Patent Application
20080063857
The invention achieves the object via provision of a transparent, biaxially oriented, sealable, antifog polyester film with a bass layer (B), with a sealable outer layer (A), with antifog coating, and with an outer layer (C), where
The term pigments (also called particles here) also includes antiblocking agents, fillers, and external or inert particles which are characterized via the parameters mentioned. Typical pigments are inorganic and/or organic particles, e.g. calcium carbonate, amorphous silica, talc, magnesium carbonate, barium carbonate, calcium sulfate, barium sulfate, lithium phosphate, calcium phosphate, magnesium phosphate, aluminum oxide, LiF, the calcium, barium, zinc, or manganese salts of the dicarboxylic acids used, titanium dioxide, kaolin, or particulate polymers, e.g. crosslinked polystyrene particles or crosslinked acrylate particles.
According to the invention, the film preferably has a three-layer structure and encompasses the base layer (B), the sealable and coated outer layer (A), and the non-sealable outer layer (C) (A-B-C,
The haze of the transparent film is preferably smaller than 5%.
The sealable outer layer (A) preferably comprises less than 0.01% by weight (based on the total weight of the outer layer (A)) of external particles, and particularly preferably absolutely no external particles. This case gives the best sealing with respect to itself and with respect to the materials listed above. In this case no bubbles form between the sealable layer (A) and the substrate, and sealing occurs over the entire area. Otherwise, cavities form, caused by the peaks of the fillers, for example, and impair sealing.
It has been found that the d50 particle diameter, the SPAN98 scatter, and the concentration of the pigments used in the non-sealable outer layer (C) are decisive for good winding, for good processibility, and for good optical properties of the film.
To achieve good winding and good processibility of the film, the outer layer (C) comprises a pigment whose median diameter (d50 value) is in the range from 2 to 5 μm, preferably from 2.1 to 4.9 μm, and particularly preferably from 2.2 to 4.8 μm, and the SPAN98 is in the range from 1.2 to 2, preferably from 1.25 to 1.9, and particularly preferably from 1.3 to 1.8.
If, in contrast, the outer layer (C) comprises a pigment whose median diameter and/or SPAN98 is outside the inventive range, this has an adverse effect on winding, processibility, and the optical properties of the film.
If the outer layer (C) comprises a pigment whose median diameter is greater than 5 μm, filter behavior becomes impaired; if the SPAN98 is greater than 2, the optical properties and the winding of the film become poorer.
If the outer layer (C) of the film comprises, as sole pigment, a pigment whose median diameter is smaller than 2 μm, haze increases, and gloss becomes poorer; if the SPAN98 is smaller than 1.2, the winding of the film becomes poorer, and the film has a tendency toward “blocking”, for example.
In order to improve winding behavior and processibility, the outer layer (C) has a high level of inert, added pigments. The concentration of the inert particles in the outer layer (C) is preferably from 0.12 to 0.4% by weight, in particular from 0.14 to 0.35% by weight, and in the particularly preferred embodiment is from 0.16 to 0.3% by weight, depending in essence on the optical properties to be achieved in the film.
It is also possible (in addition) moreover to incorporate other pigments, e.g. with d50 value smaller than that of the main pigment, into the outer layer (C). However, if the d50 values and/or SPAN98 values of the main pigment deviate from the abovementioned values, or if no such pigment is present, the disadvantages described above arise (main pigment means the pigment with the greatest concentration in (C)).
A feature of the antifog coating on the outer layer (A) is that it comprises the following components, alongside water:
polyvinylpyrrolidone (=component i),
a surfactant (=component ii), and optionally
a polymer which improves the binding of the other components to the polyester surface (adhesion-promoting polymer=component iii).
The total concentration of all of the components i) to iii) in water is preferably in the range from 1 to 8% by weight. Unless otherwise stated, all of the amounts stated are percentages by weight, based on the weight of the ready-to-use coating preparation.
The molecular weight (Mw) of the polyvinylpyrrolidone used is preferably from 20 to 2500 kilodaltons, particularly preferably from 40 to 1500 kilodaltons. The content of the polyvinylpyrrolidone in the coating solution is mostly from 0.3 to 4.0% by weight, preferably from 0.5 to 3.5% by weight. If polyvinylpyrrolidone having lower molecular weights is used, the coating becomes more susceptible to removal by washing, and at higher molecular weights the coating solution becomes too viscous.
Surfactants are molecules comprised of a hydrophobic and a hydrophilic moiety, being described as amphiphilic.
