GB2289864A - Laminate of metal and biaxially oriented polypropylene film - Google Patents

Abstract

A laminate having a layer of metal and a layer of biaxially oriented polypropylene (BOPP) is formed using bare metal temperatures which are generally between 180 DEG C and 220 DEG C. The biaxially oriented polypropylene film comprises one or more polypropylene layers and one or more acid modified polyolefin layers. Such temperatures would have been expected to cause the laminate to adhere to the lamination nip rolls since this would occur with a cast polypropylene film. The use of BOPP, however, has been found to be possible at elevated temperatures without the risk of such adhesion and these elevated temperatures also reduce any shrinkage of the BOPP to a minimum.

Description

LAMINATE This invention relates to a laminate of metal and a biaxially oriented polypropylene (BOPP) film.

BOPP is generally a co-extruded film but with copolymer heat seal layers for self-sealing to itself or other polypropylene material. However, BOPP has a tendency to shrink when heated, due to its oriented structure, which has made it undesirable for use in the manufacture of thermally bonded metal/polymer laminates.

Although laminates of polypropylene and metal have previously been described, none of the documents in the prior art describes the specific use of BOPP. A process for making a metal/polypropylene laminate is described in US-T104502 (Liu) comprising extrusion coating polypropylene onto the metal. US-3323965 (Haule et al) also describes the lamination of polypropylene (or another polyolefin) onto metal by the use of an extrusion process.

EP-A-0312302 (CarnaudMetalbox) discloses a polymer/metal/ polymer simultaneous lamination process in which one of the polymers may be a polyolefin such as polypropylene.

None of these cases, however, state what type of polypropylene film is used and US-T104502 does not describe a film process.

The examples of EP-A-0312302 give parameters which indicate that the polypropylene type used therein is likely to be a cast polypropylene, although the use of BOPP is covered in the generic claims. However, the selection of BOPP is not suggested in that document and it is apparent from that document that the problem addressed therein is to ensure good adhesion of a polyester film to one side of the metal without the risk of adhesion to the lamination rolls of the polyolefin layer on the other side, which occurs at elevated temperatures.

According to the present invention, there is provided a laminate having a metal layer and a co-extruded polypropylene layer, in which the polypropylene layer is derived from a biaxially oriented polypropylene film comprising one or more polypropylene layers and one or more polypropylene layers and one or more acid modified polyolefin layers, a polyolefin layer being adjacent to the metal.

Preferably, the polyolefin layer comprises a maleic anhydride graft modified bonding resin and may usually have a softening point which is about 12O0C. Typically, the biaxially oriented polypropylene layer comprises a co-extruded film of polypropylene and a bonding resin.

According to another aspect of the present invention, there is provided a process for producing by simultaneous lamination a laminate having a metal layer and a polypropylene layer derived from a biaxially oriented polypropylene film, the process comprising feeding a strip of biaxially oriented polypropylene and a metal strip, heated to a first temperature T1 which is above the adhesion point of the biaxially oriented polypropylene, to a lamination nip to cause intimate contact of the film with the metal strip, reheating the resultant laminate to a second temperature T2 to cause the film to interact and bond with the metal strip, and quenching the laminate rapidly and uniformly.

The reheating stage may typically involve melting of at least part of the polypropylene film.

It has surprisingly been found in the present invention, that it is possible to use BOPP rather than cast polypropylene to make a laminate of polypropylene and metal. Furthermore, the conditions which are suitable for the lamination of cast polypropylene have been found to be unsuitable for biaxially oriented polypropylene. In particular, it has always been believed to be important in the manufacture of laminates having a layer of polypropylene to ensure that the polypropylene does not adhere to the lamination rolls, whilst still enabling the adhesion of the polypropylene to the metal to be satisfactory. These low temperatures of typically 1500C-1600C for a 20 micron cast polypropylene film when used in the lamination of metal and biaxially oriented polypropylene will, however, lead to severe shrinkage of the oriented film.Cast films show no such tendency even if there is poor adhesion between the film and the metal.

