PROCESS FOR THE PREPARATION OF BLOWN FILM

Abstract: Blown film on the basis of low density polyethylene is prepared by extruding the molten polymer at a die gap of at least 1 mm at melt temperatures at the die gap of below 160 °C. Films that have been prepared this way, have good mechanical and optical properties.

Description: PROCESS FOR THE PREPARATION OF BLOWN FILM The invention relates to a process for the preparation of blown film from a polymer composition on the basis of polyethylene, which polyethylene has a density of 910 to 940 kg/m3, in which the polymer composition is, in molten condition, forced via an extruder through an annular die having a width of at least 1 mm and is made to balloon out by means of a gas, supplied through an opening within the annular die, following which the polymer composition that has formed the balloon is cooled, flattened and rolled up. Such a process for the preparation of blown film is wellknown in itself and is described in, for instance, 'Petrothene Polyolefins ....... a processing guide' (a publication of the National Distillers and Chemical Corporation, 1971, 4th ed.). tow-density polyethylene (LDPE) blown film has found largescale application as packaging material. The main requirements to be met by packaging material concern mechanical properties, such as stiffness, tear resistance, tensile strength, puncture resistance, and the like. Depending on the application, said material must often also satisfy requirements in the field of optical properties such as transparency, opacity and gloss. Film to be processed into refuse bags, for instance, will need to have a good puncture resistance and tear properties, while film for carrier bags must in addition have high stiffness. In applications such as bread packaging, it is especially stiffness and opticals that play a part. From 'Petrothene Polyolefins ...... a processing guide' it is known that the best optical properties are obtained when the polymer melt is processed at the highest possible temperatures. A melt temperature of 205 C is not exceptional. One of the factors affecting the mechanical properties is-the degree of orientation of the melt. A recent development in the field of blown film preparation from low-density polyethylene is the so-called long-stalk extrusion. This process, described in Plastics World of June 1982, pp. 62-64, starts from LDPE with a low melt index, which is processed at a temperature of 190 C with a die gap of about 1.1 mm and an extra high frost line. The height of the frost line is the distance between the annular die and the frost line. The main difference-between this long-stalk extrusion and conventional extrusion lies in the degree of orientation of the melt. This results in blown film prepared by the long-stalk process having a better impact strength than conventional blown film. A drawback is that the stiffness and the tear resistance, especially in machine direction, deteriorate substantially. In addition, it is not possible to convert linear low-density polyethylene into film this way. A method for processing linear polyethylene into blown film using die gaps of about 0,4-5 mm is described in European patent application 51358. This method uses specially formed die gaps in order to prevent melt fracture. The temperature of the melt is between about 162 and 260 OC. This method relates only to linear polyethylene. The object of the present invention is to provide a process for the preparation of blown film from a polymer composition on the basis of polyethylene with a density of 910-940 kg/m3 that is suitable for both linear and non-linear LDPE and that yields a film having good mechanical properties, notably high stiffness and good tear properties. This object is achieved by processing the polymer composition in molten condition via an extruder through an annular die having a width of at least 1 mm, at a melt temperature at the location of the die gap of below 160 C. Surprisingly, when processing is carried out under conditions according to the invention, special orientation effects occur, as a result of which good mechanical properties are imparted to the film, such as good tear resistance and tensile strength, high stiffness, etc. This effect increases when the temperature of the melt at the location of the die gap is below 150 DC, so that processing at below 150 C is to be preferred, and in particular at below 140 OC. Polymer compositions that are suitable for being processed into blown film in accordance with the invention generally contain at least 50 wt. x polyethylene homopolymer and/or one or more copolymers with a density of 910-940 kg/m3. Low-density polyethylene homopolymer, copolymers of ethylene with one or more C3-C15 alkenes, copolymers of ethylene and one or more polar comonomers with a comonomer content of at most 10 wt. Z, relative to the ethylene (e.g. vinylacetate, methacrylate) can be applied with good results. Also mixtures are suitable, like for instance mixtures of polyethylene homo-and/or copolymers with a density of 910-940 kg/m3, or mixtures of polyethylene homo- and/or copolymers having a density of 910-940 kg/m3 with lowdensity polyethylene copolymers (with a density less than 940 kg/m3), high density polyethylene (with a density in excess of 940 kg/m3) and/or polypropylene. In these mixtures preferably at least 70 wt %, relative to the total polymer content, of the homo- and/or copolymer with a density of 910-940 kg/m3 is present. For the preparation of films having good optical properties, the use of a polymer composition on the basis of polyethylene homopolymer or a copolymer of ethylene and one or more polar comonomers with a comonomer content of at most 10 wt. % relative to the ethylene, is to be recommended. The process according to the invention is particularly suitable for the preparation of films having good mechanical and optical properties from low-density polyethylene homopolymer or ethylene vinylacetate copolymer. Furthermore, various additives, such as stabilisers, slip agents, fillers, colourants, waxes and the like, may be present. The amount by weight of additives usually will not exceed 20 X of the amount by weight of polymer. The melt index of these polymer compositions, determined according to ASTM D 1238, may have the usual values, for instance between 0.3 and 30 dg/min. In view of the low processing temperature, however, it is to be recommended to choose a melt index of not lower than 2 dg/min. The favourable effect that low-temperature processing in combination with the large die gap has on the mechanical properties, decreases when the melt index exceeds 25 dg/min, and is most pronounced at a melt index of at most 20 dg/min, in particular at most 17 dg/min. The advantage of the invention is that also polymer compositions having a relatively high melt index can be processed into blown film with good mechanical properties. Often, packaging film must not only satisfy requirements with respect of its mechanical properties, but also requirements relating to its optical properties. The optical properties of the film improve when use is made of a polymer composition having a melt index of at least 5 dg/min, in particular at least 8 dg/min. There already is advantage in applying the invention at die gaps of 1 mm, but larger die gaps, for instance of 5 or 10 mm, can also be applied; in addition, the geometry of the die may be varied. It has now been found that the mechanical properties of the film improve when the die gap-used is at least 2 mm and at most 5 mm, in particular at least 3 mm. In combination with the low processing temperature, these larger die gaps result in even better mechanical properties, notably in good stiffness and tear properties. In blown film extrusion, the polymer composition exits the annular die in molten condition and is subsequently blown up. The resulting balloon is cooled by contacting it with relatively cool air or water. Balloon diameter divided by annular die diameter is called the blow-up ratio. This may vary between 1 and 6, but generally is 1.5 to 4. These values can also be applied in the process according to the invention, yielding good results. The process according to the invention is suitable especially for the preparation of monolayer film, but can also be used for multilayer film. The invention will now be elucidated on the basis of some examples. Examples and comparative examples 1 through 8 Blown film was prepared under conditions as shown in the table. Use was made of a Schwabenthan extrusion equipment with a screw diameter of 30 mm and a length/diameter ratio of 24 : 1. The diameter of the annular die was 5 cm. The melt temperature at the die gap was determined by infrared measurement. Of the film, the following properties were determined: modulus of elasticity according to ASTM D 1922; strength and tensile strength according to ISO R 527; puncture resistance, determined from the energy required to puncture the film with a plunger at a rate of 50 turn/min; notched tear resistance according to a method derived from DIN 53363, but with the notch at the centre of the film, at a drawing rate of 2.5 cm/min; gloss according to ASTM D 523; opacity according to ASTM D 1003; transparency according to the Electro Evans Ltd. method. Notes: 1. not measured 2. the notch did not tear further, so that drawing phenomena occurred 3. owing to the high opacity, the value for transparency cannot be determined in a reliable way Table comp. comp. ex. ex. ex. ex. ex. ex. ex. ex. 1 2 3 4 5 6 7 8 Polymer composition LPDE LPDE LPDE LPDE LPDE LPDE LPDE LPDE density, kg/m 922 922 921 921 921 921 921 921 melt index, dg/min 0.3 0.3 2 9 9 14.5 14.5 14.5 temperature, C 182 137 159 170 140 156 139 141 die gap, m 2000 2000 3000 2000 2000 2000 2000 2000 froat line, cm 15 15 16 8 8 10 8 14 blow-up ratio 3 3 2 2 2 2 2 2 film thickness, m 30 30 22 25 25 25 25 25 output, g/min 50 50 50 50 50 50 50 50 modulus of elasticity // 182 222 220 127 174 133 143 135 N/mmê # 239 255 217 136 199 153 176 210 yield strength, N/mmê // 10 13.5 12 9.5 11 9 9.5 n.m.Ú # 11.5 11 9 8.5 10 9 10 n.m.Ú puncturs resistance, J/m 816 895 n.m.Ú 459 424 n.m.Ú 468 n.m.Ú notched tear resistance, // 294 322 393 277 287 251 285 305 kJ/m2 # 184 1210ê 926ê 171 356ê 154 218ê 449ê tensile strength, N/mmê // 29 35 36 19 28 16.5 20 n.m.Ú # 29 24 11 11 15 10.5 13 n.m.Ú gloss, o/oo 40 32 50 85 88 80 89 82 opacity, % 9 15 9 7 4 8 4 12 transparency 18 49 18 4.1 5.8 3 5.3 4.4

