JPH0574447B2 - - Google Patents

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a method for injection molding a liquid silicone rubber composition, and more specifically, a method for injection molding a liquid silicone rubber composition, in which a liquid silicone rubber composition is prepared by mixing at a relatively low temperature. The present invention relates to a method for molding liquid silicone rubber, which can be injection molded to give silicone rubber molded products with excellent properties.

[Prior Art] Injection molded products of liquid silicone rubber compositions are generally used in large quantities for the rubber parts of calculators, computer keyboards, rubber switches, anode caps, and copying machine rollers.

Conventionally, in order to obtain such molded products, the components of a liquid silicone rubber composition were mixed at room temperature or higher to create a liquid silicone rubber composition, and the composition was heated to a temperature of 120°C or higher. It is injected into a mold and cured at that temperature (for example, JP-A-57-149354, JP-A-60-17428).
(see publication).

[Problems to be Solved by the Invention] However, in all of these conventional injection molding methods, a liquid silicone rubber composition is produced at room temperature or higher, and the liquid silicone rubber composition is cured at a high temperature. Several problems have been discovered due to the fact that molded products are obtained using

In other words, the liquid silicone rubber composition expands during curing, making it difficult for the resulting molded product to separate from the mold. There were many disadvantageous problems in molding, such as a decrease in viscosity and the tendency for burrs to form on the molded product, resulting in low productivity. In addition, bubbles are likely to occur inside the resulting molded product, and in particular, the shrinkage rate of the molded product is as high as 2 to 3%, making it extremely difficult to obtain a precise molded product.

Therefore, the present inventors investigated an injection molding method that does not have such problems, and found that a liquid silicone rubber composition was prepared by mixing the constituent components of a liquid silicone rubber composition at low temperature, and then the composition was It was confirmed that the above-mentioned problems could be largely solved by curing at a relatively low temperature, and the present invention was completed.

The purpose of the present invention is to provide an injection molding method for a liquid silicone rubber composition that has extremely high productivity with less burr generation and easy removal of the molded product from the mold. It is an object of the present invention to provide a method for injection molding a liquid silicone rubber composition by which a uniform molded article with a small molding rate can be obtained.

[Means for solving the problem and its effects] This problem can be solved by mixing the constituent components of the liquid silicone rubber composition at a temperature of -60 to +5°C to form liquid silicone rubber in the injection molding method of the liquid silicone rubber composition. This problem can be solved by preparing a composition, then injecting the composition into a mold set at a temperature in the range of +25 to +100°C and curing it to continuously obtain molded products.

To explain this, the liquid silicone rubber composition used in the present invention is a liquid or paste-like composition at room temperature that contains a liquid organopolysiloxane having a reactive group, a crosslinking agent, and/or a curing catalyst as the main ingredients. It hardens into a rubber-like elastic body when left at room temperature or heated, and may be either a sag type or a non-sag type. Regarding the curing mechanism, an addition reaction type, a radical reaction type using an organic peroxide, and a condensation reaction type can be mentioned, but the addition reaction type is preferable because it has a fast curing rate and is excellent in curing uniformity. Particularly preferred in the present invention as such an addition reaction type liquid silicone rubber composition are (A) an organopolysiloxane having at least two lower alkenyl groups in one molecule; (B) an organopolysiloxane having at least two lower alkenyl groups in one molecule; Organopolysiloxane (C) having a silicon-bonded hydrogen atom Platinum-based catalyst 0.1 to 1000 parts by weight of platinum-based metal per 1 million parts by weight of the total amount of components (A) to (B) [However, component (A) 1 The total number of alkenyl groups per molecule and hydrogen atoms per molecule of component (B) is at least 5].

