POLYMER COATING COMPOSITION FOR METAL SUBSTRATE AND USE THEREOF

Abstract

A polymer film for laminating onto a metal substrate (M), the polymer film including an adhesion layer (A) and a bulk layer (B), wherein the adhesion layer is intended for bonding to the metal substrate and includes 20 to 50 wt. % of a non-crystallisable copolyester, 50 to 80 wt. % of polybutylene terephthalate (PBT) and 0-10 wt. % of polymers and additives, and wherein the bulk layer consists of is at least 91 wt. % of PBT and at most 9% of other polymers and additives.

Claims

1. A polymer film for laminating onto a metal substrate, the polymer film comprising an adhesion layer and a bulk layer, wherein the adhesion layer is intended for bonding to the metal substrate and comprises 20 to 50 wt. % of a non-crystallisable copolyester, 50 to 80 wt. % of polybutylene terephthalate (PBT) and 0-10 wt. % of polymers and additives, and wherein the bulk layer consists of at least 91 wt. % of PBT and at most 9% of other polymers and additives.

2. The polymer film according to claim 1, further comprising a top layer on top of the bulk layer.

3. The polymer film according to claim 1, wherein the bulk layer consists of at least 95 wt. % of PBT, and at most 5% of other polymers and additives.

4. The polymer film according to claim 1, wherein the bulk layer consists of at least 99 wt. % of PBT, and at most 1% of other polymers and additives.

5. The polymer film according to claim 1, wherein the top layer consists of at least 95 wt. % of PBT, and at most 5% of other polymers and additives.

6. The polymer film according to claim 1, wherein the total PBT content in the film is at least 88 wt. %, preferably at least 90 wt. %

7. The polymer film according to claim 1, wherein the total PBT content in the film is at most 98 wt. %, preferably at most 96 wt. %.

8. The polymer film according to claim 1, wherein the thickness of the adhesion layer is between 2 and 8 m.

9. The polymer film according to claim 1, wherein the total film thickness is between 10 and 50 m.

10. The polymer film according to claim 1, wherein the non-crystallisable polyester is PETg.

11. The polymer film according to claim 1, wherein the non-crystallisable polyester is IPA-PET.

12. The polymer film according to claim 1 comprising one or more polymer layers in addition to the top, bulk and adhesion layer.

13. A process for producing a polymer film according to claim 1, comprising the steps of: melting a suitable mixture of polymer granules in two or more extruders; passing the molten polymer through a flat extrusion die to form the polymer film consisting of the said two or more layers; cooling the extruded polymer film using one or more cooling, casting or calendering rolls to form a solid polymer film; trimming the edges of the extruded polymer film; reducing the thickness of the solid polymer film by stretching the solid polymer film in a stretching unit by exerting a stretching force only in the longitudinal direction (MDO), or only in the transverse direction (TDO), or biaxially (BO); optionally trimming the edges of the stretched polymer film; optionally coiling the stretched polymer film.

14. A method of use of the polymer film according to claim 1 and a metal substrate, comprising laminating the polymer film onto the metal substrate by (i) laminating the polymer film, wherein the polymer film has been stretched, onto the metal substrate by passing the metal substrate and the polymer film through a laminating device and heat bonding the polymer film to the metal substrate to produce a polymer-coated substrate followed by post-heating the polymer-coated substrate to reduce the orientation and crystallinity of the polymer film, and subsequently cooling the post-heated polymer-coated substrate or by (ii) direct extrusion onto the metal substrate followed by cooling the polymer-coated substrate.

15. The method according to claim 14, wherein the laminate is produced into containers or container parts for packaging food or beverages for human or animal consumption.

16. The method according to claim 14, wherein: (i) the cooling subsequent to the post-heating of the polymer-coated substrate comprises fast cooling or quenching of the post-heated polymer-coated substrate or (ii) the cooling which follows the direct extrusion onto the metal substrate comprises fast cooling or quenching of the polymer-coated substrate.

