Patent Description:
Expanded polypropylene beads (EPP beads) are foamed particles. The shape of EPP beads is quasi-spherical. The diameter of polypropylene pellets prior to foaming, is typically in the range from <NUM> to <NUM> while the diameter of EPP beads is in the range of <NUM> to <NUM>. The degree of foaming can be described by the foaming ratio which is the volume ratio between the EPP beads and polypropylene pellets prior to foaming.

Currently, the vast majority of commercially available EPP beads are prepared via an autoclave process. Such autoclave process is for example described in the examples of <CIT>.

However, the main disadvantage of the autoclave process is that it is a batch process, which requires loading and unloading of the autoclave/reactor, which is time consuming and laborious. Therefore, there is a need for a process which enables continuous production of EPP beads and is less time consuming and laborious. Such a process is a bead foam extrusion process which uses an underwater granulator. <CIT> describes such a process and device for implementing the method. <CIT> discloses a method for producing a foamed granulate, wherein a thermoplastic synthetic material is placed in an extruder, the synthetic material is melted, a pressurized expanding agent is fed through one or several injection nozzles, the molten material enriched with the expanding medium is foamed as it exits through a perforated plate arranged at the outlet of the extruder and is granulated by a cutting device arranged behind the perforated plate.

EPP beads can be used to produce an article in a steam molding process. Steam molded articles made from EPP beads are known for use in various fields such as automotive, building and construction, furniture and toys due to their unique combination of properties e.g. good durability, light weight, excellent heat insulation, good chemical resistance, a good balance between tensile strength, impact resistance and compression strength and they also have excellent recycling properties.

However, the compositions currently used for the preparation of EPP beads using an autoclave process are not suitable for the preparation of EPP beads via a bead foam extrusion process, as the latter compositions cannot be foamed (enough) in a bead foam extrusion process.

The current EPP beads prepared via a bead foam extrusion process, for example the EPP beads as decribed in <CIT> and <CIT>, are not very suitable for the production of articles in a steam molding process, as these beads have at least one of the following disadvantages in a steam molding process: they show excessive shrinkage of the molded article in the mold, they show collapse of the molded article, they have a narrow molding window, they require a high steam pressure for the preparation of the molded article.

Therefore, it is an object of the invention to provide a composition suitable for a process for the production of expanded polypropylene beads via a bead foam extrusion process, which EPP beads are suitable for conversion into an article via a steam molding process.

This object is achieved by a polymer composition comprising.

It has been found that with the polymer composition of the invention, it is possible to produce expanded polypropylene beads via a bead foam extrusion process. The expanded polypropylene beads could then suitably be converted in a steam molding process.

<CIT> discloses a mineral-filled polypropylene composition comprising.

<CIT> discloses an inherently open-celled polymeric foam consisting essentially of a polymer blend having cells with an average cell size of at least four millimeters defined therein wherein the foam has an inherently open-cell content of at least <NUM> percent (according to American Society for Testing and Materials (ASTM) D2856-<NUM>) and wherein the polymer blend consists essentially of:.

wherein the polymer blend contains <NUM> weight percent (wt %) or less of (c) based on polymer blend weight; and wherein (a) makes up at least <NUM> wt % and <NUM> wt % or less of the total weight of (a) and (b).

The molded products prepared from the expanded polypropylene beads showed sufficient fusion, no excessive shrinkage and/or no collapse.

Furthermore, it was found that the polymer compositions of the invention can be used to produce expanded polypropylene beads that can be molded into a molded article in a steam molding process with a broad molding window.

The steam molding process is an energy consuming process. It was also found that by using the (polymer compositions of the) invention, it is possible to lower the energy consumption by using lower steam pressures. This is advantageous from an energy/environmental as well as a commercial point of view.

Surprisingly, if was found that molded articles prepared from the polymer compositions of the invention could be molded from expanded polypropylene beads using lower steam pressures than the polymer compositions, known in the art while the molded articles had a similar or higher compression strengths.

In the context of the invention, with 'foamed' or 'foam' or 'expanded' is meant that the shape has a lower density due to the presence of gas bubbles (such as air) as compared to the density of the same material without gas bubbles.

Preferably, in the polymer composition of the invention, the sum of the amount of the high melt strength polypropylene, the polyethylene, the first polypropylene and the second polypropylene is ≥ <NUM> wt%, more preferably ≥ 85wt%, more preferably ≥90wt%, most preferably ≥ 95wt% based on the polymer composition.

