METHOD FOR PRODUCING A FILM COMPRISING THERMOPLASTIC POLYMER AND INORGANIC FILLER

Abstract

A method for producing a film including at least 20 wt % thermoplastic polymer and 50 to 75 wt % inorganic filler, and including the steps of: providing the mixture, melting the mixture, producing a thin layer from the mixture, cooling the thin layer, producing a film, stretching the film in the longitudinal direction and in the transverse direction, wherein the particle size of the inorganic filler is at most 5 μm and the stretch ratio in the longitudinal direction and in the transverse direction is at least 3.5. The disclosure further relates to films produced by the method and to the use thereof.

Claims

1. Method for producing a film comprising at least one thermoplastic polymer and at least one inorganic filler, comprising the steps of: providing a mixture comprising at least one thermoplastic polymer and at least one inorganic filler, melting the mixture, producing a thin layer from the molten mixture, cooling the resulting thin layer to produce a film, stretching the film in the longitudinal direction and in the transverse direction, the proportion of the thermoplastic polymer in the film being at least 20 wt %, the proportion of the inorganic filler in the film being in the range from 50 to 75 wt % and the particle size of the inorganic filler being at most 5 μm, wherein the stretching ratio in the longitudinal direction is at least 3.5 and the stretching ratio in the transverse direction is also at least 3.5.

2. Method according to claim 1, wherein the stretching of the film is carried out sequentially in the longitudinal direction and in the transverse direction.

3. Method according to claim 1, wherein the stretching ratios in the longitudinal direction and in the transverse direction are in the range from 3.5 to 7.5.

4. Method according to claim 1, wherein the stretching ratios in the longitudinal direction and in the transverse direction are in the range from 4 to 7.

5. Method according to claim 1, wherein the stretching ratio in the longitudinal direction is in a range from 4 to 5.5 and the stretching ratio in the transverse direction is in a range from 4.4 to 6.

6. Method according to claim 1, wherein the at least one thermoplastic polymer is selected from the group consisting of polyethylene, polypropylene or mixtures thereof.

7. Method according to claim 1, wherein the inorganic filler is selected from the group consisting of calcium carbonate, carbon dust, powder, calcium sulphate, barium sulphate, kaolin, mica, zinc oxide, dolomite, calcium silicate, glass, silicates, chalk, talc, pigment, titanium dioxide, silicon dioxide, bentonite, clay, diatomite, and mixtures thereof.

8. Method according to claim 1, wherein the inorganic filler is calcium carbonate.

9. Method according to claim 1, wherein the particle size of the inorganic filler is at most 5 μm, preferably at most 3 μm and most preferably at most 2 μm.

10. Method according to claim 1, wherein the particle size of the inorganic filler is at least 0.1 μm, preferably at least 0.5 μm.

11. Method according to claim 1, wherein the film additionally comprises up to 25 wt % of at least one polyolefin elastomer.

12. Method according to claim 11, wherein the at least one polyolefin elastomer is selected from the group consisting of polyisobutylene, ethylene propylene rubber and ethylene propylene diene monomer rubber.

13. Method according to claim 1, wherein the film comprises up to 5% auxiliary materials, preferably in an amount in the range from 1 wt % to 5 wt %.

14. Method according to claim 1, wherein the film comprises at least one auxiliary material selected from the group consisting of adhesion promoters, dispersants, stabilisers, lubricants, antistatic agents, solid plasticisers, activators, promoters, anti-aging agents, agents for preventing burn marks, binders, heat-resistant agents, initiator agents, polymerisation catalysts, emulsifiers, plasticisers, heat stabilisers, light stabilisers, flame retardants and mould release agents.

15. Method according to claim 1, wherein the film comprises stretching aids, preferably selected from the group consisting of polyethylene waxes or polypropylene waxes.

16. Film produced by a method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG. 1 shows a film after stretching at different stretching ratios.

[0063] FIG. 2 shows a microscopic image of a film according to the invention without a dispersant.

[0064] FIG. 3 shows a microscopic image of a film according to the invention with a dispersant.

[0065] FIG. 4 shows the profile of the stretching in the transverse direction for example 4, sample roll 1 and sample roll 3.

[0066] FIG. 5 shows the temperature profile during stretching in the transverse direction for example 4, sample roll 1.

[0067] FIG. 6 shows the temperature profile during stretching in the transverse direction for example 4, sample roll 3.

[0068] FIG. 7 shows the profile of the stretching in the transverse direction for example 5.

[0069] FIG. 8 shows the temperature profile during stretching in the transverse direction for example 5.

