FOAM BEAD, MOLDED ARTICLE FORMED OF A PLURALITY OF FOAM BEADS, AND METHOD FOR PRODUCING FOAM BEADS

Abstract

A foam bead intended in particular for the production of molded parts is also intended to be particularly suitable for novel, hitherto unknown applications, especially after processing into a corresponding molded part. For this purpose, the foam bead comprises, according to the invention, a core formed by a first plastic and a shell formed by a second plastic and at least partially surrounding the core, the second plastic forming the shell having a lower melting point than the first plastic forming the core.

Claims

1. A foam bead having a core formed by a first plastic based on polyethylene terephthalate (PET) and having a shell which at least partially surrounds the core and is formed by a second plastic based on polypropylene (PP), the second plastic forming the shell having a melting point at an ambient pressure of about 1 to 5 bar, in particular in a steam atmosphere, which is at least about 100° C. lower than that of the first plastic forming the core.

2. The foam bead of claim 1, in which the second plastic forming the shell is provided with a color additive.

3. The foam bead of claim 1, in which the shell covers at least 65% of the surface of the core.

4. The foam bead of claim 1, in which the shell has a layer thickness of about 0.1 to 0.5 mm.

5. A molded part, in particular for use in/as insulating material for sanitary, heating or ventilation technology (hot water pipe insulation, boilers, castings), insulating material for automotive engineering (sound and heat insulating parts such as e.g.—engine compartment partitions, wheel housings), crash protection for automotive engineering, (bumpers), and/or crash protection for protective equipment (helmets, knee pads), formed from a plurality of the foam beads of claim 1, sintered or welded together on a surface.

6. A method for producing the foam beads of claim 1, in which a melt of the first plastic is mixed with a blowing agent or blowing agent mixture and, if appropriate, additives and is made ready for subsequent extrusion, the extrusion being carried out in a coextrusion in which the foamed first plastic emerging from a perforated nozzle expands and, in the process, is encased by a second plastic emerging from an annular nozzle at least partially surrounding the perforated nozzle, characterized in that the resulting strand of the first plastic, which is sheathed by a film of the second plastic, is fed to a rotary die cutter after cooling and is separated there by squeezing the strand locally towards its center and then separating it.

7. A method for production of the foam beads of claim 1, in which a melt of the first plastic is mixed with a blowing agent or blowing agent mixture and, if appropriate, additives and is then extruded and expanded in the process, characterized in that the resulting plastic foam strand is passed in a sheathing step through a treatment tank in which the second plastic is provided in liquid form.

8. The method of claim 7, wherein the second plastic is provided in the treatment tank at a temperature of at least 140° C.

9. A method for production of the foam beads of claim 1, in which a melt of the first plastic is mixed with a blowing agent or blowing agent mixture and, if appropriate, additives, characterized in that the polymer melt loaded with the blowing agent is passed in a sheathing step through a flow channel in which a melt of the second plastic is provided and flows around the polymer melt loaded with the blowing agent, the sheathed melt formed in the process subsequently being extruded and—expanded in the process.

10. The method of claim 7, in which a resulting strand of the first plastic, sheathed by a film of the second plastic, is fed, after cooling, to a rotary die cutter and is separated there by squeezing the strand locally towards its center and subsequently separating it.

11. The method of claim 6, in which the second plastic is provided with a color additive.

12. The method of claim 1, in which the first plastic coated with the second plastic is subjected to optical quality control, a characteristic number for a proportion of the coating of the first plastic by the second plastic being determined by means of the color additive.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] An embodiment of the invention is explained in more detail with reference to a drawing. Therein show:

[0046] FIG. 1 a foam bead in section,

[0047] FIG. 2 a molded part composed of a plurality of foam beads according to FIG. 1,

[0048] FIG. 3 schematic longitudinal section of a coextrusion die provided for the production of foam beads according to FIG. 1.

