Coated polyester film and polyester/polyolefin laminates produced therefrom

A coated polyester film suitable for producing polyester/polyolefin laminates consists of biaxially oriented poly(ethylene terephthalate) film coated with a cross-linked styrene copolymer by deposition from an aqueous coating composition which comprises (A) a copolymer containing 50-75 mol % of styrene units and 25-50 mole % of comonomer units providing carboxylic groups (10-50 mole %) and alkyl carboxylate groups (20-50 mole %) and (B) a melamine/formaldehyde cross-linking agent. The styrene copolymer (A) is preferably a water-soluble copolymer of styrene and maleic anhydride partially converted into butyl ester and partially neutralized with ammonia, and the cross-linking agent (B) is conveniently hexamethoxymethylmelamine. The polyolefin layer in the laminates produced may in particular be of low-density polyethylene.

This invention relates to coated polyester film and to polyester/polyolefin 
laminates produced therefrom and in particular to laminates having one 
layer of biaxially oriented poly(ethylene terephthalate) and another layer 
of polyethylene or of an ethylene copolymer containing up to 10 mole % of 
comonomer such as ethylene/vinyl acetate copolymer. Such laminates are 
extensively used for packaging. (For convenience poly(ethylene 
terephthalate) will be abbreviated to PET and polyethylene and ethylene 
copolymer to PE.) 
Films of biaxially oriented PET have been made and used for many years and 
one important use is for packaging. Packaging films are often sealed by 
heating to the melting point and pressing together two sheets of film so 
that the two sheets become merged (this is usually known as 
"heat-sealing"). The melting is sufficient to destroy the orientation. 
This weakens the seal and therefore homogeneous PET films are not used for 
heat-sealing. When heat-sealing is needed the PET film is laminated to a 
component with a lower melting point, e.g. a polyester copolymer or a 
polyolefin such as polyethylene. 
Thus PET/PE laminates are widely used for packaging. They are heat-sealable 
because the PE has a lower melting point than the PET. In addition the PE 
improves the permeability properties of the PET in some applications. As 
PET and PE have only poor direct adhesion it is necessary for the laminate 
to include an adhesion-promoting interlayer. The usual interlayers are 
cross-linked polyurethan resins. These are applied to one layer (usually 
the PET layer) from an organic solvent. The flammability of the solvent is 
a severe disadvantage and its restricts the methods of fabrication, e.g. 
flammable liquids cannot be used during the extrusion of the PET. This 
invention relates to interlayers which are applied from an aqueous medium. 
According to the invention a coated polyester film for producing a 
packaging laminate comprising a layer of PET and a layer of PE with an 
adhesion-promoting interlayer between them is characterized in that the 
interlayer is coated on the PET and is a cross-linked styrene copolymer as 
hereinafter defined. 
Preferably the PET layer is 6 .mu.m to 400 .mu.m thick and the PE layer is 
2 .mu.m thick to 200 .mu.m thick. The interlayer is so thin that it makes 
only a negligible contribution to the total thickness; it is usually 10 to 
1000 nm thick. 
The interlayer is a cross-linked styrene copolymer deposited from an 
aqueous coating composition comprising: 
(A) 100 parts by weight of a copolymer of: 
(i) 50-75 mole % of styrene units which may be alpha- and/or 
ring-substituted with methyl or halogen; 
(ii) 25-50 mole % of comonomer units selected from units of acrylic, 
methacrylic, itaconic and maleic acids, alkyl half-esters of itaconic and 
maleic acids, alkyl acrylates and methacrylates and dialkyl maleates; 
the comonomer units being such as to provide the copolymer with 10-50 mole 
% of carboxylic groups which may be present as salts (preferably ammonium 
salts) and 20-50 mole % of alkyl carboxylate groups in which the alkyl 
groups may each have up to 10 carbon atoms (and are preferably butyl 
groups); 
(B) 10-20 parts by weight of cross-linking agent selected from condensation 
products of formaldehyde and melamine and said condensation products 
having hydroxyl groups converted into methoxy, ethoxy, propoxy or butoxy 
groups. 
The coated films described above can be produced by conventional methods. 
For example the polyester film is conventionally extruded including 
forward and sideways draw. The interlayer is applied by offset gravure 
coating during this process as an aqueous dispersion of the styrene 
copolymer and the cross-linking agent. Heat is always used in the 
preparation of polyester film and this heat evaporates the water and 
assists the cross-linking. Because the drawing is carried out using 
radiant heat it is necessary to avoid substantial amounts of toxic or 
flammable components. Thus water is used as the vehicle of dispersion. 
