US3801535A - Damping means - Google Patents

Abstract

SOLID SLICONES HAVING A SHORE A HARDNESS OF MUCH LESS THAN 15 AT A TEMPERATURE BELOW 50*C. DAMP MECHANICAL VIBRATIONS AND ARE PARTICULARLY USEFUL IN THIS REGARD FOR SOUND PICKUPS.

Description

US. Cl. 26029.1 SB 25 Claims ABSTRACT OF THE DISCLOSURE Solid silicones having a Shore A hardness of much less than 15 at a temperature below 50 C. damp mechanical vibrations and are particularly useful in this regard for sound pickups.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of copending application Ser. 881,617 filed Dec. 2, 1968, and now abandoned.

BACKGROUND OF THE INVENTION (1) Field of the invention Damping resonances of mechanically vibrating arrangements requires reaction-free media, i.e., media which do not influence frequency behavior or sensitivity. A sound pickup for mechanical scanning of data which are fixed in grooves will serve to illustrate the circumstances involved:

The frequency dependence of the scanning behavior is characterized by a plurality of resonances (transducer resonance, stylus resonance and others) which must be damped to produce a linear frequency characteristic. Likewise, the crosstalk behavior must be improved in twochannel systems by damping. This damping, however, must not reduce system sensitivity; the pickup must operate (in order to protect phonograph records) with a specifically light contact weight and without distortion.

In addition to this damping effect the media should also provide protection against climatic influences, such as humidity. These characteristics must remain constant with time.

(2) Description of the prior art Silicone pastes consisting of highwiscosity silicone oil and filler [for example the types used in sound pickups of the firm Telefunken (type T 22), Elac and Sonotone] and rubber-elastic shaped components (also used in pickups of said firm) of higher hardness (a Shore A hardness in excess of 20) have been employed to effect damping. There is a distinct detriment in the use of damping pastes since they increase by 60 to 70 percent [in conventional systems from 3 to 4 grams (force) (p) to 5 to 6 p] the system tracking force required for distortion-free scanning. Moreover, the time constancy of the paste properties is conditional and problematical; the pastes'are mechanically mixed and they tend to separate. Their thixotropic and poor flow properties render their compounding extremely difiicult. Furthermore it is rather diflicult to get them in a cavity surrounding the stylus arm of the pickup.

Rubber-elastic shaped components lead to an even greater increase in required tracking forces. It is also difficult to place them into such cavity of the pickup.

Use of silicone oil as softener for silicone gum was limited to a maximum of percent, based on the weight of the gum, since it was believed that a solid product United States Patent O Patented Apr. 2, 1974 would not be formed if the relative proportion of oil were higher and that silicone oil would run out of any resulting (polymerized) product in time if larger amounts of oil were employed.

SUMMARY OF THE INVENTION The application relates to damping means for damping mechanical vibrations, particularly for sound pickups. These damping means consist of one or a plurality of silicone gums (polymers) having a viscosity of less than 80,000 centistokes, to which a hardener (curing or crosslinking agent) as well as low-viscosity silicone oils and fillers are added in such amounts that the mixture polymerizes in cold state (below 50 C.) into a solid having a Shore A hardness of less than preferably significantly less than fifteen.

The mixture comprises an amount of the component of low-viscosity silicone oil which is more than 100 percent by weight of the liquid polymer (gum) portion, generally from 300 to 400 precent by weight. In order to mix such amounts of silicone oil into the liquid (uncured) polymer, fillers are required; the quantity of filler is more than percent by Weight of the liquid polymer content. In order to realize a sufiicient curing of the (hitherto uncured) polymer, the amount of hardner is at least (preferably approximately twice) that necessary for normal polymer hardening.

The reactive mixture is liquid at first. It can thus be easily measured out and filled. Hardening (curing) is effected at a slightly elevated temperature (30 to 35 C.). A solid body results. The solid body is extraordinarily soft, molds easily around a component to be damped and has good damping properties. The increase (only about 15 to 20 percent) in the required tracking force for sound pickups is substantially less than with known means. For conventional constructions this represents an increase from 3 to 4 p to 3.5 to 4.5 p. The time stability is better than with the paste-type masses since a chemical polymerization (cross-linking, curing and/or hardening) has occurred. The resulting cured or hardened polymer provides better protection of transducer systems against humidity and temperature changes since it reduces the speed for water vapor diffusion.

The fiuid reaction mixture is filled into its final location of application Where curing occurs. For the rest the application of the cured damping medium of the present invention occurs in the same manner as prior silicone pastes and rubber-elastic shaped components to effect damping, but the above-noted deficiencies are eliminated with its use.