EXAMPLES

Example 1

[0070] Production of a biaxially stretched film comprising polypropylene (homopolymer?) and calcium carbonate and stretching at various stretching ratios.

[0071] 40 wt % of a polypropylene having a melt mass flow rate (MFR) of 2 g/10 min and 60 wt % of a calcium carbonate having an average particle size of 1.6 μm were mixed in a co-rotating twin-screw extruder. The mixture (compound) obtained in this way was extruded and was extruded through a wide-slot nozzle. The melt was then cooled by means of a cooling roller and rectangular film portions were cut from the film thus obtained.

[0072] A line pattern in the form of a chess board was applied to the film portions thus obtained (see FIG. 1, top left). A total of five identical films were then simultaneously stretched at different stretching ratios. A “Karo IV laboratory stretching frame” from Brückner Maschinenbau GmbH was used for this. The stretching ratios in the longitudinal and transverse directions were always the same. Stretching ratios of 2, 3, 4, 5 and 6 were used.

[0073] As can be seen in FIG. 1, this example shows that the lines in the middle of the films run more parallel at higher stretching ratios than at lower stretching ratios. In particular, at higher stretching ratios, the area of the squares surrounding the central square is approximately as large as the central square. Surprisingly, this indicates that with stretching ratios above 3 the stretching proceeds more evenly than with stretching ratios up to 3. This results in a more uniform film thickness at stretching ratios above 3 and thus also a lower standard deviation of the film thickness. This leads to a significantly better printability and better mechanical properties of the films.

Example 2

[0074] Production of biaxially stretched films comprising polypropylene and calcium carbonate having different compositions and stretching of the films at a stretching ratio in the longitudinal direction of 5 and in the transverse direction of 5.

[0075] Polypropylene and calcium carbonate and optionally dispersants and stabilisers were mixed on a co-rotating twin-screw extruder. Mixtures with 60 wt % calcium carbonate and formulations with 70 wt % calcium carbonate were produced. The mixtures (compounds) obtained in this way were extruded and pressed through a wide-slot nozzle. The thin layer (cast film) produced in this way was cooled on a cooling roller and rectangular film portions were cut from the resulting film, which were then simultaneously stretched with the above-mentioned stretching ratios in a “Karo IV laboratory stretching frame” from Brückner Group GmbH. The temperature of the film during stretching was set to 160° C. Table 1 shows the composition of the films and indicates whether a film could be obtained after stretching with a stretching ratio of 5×5:

TABLE-US-00001 TABLE 1 MFR-PP CaCO.sub.3 [g/10 particle No. min] size [μm] Dispersants Stabilisation Stretchable? Formulations with 60% CaCO.sub.3 CF1 2 1.6 No No Yes CF2 19 1.6 No No No CF3 2 1.6 Yes No Yes CF4 19 1.6 Yes No Yes CF5 2 6.5 Yes No No CF6 19 6.5 No No Yes CF7 19 6.5 Yes No Yes Formulations with 70% CaCO.sub.3 CF13 2 1.6 Yes No No CF14 2 1.6 Yes Yes Yes CF: Cast film MFR-PP: melt mass flow rate according to ISO 1133

[0076] The polypropylene homopolymer “Moplen® HP2624” from LyondellBasell, Rotterdam, Netherlands was used as the polypropylene having a melt mass flow rate of 2 g/10 min. The polypropylene homopolymer “Sabic® PP 576P” from Sabic, Riyadh, Saudi Arabia was used as the polypropylene having a melt mass flow rate of 19 g/10 min. “Omyafilm 707” from Omya GmbH, Hamburg, Germany is used as the calcium carbonate having a particle diameter d50=1.6 μm. “Omya BLH” also from Omya GmbH is used as the calcium carbonate having a particle diameter d50=6.5 μm. Both types of calcium carbonate are coated. In the experiments in which dispersant was added, 2% of the dispersant “Licowax® OP powder” from Clariant, Frankfurt/Main, Germany was added. In the experiments in which stabilisers were added, 1000 ppm of the acid scavenger “DHT-4A” from Kisuma Chemicals, Veendam, Netherlands and 500 ppm “Irganox B561” from BASF SE, Ludwigshafen, Germany were added.

