DE1244401B - Process for the production of tough organopolysiloxane elastomers by the pouring or coating process - Google Patents

Description

C 09 D -

FEDERAL REPUBLIC OF GERMANY Int. Cl .:

GERMAN

PATENT OFFICE

EDITORIAL

C08f

090 ι 8 3 /

German class: 39 b -22/10

Number: 1244 401

File number: G 33918 IV c / 39 b

Filing date: December 28, 1961

Opened on: July 13, 1967

Solvent-containing mixtures of a mixed _{polymer of R 3} SiO 0jB units (R = monovalent hydrocarbon _{radical) and SiO 2} units and of a liquid diorganopolysiloxane with a viscosity of over 200,000 cP are known, which are used as adhesives for coating adhesive tapes .

Furthermore, a method for impregnating paper is already known in which solvent-based Mixtures of resinous and rubbery organopolysiloxanes and catalysts are used will. Paper treated with such a mixture shows that the solvent has been driven off decreased adhesiveness.

Using a mixed polymer of R ₃ SiO _0, 5 units (R = methyl) and SiO ₂ units and of diorganopolysiloxanes having a viscosity of 4000, 100000 and 1000000 cP mixtures were prepared. In the case of the copolymer used, an average of 0.6 trimethylsiloxane _{units met one SiO 2} unit. The copolymer was used in the form of a 60% solution in xylene. It was found that the mixtures containing diorganopolysiloxanes with a viscosity of 100,000 and 1,000,000 cP were very viscous after the solvent had been driven off, so that they could not be used for embedding objects. From the mixture containing the diorganopolysiloxane with a viscosity of 4000 cP, however, a clear, tough, transparent and elastic embedding compound could be produced.

It has now been found, unexpectedly, that with solvent-containing mixtures of a copolymer of R ₃ SiO _0f5 units (R. = monovalent hydrocarbon _{radical) and SiO 2} units, in which an average of 0.3 to 0.8 R ₃ SiO ₀₁₈ - Units meet an SiO ₂ unit, a liquid, linear diorganopolysiloxane with Si-bonded OH groups, catalysts and optionally fillers, elastic and clear, tough organopolysiloxane elastomers, e.g. B. as embedding compounds, if you use a liquid, linear diorganopolysiloxane with a viscosity of about 100 to 400 oCP at 25 ⁰ C in the mixture.

The invention therefore relates to a process for the production of tough organopolysiloxane elastomers by the casting or coating process from solvent-containing mixtures of a copolymer of R ₃ SiO _{0> 6} units (R = monovalent hydrocarbon radical) and SiO ₂ units, in which an average of 0, 3 to 0.8 R ₃ SiO _0f5 units meet an SiO ₂ unit, a liquid, linear process for the production of tough
Organopolysiloxane elastomers after the casting or coating process

Applicant:

General Electric Company,

Schenectady, N.Y. (V. St. A.)

Representative:

Dipl.-Ing. M. Licht and Dr. R. Schmidt,

Patent Attorneys, Munich 2, Theresienstr. 33

Named as inventor:

Frank Joseph Modic, Scotia, N.Y. (V. St. A.)

Claimed priority:

V. St. v. America December 30, 1960 (79555)

Diorganopolysiloxane with Si-bonded OH groups, catalysts, hardeners and optionally fillers with shaping by removing the solvent, which is characterized in that a liquid, linear diorganopolysiloxane with a viscosity of approximately 100 to 400 ocP at 25 ⁰ C is used in a mixture.

With the method according to the invention, components, in particular electrical components, can be embedded in organopolysiloxane compounds which cure elastically and crystal-clear and can also be ^{exposed to temperatures of up to 200 °} C. for a long time without any heat aging effects occurring.

In the method according to the invention, the solvent-containing mixture is first prepared. By Stripping off the solvent gives a liquid with a viscosity approximately the same as that used liquid, linear diorganopolysiloxane corresponding viscosity. This liquid will then mixed with a catalyst and can then either at room temperature or at elevated temperatures Temperature cured after being poured over an object, e.g. B. one to be covered Electrical part.

Resin-like copolymers consisting of R ₃ SiO _{0 # B} units and SiO ₂ units are known in the art and are described, for example, in US Patents 2,736,721, 2,814,601 and 2,857,356.

