WO1983000494A1

WO1983000494A1 - Extrusion coating compositions for unprimed foil

Info

Publication number: WO1983000494A1
Application number: PCT/US1982/000573
Authority: WO
Inventors: Kodak Company Eastman; Darryl Aubrey Godfrey
Original assignee: Eastman Kodak Co
Priority date: 1981-07-28
Filing date: 1982-05-03
Publication date: 1983-02-17

Abstract

Blends of crystalline polypropylene or crystalline copolymers containing propylene, polyethylene and a tackifying resin. These blends can be used to provide extrusion coatings having excellent adhesion on aluminum foil for use in fabricating retortable pouches.

Description

EXTRUSION COATING COMPOSITIONS FOR UNPRIMED FOIL

This invention relates to blends of crystalline polypropylene or crystalline copolymers containing propylene, polyethylene and a tackifying resin. These blends can be used to provide extrusion coatings having excellent adhesion on unprimed aluminum foil for use in fabricating retortable pouches.

Various polyethylenes and blends of poly-

10 ethylenes and crystalline polypropylene have been coated onto substrates, such as paper or aluminum foil, by extrusion techniques. Although crystalline polypropylene has many desirable properties, it is -. - not a satisfactory material for coating substrates by extrusion because it is difficult to coat at high speeds and it does not have a wide range of coating weights. In order to improve the coating properties of crystalline polypropylene, blends of polyethylene

20 and crystalline polypropylene have been used in extrusion coating compositions. For example, U.S. Patent 3,418,396 describes blends of crystalline polypropylene and polyethylene that have excellent properties for coating by extrusion. Although the blends disclosed in U.S. Patent 3,418,396 are useful 5 in many applications, they are not satisfactory as extrusion coatings on unprimed aluminum foil because their adhesive properties are too low.

When coated aluminum foil is used to make 0 retortable pouches, the coating must have a bond strength or adhesive strength of at least 100 grams per 2.54 centimeters (linear inch). For some uses the bond strength should be at least 400 grams. High bond strengths are required when the pouch is filled, 5 during storage and handling and immersion in boiling water.

At the present time there are no commercial coating formulations that, when applied to umprimed aluminum foil by extrusion coating, have bond strengths that enable the coated foil to be used in retortable pouches. In order to provide satisfactory bond strengths it has been necessary to prime the foil prior to coating. Priming can be accomplished by depositing a solution of maleated amorphous polypropylene or other suitable primer on the aluminum foil and then removing solvent from the solution prior to coating. This priming procedure is expensive, time-consuming, and solvent evaporation creates environmental problems.

We have found an extrusion coating composi¬ tion that can be applied to unprimed aluminum foil, and the bond strength of coating to unprimed foil is sufficiently high to enable the coated foil to be used in retortable pouches. The coating also has excellent hardness, and it retains its bond strength for extended periods of time.

The extrusion coating compositions of this invention, when applied to unprimed aluminum foil, have bond strengths of at least 100 grams per 2.54 centimeters (linear inch) and contain: (1) 50 to 90 percent by weight of crystalline polypropylene or crystalline propylene copolymer having a melt flow of 40 to 100 dg/m at 230°C, (2) 5 to 35 percent by weight of polyethylene having a density of 0.916 to 0.921, a melt index at 190°C. of 2.5 to 4.5 and a melt index recovery greater than 50, and (3) 5 to 15 percent by weight of a rosin ester tackifying resin. The preferred coating composition contains 70 percent by weight crystalline polypropylene, 20 percent by weight polyethylene and 10 percent by weight rosin ester tackifying resin.

The rosin ester tackifying resin is an essential component of the coating composition.

Polyolefin blends, without the tackifying resin do not have sufficient adhesion to unprimed aluminum foil to permit coated foil to be used in retortable pouches. Crystalline polypropylene and crystalline propylene copolymers having a melt flow rate less than 40 dg/m at 230°C. do not extrude and coat well, and, when the melt flow rate exceeds 100 dg/m at 230°C. , the polymer is too fluid for coating. Par- ticularly useful crystalline copolymers are ethylene/- propylene copolymers containing less than 5 weight percent ethylene.

The polyethylene in the coating composition has a melt index recovery greater than 50, and preferably about 70. Polyethylene having a melt index recovery below 50 causes inadequate neck-in properties and slow coating speeds in the coating compositions.

