USE OF FLUOROPOLYMERS IN LASER SINTERING

Field of the Invention

The invention relates to the use of fluoropolymers, and in particular polyvinylidene fluoride and its copolymers, or polychlorotrifluoroethylene and its copolymers, in a laser sintering process. Fluoropolymer powder provides many advantages over polymers currently used in that it intrinsically possesses flame- retardancy, as well as excellent chemical resistance and thermal resistance.

Background of the Invention

The technology of sintering polymer powders under a laser beam is used for the manufacture of objects in three dimensions, such as prototypes and models. The selective laser sintering process is described in US Patent Number 4,863,568. In the process, a fine layer of polymer powder is deposited on a horizontal plate held in a chamber heated to a temperature lying between the crystallization temperature Tc and the melting point Tm of the polyamide powder. The laser sinters powder particles at various points of the powder layer according to a geometry corresponding to the object, for example using a computer which has the shape of the object in its memory and which reconstructs it in the form of slices. The horizontal plate is subsequently lowered by a value corresponding to the thickness of a layer of powder (for example, between 0.05 and 2 mm and generally of the order of 0.1 mm) and then a new layer of powder is deposited and the laser sinters powder particles according to a geometry corresponding to this new slice of the object. The procedure is repeated until the complete object has been manufactured. A block of powder consisting of polymer powder and melt is obtained within which the object is present. The parts that have not been sintered have thus remained in the powder state. Subsequently, the combination is gently cooled and the object solidifies as soon as its temperature falls below the crystallization temperature Tc. After cooling is complete, the object is separated from the powder, which can be recycled and used in another sintering operation.

It is recommended for the powder to have a difference Tm - Tc that is as large as possible in order to avoid deformation (or curling) phenomena during manufacture. This is because, at time to immediately after the action of the laser beam, the

temperature of the sample is greater than the crystallization temperature (Tc) of the powder but the introduction of a new colder powder layer causes the temperature of the component to rapidly fall below Tc and results in deformations. The greater the difference between Tm and Tc, the smaller the amount of shrinkage upon solidification.

Furthermore, an enthalpy of fusion (δHf) which is as high as possible is required in order to obtain good geometrical definition of the components manufactured. This is because, if the enthalpy of fusion is too low, the energy supplied by the laser is sufficient to cake, by thermal conduction, the powder particles close to the walls being constructed, and thus the geometrical precision of the component is no longer satisfactory.

US 6,245,281 discloses the use of polyamide-12 (PA 12) powders in the technology of the sintering of powders under a laser beam. These powders are such that their Tm is between 185 and 189°C, their Tc is between 138 and 143 ⁰ C and their δHf has a value of 112 ± 17 J/g. These powders are manufactured according to the process disclosed in Patent US 4 334 056.

Many different materials have been used in the laser sintering process: US 6,245 ,281 describes polyamide-11 and polyamide-12 as preferred powders, with other useful polymers being nylons, polyacetals, polypropylene, polyethylene, ionomers, polycarbonates and polystyrene. US 5,304,329 mentions semi-crystalline polymers useful in a selective laser sintering machine to also include polybutylene terephthlate.

US-2005-0197446: Describes a polyamide 12 designed specifically for a laser sintering operation. US20050003189 describes the use of a blend of a thermoplastic powder

(included polyvinylidene fluoride) with an adhesive particulate material for 3D printing. 3D printing uses an inkjet printer rather than a laser to produce prototype articles.

It has now surprisingly been found that fluoropolymers and copolymers can be used in a laser and infra-red (IR) sintering process, providing material properties to the formed article that are much better than those of currently used materials, such as flame retardency, chemical resistance and thermal resistance.

Summary of the Invention

The invention relates to a process for forming a three-dimensional object comprising the steps of: a) applying a layer of a fluoropolymer or co-polymer powder at a target surface; b) directing energy at selected locations of said polymer powder layer to sinter said powder at those selected points; c) repeating steps a) and b) over multiple layers to form an object; d) removing the unsintered powder from said object.

