GB2242485A

GB2242485A - Cover-packing built-up body for a hard-disc unit and process for producing said body

Info

Publication number: GB2242485A
Application number: GB9104845A
Authority: GB
Inventors: Noburu Shimamoto; Ryuichi Handa; Toru Takamura
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1990-03-09
Filing date: 1991-03-07
Publication date: 1991-10-02
Anticipated expiration: 2011-03-07
Also published as: JPH03259987A; GB2242485B; JPH0759697B2; GB9104845D0

Abstract

An improved cover-packing built-up body for a magnetic hard-disc unit is devised with the object of avoiding troubles caused by dust or other contamination of the gap between the magnetic recording disc and the magnetic head. Instead of using a pre-shaped rubber packing to prevent dust intrusion, the body is assembled using a curable liquid silicone rubber composition, preferably of the reactioncurable type, which after application is cured in situ to give a cured silicone rubber packing 2 containing not more than 0.05 per cent by weight of low molecular-weight organopolysiloxane constituents. In this way troubles caused by deposition of condensate of undesirable volatile by-products are minimised. As one alternative, a silicone rubber packing formed by the in situ curing of a liquid composition is subjected to a bake-out treatment by heating to reduce the content of the low molecular-weight organopolysiloxane constituents to the prescribed level. <IMAGE>

Description

COVER-PAlIING BUILT-UP BODY FOR A HXRD-DISC UNIT AND PROCESS FOR PRODUCING SAID BODY The present invention relates to a cover-packing built-up body for a hard-disc unit, more particularly to a cover-packing built-up body for a hard-disc unit formed by utilizing a selfadhesive liquid silicone rubber composition as the packing material, and to a process for producing such a body.

The hard-disc unit here implied is a device comprising a disc-shaped magnetic recording medium prepared by providing a disc of an aluminium alloy or the like with a magnetic recording layer formed thereon by coating or sputtering and a casing containing the disc in a rotatable fashion so as to magnetically write information into the magnetic recording layer or read out the recorded information by means of a magnetic head held above the rotating disc with a very narrow gap of about 0.1 to 0.5 pm therebetween. Among various requirements for such a device, it is very important to prevent troubles, such as head crashes and damages in the magnetic recording layer, caused by dust coming fron outside the device or produced inside the device.

Accordingly, it is conventional in hard-disc units to attempt to solve the aboveementioned problems due to dust by arranging that dust-free air through a high-performance filter is circulated inside the casing under a controlled pressure and any gaps between the casing and various covers are sealed by providing packings of a rubbery material. Various synthetic rubbers are used for this purpose, including polychloroprene rubbers, urethane rubbers, silicone rubbers, acrylonitrilebutadiene copolymieric rubbers and the like, and these rubbers are shaped into the form of packings either by canpression moulding in a metal mould or by punching a pre-shaped rubber sheet using a punching die.The packings can also be prepared from a sheet of foamed bodies of these rubbers or polyethylenes.

The use of the above-mentioned packings shaped in advance by compression moulding or punching is not always quite advantageous because the pre-shaped packings must be coated with a pressure-sensitive adhesive and the adhesive-coated surface must be protected temporarily with a sheet of release paper, thus entailing large labour costs; in addition the packings must be mounted on the device with an extremely high degree of precision, necessitating considerable skill and time and an unavoidable increase in production costs.

Needless to say, the packings used for the abcveementioned application must be very clean to avoid dust formation or contamination although it is a rather difficult matter to ccmpletely prevent contamination frsm the curable or pressuresensitive adhesives used thereon and, when the finishing work of a compressionenoulded packing is imperfect, burrs or fins remaining on the surface of the packing sometimes fall off and form dust particles. When the packings are prepared fram a sheet of foamed rubber body, dust particles are sometimes enclosed in the cells of the cellular foamed body and eventually cane out.