The weight proportion of silicone oil is more than percent and, ordinarily, up to 500 percent that of employed silicone gum. Yet, a stable solid (having unchanging properties over long periods of time) is produced. This solid is an extremely soft damping mass or medium, but still has the characteristics, e.g., elasticity, of a solid body.

DETAILS All liquid polysiloxanes, especially diorgano-polysiloxanes having viscosities which are not too high, i.e., less than 80,000 centistokes (See W. Noll: Chemie und Technologie der Silicone; 2. Auflage 1968; Verlag Chemie GmbH, Weinheim/Bergstrasse), which are polymerizable in cold state, i.e., below a temperature of 50 C., are suitable silicone polymers. The preparation of suitable liquid silicone polymers is known (Chemie und Technologie der Silicone, supra). Moreover, such materials are usual in the trade (suppliers are: Dow Corning, Wacker- Chemie) They need no special treatment.

The silicone polymer or silicone gum is not limited to that with a viscosity of 20,000 centistokes or more. There is not known critical lower viscosity limit, and commercially-used silicone gums of molecular weights as low as 10,000 (corresponding to viscosities of from about 200 to about 400 centistokes) are clearly suitable for this invention. The only prerequisite for the silicone gum, other than the noted upper viscosity limit, is the ability to polymerize in cold state. To satisfy this latter requirement, the molecules of the silicone gum must be difunctional, eg by having at least 2 HO-groups, within the noted temperature range. (Monofunctional molecules are not suited for polymerization.)

All silicone oils are likewise useful; the essentiality is their viscosity, which must be less than that of the liquid (uncured) silicone polymer.

Suitable low-viscosity silicone oils are also conventional in the trade. An example is dimethyl-polysiloxane. The viscosity should be less than 500 centistokes, especially 35 centistokes. For starting materials and preparation of suitable low-viscosity silicone oils, see said book, Chemie und Technologie der Silicone.

Throughout the text reference is made to silicone polymer or gum and also to silicone oil. All references to silicone polymer mean silicone gum when referring to the uncured or liquid material. This consists of liquid high molecular, generally linear (chain), polydiorganosiloxanes which are convertible to silicone rubber by polymerization at a temperature below 50 C., i.e., room temperature or a slightly elevated temperature. Such cold vulcanizable silicone gum types constitute an art-recognized and established class; they have reactive, usually OH, end groups. The particular nature of the end group is insignificant as long as the silicone gum is capable of cold vulcanization (reaction at a temperature below 50 C.).

Amplification of the foregoing and how cold vulcaniza tion (cold hardening) is effected is provided by Chemie und Technologie der Silicone, supra, pp. 332, 335 and 339. Illustrative starting products (silicone gums) are a,w-fUIlCtiOI1al polydiorganosiloxanes, e.g., of the following structure:

which consist of a plurality of siloxane (SiO) groups, each having two (di) organic radicals (R) and which are reactionable (functional) at the beginning (a) and at the end (to) of each chain. Cold polymerizable dimethylor methyl-phenyl-polysiloxanes are preferred.

An otho or polysilicic acid ester or alkyl trialkoxysilane molecule exemplify hardeners which react with a plurality of such c m-functional polydiorgano siloxanes, as illustrated in the following examples (a) through (c):

min, 60ft... \l\ ,/l/

Each organic group, R, can be the same or different; its constitution is not critical. It can be aromatic, such as phenyl, tolyl, xylyl, biphenyl, phenoxyphenyl and halophenyl, or aliphatic, such as lower alkyl, e.g., methyl and ethyl. The sole limitations of the silicone gum are:

(a) it is a liquid polysiloxane at room temperature;

(b) it has reactive end groups; and

(c) it is hardenable to a solid polysiloxane (rubber) at a temperature below 50 C.

Polymerization (cold hardening) of silicone gum is effected through Si-O-Si bridges. Such silicone gums are known and, per se, do not constitute the subject invention; they are conventional cold-hardening silicone gums, e.g., a polysiloxanediol with OH end groups. Hardeners for liquid silicone gums are, likewise, known. The sole real limitation of the hardener is that it must be a suitable hardener for the employed silicone gum.

The viscosity of the employed silicone polymer is less than 80,000 centistokes (cs.). Although there is no lower viscosity limit, the viscosity is ordinarily at least 300, and preferably more than 500, cs., but such is in no way critical.

Silicone oils are also linear polysiloxanes, i.e., SiO chains with radicals, but they are not functional; they do not have end groups which are reactive at temperatures below 50 C. The radicals, e.g., methyl, trifiuoromethyl and phenyl, are those which are very stable in the cold state, i.e., below 50 C. Walter Noll, Chemie und Technologie der Silicone, supra, pages 3 and 393 to 395, confirms the nature of silicone oils and that they are also known. Table 77 on pages 394 and 395 provides illustrative examples, and Table 1 on page 3 defines the employed symbols.

