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
2000-06-12
2002-01-15
Dawson, Robert (Department: 1712)
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...
C524S492000, C524S417000, C525S477000, C525S478000, C528S024000, C528S012000, C528S014000, C528S015000, C528S032000
Reexamination Certificate
active
06339124
ABSTRACT:
This invention relates to a silicone rubber composition having minimized plasticity recovery with time and improved storage stability and curing into a silicone rubber with improved compression set and yellowing resistance.
BACKGROUND OF THE INVENTION
Because of its weather resistance, electrical properties, compression set, heat resistance, and freeze resistance, silicone rubber has been widely employed in a variety of fields including electric, electronic, automotive, building, medical and food industries. Typical rubber parts include rubber contacts in remote controllers, keyboards, and musical instruments, building gaskets, rolls in copiers and printers such as developing rolls, transfer rolls, charging rolls, and paper feed rolls, vibration dampers in audio equipment, and compact disc packing in computers. The demand for silicone rubber is still increasing. There is a desire to have silicone rubber having further improved properties.
Such silicone rubber is usually used in the form of compositions comprising a polyorganosiloxane having a high degree of polymerization and a reinforcing filler. These compositions are prepared, for example, by mixing the base polymer with the reinforcing filler and various additives in a mixer such as a dough mixer or two-roll mill.
However, silicone rubber compositions loaded with reinforcing silica are known to undergo plasticity recovery with time. One known method for precluding this phenomenon is by treating surfaces of reinforcing silica fines with an organosilazane, organopolysiloxane terminated with a hydroxyl radical, or organosiloxane terminated with an alkoxy radical. This surface treatment is not fully satisfactory.
Also, a method for preparing a silicone rubber composition which changes little with time is disclosed in JP-A 59-176325 as comprising mixing a hydroxyl-terminated polyorganosiloxane having a low viscosity with a filler, adding sulfuric acid or sulfonic acid thereto, and subjecting the organosiloxane to polycondensation into a higher molecular weight one. With this method, the filler is readily blended in the organosiloxane. However, this method has the drawbacks that controlling a degree of polymerization is difficult due to the influence of moisture and the influence of the filler, a relatively large amount of low molecular weight siloxane is produced during polycondensation, and the residual catalyst can detract from heat resistance.
JP-A 59-176326 discloses the polycondensation of a hydroxyl-terminated polyorganosiloxane having a low viscosity in the presence of a basic catalyst. This method have similar drawbacks to the above method.
U.S. Pat. No. 4,978,705 discloses that silicone rubber is improved in flame retardance by blending therein a minor amount of an alkali metal salt of phosphoric acid. The addition of a minor amount of such a phosphoric acid salt is little effective for suppressing plasticity recovery.
Therefore, there is a desire to have a more effective means for improving the plasticity recovery of a reinforcing silica-loaded silicone rubber composition.
SUMMARY OF THE INVENTION
An object of the invention is to provide a silicone rubber composition having minimized plasticity recovery with time and improved storage stability and curing into a silicone rubber with improved compression set and minimized yellowing after secondary vulcanization.
It has been found that when a silicone rubber composition comprising (A) an organopolysiloxane of the following average compositional formula (1) containing at least two alkenyl radicals in a molecule and having a degree of polymerization of at least 100, (B) an alkoxy or hydroxyl-terminated organopolysiloxane or organosilane represented by the following general formula (2), and (C) reinforcing silica having a specific surface area of at least 50 m
2
/g is blended with (D) an alkali metal salt of phosphoric acid in an amount of 0.1 to 10 parts by weight per 100 parts by weight of the organopolysiloxane (A), the reinforcing silica (C) is uniformly dispersed in the base compound so that plasticity recovery with time is minimized and storage stability is improved. When this silicone rubber composition is cured with an organic peroxide or addition reaction curing agent, the resulting silicone rubber has improved compression set and minimized yellowing after secondary vulcanization.
