WO2005049705A1

WO2005049705A1 - Stopper based on an expanded plastic compound

Info

Publication number: WO2005049705A1
Application number: PCT/EP2004/052668
Authority: WO
Inventors: Martine Kaszacs; Claude Dehennau
Original assignee: Solvay Sa
Priority date: 2003-10-29
Filing date: 2004-10-26
Publication date: 2005-06-02
Also published as: FR2861700A1

Abstract

Stopper based on an expanded and at least partially crosslinked plastic compoud comprising at least one polyolefin and being at least partially crosslinked; process and apparatus for manufacturing such a stopper.

Description

Stopper based on an expanded plastic compound

The present invention relates to a stopper based on an expanded plastic compound and to a process and an apparatus for manufacturing such a stopper. The material used most at the present time for manufacturing stoppers intended for bottles or flasks of wine, champagne, alcoholic drinks, etc. is cork. This is because such a material has many advantages :

- it contains many gaseous inclusions, which makes it light and in fact an excellent heat insulator, vibration insulator, etc.;

- it is flexible and elastic, which gives it good sealing potential;

- it does not absorb dust nor does it cause allergies; - it is fire-resistant and can be recycled;

- its chemical composition makes it hydrophobic; and

- its honeycomb surface texture gives it an excellent friction coefficient and good abrasion resistance. However, cork is a natural material, coming from limited natural resources, whereas the demand for stoppers continues to grow. Moreover, cork also has certain drawbacks:

- its quality is not constant and is also in decline, especially owing to the growth in demand;

- grades of cork that are too soft, too porous or too irregular in shape may result in leaks;

- the taste of the wine may be progressively impaired as a result of various chemical or physical phenomena (oxidation; extraction of certain impurities by the wine, etc.);

- the mechanical properties of cork may deteriorate over time if it is not stored in a controlled-humidity atmosphere;

- finally, cork may be inhabited by larvae, which alter the physical structure thereof and may lead to impurities and to deposits in the wine. It has therefore been sought to replace cork with synthetic materials having the maximum number of advantages of cork without the abovementioned drawbacks thereof, while still maintaining the visual and tactile appearance with which consumers are accustomed. Thus, document US 4,499,141 discloses the manufacture of synthetic stoppers by expanding a plastic compound comprising polyethylene (PE) and an ethylene/vinyl acetate copolymer (EVA). Such stoppers have the advantage of being light, elastic and able to be printed by means of cork inks, but they also have certain disadvantages, in particular:

- they raise organoleptic problems that are attributed to the presence of vinyl acetate (VAc) and/or its hydrolysis products; and