Claims: CLAIMS 1. Process for the preparation of blown film from a polymer composition on the basis of polyethylene, which polyethylene has a density of 910-940 kg/m3, in which the polymer composition is, in molten con dition forced via an extruder, through an annular die having a width of at least 1 mm and is made to balloon out by means of a gas, supplied through an opening within the annular die, following which the polymer composition that has formed the balloon is cooled, flat tened and rolled up, this process being characterised in that the polymer composition is processed at a temperature of the melt at the die gap of below 160 OC. 2. Process according to claim 1, characterised in that the polymer com position is processed at a temperature of the melt at the die gap of below 150 OC. 3. Process according to claim 2, characterised in that the polymer com position is processed at a temperature of the melt at the die gap of below 140 OC. 4. Process according to claims 1-3, characterised in that use is made of a polymer composition having a melt index between 2 and 25 dg/min. 5. Process according to claim 4, characterised in that use is made of a polymer composition having a melt index between 5 and 20 dg/min. 6. Process according to claim 5, characterised in that use is made of a polymer composition having a melt index between 8 and 17 dg/min. 7. Process according to claims 1-6, characterised in that a die gap of at least 2 mm and at most 5 mm is applied. 8. Process according to claim 7, characterised in that a die gap of at least 3 mm is applied. 9. Process according to claims 1-8, characterised in that monolayer blown film is prepared. 10. Process according to claims 1-9, characterised in that use is made of a polymer composition on the basis of polyethylene homopolymer. 11. Process according to claims 1-9, characterised in that use is made of a polymer composition on the basis of a copolymer of ethylene and one or more polar comonomers, with a comonomer content of at most 10 wt. % relative to the ethylene. 12. Process according to claim 10, characterized in that use is made of polyethylene homopolymer. 13. Process according to claim 11, characterized in that use is made of ethylene vinyl acetate copolymer. 14. Process for the preparation of blown film as substantially described in the specification, in particular the examples. 15. Film, prepared according to one of claims 1-14.

Inventors: VAN DER MOLEN, THEODORUS JACOBUS
Applicants: STAMICARBON B.V.
Application year: 1985
Main classification: B29C47
IPC codes: B29C47, B29C55, B29K23, B29L7