Use of a sugar

Abstract

A use of a modified sugar or a sugar alcohol as a binder or binder component.

Description

For manufacturing compounds and molded parts from metallic powders, admixing the powder with a suitable binder or binder combination in order to achieve sufficient strength in the processing of the powder and/or of the molded parts manufactured therefrom during manufacturing, but also during use, is known.

Alloys based on, for example, iron, steel, and light metals, as well as superalloys, are particularly used as starting materials for the metallic powders. The powders originate from different preparation processes, such as carbonyl reactions, water atomization, or gas atomization.

The known binders include hydrocarbons such as montan waxes, polyolefins, amide waxes, artificial resins, and various inorganic and organic binders.

For specific applications, it is sufficient to use temporary binders. These have the function above all of ensuring the green strength of a formed body until—in the pyroprocess—the product strength is taken over by a solid bond in particular.

The use of thermoplastic binders in metallic powders is known above all in hot forming methods, such as injection molding, or in hot extrusion. The mixture made of metallic powder and binder is heated to a temperature above the melting point of the binder, so that it enters a free-flowing (viscous) form and is subsequently processed. After the forming, the binder must be removed again from the formed body (green part) before a typical sintering process follows.

The “debindering” represents a problem both from process engineering and environmental viewpoints. If solvents are used to dissolve out the binders, additional harmful materials, which must be disposed of, result in addition to the binders themselves. EP 0 362 866 A2 describes details in this regard. A method for manufacturing a sintered molded part is also described in this publication, in which an inorganic binder is used which is made of one component of a water-soluble thermoplastic organic polymer and one component of a water-insoluble thermoplastic organic polymer. After the shaping, the molded part is brought into contact with water and the water-soluble polymer component is taken out before the molded body is sintered.

If metallic powders are used, materials which are sensitive to hydrolysis, such as carbonyl iron, must necessarily be protected from oxidizing during the dwell time of the water storage through the addition of inhibitors.

The degree of extraction of binders achievable through the water storage (water treatment) is sometimes significantly below 50% in this case, so that the problems described above for the removal of the residual binder apply essentially analogously.

The present invention is thus based on the object of specifying an environmentally-friendly possibility, which is optimized in regard to process engineering, for achieving at least temporary binding in metallic powders and powders made of hard materials and molded parts produced therefrom.

For this purpose, the present invention is based on the recognition that this object may be achieved by using a modified sugar or a sugar alcohol as a binder or binder component for metallic powders or powders made of hard materials and molded parts produced therefrom.

In this case, the term “sugar” stands for monosaccharides and oligosaccharides. These low molecular weight carbohydrates are sweet-tasting, water-soluble, crystalline, and amorphous compounds of defined molecular weight.

The carbohydrates cited are to be distinguished from polysaccharides, which, like most polymers, do not have a uniform molecular weight and additionally dissolve in water only poorly or not at all.

The term sugar alcohols describes a group of polyhydroxy compounds which result from monosaccharides through reduction of the carbonyl function. These sugar alcohols are predominantly crystalline, but may also be amorphous in modified form. These are water-soluble polyols. They are differentiated as tetrites, pentites, and hexites, for example, as a function of the number of hydroxy groups contained in the molecule. The naturally occurring sugar alcohols include, among others, glycerin and adonitol.

The sugars and sugar alcohols cited share the feature that they have a “sticky consistency”, which makes them especially suitable for use as binders or binder components. A further essential advantage is that both materials have very good water solubility.

Because of this, they fulfill the following action mechanism in the treatment of metallic powders and powders made of hard materials and their subsequent processing into molded parts:

During the shaping of metallic powders and powders made of hard materials, the addition of the carbohydrates leads to gluing of the metallic particles. During the temperature increase to prepare the metallic powder admixed with the binder and/or the powder made of hard materials admixed with the binder for the subsequent shaping in the shaping method of the above-mentioned type (e.g., injection molding, hot extrusion), the binder enters a low-viscosity aggregation state, which makes the metallic powder and/or the powder made of hard materials plastic as a whole, so that even complicated ceramic molded parts may be pressed, injected, or cast easily in the desired shape.

A further advantage is that the adhesive effect of the binder is lost at elevated temperatures (approximately 140 to 150° C.) because of the low hygroscopicity, i.e., not only does it not prevent the shaping process in this phase, but rather it positively supports it.

Even when crystalline sugars were used, it was determined that they no longer recrystallized once they had been transferred into the viscous state through temperature elevation. This property is advantageous because any possible components of the modified sugar or sugar alcohol remaining in the green part may not lead to microcracks in the ceramic microstructure due to recrystallization.

