Polyethylene powder, process for its preparation and its use for rotomolding

Abstract

Polyethylene powder comprising at least one additive having: Melt Flow Index (MFI), measured at 190 C. with a load of 2.16 kg in accordance with standard ISO 1133-1:2011, ranging from 0.1 g/10 minutes to 100 g/10 minutes, preferably ranging from 5 g/10 minutes to 35 g/10 minutes; density ranging from 0.890 g/cm.sup.3 to 0.965 g/cm.sup.3, preferably ranging from 0.940 g/cm.sup.3 to 960 g/cm.sup.3; bulk density ranging from 0.35 g/cm.sup.3 to 0.60 g/cm.sup.3, preferably ranging from 0.40 g/cm.sup.3 to 0.50 g/cm.sup.3; average particle diameter (d.sub.50) ranging from 400 m to 800 m, preferably ranging from 450 m to 780 m. Said polyethylene powder comprising at least one additive can be advantageously used in rotomolding.

Claims

1. Process for the preparation of a polyethylene powder comprising at least one additive, comprising: preparing a solution comprising at least one additive, operating at a temperature ranging from 25 C. to 50 C., at a rotation speed ranging from 1 rpm to 100 rpm, for a time ranging from 10 minutes to 50 minutes; adding said solution to at least one polyethylene powder obtained through gas-phase (co)polymerization, and maintaining said solution and said at least one polyethylene powder under stirring, at a rotation speed ranging from 10 rpm to 100 rpm, at a temperature ranging from 25 C. to 50 C., for a time ranging from 10 minutes to 60 minutes.

2. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 1, wherein said polyethylene powder has: Melt Flow Index (MFI), measured at 190C with a load of 2.16 kg in accordance with ISO 1133-1: 2011, ranging from 0.1 g/10 minutes to 100 g/10 minutes; density ranging from 0.890 g/cm.sup.3 to 0.965 g/cm.sup.3; bulk density ranging from 0.35 g/cm.sup.3 to 0.60 g/cm.sup.3; average particle diameter (d.sub.50) ranging from 400 m to 800 m.

3. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 1, wherein said polyethylene is a high density polyethylene (HDPE), obtained by gas-phase (co) polymerization.

4. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 1, wherein said additive includes at least one of the following: an antioxidant, a crosslinking agent, a co-crosslinking agent, a peroxide scavenger, a UV-absorber, and a light stabilizer.

5. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 4, wherein said antioxidant includes at least one of the following: 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-nonyl-phenol, 2,2-methylene-bis-(4-methyl-6-t-butyl-phenol), 4,4-butylidene-bis-(2-t-butyl-5-methyl-phenol), 4,4-thio-bis-(2-t-butyl-5-methyl-phenol), 2,2-thio-bis (6-t-butyl-4-methyl-phenol), 2,5-di-t-amyl-hydroquinone; tris-(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate, 2,2-thiodiethyl-bis-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 1,1,3-tris-(2-methyl-4-hydroxy-5-t-butyl-phenyl)butane, 2,2-methylene-bis-6-(1-methyl-cyclohexyl)-para-cresol, 3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecil)-2H-1-benzopyran-6-ol, 2,4-dimethyl-6-(1-methylcyclohexyl)phenol, N,N-hexamethylene bis-(3,5-di-t-butyl-4-hydroxyhydrocinnamide), C.sub.3-C.sub.15 alkyl esters of 3,5-bis(1,1-dimethyl)-4-hydroxybenzenpropanoic acid, tris-(2,4-di-t-butyl-phenyl)phosphite, tris-2,4-bis(1,1-dimethylpropyl)phenylphosphite, tris-4-(1,1-dimethylpropyl)phenylphosphite, bis[2,4-bis(1,1-dimethylpropyl)phenyl][4-(1,1-dimethylpropyl)phenyl]phosphite, [2,4-bis(1,1-dimethylpropyl)phenyl] bis [4-(1,1-dimethylpropyl)phenyl] phosphite, bis [tris (2,4-di-t-butyl-phenyl) phosphite in combination with distearyl-3,3-thiodipropionate, bis-(2,4-di-t-butyl-phenyl)pentaerythritoldiphosphite, tetrakis-(2,4-di-t-butyl-phenyl)-4,4-biphenylenediphosphonite, tris-(p-nonylphenyl)phosphite, di-iso-decylphenyl phosphite, diphenyl-iso-decyl phosphite, tri-iso-decyl phosphite, trilauryl phosphite, organic phosphonate, organic phosphonite, di(stearyl)pentaerythritol diphosphite, tetrakis(2,4-di-t-butylphenyl)-4,4-biphenylene-diphosphonite, and an antioxidant based on vitamin E.

6. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 4, wherein said crosslinking agent includes an organic peroxide having general f, ##STR00003## wherein R is a divalent hydrocarbon group, selected from: ##STR00004## C.sub.1- wherein m represents a number ranging from 1 to 8; R.sub.1 and R.sub.2, mutually identical or different, represent a linear or branched C.sub.1-C.sub.12 alkyl group; R.sub.3 and R.sub.4, mutually identical or different, represent a linear or branched C.sub.1-C.sub.12 alkyl group; n is 0 or 1.

7. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 4, wherein said co-crosslinking agent includes at least one of the following allyl compounds: allyl methacrylate, diallyl itaconate, diallyl phthalate, triallyl trimellitate, triallyl trimethyl trimellitate, triallyl cyanurate (TAC), triallyl iso-cyanurate (TAlC), and triallyl phosphate.

8. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 4, wherein said peroxide scavenger includes at least one of the following: an ester of -thiodipropionic acid; mercaptobenzoimidazole or a zinc salt of 2-mercaptobenzimidazole; dibutylthiodicarbamate zinc; dioctadecyl disulfide; and pentaerythritol tetrakis (-dodecylmercapto)propionate.

9. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 4, wherein said UV absorber includes at least one of the following: triazine, benzooxaazinone, benzotriazole, benzophenone, benzoate, formamidine, cinnamate, propenoate, aromatic propandione, benzoimidazole, cycloaliphatic ketone, formanilide, oxamide, cyanoacrylate, benzopiranone and salicylate.

10. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 4, wherein said light stabilizer includes a sterically hindered amine.

11. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 4, wherein said antioxidant includes at least one of the following: sterically hindered phenol, polymeric sterically hindered phenol, a C.sub.3-C.sub.15 alkyl ester of 3,5-bis(1,1-dimethyl)-4-hydroxybenzenpropanoic acid, organic phosphite, organic phosphonate, organic phosphonite, organic phosphate, and an antioxidant based on vitamin E.

12. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 6, wherein said organic peroxide having general formula (I) includes at least one of the following: 2,5-bis(t-amylperoxy)-2,5-dimethylhexane, 2,5-bis(t-butylperoxy)-2,5-dimethylhexane, 3,6-bis(t-butylperoxy)-3,6-dimethyl-octane, 2,7-bis(t-butylperoxy)-2,7-dimethyl-octane, 8,11-bis(t-butylperoxy)-8,11-dimethyloctadecane; ,-bis(t-amylperoxy-iso-propyl)benzene, ,-bis(t-butylperoxy-iso-propyl)benzene; 2,7-dimethyl-2,7-di(t-butyl peroxy)octadiyne-3,5, 2,7-dimethyl-2,7-di(peroxyethylcarbonate)octadiyne-3,5, 3,6-dimethyl-3,6-di(peroxyethylcarbonate)octyne-4, t-butyl-peroxybenzoate, 3,6-dimethyl-3,6-di(t-butylperoxy)octyne-4, 2,5-dimethyl-2,5-di(peroxy-n-propyl-carbonate)hexyne-3, 2,5-dimethyl-2,5-di(peroxy-iso-butyl carbonate)hexyne-3, 2,5-dimethyl-2,5-di(peroxyethyl-carbonate)hexyne-3, 2,5-dimethyl-2,5-di(-cumyl-peroxy)hexyne-3, and 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3.

13. Process for the preparation of a polyethylene powder comprising at least one additive in accordance with claim 1 comprising: preparing the solution comprising at least one additive, operating at the temperature ranging from 30 C. to 40 C., at the rotation speed ranging from 20 rpm to 80 rpm, for the time ranging from 15 minutes to 45 minutes; and adding said solution to the at least one polyethylene powder obtained through gas-phase (co)polymerization, and maintaining said solution and said at least one polyethylene powder under stirring, at the rotation speed ranging from 30 rpm to 70 rpm, at the temperature ranging from 30 C. to 40 C., for the time ranging from 15 minutes to 50 minutes.

