WO2012076447A1

WO2012076447A1 - Polyolefin based corrugated boards

Info

Publication number: WO2012076447A1
Application number: PCT/EP2011/071702
Authority: WO
Inventors: Roberta Marzolla; Monica Galvan; Daniele Po
Original assignee: Basell Poliolefine Italia S.R.L.
Priority date: 2010-12-07
Filing date: 2011-12-05
Publication date: 2012-06-14

Abstract

A corrugated board comprising a polyolefin composition comprising (per cent by weight): a) from 50% to 85 of a propylene polymer having a fraction of xylene solubles measured at 25°C lower than 3%; b) from 5% to 20%, of a copolymer of ethylene and propylene, the copolymer having an amount of recurring units deriving from ethylene ranging from 30 to 60%, and being partially soluble in xylene at soluble in xylene at 25°C; the polymer fraction soluble in xylene at soluble in xylene at 25°C having an intrinsic viscosity value ranging from 2.0 to 4.0 dl/g; and c) 5% to 30%), of ethylene homopolymer having having a xylene soluble fraction at 25°C ranging from 5% to 30%; the sum of a)+b) +c) being 100; said composition having a value of melt flow rate ranging from 0.5 to 30.0 g/10 min.

Description

Title:

Polyolefin based corrugated boards

The present invention relates to corrugated board comprising a particular polypropylene- based composition said composition having particular features, to containers made of such corrugated boards and to a process for producing such corrugated boards.

Single-face and double-face corrugated board are well known material for packaging or other uses. Plastic material have been widely used for this kind of application for example US 3,893,879 relates to a process for producing a plastic corrugated board. Among the preferred resin to be used crystalline polypropylene is mentioned. US 5,842,315 relates to a corrugated board structure, wherein in particular the structure includes an upper layer of corrugated board formed by polyvinylchloride (PVC), polycarbonate (PC), polypropylene (PP) film or steel sheet, a bottom layer of PVC film 3, PP film, nylon film or wooden fiber skin and a middle layer of PU foam.

In "Packaging materials in microwave ovens" (Packaging Technology and Science)

Volume 5, Issue 6, November 1992, Pages: 291-294, Raija Ahvenainen, Thea Sipilinen- Malm, Raija-Liisa Heinio and Annukka Leppnen) it has been reported that corrugated board can be obtained by using polypropylene or polyethylene.

From another side it is well known in the art that the isotactic polypropylene, though being endowed with an exceptional combination of excellent properties, is affected by the drawback of possessing an insufficient impact resistance at relatively low temperatures. Thus it is necessary to prepare polypropylene-based composition in order to achieve a sufficient impact resistance and a good stress-whitening resistance.

It is also desirable that in order to increase the productivity of the corrugated board line, the polyolefin composition can be extruded at the highest speed possible

Thus an object of the present invention is a corrugated board comprising a polyolefin composition comprising (per cent by weight):

a) from 50% to 85%, preferably from 60% to 81%, more preferably from 70% to 79% of a propylene polymer having a fraction of xylene solubles measured at 25°C lower than 3% preferably lower than 2%;

b) from 5% to 20%, preferably from 7% to 20%, more preferably from 8 % to 15% of a copolymer of ethylene and propylene, the copolymer having an amount of recurring units deriving from ethylene ranging from 30% to 60 %, preferably 40% to 50 %, and being partially soluble in xylene at 25°C; the polymer fraction soluble in xylene at soluble in xylene at 25°C having an intrinsic viscosity value ranging from 2.0 to 4.0 dl/g; preferably from 3.0 to 4.0 dl/g and

c) from 5% to 30%, preferably from 9% to 25%, more preferably from 13% to 20% of ethylene homopolymer having a xylene soluble fraction at 25°C ranging from 2% to 30%, preferably from 3% to 20% even more preferably from 3% to 10%; the sum of a)+b) +c) being 100;

said composition having a value of melt flow rate ranging from 0.5 to 30.0 g/10 min, preferably from 1.0 to 10.0 g/10 min, more preferably from 1.5 to 5.0 g/10.