Preferably, the temperature T1 is selected to limit transverse shrinkage of the biaxially oriented polypropylene to not more than 6%.

There are advantages in using BOPP over cast polypropylene since BOPP has excellent strength at low gauge and high reel quality in comparison with cast polypropylene. In addition, 18 micron BOPP is about half the area price of 12 micron base PET, which is another popular product for the manufacture of metal/polymer laminates.

Preferably, the temperatures T1, or bare metal temperature, is at least 1800 C, more preferably greater than 2000C. Usually, the temperature T1 is less than the temperature at which the biaxially oriented polypropylene adheres to the lamination nip rollers. A particularly advantageous temperature has been found to be less than 2300C, most preferably 2200C. Although these temperatures when used with similar gauge cast polypropylene would result in the adhesion of the polypropylene to the lamination rolls, the converse of this has been found to be the case with BOPP.In addition, rather than adhere to the lamination rolls at higher temperatures, the BOPP shows the further advantage that it no longer shrinks to anything like the same degree at higher temperatures as it does at the lower temperatures used for lamination using cast polypropylene. Generally, an acceptable shrinkage of less than 3% has been achieved.

The BOPP film is adhered to the metal by a resin layer, which preferably comprises a maleic anhydride graft modified bonding resin such as maleic anhydride modified polypropylene. Usually, the BOPP film comprises a co-extrusion of polypropylene and such a bonding resin.

The resin preferably has a softening point which is in the region of 1200C. This ensures that the polypropylene adheres well to the metal before any risk of shrinkage might arise.

It is believed that there are two competing processes in operation: film shrinkage and adhesion to the metal strip, both of which increase with temperature. The adhesion itself is dependent on softening induced intimacy and chemical reaction of carboxyl groups and metal oxides.

Although the BOPP would bond satisfactorily to the metal at a lower temperature, lower temperatures give rise to substantially greater shrinkage of the BOPP. Furthermore, shrinkage may be reduced if the film is adhered to the metal before it has the opportunity to shrink.

A small proportion of the shrinkage itself is believed to arise when the film contacts the lamination roll prior to contact with the metal strip. The risk of shrinkage due to this contact can also be limited by cooling the roller. Since the heat from the metal strip will itself heat the lamination roll, force cooling of the roll enables the temperature T1 of the metal strip to be increased. Adhesion of the film to the metal strip is thereby improved whilst limiting the risk of shrinkage at the elevated temperature.

It has also been observed that, unlike cast polypropylene which is subject to a greater risk of sticking to the lamination rolls if the gauge of the film is reduced, changing the thickness of BOPP in the range 15 to 25 microns does not have any apparent effect on the properties of the BOPP when used in lamination.

It is noted that the film forming process used for BOPP enables resins to be used which have a lower melt flow rate and a higher molecular weight than those which can be used for cast polypropylene. The resins used for BOPP provide advantageous forming properties and product resistance.

The invention will now be described with reference to the examples.

Each lamination process in the examples comprised feeding a strip of BOPP and a metal strip, heated to a bare metal temperature T1 which is above the adhesion point of the BOPP, to a lamination nip to cause intimate contact of the film with the metal strip, reheating the resultant laminate to a second temperature T2 to cause the film to interact and bond with the metal strip, and quenching the laminate rapidly and uniformly.

In the examples 1 to 3, the bonding resins are maleic anhydride graft modified bonding resins supplied by Mitsui Petrochemical having the properties details in table 1.

In addition, in each of the examples 1 to 3, a BOPP film having a gauge of 22.5 microns with a 3 micron bonding resin layer was laminated to one side of an aluminium alloy 3104 sheet of 0.27mm thickness with proof stress typically 340 MPa and a polyester film was laminated to the other side. The temperature T2 was chosen so as to ensure that the polyester in the laminate was semi-crystalline, typically this required a T2 in the range of 225"C-230"C. At these temperatures the polypropylene melts and all orientation and crystallinity in the BOPP is destroyed.