[0077] Table 2 shows some properties of the films obtained having a calcium carbonate content of 60 and 70 wt %:

TABLE-US-00002 TABLE 2 Tensile Modulus of Standard strength elasticity deviation [MPa] [MPa] Density Thickness thickness No. MD TD MD TD [g/cm.sup.3] [μm] [%] Formulations with 60% CaCO.sub.3 CF1 30 20 660 534 0.524 142.40 7.36 CF3 43 31 999 855 0.692 100.71 2.64 CF4 32 18 800 651 0.570 162.29 12.11 CF6 11 8.0 284 239 0.319 250.27 16.51 CF7 12 8.5 281 242 0.325 237.03 10.81 Formulations with 70% CaCO.sub.3 CF14 16 10 322 241 0.583 134.44 28.79

[0078] As can be seen, all of the films produced have surprisingly low standard deviations in film thickness.

[0079] Cast film 1 (CF1) has a very good standard deviation of the thickness, good values for the tear resistance, and still adequate values for the modulus of elasticity. CF1 thus offers a balanced distribution of properties and can be used as a paper substitute in many applications.

[0080] Cast film 3 (CF3) performs best in every respect. It has the best standard deviation for film thickness, the lowest density reduction and the best mechanical properties. CF1 and CF3 only differ in the use of a dispersant (lubricant). FIG. 2 shows SEM images of sample cross sections of CF1 after stretching at 500-fold and 1000-fold magnification and FIG. 3 shows corresponding microscopic images of the surface of CF3. As can be seen, CF1 has a high porosity. This involves large oval openings that form around the calcium carbonate crystals. Here, the tension between the calcium carbonate particles and the polymer is apparently partially broken and relatively large openings and thus unevenness are formed. By using the dispersant, the so-called cavitation, i.e. the tearing off of the polypropylene matrix from the calcium carbonate particles, is significantly suppressed, which also has a positive effect on the density reduction. The dispersant thus prevents the formation of larger openings around the calcium carbonate particles and leads to a more uniform surface of the film and thus also to lower standard deviations in the film thickness. Surprisingly, this leads to an excellent standard deviation of the film thickness of only 2.64%.

[0081] At 0.692 g/cm.sup.3, the density of CF3 is only slightly higher than that of CF1, but is still well below 1 g/cm.sup.3. The tear resistance in the longitudinal and in the transverse direction is well over 20 MPa and the tear resistance in the transverse direction only deviates by about 28% from the tear resistance in the longitudinal direction. This should sufficiently prevent splicing of the film during further processing. At almost 1000 MPa, the moduli of elasticity are sufficiently high in both directions and hardly deviate from one another (around 14%).

[0082] In order to be able to replace classic paper as completely as possible, stone paper should have a tear resistance of at least about 20 MPa and a modulus of elasticity of about 1000 MPa. A modulus of elasticity in the range from 800 to 1200 MPa is acceptable in this context. Furthermore, foldability and splicing should resemble the behaviour of classic paper. CF3 ideally fulfils all of these conditions.

[0083] A comparison of CF4 with CF3 shows the influence of using polymers having high melt mass flow rates. The standard deviation of the film thickness increases significantly to 12.11%. However, the value is still acceptable for many printing applications. The tear resistances and the moduli of elasticity decrease slightly, but are still acceptable. The deviations in the tear resistances and the moduli of elasticity from one another increase, but are also still acceptable.

[0084] CF6 and CF7 show the effect of calcium carbonate particles having a relatively large particle size of more than 5 μm. The mechanical properties have clearly deteriorated. These films are no longer suitable for all applications in which paper is normally used. But they are still useful for many applications. However, for these examples too, the standard deviation of the film thickness is still in an acceptable range. A comparison of CF6 and CF7 also shows the positive effect of the dispersant on the standard deviation of the film thickness.

[0085] With a calcium carbonate content of 70 wt %, reasonably good mechanical properties could also be achieved (see CF14). Stabilisation is advantageous here (see comparison CF13 with CF14). Stabilisation is necessary to avoid the degradation of the polymer. Without stabilisation, stretching at such high fill levels is difficult. With these high proportions of calcium carbonate, the same standard deviations of the film thickness cannot be achieved as with 60 wt % calcium carbonate.

[0086] Some of the films were not stretchable under the conditions used. Higher stretching temperatures or higher amounts of auxiliary materials or other changes in the experimental conditions could correct this problem. However, this would impair the comparability of the tests. That is why it was dispensed with. The mechanical properties can also be improved by increasing the stretching temperatures. However, this hardly improves the standard deviations of the film thicknesses.

