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Screw cap composition

Screw cap composition description/claims

The present invention relates to screw caps comprising a composition based on multimodal ethylene polymer. It also relates to a process for manufacturing said caps and their use for the closure of bottles, in particular bottles containing foodstuffs, and more particularly fizzy drinks.

It is known to use polyethylene, and more particularly bimodal polyethylene, for the manufacture of caps. Thus patent applications U.S. Pat. No. 5,981,664 and WO 00/71615 describe caps obtained by injection of a composition comprising two polyethylenes having different molecular weight distributions. However, the compositions described in said documents do not have the optimum properties for the manufacture of caps, more particularly for caps intended for the closure of bottles containing fizzy drinks.

We have now found screw caps comprising a composition based on multimodal ethylene polymer which do not possess the above-mentioned drawbacks.

To this end, the present invention relates to screw caps comprising a composition based on a multimodal ethylene polymer having a standard density (SD) greater than 950 kg/m3 and a melt flow index MI2 of less than 10 g/10 min, said multimodal ethylene polymer comprising—

from 35 to 65 wt %, based on the total weight of the multimodal ethylene polymer, of a fraction of ethylene polymer (A) having an SD(A) of more than 965 kg/m3 and a melt flow index MI2(A) of at least 10 g/10 min, and

from 65 to 35 wt %, based on the total weight of the multimodal ethylene polymer, of a fraction of a copolymer (B) of ethylene and at least one alpha-olefin containing from 3 to 12 carbon atoms, and having a melt flow index MI2(B) of less than 10 g/10 min and a content of said alpha-olefin(s) of from 0.1 to 5 mol %.

Within the scope of the present invention, the term “screw caps” is meant screw caps possessing a threaded closure. In most cases, said screw caps are provided with a tear strip.

By “multimodal ethylene polymer” is meant an ethylene polymer comprising at least two fractions having different melt flow indices (MI2) so that it possesses a broad or multimodal molecular weight distribution.

The multimodal ethylene polymer used in the present invention has generally a standard density (SD) which does not exceed 965 kg/m3. Within the scope of the present invention, the SD is measured according to the standard ISO 1183-3 (1999). The SD preferably does not exceed 960 kg/m3, more particularly not 958 kg/m3. The SD is preferably at least 951 kg/m3.

The multimodal ethylene polymer used in the present invention preferably possesses a melt flow index (MI2) measured at 190° C. under a load of 2.16 kg according to the standard ASTM D 1238 (1998) of less than 4 g/10 min. MI2 values of less than 2 g/10 min are particularly preferred. The melt flow index MI2 is, in general, at least 0.5 g/10 min, and may be at least 0.8 g/10 min; values of at least 1.2 g/10 min being particularly recommended. Melt flow indices of 1.4 to 1.8 g/10 min are very particularly preferred.

The fraction of ethylene polymer (A) in the multimodal ethylene polymer is preferably at least 40%, more particularly at least 45% by weight compared with the total weight of the multimodal ethylene polymer. The fraction of ethylene polymer (A) preferably does not exceed 60 wt %, more particularly it does not exceed 55 wt % compared with the total weight of the multimodal ethylene polymer. Good results were obtained with a fraction of ethylene polymer (A) of 48 to 52 wt % compared with the total weight of the multimodal ethylene polymer.

The fraction of ethylene copolymer (B) in the multimodal ethylene polymer is preferably at least 40%, more particularly at least 45 wt % by weight compared with the total weight of the multimodal ethylene polymer. The fraction of ethylene copolymer (B) preferably does not exceed 60 wt %, more particularly not 55 wt % compared with the total weight of the multimodal ethylene polymer. Fractions of ethylene copolymer (B) of 48 to 52 wt % compared with the total weight of the multimodal ethylene polymer have given good results.

The composition used in the present invention generally contains at least 95%, preferably at least 98% by weight of the whole of the polymer (A) and the copolymer (B). Most particularly preferred is a composition consisting mainly of the polymer (A) and the copolymer (B).

Preferably, the polymer (A) is an ethylene homopolymer. For the purposes of the present invention, there is taken to mean by ethylene homopolymer (A) an ethylene polymer consisting mainly of monomer units of ethylene and substantially devoid of monomer units derived from other olefins.