14. Process of forming articles by rotomolding comprising preparing a polyethylene powder comprising at least one additive, comprising: preparing a solution comprising at least one additive, operating at a temperature ranging from 25 C. to 50 C., at a rotation speed ranging from 1 rpm to 100 rpm for a time ranging from 10 minutes to 50 minutes; adding said solution to at least one polyethylene powder obtained through gas-phase (co)polymerization, and maintaining said solution and said at least one polyethylene powder under stirring, at a rotation speed ranging from 10 rpm to 100 rpm, at a temperature ranging from 25 C. to 50 C., for a time ranging from 10 minutes to 60 minutes, thereby forming the polyethylene powder comprising the at least one additive; with the proviso that extrusion and grinding of the polyethylene powder comprising the at least one additive is not carried out; and adding the polyethylene powder comprising the at least one additive to a mold; and carrying out rotomolding of the polyethylene powder comprising the at least one additive to form an article.

Description

EXAMPLE 1 (COMPARATIVE)

(1) 9.8 kg of high density polyethylene (HDPE) (Eraclene MR 80 U by Versalis spa) in granular form, having the following characteristics: Melt Flow Index (MFI), measured at 190 C. with a load of 2.16 kg in accordance with ISO 1133-1: 2011, of 25 g/10 minutes; density of 0.954 g/cm.sup.3,

(2) 0.72 kg of 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 (Trigonox 145-E85 by Akzo Nobel) and 0.88 kg of triallyl cyanurate(TAC) (Luvomaxx TAC by Lehvoss), were placed in a laboratory screw mixer, at a temperature of 25 C., for 20 minutes, at a mixing speed of 60 rpm. At the end, the crosslinkable composition obtained was unloaded from the internal mixer and fed to a co-rotating twin-screw extruder (D=30 mm; L/D=28) and extruded at a temperature profile not exceeding 145 C., at a flow rate of 7.5 kg/h, and at a screw rotation speed of 100 rpm. The material extruded in spaghetti form was cooled in a water bath, dried in air, granulated using a chopper and then subjected to grinding in a mill with rotating blades, obtaining a powder with average particle diameter (d.sub.50) of 353 m and a dry flow index of 32 seconds.

(3) The mechanical properties of the polyethylene powder were measured. For that purpose, samples of said polyethylene powder obtained by compression molding at 200 C., at a pressure of 5000 kg/cm.sup.2, for 20 minutes, having 3.2 mm thickness, 150 mm height and 150 mm width, were subjected to the following measurements: yield stress in accordance with standard ASTM D638-03; stress at break in accordance with standard ASTM D638-03; elongation at break in accordance with standard ASTM D 638-03; ball drop test in accordance with standard ASTM D2463-15, at 20 C.; content of gel measured by weight loss, after 8 hours, in ethylbenzene at reflux; dry flow index in accordance with standard ASTM D1895-96(2010)e1.

(4) The results obtained are shown in Table 1. Table 1 also shows the dry flow index and the gel content.

EXAMPLE 2 (INVENTION)

(5) In a fluidized bed reactor, a polyethylene powder was prepared, in the gas-phase, by mixing ethylene at a partial pressure of 880 kPa, hydrogen at a partial pressure suitable to obtain a hydrogen/ethylene molar ratio of 0.66, 1-hexene at a partial pressure suitable for obtaining a 1-hexene/ethylene molar ratio of 0.019:dry nitrogen was also fed into said reactor for the purpose of reaching a total pressure of 2000 kPa. The reactor was brought to 110 C. and subsequently the catalyst UCAT-A by Univation Technologies and tri-iso-butylaluminum by Aldrich was fed into it, and the whole was kept, at said total pressure and at said temperature, for a time ranging from 2.5 hours to 3.5 hours. The polyethylene powder was unloaded continuously, passed into an automatic degassing and washing system and then left to cool in a container under a flow of cold nitrogen.

(6) 34 g of 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 (Trigonox 145-E85 by Akzo Nobel), 5 g of sterically hindered phenol (Anox 1315 by Addivant) and 8 g of phosphite (Weston 705 by Addivant) were loaded into a 0.5 liter flask, equipped with a magnetic stirrer: the mixture was heated, under stirring at 50 rpm, to 38 C. After reaching said temperature, 41 g of triallyl cyanurate (crystalline solid) (TAC) (Luvomaxx TAC by Lehvoss) and 28 g of Chimassorb 81 (benzophenone) (in powder) (Basf) were added: the mixture was kept, under stirring at 50 rpm, at 38 C., until complete dissolution of all the components (about 40 minutes). The faded yellow color solution obtained was left to cool to ambient temperature (25 C.) and then added to the polyethylene powder operating as described below.

(7) 4.4 kg of the polyethylene powder obtained as described above were introduced into a jacketed reactor equipped with a magnetic stirrer and, in 10 minutes, through spray nozzles, the aforementioned solution: the whole was kept under stirring at 60 rpm, for 45 minutes, at a temperature of 35 C.