The significantly increased water solubility in relation to known binders leads to the binder being able to be dissolved back out of the metallic structure easily and nearly completely through simple water storage after the shaping. This dissolving process is encouraged if the treatment occurs in heated water.

Because the binder is also a material frequently used in the food industry, this directly results in the remaining sugar solution not requiring any special disposal and particularly not containing any toxic components.

Depending on whether the binder is made entirely or partially out of the sugars and/or sugar alcohols cited, it is completely or partially removed again from the molded part during the subsequent extraction. In this case, extraction values from 60 to 99% may be achieved without anything further, depending on the duration of the water storage (typically 20 to 60 hours, preferably 30 to 50 hours) and water temperature (preferably 30 to 70° C.).

The use of the sugars and sugar alcohols cited in amorphous form represents an optimized selection.

The latter group includes, for example, sugars or sugar alcohols modified through oxyethylation or oxypropylation, oxyalkylated carbohydrates, hydrogenated and/or partially hydrogenated carbohydrates and/or oxyalkylated hydrogenated and/or partially hydrogenated carbohydrates as the water-soluble components.

Caramelization of the materials during heating, which is negative for the use as a binder, is reliably prevented through the selection cited.

As described, the binders selected may be used either alone or in combination with further auxiliary materials. In the latter case, the sugar/sugar alcohol component is typically 40 to 90 weight-percent. Additives may be, for example, lubricants, inhibitors, wetting agents, or demolding aids (external and internal), as are known per se in ceramic processing technology, for example. These include, among others:

Polymers made of polyethylenes, polypropylenes, and/or ethylene-vinyl acetate copolymers made of ethylene vinyl acetate and/or ethylene vinyl ester as lubricants, which lead to equalization of the pressure buildup during extrusion; surfactants, particularly non-ionic surfactants (e.g., fatty acid polyglycol esters) as wetting agents, or wax esters (e.g., beeswax or wool fat).

The binder proportion within a mixture made of metallic powder, a stainless-steel powder, for example, and binder and/or within a mixture made of a powder made of hard materials and binders, may be up to 20 weight-percent in relation to the total mixture.

The metallic powders may be composed, for example, of powders of the metals iron, steel, copper, aluminum, other light metals, titanium, tin, zinc, antimony, lead, their alloys or superalloys or mixtures of the above-mentioned materials.

The powders made of hard materials may be composed, for example, of powders of the hard materials carbides (such as tungsten or titanium carbide), borides, nitrides, or silicides or mixtures thereof.

Mixtures made of metallic powders and powders made of hard materials may also be provided, for example.

The present invention will be described in greater detail in the following on the basis of an example:

• • 91 weight-parts of a metallic powder based on stainless steel were mixed with 9 weight-parts of a binder having the following composition:
  • 75 weight-parts oxypropylated sorbite, 15 weight-parts ethylene vinyl ester, and 10 weight-parts beeswax. After mixing (homogenization) in typical solid mixers, the compound containing the binder was heated to 160 to 180° C., so that the binder entered a low-viscosity aggregation state. The mixture made of metallic powder and binder was subsequently pressed dry into molded parts such as gear wheels or transmission components and then had an outstanding green bond.

The molded parts were then placed for 40 hours in 50° warm water. Through the diffusion process, the sorbite components above all were practically completely extracted in this case. In parallel to this, the open pore volume of the molded part was increased proportionally in particular, the green bond remaining maintained practically unchanged because of the prior shaping process, however.

In the concrete case, a degree of extraction of the binder of approximately 75% was achieved.

The remaining binder component is harmless in regard to process technology in the subsequent pyroprocess in connection with the increased (open) porosity after the extraction. During the heating phase, a residual binder removal (debindering) occurs via the open pore structure, which is followed by the sintering of the product.

No damage was observed to the microstructure.

Claims

1. A use of a modified sugar or sugar alcohol as a binder or binder component for metallic powders or powders made of hard materials and molded parts made therefrom.

2. The use according to claim 1,

wherein the sugar or sugar alcohol is used in amorphous form.

3. The use according to claim 1,

wherein a sugar or sugar alcohol modified through oxyethylation or oxypropylation is used.

4. The use according to claim 1,

wherein a binder having 40 to 90 weight-percent sugar or sugar alcohols, with the remainder lubricants, demolding aids (internal and external), wetting agents, and inhibitors, is used.