Scrap film coated with the interlayer can be reclaimed by re-extruding into 
film with fresh polyester. 
In the aqueous composition for use in the coating, 
(A) is preferably a water-soluble copolymer of 50-75 mole % of styrene 
units and 25-50 mole % of units of maleic anhydride which have been 
partially converted into butyl ester and partially neutralized with 
ammonia, and 
(B) is preferably hexamethoxymethylmelamine or melamineformaldehyde 
condensate having a methoxymethyl:methylol ratio of at least 1:2. 
The concentration in the aqueous composition of styrene copolymer plus 
cross-linking agent is conveniently 10 to 40 g/liter. 
The aqueous composition may also contain additional ingredients, e.g. a 
catalyst to facilitate the cross-linking and a dispersant to keep the 
other ingredients in suspension. Suitable catalysts include ammonium 
chloride, ammonium nitrate, ammonium thiocyanate, ammonium dihydrogen 
phosphate, ammonium sulphate, diammonium hydrogen phosphate, 
p-toluenesulphonic acid or its ammonium or morpholinium salt, and maleic 
acid stabilized by reaction with a base. 
The coated polyester, produced as described above, is overcoated with the 
polyolefin using a conventional melt-coating extrusion method. Immediately 
before overcoating it is desirable to treat the interlayer by corona 
discharge.

The laminate illustrated in FIG. 1 comprises a polyester layer 10 and a 
polyolefin layer 12 with an interlayer 11 between them to promote 
adhesion. 
The polyester layer 10 is 10-20 .mu.m thick and it consists of 
poly(ethylene terephthalate) with balanced orientation. The polyolefin 
layer is 15-80 .mu.m thick and it consists of polyethylene. 
The interlayer is of negligible thickness, about 25 nm, and it consists of 
a cross-linked styrene copolymer. It is conveniently defined by the 
styrene copolymer and the cross-linking agent from which it was derived. 
Specific examples of interlayers are given hereinafter. 
The total thickness of the laminate is between 25 and 100 .mu.m and the 
polyester 12 comprises 15-40% of the total thickness. 
The laminate shown in FIG. 11 is conveniently produced by a two-stage 
process. The first stage, using the equipment illustrated diagrammatically 
in FIG. 2, is a process for making coated polyester film. As indicated in 
FIG. 2 molten PET is extruded through a die 20 onto a quenching drum 21 to 
produce a film 30. After quenching, the film 30 is reheated to 80.degree. 
C. and passed to a longitudinal draw station 22. This comprises a 
secondary nip 22b which runs 3.5 times as fast as the primary nip 22a. 
Thus the draw ratio in the longitudinal direction, i.e. the machine 
direction, is 1:3.5. 
The drawn film is coated on its underside using a conventional 
offset-gravure technique. The coating station comprises a bath 23 which 
contains the aqueous coating composition. A constant level is maintained 
in the bath 23; the constant level device is not shown. A steel gravure 
roller 24 rotates in the bath; it has surface grooves which pick up the 
aqueous composition and transfer it to the offset roller 25 which is made 
of rubber. The offset roller 25 transfers the aqueous composition to the 
underside of the film 30. The grooves in the gravure roller 24 meter the 
composition so that a uniform coating is obtained. 
After coating the film 30 passes between radiant heaters 26. It is gripped 
by the clips of a conventional stenter (not shown in any drawing). The 
width is increased 3.5 times, i.e. the sideways draw ratio is 1:3.5. 
During the draw the temperature is 105.degree. C. After the draw the film 
is held at the extended width with temperature in the range 150.degree. to 
230.degree. C. for heat-setting. 
As the film 30 passes between the heaters 26 the water from the coating 
composition is evaporated. The heat also assists the cross-linking of the 
styrene copolymer. It is essential to use an aqueous composition because 
flammable solvents, e.g. alcohol, would ignite during the heating. Coating 
between the draws is simple and there is no need for separate treatments 
to dry and cure the coating. In addition the draw reduces the thickness of 
the coating. Using a solids content of 30 g/liter a coating 25 nm thick is 
produced. The coated polyester film is transferred to a conventional 
melt-coating process, using equipment illustrated diagrammatically in FIG. 
3. In commercial practice it may be convenient for one manufacturer to 
make the coated polyester film and sell the composite to a second 
manufacturer who would apply the polyolefin to obtain the laminate shown 
in FIG. 1. 
As indicated in FIG. 3 the composite from FIG. 2 is supplied from roll 31 
and passed through a corona discharge station 32. The corona discharge 
modifies the coated surface to improve its adhesive properties. 