709 610/594

wrote. Such copolymers are in the known manner by "hydrolysis of a hydrolyzable Triorganosilane, e.g. B. trimethylchlorosilane, with an alkyl orthosilicate, e.g. B. ethyl orthosilicate, or the partial hydrolyzate of such an alkyl orthosilicate. Depending on the reaction conditions and the ratio of the reactants contain such resinous copolymers between about 0.1 to 5 percent by weight silicon bonded silanol or hydroxyl groups.

The ratio of triorganosilane to the alkyl orthosilicate in the preparation of the resinous copolymer as the ratio of R ₃ SiO ₀ .6 units and SiO ₂ units is desired in the final copolymer chosen. Usually, the resinous copolymer preferably contains between about 0.3 to 0.8 R ₃ SiO _{0 5} units per SiO ₂ unit, and consequently the same proportion of the units is present in the starting material.

The organic solvent used in the hydrolysis reaction to form the resinous copolymer to be used here can be selected from a large number of usable organic solvents. Usually the solvent is either an aliphatic or aromatic hydrocarbon or a halogenated hydrocarbon, since these solvents are the most readily available commercially. Particularly suitable are: benzene, toluene, xylene, trichlorethylene or both aliphatic and aromatic turpentine substitutes. If desired, the resinous copolymer of SiO ₂ units and R ₃ SiO ₀₁₆ units can be formed from a reaction mixture which contains an organic solvent from the preparation of the mixed polymer, and this solvent can be evaporated from the resinous copolymer, see above that a powdery, resinous copolymer is supplied. The powdery, resinous copolymer can then be redissolved in a second, different solvent, for which purpose one of the hydrocarbons known as solvents is used again. When carrying out the process according to the invention, the amount of the hydrocarbon solvent in which the resinous copolymer is dissolved can be varied within extremely wide limits. Satisfactory results are obtained with solutions of the resinous copolymer with a content of 10 to 90% of solid constituents. The solution of the resinous copolymer preferably has a solid component content of about 50 to 70%.

_{The R 3} SiO ₀ , ₆ units present in the resinous copolymer are usually those in which the R are methyl radicals. In the context of the invention, however, it is also possible to use those R ₃ SiO ", ₅ units in which the R are other monovalent hydrocarbon radicals: alkyl radicals, e.g. B. ethyl, propyl, butyl or octyl radicals; Aryl residues, e.g. B. phenyl, naphthyl, toluyl or xylyl radicals; Aralkyl residues, e.g. B. benzyl or phenylethyl radicals; Alkenyl radicals, e.g. B. vinyl or allyl radicals, or cyclic hydrocarbon radicals, e.g. B. cyclohexyl, cycloheptyl or cyclohexenyl radicals or various halogenated, monovalent hydrocarbon radicals including chloromethyl, chloropropyl or dibromophenyl radicals. Are in the R ₃ Sio ₀ containing ₅ units other radicals than methyl radicals, then these preferably contain at least 50% also Trime'thylsiloxaneinheiten.

The used for the method of the invention,

liquid linear diorganopolysiloxanes with a chain stopped by silanol are known to those skilled in the art

and can be represented by the following formula:

HO

R "

Si-O

R "

in which R "is a monovalent hydrocarbon radical and η has a value between about 100 and 700. These liquid diorganopolysiloxanes with a chain stopped by silanol have a viscosity of about 100 to 4000 cP at 25 ° C. Such diorgariopolysiloxanes" with a chain stopped by silanol are in U.S. Patent 2,843,555. The dioganopolysiloxane with a chain stopped by silanol which is preferred for carrying out the invention has a fine viscosity between about 300 and 3000 cP at 25 ° C. If the viscosity is more than 400 ocP, then the resulting liquid is too viscous to coat complicated parts and not suitable for the process according to the invention.

The solution of the resinous copolymer and the liquid linear diörganopolysiloxane with through Stop chain silanols are mixed together in any suitable manner. It has been found that the resin solution and the liquid with the chain stopped by silanol are completely miscible are, so that a uniform homogeneous solution of the resinous copolymer and the liquid with the chain stopped by silanol in the organic solvent of the resin solution after Mixing will get. However, the solubility of the two components is to some extent in Different depending on the content of solid components in the resin solution. In some cases several minutes are required to form a homogeneous solution, i.e. i.e., when a solution of the resinous Copolymer with a high content of solid components and a liquid with through Silanol stopped chain can be used. The ratio of resinous copolymer to the

So can liquids with a chain stopped by silanol can be varied within wide limits. However, 1 to 1 is preferred for best results 10 parts by weight of the liquid with the chain stopped by silanol per part of the resinous copolymer

SS sats (based on the solid matter content) in the mixture of the two ingredients.