- the cellular structure obtained after expansion is relatively coarse and irregular. The Applicant has found that, surprisingly, these problems can be solved by at least partially crosslinking the said plastic compound, especially by means of a peroxide. The present invention therefore relates, according to a first aspect, to a stopper based on an expanded plastic compound comprising at least one polyolefin, the said compound being at least partially crosslinked. The term "stopper" is understood according to the present invention to denote a closure of generally cylindrical shape intended to be inserted into, and to close off, the neck of a container (bottle, flask, etc.). The term "generally cylindrical" is understood to mean a shape having an external envelope substantially cylindrical in shape, but this does not exclude slightly conical shapes or even the presence of a rounded-off (chamfered) part at one of the two ends of the cylinder at least. According to the invention, the plastic compound constituting the stopper is expanded, that is to say it is in the form of a foam, preferably having closed cells. These cells are generally uniform in size and of uniform shape, generally elliptical or spherical with a major axis/minor axis ratio of greater than 0.5, preferably 0.9, or even equal to 1. In general, these ellipses (spheres) have a major axis (diameter) not exceeding 1 mm, or even 800 μm. However, it is rare for this axis (diameter) to be less than 200 μm, or even less than 100 μm. As a result of the foregoing, the density of these stoppers is less than the density of the unexpanded compound. In general, this density is not less than 0.1, or even 0.3 g/cm³. However, it generally does not exceed 0.8 or even 0.6 g/cm³. The term "polyolefin" is understood to mean any polymer having as main monomer (i.e. greater than 50 % by number) a C2-C10 olefϊn. Preferably, this olefin is ethylene or propylene, ethylene being particularly preferred. According to the invention, the polyolefin may be a blend of polyolefins, blends of ethylene-based resins being preferred. According to one particularly advantageous embodiment of this aspect of the invention, the polyolefin is a blend of polyethylene (PE) and an ethylene/vinyl acetate copolymer (EVA). The polyethylene (PE) used in the stopper according to this embodiment of the invention is a polymer (homopolymer or copolymer) which consists mainly of ethylene. It is preferably a polymer containing at least 90 wt% ethylene and having, as optional comonomer(s), one or more C₃-Cιo α-olefins. It should also be noted that it may be a blend of several different PEs and particularly blends of PE homopolymers and/or copolymers and/or resins having different densities. For example, it may be blend of HDPE (high-density PE, i.e. a PE generally having a density of 0.948 to 0.965 g/cm³) with an LDPE (low-density PE, i.e. a PE generally having a density of 0.913 to 0.933 g/cm³), with an LLDPE (linear low-density PE generally having a density of 0.918 to 0.946 g/cm³) and/or with a VLDPE (very low-density PE, i.e. a PE generally having a density of 0.882 to 0.911 g/cm³). HDPE/VLDPE blends give good results. In particular, blends of HDPE with a plastomer-type ethylene/octene copolymer VLDPE (such as, for example, EXACT^® 8210) give good results. In these blends, the homopolymer content is preferably between 1 and 50 % by weight and the plastomer content from 50 to 99 % by weight. Alternatively, in this embodiment, a VLDPE alone (without being blended with a HDPE) can be used in conjunction with an EVA. In that case, it has been observed that the more rigid the VLDPE resin is (presents a high modulus), the better the results are (namely in terms of elastic return). For instance, better results are observed with a VLDPE resin having an E modulus (according to ISO 527) of about 60 MPa (like grade EXACT^® 0210) than with a VLDPE resin having an E modulus of about 24 MPa (like grade EXACT^® 8210). Finally, it should be noted that the above mentioned PE resins may be grafted using functional monomers, such as maleic anhydride, vinylsilane, etc., but generally with a weight content of 5 % or less, preferably 3 % or less or even 1 % or less (relative to the total weight of the polymer). The ethylene/vinyl acetate copolymer (EVA) used in the stopper according to this embodiment of the invention preferably has a vinyl acetate (VAc) weight content not less than 3 %, preferably not less than 5 %, or even not less than 10 %. However, this content is generally 30 % or less, preferably 20 % or less, or even 10 % or less. It should also be noted that it may be a blend of several different EVAs. Should the EVA be used in a blend with VLDPE alone (without HDPE), then better results are obtained with more rigid resins, i.e. resins having at most 10% VAc. For instance, better results are obtained with an EVA resin having an E modulus (according to ASTM D 638) of about 82 MPa (like grade FL00909) than with an EVA resin having an E modulus of about 55 MPa (like grade UL00514). The EVA weight content in the plastic compound according to this embodiment of the invention is preferably not less than 30 %, not less than 40 %, or even not less than 45 % (relative to the total weight of the compound). However, this content is generally 90 % or less, or even 80 % or less. Should the EVA be used in a blend with VLDPE alone (without HDPE), then better results are obtained with compositions having an EVA content in between 