GB2279616A - Silicone resin keytops for membrane switches. - Google Patents

Description

-1 COVERING MEMBER FOR PUSH-BUTTON SWITCHES WITH RIGID KEY TOPS 2279616

BACKGROUND OF THE INVENTION

The present invention relates member for push-button switches with rigid key tops and more particularly relates to a covering member for pushbutton switches of which the key tops are made from a high-hardness resin so as to be capable of giving excellent and pleasant finger-touch feeling to the operator when it is used in push-button panels in desk-top electronic calculators, remote controllers, telephones, various kinds of office-automation instruments, household electric appliances and the like.

A covering member for push-button switches has a plurality of dome-like protrusions on a base plate mounted on a substrate, each dome-like protrusion forming part of a switching unit consisting of a riser part and a f lat top part called a key top. When the key top is pushed down by a f inger tip, the riser part causes buckling deformation to a coverina so that a movable contact point on the lower surf ace of the key top is brought into contact with f ixed contact points on the substrate so as to close the electric circuit; upon release of the pushing 5 force on the key top, the riser part regains the unpushed configuration and thereby opens the circuit between the f ixed contact points. For this purpose, at least the riser part of the covering member is made from a rubbery resilient material.

In of the capable of giving a pleasant finger-touch feeling of rigidness, the base plate, riser parts and movable contact points are made from a silicone rubber in view of its excellent resilience characteristics and durability, while the key top is made from a more rigid resinous material such as acrylic or methacrylic resins and unsaturated polyester resins formed by the in situ polymerization and curing of a monomeric or oligomeric composition with admixture of a polymerization initiator and/or curing agent by heating. These covering members are generally satisfactory not only in terms of the finger-touch feeling on pushing and the resilience performance but also in that little trouble is caused by contamination of the contact points a conventional model of a covering member above-mentioned type having rigid key tops by oily matters bleeding out of the resinous key tops, so that they are widely used in the push-button switch panels of various kinds of instruments having a small thickness or having a switching panel with large key tops.

The above-mentioned covering member is prepared usually in the following manner. A metal mold for the covering member is heated at a high to effect curing rubber composition and of a monomeric or oligomeric liquid composition to give the resin-made key tops, and the cavities of the metal mold for the formation of the key tops are filled with the monomeric or oligomeric liquid resin composition, which is in situ polymerized and brought into a semi-cured state without flowability but still retaining tackiness on its surface. Thereafter, the cavities for the riser parts. and the base plate part are filled with an uncured silicone rubber composition which is molded into the form of the respective parts integral with the key top parts previously molded and in a semi-cured state, followed by further heating to complete integration of the key tops with the riser parts by completely curing both the resin composition and the silicone rubber composition.

temperature sufficiently of an uncured silicone Most of these procedures are mechanized by using suitable machines and instruments but some of the steps still require manual working.

One of the problems in the above-described manufacturing method for the preparation of a covering member consisting of resinous parts and silicone rubber parts is that, since the monomeric or oligomeric liquid resin composition exhibits a large shrinkage on polymerization and curing when contained in the cavities for the formation of the key tops, so that the free surface of the key top in a semi-cured state is sometimes not flat or is irregularly rugged, the boundary line or the interface between the key top part made from the resin and the silicone rubber-made riser part is also not flat, resulting in poor dimensional accuracy of the resin-made key top parts. This problem is more serious when the liquid resin composition filling the cavities for the key tops cannot exhibit good levelling of its surface.

Another problem in the above-described covering member is that, since the resins forming the key tops are relatively unstable at elevated temperatures, which tend to cause yellowing, no sufficient thermal conditions can be adopted in the post-curing of the integrated member consisting of the semi-cured resin parts and the silicone rubber parts, that the silicone rubber parts are left in an insufficiently baked state still containing a considerable amount of low-molecular volatile matters which otherwise could have been completely removed by heating at a sufficiently high temperature for a sufficiently long time. A further consequence of the insufficiently performed post-curing of the resin-made key top parts is that, while it is usual for the top surface of the key top to be provided with various kinds of indicias such as letters, numerical figures or symbolic or pictorial signs, formed by printing with a thermosetting ink to indicate the function of the respective key tops, the adhesive bonding strength between the layer of the printing ink and the top surface of the key top is sometimes so low that the indicia are poorly durable in use of the covering member, due to rubbing, scratching and other mechanical attacks.

