Adhesive for injection molding compositions

Abstract

Binder for injection molding compositions comprising:-45 to 50 volume% of a polymer base,-at least 34 volume% of a wax mixture or a mixture of wax and palm oil,-and 11 to 16 volume% of a surfactant, wherein the polymer base contains, in addition to polyethylene, polypropylene and acrylic resins, ethylene and methacrylic acid or acrylic acid copolymers, or maleic anhydride-containing ethylene copolymers, or mixtures of these copolymers, the respective amounts of the binder components being such that they add up to no more than 100% by volume.

Description

[0001] This application is a divisional application of International Application No. PCT / EP2017 / 076486, filed by Comado S.p.A. on October 17, 2017, entitled “Binder for Injection Molding Compositions.” International Application No. PCT / EP2017 / 076486 entered the Chinese national phase on June 6, 2019, with national application number 201780075637.9. Technical Field

[0002] The present invention relates to adhesive compositions for injection molding and to injection molding compositions (raw materials) intended for the production of shaped metal parts. Background Art

[0003] Powder metallurgy techniques are implemented in the manufacture of hard materials for the jewellery and watchmaking industries or for technical applications such as medical devices, electronics, telephony, tools, machine cutting blades and the consumer goods industry. The resulting synthetic inorganic material is referred to herein as "metal", regardless of the nature of the metal or metal alloy. In general, the basic raw materials are of different natures and comprise at least: on the one hand, a metal powder and, on the other hand, an organic binder such as a resin or plastic or the like, which can be injection moulded and provides good mechanical strength for the component made from the mixture of all raw materials. Other additives may be included in the mixture. These raw materials may also have different textures: solid, powder, liquid or paste. The structure of the mixture may also change during its manufacture, in particular but not exclusively when polymerization reactions occur with the supplementary components of the resin.

[0004] The overall manufacturing method for producing a metal component by powder metallurgy comprises at least the following steps:

[0005] - Preparation of raw materials;

[0006] - Mixing of raw materials, or / and if necessary, premixing of two (or more) at a time;

[0007] - homogenization kneading;

[0008] - granulation;

[0009] - pressing, in particular in a pressing chamber, a quantity of powder or feedstock pellets obtained from the homogenization and granulation step, in order to produce a blank or "green part". This pressing operation can be carried out by injection molding under pressure, in particular in a screw injector comprising means for heating the quantity of powder or feedstock pellets obtained from the homogenization and granulation step;

[0010] - debinding in an oven to burn and / or dissolve certain components of the mixture;

[0011] - heat treatment of the green part after debinding to carry out the sintering process which gives the finished component its final cohesion. This heat treatment causes dimensional shrinkage which makes it possible to obtain a component with final dimensions,

[0012] -Surface finishing treatment of the component.

[0013] A particular object of the present invention is to propose an optimized binder for injection molding compositions which facilitates powder metallurgical mixing to obtain metals, thereby producing metal parts with complex shapes and highly reproducible quality with a controlled shrinkage coefficient.

[0014] There are known thermoplastic materials (raw materials) for producing molded metal parts, which contain a sinterable metal powder and an organic polymer binder, the latter being essentially formed from a mixture of polyoxymethylene and a polyoxymethylene-polyoxolane copolymer, such materials (raw materials) being marketed, for example, by BASF under the trade name sell.

[0015] However, these raw materials have shown a number of drawbacks, such as insufficient fluidity for injection molding, which limits the complexity of parts that can be made by this means and sometimes requires expensive mechanical reworking operations; and mechanical strength problems of the molded parts, which show cracks and lamination, especially when the shape of the part is critical. They also pose environmental problems due to the necessity to use aggressive products such as nitric acid, especially to remove the organic phase in the degreasing step.

[0016] CH Patent No. 707572 also discloses an adhesive for injection molding compositions comprising a polymer base, a mixture of wax and / or palm oil, and a surfactant. Such adhesive compositions are degreased in two separate, sequential steps: a first partial degreasing step using a solvent, followed by a second, thermal degreasing step. During the first degreasing step, the solvent-soluble adhesive components, namely the wax, surfactant, and acrylic resin mixture, are removed. In practice, this operation is carried out batchwise in a furnace containing green parts and a suitable solvent. The batch is then placed in a batch furnace where the adhesive components that did not dissolve during the chemical degreasing step are removed, typically by heating the green parts to a temperature of approximately 600°C. Once degreasing has occurred, the organic components have been removed, and only then are the parts sintered in the same furnace by heating to a temperature of approximately 1200°C.

