Plastic and nonmetallic article shaping or treating: processes – Including step of generating heat by friction
Patent
1995-08-10
1997-03-18
Fiorilla, Christopher A.
Plastic and nonmetallic article shaping or treating: processes
Including step of generating heat by friction
264 82, 419 44, C04B 4000, C04B 3564
Patent
active
056119780
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an improved process for the production of sintered moldings by molding a mixture of a sinterable ceramic or metallic powder and polyoxymethylene or a copolymer containing a majority of oxymethylene units as binder to give a compact, removing the binder by treatment with a gaseous acid, and sintering the product.
EP-A-413 231 describes a process for the production of inorganic sintered moldings in which compacts of sinterable powders and polyoxymethylene as binder are injection molded or extruded. The binder is removed from these compacts by treatment with a gaseous acid or gaseous boron trifluoride, preferably in a stream of inert carrier gas, and the resultant parts are sintered.
The earlier application P 42354293 discloses a further-developed process for the production of sintered moldings in which the binder polyoxymethylene is removed under reduced pressure and thus without using a carrier gas.
A common feature of both processes is the type of acids to be used for removing the binder, namely acids which are gaseous at room temperature, for example hydrogen halides and hydrogen sulfide, or liquid at room temperature, such as nitric acid, sulfuric acid, formic acid and acetic acid. However, these acids remain in the gas phase and corrode all apparatus parts which come into contact with the gas phase, or form liquid films which are likewise corrosive. In addition, disposal of these gases is not straightforward.
It is an object of the present invention to overcome these problems.
We have found that this object is achieved by a process for the production of sintered moldings by molding a mixture of a sinterable ceramic or metallic powder and polyoxymethylene or a copolymer containing a majority of oxymethylene units as binder to give a compact, removing the binder by treatment with a gaseous acid, and sintering the product, which comprises removing the binder using an acid which is solid at room temperature and sublimes or melts and evaporates at elevated temperatures.
Suitable sinterable powders for the novel process are oxidic, ceramic powders, such as Al.sub.2 O.sub.3 Y.sub.2 O.sub.3, SiO.sub.2, ZrO.sub.2, TiO.sub.2, Al.sub.2 TiO.sub.5 or YBa.sub.2 Cu.sub.3 o.sub.7-x as ceramic superconductor. Also suitable are oxide-free ceramic powders, such as Si.sub.3 N.sub.4, SiC, BN, B.sub.4 C, AlN, TiC, TiN, TaC and WC.
Examples of suitable metal powders are Fe, Al, Cu, Nb, Ti, Mn, V, Ni, Cr, Co, Mo, W and Si powders. The metal powders can also be employed in the form of alloys, for example as intermetallic phases such as TiAl, Ti.sub.3 Al and Ni.sub.3 Al. Graphite and carbon black are also suitable. It is of course also possible to use mixtures of said materials.
The particle size of the powders is generally from 0.005 to 100 .mu.m, preferably from 0.1 to 30 .mu.m, particularly preferably from 0.2 to 10 .mu.m.
The binders to be employed according to the invention comprise polyoxymethylene, advantageously having a molecular weight of from 10,000 to 500,000. In addition to homopolymers of formaldehyde or trioxane, copolymers of trioxane with, for example, cyclic ethers, such as ethylene oxide and 1,3-dioxolane, or formals, such as butanediol formal, are also suitable, the amounts of the comonomers generally being from 1 to 4% by weight of the polymers.
The materials to be molded usually contain from 40 to 70% by volume of the sinterable powder in addition to the binder. Furthermore, inorganic fibers or whiskers of, for example, Al.sub.2 O.sub.3, SiC, Si.sub.3 N.sub.4 or C can be added to the materials, which may also contain auxiliaries, such as dispersants, lubricants, such as polyethylene glycol or stearic acid, or further thermoplastic binders, such polyethylene, polymethyl methacrylate or polyethylene oxide.
The amount of auxiliaries is Generally from 0.1 to 12% by weight of the total material.
After all the components have been mixed, for example in a compounder or extruder, the materials are molded, for example by injection molding in conventional screw or plunger injection-mo
REFERENCES:
patent: 5043121 (1991-08-01), Wingefeld et al.
patent: 5188782 (1993-02-01), Bittler et al.
BASF - Aktiengesellschaft
Fiorilla Christopher A.
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