Typically, the polyolefin composition to be used for the corrugated board of the present invention exhibits a flexural modulus value at least 1200 MPa, preferably it is comprised between 1200 MPa and 2000 MPa.

Propylene polymer (a) is selected from a propylene homopolymer and a copolymer of propylene containing at most 3 wt% of ethylene or a C4-C10 α-olefin or combination thereof. Particularly preferred is the propylene homopolymer.

Typically the melt flow rate of propylene polymer (a) typically ranges from 1.0 to 20.0 g/10 min, preferably from 2.0 to 15.0 g/10 min, more preferably from 3.0 to 10.0 g/10 min.

Elastomeric ethylene-propylene copolymer (b) can optionally comprises a diene. When present, the diene is typically in amounts ranging from 0.5 to 10 wt% with respect to the weight of copolymer (b). The diene can be conjugated or not and it is preferably selected from butadiene, 1,4-hexadiene, 1,5-hexadiene, and ethylidene-norbornene-1.

The corrugated board object of the present invention is further endowed with an improved stress-whitening resistance. This effect improves among other things the aesthetic appearance of objects obtained by said material such as bags for example suitcases. An other advantage of the corrugated board of the present invention is the surface appearance, that is improved. Furthermore when the described polyolefin composition is used for the production of the corrugated board object of the present invention it is possible to achieve high extrusion speeds.

Therefore it is a further object of the present invention a process for the production of corrugated board comprising the step of moulding the above described polyolefin composition and forming the corrugated board.

The corrugated board object of the present invention can be obtained with processes commonly known in the art.

The corrugated boards according to the present invention can be any mono or multilayer corrugated board known in the art.

Fig 1 is an end elevational view of one layer of the corrugated board according to the present invention. The layer comprises a top surface 1 and a bottom 2 and a corrugated sheet 3 between the two planar surfaces

Fig 2 is an end elevational view of a multilayer (3 layers in this case) corrugated board according to the present invention. The multilayer board comprises a top and a bottom planar layer 21 and 22, corrugated sheets 23 (one for each layer), and intermediate sheets 24 between two adjacent corrugated sheets 23.

The composition of the present invention is obtained by means of a sequential copolymerization process. Said process comprising at least three sequential polymerization stages with each subsequent polymerization being conducted in the presence of the polymeric material formed in the immediately preceding polymerization reaction, wherein the polymerization stage of propylene to the crystalline polymer (a) is carried out in at least one stage, than a copolymerization stage of mixtures of ethylene with propylene (and optionally a diene) to elastomeric polymer (b) and finally a polymerization stage of ethylene to polyethylene (c) are carried out. The polymerisation stages may be carried out in the presence of a stereospecific Ziegler-Natta catalyst.

According to a preferred embodiment, all the polymerisation stages are carried out in the presence of a catalyst comprising a trialkylaluminum compound, optionally an electron donor, and a solid catalyst component comprising a halide or halogen-alcoholate of Ti and an electron-donor compound supported on anhydrous magnesium chloride. Catalysts having the above-mentioned characteristics are well known in the patent literature; particularly advantageous are the catalysts described in USP 4,399,054 and EP-A-45 977. Other examples can be found in USP 4,472,524.

Preferably the polymerisation catalyst is a Ziegler-Natta catalyst comprising a solid catalyst component comprising:

a) Mg, Ti and halogen and an electron donor (internal donor),

b) an alkylaluminum compound and, optionally (but preferably),

c) one or more electron-donor compounds (external donor).