20 and 30 % by weight. The expanded plastic compound may include polymers and/or additives other than the polyolefin. Preferably, it does not include polymers other than polyolefins and particularly preferably it contains only PE and EVA as polymers. However, it is advantageous for it to include one or more standard additives for plastics, such as stabilizers, pigments, lubricants, nucleating agents, etc. To facilitate the insertion (and the re-insertion) of the stopper into the neck, it is advantageous for the expanded plastic compound to include at least one slip agent, so as to avoid having to use a coating (for example a silicone coating). Examples of such slip agents are primary amines derived from long-chain fatty acids, such as erucamide, oleamide, behenamide, stearamide, etc. A slip agent giving good results is erucamide and in particular the grade L CROSLIP^® C from Croda. The weight content of lubricant in the expanded plastic compound according to this embodiment of the invention is generally not less than 0.05 %, or even not less than 0.1 % (relative to the total weight of the compound). However, it generally does not exceed 10 %, or even 5 %. Furthermore, to promote the foπnation of a fine and uniform cellular structure, the expanded plastic compound preferably includes a nucleating agent. As examples of nucleating agents, mention may be made of fine mineral particles, such as microparticles like talc, clays, etc., or nanoparticles, for example such as silicate nanoparticles. Talc gives good results, in particular with weight contents (relative to the total weight of the compound) of 10 % or less, preferably 5 % or less or even 2 % or less. Finally, to best imitate the appearance of cork, the expanded plastic compound preferably includes a pigment whose colour and concentration will both be adapted for this purpose. According to the invention, the plastic compound is at least partially crosslinked. The term "at least partial" crosslinking is understood to mean that long intermolecular and intramolecular branches are formed. However, preferably this crosslinking takes place without excessive formation of insolubles (the said insolubles being determined by dissolving the plastic in boiling xylene and successively centrifuging the undissolved material), since these are liable to impair the expansion of the plastic. Preferably, the insolubles content does not exceed 15 %, or even 10 %, or even better still 5 %. A content not exceeding 1 %, or even 0.2 % by weight (the detection limit often encountered in practice) is particularly preferable. It is most particularly advantageous for the partial crosslinking to be such that the plastic has a non-zero value of tan delta (tanδ or mean of the ratio of the viscous modulus to the elastic modulus, the moduli being measured at 190°C at between 0.1 and 1 s-1 using an ARES rheogoniometer calibrated according to the ISO standard 6721-10) of 3 or less, and preferably 1.5 or less. However, in the case of blends of VLDPE and EVA discussed above, this value may be higher than 3, but will generally remain below 6. Most particularly preferably, the plastic is both free of insolubles (i.e. its insolubles content is below the detection limit) and has a non-zero value of tanδ of 3 or less, or even 1.5 or less. In the case of blends of VLDPE and EVA, these are preferably free of insolubles and having a value of tanδ of less than 6. The at least partial crosslinking of the plastic compound may be obtained by any known means, such as by electron bombardment, a condensation reaction, an acid-base reaction, or through the action of a peroxide. Preferably, the crosslinking is carried out using a peroxide. Advantageously, the latter may be chosen from tert-butyl cumyl peroxide, l,3-di(2-tert-butylperoxyisopropyl)- benzene, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, di(tert-butyl)peroxide and 2,5-dimethyl-2,5-di(tert-butylperoxy)-hex-3-yne. Good results are given by 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (DHBP). In general, this peroxide is substantially removed during processing, that is to say that, in the stopper, it is absent or present only in trace amounts (of the order of ppm or even ppb). According to a second aspect, the present invention relates to a process for manufacturing a stopper as described above. In this process, a plastic compound comprising at least one polyolefin is at least partially crosslinked; at least one blowing agent is blended into the crosslinked compound and the blend is injection-moulded and expanded in a mould comprising at least one cavity having the shape of the stopper. The crosslinking of the plastic compound by means of the peroxide may take place by any known means. Preferably, this crosslinking takes place in the melt and most particularly preferably it takes place in an extruder coupled to a granulator, thereby making it possible to obtain granules of the crosslinked compound. According to this method of implementation, the polyolefm(s) and optional additives are preferably physically blended before they are introduced into the extruder and peroxide is injected into this blend in the extruder. The injection of the peroxide into the extruder may be carried out by any suitable means. However, owing to the low peroxide concentration used (preferably less than 1000 ppm, or less than 800 ppm, or even less than 500 ppm, although this content is generally at least 5 ppm, or 10 or even 50 ppm; in the case of blends of VLDPE and