In the polymer composition, the sum of the high melt strength polypropylene, the first polypropylene and the second polypropylene is ≥ 60wt%, for example ≥ 62wt%, for example ≥ 64wt%, for example ≥ 65wt%, for example ≥ 66wt%, preferably ≥ 67wt%, for example ≥ 68wt%, for example ≥ 69wt%, for example ≥ 70wt% based on the sum of the weight of the high melt strength polypropylene, the polyethylene, the first polypropylene and the second polypropylene.

Preferably, the polymer composition has a melt flow rate ≥ <NUM> and ≤ <NUM>/<NUM>, more preferably ≥ <NUM> and ≤ <NUM>/<NUM>, most preferably ≥ <NUM> and ≤ <NUM>/<NUM> as determined in accordance with ASTM D1238 (<NUM>) at a temperature of <NUM> under a load of <NUM>.

In the polymer composition of the invention, the polyethylene has a density ≥ <NUM>/m<NUM> and ≤ <NUM>/m<NUM>, preferably has a density ≥ <NUM>/m<NUM> and ≤ <NUM>/m<NUM>, more preferably has a density ≥ <NUM>/m<NUM> and ≤ <NUM>/m<NUM> as determined according to ISO <NUM> (<NUM>).

Such polyethylenes, also referred to herein as high density polyethylene or HDPE may be obtained either by a gas phase process, a slurry process or a solution process.

The production processes of HDPE are known and are for example summarized in "<NPL>. Suitable catalysts for the production of polyethylene include Ziegler Natta catalysts, chromium based catalysts and single site metallocene catalysts. The various processes may be divided into solution polymerisation processes employing homogeneous (soluble) catalysts and processes employing supported (heterogeneous) catalysts. The latter processes include both slurry and gas phase processes.

The ethylene polymer may be produced using ethylene as the sole monomer, or may be produced using ethylene and one or more α-olefin comonomers. In case the ethylene polymer is produced using ethylene as the sole monomer, the ethylene polymer is an ethylene homopolymer. In case the ethylene polymer is produced using ethylene and one or more α-olefin comonomers, such as propylene, <NUM>-butene, <NUM>-hexene and <NUM>-octene, preferably <NUM>-butene and/or <NUM>-hexene, the ethylene polymer is an ethylene copolymer.

The polyethylene is present in the polymer composition in an amount ≥ <NUM> wt% and ≤ <NUM> wt%, preferably in an amount ≥ <NUM> wt% and ≤ <NUM> wt%, for example in an amount ≥ <NUM> wt% and ≤ <NUM> wt%, most preferably in an amount ≥ <NUM> wt% and ≤ <NUM> wt% based on the polymer composition.

Preferably, the polyethylene has a melt flow rate ≥ <NUM> and ≤ <NUM>/<NUM>, preferably has a melt flow rate ≥ <NUM> and ≤ <NUM>/<NUM>, more preferably has a melt flow rate ≥ <NUM> and ≤ <NUM>/<NUM> as determined in accordance with ASTM D1238 (<NUM>) at a temperature of <NUM> under a load of <NUM>.

High melt strength polypropylenes are available in the art. Typically such polypropylenes are branched polypropylene. A branched polypropylene differs from a linear polypropylene in that the polypropylene backbone has side chains, whereas a non-branched (linear) polypropylene does not have side chains on its backbone. There are different ways to achieve branching in polypropylenes. For example, branching can be achieved by using a specific catalyst, for example a specific single-site catalyst, or by chemical modification. <CIT> described the preparation of a branched polypropylene obtained by the use of a specific catalyst. <CIT> describes the preparation of a branched polypropylene by chemical modification.

<CIT> discloses an irradiated polymer composition comprising at least one polyolefin resin and at least one non-phenolic stabilizer, wherein the irradiated polymer composition is produced by a process comprising mixing the polyolefin resin with the non-phenolic stabilizer and irradiating this mixture in a reduced oxygen environment. By using the process of <CIT>, branched polypropylenes can also be obtained.

Examples of commercially available high melt strength polypropylene include but are not limited to Daploy™ polypropylenes available from Borealis and Bourouge, e.g. Daploy™ WB 135HMS, Daploy™ 135HMS or Daploy™ WB260HMS.