To explain this, component (A) is the main component of organopolysiloxane that provides silicone rubber,
This is a component that undergoes an addition reaction with component (B) and hardens due to the catalytic action of component (C). Component (A) must have at least two lower alkenyl groups bonded to silicon atoms in one molecule; if the number of lower alkenyl groups is less than two, a network structure will not be formed, resulting in good curing. I can't get things. Examples of such lower alkenyl groups include vinyl, allyl, and propenyl groups. Further, such a lower alkenyl group may be present anywhere in the molecule, but is preferably present at least at the terminal end of the molecule. Further, the molecular structure of this component may be a linear structure, a linear structure including branches, a cyclic structure, a network structure, or a three-dimensional structure, but preferably a linear structure with or without a small amount of branching. It is chain-like. The molecular weight of this component is not particularly limited and can range from a liquid with a low viscosity to a raw rubber with a very high viscosity, but is preferably viscosity at 25°C of 100 centipoise or more in order for the cured product to become a rubber-like elastic body. . Of the alkenyl groups, vinyl groups are preferred; examples of organopolysiloxanes containing such vinyl groups include methylvinylpolysiloxane with both ends blocked by trimethylsiloxy groups, methylvinylsiloxane and dimethylsiloxane with both ends blocked with trimethylsiloxy groups. Copolymer, dimethylpolysiloxane with dimethylvinylsiloxy groups blocked on both ends, dimethylsiloxane-methylphenylsiloxane copolymer with dimethylvinylsiloxy groups blocked on both ends, dimethylsiloxane-diphenylsiloxane with dimethylvinylsiloxy groups blocked on both ends methylvinylsiloxane copolymer,
Dimethylsiloxane-methylvinylsiloxane copolymer with both ends blocked with trimethylsiloxy groups, dimethylsiloxane-methylphenylsiloxane-methylvinylsiloxane copolymer with both ends blocked with trimethylsiloxy groups, methyl with both ends blocked with methylvinylsiloxy groups (3 , 3,3-tofluoropropyl) polysiloxane, dimethylsiloxane-methyl (3,
3,3-trifluoropropyl)siloxane copolymer, _CH2 =CH( _CH3 ) _2SiO1 /2 units and ( _CH3 ) _3SiO
Examples include polysiloxanes consisting of 1/2 units and SiO4/2 units. In the present invention, the above organopolysiloxanes may be used in combination.

Component (B) used in the present invention is a crosslinking agent for component (A), and due to the catalytic action of component (C), silicon-bonded hydrogen atoms in this component connect with lower alkenyl groups in component (A). It is cured by addition reaction. Component (B) must have at least two silicon-bonded hydrogen atoms in one molecule in order to function as a crosslinking agent.

In the present invention, the total number of alkenyl groups in one molecule of component (A) and silicon-bonded hydrogen atoms in one molecule of component (B) is at least five. 5
If it is less than this, it is not preferable because a good molded product cannot be obtained because a network structure is not substantially formed.

There are no particular limitations on the molecular structure of this component,
It may be linear, linear including branched, cyclic, etc. There is no particular limitation on the molecular weight of this component, but (A)
In order to improve the compatibility with the components, the viscosity at 25°C is preferably 1 to 50,000 centipoise.

The amount of this component added is such that the molar ratio of the total amount of silicon-bonded hydrogen atoms in this component to the total amount of all lower alkenyl groups in component (A) is (0.5:1) to (20:1).
The amount is said to be such that the molar ratio is
If it is less than 0.5:1, good curing properties cannot be obtained.
This is because when the ratio is greater than 20:1, the hardness increases when the cured product is heated. In addition, when a siloxane containing an alkenyl group is separately added for reinforcement or other purposes, it is preferable to add this component containing silicon-bonded hydrogen atoms in an amount corresponding to the alkenyl group.

Specific examples of this component include methylhydrodiene polysiloxane with trimethylsiloxy groups blocked at both ends, dimethylsiloxane-methylhydrodiene polysiloxane copolymer with both ends blocked with trimethylsiloxy groups, and dimethylhydrodienepolysiloxane copolymer with both ends blocked with trimethylsiloxy groups. Dimethylsiloxane-methylhydrodienesiloxane copolymer, dimethylsiloxane-methylhydrodienesiloxane cyclic copolymer, (CH ₃ ) ₂ Copolymer consisting of SiO1/2 units and SiO2/4 units, (CH ₃ ) ₃ SiO1/ 2 units, (CH ₃ ) ₂ HSiO1/2
Examples include copolymers consisting of SiO4/2 units and SiO4/2 units.