17. A method of use of the polymer film produced according to the process of claim 13 and a metal substrate, comprising laminating the polymer film onto the metal substrate by (i) laminating the stretched polymer film onto the metal substrate by passing the metal substrate and the polymer film through a laminating device and heat bonding the polymer film to the metal substrate to produce a polymer-coated substrate followed by post-heating the polymer-coated substrate to reduce the orientation and crystallinity of the polymer film, and subsequently cooling the post-heated polymer-coated substrate.

18. The method according to claim 17, wherein the laminate is produced into containers or container parts for packaging food or beverages for human or animal consumption.

19. The laminate according to claim 17, wherein the cooling subsequent to the post-heating of the polymer-coated substrate comprises fast cooling or quenching of the post-heated polymer-coated substrate.

20. A laminate comprising the polymer film according to claim 1 and a metal substrate, wherein the polymer film is laminated onto the metal substrate by (i) laminating the polymer film, wherein the polymer film has been stretched, onto the metal substrate by passing the metal substrate and the polymer film through a laminating device and heat bonding the polymer film to the metal substrate to produce a polymer-coated substrate followed by post-heating the polymer-coated substrate to reduce the orientation and crystallinity of the polymer film, and subsequently cooling the post-heated polymer-coated substrate or by (ii) direct extrusion onto the metal substrate followed by cooling the polymer-coated substrate.

Description

[0045] Blushing performance was evaluated as follows (see also FIG. 3 and the description thereof). In this test, a flat sample of polymer coated metal substrate is placed on top of an open cylinder connected to a pan of generously boiling water, so that the sample covers the open cylinder and so that steam impinges on the sample within the well-defined area inside the cylinder. The test is run during 15 min., after which the material is visually inspected for signs of whitening discolouration. FIG. 3 shows the schematic set-up. A pressure cooker type pan (2) filled with water is brought to a generous boil by means of hot-plate (1) so that steam is generated. The pan is provided with a lid (3) which has a small hole in the middle where the steam escapes from the pan. On top of the lid, covering the hole, there is an open cylindrical adapter (4) of 75 mm in diameter and 50 mm in height. A flat sample of polymer-coated material (5) of 1212 cm dimensions is placed on top of the cylindrical adapter and held in place with e.g. a small weight and simulates the effect of the steam during retorting. Steam from the boiling water impinges on the test sample on the area enclosed by the cylindrical adapter. The ring where the sample touches the open ring is clearly visible, e.g. in FIG. 2, bottom right image, and the degree of blushing is determined in the circle. This method is very much a qualitative measure, but it allows to compare different polymer coated samples quickly, easily, and in a way representative to a retort operation.

[0046] The PBT is obtained by polycondensation of a terephthalic acid component (preferably 80-100%, more preferably 90-100%), and a 1,4-butanediol component as main components (preferably 80-100%, more preferably 90-100%), with or without sequential solid-state polycondensation. It has repeating butylene terephthalate structure, shows high crystallinity, has high crystallisation speed and low Tg, and therefore, is suitable for can forming. The PBT preferably has the following properties: an intrinsic viscosity of preferably 0.60-2.2, more preferably 1.0-1.5, a weight-average molecular weight of preferably 50000-200000, more preferably 80000-150000, and a distribution of molecular weight, which is a ratio of weight-average molecular weight to number-average molecular weight, of preferably 1.5-5.0, more preferably 1.5-2.5 (D=M.sub.w/M.sub.n).

[0047] In addition to pure (homopolymer) PET, PET modified by copolymerisation is also available. In some cases, the modified properties of copolymer are more desirable for a particular application. For example, cyclohexane dimethanol (CHDM) can be added to the polymer backbone in place of ethylene glycol (EG). Since this building block is much larger (6 additional carbon atoms) than the ethylene glycol unit it replaces, it does not fit in with the neighbouring chains the way an ethylene glycol unit would. This interferes with crystallisation. In general, such PET is known as PETg or PET-G (Polyethylene terephthalate glycol-modified). Eastman Chemical, SK Chemicals and Selenis are some PETg manufacturers. Although the use of CHDM in the context of this invention is preferable, examples of other comonomers that can be used to produce PETg and achieve obtain a non-crystallising PETg are 2,2,4,4-Tetramethyl-1,3-cyclobutanediol or 1,4:3,6-Dianhydro-D-glucitol (isosorbide).

