Title of Invention: POLYMERIC BINDERS BASED ON A WATER

SOLUBLE POLYMER AND A BACKBONE POLYMER FOR

INJECTION MOULDING OF METALLIC OR CERAMIC

POWDER AND METHOD FOR THE PREPARATION THEREOF

Field of invention

[1] The invention consists of binders based on a water soluble binder and a metallocene polyethylene (MPE) to use in metal or ceramic powder injection moulding process, for the industrial production of metal or ceramic parts, respectively.

[2] This technology, hereinafter designated and known as PIM technology (from Powder

Injection Moulding), combines the capabilities of the processes of plastics injection moulding and powder processing and sintering, being applicable in the production of metal and/or ceramic components. Background of the invention

1. Powder Injection Moulding

[3] Powder injection moulding is a multi step process technology, which has been applied to both ceramic and metal powders. This process can be considered as a combination of injection moulding of plastics and the consolidation processes by sintering, wherein a hot mixture of a polymeric material, acting as a binder, and a metallic or ceramic powder, is injected into a mould (injection moulding step). This way, a so- called 'green part' is obtained, which has the same shape as and is oversized than the final part (sintered).

[4] The polymeric material, which acts as a binder of metal or ceramic particles, is then removed from the part (debinding step), resulting in a so called 'brown part'. Finally, the part is sintered at a sintering temperature of the metallic or ceramic material under similar conditions to those of a process of conventional powder technology.

[5] The main role of the binder is to provide the flow of the mixture (also referred to as feedstock) into the mould. After the moulding step, the binder holds the particles together and is extracted from the parts in the debinding step. Finally, during sintering the binder is burned and the remaining particles are heated up to sintering temperature.

2. Classification of binder based on the debinding methods

[6] The debinding method is fixed according to the chemical composition of binder and thus by its physical and/or chemical characteristics. The use of different binders may require using different methods and equipment for debinding, so the choice of a binder is a decision that is part of the investment plan of a PIM manufacturing company. 2.1. Binders for thermal debinding

[7] Several documents disclose the use of binders designed to be removed by thermal degradation, a process commonly referred as thermal debinding. [8] Examples of binder formulation comprise: a) semi-synthetic wax (based on montan wax), a polyolefin wax, an ethylene- vinyl acetate copolymer (EVA), an alcohol and, as additives, an organic peroxide and an azo-ester (US5254613, US5417756, EP0599285); b) polystyrene, oil and stearic acid (US4207226); c) acrylic resin (ES2167130).

[9] The thermal debinding method is considered physically simple, but has certain disadvantages, such as the high processing time (up to 60 hours) and high rate of defects in moulded parts, so that it is a method with low productivity (German and Bose, Injection Molding of Metals and Ceramics', Metal Powder Industries Federation, 1997). The present invention discloses binder formulations designed to be removed by dissolution in water with lower processing times (up to 24 hours) and lower probability of part defect occurrence, since the removal of the soluble constituent is made at a much lower temperature than the softening temperature of the backbone constituent.

2.2 Binders for catalytic debinding

[10] US5531958 and US5531958 disclose a binder consisting of polyacetal

(polyoxymethylene), homopolymer or copolymer, a secondary component consisting of poly(butanediol-formal), polyethylene or polypropylene or a mixture of these and additives to increase the dispersion of powder in the binder. This binder is removed by catalytic degradation, by subjecting the pieces to a strong acid inert atmosphere and to temperature. The industrial process has drawbacks in terms of high operation cost due to the consumption of inert gas and high purity acid, for example, nitric acid or oxalic acid. The use of strong acids represents a large risk in the process and conditions the industry when facing increasingly restrictive legislation on environment and health and safety.

2.3 Binders for water extraction

[11] The debinding method described in this invention - water extraction - allows PIM technology to reduce its environmental impact, because there is no gaseous emissions produced by of thermal degradation product incineration, and to reduce the level of health risks, without the use of acids or solvents, but maintaining productivity and process quality.