In addition, a high melt strength polypropylene is available from SABIC as SABIC® PP UMS 561P as of <NUM> February <NUM>.

Preferably, the high melt strength polypropylene is prepared by.

How to deactivate the free radicals is known in the art, for example by heating as described in <CIT>.

Examples of non-phenolic stabilizers are known in the art and are for example disclosed on pages <NUM> - <NUM> of <CIT>, hereby incorporated by reference. Preferably, the non-phenolic stabiizer is chosen from the group of hindered amines.

More preferably, the non-phenolic stabilizer comprises at least one hindered amine selected from the group of Chimassorb® <NUM>, Tinuvin® <NUM>, Chimassorb® <NUM>, Chimassorb® <NUM>, Tinuvin® <NUM>, and mixtures thereof, separate or in combination with at least one hydroxylamine, nitrone, amine oxide, or benzofuranone selected from N,N-di(hydrogenated tallow)amine (Irgastab® FS-<NUM>), an N,N- di(alkyl)hydroxylamine produced by a direct oxidation of N,N-di(hydrogenated tallow)amine (Irgastab® FS-<NUM>), N-octadecyl-α-heptadecylnitrone, Genox™ EP, a di(C16 -C18 )alkyl methyl amine oxide, <NUM>-(<NUM>,<NUM>-dimethylphenyl)-<NUM>,<NUM>-di-tert-butyl-benzofuran-<NUM>-one, Irganox® HP-<NUM> (BFI), and mixtures thereof, and separate or in combination with at least one organic phosphite or phosphonite selected from tris(<NUM>,<NUM>-di-tert-butylphenyl) phosphite (Irgafos® <NUM>). Even more preferably, the non-phenolic stabilizers of the present subject matter can include those described in <CIT> and <CIT>, both of which are incorporated herein by reference in their entirety.

Preferably, the high melt strength polypropylene has a melt strength ≥ <NUM> cN and ≤ <NUM> cN, wherein the melt strength is determined in accordance with ISO <NUM>:<NUM> at a temperature of <NUM>, using a cylindrical capillary having a length of <NUM> and a width of <NUM>, a starting velocity v<NUM> of <NUM>/s and an acceleration of <NUM>/s<NUM>.

Preferably, the high melt strength polypropylene has a melt strength ≥ <NUM> cN , preferably ≥ 20cN, more preferably ≥ 30cN, more preferably ≥ 40cN, more preferably ≥ 45cN. Even more preferably, the melt strength of the high melt strength polypropylene is ≥ <NUM> cN, more preferably ≥ <NUM> cN, even more preferably ≥ <NUM> cN, most preferably ≥ <NUM> cN and/or preferably the melt strength of the high melt strength polypropylene composition is ≤ <NUM> cN, for example ≤ <NUM> cN, for example ≤ 87cN.

The melt strength of the high melt strength polypropylene is determined in accordance with ISO <NUM>:<NUM> at a temperature of <NUM>, using a cylindrical capillary having a length of <NUM> and a width of <NUM>, a starting velocity v0 of <NUM>/s and an acceleration of <NUM>/s2.

The high melt strength polypropylene is present in the polymer composition in an amount of ≥ <NUM> wt% and ≤ <NUM> wt% based on the polymer composition. For example, the amount of high melt strength polypropylene is ≥ 14wt%, for example ≥ 20wt%, preferably ≥ 27wt% and/or ≤ 55wt%, for example ≤ <NUM> wt%, for example ≤ 45wt%, preferably ≤ 40wt%. Preferably, the high melt strength polypropylene is present in the polymer composition in an amount of ≥ <NUM> wt% and ≤ <NUM> wt% based on the polymer composition, more preferably in an amount of ≥ <NUM> wt% and ≤ <NUM> wt% based on the polymer composition.

The high melt strength polypropylene is preferably present in the polymer composition in an amount ≥ <NUM> wt% based on the polymer composition.

With polypropylene as used herein is meant propylene homopolymer, a copolymer of propylene with an α-olefin or a heterophasic propylene copolymer.

Preferably, the high melt strength polypropylene is a polypropylene chosen from the group of propylene homopolymers and propylene copolymers comprising moieties derived from propylene and one or more comonomers chosen from the group of ethylene and alpha-olefins with ≥ <NUM> and ≤ <NUM> carbon atoms.