[0017] This batch manufacturing technique produced good results, but processing continuous batches meant that it was not possible to achieve satisfactory production rates with existing facilities.

[0018] One object of the present invention is therefore to provide a binder for injection molding compositions which avoids the above-mentioned disadvantages and is more particularly intended to improve the homogeneity and flowability of the raw material in order to manufacture metal parts with more complex shapes, reduce the production cycle times, limit or avoid the need for mechanical finishing processes and, finally, do not require the use of environmentally damaging products for the removal of the organic binder, but instead use non-polluting solvents which can be removed by a simple thermal treatment. Summary of the Invention SUMMARY OF THE INVENTION

[0020] The present invention therefore relates to a binder for injection molding compositions comprising:

[0021] - 45 to 50% by volume of polymer binder,

[0022] - at least 34% by volume of a mixture of waxes or a mixture of waxes and palm oil,

[0023] - and 11 to 16% by volume of a surfactant,

[0024] wherein the polymeric binder contains, in addition to polyethylene, polypropylene and acrylic resins, copolymers of ethylene with methacrylic acid or acrylic acid, or ethylene copolymers containing maleic anhydride, or mixtures of such copolymers, the respective amounts of the adhesive components being such that they add up to not more than 100% by volume and are preferably equal to 100% by volume.

[0025] These features make it possible to shorten the thermal debinding step and, for example, use a conveyor-type furnace, thereby optimizing productivity while ensuring complete debinding prior to sintering. The use of a conveyor-type furnace makes it possible to continuously combine the thermal debinding and sintering steps within the same facility, thereby increasing component productivity while simultaneously reducing production costs. Furthermore, this binder composition possesses a viscosity that enables the injection molding of flawless, complex, and shaped parts.

[0026] Preferably, the adhesive of the present invention comprises 4 to 5% by volume of one of the copolymers or a mixture thereof, 25 to 30% by volume of polyethylene, 3 to 5% by volume of polypropylene and 8 to 10% by volume of an acrylic resin.

[0027] Preferably, the copolymer of ethylene and methacrylic acid or acrylic acid contains 3 to 10 wt% of methacrylic acid or acrylic acid comonomer, and the copolymer of ethylene and anhydride is a statistical (random) copolymer of ethylene and maleic anhydride having a melting point of 100 to 110°C or a copolymer of HD polyethylene and a modified anhydride having a melting point of 130 to 134°C.

[0028] Preferably, the acrylic resin has a molecular weight of 50,000 to 220,000 and an intrinsic viscosity of 0.21 to 0.83 and is selected from polymers of isobutyl methacrylate, methyl methacrylate, ethyl methacrylate, and n-butyl methacrylate, and copolymers of isobutyl methacrylate, n-butyl methacrylate, and methyl methacrylate, or mixtures of these polymers and / or copolymers. "Inherent viscosity" refers to the viscosity of a solution containing 0.25 g of the polymer in 50 ml of dichloromethane, as measured at 20° C. using a size 50 Cannon-Fenske viscometer.

[0029] Advantageously, the wax mixture comprises carnauba wax and paraffin wax, or the wax mixture comprises carnauba wax and palm oil.

[0030] According to another preferred characteristic, the surfactant is N,N'-ethylenebisstearamide or a mixture of stearic acid and palmitic acid (stearin), or a mixture of these ingredients.

[0031] According to an advantageous feature, the surfactant and the acrylic resin are soluble in isopropyl alcohol and / or turpentine.

[0032] The present invention also relates to an injection molding composition (raw material) intended for the production of shaped metal parts, comprising 70 to 80% by volume of metal powder and 20 to 30% by volume of a binder comprising:

[0033] - 45 to 50% by volume of polymer binder,

[0034] - at least 35% by volume of a mixture of waxes or a mixture of waxes and palm oil,

[0035] - and 11 to 16% by volume of a surfactant,

[0036] wherein the polymeric binder contains, in addition to polyethylene, polypropylene and acrylic resins, copolymers of ethylene with methacrylic acid or acrylic acid, or ethylene copolymers containing maleic anhydride, or mixtures of such copolymers, the respective amounts of the adhesive components being such that they add up to not more than 100% by volume and are preferably equal to 100% by volume.