[12] Some binders of this type are based on: partially hydrolysed polyvinyl alcohol

(PVA), polypropylene or polyethylene, water, glycerin and other materials such as processing agents and additives (US6008281); polyalcohol, polyolefin, EVA copolymer and dispersion agents (US 6264863, PT102147); PVA, processing agents based on polyethylene glycol (PEG) and copolymer of ethylene and propylene oxide with hydroxyl end groups (US5098942). 3. Metallocene polyethylene as a PIM binder constituent

[13] The metallocene polyethylene (MPE), also known as metallocene linear low density polyethylene (mLLDPE) is a type of linear low density polyethylene (LLDPE) produced using metallocene catalysts. These polyolefin copolymers have a molecular structure different from conventional LLDPE. The unique feature of metallocene catalysts is to allow the production with the addition of a co-monomer with a random distribution in the polymer chains. The most frequent co-monomers may be butene, hexene and octene.

[14] The MPE molecular structure provides elastomeric properties to the material, so these materials have been used in products of plastics industry, especially in pipes, as an additive to modify the impact resistance, insulation of low voltage cables, elastic films, shoe soles, belts, automobile pipes, medical applications, sealants and other electrical applications.

[15] The properties of a polymer mixture are generally a combination of the properties of its constituents. Thus, with the addition of MPE in a binder formulation for PIM, it is expected that the binder exhibits an elastomeric behaviour, and that can also be observed in the mixture of binder with the powder and hence the 'green parts'.

[16] Studies of compatibility between the binder constituents have shown that the metallocene polyethylene is less compatible with PEG and waxes than the low density polyethylene in an identical mixture, which could cause more processing problems (powder segregation and resulting deformations, cracks and failures in the sintered parts (Rhee, BO and Chung, CI, 'Effects of the binder characteristics on binder separation in powder injection molding, Powder Injection Moulding Symposium, 1992)). However, it has been found that the use of MPE in PIM processing has resulted in a lower defect incidence in sintered parts related to the phase separation in the injection moulding step. General description of the invention

[17] The present invention refers to binder compositions for powder injection moulding which, after being mixed with metal or ceramic powders, allows the production of metal or ceramic components.

[18] These binders are mainly composed by a main polymer and a backbone polymer. Additionally, they may contain a lubricant and/or a dispersant. The constituents are, specifically: polyethylene glycol (PEG), as the water soluble main polymer; metallocene polyethylene (MPE) or a mixture of MPE and low density polyethylene, as the backbone polymer; polyethylene wax or a mixture of polyethylene waxes, as the lubricant; carboxylic acid or a mixture of carboxylic acids, as the dispersant.

[19] These formulations allow the use of a debinding method of extraction in water, being removed out from the moulded parts by dissolution, characterised in that it is a simple method with low equipment investment and low environmental impact and health risk.

[20] The presence of MPE in the binder formulation results in a greater productivity in the injection moulding step, by facilitating the extraction of parts and production of sintered parts with fewer defects; the use of MPE increases the yield of the PIM processing.

[21] The preparation of binders of the present invention is carried out in a mixing device consisting of a heated vessel and one or more vertical axis rotors. This method comprises the following three steps:

[22] 1. Phased addition of the constituents - The first constituent, PEG, is poured into the heated vessel, at a set temperature. The following components must be added when the following constituents are completely melted. The second constituent to be added can be the wax, then the carboxylic acid, or vice versa. Then MPE should be added.

[23] 2. Mixing - The mixing process is carried in steady state, i.e., at a constant temperature and stirring speed, for a predefined time.

[24] 3. Cooling - The resulting molten binder is poured into a cooling and solidification device. 1. Characterisation of the binders

[25] The present invention refers to binder composition for powder injection moulding which, after being mixed with metal or ceramic powders, is used for the production of metal or ceramic components. These binders are mainly composed by a main polymer and a backbone polymer. Additionally, they may contain a lubricant and/or a dispersant.

[26] Each binder constituent plays a specific function, thus influencing their properties and PIM production conditions:

The main polymer, water soluble, is the material in major content in the binder formulation and it is extracted by water extraction in the debinding step;

The backbone polymers are the polymers with higher mechanical and flow resistance, and provide the necessary shape retention of the moulded part, critical for the debinding step. These materials are then extracted during the sintering process;

The lubricants are used to modify the rheological properties of the powder and binder mixture during the injection moulding process; The dispersants are used to improve the powder dispersion in the binder, thus in- creasing the mixture homogeneity, consequently, fomenting the rheological properties and diminishing the part defect occurrence during the debinding and sintering steps.