Preferably, the propylene copolymer comprises moieties derived from one or more comonomers chosen from the group of ethylene and alpha-olefins with ≥ <NUM> and ≤ <NUM> carbon atoms in an amount of ≤ 10wt%, for example in an amount of ≥ <NUM> and ≤ <NUM>. 0wt% based on the propylene copolymer, wherein the wt% is determined using <NUM> C NMR. For example, the propylene copolymer comprises moieties derived from one or more comonomer chosen from the group of ethylene, <NUM>-butene, <NUM>-pentene, <NUM>-hexene, <NUM>-methyl-<NUM>-pentene, <NUM>-heptene, <NUM>-octene, <NUM>-decene and <NUM>-dodecene, preferably moieties derived from ethylene.

Polypropylenes and the processes for the synthesis of polypropylenes are known. A propylene homopolymer is obtained by polymerizing propylene under suitable polymerization conditions. A propylene copolymer is obtained by copolymerizing propylene and one or more other comonomers, for example ethylene, under suitable polymerization conditions. The preparation of propylene homopolymers and copolymers is for example described in <NPL>.

Propylene homopolymers, propylene copolymers and heterophasic propylene copolymers can be made by any known polymerization technique as well as with any known polymerization catalyst system. Regarding the techniques, reference can be given to slurry, solution or gas phase polymerizations; regarding the catalyst system reference can be given to Ziegler-Natta, metallocene or single-site catalyst systems. All are, in themselves, known in the art.

In the polymer composition of the invention, the largest difference between the highest melting temperature (Tm) of the high melt strength polypropylene (Tmelt HMS) and the highest melting temperature of a polypropylene (Tmelt PP) is ≥ <NUM> and ≤ <NUM>, preferably ≥ <NUM> and ≤ <NUM>, more preferably ≥ <NUM> and ≤ <NUM>, wherein the melting temperatures are determined using a differential scanning calorimeter on the second heating cycle using a heating rate of <NUM>/min and a cooling rate of <NUM>/min. This has the advantage that the steam pressure needed in a steam-molding process to prepare articles produced from expanded beads comprising the polymer composition of the invention can be reduced.

In case - apart from the high melt strength polypropylene, there are two or more further polypropylenes present in the composition of the invention, the largest difference in melting temperatures should be taken.

For example, in case two polypropylenes are present in the polymer composition of the invention, wherein one polypropylene has a Tm of <NUM> and the other polypropylene has a Tm of <NUM> and the high melt strength polypropylene has a Tm of <NUM>, the difference Tmelt UMS and T melt PP1 is <NUM> - <NUM> = <NUM> (and not <NUM> - <NUM> = <NUM>).

Preferably, the high melt strength polypropylene has a VOC value as determined in accordance with VDA278 (<NUM>-<NUM>) ≤ <NUM>µg/g, preferably a VOC value ≤ <NUM>µg/g and/or an FOG value as determined in accordance with VDA278 (<NUM>-<NUM>) ≤ <NUM>µg/g, preferably an FOG-value ≤ <NUM>µg/g.

Preferably, the high melt strength polypropylene has a melt flow rate ≥ <NUM> and ≤ <NUM>/<NUM>, more preferably ≥ <NUM> and ≤ <NUM>/<NUM>, most preferably ≥ <NUM> and ≤ <NUM>/<NUM> as determined in accordance with ASTM D1238 (<NUM>) at a temperature of <NUM> under a load of <NUM>.

The polymer composition may further comprise additives, such as for example flame retardants, pigments, lubricants, slip agents flow promoters, antistatic agents, processing stabilizers, long term stabilisers and/or UV stabilizers. The additives may be present in any desired amount to be determined by the man skilled in the art, but are preferably present ≥ <NUM> wt% and ≤ <NUM> wt%, more preferably ≥ <NUM> wt% and ≤ <NUM> wt%, even more preferably ≥ <NUM> wt% and ≤ <NUM> wt%, even more preferably ≥ <NUM> wt% and ≤ <NUM> wt% based on the polymer composition.

Preferably, the first polypropylene is chosen from the group of propylene homopolymers and propylene copolymers and the second polypropylene is chosen from the group of propylene homopolymers, propylene copolymers and heterophasic propylene copolymers, preferably wherein the second polypropylene is a propylene homopolymer.

For example, the first polypropylene may have a melt flow rate ≥ <NUM> and ≤ <NUM>/<NUM> as determined in accordance with ASTM D1238 (<NUM>) at a temperature of <NUM> under a load of <NUM>.

