COATING OF SUBSTRATES

Abstract: Substrates are coated by a process which comprises co-extruding onto the substrate, e.g. aluminium foil, a layer of an olefin terpolymer and a layer of polypropylene, the olefin terpolymer being adjacent to the substrate and the polypropylene being adjacent to the olefin terpolymer. The olefin terpolymer is a terpolymer of an olefin, e.g. ethylene, an unsaturated carboxylic acid or anhydride, e.g. acrylic acid, and an unsaturated ester, e.g. methyl acrylate, in which the amount of carboxylic acid or anhydride groups in the terpolymer is at least 3 mole % and the amount of ester groups in the terpolymer is at least 3 mole %.

Description: This invention relates to a process for the coating of substrates, eg metal foils, with polypropylene. Laminates based on polypropylene and aluminium foil are becoming increasingly important. Compared with polyethylenes, polypropylene provides higher stiffness and tensile strength, a higher melting point (160°C) and very good resistance to attack by chemicals. For its part aluminium provides an outstanding barrier to gases, moisture and light, does not rust and can be readily shaped into functional products. Hitherto, the limit to the extended use of processes for laminating polypropylene and metal films such as aluminium has been the relatively poor adhesion of polypropylene to metals for low cost high speed, extrusion coating processes. The chemically modified polyolefins commercially available can adhere to both polypropylene and metals by heat lamination processes. However to develop adequate adhesion the contact time at temperatures in the region of 200°C has to be at least 20 seconds. This excludes the use of the high speed extrusion coating process where although application temperatures are high, the contact time in the cooling nip is usually a fraction of a second when operating at economic running rates of at least 100 m/min of web. Various patents have described the coating of substrates such as metal foil with polypropylene, eg US 4313996 and US 3721597, but none of them teach processes where high speeds of coating of over 50 m/min are possible. Also there is no evidence that good adhesion of the polypropylene to the substrate is achieved and no teaching of the necessity for having certain minimum quantities of acid or anhydride groups and ester groups in the terpolymer. GB 1214820 discloses the use of terpolymers of ethylene and acrylic acid and vinyl acetate in which the total of the non-ethylene monomer is from 20-30 wt% for adhering polyolefins including polypropylene to vinylidene chloride polymers. Coextrusion is used in the examples. GB 1263218, GB 1405769, EP 2240 discuss the combination of sheets of materials by pressing and heating. DE 1669685 disclose how sheets of polypropylene (Example 10) can be joined to steel plate using a sheet made of a mixture of joined to steel plate using a sheet made of a mixture of polypropylene and a terpolymer. DE 2138231 teaches the use of copolymers as an adhesive layer for a polypropylene and aluminum foil layer with internal combination of the thermoplastic polypropylene and adhesive layer in an extrusion die. There is no disclosure of how a thin layer of tie resin could be coextruded at high speed with polypropylene to provide a strong bond to substrates such as aluminium at high operating speeds. We have now devised a process which enables substrates eg metal foil, to be coated with polypropylene at high speed and with good adhesion. According to this invention there is provided a process for coating an aluminum foil or paper/aluminum foil which includes the steps of (1) supplying the foil at a line speed of at least 50 m/min (2) co-extruding from a die a tie layer of an olefin terpolymer and a layer of polypropylene and combining the terpolymer and polypropylene in the die (3) contacting the coextruded combined materials with the foil with the terpolymer contacting the foil over a nip roll and through a nip formed with a chill roll wherein the terpolymer is a terpolymer of an olefin, an unsaturated carboxylic acid or anhydride and an unsaturated ester in which the amount of carboxylic acid or anhydride groups in the terpolymer is at least 3 mole % and the amount of ester groups in the terpolymer is at least 3 mole %. The olefin terpolymer may be a terpolymer of an olefin, an unsaturated carboxylic acid or anhydride and an unsaturated ester. The amount of carboxylic acid or anhydride groups in the terpolymer is at least 3 mole % and the amount of ester groups in the terpolymer is at least 3 mole %. A broad range of substrates can be coated by the process of this invention. Examples are paper, board, calendered paper/board, metals and plastics, eg polyolefins and nylon. Suitable metals especially foil, include aluminium and its alloys, copper, bronze, magnesium and its alloys, steel, tin-plate steel, stainless steel and galvanised steel. Especially preferred is aluminium. Also suitable are paper/aluminium foil