AU2001100093A4 - Injection moulding - Google Patents

Description

INJECTION MOULDING The present invention relates to an injection moulding process for producing flexible thin walled injection moulded articles such as tubular containers as used in the cosmetics industry and polymers for the use therein.

Tubular containers are currently produced by a combination of extrusion moulding, injection moulding and welding (this combination referred to herein as the "extrusion process"). The body of the tube is extruded in the form of a continuous cylinder that is then cut into the desired length to form the body of the container. In a separate injection moulding process the "head and shoulders" of the tube are produced. The injection moulded "head and shoulders" are then welded to the extruded tube to form the container. Once the container is filled with product the tail end of the container is sealed by a further welding process.

The extrusion process has a number of limitations, these being the high capital investment required for extrusion equipment, an inability to commercially manufacture a variety of tube shapes, an inability to texture surface finishes or emboss the container, and an inability to incorporate attachments such as closures and hooks during the extrusion process.

PCT/AU98/00255 (hereafter referred to as 'the '255 patent'), and which is incorporated herein by reference, describes a process and materials for the injection moulding of flexible thin walled articles having thin sections and which provide a number of advantages hitherto unattainable through the extrusion process due to technical constraints. The '255 patent describes a wide variety of polymer types and blends that may be used to injection mould thin walled flexible articles, and a method for selecting suitable blends. The polymers and blends described may be injection moulded using a variety of different injection moulding processes. The '255 patent also describes means by which the ESCR of many polymer types in particular, relatively high MFI grades of these polymers may be improved in order to increase their utility for the injection moulding of thin walled flexible articles. In the context of this patent application, ESCR is taken to mean the ESCR of a polymer blend which passes the specially designed ESCR test for testing the ESCR of thin walled moulding, and which is described below.

The ESCR TestSix thin sections of injection moulded polymer blend, 0.65mm thickness were used to determine environmental stress crack resistance.

Sections 10mm wide are cut transverse to the major direction of flow of the polymer blend in the mould and are subsequently treated with any post-mould treatments. Each section is bent back on itself and stapled 3mm from the bend. The bent sections are immersed in a 10% Teric N9 solution at (Teric is a trademark of Orica Australia Pty Ltd). The strips are then regularly checked for signs of cracking. Any sign of cracking is regarded as a failure.

The time at which 50% of the sections have failed is regarded as the time to failure of the polymer blend. The test is concluded after 360 hours if the polymer has yet to fail.

Among the polymer types that may be suitable for injection moulding thin walled flexible articles are star, comb and other polymers such as are described in US 6,084,030, which is incorporated herein by reference. This patent describes highly branched polyolefin polymers in the form of a comb, star, nanogelor structural combinations thereof, whereby a plurality of polyolefin arms are linked to a polymeric backbone to provide a highly branched structure in which the properties of the highly branched structure can be conveniently tailored to the application for which the polymer is used.

Unlike the prior art it describes, the concepts of the invention described in US 6,084,030 use as the backbone a polymeric structure to which polyolefin arms are linked. Through the use of a polymeric backbone, it is possible to obtain very highly branched polymeric structures in which a relatively high number of branches or arms can be provided on the polymeric backbone. The choice of specific reactive polymeric backbone and/or its manner of preparation controls the branched structure as to comb, star, nanogel or structural combinations thereof. That allows for the preparation of polymers having relatively low viscosities compared to their linear counterparts at the same absolute molecular weight. These polymer types and blends made therefrom may be particularly suitable for the production of injection mould thin walled flexible articles.

Other polymer types that may be suitable for injection moulding flexible thin walled articles include polymers that are made up of or include in their structure functional groups and/or monomers that facilitate the breakdown of the polymer molecules into smaller molecules by various means such as the activities of organisms such as bacteria or fungi, the activities of nature such as light, oxidation, etc. Such polymers may include natural components or chemical types such as cellulose, starch, lactic acid and their various derivatives and/or other biologically active moieties in their molecular structure or blends and/or may be polymerised with chemical structures that are designed to make the polymer susceptible to molecular size reduction. Thin walled flexible articles made from such polymers may be particularly advantageous because of their ability to be disposed of more easily than nondegradable polymers, and/or with less environmental impact.