Component (C) is a catalyst that causes an addition reaction between a silicon-bonded hydrogen atom and an alkenyl group. Specific examples include chloroplatinic acid, a solution of this in alcohol or ketones, and an aged solution of the same. , complex compounds of chloroplatinic acid and olefins,
These include complex compounds of chloroplatinic acid and alkenylsiloxane, complex compounds of chloroplatinic acid and diketone, platinum black, and those in which platinum is supported on a carrier.

The amount of this component added is 0.1 to 1000 parts by weight as a platinum-based catalyst per 1 million parts by weight of the total amount of components (A) to (B). This is because it is uneconomical if the amount exceeds 1000 parts by weight. When used normally, the amount of platinum-based catalyst added is preferably about 1 to 100 parts by weight.

The liquid silicone rubber composition used in the present invention may contain a filler in order to adjust the fluidity and improve the mechanical strength of the molded product. Such fillers include reinforcing fillers such as precipitated silica, fumed silica, pyrogenic silica, fumed titanium oxide, ground quartz, diatomaceous earth, asbestos, aluminosilicate, iron oxide, zinc oxide, calcium carbonate, etc. Examples include non-reinforcing fillers, which may be used as is or may be surface-treated with an organosilicon compound such as hexamethylsilazane, trimethylchlorosilane, or polymethylsiloxane. In addition, it is possible to add trace or small amounts of acetylene compounds, hydrazines, triazoles, phosphines, mercaptans, etc. to the organopolysiloxane composition used in the present invention as additives for suppressing the curing reaction. There is no problem as long as it does not impair the purpose of the present invention. In addition, pigments, heat resistant agents, flame retardants, plasticizers, etc. may be added as necessary.

Next, as a preferred liquid silicone rubber composition, there is a radical reaction type liquid silicone rubber composition using an organic peroxide. Inorganic filler if necessary,
For example, fumed silica, precipitated silica heat-resistant agents, pigments, etc. are added. It is preferable to select an organic peroxide having a decomposition temperature in the range of +25°C to +100°C.

In the present invention, it is necessary to prepare the liquid silicone rubber composition by mixing the constituent components of the liquid silicone rubber composition at low temperature. The mixing temperature is preferably in the range -60°C to +5°C, preferably in the range -30°C to 0°C.
This is because the organopolysiloxane used in the present invention tends to become gel-like at temperatures below -60°C and cannot be injection molded.
This is because if the temperature exceeds 5°C, the curing reaction progresses during mixing, making injection molding difficult, and the mechanical strength of the resulting cured product decreases significantly.

The liquid silicone rubber composition thus obtained is then injected into a mold set at a temperature in the range of +25 to +100°C, preferably +40 to +75°C, and cured. If this curing temperature is less than 25°C, the curing speed of the liquid silicone rubber composition used in the present invention will decrease, resulting in a decrease in productivity.
If the temperature exceeds 100°C, air bubbles will occur inside the molded product, which will reduce the withstand voltage and make it difficult to obtain a homogeneous molded product. Furthermore, this is not preferable because it increases the occurrence of burrs and at the same time makes it difficult for the molded product to be released from the mold.

The injection molding method of the present invention can be carried out by connecting a cooling device to a well-known injection molding machine for liquid silicone rubber.

By using the injection molding method of the present invention,
Keyboard, rubber switch, anode cap,
Packing, gaskets, and rubber plugs can be produced efficiently.

[Example] Next, the present invention will be explained with reference to an example. The middle part of the example indicates parts by weight, and the viscosity is the value at 25°C.