After preparing the solution from the resinous copolymer and the liquid with the chain stopped by silanol in the organic solvent, the decisive step of the process according to the invention is carried out. This process step is the removal of the organic solvent from the solution.
Solvent removal is accomplished by heating the solution to an elevated temperature, e.g. B. causes a temperature between about 40 ⁰ C and the boiling point of the solvent, which is usually less than 150 ⁰ C. Preferably the solvent is

5 6

by heating the solution to a temperature far from a primary, secondary or tertiary amine and

below 100 ° C, preferably to a temperature of 40 of an organic epoxy-containing compound. That

up to 80 ° C, with the solution under reduced primary, secondary or tertiary organic amine

tem pressure ', e.g. a pressure of 1 to 50 mm ", e.g. 1,3-difriefhylbutylamine, dimethylsoya-

will hold. Riiäri holds the solution on this ratio- 5 amine, Dibutylbenzylam "in, Tetrriethylguanidin, N-Me-

moderately low temperature, the possibility becomes thylmorpholiri, triethaholftiin or dibutylamiri.

premature gelation of the solution largely The epoxy used in the Kätalysatorsystefri

switched off. compounds are, for example, the glycidyl ethers,

Since the solvent has an adverse effect on e.g. B. Phenylglycidylether, butylglycidyläthef, the Didie Properties of the solvent-free coating io glycidyl ether of bis-ljS-oxypropylteträme'thyldisilnach the "invention exercises, it is advisable to use the oxane, cyclohexerioxide or the resinous substances Solvent almost completely removed. Epoxy groups, such as the resinous reaction product This means that such an amount of the solution of epichlorohydrin and ρ, ρ'-dioxyphenyldimethyimittels is withdrawn that the resulting methane solution. If this hardening catalyst is made of a with sulfur-free liquid at most about 2% solution 15 primary, secondary or tertiary amine and one medium as the remainder, based on the weight of the solid epoxy compound, then both Components of the resinous mixed polymer and the amine as well as the 'epoxy' compound in one the liquid with the chain stopped by Silahol contains an amount of approximately 0.1 to 5 percent by weight, preferably less than 1%, usually based on the weight of the solvent-free, liquid the solvent is added within about 2 to 20 coating mass, the ratio of 24 hours, depending on the respective "frequency to the epoxy compound" must be sufficient to applied temperature and deprived of pressure. about "0.2 to 5 moles of amifi per mole of the epoxy compound

Define removal of the solvent under the conditions described.

Conditions provides a final, solvent-free The choice of the respective Kafäly-

Coating compound with a "viscosity, which 'essentially 25 sators, which is used to harden the solvent-free, liquid

the amount with which the initial liquid is added with the coating mass depends on the

Silanol stopped chain matches; as already different sizes. Though the hardener

mentioned, this is a viscosity of about 100 to from the group of metal salts the liquid coating

400OcP at 25 ⁰ C. fabrics harden very well, it has been observed that the

Although the exact chemical structure of the acid residue of the catalyst has an adverse effect standing, solvent-free, liquid material exerts on certain metal parts, e.g. B. on copper parts, cannot be specified with certainty, a board with printed circuits and knows that the solution has a number of silicon-bonded copper parts on the surface according to the invention Contains hydroxyl or silanol groups. Usually coated with a metal salt analyzer then covered the proportion of hydroxyl groups is about 0.1 to 35, it is observed that the acid residue tarnishes the copper parts 3 percent by weight of the total solvent-free or discolored, reducing both the appearance and ' Liquid. The presence of these silicon-bonded parts will affect the performance of such parts. 1st those hydroxyl groups in the solvent-free on the other hand the substance to be coated already from Liquid provides the responsiveness by which it surrounds another or consists of impossible is, the liquid after pouring over 40 stainless steel, then the acid residue has the metal earth to harden the object to be coated. These salt catalysts have no adverse effect on the too Liquid coating materials can, like already hardened parts. The quaternary amrrionium connects be cured with the help of catalysts. fertilizers offer certain hardening catalysts