The internal donor is preferably selected from the esters of mono or dicarboxylic organic acids such as benzoates, malonates, phthalates and certain succinates. They are described in US patent 4522930, European patent 45977 and international patent applications WO 00/63261 and WO 01/57099, for example. Particularly suited are the phthalic acid esters and succinate acids esters. Alkylphthalates are preferred, such as diisobutyl, dioctyl and diphenyl phthalate and benzyl-butyl phthalate. Among succinates, they are preferably selected from succinates of the formula (I):

(I)

wherein the radicals Ri and R₂, equal to or different from each other, are a C₁-C₂₀ linear or branched alkyl, alkenyl, cycloalkyl, aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms; the radicals R3 to 5, equal to or different from each other, are hydrogen or a C₁-C₂₀ linear or branched alkyl, alkenyl, cycloalkyl, aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms, and the radicals R3 to 5 which are joined to the same carbon atom can be linked together to form a cycle; with the proviso that when R₃ to R₅ are contemporaneously hydrogen, 5 is a radical selected from primary branched, secondary or tertiary alkyl groups, cycloalkyl, aryl, arylalkyl or alkylaryl groups having from 3 to 20 carbon atoms;

or of formula (II):

(Π)

O

wherein the radicals Ri and R₂, equal to or different from each other, are a C₁-C₂₀ linear or branched alkyl, alkenyl, cycloalkyl, aryl, arylalkyl or alkylaryl group, optionally containing heteroatoms and the radical R3 is a linear alkyl group having at least four carbon atoms optionally containing heteroatoms.

The Al-alkyl compounds used as co-catalysts comprise Al-trialkyls, such as Al-triethyl, Al- triisobutyl, Al-tri-n-butyl, and linear or cyclic Al-alkyl compounds containing two or more Al atoms bonded to each other by way of O or N atoms, or SO₄ or SO₃ groups. The Al-alkyl compound is generally used in such a quantity that the Al/Ti ratio be from 1 to 1000.

External donor (c) can be of the same type or it can be different from the succinates of formula (I) or (II). Suitable external electron-donor compounds include silicon compounds, ethers, esters such as phthalates, benzoates, succinates also having a different structure from those of formula (I) or (II), amines, heterocyclic compounds and particularly 2,2,6,6-tetramethylpiperidine, ketones and the 1,3-diethers of the general formula (III):

R!-O— CH₂ CH₂O-R^IV

(III)

wherein R¹ and Rⁿ are the same or different and are Ci-Ci₈ alkyl, C₃-Ci₈ cycloalkyl or C₇-Ci₈ aryl radicals; Rⁱⁿ and R^IV are the same or different and are C₁-C₄ alkyl radicals; or the 1,3- diethers in which the carbon atom in position 2 belongs to a cyclic or polycyclic structure made up of 5, 6 or 7 carbon atoms and containing two or three unsaturations.

Ethers of this type are described in published European patent applications 361493 and 728769.

Preferred electron-donor compounds that can be used as external donors include aromatic silicon compounds containing at least one Si-OR bond, where R is a hydrocarbon radical. A particularly preferred class of external donor compounds is that of silicon compounds of formula Ra⁷Rb⁸Si(OR⁹)c, where a and b are integer from 0 to 2, c is an integer from 1 to 3 and the sum (a+b+c) is 4; R⁷, R⁸, and R⁹, are Ci-Ci₈ hydrocarbon groups optionally containing heteroatoms. Particularly preferred are the silicon compounds in which a is 1, b is 1, c is 2, at least one of R⁷ and R⁸ is selected from branched alkyl, alkenyl, alkylene, cycloalkyl or aryl groups with 3-10 carbon atoms optionally containing heteroatoms and R⁹ is a Ci-Cio alkyl group, in particular methyl. Examples of such preferred silicon compounds are cyclohexyltrimethoxysilane, t- butyltrimethoxysilane, t-hexyltrimethoxysilane, cyclohexylmethyldimethoxysilane, 3,3,3- trifluoropropyl-2-ethylpiperidyl-dimethoxysilane, diphenyldimethoxysilane, methyl-t- butyldimethoxysilane, dicyclopentyldimethoxysilane, 2-ethylpiperidinyl-2-t- butyldimethoxysilane, (l, l, l-trifluoro-2-propyl)-methyldimethoxysilane and (l, l, l-trifluoro-2- propyl)-2-ethylpiperidinyldimethoxysilane. Moreover, are also preferred the silicon compounds in which a is 0, c is 3, R⁸ is a branched alkyl or cycloalkyl group, optionally containing heteroatoms, and R⁹ is methyl. Particularly preferred specific examples of silicon compounds are (tert-butyl)₂Si(OCH₃)2, (cyclohexyl)(methyl) Si(OCH₃)₂, (phenyl)₂Si(OCH₃)₂ and (cyclopentyl)₂Si(OCH₃)₂.