An alternative proposal which has been made and partly practised is that, instead of using a pre-shaped rubber packing, a liquid rubber copposition curable into rubber is spread over the covering member or the interstices between the device and the covering are filled therewith, followed by in situ curing into rubber. This method is still not completely free from the problem of contamination.Since the gap between the surface of the magnetic recording layer and the magnetic head is required to be smaller and smaller as a consequence of a trend in recent years to comply with a requirement for improved performance of the device, any extremely fine foreign bodies on the surface of the packing are detrimental to the performance of the device; even if the surface of the packing is absolutely free from dust particles, the surface of the in situ cured rubber packings in general is not free from bleeding of an oily material or blooming of a powdery material or sometimes emits a volatile matter which is eventually condensed into particles or a layer of an oily matter, resulting in frequent troubles between the disc surface and the magnetic head.It is generally accepted in this regard that silicone-based curable rubber compositions are most preferable among various types of curable rubber compositions. Silicone rubber canpositions, however, are still not completely free from the problem of contamination since curable silicone rubber compositions usually contain a substantial amount of low molecular-weight organopolysiloxane constituents having volatility. when condensation of the vapor of such volatile organopolysiloaane materials takes place in the gap between the disc surface and the magnetic head, floating of the magnetic head above the disc surface with a definite and extremely small gap space therebetween would be heavily disturbed, leading to a head crash or damage in the magnetic recording layer.

The present invention accordingly has as an object to provide an improvement in a cover-packing built-up body for a hard-disc unit, free fram the disadvantages and problems in the prior art units.

Thus, the improvement of the invention comprises, in a cover-packing built-up body for a hard-disc unit in which a packing for sealing is formed by the in situ curing of a curable liquid silicone rubber composition, keeping at not more than 0.05 per cent by weight the content of the low molecular-weight organopolysiloxanes having a degree of polymerization of 10 or less in the packing.

As is described above, the present invention consists in the improvement obtained by using a silicone rubber packing containing little low molecular-weight organopolysiloxanes as a packing in the assemblage of a cover-packing built-up body for a hard-disc unit. Such a packing can be obtained by curing a curable liquid silicone rubber composition, which is then cured in situ into a silicone rubber, containing little low molecular-weight organopolysiloxanes Alternatively, the content of the low molecular-weight organopolysiloxanes in the silicone rubber packing formed by the in situ curing of a curable liquid silicone rubber composition can be decreased as desired by subjecting the silicone rubber packing as cured to a bake-out treatment by heating.

The advantages obtained by the use of the above-specified very specific curable liquid silicone rubber composition are not limited to the advantages obtainable by the use of a silicone rubber composition in general, which is inherently free from the problems of bleeding and blocming due to plasticizers or other ingredients cotained therein, and the advantages obtained by the use of an in situ cured rubber packing free from the use of an adhesive liable to cause contamination, but include the advantage obtained by the absence of any condensate of a volatile material emitted from conventional silicone rubber compositions containing a substantial amount of volatile organopolysiloxane materials of low molecular weights.

Various types of curable liquid silicone rubber compositions are known depending on the mechanism of the crosslinking reaction by 'which the liquid composition is cured into a silicone rubber, including those curable by using an organic peroxide as a curing agent, condensation reaction-curable ones and addition reaction-curable ones. Although any of these compositions can be used in the invention, the addition reaction-curable silicone rubber compositions are preferred among them in view of the good shape-retention when the composition is put on the covering member, as well as the relatively high curing velocity.It is also desirable that the silicone rubber composition has self-adhesiveness, which means that firm adhesive bonding can be obtained between the cured silicone rubber and the substrate surface on which the silicone rubber composition has been cured in contact therewith, even without a pretreatment of the substrate surface with a primer composition for improving the adhesion.

The above-mentioned addition reaction-curable organopolysiloxane composition comprises, as the essential ingredients, (a) a diorganopolysilcaane of a linear molecular structure having in the molecule at least two aliphatically unsaturated groups or alkenyl groups such as vinyl and allyl groups, (b) an organohydrogenpolysiloxane having in the molecule at least two, or preferably at least three, hydrogen atoms directly bonded to the silicon atoms, and (c) a small amount of a platinum cccpnd which serves as a catalyst for promoting the addition reaction or so-called hydrosilation reaction between the alkenyl groups and the silicon-bonded hydrogen atoms. The composition may optionally contain an adhesion improves and a reinforcing filler such as a fumed or precipitated silica filler.

Besides the aboveimentioned alkenyl groups, the diorganopolysiloxane contains silicon-bonded monovalent hydrocarbon groups of 1 to 10 carbon atoms including alkyl groups such as methyl, ethyl, propyl and butyl groups and aryl groups such as phenyl and tolyl groups, as well as those substituted hydrocarbon groups obtained by replacing a part or all of the hydrogen atoms in the above-named hydrocarbon groups with halogen atoms and the like such as chlorsmethyl and 3,3,3trifluoropropyl groups. It is preferable that all or most of the organic groups other than the alkenyl groups are methyl groups.