The concentration used of the surfactant mentioned in the coating composition described above is from about 0.1 to 2.5% by weight, preferably from 0.3 to 2.0% by weight, and the surfactant is preferably ionic, particularly preferably anionic, and is particularly preferably selected from the group of the alkyl sulfates, alkylbenzene sulfates, alkyl ether sulfates, or sulfosuccinic esters or their salts.
The polymers which improve the binding of the polyvinylpyrrolidone to the polyester surface are preferably used in the form of an aqueous solution or dispersion. The concentration of these polymers in the finished coating solution is from about 0.3 to 4.0% by weight, preferably from 0.5 to 3.5% by weight. Suitable polymers of this type are acrylates, for example those described in WO 94/13476, hydrophilic polyesters (e.g. PET/IPA polyesters containing the sodium salt of 5-sulfoisophthalic acid, for example those described in EP-A-0 144 878 equivalent to U.S. Pat. No. 4,493,872A1, U.S. Pat. No. 4,252,885, or EP-A-0 296 620, dendritic polyesters having alcohol or acid end groups), polyurethanes, butadiene copolymers with acrylonitrile or methyl methacrylate, methacrylic acid, or an ester thereof.
The ready-to-use coating composition is therefore preferably comprised solely of water and of components i) and ii), or i), ii), and iii), and also, if appropriate, of antiblocking agents. “Comprised” here means that the composition is comprised of at least 90% by weight, preferably at least 95% by weight, and particularly preferably at least 99% by weight, of the components mentioned.
After in-line coating with the coating composition, the finished coating on the outer layer (A) is comprised of the dried residue (drying product) of the coating composition, which then equally preferably is comprised solely of the drying product of components i) and ii), or i), ii), and iii), and also, if appropriate, of antiblocking agents. The excess water or the excess solvent used has evaporated in the process.
Surprisingly, it has been found that the coating brings about not only the antifog effect but also a marked reduction in the coefficient of friction of the sealable layer (A) with respect to itself and with respect to the outer layer (C) of the film.
The base layer (B) and the outer layer (C) of the film are preferably comprised of at least 90% by weight of a thermoplastic polyester. Materials suitable for this purpose are polyesters comprised of ethylene glycol and terephthalic acid (=polyethylene terephthalate, PET), comprised of ethylene glycol and naphthalene-2,6-dicarboxylic acid (=polyethylene-2,6-naphthalate, PEN), comprised of 1,4-bishydroxymethylcyclohexane and terephthalic acid (=poly(1,4-cyclohexanedimethylene terephthalate, PCDT) or else comprised of ethylene glycol, naphthalene-2,6-dicarboxylic acid and biphenyl-4,4-dicarboxylic acid (=polyethylene-2,6-naphthalate bibenzoate, PENBB). Particular preference is given to polyesters comprised of at least 90 mol %, preferably at least 95 mol %, of ethylene glycol units and terephthalic acid units, or of ethylene glycol units and naphthalene-2,6-dicarboxylic acid units. The remaining monomer units derive from other aliphatic, cycloaliphatic, or aromatic diols and, respectively, dicarboxylic acids.
Other examples of suitable aliphatic diols are diethylene glycol, triethylene glycol, aliphatic glycols of the formula HO—(CH2)n—OH, where n is an integer from 3 to 6 (in particular 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol) and branched aliphatic glycols having up to 6 carbon atoms. Among the cycloaliphatic diols, mention should be made of cyclohexanediols (in particular 1,4-cyclohexane-diol). Examples of other suitable aromatic diols have the formula HO—C6H4-X-C6H4—OH, where X is —CH2—, —C(CH3)2—, —C(CF3)2—, —O—, —S— or —SO2—. Bisphenols of the formula HO—C6H4—C6H4—OH are very suitable.
One way of preparing the polyesters is the transesterification process. Here, the starting materials are dicarboxylic esters and diols, which are reacted using the customary transesterification catalysts, such as the salts of zinc, of calcium, of lithium, of magnesium or of manganese. The intermediates are then polycondensed in the presence of well-known polycondensation catalysts, such as antimony trioxide or titanium salts. Another equally good preparation method is the direct esterification process in the presence of polycondensation catalysts. This starts directly from the dicarboxylic acids and the diols. Inventive polyesters are commercially available.