[0049] FIGS. 4,5 each a schematic of a system for producing foam beads according to FIG. 1,

[0050] FIG. 6 schematic of a separation station,

[0051] FIG. 7 some variants of foam beads in longitudinal section, and

[0052] FIG. 8 also shows some variants of foam beads in longitudinal section.

[0053] Identical parts are marked with the same reference signs in both figures.

DETAILED DESCRIPTION OF THE DRAWINGS

[0054] The foam bead 1 according to FIG. 1, also referred to as a ball of foamed plastic or also as a “bead”, is suitable and intended for a variety of applications, for example for the production of a molded part 2, as shown in FIG. 2. To produce the molded part 2 from a plurality of such foam beads 1, they are sintered or welded together.

[0055] The foam bead 1 is specifically designed for an extended range of applications in which—if possible with recourse to existing plant and process technologies—novel materials, which for various reasons could not previously be used for these purposes, are also to be made accessible for possible use. In order to make this possible, the foam bead 1 is designed in the manner of a hybrid design with two or more components; it comprises a core 4 formed by a first plastic and, in addition to this, a jacket 6 formed by a second plastic and at least partially surrounding the core 4. This two- or multi-component design means that the core 4 can be designed independently of the jacket 6 in terms of material selection and other properties. This is used in the present case to also be able to use PET as a base material for the provision of foam beads 1.

[0056] In the embodiment example, the core 4 of the foam bead 1 is thus made of a polyester, namely PET. This means that the numerous advantages and favorable properties, in particular the possible integration into the recycling chain, can also be utilized for foam parts. However. PET has a melting point of around 260° C., so that further processing by sintering or welding to produce a molded part 2 would not be possible using conventional plant technology. In fact, conventional molding plants usually operate at working temperatures of about 120° C. to 150° C.; this would not be sufficient to bring about the sintering of the PET foam beads necessary for the production of a molded part 2 from PET beads.

[0057] To remedy this situation, the PET core 4 in foam bead 1 is surrounded by the jacket 6 made of the second plastic. The material is selected in such a way that the second plastic forming the jacket 6 has a significantly lower melting point than the first plastic forming the core 4. In the embodiment example, polypropylene (PP) is provided as the material for the sheath 6; the foam bead 1 is thus a PP-sheathed PET bead. Polypropylene has a melting point of about 160° C. and thus a significantly lower melting point than PET. The two-component structure of foam bead 1 thus ensures that the volume properties (for example elasticity, thermal conductivity, etc.) are imprinted by the material forming the core 4, i.e. PET,—whereas the surface properties relevant for the sintering or welding of the foam beads to one another are imprinted by the PP forming the shell 6. The foam bead 1 is thus a foam bead 1 that essentially consists of PET, but can be treated during further processing and sintering in framework conditions and corresponding plant technology, as is available and already established for EPP beads.

[0058] The molded part 2 obtained by sintering or welding a plurality of foam beads 1 is shown in FIG. 2. The predominantly interlocking connection of adjacent foam beads 1 to one another is achieved via the respective shell 6, with the cores 4 remaining intact. After its manufacture, the molded part 2 thus adopts the matrix structure shown in section in FIG. 2, in which a plurality of foamed plastic bodies 10 corresponding to the original cores 4 and formed from the first plastic, namely PET in this case, are embedded in a matrix 12 formed from the second plastic, namely PP in this case. The matrix 12 is created by the sealing process. The matrix 12 is thereby formed by fusing, sintering or welding the jackets 6 of the foam beads 1 to one another.

[0059] The production of foam beads 1 of the embodiment shown is possible in several ways, of which only two, each considered independently inventive, are explained in more detail by way of example below. The two exemplary embodiments listed have in common that they are based on the extrusion of continuous strands with subsequent separation, for example by cold cutting, in a departure from the water granulation processes actually commonly used in the production of foam beads or beads. The following embodiments are explained by way of example on the basis of the particularly preferred material pairing of PET as the first plastic for forming the cores 4 and polypropylene (PP) as the second plastic for forming the cladding 6, which is considered to be independently inventive; of course, other material pairings are also conceivable and covered by the present invention, depending on the requirements which may be specified specifically for the application.