It has been demonstrated (as for example by Zhou et al, Polymer, Vol 24, p.

2520 (1993)), that large strain properties of polymer blends, such as toughness tear, impact and ESCR, can be improved by the presence of "tie molecules" in the blend, and that as a generalisation, the greater the proportion of tie molecules in the blend, the greater the improvement in properties. High molecular weight molecules with the highest comonomer content (that is the highest degree of short chain branching) are responsible for the formation of most of the tie molecules upon crystallization. Thus attempts to maximize properties such as toughness, modulus, impact strength and ESCR without sacrificing processability has resulted in the preparation and use of blend compositions made out of two or more polymer components of differing molecular structures. Blends consisting of a high molecular weight low MFl) copolymer in combination with a low molecular weight high MFl) homopolymer or copolymer with higher crystallinitythan the high molecular weight polymer have been found to possess good processability and excellent low temperature mechanical properties, and provided they pass the ESCR test described in the '255 patent, are particularly suited for the manufacture of injection moulded flexible thin walled articles. This is particularly the case for blends with an MFI (12) greater than 10, more preferably greater than 20 and most preferably greater than 30. While not wishing to be bound by any theory, it is believed that the low MFI copolymers-preferably polyolefin copolymers, more preferably polyethylene or polypropylene copolymers, even more preferably polyethylene copolymers and most preferably polyethylene copolymers characterised by super-random distribution of comonomers within the polymer chains-produce a high number of 'tie' molecules within the blend, resulting in improved ESCR and other physical and chemical properties. The high MFI component(s) of such blends are preferably homo or copolymers with a lower copolymer content than the low MFI component(s), and are preferably polyolefin homopolymers or copolymers, more preferably polyethylene or polypropylene homopolymers or copolymers and most preferably polyethylene copolymers not characterised by super-random distribution of comonomers within the polymer chains (eg. Z- N catalysed polymers). When the high MFI component(s) are polyethylenes, they may be ultra-low, low, medium and/or high density polyethylene homo or copolymers. It is preferable that the high and low MFI blend components are homo and/or copolymers of the same olefin, eg. both are either polyethylene polymers or polypropylene polymers, though this is not essential. It is believed that the high MFI component(s) provide the blend with improved processing characteristics relative to the low MFI component, and in addition improve the cycle time of the moulding process by accelerating the crystallisation of the low MFI blend component by acting as nucleating agents, thereby enabling the moulding to be removed from the mould sooner than would otherwise be the case. Also, because the high MFI component(s) usually has a higher crystallinity than the low MFI component, they usually make removal of the moulding from the tool easier than would otherwise be the case if the low MFI component(s) alone comprised the blend. An additional advantage of the high MFI component(s) is that it raises the MFI of the blend, thereby lowering the injection pressure necessary to fill the mould and minimising the chances of core flexing and other problems associated with high injection pressures associated with low MFI blends, and which may be a particular problem with regard to the process of injection moulding thin walled articles, and tubes in particular. The blends may contain 2-98% low MFI component(s) as described above and 98-2% high MFI component(s) as described above, preferably 10-80% low MFI component(s) and 90-20% high MFI component(s) and most preferably 20-40% low MFI component(s) and 80-60% high MFI component(s). The blends preferably have an MFI of greater than 10, more preferably greater than 20 and most preferably greater than 30. Illustrative non-limiting examples of blends meeting the above criteria are as follows: Example 30% Affinity 8407 (Dupont-Dow) a MFI, 0.87 density mPE copolymer 70% WSM 168 (Qenos) a 60 MFI .918 density LDPE Example 30% Affinity 8407 (Dupont-Dow) a 30 MFI, 0.87 density mPE copolymer 70% HD9952 (Qenos) a 36 MFI, 0.956 density HDPE Example 30% Affinity 8402 (Dupont-Dow) a 30 MFI, 0.902 density mPE copolymer 70% DOWLEX IP-60 (Dow) a 60 MFI, 0.952 density HDPE Blends meeting the above criteria blends containing both high and low MFI components made of substantially similar base polymer types) and containing solely Ziegler catalyst products are described in a number of patents. For example, US 3,280,220 teaches that a blend of an ethylene homopolymer of low molecular weight and an ethylene-butene-1 copolymer of high molecular weight provides higher ESCR advantageous for containers (bottles) and pipe than similar blends of copolymers. US 3,660,530 teaches a method where part of the homopolymer produced after a first reaction step is subjected to 1 -butene. The still-active catalyst then produces a block copolymer of polyethylene and polymerized butene-1. Both components are then admixed. The resultant blend has improved ESCR properties. US 4,438,238 discloses blends consisting of components with densities between 0.910 and 0.940 g/cm3 and broad molecular weight and blend distributions substantially not having long chain branches that have been found to have good processability similar to high pressure polyethylene. US 4,547,551 teaches that ethylene polymer blends of a high molecular weight ethylene polymer, preferably an ethylene-alpha-olefin copolymer, and a low molecular weight ethylene polymer, preferably an ethylene homopolymer, both preferentially with a narrow molecular weight distribution and low levels of long chain branching exhibit excellent film properties and a better balance of stiffness and impact and environmental stress cracking resistance (ESCR), superior to that expected for polyethylene of comparable density and flow. US 5,189,106, and 5,260,384 disclose blends consisting of a high molecular weight copolymer in combination with a low molecular weight homopolymer have been found to possess good processability and excellent low temperature mechanical properties. Polymer blends of the abovementioned patents, all of which are incorporated by reference, may be suitable for use in the present invention for the production of thin-walled products, and especially for the production of flexible tubes as described in the '255 patent.