The catalysts that can be used include difficulties. These connections are such

For example, the metal salts of organic carboxylic acids 45 active that an exceptionally fast hardening of the

and in particular salts of organic carboxylic acids solvent-free coating material occurs, so that

the following metals: lead, tin, zinc, iron, cobalt, bubbles can form in the coating. One can

Chromium or manganese. The organic carboxylic acid residue eliminates this difficulty by having a lower concentration

these salts stem from the known organic tration of the catalyst; but even then

Carboxylic acids, the metal salts of which are more conventional, often lead to an uneconomically rapid hardening.

wisely used as a siccative. These acid residues on the other hand, the catalyst system from the

are for example acetates, octoates, butyrates or both components of the German "patent

Naphthenates. Regarding the hardening catalysts regulation 1188 106 of the solvent-free coating

The metal salts included are, for example, zinc solution which has proven to be extremely well suited. the

octoate, lead acetate, lead octoate, lead naphthenate or 55 speed of hardening is so favorable that

Dibutyltin dilaurate. Are such metal salts as thin as well as extraordinarily thick layers,

Hardening agents are used, then they are added in an amount e.g. B. of a thickness of about 5 cm or more,

from about 0.01 to 2 percent by weight, based on proportionally can be cured quickly without

Metal, added. that bubbles "in the resulting tough elastomers

Other useful catalysts include 60 occurrences.

Quaternary ammonium or quaternary phospho- Da the solvent-free coating solution

nium compounds, e.g. tetramethylammonium hy- tends to harden even at room temperature, if

hydroxide, benzyltrimethylammonium hydroxide or the catalyst was added, it is recommended to use the

Tetrabutylphosphonium hydroxide. The for the through-catalyst only immediately before the coating of the

The preferred catalyst for carrying out the invention is to add 65 desired object. To this

consists of two components and is used in the German manner to prematurely harden the coating solution

Patent 1188,106 described and claimed. avoided - and the storage offers no difficulty

This complex catalyst system consists of skills.

7 8

For covering objects, e.g. B. Electrically stopped chain and a viscosity of about 3500 cP

dry, it is recommended to mix ^{the catalyst at 25 0 C.} The resulting solution was

Add solvent-free coating solution, and then to a temperature of approximately 55 ° C

then the liquid over the to be coated. at a reduced pressure of about 20 mm 5 hours

Pour Gcgcnsland. Heated to remove all traces of air or 5 den long. During this time the

To remove moisture, which when pouring the xylene content of the mixture to about 0.5 weight per

standing, one should reduce the poured-over percent of the total mixture. The resulting

The article in a vacuum of fabric for about 30 minutes had a viscosity of about 3500 cP at 25 ° C

for example give 1 to 500 mm of pressure. After removing and contained 0.1 percent by weight hydroxyl groups,

During this time, the poured-over object becomes 0.5 percent by weight

'Room temperature or slightly elevated temperature added dibutyltin dilaurate, and the catalyzed

kept so that the hardening of the coated liquid was in and around an electrical coil

The object can take place and become a tough one, poured into a container so that layers out of the

clear, translucent and resilient housing around coating solution up to 5 cm thick,

can train the object. Although the hardening 15 This poured-over object was

bci room temperature can be achieved, recommends temperature for about 30 minutes on a print of

brought it to an elevated temperature of about 50 to 150 ⁰ C 10 mm, so that the possibly

zui "to accelerate the hardening process. Closed air or moisture has been removed, and

It can be cured in this way layers of the over- subsequently for 2 hours at 15O ⁰ C, so that a

tensile solution of about 5 cm thick or more with a 20 clear, tough, transparent and resilient coating

Temperature of about 8O ⁰ C was delivered on the spool in approximately 10 hours. The subject was

or at a temperature of about 150 ^° C. in an- then placed in an oven of 150 ^° C. and 72 hours

cured for approximately 2 hours. The resultant was kept at this temperature for a long time so that the Wi 1-