EVA, this concentration is preferably higher than 1000 ppm, but lower than 1400 ppm), it is often advantageous to use a suitable spray device, such as a nitrogen sprayer coupled to a pump. Moreover, to promote adhesion of the peroxide to the polymer granules, it may be advantage to use at least one liquid agent (for example polyethylene glycol) and/or solid agent (for example PE powder) that is then advantageously introduced during the abovementioned physical blending and this in contents of the order of 1 tenth of a per cent (relative to the total weight of the compound). To avoid the abovementioned organoleptic problems, a vacuum degassing operation is advantageously carried out in the extruder and/or on the granules obtained. Particularly advantageously, the degassing in the extruder is carried out using a vacuum pump. Thanks to the latter, the degassing pressure is generally 100 mbar or less, or even 50 mbar or less. This method of carrying out the process makes it possible to remove the volatile materials that would be present in the compound (residual peroxide, monomers and/or free substances such as VAc, and/or its hydrolysis products, etc.). As regards degassing the granules, this may be carried out in an oven, by steam stripping, or hot-water stripping, etc. In another way of implementing the process according to the invention, the at least partially crosslinked plastic compound is not injection-moulded and expanded as such, but is firstly blended with a similar compound (i.e. one comprising identical polymers, preferably PE and EVA) but which is not crosslinked. In this blend, the weight content of crosslinked compound is advantageously equal to 5 % or higher, or even 10 % or higher (relative to the total weight of the compound). However, this content does not generally exceed 50 %, or even 40 %. When a process according to this method of implementing the invention is carried out, measures will preferably be taken to ensure that the crosslinked compound is sufficiently crosslinked to still be effective when diluted, but not crosslinked so much as to not introduce insolubles into the compound to be injection-moulded and expanded. The above crosslinked compound/uncrosslinked compound blend may be produced by any known means. Preferably, this blend is produced by melt- blending in an extruder coupled to a granulator for obtaining granules comprising a crosslinked fraction and an uncrosslinked fraction. According to this method of implementation, the various constituents of the blend (crosslinked fraction, uncrosslinked fraction (or its components taken separately) and optional additives) are preferably physically blended firstly in a mechanical blender and the physical blend thus obtained is then homogenized in the extruder. In this method of implementation, it is also advantageous to carry out a vacuum degassing operation in the extruder and/or on the granules obtained, as described above. Finally, it is worth noting that the prefabrication of crosslinked/uncrosslinked compounds as described above is not required with blends of EVA and VLDPE. The blowing agent used in the process according to the invention may be of any known type. It may be what is called a "physical" blowing agent, that is to say a gas dissolved in the plastic and under pressure and which causes its expansion during the expansion on leaving the extruder. Examples of such gases are CO₂, nitrogen, steam, HFCs (such as the mixture SOLKANE^® XG87 based on HFC- 134a and HFC- 152a, sold by SOLVAY), hydrocarbons (such as butane and pentane) or a mixture of these. It may also be what is called a "chemical" blowing agent, that is to say a substance (or a mixture of substances) which is dissolved or dispersed in the plastic and which, under the effect of temperature, releases the gas or gases that will serve to expand the plastic. Examples of such substances are azodicarbonamide, a sodium bicarbonate/citric acid mixture, etc., and also mixtures thereof. Preferably, the blowing agent is a chemical agent and most particularly preferably it is blended into the at least partially crosslinked plastic compound before the said compound is injected into the mould. This blend may be produced by any known process but does not result in decomposition of the said chemical agent. Preferably, in the process according to the invention, the injection moulding and expansion take place simultaneously, as described in Patent US 4,499,141, the teaching of which in this regard is introduced by reference into the present application. In summary, according to that process, to injection- mould and expand the blend that includes the blowing agent, a strand of the molten blend is injected through an orifice made in the wall of the mould and communicating with the cavity; the strand then expands, turning on itself in order to adopt the shape of the cavity and form the stopper. In that process, the pressure, the temperature (of the molten material and of the mould), the injection-moulding time, the cooling of the stopper before ejection, etc. and other process parameters will be readily adapted by a person skilled in the art to the compounds used, the mould geometry, etc. Furthermore, in the process according to the invention it is advantageous to include at least one step of making the said stopper impermeable, consisting in applying thereto an impermeable coating over at least part of its surface. This coating is generally based on a barrier polymer such as PVDC (polyvinylidene