laminates, for example kraft paper extrusion laminated to aluminium foil. The thickness of the substrate can vary depending on its use and its composition. Since the olefin terpolymer and the polypropylene are co-extruded onto the substrate, for all practical purposes the substrate must be capable of being coiled. Hence the thickness of the substrate may be limited by its composition. If the substrate is an aluminium foil such foils can have thickness varying from 5 to 100 µm for example about 40 µm. The olefin terpolymer acts as a tie resin between the substrate and polypropylene and the olefin from which the olefin terpolymer is derived is preferably of low molecular weight and C₂ to C₅ olefins are preferred, in particular propylene and especially ethylene. The unsaturated carboxylic acid is preferably an ethylenically unsaturated carboxylic acid and should contain one or more double bonds. It may be a polycarboxylic acid, eg an alph-beta ethylenically unsaturated carboxylic acid preferably containing 4 to 8 carbon atoms eg 4 to 6 carbon atoms per molecule, such as maleic acid, fumaric acid or itaconic acid. Preferably however, it is a monocarboxylic acid, eg an alpha-beta ethylenically unsaturated carboxylic acid having 3 to 8 carbon atoms, eg 3 to 5 carbon atoms per molecule, for instance, acrylic acid, methacrylic acid, crotonic acid or ethacrylic acid. In general ethylenically unsaturated acids having 2 to 5 carbon atoms (not including the acid group carbon(s)) per molecule are preferred. Acrylic acid is the most preferred acid. Alternatively an unsaturated carboxylic acid anhydride may be used for example maleic anhydride. The unsaturated ester may be an ester derived from an unsaturated carboxylic acid or one derived from an unsaturated alcohol. When it is derived from an unsaturated carboxylic acid it is preferably derived from an ethylenically unsaturated carboxylic acid and preferred classes, preferred acids and specific acids are those given above. Particularly preferred esters are those derived from acrylic acid and methacrylic acid. Suitable alcohols from which these esters are derived include monohydric and less desirably polyhydric alcohols. Examples of suitable monohydric alcohols are C₁ to C₁₀ aliphatic alcohols, for example methanol, ethanol, isobutanol and n-hexanol. The esters could be derived from alcohols containing an aryl group, eg benzyl alcohol or 4-phenyl-butanol-1. Examples of diyhydric and trihydric alcohols are ethylene glycol and glycerol. The most suitable examples of esters derived from ethylenically unsaturated carboxylic acid are C₁-C₁₀ alkyl acrylates and methacrylates, eg methyl, ethyl and n-butyl acrylate and methyl, ethyl and n-butyl methacrylates. Other possible esters are those derived from unsaturated alcohols and saturated acids. The unsaturated alcohol may for example be vinyl alcohol or allyl alcohol. The saturated acid is preferably a monocarboxylic acid, for example a C₁ to C₁₀ monocarboxylic acid such as acetic acid, propionic acid, n-hexanoic acid or phenyl acetic acid. Less desirably the ester is derived from a polycarboxylic acid eg oxalic acid or succinic acid. The more suitable ester is vinyl acetate. The most preferred copolymers are terpolymers of ethylene, methyl acrylate and acrylic acid and of ethylene, methyl methacrylate and methacrylic acid. In order to act effectively as a tie resin there must be minimum of 3 mole % of carboxylic acid or anhydride groups and 3 mole % of ester groups in the terpolymer. It is preferred that the amount of olefin is 80 to 94 mole %, for example about 92 mole %. For the other monomers, the preferred amounts are each 3 to 5 mole %, eg about 4 mole %. It is usual for the melt index (dg/min at 190°C) to be between 2 and. 20 especially between 4 and 7. For good melt stability and drawdown in the extrusion coating process it is highly desirable that the melt flow rate should be in the range of 10-40 dg/min at 230°C (2.16 kg), for example from 20 to 30 dg/min at 230°C (2.15 kg). Other physical property ranges are usually as follows:- Melting : Peak °C 60 to 100 preferably 65 to 85 Crystallisation (1) : Peak °C 45 to 75 preferably 50 to 70 Ultimate Tensile: (M.Pa) typically 10 to 20 Elongation at Break: (%) typically 300 to 600 The terpolymers are usually prepared by polymerising the monomers in bulk at high pressure, usually 1500 to 2500 bars, at a temperatures usually between 150°C and 250°C preferably between 190°C and 220°C, using a free radical initiator. Suitable initiators are peroxyesters such as peroxypivalates, peroctoates or perbenzoates or a mixture of these or peroxides. The polymerisation reaction can be continuous and the product can be continually withdrawn from the reactor as it is formed. The viscosity of the product is controlled by using a transfer agent such as isobutylene. Other suitable transfer agents are compounds such as an alcohol, a ketone, an aldeyde or an aliphatic or aromatic hydrocarbon. The layer which is adjacent to the tie-resin