A further problem in the covering member of the above-described type is that, though not always so and usually not so serious, the liquid resin composition, before curing or when insufficiently cure d, exhibits irritativeness against human skin, causing dermatitis, or emits an irritating or offensive odor in the course of curing by heating, thus causing pollution of the working environment.

SUMMARY OF THE INVENTION

The present invention accordingly has as an object to provide a novel and improved covering member for push-button switches with resinous rigid key tops, free from the above-described problems and disadvantages in the prior art covering member.

The present invention provides a covering member for push-button switches with resinous rigid key tops which is an integral body comprising: (a) a base plate made from a silicone rubber having 15 elastic resilience; (b) a riser part also made from a silicone rubber having elastic resilience and integrally bonded to the base plate; and (c) a key top made from a cured silicone resin having a Shore D hardness of 40 or higher and integrally bonded to the top of the riser part. BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a cross-sectional view illustrating impregnation of a metal mold cavity for a key 25 top, using a conventional liquid resin composition.

Figure 3 is impregnation of member for Figure 2 is a cross-sectional view of a covering member for push-button switches with rigid key tops made from a conventional resin.

a cross-sectional view illustrating a metal mold cavity for a key top, using a liquid silicone resin composition according to the invention.

Figure 4 is a cross-sectional view of a covering push-button switches with rigid key tops made from a high-hardness silicone resin according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The covering member for push-button switches with rigid key tops according to the present invention as well as the method for the preparation thereof are described in detail with reference to Figures 3 and 4 of the accompanying drawing by comparison with a conventional covering member for push-button switches with rigid key tops by using an organic resin, as well as the method for the preparation thereof, illustrated in Figures 1 and 2-.

In Figure 1, illustrating in vertical crosssection the starting step of the method of preparation of a conventional covering member, a metal mold 1 having a cavity la for a key top is heated at such an elevated temperature that the monomeric or oligomeric liquid resin composition can be polymerized and cured. The liquid resin composition 3 is poured into the cavity la of the metal mold 1 and polymerized and semi-cured therein until it loses flowability but still retains surface tackiness. Since conventional liquid resin compositions exhibit poor levelling behaviour and cause great. shrinkage on polymerization and curing, the thus-formed key top part 3 in a semi-cured state usually has a free surface 3a which is not flat but irregularly rugged as is shown in Figure 1.

In the next step, an uncured silicone rubber composition, usually in the form of a sheet, is put on the metal mold 1 to cover the key top part 3 and the metal mold is closed and compressed to effect molding and curing of the silicone rubber composition and to integrate it with the key top part 3, which is also cured completely in this step. Figure 2 illustrates a vertical cross-sectional view of the thus- prepared covering member, which is an integral body consisting of a base plate 5 made from a cured silicone rubber, a riser part 4 also made from a cured silicone rubber integrally bonded to the base plate 5 around the lower peripheral line thereof and a key top 3 made f rom. the cured resin having a high hardness bonded to the riser part 4 through a key top base 6 of the silicone rubber, which is integral with the riser part 4. Usually a movable contact point 7 made from 5 an electroconductive rubbery material is bonded to the lower surf ace of the key top base 6. Since the free surface 3a of the semi-cured key top part 3 in the cavity la of the metal mold 1 is not flat but irregularly rugged as is mentioned 10 above, the interface 3a between the cured resinous key top 3 and the key top base 6 of the cured silicone rubber is also necessarily not flat but irregularly rugged, as is shown in Figure 2, and badly affects the dimensional accuracy of the key top 3 or the covering member as a whole. The covering member according to the present invention for push-button switches with rigid key tops can be prepared in a similar way to the above. As is shown in Figure 3 in a vertical 20 cross-sectional view, the cavity la of the metal mold 1 heated at an elevated temperature is filled with a curable liquid silicone resin composition 13 which is brought into a semi-cured state by heating. Unlike as in Figure 1 showing the process using a conventional liquid resin composition, the key top part 13 in a semi-cured state as contained in the cavity la of the metal mold 1 has a f ree surface 13a which is excellently flat, by virtue of the flowing behaviour of the liquid silicone resin composition, and exhibits excellent levelling along with very little shrinkage of the silicone resin on curing. As a consequence, the finished covering member prepared in the same manner as in the conventional method and illustrated in Figure 4 in vertical crosssectional view has a very flat interface between the cured resinous key top 13 and the key top base 6 made f rom the cured silicone rubber, resulting in ment in the di ensiona a great improve1 accuracy of the key top 13 and the covering member as a whole along with an improvement in the adhesive bonding strength between the silicone resin-made key top 13 and the silicone rubber-made key top base 6, by virtue of the better affinity of the silicone rubber with a silicone resin than with a conventional resin of different chemical nature.