The term silicone oil is not a chemical definition, but it is definitive of an art-recognized class. For a silicone oil to remain an oil, it must not polymerize or turn into a. resin; it must be chemically inert, i.e., it must not have any end groups which are cold reactionable. Heat resistant silicone oils must also be free from heat reactionable radicals, i.e., radicals which are reactive at temperatures of 50 C. and above. Silicone oils are thus liquid, nonfunctional (non-reactive) polydiorganosiloxanes. As in the case of the silicone gums, each organo group can be the same or different; its constitution is not critical. The sole limitations of the silicone oil are:

(a) it is a liquid at room temperature;

(b) it is either a monoor polyorganosiloxanes;

'(c) it lacks end groups which are reactive with silicone gum or hardener at temperatures below 50 C. and, preferably, at room temperature; and

((1) its viscosity is lower than that of employed silicone gum.

A brochure (Siliconiile AK, Nr. 2559. 697) on one series (structure set forth on page 3) of known silicone oils was published by Wacker-Chemie GmbH in 1968.

Although polymeric silicone oils are preferred, monomeric silicone oil is also useful as softener for the subject invention. The higher viscosity of polymeric silicone oils tends to prevent their separation from the silicone gum in silicone oil/silicone gum mixtures during hardening. (The heavy silicone gum precipitates.)

The viscosity of the employed silicone oil is not critical, but a low viscosity, i.e., less than 500 cs., silicone oil is generally used. There is no lower limit for the viscosity of suitable silicone oil.

The viscosity of employed liquid silicone (uncured) polymer may be, e.g., 20,000 centistokes; the viscosity of silicone oil, e.g., 35 centistokes. The viscosity may be controlled with a viscosity measuring instrument (e.g., Rotobisko of the firm Haake, Berlin, Germany).

Different materials, e.g., powdered quartz, chalk, kieselguhr, graphite, aluminum oxide and kaolin may be used, as fillers. Of real importance is the average grain size of the filler. This grain size should be fine (small) enough to avoid any substantial sinking of filler grains during hardening (standard procedure in synthetic polymer art), but must not be so small that insuflicient interlacing occurs. In the hereafter described mixtures the average grain size is 4 or 5 microns (,u.). Since contemplated gum/ oil mixtures have very low viscosities prior to hardening (polymerization), the upper limit of average grain diameter is ordinarily approximately 20;. The filler has essentially the following effects:

(a) It influences the viscosity of the mixture in its fluid state. This viscosity is important when filling out the mixture in a predetermined cavity of a pickup for instance.

(b) It influences the interlacing in reducing the degree of interlacing which is responsible for the resulting hardness of the mixture in its solid state. That is the reason for the fact that certain grain sizes are advantageous. A suitable grain size is found by a few simple experiments.

There is no chemical influence of the filler, which is essentially inert with respect to the silicone gum, the silicone oil and the hardener at temperatures below 50 C.

The quantity of filler is more than 80 (especially 150 and even up to about 200) percent by Weight of the polymer content.

Suitable hardners or hardening systems (polymer and catalyst) are known and are described, e.g., in said book Chemie und Technologie der Silicone (page 340). For instance the combination of hardeners named Harter T of the firm Wacker-Chemie is suitable. The quantity of hardener is more than 5, especially equal to 11, percent by weight of the polymer content.

The mixture of liquid silicone (uncured) polymer and silicone oil should have a viscosity of less than 500 centistokes, which can be measured by an instrument, such as that mentioned heretofore. The ratio of gum/hardener/ oil/filler is such that the mixture of these four ingredients is liquid prior to the onset of and solid (having a Shore A hardness less than after hardening. Absolute ranges of proportions for each constituent are not fixed as they are dependent on the respective viscosities of the components. For any given system, however, suitable proportions are readily determined. The solid product has a Shore A hardness which is preferably materially (significantly) less than 15 and ordinarily much less than 15 15 The weight proportion of oil in the prehardened composition is in excess (preferably from more than 100 to about 500 percent) of that of gum; the weight proportion of filler is in excess of 80 percent (preferably from more than 80 to about 200 percent) of that of gum; and the amount of hardener for the gum is at least equal to or more than (preferably 200 percent by weight of) that normally required to cure the gum.

The measurement of the hardness of the damping mass is possible, for instance, by way of an instrument described in German patent application P 19 04 520.6 of Giinter loschko.