[0048] Another common modifier is isophthalic acid, replacing some of the 1,4-(para-) linked terephthalate units. Replacing terephthalic acid with isophthalic acid creates a kink in the PET chain, interfering with crystallisation and lowering the polymer's melting point. The 1,2-(ortho-) or 1,3-(meta-) linkage produces an angle in the chain, which also disturbs crystallinity. These copolyesters are generally referred to as IPA-PET, A-PET or PETA (Polyethylene terephthalate acid-modified) and are also non-crystallising polyesters that can be used in the polymer films according to the invention, such as IPA-PET with 30% IPA.

[0049] According to the invention the polymer film contains at least two layers: The layer contacting the metal substrate (adhesion layer) contains a copolyester resin in an amount of 20-50 wt. % and PBT in an amount of 50-80 wt. % and optionally other polymers and additives in an amount of 0-10 wt. %. The copolyester resin has the property that it is non-crystallisable. An example of a suitable copolyester is glycol-modified polyester PETg, such as commercially available Eastar Copolyester 6763 from Eastman Chemical Co. or Skygreen S2008 from SK Chemicals. Also, IPA-modified polyester resins may be used of the amount of IPA modification is sufficiently high to ensure the IPA-PET is non-crystallisable. Whether or not the polyester is non-crystallisable can be determined as described above.

[0050] The non-crystallising PETg used in the present invention has TPA as diacid component and a mixture of EG and CHDM as diol component in which the latter represents about 30 wt. % of the diol component.

[0051] The one or more layers on top of the adhesion layer consist essentially of PBT. Examples of suitable PBT resins are Crastin FG6129 by DuPont, Valox 315 by SABIC, Pocan B1600 by Lanxess or Ultradur B4500 FC by BASF. Certain minor amounts of additives may be used in the upper layers, such as antiblock or release additives which can be added in the form of additive masterbatches. Therefore the upper layers consist of PBT in an amount of 91-100 wt. % and other polymers and additives in an amount of 0-9 wt. %.

[0052] In case of three or more layers the bulk layer consists substantially of PBT, and it has to be understood that this means that insignificant amounts of other compounds may be present that do not significantly affect the favourable properties of PBT.

[0053] The coating composition can be applied to a metal substrate in various forms and by various methods, which is not restricted. For instance, the coating composition can be applied to the metal substrate by a process of multi-layer extrusion coating.

[0054] The composition can also be provided in the form of a pre-fabricated polymer film that is made by multi-layer extrusion and casting, optionally followed by stretching in one or two directions. Thus, the coating composition is provided as an unoriented cast film, a machine direction oriented (MDO) film, a transverse direction oriented (TDO) film or a biaxially oriented (BO) film. The pre-fabricated film is then laminated to the metal substrate by unwinding the film, transporting the film to a laminating nip and laminating the film to the metal substrate.

[0055] The thickness of the coating is not particularly restricted but will be optimised between cost and functionality. Typically total polymer film thickness is between 10 and 50 m, preferably between 12 and 40 m and more preferably between 15 and 35 m. The thickness of the adhesion layer is typically between 2 and 8 m, preferably between 3 and 6 m. The minimum number of sublayers in the coating is two, an adhesion layer and a bulk layer (see Figure la), and in use the adhesion layer is positioned between the metal substrate and the bulk layer. More preferred is a three layer coating consisting of an adhesion layer A, a bulk layer B and a top layer T, where the B and T layers consist of PBT in an amount of 91-100 wt. %, but do not necessarily have the same composition. For example, the B layer may consist of 100 wt. % of PBT, while the top layer consists of 95-100 wt. % of PBT and 0-5 wt. % of a masterbatch containing an antiblock or release agent additive. In use the adhesion layer is positioned between the metal substrate and the bulk layer, and the top layer is positioned on top of the bulk layer. Additional layers may be present beside the A-, B- and T-layer. Figure lc shows an additional layer X on top of the top layer, and figure id shows an additional layer X between the top layer and the bulk layer. These (and other) additional layers may be used if certain specific functionality is needed that cannot be provided by the layers already present according to the invention.