Hardened material possesses at 125 to 200 ⁰ C an almost kung of the heat on this coated coil

unlimited lifespan. 25 could be. After this time you could

In some cases it is advisable to have a coating - no change in the properties of the cured one

to have liquid available, which is not so determined by the substance,

hardens like a normal gel. Such a B is ό ie I 2
Liquid can be removed by modifying the coating

liquid with a conventional linear or 30 100 parts of the resinous. Copolymer from

branched-chain, liquid organopolysiloxane with Example 1 were dissolved in such an amount of xylene,

chain stopped by triorganosilyl, that a solution with 60 ° / ₀ of solid constituents is obtained

found in U.S. Patents 2,469,888; 2,469,890. 167 parts of this solution were mixed with 400 parts

to be discribed. These modified liquids of dimethylpolysiloxane with quenched by silanol

preferably have a viscosity of about 50 to 35 chain of Example 1 mixed. After almost the

Such 500OcP at 25 ⁰ C. fluids have removed the entire xylene at 50 ⁰ C and 40 mm pressure

the following general formula had been, the resulting solution was with 1 Ge

_ _._ _f weight percent lead octoate catalyzed. This mixture

Κ _β & ιυ ₄ _α (A) was replaced by a printed current path

² 40 see control panel cast, and the coated one

in which R has the same meaning as above and α subject was subsequently at room temperature

a value between about 2.003 and 2.040. Such evacuated for 30 minutes so that all traces of

liquid organopolysiloxanes become in a lot of possibly entrapped air or humid

of up to approximately 50 percent by weight of the removal was withdrawn, and then on for 2 hours

converted coating solution applied. Usually 45 100 ⁰ C heated, so that a product was created that

The R of the organopolysiloxanes with stopped are predominantly the same in appearance and properties

Chain chosen so that it is compatible with that of Example 1.

with the R of the R ₃ SiOo, ₆ units or the R ″ des for R ^- Ii

the coating solution used, liquid linear example 3

Organopolysiloxane with 50 150 parts of a solid, resinous mixed polymer stopped by silanol

Kcttc match. sats with an average of one trimethylsiloxane unit

The following examples are used for each SiO ₂ unit were toluene in such an amount

more detailed description of the invention. All anions that a solution with a content of

given refer to units of weight. 60 ° / ₀ of solid components formed. 250 parts of this

_R · · 1 1 55 solution were mixed with 250 parts of a linear dimethyl

^{a e} ' ^sp ' ^e ' ^L polysiloxane with chain stopped by silanol and

A resinous copolymer was mixed with a viscosity of 50OcP at 25 ⁰ C. These

the method of the aforementioned Patent ÜSA. solution was heated to 65 ⁰ C and 3 hours

2,857,356 by reaction of trimethylchlorosilane and kept under a vacuum of about 100 mm, so

Ethyl orthosilicate, so that a mixed 60 that the toluene as a solvent is almost completely

Polymcrisat from trimethylsiloxane and SiO ₂ units was pulled and a clear liquid with a

formed, in which the ratio of trimethylsiloxane viscosity of about 500 cP and a content of an-

to SiOa units was about 0.5 to 1. approximately 1 percent by weight of hydroxyl groups

A solution of 50 parts of the copolymer remained. This solution was with 0.25 ° / ₀ 1,3-dimethyl

in a sufficient amount of xylene was prepared, so 65 butylamine and butylglycidyl ether catalyzed and over

that the solution contained 60% solids. a lightning bolt equipped with a battery and capacitor

83.5 parts of this solution were poured with 300 parts of a light tube, and the overmolded assembly

linear dimethylpolysiloxane with silanol was then used at room temperature under one

Maintained vacuum of about 50 mm for 1 hour to remove all traces of air or moisture. This arrangement was subsequently heated to a temperature of 70 ^° C. for 12 hours, so that an article was provided with a tough, flexible, transparent, bubble-free coating. The procedure of this example was repeated with the difference that a dimethylpolysiloxane having a chain stopped by trimethylsilyl and a viscosity of 100 cP at 25 ° C. was used to dilute the coating solution. This trimethylsilyl group stopped liquid was present in an amount of 10% by weight of the total weight of the diluted solution. If another flash tube equipped with a battery and a capacitor was coated with this compound, the end product was somewhat softer than that which was first described in this example.

comparison

100 parts of a solid, resinous copolymer with 40 mole percent of trimethylsiloxane units and 60 mole percent SiO _a units was 250 parts of a linear dimethylpolysiloxane having stopped by silanol chain and a viscosity of 2,000 cps at 25 ° C was added and the mixture was heated to 70 ⁰ C. because you wanted to try to get a homogeneous solution that can be used as a coating mass. After this material was heated, it was stirred at room temperature for 2 hours. The resin had not dissolved in the liquid with the chain stopped by silanol, but rather the mixture began to gel and gave a substance with a viscosity of about 30,000 cP and the appearance of a poorly dispersed solid mixture.