[27] The present invention consists in binder formulations which are distinguished from others by comprising a main water soluble polymer and a backbone polymer comprising a metallocene polyethylene - MPE.

[28] In a preferred embodiment of the invention, the backbone polymer can be a MPE or a mixture of MPE and low density polyethylene.

[29] In another preferred embodiment of the invention, the binder compositions may also contain a lubricant and/or a dispersant.

[30] Still in another preferred embodiment of the invention, the binder formulations compromise polyethylene glycol (PEG) as the main polymer.

[31] In another preferred embodiment of the invention, the lubricant used in the binder formulations is a polyethylene wax and/or a mixture of polyethylene waxes.

[32] In another preferred embodiment of the invention, the dispersant used in the binder formulations is a carboxylic acid and/or a mixture of carboxylic acids.

[33] According to the present invention, the binder compositions comprise a mixture of polyethylene glycol (PEG) and a metallocene polyethylene (MPE) or a mixture thereof with low density polyethylene (LDPE), with the possible addition of one polyethylene wax or a mixture of polyethylene waxes. One carboxylic acid or a mixture of carboxylic acids can also be added. The binder formulations comprise the constituents with the following concentrations and features:

The water soluble main polymer in an amount ranging 30 to 90 % by weight of binder is a polyethylene glycol (PEG) characterised by a weight average molecular weight (M _w ) from 500 to 40,000, a density from 1200 to 1260 kg/m ³ and a solidification temperature from 15 to 60 ⁰ C;

The backbone polymer in an amount ranging 5 to 60 % by weight of binder is a metallocene polyethylene (MPE) characterised by M _w from 40,000 to 500,000 and a density from 886 to 940 kg/m ³ , or a mixture thereof with a low density polyethylene (LDPE) characterised by M _w from 30,000 to 400,000 and a density from 910 to 935 kg/m ³

The lubricant in an amount ranging 1 to 30 % by weight of binder is a polyethylene wax characterised by M _w from 1000 to 10,000 and a density from 910 to 996 kg/m ³ , or an oxidised polyethylene wax characterised by M _w from 1000 to 8500 and a density from 915 to 990 kg/m ³ , or a mixture of both;

The dispersant in an amount ranging 1 to 15 % by weight of binder can be a carboxylic acid or a mixture of several carboxylic acids. These acids are octadecanoic acid (also known as stearic acid) and 9-octadecenoic acid (also known as oleic acid) 2. The method for binder preparation [34] In order to obtain the binders, a stirrer consisting of a heated vessel and one or more vertical axis rotors are used. The rotor must be able to produce a turbulent flow of the mixture; these mixing devices are commonly designated overhead stirrers, with shaft speed up to 3000 rpm.

[35] The procedure for binder preparation comprises three stages:

[36] 1. Phased addition of the constituents - PEG is first poured in the mixer vessel. The vessel is heated to a predefined set temperature, above all melting temperature of all binder constituents, in this case, from 65 to 195 ⁰ C. The following constituents must be added after the previous constituent are completed melted. The second constituent to add can be the wax, followed by carboxylic acid, or vice- versa. Then, MPE should be added by means of a particular procedure. It should be added portion-by-portion in order to be gradually melted and form micro-spheres dispersed in the PEG medium. The following MPE portion should only be added after ensuring that the previous portion has been completely dispersed.

[37] 2. Mixing in stationary state - The mixing process is carried out in stationary state, i.e., at a constant temperature and stirring speed, for a predefined time. Mixing time ranges 10 minutes to 4 hours.

[38] 3. Cooling - The resulting molten binder is poured into a cooling and solidification device. Granulation or grinding operation can be carried out to make the storage and handling easier during the following mixing process with powder.

[39] The binders are used in PIM process as a component of the mouldable feedstock, whose main role is to transport and allow the flow and packing the powder into the cavity of a mould. Downstream, the binders also ensure the moulded part integrity till sintering step.

[40] The debinding method described in the present invention - water extraction - allows the PIM technology to reduce environmental impact, so no gaseous emissions by incineration of degradation products are produced, and to reduce human health risks, without the need to use of acids or solvents. Compared to thermal debinding, the removal by dissolution in water is associated with lower processing times (up to 24 hours) and less probability of part defect occurrence, since the removal of constituent content is made at a lower temperature than the softening temperature of the backbone constituent. This method is thus characterised as simple and with low environmental impact and human health risk, and requires less expensive equipment than other binder removal process.