Blend compositions of homogeneous interpolymers having narrow molecular weight distribution and narrow composition distributions are also known.

Stehling et al. in US 5,382,630 and 5,382,631 describe polymer compositions made by blending components which have Mw/Mn of less than 3 and a Composition Distribution Breadth Index of equal-to-or-greater-than 50 percent.

The components are said to be produced by using metallocene catalyst systems known to provide narrow composition distributions and narrow molecular weight distributions. Polymer blends of the abovementioned patents may be suitable for use in the present invention for the production of thin-walled products, and especially for the production of flexible tubes as described in the '255 patent.

Blend compositions containing both Ziegler and single site catalyst products have also been disclosed. Research Disclosure No. 310163 (Anonymous) teaches that blends of Ziegler Natta- and metallocene-catalysed ethylene copolymers when fabricated into cast films have improved optical, toughness, heat sealability, film blocking and unwind noise properties when compared with metallocene-catalyzed polymer alone. Research Disclosure No. 37644 (Anonymous) teaches that blends of traditionally (Ziegler-Natta) catalysed resins and resins made by single site metallocene catalysts display superior transverse direction tear and machine direction ultimate tensile properties useful in cast film applications. WO 94/25523 teaches that films having synergistically enhanced physical properties can be made when the film is a blend of at least one homogeneously branched ethylene/alpha-olefin interpolymer and a heterogeneously branched ethylene/alpha-olefin interpolymer. Films made from such formulated compositions have surprisingly good impact and tensile properties, and an especially good combination of modulus and toughness. Polymer blends based on the abovementioned Research Disclosures and patents may be suitable for use in the present invention for the production of thin-walled products, and especially for the production of flexible tubes as described in the '255 patent.

