Coextruded polyethylene film

Abstract

A coextruded polyethylene blown film, particularly for packaging, has a thickness between 20 ?m and 250 ?m and is formed by a foamed, particle-containing core layer between a first unfoamed weldable outer layer and a second unfoamed outer layer. The core layer has a polyethylene or a polyethylene-based mixture as a polymer component having a melt mass-flow rate (MFR) per DIN ISO EN 1133 of greater than 5 g/10 min at 190? C. and 2.16 kg.

Claims

1. A coextruded polyethylene blown film for packaging and with a thickness between 20 ?m and 250 ?m, the film comprising: a first unfoamed weldable outer layer; a second unfoamed outer layer, the outer layers each of a polyethylene or a polyethylene-based mixture as a polymer component having a melt mass-flow rate (MFR) per DIN ISO EN 1133 of less than 3 g/10 min; and a foamed, particle-containing core layer of a polyethylene or a polyethylene-based mixture as a polymer component having a melt mass-flow rate (MFR) per DIN ISO EN 1133 of greater than 5 g/10 min at 190? C. and 2.16 kg.

2. The coextruded polyethylene blown film defined in claim 1, wherein the mass-flow rate of the outer layers is less than 1 g/10 min at 190? C. and 2.16 kg.

3. The coextruded polyethylene blown film defined in claim 1, wherein the first outer layer has a higher melt mass-flow rate (MFR) than the second outer layer.

4. The coextruded polyethylene blown film defined in claim 1, wherein the foamed core layer has closed cells filled with nitrogen or carbon dioxide.

5. The coextruded polyethylene blown film defined in claim 1, wherein the foamed core layer has closed cells with a volume of less than 50000 ?m.sup.3.

6. The coextruded polyethylene blown film defined in claim 1, wherein the core layer has a density between 0.2 g/cm.sup.3 and 0.8 g/cm.sup.3.

7. The coextruded polyethylene blown film defined in claim 1, wherein the thickness of the foamed core layer is between 40% and 70% of a total thickness of the blown film.

8. The coextruded polyethylene blown film defined in claim 1, wherein the polymer component of the core layer contains a linear polyethylene as the main component.

9. The coextruded polyethylene blown film defined in claim 8 wherein the polymer component includes metallocene catalysts.

10. The coextruded polyethylene blown film defined in claim 8, wherein the polymer component of the core layer contains at least one other low-density polyethylene besides the linear polyethylene.

11. The coextruded polyethylene blown film defined in claim 1, wherein the outer layers are immediately adjacent and flank the core layer.

12. The coextruded polyethylene blown film defined in claim 1, wherein at least one of the outer layers contains colored particles.

13. The coextruded polyethylene blown film defined in claim 1, wherein the second outer layer has a matte surface with a reflectometer value per DIN 67530 of less than 40 at an angle of measurement of 85?.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the sole FIGURE of accompanying drawing that is an end view of a three-layer tube according to the invention.

SPECIFIC DESCRIPTION OF THE INVENTION

(2) As seen in the drawing a coextruded tube comprises thin unfoamed outer and inner layers 1 and 2 that sandwich a foamed core layer 3 having cells as shown partially at 4.

(3) The invention is explained below in more detail with reference to several embodiments.

(4) In preliminary tests, three respective formulations were considered for the first unfoamed, weldable outer layer 1 as the sealing layer, the core layer 3 and the second unfoamed outer layer 2.

(5) In order to provide a sealing layer with the usual sealing characteristics, the first outer layer 1 can be made of a material composition A1 having polyethylene with a density between 0.92 to 0.94 g/cm.sup.3, with mixtures of linear and nonlinear polyethylenes being particularly worthy of consideration. In order to provide a low-melting first outer layer 1 as the sealing layer, a polyethylene with a density of 0.905 to 0.917 g/cm.sup.3 (ULDPE) can be used as material A2. For a sealing layer with a very low melting temperature, on the other hand, a polyethylene-based polymer material A3 with a density of less than 0.905 g/cm.sup.3 can be used.

(6) The composition of the core layer 3 can be based on the desired characteristics of the coextruded polyethylene blown film.

(7) Particularly in order to form relatively thick films, where a slightly higher level of surface roughness can be accepted and where a high level of weld strength is desired, a material B1 can be used for the core layer 3 with 20% by weight talc as a filler and the rest LLDPE C8 with a melt mass-flow rate of 6 to 8.