Preferably electron donor compound (c) is used in such an amount to give a molar ratio between the organoaluminum compound and said electron donor compound (c) of from 0.1 to 500, more preferably from 1 to 300 and in particular from 3 to 100.

As explained above, the solid catalyst component comprises, in addition to the above electron donors, Ti, Mg and halogen. In particular, the catalyst component comprises a titanium compound, having at least a Ti-halogen bond, and the above mentioned electron donor compounds supported on a Mg halide. The magnesium halide is preferably MgCl₂ in active form, which is widely known from the patent literature as a support for Ziegler-Natta catalysts. Patents USP 4,298,718 and USP 4,495,338 were the first to describe the use of these compounds in Ziegler-Natta catalysis. It is known from these patents that the magnesium dihalides in active form used as support or co-support in components of catalysts for the polymerisation of olefins are characterized by X-ray spectra in which the most intense diffraction line that appears in the spectrum of the non-active halide is diminished in intensity and is replaced by a halo whose maximum intensity is displaced towards lower angles relative to that of the more intense line.

The preferred titanium compounds are TiCl₄ and TiCl₃; furthermore, also Ti-haloalcoholates of formula Ti(OR)n-yXy can be used, where n is the valence of titanium, y is a number between 1 and n, X is halogen and R is a hydrocarbon radical having from 1 to 10 carbon atoms.

The preparation of the solid catalyst component can be carried out according to several methods, well known and described in the art.

According to a preferred method, the solid catalyst component can be prepared by reacting a titanium compound of formula Ti(OR)n-yXy, where n is the valence of titanium and y is a number between 1 and n, preferably TiCl₄, with a magnesium chloride deriving from an adduct of formula MgCl₂-pROH, where p is a number between 0.1 and 6, preferably from 2 to 3.5, and R is a hydrocarbon radical having 1-18 carbon atoms. The adduct can be suitably prepared in spherical form by mixing alcohol and magnesium chloride in the presence of an inert hydrocarbon immiscible with the adduct, operating under stirring conditions at the melting temperature of the adduct (100-130° C). Then, the emulsion is quickly quenched, thereby causing the solidification of the adduct in form of spherical particles.

Examples of spherical adducts prepared according to this procedure are described in USP 4,399,054 and USP 4,469,648. The so obtained adduct can be directly reacted with the Ti compound or it can be previously subjected to thermally controlled dealcoholation (80-130° C) so as to obtain an adduct in which the number of moles of alcohol is generally lower than 3, preferably between 0.1 and 2.5. The reaction with the Ti compound can be carried out by suspending the adduct (dealcoholated or as such) in cold TiCl₄ (generally 0° C); the mixture is heated up to 80-130° C and kept at this temperature for 0.5-2 hours. The treatment with TiCl₄ can be carried out one or more times. The electron donor compound(s) can be added during the treatment with TiCl_4.

Regardless of the preparation method used, the final amount of the electron donor compound(s) is preferably such that the molar ratio with respect to the MgCl₂ is from 0.01 to 1, more preferably from 0.05 to 0.5.

The said catalyst components and catalysts are described in WO 00/63261 and WO 01/57099. The catalysts may be precontacted with small quantities of olefin (prepolymerisation), maintaining the catalyst in suspension in a hydrocarbon solvent, and polymerising at temperatures from ambient to 60° C, thus producing a quantity of polymer from 0.5 to 3 times the weight of the catalyst. The operation can also take place in liquid monomer, producing, in this case, a quantity of polymer 1000 times the weight of the catalyst.