(8) At the end, the reactor was left to cool to ambient temperature (25 C.) and the polyethylene powder comprising the additives was unloaded. Said polyethylene powder was found to have the following characteristics: Melt Flow Index (MFI), measured at 190 C. with a load of 2.16 kg in accordance with ISO 1133-1: 2011, of 27 g/10 minutes; density of 0.954 g/cm.sup.3, bulk density of 0.44 g/cm.sup.3; (d.sub.10) of 207 m; average particle diameter (d.sub.50) of 518 m; (d.sub.90) of 1082 m; Span of 1.69; dry flow index of 16 seconds.

(9) The mechanical properties of the polyethylene powder obtained were measured, operating as specified in Example 1. The results obtained are shown in Table 1. Table 1 also shows the dry flow index and the gel content measured as described above.

EXAMPLE 3 (INVENTION)

(10) In a fluidized bed reactor, a polyethylene powder was prepared, in gas-phase, by mixing ethylene at a partial pressure of 880 kPa, hydrogen at a partial pressure suitable to obtain a hydrogen/ethylene molar ratio of 0.30:dry nitrogen was also fed into said reactor for the purpose of reaching a total pressure of 2000 kPa. The reactor was brought to 108 C. and subsequently the catalyst UCAT-A by Univation Technologies and tri-iso-butylaluminum by Aldrich was fed into it, and the whole was kept, at said total pressure and at said temperature, for a time ranging from 2.5 hours to 3.5 hours.

(11) The polyethylene powder was unloaded continuously, passed into an automatic degassing and washing system and then left to cool in a container under a flow of cold nitrogen.

(12) 34 g of 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3 (Trigonox 145-E85 by Akzo Nobel), 5 g of sterically hindered phenol (Anox 1315 by Addivant) and 8 g of phosphite (Weston 705 by Addivant) were loaded into a 0.5 liter flask, equipped with a magnetic stirrer: the mixture was heated, under stirring at 50 rpm, to 38 C. After reaching said temperature, 41 g of triallyl cyanurate (crystalline solid) (TAC) (Luvomaxx TAC by Lehvoss) and 28 g of Chimassorb 81 (benzophenone) (in powder) (Basf) were added: the mixture was kept, under stirring at 50 rpm, at 38 C., until complete dissolution of all the components (about 40 minutes). The faded yellow color solution obtained was left to cool to ambient temperature (25 C.) and then added to the polyethylene powder as described below. 4.4 kg of the polyethylene powder obtained as described above were introduced into a jacketed reactor equipped with a magnetic stirrer and, in 10 minutes, through spray nozzles, the aforementioned solution: the whole was kept under stirring at 60 rpm, for 45 minutes, at a temperature of 35 C.

(13) At the end, the reactor was left to cool to ambient temperature (25 C.) and the polyethylene powder comprising the additives was unloaded. Said polyethylene powder had the following characteristics: Melt Flow Index (MFI), measured at 190 C. with a load of 2.16 kg in accordance with ISO 1133-1: 2011, of 9 g/10 minutes; density of 0.959 g/cm.sup.3, bulk density of 0.47 g/cm.sup.3; (d.sub.10) of 320 m; average particle diameter (d.sub.50) of 765 m; (d.sub.90) of 1360 m; Span of 1.408; dry flow index of 14 seconds.

(14) The mechanical properties of the polyethylene powder obtained were measured, operating as specified in Example 1. The results obtained are shown in Table 1. Table 1 also shows the dry flow index and the gel content measured as described above.

(15) TABLE-US-00001 TABLE 1 Example 1 Example 2 Example 3 Mechanical properties (comparison) (invention) (invention) Yield stress 18.5 17.8 19.2 (MPa) Stress at break (MPa) 13.0 13.6 13.0 Elongation at break 205 300 200 (MPa) Peak energy (Ball drop) 32 n.d..sup.(*.sup.) 36 (T = 20 C.) (J) Total energy (Ball drop) 51 n.d..sup.(*.sup.) 58 (T = 20 C.) (J) Maximum peak force 4500 n.d..sup.(*.sup.) 5010 (Ball drop) (T = 20 C.) (N) Gel content (%) 76.5 77.5 78.5 Dry flow index (sec.) 32 16 14 .sup.(*.sup.)n.d.: not determined.

(16) From the data shown in Table 1 it can be inferred that the polyethylene powder comprising at least one additive according to the present invention [Example 2 and Example 3 (invention)] has similar mechanical properties to those of the polyethylene powder comprising the same additives, obtained by mixing in an extruder and subsequent granulation [Example 1 (comparison)] and a better dry flow index.