The organohydrogenpolysiloxane, which serves as a crosslinking agent for the above-described diorganopolysiloxane, may have a molecular structure which can be linear, cyclic or branched. An organohydrogenpolysiloxane having only two silicon-bonded hydrogen atoms in the molecule serves as a chain extender of the alkenyl-containing diorganopolysiloxane so as to contribute to the improvement of the cured silicone rubber by decreasing the hardness or increasing the tensile strength.Accordingly, it is a possible way to adequately control the mechanical properties of the cured silicone rubber, such as the hardness, elastic modulus and the like, by the combined use of organohydrogenpolysiloxanes having, on the one hand, only two silicon-bonded hydrogen atoms in the molecule and, on the other hand, three or more siliconbonded hydrogen atoms in the molecule. The amount of the organohydrogenpolysiloxane contained in the addition reactioncurable silicone rubber composition should be sufficient to provide from 0.5 to 4.0 moles or, preferably, fran 1.0 to 2.0 moles of the silicon-bonded hydrogen atoms per mole of the alkenyl groups in the diorganopolysiloxane.The organic groups in the organohydrogenpolysiloxane can be the same ones as in the diorganopolysiloxane apart from alkenyl groups.

The platinum compound used as a catalyst includes chloroplatinic acid, alcohol solutions of chloroplatinic acid, complexes of chloroplatinic acid with an olefin or vinyl siloxane and the like.

Copounds used as the above-mentioned adhesion improver in the composition include organosilicon compounds having functional groups to have an improving effect on the adhesiveness of the composition, such as epoxy groups, alkoxy groups and the like.

The addition reaction-curable silicone rubber composition used in the invention can be prepared by uniformly blending the ab#e-described essential and optional ingredients each in a specified amount. It is desirable that the thus-prepared composition has a viscosity in the range fran 1,000 to 100,000 poise at 250c as determined with a rotation viscosimeter to ensure workability and shape retention when the composition is applied to the covering member of the hard-disc unit.

It should be noted that diorganopolysiloxanes compounded in the composition usually contain about 2 to 7% by weight of low molecular-weight organopolysiloxanes having a degree of polymerization of 3 to 10 as by-products. Organohydrogenpolysiloxanes also contain a substantial amount of such low molecular weight constituents. These low molecular-weight organopolysiloxanes having volatility are vaporized when the composition is applied to the covering member of the hard-disc unit and eventually condense on various parts of the surface and cause various troubles. Therefore these volatile materials must be removed as completely as possible prior to the assembly of the hard-disc unit or preparation of the composition by compounding the respective ingredients. A preferred way for the treatment of the diorganopolysiloxane in this regard is to subject the diorganopolysiloxane to stripping by heating under reduced pressure while it is in the form of a thin film. The organohydrogenpolysiloaane should also be treated in a similar way although not essentially because the organohydrogenpolysiloxane, which is used in a relatively small amount, is little responsible for the increase in the content of volatile matter in the composition even when it is used without the stripping treatment.

Decrease in the content of the low molecular-weight organopolysiloxanes in the composition has an additional advantage that the curing velocity of the composition can be increased so much that complete curing of the composition can be achieved by heating, for example, for 30 to 60 minutes at 1500C; so that the adverse influences caused by heating of the covering nxmiber to be assembled after coating with the composition can be greatly mitigated and the range of material selection can be more broad, including certain plastics which otherwise are not suitable for the purpose.

The low molecular-weight organopolysiloxanes removed by the above-mentioned stripping treatment from the diorganopolysiloxane or, in particular, dimethylpolys iloxane include those having linear and cyclic molecular structures, although most of than are cyclic. Namely, they are cyclic oliganers of dimethylsiloxane units having 3 to 25 silicon atoms in the molecule such as hexamethyl cyclotrisiloxane, octamethyl cyclotetrasiloxane, decamethyl cyclopentasiloxane and so on, of which, however, those having a degree of polymerization greater than 10 have an extremely low vapor pressure and rarely cause troubles even when a composition containing an amount of than is used in assembling the harddisc unit according to the invention.In other words, it is essential in the invention that the low molecular-weight organopoly silcxanes having a degree of polymerization of 10 or less must be removed as completely as possible or at least to such an extent as not to exceed 0.05% by weight in the composition.