For example, the second polypropylene may have a melt flow rate ≥ <NUM> and ≤ <NUM>/<NUM> as determined in accordance with ASTM D1238 (<NUM>) at a temperature of <NUM> under a load of <NUM>.

Preferably, the sum of the amount of the first and the second polypropylene is ≥ <NUM> and ≤ <NUM> wt% based on the polymer composition.

The polymer composition may further comprise a nucleating agent. A nucleating agent may be desired to increase the cell density and to modify the dynamics of bubble formation and growth.

The amount of nucleating agent may for example be ≥ <NUM> wt% and ≤ <NUM> wt%, for example ≥ <NUM> wt% and ≤ <NUM> wt%, for example ≥ <NUM> wt% and ≤ <NUM> wt%, preferably ≥ <NUM> wt% and ≤ <NUM> wt%, more preferably ≥ <NUM> wt% and ≤ <NUM> wt% based on the polymer composition, most preferably ≥ <NUM> wt% and ≤ <NUM>. 2wt% based on the polymer composition.

Suitable nucleating agents include but are not limited to talc, silica and a mixture of sodium bicarbonate and citric acid. Other suitable nucleating agents include amides, for example azo dicarbonamide, amines and/or esters of a saturated or unsaturated aliphatic (C<NUM>-C<NUM>) carboxylic acid.

Examples of suitable amides include fatty acid (bis)amides such as for example stearamide, caproamide, caprylamide, undecylamide, lauramide, myristamide, palmitamide, behenamide and arachidamide, hydroxystearamides and alkylenediyl-bis-alkanamides, preferably (C<NUM>-C<NUM>) alkylenediyl-bis-(C<NUM>-C<NUM>) alkanamides, such as for example ethylene bistearamide (EBS), butylene bistearamide, hexamethylene bistearamide, ethylene bisbehenamide and mixtures thereof. Suitable amines include or instance (C<NUM>-C<NUM>) alkylene diamines such as for example ethylene biscaproamine and hexamethylene biscaproamine Preferred esters of a saturated or unsaturated aliphatic (C<NUM>-C<NUM>) carboxylic acid are the esters of an aliphatic (C<NUM>-C<NUM>) carboxylic acid.

Preferably, the nucleating agent is chosen from the group of talc, sodium bicarbonate, citric acid, azodicarbonamide and mixtures thereof, more preferably the nucleating agent is talc.

For the preparation of the expanded polypropylene beads, it may be desired to use a cell stabilizer. Therefore, the invention also relates to a polymer composition of the invention further comprising a cell stabilizer. Cell stabilizers are permeability modifiers which retard the diffusion of for example hydrocarbons such as isobutane to create dimensionally stable foams. (<NPL>) Preferred cell stabilizers include but are not limited to glycerol monostearate (GMS), glycerol monopalmitate (GMP), palmitides and/or amides. Suitable amides are for example stearyl stearamide, palmitide and/or stearamide. Suitable mixtures include for example a mixture comprising GMS and GMP or a mixture comprising stearamide and palmitamide. Preferably, in case a cell stabilizer is used, the cell stabilizer is glycerol monostearate or stearamide.

The amount of cell stabiliser to be added depends on desired cell size and the polymer composition used for the preparation of the expanded polypropylene beads. Generally, the cell stabiliser may be added in an amount ≥ <NUM> and ≤ <NUM> wt %.

The polymer composition of the invention can suitably be used for the preparation of expanded polymer beads. Therefore, in another aspect, the invention relates to expanded polypropylene beads comprising the polymer composition of the invention.

Preferably, the polymer composition of the invention is present in an amount ≥ <NUM> wt% , for example ≥ 96wt%, for example ≥ 97wt%, preferably ≥ 98wt%, for example ≥99wt%, based on the expanded polypropylene beads. Most preferably, the expanded polypropylene beads consist of the polymer composition of the invention.

In another aspect, the invention relates to a process comprising the sequential steps of:.

Processes for the preparation of expanded polypropylene beads are known in the art and include autoclave and extrusion processes. Preferably, the expanded polypropylene beads of the invention are obtained by a foam extrusion process, which process is known per se.