(8) On the other hand, for thinner films with an improved, lesser surface roughness and a low weld strength, a material B2 with 20% by weight talc, 20% LDPE and the rest LLDPE is preferably used, the two cited polyethylene types having a melt mass-flow rate between 6 and 8, so that, as expected, the melt mass-flow rate of the polymer component made of LLDPE and LDPE also has commensurate values.

(9) As the third material B3, a composition of 20% talc, 20% mLLDPE with an MFI of 15 to 20 and the rest LLDPE with an MFI of 6 to 8 is provided. The formulation B3 is suited both to thin and thick films and leads to a good weld strength and good surface characteristics with little surface roughness. Compared to materials B1 and B2, however, process management during coextrusion is somewhat difficult, and higher material costs are involved.

(10) Together with the first unfoamed outer layer 1, the first unfoamed outer layer 2 contributes to the stiffness and strength of the coextruded polyethylene blown film. Consequently, the mechanical characteristics of the entire coextruded polyethylene blown film can be substantially influenced by the material of the second outer layer 2. A polyethylene mixture of LDPE and LLDPE with a density of about 0.92 g/cm.sup.3 is provided as the first material C1 for the second outer layer 2.

(11) For increased stiffness, a mixture of LDPE and LHDPE with a density between 0.925 and 0.935 g/cm.sup.3 can be used as material C2. Finally, a relatively high level of stiffness can be achieved if LDPE is mixed down with linear high-density polyethylene (LHDPE) in a third material C3 for the first unfoamed outer layer 2, in which case the total density of the mixture C3 lies between 0.935 and 0.95 g/cm.sup.3.

(12) Independently of the requirement profiles, the different formulations for the individual layers can be freely combined with one another. For example, if a relatively thin film with little surface roughness is desired that has a low melting point with a low weld strength and a high level of stiffness, a layer construction A2/B2/C3 is preferably used. As will readily be understood, all other compositions are also possible depending on requirements.

(13) Another example of layer construction is described in Table 1. The thickness ratios are shown with reference to an unfoamed core layer 3. On foaming of the core layer 3, the thickness ratio thus changes accordingly.

(14) TABLE-US-00001 TABLE 1 Layer: First outer layer 1 Core layer 3 Layer thickness, 29% 42% 29% unfoamed: Components: 57% mLLDPE-C8 77% LLDPE-C8 Second outer layer 2 Density: 0.925 g/cm.sup.3/ Density: 0.919 g/cm.sup.3/ MFI 0.85 MFI 6 28% LDPE 20% talc batch (with 26% LDPE Density: 0.929 g/cm.sup.3/ 60% talc) Density: 0.923 g/cm.sup.3/ MFI 0.55 MFI 2 14% white batch 2% lubricant anti-block 2% lubricant anti-block (with 60% TiO2) batch batch 1% fluoropolymer batch 1% thermostabilizer 1% fluoropolymer batch (processing additive) batch (processing additive)

(15) TABLE-US-00002 TABLE 2 Coef- ficient of Force at friction Water breaking Elonga- COF vapor (DIN tion (DIN EN trans- Meas- EN ISO at Flexural ISO mission Pre- ured 527-1) breaking rigidity Tear 8295) rate scribed thick- Mass Manufac- Trans- (DIN 30?/ resistance First WVTR thick- ness per turing verse EN 10 nm per Opacity First outer Weld (DIN ness (DIN unit direc- direc- ISO (DIN Elemdorf (DIN outer layer 1 strength 53122-1) unfoamed/ 53370) area tion tion 527- 1) 53121) (ISO 53146) layer 1 against 150? C./ x? C./ foamed MD CD MD CD MD CD MD CD against metal 1.5 s x% RH ?m/?m ?m g/m.sup.2 N/Inch N/Inch % % mN mN mN mN % 1 1 N/Inch g/(m.sup.2*day) Compar- 100/100 98.9 94.7 73.9 69.1 692 919 25.4 36.6 6954 34519 78 0.15 0.20 44.5 2.8 ative Film 1 90/100 98. 86.3 59.5 38. 639 607 26.4 34.4 7168 28352 80 0.1 0.1 40.3 3.2 Film 2 80/100 103.3 75.5 41.4 27. 451 500 25.6 35.5 1961 22267 77 0.1 0.2 35.2 4.9 Film 3 70/100 104.6 65.6 36.7 23. 432 497 22.4 28.7 1398 13802 75 0.1 0.2 28.9 3.2 Film 4 60/100 105.1 59.0 27.1 13. 293 125 19.7 17.7 999 10632 77 0.2 0.3 29.3 7.2 Film 5 100/110 107.7 97.4 65.9 41. 672 643 34.3 44.4 6669 32578 81 0.1 0.2 42.3 2.9 Film 6 100/120 117.6 97.3 54.6 34. 554 513 47.1 58.5 4286 31272 82 0.1 0.2 39.7 3.5 Film 7 100/130 127.7 97.1 55.0 30. 497 486 54.6 68.9 2321 29224 85 0.1 0.2 37.8 3.9 Film 8 100/140 138.0 98.3 48.3 28. 465 419 65.7 74.3 2067 23923 84 0.1 0.2 35.7 4.7