Example 1 100 parts of dimethylpolysiloxane (vinyl group content: 0.5% by weight) with a viscosity of 2000 centipoise, both molecular chain ends blocked with dimethylvinylsiloxy groups, 40 parts of wet silica with a specific surface area of 200 m ³ /g, and both molecular chains Methylhydrodiene polysiloxane with a viscosity of 10 centipoise [SiH content 1% by weight] terminal-capped with trimethylsiloxy groups was added and mixed to obtain a mixture with a viscosity of 3000 poise at a shear rate of 10 sec ^-1 (mixture A). Next, 40 parts of the same wet silica as above and 0.02 part of an isopropyl alcohol solution of chloroplatinic acid [platinum content 3% by weight] were added to 100 parts of the same dimethylpolysiloxane as above and mixed.
A mixture similar to the above was obtained (mixture B).

This mixture A was placed in tank 1 for liquid silicone rubber composition, and mixture B was similarly placed in tank 2 for liquid silicone rubber composition and set. Next, these were fed into a static mixer 8 which had been previously cooled to -5° C. by a refrigerant circulation device 11 using pressure pumps 4 and 5, and mixture A and mixture B were mixed at a ratio of 1:1. The obtained cured product was continuously injected into molds 14a and 14b whose temperature was previously adjusted to 50°C using a refrigerant circulation device 16, and was cured under conditions of an injection time of 5 seconds and a curing time of 20 seconds. 100 injection molded products were obtained. The molded product obtained here was free of burrs and could be easily demolded from the mold. Moreover, the molded product had a smooth surface with no air bubbles observed inside. Next, physical properties of this injection molded product were measured according to JISK6301, and the hardness was 61, tensile strength was 67Kg/cm ² , and tensile elongation.
It was 300%.

For comparison, the mixture mixed in the static mixer 8 was placed in a mold 1 preset at 120°C.
When the molded products were injected into the interiors of 4a and 14b, burrs were generated in the molded products obtained, and demolding from the mold was not as good as in the above. In addition, the molded product contained fine air bubbles inside.

Furthermore, the mixture obtained in the same manner as above except that the passage of the refrigerant through the jacket 9 of the static mixer 8 was stopped and the temperature was kept at room temperature (20°C) was transferred to the mold 14a set at 50°C in the same manner as above. and 14
When the injection molded product was injected into the inside of the molded product and cured under the same conditions as above, the surface of the obtained molded product had irregularities and the inside contained air bubbles.

Example 2 100 parts of dimethylpolysiloxane (vinyl group content 0.15% by weight) having a viscosity of 10,000 centipoise and having both molecular chain ends blocked with dimethylvinylsiloxy groups was mixed with dry silica having a surface area of 200 m ² /g [manufactured by Nippon Aerosil Co., Ltd.] 30 parts of Aerosil 200] and 1.5 parts of methylhydrodiene polysiloxane with a viscosity of 12 centistokes [SiH content 0.9% by weight] with both ends of the molecular chain capped with trimethylsiloxy groups were added and mixed at a shear rate of 10 sec ^-1 . A mixture was obtained having a viscosity of poise (mixture A). Next, add the same dry silica as above to 10 parts of the same dimethylpolysiloxane as above.
30 parts of chloroplatinic acid and 0.02 parts of isopropyl alcohol solution [platinum content: 3% by weight] were added and mixed to obtain a mixture similar to the above (mixture B).

This mixture A was placed in a raw material tank 1 for a liquid silicone rubber composition, and the mixture B was similarly placed in a raw material tank 2 for a liquid silicone rubber composition. Next, these are preheated to -20℃ in the refrigerant circulation device 11.
Feed mixture A and mixture B were mixed in a 1:1 ratio using pressure pumps 4 and 5 in a static mixer 8 that had been cooled to a temperature of 100 mL.

