Mother mold-forming silicone rubber composition and mother mold

Details Mother mold-forming silicone rubber composition and mother mold Mother mold-forming silicone rubber composition and mother mold

2000-04-05

2002-04-16

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C524S267000, C524S731000, C524S700000, C525S106000, C525S478000, C525S479000, C264S331150, C264S331110, C528S032000, C528S031000

Reexamination Certificate

active

06372860

ABSTRACT:

This invention relates to a silicone rubber composition for forming mother molds (master negatives) having improved parting properties and a mother mold.
BACKGROUND OF THE INVENTION
A process of producing a plurality of parts by using a silicone rubber mold as a master negative, casting a resin material in the mold, and curing the material therein has been widely employed in the industry covering from the manufacture of hobby miniature models to the manufacture of industrial trial models.
The silicone rubber used for that purpose has the advantage that a master negative can be formed with ease because it is liquid prior to curing and can be cured simply by mixing with a curing agent and holding at room temperature or heating. Nevertheless, the resin cast and molded in the silicone rubber mold can tightly stick to the mold and is difficult to remove from the mold because the mold is of rubber nature. This problem is commonly solved by using a parting agent (releasing agent). The application of a parting agent is a consideration when it is desired to reduce the process time.
One solution is disclosed in JP-A 7-118534 wherein a crosslinkable base polymer is compounded with a non-crosslinkable silicone polymer having a greater chain length than the base polymer, achieving an improvement in parting properties (releasing properties).
However, this approach still suffers from several problems. Since the silicone fluid added as the parting agent has a greater viscosity than the base polymer, the composition has a higher viscosity than ordinary compositions. Some blending methods can restrain the migration of the non-reactive silicone fluid (parting agent) to the surface. Only a small amount of the parting agent migrates to the surface at the initial of molding.
SUMMARY OF THE INVENTION
An object of the invention is to provide a silicone rubber composition which forms mother molds exhibiting good parting properties from the initial and has a low viscosity. Another object of the invention is to provide a mother mold manufactured therefrom.
It has been found that when an organopolysiloxane base polymer from which a silicone rubber mother mold is to be formed is compounded with a non-crosslinkable oily substance having a refractive index difference of at least 0.01 from the base polymer and a melting point of up to 100° C., the substance migrates to the mold surface, which eliminates a need for parting agent or significantly reduces the number of applications of parting agent. The initial migration of the substance to the mold surface is improved. Since the oily substance need not have a greater molecular weight than the base polymer, the composition can have a low viscosity.
Accordingly the invention provides a silicone rubber composition for forming mother molds comprising an organopolysiloxane as a base polymer wherein up to 50 parts by weight of an oily substance having a melting point of up to 100° C. and a refractive index which differs by at least 0.01 from the refractive index of the base polymer is blended per 100 parts by weight of the composition.
Also provided is a mother mold obtained by casting and curing the composition.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The base polymer of the silicone rubber composition for forming mother molds according to the invention is an organopolysiloxane. Preferably, it is an alkenyl-containing organopolysiloxane and more preferably an organopolysiloxane of the following average compositional formula (1) containing on the average at least two alkenyl radicals in a molecule.
R
1
a
SiO
(4−a)/2
  (1)
Herein R
1
is a substituted or unsubstituted monovalent hydrocarbon radical, 0.01 to 10 mol % of R
1
being alkenyl radicals, and “a” is a positive number of 1.9 to 2.4.
Preferably R
1
stands for a substituted or unsubstituted monovalent hydrocarbon radical of 1 to 12 carbon atoms, and especially 1 to 8 carbon atoms, attached to a silicon atom. Examples include alkyl radicals such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, and dodecyl, cycloalkyl radicals such as cyclopentyl, cyclohexyl, and cycloheptyl, alkenyl radicals such as vinyl, allyl, propenyl, isopropenyl, butenyl, isobutenyl, hexenyl and cyclohexenyl, aryl radicals such as phenyl, tolyl, xylyl, naphthyl, and biphenyl, aralkyl radicals such as benzyl, phenylethyl, phenylpropyl and methylbenzyl, and substituted ones of the foregoing hydrocarbon radicals in which some or all of the hydrogen atoms are replaced by halogen atoms (e.g., F, Cl and Br) and cyano radicals, such as chloromethyl, 2-bromoethyl, 3,3,3-trifluoropropyl, 3-chloropropyl and cyanoethyl.
In the above organopolysiloxane, preferably methyl accounts for at least 90 mol % (i.e., 90 to 99.99 mol %), preferably at least 95 mol % (i.e., 95 to 99.98 mol %) of the substituted or unsubstituted monovalent hydrocarbon radicals represented by R
1
. The organopolysiloxane contains on the average at least two alkenyl radicals in a molecule. Specifically, 0.01 to 10 mol %, especially 0.02 to 5 mol % of the entire R
1
radicals are alkenyl radicals. If the content of alkenyl radicals is too low, the resulting composition may become less curable. If the content of alkenyl radicals is too high, cured products may have poor physical properties including tensile strength, tear strength and elongation. The alkenyl radical may attach to either a silicon atom at the end of the molecular chain or a silicon atom intermediate of the molecular chain or both. The preferred alkenyl radical is vinyl. The radicals other than the methyl and vinyl radicals are preferably phenyl and 3,3,3-trifluoropropyl radicals.
In formula (1), “a” is a positive number of 1.9 to 2.4,and preferably 1.95 to 2.05.The preferred molecular structure of this organopolysiloxane is a linear diorganopolysiloxane consisting essentially of recurring diorganosiloxane units, especially a linear dimethylpolysiloxane having at least two alkenyl radicals in a molecule Also preferably it has alkenyl radicals at both ends of the molecular chain. It is also acceptable that the organopolysiloxane have a branched structure containing in part R
1
SiO
3/2
units or SiO
4/2
units or both in a small portion. Further preferably, the organopolysiloxane is end-blocked with triorganosilyl radicals such as trivinylsilyl, methyldivinylsilyl, dimethylvinylsilyl or trimethylsilyl.
The organopolysiloxane has a wide choice of molecular weight. However, the preferred organopolysiloxane has a viscosity of 500 to 1,000,000 centipoise, and especially 1,000 to 100,000 centipoise at 25° C. because it will cure into a rubbery elastomer and gives a liquid silicone rubber composition.
Illustrative examples of the alkenyl-containing organopolysiloxane are diorganopolysiloxanes shown by the following general formulae.
Herein, R is a monovalent hydrocarbon radical excluding alkenyl, and preferably methyl, n is a positive number inclusive of 0, m is such a positive number that the sum of CH═CH
2
radicals in the molecule may be two or more, and n or n+m is a number that satisfies the above viscosity range.
An inorganic filler may be blended in the silicone rubber composition of the invention. Typical inorganic fillers are silica fillers in microparticulate form including dry silicas such as fumed silica and wet silicas such as precipitated silica. The filler is used in order to impart a sufficient strength to the cured composition, that is, as a reinforcing filler. Preferably the filler has a specific surface area of at least 10 m
2
/g, more preferably 50 to 500 m
2
/g and most preferably 150 to 350 m
2
/g, as measured by the BET method. Commercially available silicas may be used as the filler. Of the dry silicas, hydrophilic silica is available under the trade name of Aerosil 200 and 300,and hydrophobic silica is available under the trade name of R972, R974 and R976 which are silicas whose surface having methyl, ethyl or vinyl radicals is treated with organosilane or organosilazane compounds having the above radic

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