Immediately after treatment the polyolefin is extruded onto the coated 
surface from a die 32. The two layers are pressed together by rollers 34 
and the final laminate, described in FIG. 1, is wound up at 35. 
Specific examples of interlayers will now be described. In each case the 
interlayer will be defined in terms of the aqueous composition in which it 
is coated onto the polyester. In these examples (i) is the styrene 
copolymer, (ii) is the cross-linking agent, (iii) is the emulsifier and 
(iv) is the cross-linking catalyst. (The hexamethoxymethylmelamine used in 
these examples was obtained from American Cyanamid under the Trade Mark 
"Cymel" 300. The surfactant `Synperonic` N was a nonyl phenol condensate 
with about 9.5 moles of ethylene oxide, commercially available from 
Imperial Chemical Industries.) 
EXAMPLE 1 
(i) Styrene/maleic anhydride copolymer in mole ratio 50/50 partially 
converted into an n-butyl ester (degree of esterification 35-50%) and 
partially neutralized with ammonia (pH 9), commercially available as SMA 
144OH resin (Arco Chemical Co). 
(ii) Hexamethoxymethylmelamine, 15% w/w on (i). 
(iii) `Synperonic` N, 12.5% w/w on (i). 
(iv) Ammonium p-toluene sulphonate, 0.07% w/v. 
EXAMPLE 2 
(i) Styrene/butyl acrylate/itaconic acid copolymer in mole ratio 75/20/5. 
(ii) Hexamethoxymethylmelamine, 15% w/w on (i). 
(iii) `Synperonic` N, 12.5% w/w on (i). 
(iv) Ammonium p-toluene sulphonate, 0.07% w/v. 
EXAMPLE 3 
(i) Styrene/maleic anhydride copolymer in mole ratio 50/50 partially 
converted into a butyl ester, commercially available as SMA 144OH resin 
(Arco Chemical Co). 
(ii) Melamine-formaldehyde resin with a methoxymethyl:methylol ratio of 
about 1:2, commercially available as `Beetle` BE 336 (BIP Chemials Ltd), 
15% w/w on (i). 
(iii) `Synperonic` N, 12,5% w/w on (i). 
(iv) Ammonium p-toluene sulphonate, 0.07% w/v. 
Each of these compositions was coated onto poly(ethylene terephthalate) 
film as described with reference to FIG. 2 using a coating solution 
containing 30 g/liter of active (solid) ingredients. In each case the 
polyester was 12 .mu.m thick with a coating about 25 nm thick. A layer of 
low-density polyethylene was overcoated as described with reference to 
FIG. 3 giving a laminate as illustrated in FIG. 1. The thickness of the 
polyolefin layer and the nature of the interlayer are given in Table 1. 
TABLE 1 
______________________________________ 
Interlayer Thickness Total 
Example 
from Example 
Polyethylene 
Thickness 
% PET 
______________________________________ 
1A 1 50 .mu.m 62 19% 
2A 2 20 .mu.m 32 37% 
3A 3 50 .mu.m 62 19% 
______________________________________ 
The bond strength of each of these laminates was assessed using an Instron 
tensile tester to pull the layers apart horizontally while the remaining 
laminate was allowed to hang vertically. Separation was continued until 
one or both layers tore; the force per cm of strip applied at this time is 
given in Table 2. 
TABLE 2 
______________________________________ 
Example Bond Strength Failure Mode 
______________________________________ 
1A 660 PE tore 
2A 360 PE yield 
3A 500 PE tore 
______________________________________ 
(Bond strengths below 300 g/cm would not be regarded as satisfactory.) 
Laminates having satisfactory bond strengths were also made from PET coated 
with the interlayer of Example 1 and an overcoat of ethylene/vinyl acetate 
copolymer containing 5.5 mole % of vinyl acetate or an overcoat of 
high-density polyethylene. 
PET was satisfactorily coated with the interlayer of Example 1 using an 
aqueous coating composition containing 15 g/liter of active (solid) 
ingredients instead of 30 g/liter. At this concentration the composition 
was less viscous and enabled higher coating speeds to be achieved; by 
coating at 160 m/min a dry coat thickness of 6.3 nm was achieved. The 
coated film was overcoated with low-density polyethylene as described 
above to give laminates of satisfactory bond strength. 
PET film coated with the interlayer of Example 1 at 30 g/liter was 
re-extruded and re-filmed to give new film of acceptable quality when 
tested for yellowness and haze. It is therefore possible to recover scrap 
coated PET film produced according to the invention by re-extrusion into 
the film-making process.