chloride), silica, amorphous carbon, etc. A PVDC-based coating (i.e. one having at least one layer mainly consisting of PVDC) gives good results. In particular in the case in which the stoppers contain EVA, an EVA/PVDC/EVA multilayer coating gives good results and makes it possible to dispense with the use of a surface treatment or with a primer in order to ensure adhesion to the stopper. This is because the presence of EVA both in the stopper and in the external layers of the coating ensures compatibility between these elements in a very effective and simple manner. Furthermore, such films are commercially available as they are already used for sealing polyolefin-based food packages. Another type of coating that gives good results is a coating consisting of PVDC (for its barrier effect )/elastic material (for elastic recovery, very important at the moment of stoppering). The elastic material may be an SEBS-type polymer. The abovementioned impermeabilizing step may take place during or after the abovementioned injection-moulding and expansion steps. With a view in particular to reducing the production cycle time, this step may take place at the same time as the injection and/or the expansion of the plastic compound. One way of carrying out this method of implementation consists in placing, in the mould cavity, prior to the injection- moulding/expansion, an impermeable coating. One very suitable means consists in using the device for ejecting the stopper from the mould (i.e. the ejector) to deposit, inside the mould cavity, the said coating and to then injection- mould/expand the abovementioned compound on this coating. According to another method of implementation, the impermeablizing step takes place on the stopper obtained after the injection-moulding/expansion of the said compound. One very suitable means for carrying out this method of implementation consists in heating an impermeable coating and thermoforming it with the aid of the stopper, that is to say in making the stopper pass through this coating but making sure that the thermoformed part remains on the stopper and is detached from the base coating so that the stopper is as it were "topped" by an impermeabilizing cap (preferably on that side intended to be pushed right into the neck). In the case of an impermeable film of the EVA/PVDC/EVA type, the adhesion between the said film and the stopper is provided by the thermal melting of the EVA layer and by the chemical compatability between this layer and the stopper. The abovementioned thermoforming may also take place during the ejection of the stopper out of the mould cavity, which makes it possible to reduce the manufacturing cycle time. Another very suitable means in the case of silica or amorphous carbon coatings consists in placing the stopper in a plasma chamber in order to deposit the said coating therein. The process according to the invention may be implemented by any suitable apparatus comprising at least one mould having a stopper-shaped cavity. This apparatus preferably comprises several moulds so as to minimize the manufacturing cycle time. According to this method of implementation, the moulds are presented in succession to an injection head, to a device applying an impermeable coating to the mould on the stopper ejection side, and to a device for ejecting the stopper through the impermeable coating. In this device, the said cavities may be placed around the periphery of a "wheel" (device which rotates about itself but does not necessarily have a circular shape) so as to present in succession a given cavity to the various aforementioned (injection, cooling and coating application; ejection) stations. In general, whether the stoppers are natural or synthetic, the presence of a chamfer makes it easier to introduce the stopper into the bottle but is unfavourable as regards the sealing obtained. Consequently, according to a 3^rd aspect, the present invention relates to a stopper provided with a chamfer at only one of its ends (that intended to be introduced into the neck). This way of carrying out the process is particularly suitable when the manufacture of the stoppers takes place in line with the filling of the containers with their contents, so as to be able to dispense with a shape recognition device (for locating the side provided with a chamfer, which has to be pushed right into the neck). In particular to improve the impermeability to gases (mainly oxygen and/or CO₂ in the case of carbonated liquids) of synthetic stoppers and to prevent the migration of various additives and oligomers present in these stoppers into the liquid contained in the container, it is known to provide these stoppers with a coating, especially one which is impermeable to gases and to polymeric additives, oligomers, etc. Such coatings are often based on barrier polymers, silica or amorphous carbon. Consequently, according to a 4^th aspect, the present invention relates to a stopper provided, over at least part of its surface (and preferably at least over that surface which faces the content of the container, i.e. on that face of the stopper which is pushed right into the neck), with a coating based on a barrier polymer such as that described above or based on silica or amorphous carbon. An EVA/PVDC/EVA multilayer coating is preferred when the stopper comprises EVA. Examples The present invention is illustrated in a non- limiting manner by Examples 1 to 3, which were produced under the following conditions :

