Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – Composite article making
Reexamination Certificate
2002-02-13
2003-12-23
Foelak, Morton (Department: 1711)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
Composite article making
C264S045900, C264S046100, C521S079000, C521S081000, C521S149000
Reexamination Certificate
active
06666998
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for producing an at least partially open-celled foam foil from styrene polymers, as well as a device for producing the same, using a twin-screw extruder.
2. Description of Related Art
At least partially open-celled foam foils are used where absorption capabilities, for example, are required, in particular for fluids, or foils permeable to water vapor and air are desired. A preferred area of application for such partially open-celled foam foils made of styrene polymers is in connection with packaging and containers. Foam foils are also processed in many ways into end products, for example shells, by subsequent thermal deformation.
A method for producing an extruded, open-celled foam material on the basis of styrene polymers is known from PCT International Publication WO 96/00258, wherein the polymer is melted in a single-screw extruder. The open-celled foam foils produced are used in particular as insulating elements for roofs.
Furthermore, a method for producing an open-celled polystyrene foil for packaging shells is known from European Patent Reference EP 0 849 309 A1, wherein a mixture of polystyrene, which contains incorporated aliphatic hydrocarbons with 4 to 6 carbon atoms and, if required, additional polystyrene, as well as nucleating agents, is melted in a twin-screw extruder at temperatures between 130 to 150° C. and is extruded, wherein twin-screw extruders with an L/D ratio of 20 are employed.
SUMMARY OF THE INVENTION
One object of this invention is to provide an efficient method for producing at least partially open-celled foam foils on the basis of styrene polymers, whose properties can be flexibly matched to various requirements, such as the degree of openness of the cells, density, thickness and polymer composition. It is another object to provide an open-celled foam structure which allows the passage of gases and fluids and allows a selectively adjustable, specific absorption of polar
on-polar fluids, as well as a specific absorption capability for polar fluids.
In accordance with this invention, this object is attained with a method in accordance with the features of this specification and the claims.
A method in accordance with this invention for producing an at least partially open-celled foam foil on the basis of styrene polymers uses a mixture, which contains 0.5 to 3 parts by weight of at least one nucleating agent, 0.5 to 3.5 parts per weight of at least one surfactant, and 1 to 3 parts per weight of a foreign polymer, which cannot be mixed with polystyrene, per 100 parts per weight of styrene polymer. The mixture is melted in a twin-screw extruder with screws rotating in the same direction at a temperature of up to 270° C., and 2 to 6 wt.-% of at least one physical foaming agent in relation to the amount of styrene polymer employed is injected into the polymer melt at an injection pressure, which is above the cylinder pressure of the twin-screw extruder at the injection point and is higher than that for dissolving the foaming agent in the polymer, or sufficiently high for making possible a homogeneously distributed, two-phase system of melt/foaming agent. The polymer melt is brought to a material temperature between 160 and 180° C. while being homogeneously mixed and prior to moving out of the twin-screw extruder into the nozzle tool with an annular nozzle, and is extruded under a pressure of up to 55 bar out of the nozzle into the atmosphere in the form of a tube. The tube is cooled on the inside and the outside while expanding and is pulled over a cooling and calibration mandrel, and is cut open after it has become sufficiently firm and is laid flat as the foamed foil.
The injection pressure is preferably in the range between 30 and 160 bar.
Advantageous further embodiments of this invention are taken from the method claims.
A further object of this invention is to provide a device for producing an at least partially open-celled foam foil on the basis of styrene polymers, nucleating agents, surfactants, physical foaming agents and, if required, further additives, such as foreign polymers and auxiliary materials, comprising a twin-screw extruder with a draw-in and plastification zone, mixing zone and cooling zone. The styrene polymers with the nucleating agent, surfactant, additives are conducted, if required at least partially as a pre-mix, to the twin-screw extruder and are melted to form a polymer melt. The physical foaming agent is injected under increased pressure into the polymer melt, the polymer melt already injected with gas is cooled to a temperature suitable for foaming and lying below the melt temperature of the styrene polymer, and thereafter exits into the atmosphere from the nozzle of the twin-screw extruder in the form of a tube while expanding. The tube is cooled and cut open in a longitudinal direction and laid flat as the foamed foil.
In accordance with this invention, the extruder for melting the polymer, mixing the polymer melt with the physical foaming agent and cooling the expandable polymer melt thus obtained is a twin-screw extruder of a length of at least 25 D, in particular 30 to 42 D and having identical parallel screws rotating in the same direction. The physical foaming agent can be introduced into the melted polymer by the conveying device at an injection pressure of less than 160 bar, in particular between 30 to 150 bar, at the end of the draw-in and plastification zone of an approximate length of 10 to 15 D of the twin-screw extruder. The physical foaming agent can be mixed with the molten polymer in the mixing zone of a length corresponding to 3 to 6 D of the twin-screw extruder adjoining the draw-in and plastification zone to form an expandable polymer melt.
Advantageous further embodiments of a device of this invention are taken from features of the dependent claims.
In accordance with this invention, it is possible to create polystyrene foam with an pen-celled foam structure for the passage of gases and fluids, and with a selectively adjustable specific absorption of polar and non-polar fluids. The absorption capability for fluids is a function of the surface tension of the retained fluid. With an initially high surfactant absorption, an initially high wetting effect results, however, the absorption capability for polar fluids rises with a decrease in the concentration of the surfactant at the cell wall. The surfactant is required for absorptive wetting of polar fluids.
The homo-, as well as the copolymers of styrene are employed as the styrene polymer for the foam foils to be produced, as well as mixtures or blends of polystyrene with other substances, as well as high-molecular substances produced by the polymerization of styrene by heat or with peroxide catalysts, such as products made by substance polymerization, as well as the copolymers of styrene with acrylonitrile and/or butadiene, or acrylonitrile and acrylic esters, or the graft polymers, wherein styrene is polymerized with the addition of natural or synthetic caoutchouc, such as styrene butadiene caoutchouc. Styrene homopolymers are preferred for the foam foils in accordance with this invention.
With the method in accordance with this invention, a physical foaming process and the use of twin-screw extruders produce foam foils made of styrene polymers having a proportion of at least 25% of open cells. During this, foam foils with a density of 40 to 100 kg/m
3
or more are obtained, wherein foam foils of a thickness from 0.3 to 10 mm, preferably 1 to 5 mm, are produced. The foam foils preferably have a cell size in the range between 150 &mgr;m to 900 &mgr;m.
In accordance with this invention, an extrusion process with an expansion process in the manner of a blow molding process is employed. The expandable polymer melt, which exits a ring-shaped nozzle of the extruder in the form of a tube, is immediately cooled on the inside and the outside, for example by cooling air, and is charged with pressure on the inside in order to achieve an expansion ratio greater than 1, in p
Berghaus Ulrich
Homer Stephan
Jansen Hubert
Foelak Morton
Pauley Petersen Kinne & Erickson
Reifenhäuser GmbH & Co. Maschinenfabrik
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