As will be understood from the above description, a characteristic feature of the covering member according to the invention is that the key top 13 of the covering member is formed from a silicone resin composition having, after complete curing, a Shore D hardness of 40 or higher so that various - 11 advantages are obtained as mentioned above as compared with conventional covering members of which the key top is shaped from an organic resin composition.

The high-hardness silicone resin used in the covering member according to the present invention consists chemically of three-dimensional siloxane linkages expressed by the formula -Si-O-Si7 forming the backbone skeleton of the molecules with various kinds of organic or, in particular, monovalent hydrocarbon groups bonded to the silicon atoms. Various grades of silicone resins are commercially available with different silicon-bonded organic groups including methyl silicone resins, phenyl silicone resins, dimethyl diphenyl silicone resins, methyl phenyl silicone resins, polyester-modified silicone resins, acrylic-modified silicone resins, epoxy-modified silicone resins and the like. These silicone resins have an advantageously low viscosity even when they are in an oligomeric form of a relatively high molecular weight so that the cavities in the metal mold for the formation of key tops can be easily and completely filled with the liquid resin composition exhibiting excellent levelling behaviour on the surface even without undertaking any particular means therefor. The viscosity of the liquid silicone resin composition can be further decreased if it is heated at an elevated temperature of, for example, 100 0 C to facilitate the impregnation, while curing of the liquid silicone resin proceeds when it is heated at a higher temperature to give a cured body having a Shore D hardness of 40 or higher, giving excellent and pleasant finger-touch feeling to the operator of the push-button switches.

As an inherent physiological property of silicones in general made from organopolysiloxanes, the silicone resin used in the invention is also free from irritation against human skin either before or after curing so that the operator of the push-button switch panel can be safe from any dermatitis. Further, cured silicone resin is highly heat-resistant without yellowing and is capable of withstanding a post-curing heat treatment at a temperature much higher than with conventional organic resins so that a full adhesive bonding strength can be easily obtained between the top surface of the silicone resin-made key top and the layer of a printing ink applied thereon to form indicias such as letters, numerical figures, symbols and the like. Moreover, the key top of the cured silicone resin exhibits high heat-and-cold- 81 silicone problem of emission of resistance, ozone resistance and chemical resistance to ensure durability of the covering member.

In addition resins are free from the any offensive odor in the course of curing, so do not cause the problem of environmental pollution, and also are free from bleeding of an oily matter after curing and therefore do not cause troubles due to contamination of the contact points therewith.

The liquid silicone resin composition to form the key top parts in the inventive covering member preferably has a viscosity not exceeding 500 poise at 250C so that various types of liquid-dispenser machines can be used from which the liquid resin composition can be introduced drop-wise into the cavities of the metal mold. Of course the type of the silicone resin relative to the chemical mechanism for the crosslinking and curing reaction of the liquid silicone resin composition into cured key tops is not limited to a particular one and includes those curable- by the mechanisms of addition reaction and radical reaction as well as combinations thereof.

As is the case of most covering members for push-button switches, the top surface of the silicone resin-made key tops is usually provided with indicias such as letters, numerical figures, signs and the like by printing with a printing ink or by coating with a coating composition. Since the key tops are made from a silicone resin, it is preferable that the printing ink or coating composition for the indicias is also of a siliconebased one of the same or similar type so as to ensure good adhesive bonding strength thereof to the top surface of the key tops. When the key top is made from a methyl silicone resin or a methyl phenyl silicone resin for example, the vehicle of the printing ink is preferably also a methyl silicone resin or resin respectively. The a methyl phenyl silicone printing ink or coating composition is prepared by compounding such a silicone resin with a colouring agent, crosslinking agent, diluent and other known additives.