The sealable outer layer (A) preferably applied via coextrusion to the base layer (B) is preferably in essence comprised of copolyesters which are mainly comprised of isophthalic acid units and of terephthalic acid units and of ethylene glycol units. The remaining monomer units derive from other aliphatic, cycloaliphatic, or aromatic diols and, respectively, dicarboxylic acids that can also occur in the base layer. The preferred copolyesters which provide the desired sealing properties are those comprised of ethylene terephthalate units and of ethylene isophthalate units. The proportion of ethylene terephthalate is preferably from 60 to 95 mol %, and the corresponding proportion of ethylene isophthalate is from 40 to 5 mol %. Preference is moreover given to copolyesters in which the proportion of ethylene terephthalate is from 65 to 90 mol % and the corresponding proportion of ethylene isophthalate is from 35 to 10 mol %, and great preference is given to copolyesters in which the proportion of ethylene terephthalate is from 70 to 85 mol % and the corresponding proportion of ethylene isophthalate is from 30 to 15 mol %.
The desired sealing properties of the outer layer (A) are believed to be obtained from the combination of the chemical constitution of the copolyester used, the thickness of the outer layer, the topography (smooth surface), and the hydrophilic coating.
The sealable outer layer (A) has
The best sealing properties of the film are obtained when no further additives, in particular no inorganic or organic fillers, are added to the copolyester. This case give the lowest minimum sealing temperature and the highest seal seam strength for a given copolyester and a given layer thickness.
The base layer (B) can also comprise conventional additives, examples being UV stabilizers, hydrolysis stabilizers, antiblocking agents (e.g. by way of the regrind). The other outer layer (C) can likewise also comprise conventional additives, examples being stabilizers. These additives are advantageously added to the polymer or to the polymer mixture before melting begins. Examples of stabilizers advantageously used are phosphorus compounds, such as phosphoric acid or phosphoric esters.
The thickness of the outer layer (C) of the film is generally greater than 0.4 μm and is preferably in the range from 0.5 to 10 μm, in particular in the range from 0.8 to 9 μm, and particularly preferably in the range from 1.0 to 8 μm.
The thickness of the outer layer (A) of the film is generally greater than 0.4 μm and is preferably in the range from 0.5 to 10 μm, in particular in the range from 0.8 to 9 μm, and particularly preferably in the range from 1.0 to 8 μm.
The total thickness of the inventive polyester film can vary widely. It is preferably from 5 to 500 μm, in particular from 7 to 300 μm, with preference from 10 to 100 μm.
The invention also provides a process for production of the inventive polyester film by coextrusion methods known per se from the literature.
First, as is conventional in the coextrusion process, the polymers or the polymer mixtures for the individual layers are compressed and plasticized in an extruder, and the added materials intended, if appropriate, as additives here can be present in the polymer or in the polymer mixture by this stage. The melts are then simultaneously pressed through a flat-film die, and the extruded multilayer melt is drawn off on one or more take-off rolls, whereupon the melt cools and hardens to give an unoriented prefilm.
The biaxial orientation process is generally carried out sequentially. In this process, the prefilm is preferably first stretched longitudinally (i.e. in machine direction=MD) and then stretched transversely (i.e. perpendicularly to machine direction=TD). This results in spatial orientation of the polymer chains. The longitudinal stretching can be carried out with the aid of two rolls rotating at different speeds corresponding to the desired stretching ratio. The transverse stretching process generally uses an appropriate tenter frame in which the two edges of the film are clamped, the film then being subjected to tension in the direction of the two sides at an elevated temperature.
The temperature at which the stretching process is carried out can be varied relatively widely and depends on the desired properties of the film. The longitudinal stretching process will generally be carried out at a temperature in the range from 80 to 130° C. and the transverse stretching process will generally be carried out in the range from 90 to 150° C. The longitudinal stretching ratio is generally in the range from 2.5:1 to 6:1, preferably from 3.0:1 to 5.5:1. The transverse stretching ratio is generally in the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to 4.5:1.
In the heat-setting process which follows, the film is kept at a temperature of from about 150 to 250° C. for a period of from about 0.1 to 10 s. The film is then wound up conventionally.
After the biaxial stretching process, it is preferable that the non-sealable surface of the film is corona—or flame-treated in accordance with one of the known methods. The intensity of treatment is generally in the range above 50 mN/M.
The inventive biaxially oriented polyester film is in-line coated on the sealable layer, and this means that the coating is preferably applied during the film-production process prior to longitudinal and/or transverse stretching. In order to achieve good wetting of the polyester film by the aqueous coating solution or aqueous coating dispersion, the surface is preferably first corona-treated. The coating can be applied by any familiar suitable process, for example using a slot coater or a spray method.