Example 4

. 60 parts of the resinous copolymer from example 1 were dissolved in 40 parts of xylene and mixed with 300 parts of a linear dimethylpolysiloxane having silanol stopped by chain and a viscosity of approximately 200OcP at 25 ⁰ C. This mixture was heated to a temperature of about 40 ⁰ C at a reduced pressure of about 100 mm, so that about 99 ° / ₀ of xylene were removed, so that a flowable coating liquid having a viscosity of about 200OcP at 25 ⁰ C with an average of about 0.25 weight percent hydroxyl groups was provided. 0.25 part of N, N-dimethylsojaamine and 0.25 part of butyl glycidyl ether were added to 100 parts of this liquid. This catalyzed liquid was then poured around a control panel with printed circuit paths and the entire assembly was placed in an oven with circulating hot air at 80 ^° C. for 10 hours. At the end of this time, the organopolysiloxane coating solution had hardened to a firm, rubbery, transparent gel which had sufficient compliance to protect the assembly from mechanical shock and vibration.

Example 5

A solution was formed by dissolving 300 parts of a linear dimethylpolysiloxane with a chain stopped by silanol and a viscosity of about 300 ocP at 25 ° C. in a solution of 300 parts of the resinous copolymer according to Example 1, dissolved in 200 parts of xylene. The resulting solution was heated to a temperature of about 45 ° C. at 10 mm to remove all but about 1% of the xylene. Part of this solution was catalyzed with 0.25 percent by weight of dimethyl soya amine and 0.25 percent by weight of dibutyl glycidyl ether, based on the weight of the solvent-free liquid. The catalyzed mixture was then poured into a container and heated to 8O ⁰ C for 5 hours. ίο After this time the catalyzed system had changed from a liquid with a viscosity of about 300OcP into a solid, transparent, elastic material.

B e i s ρ i e 1 6

400 parts of a linear dimethylpolysiloxane with a chain stopped by silanol and a viscosity of 300OcP at 25 ° C were made with a solution

50 parts of the resinous copolymer from Example 1 mixed in 17 parts of xylene. The resulting solution was subsequently freed from the solvent under the same conditions as in the previous example, so that a liquid with a viscosity of about 3000 cP and a content of less than 1 percent by weight of solvent and approximately 0.5 percent by weight of OH groups resulted. This material was divided into two parts, and one part was added 0.6 weight percent of a Bleioctoatlösung ₀ lead is added at a level of 6 ° /. This part was held at 75 ° C for 48 hours. During this time a clear, compliant, hardened, translucent gel formed. The second part of the solvent-free coating solution were ₀ added tin with a content of 6 ° / 0.6 parts of a dibutyltin dilaurate. This catalyzed liquid was ^{heated to 75 °} C. for 72 hours and produced a transparent, resilient, hardened gel.
If desired, fillers can be added to the process according to the invention if these fillers have no adverse effects on the object to be coated. Suitable fillers are, for example, silicon dioxide airgel, sublimed silicon dioxide, precipitated silicon dioxide, diatomaceous earth or calcium carbonate.

Claims (1)

Claim: Process for the production of tough organoso polysiloxane elastomers by the casting or coating process from solvent-containing mixtures of a copolymer of R ₃ SiO _{0> 6} units (R = monovalent hydrocarbon radical) and SiOjj units, with an average of 0.3 to 0.8 R. ₃ SiO ₀ , B units meet an SiO ₂ unit, a liquid, linear diorganopolysiloxane with Si-bonded OH groups, catalysts, hardeners and optionally fillers, with shaping by removing the solvent, characterized in that a liquid linear diorganopolysiloxane with a viscosity of approximately 100 to 4000 cP at 25 ⁰ C is used in the mixture. Considered publications:
German Patent No. 909 995;
German Auslegeschriften Nos. 1 008 912, 1 091161; French patent specification No. 1 228 491. 709 610/594 7.67 © Bundesdruckerei Berlin