For example, <CIT> describes such a process and device for implementing the method. <CIT> discloses a method for producing a foamed granulate, wherein a thermoplastic synthetic material is placed in an extruder, the synthetic material is melted, a pressurized expanding agent is fed through one or several injection nozzles, the molten material enriched with the expanding medium is foamed as it exits through a perforated plate arranged at the outlet of the extruder and is granulated by a cutting device arranged behind the perforated plate.

Preferably, the density of the expanded polypropylene beads is ≥ <NUM> and ≤ <NUM>/m<NUM>, preferably ≥ <NUM> and ≤ <NUM>/m<NUM>, for example ≥ <NUM> and ≤ <NUM>/m<NUM>, for example ≥ <NUM> and ≤ <NUM>/m<NUM>.

Preferably, the expanded polypropylene beads have an open cell content of ≤ <NUM> %, preferably ≤ <NUM> %, more preferably ≤ <NUM>%, for example ≤ <NUM>%, for example ≤ <NUM>% wherein the open cell content is determined according to ASTM D6226-<NUM>.

The expanded polypropylene beads can be molded into an article using a molding process known per se, for example by a steam molding process. Therefore, in another aspect, the invention also relates to a molded article comprising the expanded polypropylene beads of the invention.

In another aspect, the invention relates to the use of the polymer composition of the invention in a foam extrusion process. In another aspect the invention relates to the use of the polymer composition of the invention for the preparation of expanded polypropylene beads. In yet another aspect, the invention relates to the use of the expanded polypropylene beads of the invention to prepare molded articles, for example for molded articles for use in the field of automotive, building and construction, furniture or toys.

It is noted that the invention relates to the subject-matter defined in the independent claims alone or in combination with any possible combinations of features described herein, preferred in particular are those combinations of features that are present in the claims. It will therefore be appreciated that all combinations of features relating to the composition according to the invention; all combinations of features relating to the process according to the invention and all combinations of features relating to the composition according to the invention and features relating to the process according to the invention are described herein.

It is further noted that the term 'comprising' does not exclude the presence of other elements. However, it is also to be understood that a description on a product/composition comprising certain components also discloses a product/composition consisting of these components. The product/composition consisting of these components may be advantageous in that it offers a simpler, more economical process for the preparation of the product/composition. Similarly, it is also to be understood that a description on a process comprising certain steps also discloses a process consisting of these steps. The process consisting of these steps may be advantageous in that it offers a simpler, more economical process.

When values are mentioned for a lower limit and an upper limit for a parameter, ranges made by the combinations of the values of the lower limit and the values of the upper limit are also understood to be disclosed.

The invention is now elucidated by way of the following examples, without however being limited thereto.

The melt flow rate of the polymers was determined in accordance with ASTM D1238 (<NUM>) at a temperature of <NUM> (MFR<NUM>) or <NUM> (MFR<NUM>) under a load of <NUM>.

The density of the polymers was determined in accordance with ASTM D792 (<NUM>).

The amount of comonomer in the polymer was determined using <NUM> C NMR.

The melting temperature of the polymers was measured using a differential scanning calorimeter (DSC, TA Instruments Q20) on the second heating cycle using a heating rate of <NUM>/min and a cooling rate of <NUM>/min and a temperature range of -<NUM> to <NUM>.

In case a the polymers has multiple melting temperatures, the highest melting temperature is reported as the melting temperature (Tm).

Difference Tmelt UMS and T melt PP1 is the largest difference between the Tm of the high melt strength polypropylene and the Tm of the other polypropylenes present in the polymer composition. For example, in case two polypropylenes are present, wherein one polypropylene has a Tm of <NUM> and the other polypropylene has a Tm of <NUM> and the high melt strength polypropylene has a Tm of <NUM>, the difference Tmelt UMS and T melt PP is <NUM> - <NUM> = <NUM>.

Melt strength was measured according to ISO standard <NUM>:<NUM>. Melt strength is defined as the maximum (draw-down) force (in cN) by which a molten thread can be drawn before it breaks, e.g. during a Rheotens measurement. Measurements were done on a Göttfert Rheograph <NUM> at a temperature of <NUM> with a setup like shown in figure <NUM> of ISO standard <NUM>:<NUM>. The rheometer has an oven with a diameter of <NUM>. A capillary of <NUM> length and <NUM> width was used. The entrance angle of the capillary was <NUM> ° (flat). The piston in the rheometer moved with a velocity of <NUM>/s to obtain an exit velocity v0, of <NUM>/s. After filling the rheometer, the melt was held in the rheometer for <NUM> minutes, to stabilize the temperature and fully melt the polymer. The strand that exits the capillary was drawn with a Rheotens II from Goettfert with an acceleration of <NUM>/s<NUM> until breakage occurred. The distance between the exit of the capillary and the uptake wheels of the Rheotens II (= draw length) was <NUM>.