The in situ formation of a packing by using the liquid silicone rubber composition in the invention can be performed according to a known procedure in which the liquid composition in a reservoir vessel is transferred to a coating nozzle by pumping and a covering member of the hard-disc unit is coated with the liquid composition extruded out of the nozzle on a specified area, followed by curing of the composition, if necessary under heating.The coating work can be performed accurately and efficiently by using a robotized coating machine operated by an input memory for the pattern of the areas to be coated with the silicone rubber composition. Flhen a relatively narrow line area on the substrate surface is coated with the liquid composition, the coating layer on the line usually has a cross-sectional profile of a semicircle, although the head line of the coating layer of a sgnicircular cross-section can be flattened by controlling various factors.

The coating layer of the liquid composition can be readily cured into a silicone rubber packing by heating at a temperature of 50 to 2200C on the covering member. The curing can be performed also by irradiation with ultraviolet light or electron beams, depending on the type of the curable silicone rubber composition. At any rate, it is desirable that the silicone rubber packing formed in situ by curing the liquid composition has a hardness of 5 to 60 in the JIS A scale.

The covering mnber which is integral with the silicone rubber packing formed in situ from the liquid silicone rubber composition is then fastened to a casing of the hardvdisc unit, for example by screwing, to give a cover-packing built-up body for the hard-disc unit. When the thus in situ formed silicone rubber packing has a hardness in the above-mentioned range, it serves to effectively reduce the stress load on the covering. The cover-packing built up body for a hard-disc unit is then subjected according to need to the cleaning treatments of washing with water, washing with an air stream, drying and the like prior to fastening to the casing body with screws to ensure a tightly sealed condition.By virtue of the great decrease in the content of volatile organopolysiloxanes of low molecular weight in the composition, the thus-assembled built-up body can be. used as a hard-disc unit without any troubles due to dust. When the silicone rubber packing is freed from the volatile constituents only incompletely, the covering member integrally bearing the in situ formed silicone rubber packing may be subjected to a bake-out treatment by heating, for example at 2000C for 4 hours, under normal pressure or reduced pressure or to a leaching-out treatment by dipping in an organic solvent, until the content of the volatile materials therein is decreased not to exceed the above-mentioned upper limit prior to the assembly work.

The invention is illustrated, by way of example only, in the acccmpanying drawings, wherein: Fig. 1 is a perspective view of one preferred embodiment of a cover-packing built-up body according to the present invention, for a harddisc unit; and Fig. 2 is a cross-sectional view of a part of the body of Fig. 1, in the direction of the arrows A, A.

The illustrated body comprises a top cover 1 and a packing 2 applied to form a continuous linear layer along a peripheral flange 3 of the cover 1. The packing 2 is formed of an in situ curable or cured silicone rubber composition, for example as described in the following examples.

The e invention is hereinafter further described and illustrated in more detail by way of examples and comparative examples.

Example A number of top cover members of aluminium, after coating by electrodeposition with an epoxy-based coating composition, were each coated on a specified area by using a robotized coating machine following the pattern of the inputted memory, either with a first addition reaction-curable silicone rubber composition having self-adhesiveness (X-65-273A, a product by Shin-Etsu Chemical Co., referred to as the composition A hereinbelow), of which the content of low molecular-weight organopolysiloxanes having a degree of polymerization of 10 or less was 0.04% by weight and the viscosity was 14,000 poise at 25 & or with a second addition reaction-curable silicone rubber composition having self-adhesiveness (X-65-273B, a product by Shin-Etsu Chemical Co., referred to as the composition B hereinbelow), of which the content of low molecular-weight organopolysiloxanes having a degree of polymerization of 10 or less was 0.45% by weight and the viscosity was 12,000 poise 0 at 25 C. The abovegiven contents of the volatile organo- polysiloxane constituents were determined by gas chromatographic analysis for the species having 10 or fewer silicon atoms in the molecule. The operating conditions of the robotized coating machine included: inner diameter of the 2 coating nozzle 1.4 mm; pumping pressure 2.0 kgf/cm ; line- wise coating speed 4 metershminute; and nozzle height above the surface of the covering 2.1 mm.