TABLE 1 Property QF500 QF551 QF560 melt flow rate (g/lOmins) 3.0 5.7 6.0 density 0.9 0.89 0.9 Vicat softening (C) 143 119 126 melting point (C) 160 140 140 EXAMPLE 1 Two co-extruded BOPP films were examined, one containing QF500 co-extruded bonding resin and other containing OF551. The widths of the films were measured after lamination. The starting film width was 422mm. The results in table 2 show the shrinkage over a bare metal temperature range 1700C-1900C and at 2200C.

TABLE 2 Laminate Bonding Bare metal Bonding layer Bulk layer resin temperature width (mm) width (mm) 1 QF551 1700C 411.7 408.75 2 QF500 1700C 380.7 380 3 QF551 1800C 417 414 4 QF550 1800C 395 393 5 QF551 1900C 417 414 6 QF550 1900C 407 405.5 7 QF551 2200C 416.7 415.25 8 QF550 2200C 417 415.25 It is clear from these results that QF500 based BOPP film shrinks more severely at lower temperatures than does QF551. QF551 appears to be stable over the range of 1800C - 2200C whereas the QF5OO shrinks severely even at 1900C although it is stable at 2200C and the shrinkage of QF500 at 1900C is more severe than that of QF551 at 1700C.

The shrinkage of BOPP film is thus shown to be dependent on bonding resin characterics but alway exhibits the unexpected trend of shrinking much less than at higher metal temperatures. The mechanism involved is believed to be a balance between adhesion and shrinkage since both increase with temperature. Shrinkage however, would seem to dominate at those temperatures which are associated with thin cast polypropylene lamination, i.e.

1500C-1700C, and adhesion seems to dominate at 2100C-2200C, where thin cast polypropylene would be melted in the lamination nip and would therefore adhere to the rollers.

EXAMPLE 2 A BOPP film containing QF560 was laminated across a temperature range of T1 = 1700C-2200C with an initial film width of 452mm. The results obtained are shown in table 3.

TABLE 3 Laminate Bare metal Bonding layer Bulk layer temperature width (mm) width (mm) 1 1700C 450 447.5 2 lSO0C 450 447.5 3 1900C 449.5 448 4 2000C 450 448.2 5 2100C 449.5 448.2 6 2200C 449.5 448.5 The QF560 exhibits almost no shrinkage over these temperatures, even at the lower temperatures where both the other bonding resins showed shrinkage.

The results of both examples 1 and 2 are shown graphically in figure 1.

EXAMPLE 3 206 diameter beverage can ends were manufactured using different laminates of aluminium alloy 3104 at a thickness of 0.27mm and proof stress 340 MPa, and BOPP.

Each of these was pasteurised in water at 700C for one hour and the coating integrity was remeasured to determine if the pasteurisation process induced any degradation.

The results of these trials for ten different can end shells is shown in table 4. The enamel rater current is a measure of coating continuity or metal exposure which is well established test in beverage can and end manufacture.

TABLE 4 Laminate Bonding Bare Enamel Enamel resin metal rater rater type temp. before after pasteurisation pasteurisation 1 QF551 1700C 0.3mA 0.3mA 2 QF500 1700C 0.lmA 0.2mA 3 QF551 1800C 0.1mA 0.lmA 4 QF500 180 C 0.lmA 0.lmA 5 QF551 1900C OmA OmA 6 QF5OO 1900C 0.2mA 0.3mA It can be seen that the BOPP coating provides excellent protection. In addition, no adhesion failure was seen on the curl of the ends, where such failure might normally arise in an unsatifactory coating.

EXAMPLE 4 QF551A and QF500 films (see table 1) were laminated onto 0.24mm DR8 (double reduced) tin free steel plate.

The bond layer for each was maleic anhydride graft modified resin.

The lamination process was carried out as for the other examples and the widths of the films were measured after lamination at ten different positions and an average reading taken. The shrinkage behaviour of the two films was noted over a temperature T1 ranging from 1700C to 2350C. These results are shown in figure 2, which is a graph of percentage film width against temperature T1 for both films.