[41] This process is advantageous for the production of parts in materials which are very sensitive to contamination from the binder elements, namely carbon, oxygen and hydrogen (e.g. titanium, magnesium or aluminium). The replacement of thermal processes for binder removal, such as thermal and catalytic degradation which can reach temperatures from 100 and 500 ⁰ C, by an extraction process with water, with temperatures below 80 ⁰ C, decreases the absorption of contaminants by the powder present in the moulded parts and minimises the property loss of the final pieces.

[42] In the injection moulding step, the frequent problems related to breaking of moulded parts during ejection, because these are too brittle compared to plastic parts, influence the design of the mould and the process of injection moulding. Thus it results in costs associated with the required finishing operations of the mould and precise control of the process of injection moulding. The fact that these parts may have elastomeric properties, resulting from the presence of MPE in the binder composition, can contribute for the non-breaking part ejection and, consequently, an increase in process productivity.

[43] The use of MPE in the binder formulations for PIM object of the present invention, based on polyethylene glycol (PEG) has resulted in an optimisation of the formulation. The MPE - PEG mixture is extremely difficult, but becomes possible with the addition of polymers of low molecular weight, in order to increase the mixing entropy. The MPE - PEG mixture and addition of polyethylene wax produces a liquid bi-phasic mixture at 155 ⁰ C characterised in that it comprises a dispersed phase, MPE-rich, in a dispersive medium, PEG-rich. Detailed description of the invention

[44] The present invention refers to binder compositions for powder injection moulding which, after being mixed with metal or ceramic powders, allows the production of metal or ceramic components. These binders are mainly composed by a main polymer and a backbone polymer. Additionally, they may contain a lubricant and/or a dispersant.

[45] The preparation of mechanical characterisation standard specimens by metal powder injection moulding, according to ISO 2740: 199(E), was made according to the following procedure:

[46] The reference binder, identified by L-I, was composed in weight basis by: 70 %

PEG; 17.5 % low density polyethylene (LDPE); 7.5 % polyethylene wax (PEW) and 5 % stearic acid.

[47] The binder, identified by L-2, was composed in weight basis by: 70 % PEG; 17.5 %

MPE; 7.5 % PEW and 5 % stearic acid. The constituent characteristics of both binder are shown in Table 1.

[48] The mixtures were prepared in a rotor stirrer at 2000 rpm and 155 ⁰ C.

[49] Table 1 Characteristics of binder constituents. [Table 1] [Table ]

[50] Powder-binder mixtures were prepared with AISI 316L stainless steel powder, with spherical shape, median diameter of 11 μm and real density of 7.93 g/cm ³ . By adding the binders, mixtures of 66 % in powder volume concentration was made, using a 'Z'-blade batch kneader and a shear roll compounder. The resulting feedstock mixtures presented adequate homogeneity and rheological features for PIM process.

[51] The mixture was moulded by a 500 kN clamping force hydraulic machine, with nozzle temperature of 120 ⁰ C and tool temperature of 27 ⁰ C. The moulded parts had low weight variability, therefore the process was stable and reproducible. The parts were immersed in demineralised water at 50 ⁰ C for 15 hours. Sintering was carried out in a graphite furnace at 1360 ⁰ C in vacuum (absolute pressure of 7 Pa).

[52] The sintered parts produced by both binders showed similar characteristics in terms of density, mass, dimensional accuracy and chemical composition. However, the samples obtained with binder L-I showed cracks typically resulting from the phase separation effect, or segregation, during mould filling. The binder L-2 produced parts without visible defects. This example confirms the L-2 binder advantage in obtaining defect free parts comparing to a reference formulation.

[53] The sintered parts obtained with binder L-2 showed nearly-full density (bulk density of 7.95 g/cm ³ ). The standard deviation of part weight was of 0.11 % from average and the standard deviation of the length and test section diameter was 0.2 % of average. Carbon and oxygen content were 0.03 % and 0.012 % by weight, respectively. Tensile test results were: Young modulus of 166 GPa, yield stress of 246 MPa, ultimate tensile stress of 647 MPa and elongation at rupture of 58 %. It was confirmed that the part defects obtained with L-I were detrimental to mechanical properties, therefore hindering mechanical resistance and ductility, when compared to L-2 parts.