Detailed description of the Invention

The invention relates to the use of fluoropolymer and copolymers in a laser sintering process or other means of achieving a layer-by-layer construction, such as by IR sintering. The term "fluoromonomer" or the expression "fluorinated monomer" means a polymerizable alkene which contains at least one fluorine atom, fluoroalkyl group, or fluoroalkoxy group attached to the double bond of the alkene that undergoes polymerization. The term "fluoropolymer" means a polymer formed by the polymerization of at least one fluoromonomer, and it is inclusive of homopolymers, copolymers, terpolymers and higher polymers which are thermoplastic in their nature, meaning they are capable of being formed into useful pieces by flowing upon the application of heat, such as is done in molding and extrusion processes. The thermoplastic polymers typically exhibit a crystalline melting point. Especially preferred fluoropolymers for a laser sintering application include polymer and copolymers of polyvinylidene fluoride or polychlorotrifluoroethylene

The term "polyvinylidene fluoride" (PVDF) as used herein includes both normally solid, high molecular weight homopolymers, copolymers and terpolymers. Such copolymers and terpolymers include those containing at least 50 mole percent of vinylidene fluoride copolymerized with at least one comonomer selected from the group consisting of tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropene, vinyl fluoride, pentafluoropropene, perfluoromethyl vinyl ether, perfluoropropyl vinyl ether and any other monomer that would readily copolymerize with vinylidene fluoride. Particularly preferred are copolymers composed of from at least about 70 and up to 99 mole percent vinylidene fluoride, and correspondingly

from 1 to 30 percent tetrafluoroethylene; and about 70 to 99 percent vinylidene fluoride and 1 to 30 percent hexafluoropropene (as described in U.S. Patent No. 3,178,399); and about 70 to 99 mole percent vinylidene fluoride and 1 to 30 mole percent trifluoroethylene. Terpolymers of vinylidene fluoride, hexafluoropropene and tetrafluoroethylene such as described in U.S. Patent No. 2,968,649 and terpolymers of vinylidene fluoride, trifluoroethylene and tetrafluoroethylene are also representatives of the class of vinylidene fluoride copolymers which can be used in the process embodied herein.

A preferred PVDF copolymer for laser sintering would have a reasonable amount of crystalinity to present a distinct melt point, yet would have a small amount of non-crystallinity to reduce the brittleness of a formed article. An example of such a copolymer would be a random copolymer composed of 80-97 weight percent of vinylidene fluoride monomer units and 3-20 parts by weight of hexafluoropropane monomer units. The term "polychlorotrifluoroethylene", as used herein includes both normally solid, high molecular weight homopolymers, copolymers and terpolymers. Such copolymers and terpolymers include those containing at least 50 mole percent of chlorotrifluoroethylene copolymerized with at least one comonomer selected from the group consisting of tetrafluoroethylene, trifluoroethylene, vinylidene fluoride, hexafluoropropene, vinyl fluoride, pentafluoropropene, perfluoromethyl vinyl ether, perfluoropropyl vinyl ether and any other monomer that would readily copolymerize with vinylidene fluoride.

The fluoropolymers useful for laser sintering could also include a blend of a fluoropolymers or copolymer with an acrylic or methacrylic polymer. The fluoropolymer powder may be blended with small amounts up to 10% by weight, based on the amount of polymer, of a reinforcing powder whose melting point is considerably higher than that of the polymer, or a glass powder. This blend forms a polyvinylidene fluoride laser sinterable composition.

Fluoropolymer powders of the present invention can be directly formed into powders from emulsion or suspension polymerization through the use of spray drying, freeze-drying, and other methods of powder formation. The powder average particle size is less than 100 microns, typically in the range of 40 to 80 microns. Particle size can be adjusted and optimized for the laser sintering process by known means, such as by cryogenic grinding and by sieving and/or classifying.

The polyvinylidene fluoride polymers and copolymers of the invention show melting and recrystallization characteristics with minimum overlap. They can be used in place of currently used powders in the laser sintering process. A homopolymer or copolymer of PVDF can be selected to match the physical properties desired in the laser sintering operation, and in the formed article.

The use of a fluoropolymer or copolymer in a laser sintering operation has several advantages over currently used materials, such as polyamides. PVDF powders have intrinsic flame retardant properties, and PVDF show higher chemical and thermal resistance than polyamide. PVDF powders are far more flame retardant than polyamides any other materials currently used in a laser sintering operation.

Sintered parts made of polyvinylidene fluoride powder can be used in aggressive chemical and thermal environments, and also whenever flame retardant properties are required. Additionally, parts made of VF ₂ -VF ₃ copolymers (80/20 to 60/40) can show piezoelectric properties.

Example

A three-dimensional object is formed by a) applying a layer of a fluoropolymer or co-polymer powder at a target surface, then b) directing energy at selected locations of said polymer powder layer to sinter said powder at those selected points, and then repeating steps a) and b) over multiple layers to form an object. The unsintered powder form said object.