By using the above mentioned catalysts, the polyolefin compositions are obtained in spheroidal particle form, the particles having an average diameter from about 250 to 7,000 microns, a flowability of less than 30 seconds and a bulk density (compacted) greater than 0.4 g/ml.

The polymerisation stages may occur in liquid phase, in gas phase or liquid-gas phase. Preferably, the polymerisation of crystalline polymer (a) is carried out in liquid monomer (e.g. using liquid propylene as diluent), while the copolymerisation stages of elastomeric copolymer (b) and polyethylene (c) are carried out in gas phase. Alternatively, all the three sequential polymerisation stages can be carried out in gas phase.

The reaction temperature in the polymerisation stage for the preparation of crystalline polymer (a) and in the preparation of elastomeric copolymer (b) and polyethylene (c) be the same or different, and is preferably from 40 to 100° C; more preferably, the reaction temperature ranges from 50 to 80° C in the preparation of polymer (a), and from 70 to 100° C for the preparation of polymer components (b) and (c).

The pressure of the polymerisation stage to prepare polymer (a), if carried out in liquid monomer, is the one which competes with the vapor pressure of the liquid propylene at the operating temperature used, and it may be modified by the vapor pressure of the small quantity of inert diluent used to feed the catalyst mixture, by the overpressure of optional monomers and by the hydrogen used as molecular weight regulator.

The polymerisation pressure preferably ranges from 33 to 43 bar, if done in liquid phase, and from 5 to 30 bar if done in gas phase. The residence times relative to the two stages depend on the desired ratio between polymers (a) and (b) and (c), and can usually range from 15 minutes to 8 hours. Conventional molecular weight regulators known in the art, such as chain transfer agents (e.g. hydrogen or ZnEt₂), may be used.

Conventional additives, fillers and pigments, commonly used in olefin polymers, may be added, such as nucleating agents, extension oils, mineral fillers, and other organic and inorganic pigments. In particular, the addition of inorganic fillers, such as talc, calcium carbonate and mineral fillers, also brings about an improvement to some mechanical properties, such as flexural modulus and HDT. Talc can also have a nucleating effect.

The nucleating agents are added to the compositions of the present invention in quantities ranging from 0.05 to 2% by weight, more preferably from 0.1 to 1% by weight, with respect to the total weight, for example, however the absence of nucleating agent is preferred.

The particulars are given in the following examples, which are given to illustrate, without limiting, the present invention.

The following analytical methods have been used to determine the properties reported in the detailed description and in the examples.

Examples

- Ethylene (C2): By IR spectroscopy.

The spectrum of a pressed film of the polymer is recorded in absorbance vs. wavenumbers (cm^"1). The following measurements are used to calculate C2 content:

a) Area (A_t) of the combination absorption bands between 4482 and 3950 cm^"1 which is used for spectrometric normalization of film thickness.

b) Area (Ac₂) of the absorption band due to methylenic sequences (CH₂ rocking vibration) after a proper digital subtraction of an isotactic polypropylene (IPP) reference spectrum. The range 660 to 790 cm^"1 is used for both heterophasic and/or random copolymers

- Fractions soluble and insoluble in xylene at 25 °C: 2.5 g of polymer are dissolved in 250 mL of xylene at 135° C under agitation. After 20 minutes the solution is allowed to cool to 25° C, still under agitation, and then allowed to settle for 30 minutes. The precipitate is filtered with filter paper, the solution evaporated in nitrogen flow, and the residue dried under vacuum at 80° C until constant weight is reached. Thus one calculates the percent by weight of polymer soluble and insoluble at room temperature (25° C).

- Intrinsic Viscosity [η] : Measured in tetrahydronaphthalene at 135° C.

- Melt flow rate: Determined according to ISO method 1 133 (230° C and 2.16 kg).

- Flexural modulus: Determined according to ISO method 178.

- Izod impact resistance: Determined according to ISO method 180/1 A.