[0060] In a first embodiment for the production of foam beads 1, a PET melt is first mixed with a blowing agent or blowing agent mixture and, if necessary, additives and then fed to a coextrusion. In particular, the extrusion of foam strands from the PET melt in the density range 100-200 kg/m.sup.3 can first be carried out for this purpose by means of known extrusion technology, for example in the manner of an extrusion first via a twin-screw melting screw with gas flushing and then a downstream single-screw unit for homogenization.

[0061] Subsequently, as shown schematically in FIG. 3, this pre-extruded PET foam strand 20 is then fed into a co-extrusion line driven by a co-extruder. A perforated nozzle 22 surrounded by an annular nozzle 24 is provided. A material flow 26 of polypropylene (PP) is applied to the annular nozzle 24. As soon as the PET foam strand 20 emerges from the perforated nozzle 22, it expands to approximately twice the diameter of the perforated nozzle 22. The PET foam strand 20′ expanded in this way is then coated with a PP film 28 emerging from the perforated nozzle 24 in the form of a tube during coextrusion.

[0062] Immediately after its exit from the perforated nozzle 20, the expanded PET foam strand 20′ still has a comparatively high process temperature. As a result, the PP film 28 is firmly welded to the foamed PET strand 20′ during wrapping. This creates an intimate material bond between the foamed PET forming the core 4 and the PP forming the jacket 6.

[0063] After subsequent cooling, for example in a water bath, the coextruded strands, i.e. the PP-sheathed PET foam strands, are separated by means of a rotary die cutter by locally squeezing the sheathed strand towards its center and then separating it. This results in individual bodies that deviate from an actual cylindrical shape and have pointed or tapered end areas with comparatively small end faces. These individual bodies then form the actual foam beads 1, in which a PP jacket 6 surrounds the foamed PET core 4.

[0064] The concept of an alternative process for the production of foam beads 1 is shown schematically in FIGS. 4 and 5 on the basis of the plant provided for the production in two embodiments. In these embodiments, which are considered to be independently inventive, a PET melt is also first mixed with a blowing agent or blowing agent mixture and optionally additives. The resulting mixture 30 is then extruded in a die 32 and expanded in the process. The resulting PET foam strand 34 is subsequently passed through a treatment basin 36 in a sheathing step. In the treatment basin 36, polypropylene (PP) is provided as the second plastic. The treatment tank 36 is heated and suitably tempered by means of a heating device not shown in more detail, so that the PP in the treatment tank 36 is in the liquid state.

[0065] The process parameters, in particular the process temperature, are selected in such a way that a PP coating 38 is deposited on the surface of the PET foam strand 34 when the PET foam strand 34 is passed through the PP bath. Due to the comparatively high temperatures of the PET foam strand 34 in this phase, the sheathing 38 is firmly welded to the PET foam strand 34 during deposition. In the particularly preferred embodiment, the second plastic is thereby provided in the treatment tank 36 at a temperature of at least 140° C., particularly preferably at least 150° C. This ensures that when a PP bath is used for the second plastic, its melting temperature is safely exceeded.

[0066] Since the molten bath provided in the treatment tank 36 for coating the PET foam strands 34 is kept at a temperature of about 150° C. in order to keep the PP as fluid as possible, the PET foam strands 34 expand in all three spatial directions through contact with the PP as a result of its temperature. Thus, as a side effect of the wrapping step, so-called post-foaming is possible by temperature impact, whereby the air-gas mixture in the cells expands and the plasticized PET plastic yields to the pressure and expands further. This effect is possible in the present process because the PET has not yet become crystalline again directly after extrusion and expands again at temperatures from about 110° C.

[0067] It is particularly advantageous that this type of post-expansion in the melt bath ensures that the bead continues to be fully encased in PP melt, i.e. even during post-expansion. Alternatively, if the bead were first coated with PP and then post-expanded in the oven, the bead would also expand, but the coating would break open and the PP skin would no longer fully cover the surface.