There is a clear difference in the shrinkage behaviour of the two films, although both films provided a satisfactory coating. As was observed in example 1, the overall performance of the QF551 resin film is superior to that of the QF500. QF551 was stable over a T1 of from 1700C to 2300C, whilst QF500 only showed stability above a T1 of 2100C, Above a T1 of 2300C, some shrinkage was restarting for both films.

EXAMPLE 5 Beverage cans of 12 American fl oz were manufactured from laminates of 0.3mm 3004 aluminium alloy coated with: (i) an inner coating (i.e. inside the can) of 20 micron cast polypropylene with bonding resin of Idemitsu M100; and an outer coating (i.e. outside the can) of 12 micron co-extruded PET; (ii) an inner coating of 17 micron maleic anhydride graft modified QF500 bonding resin; and an outer coating of 12 micron co-extruded PET.

Lamination conditions were such that the PET melted and therefore became amorphous. The temperature T1 was 2200C and the temperature T2 was 2600C in each case.

Cans were manufactured on a body maker having an ironing die with a 2 degree inlet angle, using conventional cupping lubricant and can forming coolant.

After forming, the cans were inspected and enamel rater values were taken.

Both of the can types (i) and (ii) had satisfactory adhesion. The enamel rater values for the cans with cast polypropylene were in excess of 199 mA, whereas those for the BOPP cans were on average only 2 mA. In addition, the cast polypropylene cans exhibited difficulty in stripping from the punch. Although the use of an amide slip additive to the cast polypropylene improved the ability to strip these cans from the punch, the enamel rater values remained greater than 199 mA.

Further can tests were carried out using BOPP as an outer coating which provided visually acceptable cans without stripping problems and having a clear unimpaired coating of BOPP.

The use of BOPP film on both aluminium and steel plate exhibits the same performance as regards shrinkage and provides a visually aceeptable coating.

Although the use of BOPP would not have been considered hitherto, particularly because of the expected shrinkage of such films, it can be seen from the examples above that its use offers several unexpected advantages over cast polypropylene. This is especially the case when the BOPP used includes a bonding resin having a softening point in the region of 1200C. All of the laminates produced gave a good visual appearance and all the films handled well in lamination without creasing which would not be possible for a cast film of similar gauge.

Claims (10)

1. A laminate having a metal layer and a co-extruded polypropylene layer, in which the polypropylene layer is derived from a biaxially oriented polypropylene film comprising one or more polypropylene layers and one or more acid modified polyolefin layers, a polyolefin layer being adjacent to the metal.

2. A laminate according to claim 1, in which the polyolefin layer comprises a maleic anhydride graft modified bonding resin.

3. A laminate according to claim 2, in which the bonding resin has a softening point which is about 1200C.

4. A laminate according to any one of claims 1 to 3, in which the biaxially oriented polypropylene layer comprises a co-extruded film of polypropylene and a bonding resin.

5. A process for producing by simultaneous lamination a laminate having a metal layer and a polypropylene layer derived from a biaxially oriented polypropylene film, the process comprising feeding a strip of biaxially oriented polypropylene and a metal strip, heated to a first temperature T1 which is above the adhesion point of the biaxially oriented polypropylene, to a lamination nip to cause intimate contact of the film with the metal strip, reheating the resultant laminate to a second temperature T2 to cause the film to interact and bond with the metal strip, and quenching the laminate rapidly and uniformly.

6. A process according to claim 5, further comprising selecting temperature T1 to limit transverse shrinkage of the biaxially oriented polypropylene to not more than 6%.

7. A process according to claim 6, in which the temperature T1 is at least 1800C.

8. A process according to any one of claims 5 to 7, in which the temperature T1 is less than the temperature at which the biaxially oriented polypropylene adheres to the lamination rollers.

9. A process according to any one of claims 5 to 8, in which the temperature T1 is not more than 2300 C.

10. A can or can end manufactured from the laminate of any one of claims 1 to 4.