[0087] Table 3 shows two of the optical properties of the films, opacity and whiteness:

TABLE-US-00003 TABLE 3 Opacity Whiteness No. [%] [%] Formulations with 60% CaCO.sub.3 CF1 98.4 81.7 CF3 96.3 79.1 CF4 98.7 82.3 CF6 96.9 75.3 CF7 96.5 73 Formulations with 70% CaCO.sub.3 CF14 99.7 84.3

Example 3

[0088] Production of two films like CF1 and CF3 in example 2, but with a stretching ratio of 7 in the longitudinal direction and 7 in the transverse direction. The stretching also took place simultaneously on a “Karo IV laboratory stretching frame” from Brückner Maschinenbau GmbH. Table 4 shows the properties of the films obtained:

TABLE-US-00004 TABLE 4 Stretching to 7 × 7 Description Unit CF1 CF3 Final film thickness μm 102.24 69.74 Standard deviation % 8.19 1.90 thickness Tensile strength MD MPa 34 47 TD MPa 29 37 Elongation until break MD % 50 54 TD % 59 64 Modulus of elasticity MD MPa 763 1394 TD MPa 763 1177 Opacity % 96.2 91.6 Whiteness — 82.1 80.3 Surface weight g/m.sup.2 58.49 53.26 Density g/cm.sup.3 0.568 0.727

[0089] As can be seen, with the CF3 the standard deviation of the film thickness is further improved by the higher stretching ratios, which leads to an extremely low value of only 1.90%. With the CF1, the effect of the higher stretching ratios is very small. The use of a dispersant thus increases the positive effect of a high stretching ratio.

Example 4

[0090] A mixture consisting of 32 wt % polypropylene homopolymer having an MFR of 2 g/10 min (“Moplen® HP PP-520H” from LyondellBasell, Rotterdam, Netherlands), 65 wt % of calcium carbonate having a diameter d.sub.50 of 1.4 μm (“Filmlink® 400”, Imerys Minerals, Cornwall, UK), 0.16 wt % of additives (400 ppm of the acid scavenger “DHT-4A” from Kisuma Chemicals, Veendam, Netherlands and 1200 ppm “Irganox B561” from BASF SE, Ludwigshafen, Germany) and 1.84 wt % dispersant (“Licowax OP® powder” from Clariant, Frankfurt/Main, Germany) was diluted with the above pure polypropylene homopolymer to a total calcium carbonate content in the film of 25 wt % (A) or 59 wt % (B). Two different mixtures were thus made with different calcium carbonate contents. A multilayer film having an ABA structure was produced from these two mixtures, with B representing the core and A the coatings. The calcium carbonate content in the core was accordingly 59 wt % and the calcium carbonate content in the coatings 25 wt %. The film was produced in accordance with example 1 by co-extrusion.

[0091] After co-extruding and cooling, the film was stretched sequentially. First stretched with a stretching ratio of 5 in the longitudinal direction and then with a stretching ratio in the transverse direction. After the stretching in the transverse direction, a heat treatment was carried out with a relaxation of 10% in the transverse direction, i.e. a reduction in the stretching ratio from 5 to 4.5 in the transverse direction. The temperatures during stretching and during heat treatment are shown in Table 5.

TABLE-US-00005 TABLE 5 Sample roll 1 Sample roll 2 Process conditions MDO temperatures [° C.] Preheat roller 1 110 Preheat roller 2 120 Preheat roller 3 142 Preheat roller 4 140 Preheat roller 5 146 Preheat roller 6 142 Stretching roller 1 148 Stretching roller 2 146 Stretching roller 3 115 Stretching roller 4 115 Annealing roller 1 120 Annealing roller 2 120 MD stretching 5

[0092] FIG. 4 shows the stretching profile in the stretching oven during stretching in the transverse direction for the two tests, sample roll 1 and sample roll 2, which were both carried out with the film obtained as described above. The stretching oven is divided into zones Z1 to Z9, wherein Z1 to Z3 represent preheating zones (preheating), Z4 and Z5 zones in which stretching is carried out (stretching), Z6 and Z7 zones in which heat treatment (annealing) is carried out without stretching, Z8 a zone in which the film is relaxed (relaxation) and zone 9 a zone in which both relaxation and cooling (relaxation & cooling) are carried out. As can be seen from FIG. 4, after stretching in the longitudinal direction, the films were first stretched to a stretching ratio in the transverse direction of 5, then held at this stretching ratio for a while and then relaxed to a final stretching ratio of 4.5. FIG. 4 shows in which zone the stretching is done and by how much. FIG. 5 shows the temperature profile of the oven temperature for sample roll 1 and FIG. 6 shows the temperature profile of the oven temperature for sample roll 2. The oven temperatures for the individual zones Z1 to Z9 are specified separately. In the overview of FIGS. 4 to 6, the correlation of the zone of the stretching oven, oven temperature and stretching ratio is therefore given for both experiments. As can be seen from a comparison of FIGS. 5 and 6, the temperature in sample roll 1 is increased before the start, at the start and during relaxation (cf. zones Z7 and Z8).