1. Compounding: the blends given in column 1 of Table 1 below were produced under the following conditions : * barrel temperature: 100-170-190-190-170-170 °C, * screw speed: 200 rpm and * output: 6 kg/h;

2. Manufacture of the stoppers: the compounds manufactured in the previous section were injection- moulded in the form of stoppers under the following conditions : * barrel temperature: (hopper side) 160 °C - 170 °C - 190 °C -190 °C (injection side), * mould temperature: 15 °C and * productivity: 1500 stoppers per hour;

Testing of the stoppers obtained: the results obtained are given in column 2 of Table 1 below, the measurement conditions being the following : Measurement of the compressive force : apparatus : an instrumented semi-automatic laboratory quadruple- compression stoppering machine; compression conditions: stopper diameter reduction from 22 to 16 mm. Measurement of the elastic return force : apparatus : hydraulic compression machine; compression conditions : 33 % stopper diameter reduction along two perpendicular axes; relaxation conditions : release of the compression piston along one axis at a rate of 8000 mm/min. qualitative assessment (3 better than 2, 2 better than 1) Rheolo ical measurement : see descri tion above tanδ

* phr = % resin, i.e. parts by weight per 100 parts by weight of all the resins or plastics.

Claims

C L A I M S

1 - Stopper based on an expanded plastic compound comprising at least one polyolefin, characterized in that the compound is at least partially ' crosslinked. 2 - Stopper according to the preceding claim, characterized in that the polyolefin is a blend of polyethylene (PE) and an ethylene/vinyl acetate copolymer (EVA).

3 — Stopper according to the preceding claim, characterized in that the PE is a VLDPE (very low-density polyethylene) or an HDPE (high-density polyethylene)/VLDPE blend.

4 - Stopper according to either of Claims 2 and 3, characterized in that the weight content of VAc (vinyl acetate) of the EVA is from 5 to 30 %.

5 - Stopper according to any one of Claims 2 to 4, characterized in that the EVA weight content in the expanded plastic compound is from 30 to 90 % relative to the total weight of the compound, or in between 20 and 30 % in the case of EVA/VLDPE blends without HDPE.

6 - Stopper according to any one of the preceding claims, characterized in that the expanded plastic compound includes at least one slip agent.

7 — Stopper according to any one of the preceding claims, characterized in that it is provided with a chamfer at only one of its ends.

8 — Stopper according to any one of the preceding claims, characterized in that it is provided, over at least part of its surface, with an EVA/PVDC/EVA multilayer coating, with a PVDC/SEBS coating or with a silica or amorphous carbon layer. 9 - Process for manufacturing a stopper according to any one of the preceding claims, in which a plastic compound comprising at least one polyolefin is at least partially crosslinked, at least one blowing agent is blended into the crosslinked compound and the blend is injection-moulded and expanded in a mould comprising at least one cavity having the shape of the stopper. 10 - Process according to the preceding claim, characterized in that to injection-mould and expand the blend that includes the blowing agent, a strand of the molten blend is injected through an orifice made in the wall of the mould and communicating with the cavity, and in that the strand expands and turns on itself in order to adopt the shape of the cavity and form the stopper.

11 - Process according to any one of Claims 9 or 10, characterized in that it includes an impermeabilizing step that consists in placing, in the mould cavity, prior to the injection-moulding/expansion an impermeable coating.

12 — Process according to any one of Claims 9 to 11, characterized in that it includes an impermeabilizing step which consists:

- either in heating an impermeable coating and thermoforming it with the aid of the stopper obtained after the injection-moulding/expansion

- or in depositing a layer of silica or amorphous carbon on the stopper obtained after the injection-moulding/expansion, by placing it in a suitable plasma chamber.

13 - Apparatus for implementing the process according to the preceding claim, characterized in that it comprises several moulds having a stopper-shaped cavity and in that these moulds are presented in succession to an injection head, to a device applying an impermeable coating to the mould on the stopper ejection side, and to a device for ejecting the stopper through the impeπneable coating.