It is particularly preferable to apply the coating by means of reverse gravure-roll coating, which can apply the coating extremely homogeneously with application weights of from 1 to 5 g/m2 (wet). Preference is likewise given to application via the Meyer rod method, which can achieve relatively high coating thicknesses. The thickness of the coating on the finished film is from about 5 to 500 nm, preferably from 30 to 200 nm.
The inventive film features excellent sealability, very good winding and optical properties, very good processing behavior, and very good antifog effect. The sealable outer layer (A) seals with respect to itself, with respect to the non-sealable outer layer (C), and with respect to substrates comprised, for example, of APET, A/CPET, and CPET. This makes the film useful in many sectors, for example as lid film for (ready-meal) trays, or for bags, or generally as packaging material for foods and other consumable items.
Alongside this, it has been ensured that, during production of the film, an amount in the range from about 20 to 60% by weight, based on the total weight of the film, of the cut material (regrind) can be reintroduced into the extrusion process without any significant resultant adverse effect on the physical and optical properties of the film, in particular on its appearance.
The tables below (tables 1 and 2) again collate the most important preferred properties of the film.
For the purposes of the present invention, the following test methods were used for characterization of the raw materials and of the films:
Contact angle α with respect to water (see
Antifog properties of the polyester films were determined as follows:
Film samples were welded onto a ready-meal tray (length about 17 cm, width about 12 cm, height about 3 cm) comprised of amorphous polyethylene terephthalate (=APET) and comprising about 50 ml of water, in a laboratory temperature-controlled to 23° C. with 50% relative humidity.
The trays were stored in a refrigerator temperature-controlled to 4° C. and removed for assessment after, respectively, 10 min, 30 min, 4 h, 8 h, and 24 h. The test assessed condensation resulting when air at 23° C. was cooled to refrigerator temperature. A film equipped with effective antifog agent is transparent even after condensation has occurred, since, for example, the condensate forms a coherent, transparent film. Without effective antifog agent, formation of a fine mist of droplets on the film surface leads to reduced transparency of the film; in the most disadvantageous case the contents of the ready-meal tray become invisible.
Another test method is known as the hot-fog test. For this, a 250 ml glass beaker containing 50 ml of water and covered by the film to be tested is placed in a water bath temperature-controlled to 70° C. The assessment is identical with that described above. In addition, this test can test long-term antifog action and, respectively, leaching resistance of the film, since steam continuously condenses on the film and then runs off or drips off. The result is leaching of readily soluble substances and decreased antifog action. This test was likewise carried out in a laboratory temperature-controlled to 23° C. with 50% relative humidity.
Median particle diameter d50 was determined by laser on a Master Sizer (Malvern Instruments, UK) by the standard method (examples of other test equipment being the Horiba LA 500 (Horiba Ltd., Japan) or Helos (Sympatec GmbH, Germany), which use the same principle of measurement). For this, the specimens were placed in a cell with water and this was then inserted into the test equipment. The test procedure is automatic, and also includes the mathematical determination of the d50 value. The d50 value is determined here in accordance with its definition from the (relative) cumulative particle size distribution curve: the point of intersection of the 50% by weight ordinate value with the cumulative curve gives the desired d50 value (cf.
The test equipment used to determine SPAN98 was the same as that described above for the determination of median diameter d50. SPAN98 is defined here as follows:
Determination of d98 and d10 is in turn based on the cumulative particle size distribution curve. The point of intersection of the 98% ordinate value with the cumulative curve gives the desired d98 value on the abscissa axis, and the point of intersection of the 10% ordinate value with the cumulative curve gives the desired d10 value on the abscissa axis (cf.
To determine seal seam strength, a strip of film (length 100 mm×width 15 mm) is placed on the APET side of a corresponding strip of a ready-meal tray and sealed with a sealing time of 0.5 s and a sealing pressure of about 3 bar (HSG/ET sealing equipment from Brugger, Germany, bilaterally heated sealing jaws) with the temperature set at 180° C. As in
Heat-sealed specimens (seal seam 15 mm×100 mm) are produced as described above under measurement of seal seam strength using Brugger HSG/ET sealing equipment, by sealing the film at different temperatures with the aid of two heated sealing jaws at a sealing pressure of 3 bar and with a sealing time of 0.5 s. Seal seam strength was measured as in the determination of 180° seal seam strength. The minimum sealing temperature is the temperature at which a seal seam strength of at least 0.5 N/15 mm is achieved.