[0037] According to a particular feature, the metal powder of the injection molding composition can be chosen from tungsten metal powder, stainless steel powder, titanium metal powder, precious metal powders such as gold, platinum, silver, palladium, rhodium and ruthenium, or mixtures of these metal powders.

[0038] In particular, the present invention relates to the following embodiments.

[0039] 1. A binder for an injection molding composition comprising:

[0040] - 45 to 50% by volume of polymer binder,

[0041] - at least 34% by volume of a mixture of waxes or a mixture of waxes and palm oil,

[0042] - and 11 to 16% by volume of a surfactant,

[0043] The polymer base contains, in addition to polyethylene, polypropylene and acrylic resin, copolymers of ethylene with methacrylic acid or acrylic acid, or copolymers of ethylene with maleic anhydride, or mixtures of such copolymers, the respective amounts of the adhesive components being such that they add up to not more than 100% by volume.

[0044] 2. The adhesive according to embodiment 1, comprising 4 to 5% by volume of one of the copolymers or a mixture thereof, 25 to 30% by volume of polyethylene, 3 to 5% by volume of polypropylene and 8 to 10% by volume of an acrylic resin.

[0045] 3. The adhesive according to embodiment 1 or embodiment 2, wherein the copolymer of ethylene and methacrylic acid or acrylic acid contains 3 to 10 wt.% of methacrylic acid or acrylic acid comonomer, and the copolymer of ethylene and anhydride is a statistical (random) copolymer of ethylene and maleic anhydride having a melting point of 100 to 110°C or a copolymer of HD polyethylene and a modified anhydride having a melting point of 130 to 134°C.

[0046] 4. The adhesive according to any one of the preceding embodiments, wherein the acrylic resin has a molecular weight of 50,000 to 220,000 and an intrinsic viscosity of 0.21 to 0.83 and is selected from polymers of isobutyl methacrylate, methyl methacrylate, ethyl methacrylate and n-butyl methacrylate, and copolymers of isobutyl methacrylate, n-butyl methacrylate and methyl methacrylate, or mixtures of these polymers and / or copolymers.

[0047] 5. The adhesive according to any one of the preceding embodiments, wherein the wax mixture comprises carnauba wax and paraffin wax, or the wax mixture comprises carnauba wax and palm oil.

[0048] 6. The adhesive according to any one of the preceding embodiments, wherein the surfactant is N,N'-ethylenebis(stearamide) or a mixture of stearic acid and palmitic acid (stearin), or a mixture of these ingredients.

[0049] 7. The adhesive according to any one of the preceding embodiments, wherein the surfactant and the acrylic resin are soluble in isopropyl alcohol and / or turpentine.

[0050] 8. Injection molding composition (raw material) intended for producing shaped metal parts, comprising 70 to 80% by volume of metal powder and 20 to 30% by volume of a binder according to any one of embodiments 1 to 7.

[0051] 9. The molding composition according to embodiment 8, wherein the metal powder is selected from tungsten metal powder, stainless steel powder, titanium metal powder, precious metal powder or a mixture of said metal powders, wherein the precious metal powder is selected from gold, platinum, silver, palladium, rhodium and ruthenium. DETAILED DESCRIPTION

[0052] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0053] The invention will now be illustrated in more detail by the following non-limiting examples.

[0054] The polymer portion of the binder is mixed with stainless steel powder (typically 316L stainless steel powder) at a temperature of approximately 150°C to produce a premix. Wax and surfactant are added to this premix, and the temperature is raised again to approximately 180°C to form a homogeneous paste. This paste is then simultaneously ground and cooled to ambient temperature to form a powder. This powder is then fed into a calender, which cuts and granulates the premix. The product of this final step is stored to form the raw material for manufacturing molded parts by injection molding according to known techniques.

[0055] In this example, more specifically, 46.5 kg (316L) stainless steel metal powder (78% by volume) and 3.5 kg binder (approximately 22% by volume) having the following volume composition were used:

[0056] -28% HD polyethylene

[0057] -4% polypropylene

[0058] - 4.5% of a copolymer of ethylene and methacrylic acid (containing 6.5% by weight of methacrylic acid, such as the type produced by DuPont as "Nucrel(TM)"

[0059] - 8.5% of an isobutyl methacrylate polymer resin having a molecular weight of 195,000 (e.g. of the type 'Elvacite(TM) 2045' manufactured by Lucite International)

[0060] - 1% of a polymer resin of n-butyl methacrylate and isobutyl methacrylate having a molecular weight of 165,000 (e.g. of the type 'Elvacite(TM) 2046' manufactured by Lucite International)

[0061] -9% Carnauba wax

[0062] - 31% paraffin wax (e.g. type 'Carisma 56T(TM)' manufactured by Alpha Wax BV)

[0063] - 14% of N,N'ethylene-bis(stearamide).