[0056] The adhesion layer shows different amounts of added PETg, which is the non-crystallisable copolymer in this example. Example D1S contained no PETg, D2S to D4S contained 30 wt. %, 45 wt. % and 70 wt. % respectively. The remainder of the adhesion layer consisted of PBT and 2 wt. % of the same release agent masterbatch as in the top layer. The presence of the release agent in one or both layers of the A and T is important for the windability of the polymer film after casting and stretching. As soon as the polymer film is laminated onto a substrate, the release agent in the adhesion layer has no further particular function, whereas the release agent in the top layer is beneficial for product release (if the polymer film is used for the inside of a can).

[0057] In FIG. 2, test panels after exposure to steam in the bespoke blushing test are shown. In material D2S (top left) (30% PETg adhesion layer) no blushing is observed at all while in material D3S (top right) (45% adhesion layer) only a very light blushing is observedthe effect is more pronounced in the photograph than it is to the eye. This level of blushing is considered acceptable. In material D4S (bottom left)(70% PETg adhesion layer) blushing can be observed quite clearly, and this is not acceptable. Finally, the backside of the material, coated with Mitsubishi RHST15 film, was also exposed to the test as PET reference material. In this case, very pronounced blushing is observed. The results clearly demonstrate that PETg in the adhesion layer has a negative effect on the blushing performance of PBT coatings, but this effect is negligible when the amount of PETg remains below 50%.

[0058] The laminate produced according to the invention is postheated and cooled after providing the polymer film according to the invention onto the metal substrate. This postheating and cooling affects (reduces) the roughness of the laminate. Surface roughness of the as-received materials was determined using the BMT Expert system equipped with a 2 m radius skidless tip, operated at 0.5 mm/s traverse speed. Measuring length and cut-off value were set to 2.4 mm and 0.8 mm, respectively, in accordance with JIS B 0601:2001. The roughness profile was determined parallel and perpendicular to the steel substrate rolling direction, three times for a given sample, and the arithmetic mean roughness Ra values from each of these six measurements was averaged. The Ra values, averaged over rolling and transverse directions, are in all cases below 0.20 m (Table 3).

TABLE-US-00003 TABLE 3 Surface roughness in rolling direction (RD) and transverse direction (TD), and average value expressed as arithmetic mean roughness Ra. Sample RD (m) TD (m) Average (m) D2S 0.08 0.19 0.13 D3S 0.12 0.19 0.15 D4S 0.12 0.20 0.16

[0059] In FIGS. 4a to 4c examples are given of the thermal analysis tests as described herein above that enable to determine whether a polymer is crystallisable in the context of this invention.

[0060] FIGS. 4a-4c below show differential scanning calorimeter (DSC) thermograms of various polymer resins. A DSC thermogram is a plot of heat flow versus temperature. In the present graphs, the positive y-axis corresponds to the exothermic heat flow (exo up representation), which means that endothermic (melting) peaks are observed as peaks pointing downward. The DSC thermograms were obtained by heating a 5-20 mg sample of the as-received polymer resin material from 0 to 300 C. in a Mettler Toledo DSC821e instrument at a heating rate of 10 C./min. Both the PBT (FIG. 4a) and PET (FIG. 4b) resins exhibit one or more endothermic (melting) peaks in the temperature range from 180 to 280 C., whereas PETg (FIG. 4c) does not exhibit any peak in this temperature range. As described above the presumption is that if a melting peak is observed, then there was a crystalline phase, and if there is a crystalline phase , then the polymer is crystallisable (see FIG. 4a-4c for examples). So in the context of this invention the PETG is marked non-crystallisable and the PBT and PET are crystallisable.