- Stress-whitening resistance: The resistance to whitening is determined by subjecting small disc, which have a 4 cm diameter and prepared by injection moulding, prepared from the polymer being tested to the impact of a ram having a 76 g weight. Both the minimum height (h) up to the maximum height allowed by the apparatus necessary to obtain whitening, and the width (diameter) of the whitened area are recorded.

Examples 1 and 2

In a plant operating continuously according to the mixed liquid-gas polymerization technique, runs were carried out under the conditions specified in Table 1.

The polymerization was carried out in the presence of a catalyst system in a series of three reactors equipped with devices to transfer the product from one reactor to the one

immediately next to it.

Preparation of the solid catalyst component

Into a 500 ml four-necked round flask, purged with nitrogen, 250 ml of TiCl₄ are introduced at 0° C. While stirring, 10.0 g of microspheroidal MgCl₂- 1.9C₂H₅OH (prepared according to the method described in ex.2 of USP 4,399,054 but operating at 3000 rpm instead of 10000 rpm) and 9.1 mmol of diethyl 2,3-(diisopropyl)succinate are added. The temperature is raised to 100° C and maintained for 120 min. Then, the stirring is discontinued, the solid product was allowed to settle and the supernatant liquid is siphoned off. Then 250 ml of fresh TiCL. are added. The mixture is reacted at 120° C for 60 min and, then, the supernatant liquid is siphoned off. The solid is washed six times with anhydrous hexane (6x 100 ml) at 60° C. Catalyst system and prepolymerization treatment

The solid catalyst component described above was contacted at 12° C for 24 minutes with aluminium triethyl (TEAL) and dicyclopentyldimethoxysilane (DCPMS) as outside-electron- donor component. The weight ratio between TEAL and the solid catalyst component and the weight ratio between TEAL and DCPMS are specified in Table 1.

The catalyst system is then subjected to prepolymerization by maintaining it in suspension in liquid propylene at 20° C for about 5 minutes before introducing it into the first

polymerization reactor.

Polymerization

The polymerisation run is conducted in continuous in a series of three reactors equipped with devices to transfer the product from one reactor to the one immediately next to it. The first reactor is a liquid phase reactor, and the second and third reactors are fluid bed gas phase reactors. Polymer (a) is prepared in the first reactor, while polymers (b) and (c) are prepared in the second and third reactor, respectively.

Temperature and pressure are maintained constant throughout the course of the reaction. Hydrogen is used as molecular weight regulator.

The gas phase (propylene, ethylene and hydrogen) is continuously analysed via gas- chromatography.

At the end of the run the powder is discharged and dried under a nitrogen flow. The polymerization parameters are reported on table 1.

Then the polymer particles are introduced in a twin screw extruder (Werner-type extruder), wherein they are mixed with 635 ppm of Irganox 1010, 635 ppm of Irgafos 168, 2450 ppm of distearyl thio-diproprionate and 270 ppm of synthetic hydrotalcite. The previously said Irganox 1010 is pentaerytrityl tetrakis 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propanoate, while Irgafos 168 is tris (2,4-di-tert-butylphenyl) phosphite, both marketed by Ciba-Geigy. Particles are extruded under nitrogen atmosphere in a twin screw extruder, at a rotation speed of 250 rpm and a melt temperature of 200-250° C.

Table 1 Example 1 2

TEAL/DCPMS weight ratio 2.8 10

1^st liquid phase reactor

Polymerisation temperature, °C 75 75

Pressure, bar 40 39

H₂ bulk, mol ppm 2800 1200

1^st gas phase reactor

Polymerisation temperature, °C 65 70

Pressure, bar 13 12

C₂7(C₂-+C₃ ^"), % 30 0.33

H₂ /C₂; % 5.2 0.035

2^ns gas phase reactor

Polymerisation temperature, °C 75 95

Pressure, bar 18 12

C₂7(C₂-+C₃-), % 99.0 0.97

H₂ /C₂; % 18.0 0.1

ethylene; C₃ ^" propylene; H₂ hydi

Comparative example 3

The polymerization has been carried out by using the same catalyst system and the same polymerization reactors of examples 1 and 2 excepting that the third gas phase reactor has not been used. Temperature and pressure are maintained constant throughout the course of the reaction. Hydrogen is used as molecular weight regulator.