is polypropylene. Various forms of polypropylene can be used, for example isotactic polypropylene usually having a density of from 0.86 to 0.91 g/cm 3 and a molecular weight of from 50,000 to 500,000 as determined by the intrinsic viscosity method. It can also. be biorientated polypropylene, uniaxially stretched polypropylene film or cast (unstretched) polypropylene film. Preferably the polypropylene should have a melt flow rate of from 30 to 50 dg/min, eg about 40 dg/min to ensure a good rheology match with the tie-resin. Typical properties of polypropylene are as follows: Density 0.905 g/cm³ Vicat softening point 155°C Tensile yield 35 N/mm² Tensile modulus 1600 N/mm² Flexural modulus 1500 N/mm² Elongation at yield 10% Izod Impact Strength 40 J/m at 23°C The tie-resin (ie the olefin terpolymer) and polypropylene are co-extruded onto the moving substrate. This co-extrusion can be achieved by the use of a co-extrusion machine having two extruders, a main extruder for the polypropylene and a satellite extruder for the olefin terpolymer. It is preferred to employ a combining feedblock system rather than a dual manifold die. Both extruders are usually set at about the same temperature, typically 270 to 290°C and the coating thickness can be controlled by the extruder screw speed and the web transport rate. The substrate to be coated (eg aluminium foil) is preferably fed from an unwinder into the coating nip of the co-extruder machine. Preferably the temperature of the chill roll with which the polypropylene is in contact is set at 35 to 45° C, eg about 40° C. The molten co-extruded web is fed tangentially onto the moving substrate feeding into the nip where the molten extrudate is pressed onto the substrate. The air gap (the distance between the die exit and nip) should preferably be about 100 mm. When the substrate comprises aluminium it is desirable to subject the aluminium surface to corona treatment prior to its entry into the coating nip. The coated substrate can be edge trimmed and wound onto rolls using tension controlled winding systems. If desired both surfaces of the substrate can be coated by the co-extruded olefin terpolymer and polypropylene. To achieve this two co-extrusion machines designed to operate in tandem are used. Another way is to pass the coated substrate a second time through the same machine ensuring that the sides are reversed. The thickness of the tie-resin using a dual manifold die can be as thin as 5 microns. A typical product is one which an aluminium foil of thickness about 40 µm is coated with an ethylene terpolymer layer of thickness about 5 µm and a polypropylene layer of thickness about 30 µm. By the process of this invention it is possible to achieve coating speeds of at least 100 m/min and even above about 200 m/min such as from 300 to 400 m/min. At the same time one can achieve adhesion values of at least 400 g/15 mm width of specimen and even up to 800 g/15 mm. The time available for adhering the layers together is inversely related to the line speed and may be from 0.1 to 0.5 seconds. The process of this invention which enables one to apply polypropylene at high speed onto a wide range of substrates ranging from paper and board to plastics and metals has great advantages over prior art processes. Hitherto coating by polypropylene has been limited by the fact that no adhesion to any substrate is sufficiently porous to allow the polypropylene melt to penetrate into its pores and this has limited the substrate to fabrics and porous papers. Also in the past, due to the fact that the rheological properties inherent in polypropylene (PP) result in poor melt stability in the extrusion coating process, line speeds have been restricted to a maximum of 50 m/min whereas for low density polyethylene (LDPE) the line speeds can be several hundred m/min. Surprisingly, with the invention the tie-resin layer supports the PP in the air gap sufficiently to stabilize the PP melt stream and permit the higher line speeds referred to previously. To overcome this limitation blends of PP with inter alia LDPE have been developed but this dilutes the PP properties and increases the fabricating cost. Dilution of PP properties is a disadvantage because compared with LDPE, ethylene/vinyl acetate copolymers, ethylene/acrylic acid copolymers, ethylene/methyl acrylate copolymers and ionomers commonly used in the extrusion coating process, PP has the following advantages : higher heat resistance, a better barrier to gases and moisture; excellent resistance to oils and fats; greater stiffness, hardness and abrasion resistance, and good surface release properties. The invention is now described with reference to the following Examples. Example A paper/foil laminate was coated with polypropylene using a co-extrusion machine illustrated diagrammatically in Fig. 1 of the accompanying drawings. Referring to Fig. 1 the co-extrusion machine 1 comprised a satellite extruder 2 containing granules of the olefin terpolymer tie-resin 3 and a main