The pressure required to push the melted polymer through the capillary, the maximum drawing force (= Melt strength) and the maximum draw ratio at breakage were recorded.

The bulk density was determined by using a measuring beaker with an effective volume of 1liter. The beaker was filled with expanded polypropylene beads to the top and the weight was measured, resulting value is the bulk density in g/l.

Density of the molded article (kg/m<NUM>) is the apparent overall density and was determined according to ISO <NUM>:<NUM>.

To this end, first the weight (m) of a handful expanded polypropylene beads was determined with a scale in air. Secondly, the handful of the expanded polypropylene beads were placed into a perforated metal cage. The volume (V) of the expanded polypropylene beads was determined underwater by measuring buoyancy force using a scale. The (buoyancy) V is directly related with the geometric volume of the sample. The foam density can be calculated using following equation: <MAT>.

The open cell content was determined by using a Quantachrome Pentapyc 5200e gas pycnometer using a method based on ASTM D6226-<NUM>. The volume from the external dimensions of the sample was determined by using the Archimedes' principle as described for the determination of the volume of the expanded polypropylene beads above. It was assumed that the uptake of water by the sample can be neglected. After drying the sample from adhering water, the sample volume (VSPEC) was determined by the pycnometer according ASTM D6226-<NUM> at different pressures.

All applied pressures were below <NUM> bar to minimize compression of the foam. <MAT> wherein:.

The sample volume of the foam was plotted against the applied pressures (<NUM> bar; <NUM> bar, <NUM> bar, <NUM> bar, <NUM>. 035bar, <NUM> bar and <NUM> bar). A straight line was fit through the measurement points, using linear regression. The interception of the linear regression line with the Y-axis at p = <NUM> bar is the volume (VSPEC_0) used in equation below.

The open cell content Vopen (%) was calculated using the following formula: <MAT> wherein:.

The evaluation of extrusion foaming was determined based on the processing observations. During cooling of the melt the viscosity increases resulting into a pressure build-up at the die. When die pressures reached pressure > <NUM> bar, the processing was considered poor and a `-` was noted for 'extrusion processing'. If die pressures were ≤ <NUM> bar, the extrusion processing was noted as '+ `.

With the use of a sharp razorblade a cut of approximately <NUM> in depth was made over the width of the obtained molded article. The product was broken along the precut and the resulted surface area was evaluated. The ratio of the number of beads that were broken through the beads to the total number of all beads observed at the surface area was calculated (= rate of fusion). Whenever the rate of fusion was more than <NUM>% the fusion was regarded as acceptable.

The minimal steam pressure needed for reaching a rate of fusion > <NUM>% was noted in Table as Steam pressures [bar] (><NUM>% fusion).

The compression strength was determined with the use of a ZwickRoell tensile testing machine using ISO844:<NUM>. The skin layer of the molded article was removed and samples of 50x50x50mm were taken. 1N pre-load was applied before measurement. During the measurement, the applied compression speed was <NUM>/min was until a compression of <NUM>% was reached. The compression stress at <NUM>%, <NUM>% and <NUM>% compression was recorded and the value obtained was reported as compression strength.

The properties of the polypropylenes used as listed below in Table <NUM>. The properties of the polyethylenes are listed below in Table <NUM>. All polypropylenes and polyethylenes are commercially available from SABIC. SABIC® PPUMS 561P is commercially available as of <NUM> February <NUM> without confidentiality restrictions.

Talc: stands for POLYBATCH® FPE <NUM> T, which is a <NUM>% masterbatch of talcum in polyethylene and which is commercially available fom LyondellBasell.

GMS stands for Atmer™ <NUM><NUM>%MB, which is a <NUM>% concentrate in polyethylene containing an anti-static agent (glycerol monostearate) and which is commercially available from Croda.

PBA: stands for physicial blowing agent. In the below foaming experiments, iso-butane was used as physical blowing agent.

