysiloxane (A).
Additives (C) include processing auxiliaries such as plasticizers, pigments, and stabilizers, for example, heat stabilizers. Additives (C) can be added when necessary to the organopolysiloxane compositions vulcanizable to elastomers, depending on the particular use contemplated for the elastomer product. Non-limiting examples of plasticizers which can be employed as additives (C) include polydimethylsiloxanes which have terminal trimethylsilyl groups or hydroxyl groups and have a viscosity of not more than 1000 mm
2
/s at 25° C., and diphenylsilanediol. Non-limiting examples of heat stabilizers which can be employed as additives (C) include transition metal salts of fatty acids such as iron octoate, transition
Moore Margaret G.
Zimmer Marc S.
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