The gas phase (propylene, ethylene and hydrogen) is continuously analysed via gas- chromatography. At the end of the run the powder is discharged and dried under a nitrogen flow. The polymerization parameters are reported on table la

Table la

Comparative example 3 3

TEAL/DCPMS weight ratio 5

1^st liquid phase reactor

Polymerisation temperature, °C 70

Pressure, bar 40 H₂ bulk, mol ppm 1100

1^st gas phase reactor

Polymerisation temperature, °C 80

Pressure, bar 14

C₂7(C₂-+C₃ ^"), mol 0.34

H₂ /C₂ ^", mol 0.06

ethylene; C₃ ^" propylene; H₂ hydi

Then the polymer particles are introduced in a twin screw extruder (Werner-type extruder), wherein they are mixed with 635 ppm of Irganox 1010, 635 ppm of Irgafos 168, 2450 ppm of distearyl thio-diproprionate and 270 ppm of synthetic hydrotalcite. The previously said Irganox 1010 is pentaerytrityl tetrakis 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propanoate, while Irgafos 168 is tris (2,4-di-tert-butylphenyl) phosphite, both marketed by Ciba-Geigy. The polymer particles are extruded under nitrogen atmosphere in a twin screw. The characteristics of the polymers have been reported in table 2

Table 2

162.6 164.7

Tm °C 163

111 116.6

Tc °C 106

70

Whitening 5 cm height mm 60 90 resistance: 100

10 cm height mm 90 130 diameter

110

(mm* 10) of the 20 cm height mm 112 165 whitening area 130

30 cm height mm 126 170 due to a ram

140

falling from a 76 cm height mm 160 195

* sum of xylene solubles of a) + b) or a) +b)+c)

The polymers of examples 2 and comparative example 3 have been added with respectively 1% and 0.85 % of talc. The polymers have been extruded to obtain a monolayer corrugated board 2 mm thick and weighting 250 gr/m². The results have been reported in table 3

Table3

From table 3 clearly results that the corrugated boards obtained by using the materials of examples 1 and 2 have a good surface appearance and are extruded with ah higher speed with respect to the comparative example.

Claims

A corrugated board comprising a polyolefin composition comprising (per cent by weight):

a) from 50% to 85 of a propylene polymer having a fraction of xylene solubles measured at 25°C lower than 3%;

b) from 5% to 20%, of a copolymer of ethylene and propylene, the copolymer having an amount of recurring units deriving from ethylene ranging from 30 to 60%, and being partially soluble in xylene at soluble in xylene at 25°C; the polymer fraction soluble in xylene at soluble in xylene at 25°C having an intrinsic viscosity value ranging from 2.0 to 4.0 dl/g; and

c) 5%> to 30%), of ethylene homopolymer having a xylene soluble fraction at 25°C ranging from 5% to 30%>; the sum of a)+b) +c) being 100;

said composition having a value of melt flow rate ranging from 0.5 to 30.0 g/10 min. The corrugated board according to claim 1 wherein in the polyolefin composition the crystalline propylene (a) is from 60wt%> to 81 wt%>, the copolymer (b) is from 7 wt%> to 20 wt%> and the polyethylene (c) is from 9 wt%> to 25 wt% with respect to the whole polymer composition.

The corrugated board according to claims 1 or 2 being mono or multilayer.

The corrugated board according to claim 3 wherein the board is a monolayer board comprising a top surface 1, a bottom surface 2 and a corrugated sheet 3 between the two planar surfaces.

The corrugated board according to claim 4 wherein the board is a multilayer board comprising a top and a bottom planar layer 21 and 22, corrugated sheets 23 ,one for each layer, and intermediate sheets 24 between two adjacent corrugated sheets 23 A process for the production of corrugated boards according to any of claims 1 to 6, comprising the step of moulding a polyolefin composition according to any of claims 1 to 2 and forming the corrugated board