Accordingly, the invention provides a silicone rubber composition comprising
(A) 100 parts by weight of an organopolysiloxane containing at least two alkenyl radicals in a molecule, represented by the following average compositional formula (1):
R
n
SiO
(4−n)/2
(1)
wherein R which may be the same or different is a substituted or unsubstituted monovalent hydrocarbon radical and n is a positive number of 1.98 to 2.02, said organopolysiloxane having a degree of polymerization of at least 100,
(B) 0.5 to 50 parts by weight of an organopolysiloxane or organosilane represented by the following general formula (2):
R
2
O(SiR
1
2
O)
m
R
2
(2)
wherein R
1
which may be the same or different is a substituted or unsubstituted monovalent hydrocarbon radical, R
2
is hydrogen or an alkyl radical, and m is a positive number of 1 to 50,
(C) 5 to 100 parts by weight of reinforcing silica having a specific surface area of at least 50 m2/g, and
(D) 0.1 to 10 parts by weight of an alkali metal salt of phosphoric acid.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Component (A) of the silicone rubber composition according to the invention is an organopolysiloxane containing at least two alkenyl radicals in a molecule, represented by the following average compositional formula (1).
R
n
SiO
(4−n)/2
(1)
Herein R, which may be the same or different, is a substituted or unsubstituted monovalent hydrocarbon radical. Included are monovalent hydrocarbon radicals of 1 to 12 carbon atoms, preferably 1 to 8 carbon atoms, for example, alkyl radicals such as methyl, ethyl, propyl and butyl, cycloalkyl radicals such as cyclohexyl, alkenyl radicals such as vinyl, allyl, butenyl and hexenyl, aryl radicals such as phenyl and tolyl, aralkyl radicals such as benzyl and &bgr;-phenylpropyl, and substituted ones of these radicals in which some or all of the hydrogen atoms attached to carbon atoms are replaced by halogen atoms or cyano radicals, such as chloromethyl, trifluoropropyl and cyanoethyl. The organopolysiloxane of formula (1) should contain at least two alkenyl radicals in a molecule. Preferably alkenyl radicals, especially vinyl radicals, account for 0.001 to 10 mol%, especially 0.01 to 5 mol% of the R radicals. The preferred organopolysiloxanes are those end-blocked with a triorganosilyl or diorganohydroxysilyl radical, for example, those end-blocked with a trimethylsilyl, dimethylvinylsilyl, dimethylhydroxysilyl or trivinylsilyl radical, and especially having at least one vinyl radical as well. Letter n is a positive number of 1.98 to 2.02.
The organopolysiloxane of formula (1) should also have a degree of polymerization of at least 100, preferably 100 to 100,000, especially 3,000 to 20,000. It is noted that a mixture of organopolysiloxanes which are different in degree of polymerization or structure is also useful as component (A).
Component (B) is an alkoxy or hydroxyl-terminated organopolysiloxane or organosilane represented by the following general formula (2). This component is an agent for treating the reinforcing silica (C) and assists in dispersing the reinforcing silica in the base compound. Additionally, component (B) synergistically cooperates with component (D) for restraining plasticity recovery with time and reducing yellowing after secondary vulcanization.
R
2
O(SiR
1
2
O)
m
R
2
(2)
Herein R
1
, which may be the same or different, is a substituted or unsubstituted monovalent hydrocarbon radical. Included are monovalent hydrocarbon radicals of 1 to 12 carbon atoms, preferably 1 to 8 carbon atoms, for example, alkyl radicals such as methyl, ethyl, propyl and butyl, cycloalkyl radicals such as cyclohexyl, alkenyl radicals such as vinyl, allyl, butenyl
Hagiwara Yutaka
Igarashi Minoru
Dawson Robert
Millen White Zelano & Branigan P.C.
Peng Kuo Liang
Shin-Etsu Chemical Co. , Ltd.
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