Claims

1. A binder for an injection molding composition comprising: - 45 to 50% by volume of polymer binder, - at least 34% by volume of a mixture of waxes or a mixture of waxes and palm oil, - and 14 to 16% by volume of a surfactant, The polymer base contains, in addition to polyethylene, polypropylene and acrylic resin, copolymers of ethylene with methacrylic acid or acrylic acid, or copolymers of ethylene with maleic anhydride, or mixtures of such copolymers, the respective amounts of the adhesive components being such that they add up to not more than 100% by volume.

2. Adhesive according to claim 1, comprising 4 to 5% by volume of one of the copolymers or a mixture thereof, 25 to 30% by volume of polyethylene, 3 to 5% by volume of polypropylene and 8 to 10% by volume of an acrylic resin.

3. The adhesive according to claim 1 , wherein the copolymer of ethylene and methacrylic acid or acrylic acid contains 3 to 10% by weight of methacrylic acid or acrylic acid comonomer, and the copolymer of ethylene and anhydride is a statistical (random) copolymer of ethylene and maleic anhydride having a melting point of 100 to 110° C. or a copolymer of HD polyethylene and a modified anhydride having a melting point of 130 to 134° C.

4. The adhesive according to claim 2, wherein the copolymer of ethylene and methacrylic acid or acrylic acid contains 3 to 10 wt% of methacrylic acid or acrylic acid comonomer, and the copolymer of ethylene and anhydride is a statistical (random) copolymer of ethylene and maleic anhydride having a melting point of 100 to 110°C or a copolymer of HD polyethylene and a modified anhydride having a melting point of 130 to 134°C.

5. The adhesive according to claim 1 , wherein the acrylic resin has a molecular weight of 50,000 to 220,000 and an intrinsic viscosity of 0.21 to 0.83 and is selected from polymers of isobutyl methacrylate, methyl methacrylate, ethyl methacrylate and n-butyl methacrylate, and copolymers of isobutyl methacrylate, n-butyl methacrylate and methyl methacrylate, or mixtures of these polymers and / or copolymers.

6. The adhesive according to any one of claims 1 to 4, wherein the wax mixture comprises carnauba wax and paraffin wax, or the wax mixture comprises carnauba wax and palm oil.

7. The adhesive according to claim 5, wherein the wax mixture comprises carnauba wax and paraffin wax, or the wax mixture comprises carnauba wax and palm oil.

8. An adhesive according to any one of claims 1 to 4 or claim 7, wherein the surfactant is N,N'-ethylenebis(stearamide) or a mixture of stearic acid and palmitic acid (stearin), or a mixture of these ingredients.

9. The adhesive according to claim 5, wherein the surfactant is N,N'-ethylenebis(stearamide) or a mixture of stearic acid and palmitic acid (stearin), or a mixture of these ingredients.

10. The adhesive according to claim 6, wherein the surfactant is N,N'-ethylenebis(stearamide) or a mixture of stearic acid and palmitic acid (stearin), or a mixture of these ingredients.

11. The adhesive according to any one of claims 1 to 4 or any one of claims 7, 9, 10, wherein the surfactant and the acrylic resin are soluble in isopropyl alcohol and / or turpentine.

12. The adhesive according to claim 5, wherein the surfactant and the acrylic resin are soluble in isopropyl alcohol and / or turpentine.

13. The adhesive according to claim 6, wherein the surfactant and the acrylic resin are soluble in isopropyl alcohol and / or turpentine.

14. The adhesive according to claim 8, wherein the surfactant and the acrylic resin are soluble in isopropyl alcohol and / or turpentine.

15. Injection molding composition (raw material) intended for producing shaped metal parts, comprising 70 to 80% by volume of metal powder and 20 to 30% by volume of a binder according to any one of claims 1 to 14.

16. The molding composition according to claim 15, wherein the metal powder is selected from tungsten metal powder, stainless steel powder, titanium metal powder, noble metal powder or mixtures of said metal powders, the noble metal powder being selected from gold, platinum, silver, palladium, rhodium and ruthenium.