The employed liquid (uncured) polymers are preferably diorgano polysiloxanes, e.g., those of the formula (R SiO) wherein each R is, independently, a lower alkyl, such as methyl and ethyl, or aryl, such as phenyl; and n is a positive whole number, whose value is not of interest because the viscosity of the (uncured) polymers is the deciding factor. The absolute value of n is not critical and varies considerably with degree of desired crosslinking. The criterion is not the value of 11, but the viscosity of the liquid polymer.

The oil is exemplified by dimethyl polysiloxane having a viscosity of 35 centistokes. Suitable hardeners are mixtures of silicic acid esters with an organic tin compound, e.g. dibutyl tin dilaurate (see Walter Noll: Chemie and Technologie der Silicone, page 340 and the following). Illustrative fillers are chalk, powdered quartz, diatomaceous earth (kieselguhr) and graphite, each having an average particle diameter from 5 to 50 microns. The following (in parts by weight) are particularly suitable reaction mixtures:

1 The viscosity is defined in centistokes. i The hardener is Harter T, a product of Wacker-Chemie, Munich, Germany.

The hardness of the polymerized (cured) mixture is so low that conventional instruments (with which hardness is measured in Shore units) fail. The minimum hardness measurable with such measuring instruments is approximately 15 Shore A units. To measure the hardness of the damping means according to the present invention, it was necessary to develop a special measuring instrument while maintaining the ball pressure process employed with the Shore measuring instrument. The following numerical example serves as a reference point for the softness of the material according to the present invention. In conventional Shore measuring instruments a ball having a diameter of 2.5 mm. and charged with a weight of 580 grams (g.) causes an impression of 2 millimeters (mm.) in material which corresponds to 25 Shore units.

With the damping material according to the instant invention an impression of 2 mm. depth is produced by a ball of 5 mm. diameter loaded with only 3 g.

Throughout the disclosure all references to polymer, particularly liquid and/or uncured polymer, or gum are to silicone gum and all references to oil are to silicone oil unless otherwise designated.

Without further elaboration, one skilled in the art can, using the preceding description, practice the present invention. The following preferred specific embodiment is merely illustrative and not limitative of the remaining disclosure in any way whatsoever.

EXAMPLE The following ingredients are used (in parts by weight):

1.-- 60 liquid silicone polymer (20,000

centistokes).

2. 300 silicone oil (35 eentistokes) 3...-. 70 Kaolin o1 4-.- 20 Vedarkreid 5- BHarterT Supplier: WackenChemie",

Munich.

Supplier: 0mya", Cologne. Supplier: Waeker-Chemie".

is extremely soft and clamps mechanical oscillations when in contact with a part to be damped. The damping medium physically consists of a lattice-work of interlaced silicone rubber, in which particles of the filler and the silicone oil are enclosed. It is supposed that the oil encloses the filler particles like a skin.

The application of the damping medium corresponding to the invention occurs as follows:

After being mixed, the liquid reaction mixture is filled into the location of its final application. This final destination, for example, may be in a cavity of a sound (or other, e.g. picture, signal) pickup, which cavity surrounds a part of the stylus arm of the pickup. Into this cavity there may be filled such a quantity of the mixture that at least a part of the stylus arm is embedded in the mixture. After this, curing of the mixture is awaited. As a result of curing the fluid mixture is a solid damping medium in contact with the stylus arm or any other arrangement to be damped. The solid damping medium is now destined to rest in said cavity or any other location of application for purposes of damping mechanical vibrations.

Those skilled in the art know (Chemie und Technologie der Silicone, page 340) that the reaction velocity of the mixture curing in cold state is influenced by the water content of the mixture. If fillers which tend to absorb water are used, the reaction velocity can be changed essentially by adding a definite quantity of water. In the case of the example mentioned herebefore, an added quantity of 2.5 parts by weight (0.6 percent) of the total quantity of the reaction mixture) results in a stabilizing effect respective to the reaction time of the mixture. This is caused by the eifect that the different water absorption by different fillers or the different water contents of different components of the mixture is fully equalized in its effect by the added quantity of water. After having added said quantity of water, a change of reaction time no longer occurs, even if the mixture (without curing agent) is stored for some days. The quantity of water to be added is dependent on the quantity of filler and its average grain size. The mentioned quantity of water is suitable for the given example in which kaolin with an average grain size of microns is used.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

What is claimed is:

1. A liquid reaction mixture consisting essentially of:

(a) liquid silicone polymer which has a viscosity of less than 80,000 centistokes,

(b) hardener for (a),

(c) silicone oil having a viscosity less than that of the silicone polymer, and

(d) filler which is essentially inert at temperatures below 50 C. to each of (a), (b) and (c); the silicone polymer being silicone gum; and the ratio of gum/hardener/oil/filler being such that the mixture, prior to the onset of hardening, is a liquid which hardens, at a temperature below 50 C., to a solid having a Shore A hardness significantly less than 15.