The temperature of the obtained cured product was adjusted in advance to 50°C using a refrigerant and heat medium circulation device 16, and then the mold 14 was set.
The mixture was continuously injected into the interiors of tubes a and 14b and cured under conditions of an injection time of 5 seconds and a curing time of 20 seconds to obtain 100 injection molded products. The molded product obtained here was free of burrs and could be easily demolded from the mold. Next, we measured the physical properties of this injection molded product according to JISK6301, and found that the hardness was 45 and the tensile strength was 70.
The measured value was Kg/cm ² and tensile elongation 500%.

For comparison, when the mixture mixed under cooling with the static mixer 8 was injected into the molds 14a and 14b, which were preset at 190°C, the liquid silicone rubber composition as the mixture was found to be in the mold cavity. The inside was not completely filled, causing a so-called short shot phenomenon, and a molded product having a perfect shape could not be obtained.

Example 3 The mixture mixed under cooling with the static mixer 8 in Example 1 was continuously injected into a mold preset at 40°C, with an injection time of 20 seconds and a curing time of 3.
It was cured under conditions of 1 minute to obtain a molded article. The molded product obtained was free from burrs and could be easily demolded from the mold.

[Effects of the Invention] In the present invention, a liquid silicone rubber composition is prepared by mixing the constituent components of a liquid silicone rubber composition at a temperature of -60°C to +5°C, and then the composition is mixed at a temperature of +25°C to +100°C. Since the molded product is obtained by injection into a mold set at a certain temperature, there are few burrs and the molded product can be easily removed from the mold, resulting in extremely high productivity. Furthermore, the obtained silicone rubber molded product has few air bubbles and has a low molding shrinkage rate, so it has the characteristics that a homogeneous molded product with excellent withstand voltage can be obtained.

Therefore, the injection molding method of the present invention is suitable for practical use in efficiently molding silicone rubber molded products from liquid silicone rubber compositions, and provides silicone rubber molded products with excellent appearance and physical properties to the industry. can be provided.

[Brief explanation of the drawing]

FIG. 1 is a schematic process diagram for carrying out the present invention. 1... Raw material tank for liquid silicone rubber composition, 2... Raw material tank for liquid silicone rubber composition, 3a... Raw material for liquid silicone rubber composition,
3b...Raw material for liquid silicone rubber composition, 4...
... Pressure pump, 5 ... Pressure pump, 6 ... Raw material supply pipe for liquid silicone rubber composition, 7 ... Raw material supply pipe for liquid silicone rubber composition, 8 ... Static mixer, 9 ... Static mixer. Mixer cooling jacket, 10... Injection nozzle, 11...
Refrigerant circulation device for static mixer cooling, 1
2... Static mixer cooling refrigerant supply pipe, 13... Static mixer cooling refrigerant supply pipe, 14a... Injection mold, 14b... Injection mold, 15... Injection mold Refrigerant/heat medium inlet for mold temperature adjustment, 16...Refrigerant/heat medium circulation device for injection mold temperature adjustment, 17... Refrigerant/heat medium supply pipe for injection mold temperature adjustment, 18...Injection Coolant/heat medium supply pipe for mold temperature adjustment.

Claims (1)

[Claims] 1. In an injection molding method for a liquid silicone rubber composition, the constituent components of the liquid silicone rubber composition are mixed at a temperature in the range of -60 to +5°C to form a liquid silicone rubber composition, and then the liquid silicone rubber composition is A method for injection molding a liquid silicone rubber composition, which comprises injecting the composition into a mold set at a temperature in the range of +25 to +100°C and curing it to continuously obtain molded products. 2. The liquid silicone rubber composition is (A) an organopolysiloxane having at least two lower alkenyl groups in one molecule (B) an organopolysiloxane having at least two silicon-bonded hydrogen atoms in one molecule (C) Platinum-based catalyst 0.1 to 1000 parts by weight of platinum-based metal per 1 million parts by weight of the total amount of components (A) to (B) [However, alkenyl group per molecule of component (A) and per molecule of component (B)] The total number of hydrogen atoms is at least 5.] A method for injection molding a liquid silicone rubber composition according to claim 1.