[0068] Subsequently, the PET foam strand 34 provided with the jacket 38 is passed through a water bath 40 for cooling and then separated in a separating station 42 in the manner of a cold cut and cut into cylinders or also separated by squeezing in a rotary die cutter as described above. The individual parts obtained in this way then form the actual foam beads 1, in which a PP jacket 6 surrounds the foamed PET core 4.

[0069] As can be clearly seen from the illustration in FIG. 5, in this embodiment the PET foam strand 34 provided with the jacket 38 is directed out of the treatment tank 36 in a substantially vertical direction indicated by the arrow 44. This aspect is particularly preferred and is considered to be independently inventive, because this orientation causes excess residual amounts of the jacket material (PP) still on the surface of the foam strand 34 to be automatically conveyed by gravity back into the treatment basin 36, where they are available for further coating.

[0070] In this variant, or optionally also in the variant described above, it is intended to mix a dye additive into the plastic provided for forming the shell 6, so that the shell 6 has a correspondingly predeterminable coloring. This makes subsequent quality control possible in a particularly simple manner by optical means, whereby, for example, with the aid of the colored identification it can be recognized how much of the surface of the core 4 is covered by the jacket 6, whether the covering is continuous and the like. In the embodiment example according to FIG. 5, this check is automated, with a corresponding detection system 50 being provided in the area of the separating station 42. In the embodiment example, color coding is provided in the color spectrum not visible to the human eye, for example in the ultraviolet by means of a UV tracer.

[0071] As mentioned above, the separating station 42 could, for example, be designed in the manner of a conventional cold cut. This results in foam beads 1 with an essentially cylindrical basic shape. With regard to the respective intended application, the geometry parameters for the core 4 and the shell 6 in particular are advantageously selected to be suitable in each case. Some particularly preferred examples of embodiments for the jacketed PET foam beads in cylindrical form produced by one of the aforementioned processes and forming the foam beads 1 are shown in FIG. 6, in each case both in transverse and in longitudinal section. The foam bead 1 shown in FIG. 6a has a length L of 3.0 mm and a core diameter of 3.0 mm; the shell thickness d is 0.25 mm. In contrast, foam bead 1′ according to FIG. 6b has a length L of 4.0 mm, a core diameter of 3.0 mm and a shell thickness d of 0.1 mm, and foam bead 1″ according to FIG. 6c has a length L of 4.0 mm, a core diameter of 2.0 mm and a shell thickness d of 0.5 mm. The geometry parameters of these particularly preferred embodiments are summarized in the following table:

TABLE-US-00001 Foam bead 1 Foam bead 1′ Foam bead 1″ Core diameter 3.0 mm 3.0 mm 2.0 mm Length 3.0 mm 4.0 mm 4.0 mm Total surface core 42.4 mm.sup.2 51.8 mm.sup.2 31.4 mm.sup.2 Overlap: Proportion of 66.7% 72.7% 80.0% shell surface Sheath thickness 0.25 mm 0.1 mm 0.5 mmm Mass core 2.1 mg 2.8 mg 1.3 mg Mantle mass 0.7 mg 0.4 mg 1.3 mg Mass ratio shell/core 33.3% 13.3% 100%

[0072] In an embodiment regarded as independently inventive, however, instead of the cylindrical body shape, a body shape tapering at the ends is provided as the basic shape for the foam bead 1. Such a foam bead 1′″ shown in longitudinal section in FIG. 7 has, in deviation from an actual cylindrical shape, pointed or conically tapering end regions 52 with comparatively small end faces 54. This ensures that the exposed surface parts of the foamed PET core 4 not surrounded by the PP sheathing 6 can be kept particularly small. This is particularly favorable for the sintering process envisaged later, since the portion available for this purpose formed by the jacket 6 is then particularly large.