By ethylene copolymer with one or more alpha-olefins containing from 3 to 12 atoms of carbons (copolymer (B)) is taken to mean a copolymer comprising monomer units derived from ethylene and monomer units derived from at least one alpha-olefin containing from 3 to 12 atoms of carbon. The alpha-olefin may be selected from among olefinically unsaturated monomers such as butene-1, pentene-1, hexene-1, octene-1. butene-1 is particularly preferred. The content of alpha-olefin in the copolymer (B) is with advantage at least equal to 0.2 molar %, in particular at least equal to 0.3 molar %. The content of alpha-olefin in the copolymer (B) is preferably at most equal to 4 molar %, more precisely at most equal to 3 molar %. Particularly good results are obtained with alpha-olefin contents in the copolymer (B) of 0.5 to 2 molar %.

The SD of the polymer (A) (SD(A)) is preferably at least 968 kg/m3, more particularly at least 970 kg/m3. With advantage, the polymer (A) is characterised by a value of MI2(A) of at least 30 g/10 min, more particularly at least 50 g/10 min. Preferably, the value of MI2(A) does not exceed 500 g/10 min, values of less than 400 g/10 min being particularly preferred. Melt flow indices MI2(A) of at least 80 g/10 min, particularly 80 to 200 g/10 min, have given good results.

Preferably, copolymer (B) is characterised by a value of MI2(B) of at least 0.03 g/10 min, more particularly of at least 0.06 g/10 min. There is preferred most particularly a value of MI2(B) of at least 0.08 g/10 min. Preferably, the value of MI2(B) does not exceed 2 g/10 min, values of at most 1 g/10 min being particularly preferred. There is preferred most particularly an MI2(B) value of at most 0.8 g/10 min, more preferably no more than 0.5 g/10 min. Melt flow indices MI2(B) of 0.08 to 0.8 g/10 min have given good results.

The multimodal ethylene polymer used in the present invention may be obtained by any suitable technique. It is possible, for example, to perform the mixing of the polymer (A) and the copolymer (B) by any known process such as, for example, the molten mixing of the two preformed polymers. Preferred, however, are processes in the course of which the polymer (A) and the copolymer (B) are prepared in at least two successive polymerisation stages. In general, first of all the preparation of the polymer (A) is performed and then the preparation of the copolymer (B) in the presence of the polymer (A) obtained from the first polymerisation stage. The polymerisation stages may each be carried out, independently of one another, in suspension in an inert hydrocarbon diluent or in gaseous phase. A process comprising at least two polymerisation stages in suspension in a hydrocarbon diluent is preferred. The hydrocarbon diluent is generally chosen from among aliphatic hydrocarbons containing from 3 to 10 carbon atoms. Preferably, the diluent is chosen from among propane, isobutane, hexane or their mixtures.

In addition to the multimodal ethylene polymer, the composition used in the present invention may contain conventional additives such as antioxidants, antacids, UV stabilisers, dyes, fillers, antistatic agents and lubricating agents. The total content of additives generally does not exceed 5 wt % compared with the total weight of the composition used in the present invention. Preferably it does not exceed 2 wt %.

The composition used for the manufacture of caps according to the invention may be obtained by any suitable known means. It is possible, for example, to employ two successive stages, the first comprising mixing the multimodal ethylene polymer and where applicable the additives at ambient temperature, the second stage comprising continuing the mixing in the molten state in an extruder. The temperature of the second stage is generally from 100 to 300° C., in particular from 120 to 250° C., more particularly from about 130 to 210° C. An alternative method comprises introducing the additives and where applicable the other compounds into the already molten multimodal ethylene polymer.

It is also possible to prepare, in an initial stage, a master batch comprising a first fraction of the multimodal ethylene polymer plus any additives, said master batch being rich in additives and optionally in other compounds. The master batch is then mixed with the remaining fraction of the multimodal ethylene polymer, for example during the manufacture of granules of the composition.

The screw caps according to the invention may be obtained by any known technique for the manufacture of objects. Injection moulding is particularly well suited.

The screw caps according to the present invention have good organoleptic properties which render them suitable to be used on bottles containing foodstuffs. In addition, they possess a good resistance to slow cracking. The screw caps according to the present invention have an acceptable opening torque. They possess in addition good dimensional tolerances. They are therefore particularly well suited to being used on bottles containing fizzy drinks. The invention consequently also relates to the use of the caps according to the invention for the closure of bottles containing foodstuffs, more particularly for the closure of bottles containing fizzy drinks. A further aspect of the invention relates to screw caps comprising a composition based on a multimodal ethylene polymer and having an ESCR(B) greater than 800 hours, a notched Charpy impact resistance greater than 7 kJ/m2, and an injectability greater than 2.8 s. Preferably the screw caps of this aspect of the invention comprise composition is having a standard density greater than 950 kg/m3, and/or a melt flow index MI2 of less than 10 g/10 min. It is also preferred that they are based on compositions according to the first aspect of the present invention.

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• Utility Patent

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