WO 01/32771 describes an ethylene polymer blend comprising at least two diverse ethylene interpolymers wherein one interpolymer has a lower number of carbons than the at least one other interpolymer. The ethylene polymer blend preferably comprises at least one homogeneously branched ethylene/alpha-olefin interpolymer blended with at least one heterogeneously branched ethylene alpha-olefin interpolymer and is characterized as having a density greater than or equal to 0.90 g/cm3 and in particularly preferred embodiments is further characterized as having an intrinsic tear value greater than or equal to 150 grams. Polymer blends of the abovementioned patent, as well as similar blends with densities lower than 0.90g/cm3, may be suitable for use in the present invention for the production of thin-walled products, and especially for the production of flexible tubes as described in the '255 patent.

United States Patent: 5,082,902 teaches that a single component of a blend, preferably a polyolefin component, more preferably a polyethylene component and even more preferably a medium or high density (>0.935 g/cm3) polyethylene component, may advantageously be substituted with a blend of 2 or more of the same polymer type to achieve a density similar to that of the single substituted component. The effect of this substitution will often result in the reduction of the crystallization half-time and mold cycle time of polymer blends. While not wishing to be bound to any theory, we believe the higher density polyethylene nucleates the lower density polyethylene accelerating the rate of crystallization. Since the blend freezes faster and at a higher temperature, than a similar density single polymer, the molded article can be removed or ejected from the mold sooner, resulting in a shorter overall molding cycle time. Also, the modified crystal morphology of the molded blend provides improved physical properties total impact energy, ESCR, etc.) compared to a molded similar density single polymer. Polymer blends made according to, or incorporating components made according to, the abovementioned patent may be suitable for use in the present invention for the production of thin-walled products, and especially for the production of flexible tubes as described in the '255 patent.

WO 00/01766 describes blends designed for recoverability and which contain a dispersed phase of a greater crystallinity and a continuous phase of lesser crystallinity. The sizes of the individual domains of the dispersed phase are very small. The components of the blend in both cases are also compatible to the extent that no compatibilizer needs to be added to attain and retain this fine morphology. One of the components is a polymer comprising predominately stereospecific polypropylene, preferably isotactic polypropylene. This is the component with greater crystallinity. A second component is a copolymer of propylene and at least one C2, C4-C20 alphaolefin, preferably ethylene. This is the component with lesser crystallinity. In the copolymer the propylene is preferably polymerized substantially stereospecifically. Preferably the copolymer has a substantially uniform composition distribution, preferably as a result of polymerization with a metallocene catalyst. Most preferably, said second component is an ethylene propylene copolymer, e.g. ethylene propylene semicrystalline elastomer.

More recently several authors have shown the formation of more refined structures of partially atactic, partially isotactic polypropylene which have elastomeric properties. It is believed that in these components each molecule consists of portions which are isotactic, and therefore crystallisable, while the other portions of the same polypropylene molecule are atactic and therefore amorphous. Examples of these propylene homopolymers containing different levels of isotacticityin different portions of the molecule are described by R.

Waymouth in U.S. Patent 5,594.080, in the article in the Journal American Chemical Society' (1995), Vol. 117, page 11586, and in the article in the Journal American Chemical Society (1997), Vol. 119, page 3635, 3. Chien in the journal article in the Journal of 20 the American Chemical Society (1991), Vol. 113, pages 8569-8570; and S. Collins in the journal article in Macromolecules (1995) Vol. 28, pages 3771-3778. These articles describe a specific polymer, but do not describe the blends with a more crystalline polymer such as isotactic polypropylene. Such polymers, and blends incorporating such polymers, may be suitable for use in the present invention for the production of thin-walled products, and especially for the production of flexible tubes, and as a component(s) of blends produced according to WO 00/01766 and similar developments.

It has been found that by blending a crystalline propylene polymer, hereinafter referred to as the "first polymer component" (FPC) and a crystallizable propylene alpha olefin copolymer polymer, hereinafter referred to as the "second polymer component" (SPC), advantageous processing characteristics result while still providing a composition having decreased flexural modulus and increased tensile strength, elongation, recovery and overall toughness. It is possible to have the addition of a third polymeric component which is another crystallizable propylene alpha olefin copolymer indicated as SPC2 in the text below which has crystallinity intermediate between the FPC and the SPC. The SPC2 may also have a narrow composition distribution and be made with a metallocene catalyst. The addition of SPC2 leads to a finer morphology and improvements in some of the properties of the blend of FPC and SPC.