The silicone rubber to form the riser part and the base plate of the covering member of the invention may be any one conventionally used in the prior art covering members for push-button switches with rigid key tops. The principal ingredient in a silicone rubber composition is a diorganopolysiloxane having a linear molecular structure as represented by the average unit formula R a Sio (4-a)/2, in which R is a monovalent hydrocarbon group exemplified by alkyl groups, such as methyl, ethyl, propyl and butyl groups, alkenyl groups, such as vinyl and allyl groups, and aryl groups, such as phenyl and tolyl groups, as well as those substituted groups obtained by replacing a part or all of the hydrogen atoms in these hydrocarbon groups with halogen atoms, cyano groups and the like, such as chloromethylr 3,3,3-trifluoropropyl and 2-cyanoethyl groups, and the subscript a is a positive number of 1.90 to 2.05. The diorganopolysiloxane should have a viscosity of at least 100 centistokes or, preferably, at least 1000 centistokes at 25 0 C.

A silicone rubber composition can be prepared 15 by compounding the abovedescribed diorganopolysiloxane with various kinds of additives including, usually, a silica filler, which can be a fumed silica filler, precipitated silica filler, fine quartz powder, diatomaceous earth or the like.

When the silica filler reinforcing effect, the is designed to impart a particle diameter thereof is preferably 50 nm or smaller. The amount of the silica filler in the silicone rubber composition is usually in the range from 20 to 200 parts by weight per 100 parts by weight of the diorgano- polysiloxane. other optional ingredients include dispersing aids such as organosilane compounds having an alkoxy group, hydroxy group or the like bonded to the silicon atom, colouring agents such as inorganic and organic pigments and so on.

The silicone rubber composition must be provided with a curing means which is obtained by compounding the composition with a crosslinking agent such as, typically, an organic peroxide. It is preferred that the organic peroxide have a half-life period of 1 minute at a temperature in the range from 120 to 180 0 C. Such an organic peroxide is exemplified by benzoyl peroxide, dichlorobenzoyl peroxide, butylperoxy-2-octyl hexanoate, 2,5-dimethyl-2,5dibenzoylperoxy hexane, butylperoxy laurate, cyclohexane peroxide, butylperoxy isopropyl carbonate, butylperoxy allyl carbonate and the like, though is not particularly limited thereto. A silicone rubber composition of an alternative type relative to the mechanism of the crosslinking reaction is that of the so-called addition-reaction type in which the diorganopolysiloxane as the principal ingredient has a plurality of alkenyl groups, e.g. vinyl groups, in a molecule as a part of silicon-bonded monovalent hydrocarbon groups this alkenyl-containing diorganopolysiloxane compounded with an organohydrogenpolysiloxane the and is - 17 having at least two silicon-bonded hydrogen atoms in a molecule as the crosslinking agent and a trace amount of a platinum compound as a catalyst for promoting the addition reaction between the alkenyl groups and the silicon-bonded hydrogen atoms.

In the following, examples and comparative examples are given to describe the covering member for push-button switches with rigid key tops according to the invention in more detail. Example 1.

A liquid silicone resin composition was prepared by compounding 100 parts by weight of a commercial product of a dimethyl diphenyl silicone resin (X321195, a product of Shin-Etsu Chemical Co.) with 10 parts by weight of a crosslinking agent (CX 1195, a product of the same company as above), 0. 4 part by weight of a curing catalyst (X-93-55, a product of the same company as above) and 1 part of a pigment (STVX 683, a product of Dainichi Seika Kogyo Co.). The thus-prepared silicone resin composition was drop-wise introduced into the cavities f or key tops of a metal mold heated at 1500C by using an. air dispenser (Model AD 3000VH, a product of Iwashita Engineering Co.) to be filled therewith.

When the liquid silicone resin, after 30 seconds, had been semi-cured sufficiently to lose flowability but still retain surface tackiness, a sheet of a curable silicone rubber composition obtained by compounding 100 parts by weight of a silicone rubber compound (KE 9510U, a product of Shin-Etsu Chemical Co.) with 1 of an organic peroxide-based curing 25B, a product of Nippon Oil and mounted on the metal mold and the then closed to fill the cavities parts and the base plate of a with the silicone part by weight agent (Perhexa Fat Co.) was metal mold was for the riser covering member rubber composition, which was then cured by compression molding at the same temperature for 10 minutes under a compressive pressure of 100 kgf/cm2 to give an integral covering member consisting of the key tops made from the silicone resin and the riser parts and the base plate made from the silicone rubber, as is illustrated in Figure 4 by a vertical cross-sectional view.