Seal seam strength with respect to itself (=FIN sealing)
To determine seal seam strength, two film strips of width 15 mm were mutually superposed and sealed with a sealing time of 0.5 s and a sealing pressure of 3 bar (equipment; Brugger NDS, single-side heated sealing jaws) at 180° C. The seal seam strength was determined by the T-Peel method=2·90°).
HSG/ET sealing equipment from Brugger was used to produce heat-sealed specimens (seal seam 20 mm×100 mm), where the film is sealed with a sealing time of 0.5 s and at a sealing pressure of 3 bar with the aid of two heated sealing jaws at various temperatures. Test strips of width 15 mm were cut from the sealed specimens. T-seal seam strength was measured as in the determination of seal seam strength. The minimum sealing temperature is the temperature at which a seal seam strength of at least 0.5 N/15 mm is achieved.
Haze is determined to ASTM D1003-52.
Standard viscosity SV (DCA) is measured by a method based on DIN 53726 at 25° C. in dichloroacetic acid. The intrinsic viscosity (IV, measured in dl/g) of polyethylene terephthalate is calculated as follows from the standard viscosity
rV=[n]=6.907·10−4 SV(DCA)+0.063096 [dl/g]
The following components were dissolved in water to produce the coating solution:
This coating solution was applied to the coextruded polyester film by the following method:
The coextruded polyester film comprised the following layers and raw materials:
Outer layer (A) 100% by weight of copolyester comprised of 78 mol % of ethylene terephthalate and 22 mol % of ethylene isophthalate whose SV value was 800
Base layer (B) 100% by weight of polyethylene terephthalate whose SV value was 800
Outer layer (C) mixture comprised of 80% by weight of polyethylene terephthalate whose SV value was 800 and 20% by weight of masterbatch comprised of 99% by weight of polyethylene terephthalate (SV value 800) and 1.0% by weight of SYLOBLOC® 44 H (synthetic SiO2 from Grace, d50; 2.5 μm, SPAN98: 1.8)
The unoriented film was stretched longitudinally with a stretching ratio of 3.8:1, and was kept here at a temperature of 115° C. The longitudinally stretched film was coated on the outer layer (A) by reverse gravure coating with the solution described above comprised of polyvinylpyrrolidone and the sodium salt of diethylhexyl sulfosuccinate. The longitudinally stretched, coated film was dried at a temperature of 100° C. The film was then transversely stretched with a stretching ratio of 3.8:1, thus giving a biaxially stretched film. The biaxially stretched film was heat-set at 230° C. The final film thickness was 25 μm, the thickness of each of the outer layers here being 2 μm. The dry weight of the coating was about 0.04 g/m2.
The film exhibited very good antifog properties, i.e. no formation of fine droplets was observed. The contact angle measured was α=12°, contrasting with an uncoated film with an angle of α=64°. There was no change to the transparency and the haze of the film during the antifog test.
In contrast to Example 1, the thickness of the sealable outer layer (A) was raised from 2.0 to 3.0 μm, but the film structure and method of production were otherwise identical. The result was an improvement in sealing properties, in particular a marked increase in seal seam strength. As in Example 1, this film, too, exhibited very good antifog properties.
In contrast with Example 1, the following constitution was now used for the coating solution:
The dry weight of the coating was about 0.05 g/m2.
As in Example 1, this film too, exhibited very good antifog properties, with simultaneous reduction in the susceptibility of the coatings for removal by washing, meaning that the antifog properties were retained even after treatment with steam for a number of hours. The contact angle measured was α=11°, contrasting with an uncoated film with an angle of α=64°. There was no change in the transparency and the haze of the film during the antifog test.
In contrast to Example 1, the level of pigmentation in the sealable outer layer (A) was now as high as in the non-sealable outer layer (C). This measure gave a marginal improvement in the winding of the film and its processing properties, but there was marked impairment of the sealing properties of the film and its optical properties.
By analogy with Example 1, a biaxially oriented polyester film was produced, but without coating. Omission of the coating resulted in a rise in the coefficient of friction in the film.
The film exhibited marked droplet formation in the antifog test, i.e. the film had no antifog effect.
The most important properties of the films thus prepared are summarized in table 3 below:
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 025 281.0 | May 2006 | DE | national |