Blends made with SPC, FPC and optionally SPC2 have heterophase morphology formed by blending a FPC which is a predominately crystalline stereoregular polypropylene with an SPC which is a crystallizable copolymer of at least one 02, 04-020 alpha-olefin (preferably ethylene) and propylene.

Optional components of the blend are SPC2, a crystallizable copolymer of at least one 02, 04-020 alpha-olefin (preferably ethylene), and process oil.

Other optional components are fillers, colorants, antioxidants, nucleators and flow improvers. The SPC and the SPC2, if used, have stereoregular propylene sequences long enough to crystallize. These stereoregular propylene sequences of SPC and SPC2 should advantageously and substantially match the stereoregularity of the propylene in the FPC. For example, if the FPC is predominately isotactic polypropylene, then the SPC, and SPC2 if used, is preferably a copolymer having isotactic propylene sequences. If the FPC is predominately syndiotactic polypropylene, then the SPC, and the SPC2 if used, is a copolymer having syndiotactic sequences.

Therefore, SPC and SPC2 have similar, preferably substantially identical, tacticity to the FPC. It is believed that this matching of stereoregularity increases the compatibility of the components and results in improved adhesion at the interface of the domains of the polymers of different crystallinities in the polymer blend composition. Narrow intermolecular and intramolecular compositional distribution in the copolymer is preferred. The aforementioned characteristics of the SPC, and SPC2 if used, are preferably achieved by polymerization with achiral metallocene catalyst.

One preferable embodiment is blending isotactic polypropylene (FPC) with ethylene propylene copolymers (SPC) having 4 wt. to 35 wt. ethylene (to ensure high compatibility with the FPC). Both the FPC and the SPC have isotactic propylene sequences long enough to crystallize. Resulting blends of isotactic polypropylene with ethylene propylene copolymers according to WO 00/01766 and similar developments have improved properties as compared to isotactic polypropylene blends with prior art ethylene propylene rubbers.

These and similar blends may be particularly suitable for the manufacture of flexible tubes and other containers that are subjected to heating by such methods as heat-filling with the product the container is required to contain and/or heat treating the filled container by methods such as retorting.

A preferred blend comprises 1% to 95% by weight of FPC and a SPC with greater than 65 percent by weight propylene and preferably greater than by weight propylene. Polymer blends made according to the abovementioned patent and similar developments may be suitable for use in the present invention for the production of thin-walled products, and especially for the production of flexible tubes as described in the '255 patent. These and similar blends may be particularly suitable for the manufacture of flexible tubes and other containers that are subjected to heating by such methods as heat-filling with the product the container is required to contain and/or heat treating the filled container by methods such as retorting.

It has been found that polymer blends in which at least one of the polymers is an ethylene, propylene and/or other olefin interpolymer that is characterised by super-random distribution of one or more of the copolymers within and amongst the polymer chains are particularly suitable for the manufacture of flexible thin walled mouldings that are to be treated with a barrier-improving coating such as chemical vapour deposition. Whilst not wishing to be bound by theory, it is believed that the polymer characterised by super-random distribution of the copolymer(s) within and amongst the polymer chains enables the barrier treatment to adhere more effectively to the article's surface than is the case in articles made from by similar polymers but that are not characterised by super-random distribution of comonomers within and amongst the polymer chains.