For examples E1 - E4 and comparative examples CE1 - CE4, the polymers and ingredients were dosed in amounts as indicated in Table <NUM>. in a co-rotating twin-screw extruder. The extruder was a <NUM> double screw foam extruderfrom Theysohn having a length over diameter ratio (l/d) of <NUM>. This extruder consists of nine electrical heating zones equipped with water cooling followed by a cooling section a static mixer and an underwater pelletizing system. The polymer, talc and GMS were dosed at the start of the extruder. The PBA was dosed in an amount of 12wt% based on the polymer composition in zone <NUM>. The molten mixture as obtained was then cooled using a melt cooler set at <NUM>. After cooling, the melt was extruded through a perforated die plate with <NUM> holes having a diameter of <NUM> and a landlength of <NUM> at a throughput of <NUM>/hour. The die plate was controlled at a temperature of <NUM>, except for CE4 where the die plate was controlled at a temperature of <NUM>. The melt was cut by a <NUM>-blade angled cutter head rotating at <NUM> RPM with the use of an underwater pelletizing system. The temperature of the water was controlled at <NUM>. The pressure was controlled at <NUM> bar. And the flow of the water was controlled at <NUM><NUM>/h. thereby obtaining the expanded polypropylene beads.

The expanded polypropylene beads obtained in examples E1-E4 and comparative examples CE1-CE4 were stored in the pressure vessel connected to the steam chest molding machine at an air pressure of <NUM>. The block-shape mold (300x200x60mm; Ixbxh) was filled with the expanded polypropylene beads. In the mold cavity, an air pressure of <NUM>. 2bar was applied during filling. Subsequently the air in the mold was replaced by steam and a steam pressure was applied for <NUM> seconds. The lowest steam pressure applied was <NUM> bars and the steam pressure was then increased stepwise with steps of <NUM> bar and after each step increase, the mold was held at the steam pressure for <NUM> seconds. The resultant molded article was put in an oven at <NUM> for <NUM> hours to allow drying. The molded article was then stored at <NUM> for <NUM> hours before evaluation of the properties of the molded article.

The results of experiments E1-E4 and CE1-CE4 are presented below. CE1 presented excessive shrinkage in the mold. CE2 presented a narrow molding window; and CE3 presented collapse above ><NUM> bar steam pressure. CE4 presented a narrow window.

Excessive shrinkage: the volume of the molded article is more than <NUM>% smaller than the volume of the mold cavity.

A sample shows collapse if the molded article did not retain the shape of the mold and/or shows sinkholes.

A sample has a narrow molding window if the range of the steam pressure in which a good molded article is obtained (> <NUM>% rate of fusion, no excessive shrinkage and no collapse) is <<NUM> bar.

As can be seen from the above results in Table <NUM>, the polymer compositions of the invention (E1-E4) allow for the preparation of expanded beads by extrusion. The expanded beads show an acceptable amount of open cells (< <NUM> %) and the extrusion processing went well.

The expanded beads of the invention (E1-E4) could be steam-molded into molded articles showing a rate of fusion > <NUM>% at lower steam pressures than those of the comparative examples (CE1-CE4). In addition, the molded articles prepared from the expanded beads of the invention showed a good compression strength.

When using an amount of ≥ <NUM> wt% of high melt strength polypropylene (PP-UMS) in the polymer composition of the invention, the amount of open cells in the expanded beads prepared therefrom is decreased (compare E2 and E3 having an amount of 32wt% PP-UMS in the polymer composition to E1 and E4).

By the use of a propylene homopolymer in the polymer composition (E3), the compression strength of the molded article can be increased.

Claim 1:
Expanded polypropylene beads comprising a polymer composition, wherein the polymer composition comprises
a) ≥ <NUM> wt% and ≤ <NUM> wt% of a high melt strength polypropylene,
b) ≥ <NUM> wt% and ≤ <NUM> wt% of a polyethylene
c) ≥ <NUM> wt% and ≤ <NUM> wt% of a first polypropylene, wherein the first polypropylene is chosen from the group of propylene homopolymers, propylene copolymers and/or mixtures thereof
d) ≥ <NUM> wt% and ≤ <NUM> wt% of a second polypropylene
wherein the sum of the high melt strength polypropylene, the first polypropylene and the second polypropylene is ≥ 60wt% based on the sum of the weight of the high melt strength polypropylene, the polyethylene, the first polypropylene and the second polypropylene and
wherein the polyethylene has a density ≥ <NUM>/m<NUM> and ≤ <NUM>/m<NUM> as determined according to ISO <NUM> (<NUM>).