Thereafter, the top surface of the key tops of the thus-prepared covering member for push-button switches was provided with indicias by screen printing with a white pasty printing ink having a viscosity of 300 poise at 25 0 C prepared by compounding 100 parts by weight of the same dimethyl - 19 diphenyl silicone resin as used above with 20 parts by weight of a titanium dioxide powder (A-280, a product of Ishihara Sangyo Co.) as diluted with toluene. After drying of the printing ink, the covering member was subjected to a post-baking treatment for 60 minutes in an air oven at 200 0 cl whereby to finish the covering member for push- button switches with rigid key tops which had a Shore D hardness of 40 or higher. By virtue of this high hardness of the key tops made from the silicone resin, this covering member, when used in a switching panel, could give excellent and pleasant finger-touch feeling to the operator of the switches and could serve for a long period of time due to the high adhesive bonding strength between the key tops and the riser parts and between the indicia layer and the top surface of the key tops.

To examine the reliability and reproducibility of the products, inspection was undertaken for 100 covering members prepared in the above- described manner; this inspection detected yellowing of the key tops in none of the 100 covering members and incomplete finishing of the key tops relative to the riser parts -and poor dimensional accuracy of the key top arrangement or the so-called key pitch in only five of the 100 covering members.

- 20 Further, the content of low-molecular volatile organopolysiloxanes contained in the silicone rubber parts after the post-baking treatment at 200 OC for 60 minutes was gas-chromatographically determined and showed that the content was less than 500 ppm by weight. The adhesive bonding strength between the indicia layer and the top surf ace of the key top was excellent to withstand a nail-scratch test.

Comparative Example 1.

For comparison, 100 covering members for push-button switches with rigid key tops were prepared in essentially the same manner as above except for replacement of the silicone resin composition with a conventional acrylic resin-based composition. These comparative covering members were subjected to the same inspection as above. This inspection showed yellowing of the key tops in all of the 100 covering members and incomplete finishing of the key tops relative to the riser parts and poor dimensional accuracy of the key top arrangement or so-called key pitch in as many as 85 of the 100 covering members.

Example 2 and Comparative Example 2.

In Example 2, covering members were prepared in the same manner as in Example 1 except that 21 the post-baking treatment of the covering members taken out of the metal mold was conducted at 150 0 C for or 60 minutes (conditions I or II), at 180 0 C for 15 or 60 minutes (conditions III or IV) and at 200 0 C for 60 minutes (conditions V). Needless to say, yellowing of the key tops was found in none of the thus-prepared covering members, while the adhesive bonding strength between the indicia layer and the top surface of the key tops was less satisfactory than in Example 1 and unacceptable under the conditions I. The contents of low-molecular volatile organopolysiloxanes in the silicone rubber parts were 12,000 ppmr 6000 ppm, 6000 ppm, 3000 ppm and 1000 ppm by weight under the post-baking conditions of I, II, III, IV and V, respectively, indicating that the content of volatile organopolysiloxanes can be decreased as low as desiredby suitably selecting the conditions f or the post-baking treatment.

In Comparative Example 2, the same test as above was concurrently undertaken with the covering members prepared in the same manner as in Comparative Example 1 but with varied conditions I to V of the post-baking treatment. The results were substantially the same as in Example 2 described above relative to the adhesive bonding strength between the indicia layer and the key tops and the content of volatile organopolysiloxanes in the silicone rubber parts. Yellowing of the key tops, however, was found under the conditions IV and V to the same extent as in Comparative Example 1 and yellowing was noticeable under the conditions II and III while no yellowing took place under the conditions I.

Example 3 and Comparative Example 3.

An organoleptic test was undertaken by seven panel members for any irritating or offensive odor emitted in the process of preparation of the covering members in the same manner as in Example 1 (Example 3) and in Comparative Example 1 (Comparative Example 3), including the compression molding in the metal mold and post-baking treatment in the air oven.

The results reported by the panel members were that nobody felt any slightest offensive odor in Example 3 while six members felt a very intense offensive odor and one member felt a clearly noticeable offensive odor in Comparative Example 3.

Claims (2)

1. A covering member for push-button switches with resinous rigid key tops which is an integral body comprising:

(a) a base plate made from a silicone rubber having elastic resilience; (b) a riser part made from having elastic resilience and to the base plate; and (c) a key top made from a cured silicone resin having a Shore D hardness of 40 or higher and integrally bonded to the top of the riser part.

a silicone rubber integrally bonded

2. A covering member for push-button switches with resinous rigid key tops, which covering member is substantially as hereinbefore described with reference to Figs. 3 and 4 of the accompanying drawings.