extruder 4 containing granules of polypropylene 5. The extruded resins passed through conduits 7 and 8 and combined in a die 9 and the combined co-extrudate emerged at 10 and passed to a coating nip 11 where it came in contact with the laminate (substrate) 12 fed from an unwinder (not shown). The machine had a standard rubber nip roll 13 and a matte chill roll 14 assembly. The substrate coated with tie-resin/polypropylene extrudate 15 passed from the chill roll 14 to be wound on a drum (not shown). The diameters of the extruders 2 and 4 were respectively 63.5 mm and 88.9 mm and their temperatures held at 275°C. The layer thicknesses were controlled by the extruder screw speeds or output of each extruder to produce a web transport speed of 100m/min. The die 9 was 650 mm wide, deckled down to 500 mm at 10. An air gap between nip 13 and 14 and die exit of 110 mm was to maximise adhesion. The temperature of the chill roll 14 was raised to 40°C. Since the polypropylene was in contact with the surface of the chill roll 14, no sticking occurred. All the operating conditions were controlled and monitored by a microcomputer linked to the machine 1. The tie-resin 3 was a terpolymer of ethylene, acrylic acid and methyl acrylate of the following composition: Monomer Moles % ethylene 94 acrylic acid 3 methacrylate 3 and its melt index was 5.5 dg/min (190°C). The polypropylene 5 had the following physical characteristics: Melt flow rate 40 dg/min (230°C/2.16 kg) Density 0.905 g/cm² Vicat softening point 100°C The substrate was 35 g/m² bleached kraft paper extrusion laminated to 9 µm aluminium foil, corona treated. The above procedure was repeated using 40 µm aluminium foil instead of the laminate, extruder temperatures of 285°C and different thicknesses of tie-resin and polypropylene. The results obtained are shown in Table 1 after adhesion measurements had been carried out. The peel adhesion (180°) was determined on 15 mm wide specimens at ambient temperature after aging for 8 days. The extension rate of the Instron tensometer was 500 mm/min. The adhesion values in Table 1, show that with as little as 5 grams/m² (4.8 µm) tie-resin, peel adhesion values up to 800 g/15 mm width are developed. The PP, tie-layer interface developed average values of 200 g/15 mm. The adhesion to the 40 µm Al foil was so high at 285°C, that only the PP layer could be stripped off. Heat seal was determined over the temperature range 160-200°C. Both sealing jaws were heated and dwell time was fixed at 0.5 sec. The samples were conditioned for 24 hrs. and the peel strength to break the seals were measured on an Instron at an extension rate of 500 mm/min. The measurements are shown in Fig. 2. The seal strength values were typically 16 N for 15 mm wide specimens. With PP as the seal layer, minimum jaw temperatures of 175°C are required. Failure occurred at the PP/tie-resin interface as the seal ruptured.

Claims: A process for coating an aluminum foil or paper/aluminum foil which includes the steps of (1) supplying the foil at a line speed of at least 50 m/min (2) co-extruding from a die a tie layer of an olefin terpolymer and a layer of polypropylene and combining the terpolymer and polypropylene in the die (3) contacting the coextruded combined materials with the foil with the terpolymer contacting the foil over a nip roll and through a nip formed with a chill roll wherein the terpolymer is a terpolymer of an olefin, an unsaturated carboxylic acid or anhydride and an unsaturated ester in which the amount of carboxylic acid or anhydride groups in the terpolymer is at least 3 mole % and the amount of ester groups in the terpolymer is at least 3 mole %. A process according to either of claim wherein the terpolymer is an ethylene terpolymer. A process according to claim 1 or claim 2 wherein the carboxylic acid of the terpolymer is an ethylenically unsaturated acid having 2 to 5 carbon atoms (not including the acid group carbon(s)) per molecule. A process according to any one of the preceding claims wherein the ester is a C₁ to C₁₀ alkyl acrylate or methacrylate. A process according to any one of the preceding claims wherein the melt flow rate of the terpolymer should be in the range of 10-40 dg/min at 250° C (2.16 kg). A process according to any one of the preceding claims wherein the co-extrusion is attained by the use of a co-extrusion machine having two extruders, a main extruder for the polypropylene and a satellite extruder for the terpolymer and by the use of a combining feedblock system. A process according to any of the preceding claims in which the amount of carboxylic acid or anhydride groups containing monomer in the terpolymer is from 5 to 10 wt% and the amount of ester group containing monomer is from 5 to 10 wt% A laminate obtained by the process according to any one of the preceding claims.

Inventors: GREGORY, BERTRAM HUBERT
Applicants: EXXON CHEMICAL PATENTS INC.
Application year: 1988
Main classification: B32B15
IPC codes: B05D1, B32B37, B29C47, B32B15, B29K23, B29L7