[0073] Such a body shape could be produced in the separating station 42, for example, by suitably shaped and contoured cutting knives 56, as shown schematically in FIG. 8. The cutting knives 56 each have a concave cutting edge 58. In the open position of the cutting knives 56 (FIG. 8a), the sheathed strand 34 can then be introduced into the opening 60 between the cutting edges 58. When the desired length is cut, the cutting blades 56 are then moved toward each other so that the opening 60 closes (FIG. 8b). In the process, the strand 34 is squeezed at the desired location and finally separated, creating the desired body shape due to the material deformation.

[0074] Quite preferably and in an embodiment regarded as independently inventive, however, the separating station 42, in particular for producing the body shape mentioned, is designed in the manner of a rotary die cutter 62, which is assumed to be known per se, as shown in sketch form in FIG. 9 on the basis of its essential components. In the embodiment example, the rotary die cutter 62 comprises two rotary elements 64 arranged at a distance from one another, which in the embodiment example are preferably designed as rotary rollers. A number of punching elements, in particular punching grooves 66, are arranged on their outer cylinder surface. In particular, the rotary rolls 64 can be operated synchronized with one another in such a way that in each case a punching groove 66 of a first rotary roll 64 assumes its reversal point in the gap formed by the roll spacing at the same time as an associated punching groove 66 of the second rotary roll 64, so that when the rolls 64 rotate, the clear width of this gap assumes a minimum value.

[0075] The PET foam strand 34 provided with the sheathing 38 is passed through this gap, where it is squeezed locally towards its center by the punching grooves 66, which cooperate in pairs, and thus separated. As a result of the shape of the punching grooves and the arrangement, individual bodies with the desired body shape are produced. These individual bodies, as shown in enlarged form in FIG. 7, then form the actual foam beads 1″″, in which the PP jacket 6 surrounds the foamed PET core 4.

[0076] The PET foam cylinders with PP coating 38, which form the foam beads 1 and are produced by one of the two processes described above—or by another process—can then be stored temporarily and, if necessary, loaded with ambient air under pressure, depending on the requirements of the application. The foamed cylindrical granules can then later be further processed using normal, conventional EPP plant technology. In particular, the steam pressure and machine technology is sufficient to expand the PET foam material and, at the same time, to obtain a well-welded particle foam block or a well-welded molded part 2 due to the welding in the PP sheathing 38.

[0077] The design of foam bead 1 as a PP-sheathed PET bead allows, among other things, the following scope for subsequent application, thus expanding the range of applications overall:

[0078] The foamed cylindrical raw material can also be produced in relatively high densities in the range of 100-200 kg/m.sup.3, depending on requirements.

[0079] If necessary, the PET foam core 4 can be pre-expanded using a pre-expansion process at temperatures of about 120° C. without the PP jacket 38 causing welding of the foam beads 1.

[0080] The foam beads 1 can be temporarily stored like conventional EPS or EPP beads and, if necessary, loaded with compressed air.

[0081] The foam beads 1 can be fed to conventional EPP molding equipment and processed there, in particular foamed and welded with steam in the temperature range of about 150° C.

[0082] Overall, the EPP foam industry will be able to process E-PET with existing technology and open up new areas of application.

LIST OF REFERENCE SIGNS

[0083] 1 Foam bead [0084] 2 Mold part [0085] 4 Core [0086] 6 Coat [0087] 10 Plastic body [0088] 12 Matrix [0089] 20,20′ Foam strand [0090] 22 Hole nozzle [0091] 24 Ring nozzle [0092] 26 Material flow [0093] 28 Slide [0094] 30 Mixture [0095] 32 Nozzle [0096] 34 Foam strand [0097] 36 Treatment basin [0098] 38 Sheathing [0099] 40 Water bath [0100] 42 Single station [0101] 44 Arrow [0102] 50 Recognition station [0103] 52 End range [0104] 54 Face [0105] 56 Cutting knife [0106] 58 Cutting edge [0107] 60 Opening [0108] 62 Rotary punch [0109] 64 Rotation elements [0110] 66 Punching grooves [0111] L Length [0112] D diameter [0113] d Sheath thickness