(16) Starting from the material composition according to Table 1, the total thickness relative to the unfoamed state (i.e. the mass per unit area) on the one hand and the degree of foaming of the core layer 3 on the other hand were varied in preliminary tests. It was found that, as a result of foaming, the flexibility of the polyethylene coextruded film increases both in the manufacturing direction (MD) and in the transverse direction (CD).

(17) In order to achieve a predetermined value for the stiffness compared to an unfoamed film, it is thus necessary to use less material. As a result of foaming, the weld strength decreases, and the coextruded polyethylene blown film is also more easily expanded as a result of the foaming of the core layer 3, which results in minimal impairment of the mechanical characteristics. In addition, the water-vapor transmission rate increases to a certain extent as a result of foaming.

(18) The characteristics of the material composition according to Table 1 can be seen in Table 2.

(19) In the first column of Table 2, the thickness in the foamed state is compared to the thickness that would result from the same material usage (without a foaming agent). The first column refers to prescribed values. The comparative example thus relates to a film that is not foamed. The degree to which the core layer 3 is foamed follows from the indicated thicknesses for an unfoamed and a foamed design.

(20) The second column relates to the actually determined thickness in a film formed specifically according to the specifications.

(21) The other columns disclose various characteristic parameters.

(22) The coextruded polyethylene blown film according to the invention is particularly intended for use in packaging, although other applications are also conceivable. For instance, the coextruded polyethylene blown film can be used as a label film, in which case an especially smooth surface is then desired. The same applies to the use of the coextruded polyethylene blown film as a surface-protecting film, in which case the foamed core layer 3 can additionally provide mechanical damping and, through such a buffer effect, greater protection. Finally, the coextruded polyethylene blown film according to the invention also has a more pleasant, softer feel than an unfoamed film, thus imparting more of a textile character to it. The foamed film therefore also be used to a certain extent as a replacement for a textile or at least suggest textile characteristics, which are desirable, for example, in articles of clothing, disposable hygiene products or the like.

(23) Use for an adhesive tape is also possible, in which case mechanical stresses can also be compensated for to a certain extend by the foamed core layer 3.

(24) As a result of the relatively large thickness with a low mass per unit area, the coextruded polyethylene blown film can also be used as a kind of seal in the form of a closure label in order to serve as an original closure in screw closures or other containers, for example, with such a closure label being torn when opened for the first time.

(25) Finally, due to its softer character compared to a film, the coextruded polyethylene blown film can also be used as a kind of waterproof replacement for paper, in which case fillers and additives can optionally be used in at least one of the outer layers 1 or 2 in order to improve writability. Kaolin and chalk are examples of additives that are suitable for this purpose.

(26) Finally, it is also possible to introduce active substances into the core layer 3 during the foaming process which are then active over a long period of time. Relative to packaging, these can particularly be aromatic agents, absorbers, or the like. Such substances are preferably introduced with the foaming agent during extrusion.

(27) The object of the invention is also a method of manufacturing a coextruded polyethylene blown film, where a polyethylene or a polyethylene-based mixture is provided in a blown film coextrusion system to form a core layer 3 that has a melt flow rate (MFR) per DIN ISO EN 1133 of greater than 5 g/10 min at 190? C. and 2.16 kg, a foaming substance being added to the core layer 3, and other polyethylene-based polymer components are provided to form a first weldable outer layer and a second outer layer 2, wherein

(28) the polymer components are coextruded from the gap of a coextrusion die under the formation of a blown-film tube, forming a first unfoamed weldable outer layer, a core layer 3 and a first unfoamed outer layer 2,

(29) the core layer 3 foams up and expands under the formation of core layer 3 immediately after emerging from the extrusion gap, and

(30) a total thickness of the coextruded polyethylene blown film of between 20 ?m and 250 ?m is produced as a result of the free foaming supported from below.