Polymer blends suitable for the present invention may also incorporate a variety of other additives. Examples of additional additives include further polymers, nucleating agents, pigments, dyes, fillers, antioxidants, plasticisers, oils, UV protection, viscosity modifying agents, additives capable of reacting with or absorbing deleterious chemicals such as oxygen and other mould release agents and melt strength modifiers amongst others. These additives may be added to one or more components of the polymer blend or the polymer blend as a whole prior to moulding in order to modify its properties to suit specific applications or to achieve specific effects in the end product. The polymer blend may be prepared by extrusion of some or all of the components of the polymer blend and the resulting chopped extrusion used in the injection moulding process of the present invention. Alternatively, the polymer blend may be provided in its component form and subjected to mixing before and during the melting of the polymer blend in the present process. Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications that fall within its spirit and scope. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.

Injection Moulding Page 8 of 8

Claims (10)

1. A process for the manufacture of flexible, thin-walled articles comprising the steps of: 1) selecting a polymer blend having an ESCR of greater than 10 hours when tested according to the following procedure; i) a plurality (preferably 6 or more) strips of the polymer blend incorporating any post moulding treatment intended for the final article having the cross-sectional dimensions of 0.65 mm in thickness and mm in width are injection moulded under high shear, long flow length conditions, similar to those intended for use in the manufacture of the flexible thin-walled article.; ii) the strips are bent back upon themselves and stapled 3 mm from the bend; iii) the bent strips are immersed in a solution of a stress crack agent and held at a temperature of iv) the strips are observed for signs of cracking, any signs of cracking are regarded as a failure; and v) the time to failure is when 50% of the strips show signs of cracking[.]; 2) melting said polymer blend; 3) ramming the molten polymer blend into a mould said mould having a cavity which produces a thin-walled article having a thin section 1 mm or less in thickness and wherein the thin section is substantially continuous for greater than 50mm in the direction of flow of the molten polymer blend in the mould; and 4) removing from the mould the thin-walled article formed from the polymer blend.

2. A process according to claim 1 wherein the stress crack agent is an ethoxylated nonylphenol.

3. A process according to claim 1 wherein the solution of a stress crack agent is a solution of nonylphenol ethoxylated with 9 moles of ethylene oxide.

4. A process according to claim 3 wherein the polymer blend has an ESCR of greater than 100 hours. A process according to claim 3 wherein the polymer blend has an ESCR of greater than 200 hours.

6. A process according to claim 3 wherein the polymer blend has an ESCR of greater than 360 hours.

7. A process according to claim 1 for injection moulding a flexible, thin- walled article comprising the steps of: 1) melting a polymer blend wherein said polymer blend comprises at least one polymer with a high molecular weight low MFI) copolymer in combination with a low molecular weight high MFI) homopolymer or copolymer with higher crystallinity than the high molecular weight polymer. 2) ramming the molten polymer blend into a mould said mould having a cavity which produces a thin-walled article having a thin section 1 mm or less in thickness and wherein the thin section is substantially continuous for greater than 50mm in the direction of flow of the molten polymer blend in the mould; and 3) removing from the mould the thin-walled article formed from the polymer blend.

8. A process according to claim 7 wherein the at least one polymer is selected from the group consisting of plastomers, polyethylenes, copolymers of ethylene and one or more unsaturated olefins, 'substantially linear' polyethylenes, [and] branched polyethylenes, polymers and copolymers of ethylene manufactured using metallocene or other catalysts producing copolymers characterised by super- random distribution of comonomers within the polymer chains, polypropylenes, copolymers of propylene and ethylene and/or one or more unsaturated olefins, terpolymers of ethylene, propylene and ethylene and/or one or more alpha- olefin, polymers and copolymers of propylene manufactured using metallocene or other catalysts producing copolymers characterised by super-random distribution of comonomers within the polymer chains, polylactic acid polymers, silane polymers and mixtures thereof.

9. A process according to claim 7 wherein both the high MFI and low MFI polymers are ethylene-based polymers. A process according to claim 7 wherein both the high MFI and low MFI polymers are propylene-based polymers.

12. A process according to claim 1 wherein the thin-walled article is a tube.

13. A thin-walled tube produced in accordance with the process of claim 1.