The covering members, after coating in the above-described manner, were heated for 60 minutes in a hot-air circulation oven at 1500C to cure the composition A or B into a silicone rubber forming a packing on the covering member. The thus-formed linear silicone rubber packing on the covering member had a semicircular arching cross-section having a height of 1.5 mm and a base width of 2.2 mn. Adhesion of the silicone rubber packing layers to the substrate surface was very firm. The cured silicone rubbers obtained fran the compositions A and B had the mechanical properties shown in the table below, determined using test specimens prepared by curing under the same curing conditions as above. The content of the low molecular-weight organopolysiloxanes in the cured silicone rubbers are also shown in the table.

Table Composition A B Hardness, JIS A 28 26 Ultimate elongation, % 580 650 Tensile strength, kgf/cm2 33 34 Permanent compression set, %, 15 12 after 20 hours at 120 C Content of lowrnolecular organo- 0.03 0.32 polysiloxanes, % by weight The cover-packing built-up bodies prepared in the above described manner were rinsed with pure water having an electroconductivity of 10 6 mho or less after filtration through a membrane filter of 0.3 F pore diameter, dried in a clean roan of cleanness 100 (0.3 F) and clean-packed.These clean-packed built-up bodies were brought into an assembly line of hard-disc units where they were built into the respective hard-disc units, all of which could pass the tests for leakage of current and cleanness. In the running test, the hard-disc units assembled with the in situ cured silicone rubber packing from the composition A caused no troubles over a long period of the running time, completely without troubles such as head crashes and damages in the magnetic recording layer, while the harddisc units assembled with the in situ cured silicone rubber packing from the composition B caused troubles even before the long-running test.These troubles were found by inspection to be due to the deposition of sane condensation product of the volatile organopolysiloxanes of low molecular weights in the gap space between the surface of the magnetic recording disc and the magnetic head.

Separately, the cover-packing built-up bodies having the integral silicone rubber packing formed by the in situ curing of the composition B were subjected to a bake-out treatment by heating for 4 hours at 2000C prior to assembly into hard-disc units. The hardness of the silicone rubber packing was increased to 30 in the JIS A scale and the content of the volatile organopolysiloxanes of low molecular weights had dropped to 0.02% by weight. The hard-disc units assembled with these cover-packing built-up bodies after the bake-out treatment were found to work as satisfactorily as those prepared by using the composition A.

Claims

CLAMS

1. A cover-packing built-up body for a hardsdisc unit in which a packing for sealing the unit has been formed by curing in situ a curable liquid silicone rubber composition, wherein the packing contains not more than 0.05 per cent by weight of low molecular-weight organopolysiloxanes having a degree of polymerization of 10 or less.

2. A cover-packing built-up body as claimed in claim 1, wherein the packing has been formed by curing a curable liquid silicone rubber composition which contains not more than 0.05 per cent by weight of said low molecular-weight organopolysiloxanes.

3. A cover-packing built-up body as claimed in either of claims 1 and 2, wherein the curable liquid silicone rubber composition is an addition reaction-curable composition comprising (a) a diorganopolysiloxane of a linear molecular structure having in the molecule at least two alkenyl groups, (b) an organohydrogenpolysiloxane having in the molecule at least two hydrogen atoms directly bonded to the silicon atoms, and (c) a catalytic amount of a platinum cound which serves as a catalyst for promoting the addition reaction between the alkenyl groups in the component (a) and the silicon-bonded hydrogen atoms in the component (b).

4. A cover-packing built-up body as claimed in any of the preceding claims, wherein the curable liquid silicone rubber composition has a viscosity in the range from 1,000 to 100,000 0 poise at 25 C.

5. A cover-packing built-up body as claimed in claim 1, wherein the packing for sealing formed by curing in situ a curable liquid silicone rubber composition is subjected to a bake-out treatment by heating in order to reduce to not more than 0.05 per cent by weight overall the content of said low molecular-weight organopolysiloxanes.

6. A cover-packing built-up body for a hard-disc unit, as claimed in any of the preceding claims and substantially as hereinbefore described in the Example.

7. A process for the production of a cover-packing built-up body for a hard-disc unit, as claimed in any of the preceding claims, said process being substantially as hereinbefore described.