2. A liquid reaction mixture according to claim 1 consisting essentially of:

(a) liquid silicone gum which has a viscosity of less than 80,000 centistokes,

(b) hardener for (a),

(c) silicone oil having a viscosity of less than 500 centistokes, and

(d) filler which is essentially inert at temperatures below 50 C. to each of (a), (b) and (c); the amount of hardener being suflicient to transform the mixture, at a temperature below 50 C., to a solid having a Shore A hardness of materially less than 15; the weight proportion of (0) being in excess of that of (a); and the 8 weight proportion of (d) being in excess of 80- percent that of (a).

3. Solid damping medium which consists essentially of polymerizate of a reaction mixture according to claim 1 and which has a Shore A hardness of much less than 15 at a temperature below C.

4. Solid damping medium according to claim 3 wherein the filler ('d) is chalk (CaCO 5. Solid damping medium according to claim 3 wherein the filler (d) is finely divided silicon dioxide (SiO 6. Solid damping medium according to claim 3 Wherein the filler (d) is graphite.

7. A reaction mixture according to claim 1 wherein the amount of hardener (b) is more than that normally required to cure silicone polymer (a).

8. A composition according to claim 2 wherein the weight of silicone oil (c) is from 3 to 5 times that of liquid silicone polymer (a).

9. A composition according to claim 2 wherein the amount of hardener (b) is from 5 to 11 percent the weight of liquid silicone polymer (a).

10. A composition according to claim 2 wherein the amount of filler is in excess of percent and up to 200 percent the weight of liquid silicone polymer (a).

11. A composition according to claim 2 wherein silicone polymer (a) and silicone oil (c), in admixture, have a viscosity of less than 500 centistokes.

12. A liquid reaction mixture according to claim 1 containing a weight proportion of Water which is about 0.6 percent that of the liquid silicone gum.

13. Solid damping medium which consists essentially of polymerizate of a reaction mixture according to claim 12 and which has a Shore A hardness of much less than 15 at a temperature below 50 C.

14. A reaction mixture according to claim 1 wherein the silicone oil is a dimethyl polysiloxane.

15. A reaction mixture according to claim 1 wherein the silicone oil is a liquid, nonfunctional polydiorganosiloxane.

16. Solid damping medium which consists essentially of polymerizate of a reaction mixture according to claim 14 and which has a Shore A hardness of much less than 15 at a temperature below 50 C.

17. Solid damping medium which consists essentially of polymerizate of a reaction mixture according to claim 8 and which has a Shore A hardness of much less than 15 at a temperature below 50 C.

18. A liquid reaction mixture according to claim 1 which comprises an amount of (b) which is suflicient to harden the mixture at a temperature below 50 C., an amount of (c) which is in excess of and up to five times that of (a), and an amount of (d) which is in excess of 80 percent and up to about 200 percent that of (c).

19. A liquid reaction mixture according to claim 18 wherein the amount of (b) is from 5 to 11 percent by weight that of (a).

20. In damping resonance of a mechanically vibrating arrangement with a damping medium, the improvement wherein the damping medium is solid damping medium according to claim 3.

21. A process for minimizing increased tracking force in a damped pickup which comprises damping the pickup with solid damping medium according to claim 3.

22. A process according to claim 21 wherein the pickup is a sound pickup.

23. A process for preparing a solid damping medium having a Shore A hardness of less than 15 at a temperature below 50 C. which comprises maintaining a reaction mixture according to claim 1 at a temperature of from about 30 to about 35 C. until the solid damping medium is produced.

24. A process according to claim 23 which comprises filling the reaction mixture into a location of its final destination or application and maintaining it there until and References Cited after curing- UNITED STATES PATENTS 25. In damping vibrations of at least one movable part 3,077,521 2/1963 Ahrens et a1 179 100'41 in a pickup by afiixing a composition at a location suitable 3,184,555 5/ 1965 Marshall 179 100.41 to effect such damping, the improvement which comprises 5 3,19 ,175 6/1965 Russell 260-29.1 SB applying to the location a reaction mixture according to 3234175 2/1966 Pike SB claim 1 and maintaining the reaction mixture at a tem- LEWIS JACOBS, Primary Examiner perature of from about 30 to about 35 C. until said reaction mixture solidifies at said location into a solid damp- 10 ing substance. 179--100.41; 260-37 SB