[0093] Table 6 shows the properties of the films thus obtained:

TABLE-US-00006 TABLE 6 Description Unit Sample roll 1 Sample roll 2 Final film thickness um 60 89.4 Standard deviation % 5.98 9.62 thickness Tensile strength MD MPa 20 32 TD MPa 26 29 Modulus of MD MPa 1590 525 elasticity TD MPa 1622 583 Opacity % 75 96 Whiteness — 81 86 Density g/cm.sup.3 1.024 0.65 Foldability very good very bad

[0094] Due to the higher temperatures during the heat treatment, the density reduction could be significantly revised and the mechanical properties could be significantly improved.

Example 5

[0095] From a mixture consisting of 65 wt % polypropylene homopolymer having a MFR of 2 g/10 min (“Moplen® PP-HP 520H” from LyondellBasell, Rotterdam, Netherlands), 15.84 wt % calcium carbonate having an diameter d.sub.50 of 1.4 μm (“Filmlink® 400”, Imerys Minerals, Cornwall, UK), 16 wt % polyolefin elastomer (POE) (Engage 8137, The Dow Chemical Company, Midland, USA), 0.16 wt % additives (400 ppm “DHT-4A” from Kisuma Chemicals, Veendam, Netherlands and 1200 ppm “Irganox B561” from BASF SE, Ludwigshafen, Germany) and wt % dispersant (“Licowax OP® powder” Clariant, Frankfurt/Main, Germany), two different mixtures having a calcium carbonate content of 58 wt % (b) and 25 wt % (A) are produced by adding the aforementioned calcium carbonate. A multilayer film having an ABA structure was produced from this by co-extrusion, with B representing the core and A the coatings. The calcium carbonate content in the core was 58 wt % and the calcium carbonate content in the coatings was 25 wt %. The film was otherwise produced in accordance with example 1.

[0096] First the film was co-extruded and then cooled. The film was then stretched sequentially, first with a stretching ratio of 5 in the longitudinal direction and then with a stretching ratio of 4.7 in the transverse direction. A heat treatment was carried out after the stretching in the transverse direction. A relaxation of 6% in the transverse direction was carried out during the heat treatment, that is to say to a final stretching ratio of 4.4 in the transverse direction. The temperatures during stretching and during heat treatment are shown in Table 7.

TABLE-US-00007 TABLE 7 Sample roll 3 Process conditions MDO temperatures [° C.] Preheat roller 1 110 Preheat roller 2 110 Preheat roller 3 126 Preheat roller 4 128 Preheat roller 5 128 Preheat roller 6 130 Stretching roller 1 142 Stretching roller 2 140 Stretching roller 3 115 Stretching roller 4 115 Annealing roller 1 120 Annealing roller 2 120 MD - stretching 5

[0097] FIG. 7 shows the stretching profile in the stretching oven during stretching in the transverse direction for sample roll 3. Here it can clearly be seen in which zone and by which factor stretching is carried out. The stretching oven is divided into zones Z1 to Z9, wherein Z1 to Z3 represent preheating zones (preheating), Z4 and Z5 zones in which stretching is carried out (stretching), Z6 and Z7 zones in which heat treatment (annealing) is carried out without stretching, Z8 a zone in which the film is relaxed (relaxation) and zone 9 a zone in which both relaxation and cooling (relaxation & cooling) are carried out. As can be seen from FIG. 4, the film was first stretched to a stretching ratio in the longitudinal direction of 4.7, then held at this stretching ratio for a while and then relaxed to a final stretching ratio of 4.4. FIG. 7 shows in which zone the stretching is done and by how much. FIG. 8 shows the temperature profile of the oven temperature for sample roll 3. The oven temperatures for the individual zones Z1 to Z9 are specified separately. In the overview of FIGS. 7 and 8, the correlation of the zone of the stretching oven, oven temperature, and stretching ratio is therefore given.

[0098] Table 8 shows the properties of the film thus obtained:

TABLE-US-00008 TABLE 8 Description Unit Sample roll 3 Final film thickness um 70.4 μm Standard deviation % 2.7 thickness Tensile strength MD MPa 26.9 TD MPa 19.8 Modulus of elasticity MD MPa 196 TD MPa 203 Opacity % 93 Whiteness % 87 Density g/cm.sup.3 0.77 Foldability very good

[0099] The mechanical properties of this film are no longer quite as good as with the pure PP as a matrix. However, the foldability (dead-fold properties) could be significantly improved. Polyolefin elastomers can therefore be used in order to improve the foldability of the films according to the invention.