The tackifying resins in the coating compositions are rosin esters which include ethylene glycol, polyethylene glycol, glycerol and pentaerythritol rosin esters, hydrogenated rosin esters or methylated rosin esters, for example, the commercially available materials ZONESTER 85 (glycerol esters of tall oil rosin) , ZONESTER 100

(pentaerythritol ester of tall oil rosin) , and Foral 85 and 105 (highly stabilized ester resins of pentaerythritol and rosin) . These resins have softening points of 85 to 110°C. (Ring and Ball ASTM -4-

E28-67) and a specific gravity of .99 to 1.07 (ASTM D1963-61). The ZONESTER resins are commercially available from Arizona Chemical Company and the Foral resins are commercially available from Hercules, Incorporated. The specific examples demonstrate that some other tackifying resins are not useful in these coating compositions.

The compositions of the invention may be pre¬ pared in various ways such as dry blending and then passing through a compounding extruder, compounding on a milling roll or in a Banbury mixer or by fusion. Any method whereby the components can be blended together will produce the desired blend. For example, pellets of each polymer are blended mechanically and the blend is fed to an extruder wherein it is fused and extruded.

Stabilizers, fillers and other additives can be added to the coating compositions. Such materials can be present in the components forming the polymer blend, or may be added when the polymers are blended to form the extrusion coating composition. EXAMPLE 1

A blend of polypropylene, polyethylene and tackifying resin having a flow rate of 66 dg/m at 230°C. was prepared by blending 70% by weight of polypropylene having a 40 dg/m flow rate at 230°C. with 20% by weight of polyethylene (melt index of 3.5 at 190°C, density of 0.917, 70 MIR) and 10% by weight pentaerythritol rosin ester having a softening point of 102-110, an acid number of 7-16, and a specific gravity of 1.07 (Foral 105). This blend of materials was evaluated as follows: each component was fed to a 88.9 mm (3.5 inch) Egan extruder having a barrel length to diameter ratio of 24:1. The four

FRE -5- zones of the extruder were maintained, from back to front, at 204°C. (400°F.), 260°C. (500°F), 304°C. (580°F), and 332°C. (630°F.). A metering type screw having six compression flights, and 12 metering 5 flights was used. Prior to entering the die the melt passed through one screen of 9 x 9 strands per square cm (24 x 24 mesh). The die was an Egan die, center- fed with 2.54 cm (1-inch) long lands, with an opening of 40 x 0.5 mm (24" x 0.020"). The temperature of θ the die was held at 304°C. (580°F.). The extrusion rate was held constant at 97.5 kg (215 pounds per hour). The resulting film extrudate was passed through a 11.4 cm (4-1/2-inch) air gap into the nip formed by a rubber-covered pressure roll and a chill _^5 roll. At the same time, .001 gauge unprimed aluminum foil 40.6 cm. (13.25 inches) wide was fed into the nip with the pressure roll in contact with the foil. The nip pressure applied was 4.5 kg per 2.54 cm. (10 pounds per linear inch) . The chill roll was a 610 mm 0 (24-inch) diameter chrome-plated steel roll, water cooled to maintain a temperature of 15.5°C. (60°F.) on the roll. The coated foil was taken off the chill roll at a point 180° from the nip formed by the pressure roll and chill roll. The chill roll was 5 operated at linear speeds of 92 to 123 meters (300 to 400 feet) per minute which is an accepted range for commercial foil coatings. For example, at a coating speed of 100 meters (325 feet) per minute the unprimed foil had a coating of about 1.25 mils. The 0 coating had an excellent adhesion to the unprimed foil at an average bond strength of 400 grams per 2.54 cm. (linear inch) as measured on an INSTRON tensile tester at a jaw speed of 50.8 mm (20 inches) per minute. 5 Another extrusion coating composition was prepared and tested in the same manner except that the composition contained 20% polyethylene, 20% tackifier (Foral 105) and 60% crystalline poly- propylene. The composition could not be extrusion coated satisfactorily as the coating adhered to the metal chill roll and polished chrome, showing that increasing the tackifier level to 20% provides a composition which has too much tack to be useful. Another extrusion coating composition was prepared and tested in the same manner except that 15% of a hydrocarbon tackifier resin having a Ring and Ball softening point of 130°C, a density of 1.04 and a Brookfield Viscosity at 190°C. of 1200 and available commercially as Eastman Resin H-130 from Eastman Chemicals Products, Inc., was used in place of the ester resin of pentaerythritol and rosin (Foral 105). The composition was extrusion coated to unprimed aluminum foil. The bond strength of coating to foil was less than 20 grams per linear inch and the coating delaminated from the foil on handling.

Another extrusion coating composition was prepared and tested in the same manner except that

15% of a synthetic polyterpene tackifier resin (Wingtack 110) having a Ring and Ball softening point of 110°C. and a molecular weight of 1000 to 1200 and available commercially from Goodyear Tire and Rubber Company was used in place of the ester resin of pentaerythritol and rosin (Foral 105) . The bond strength of coating to foil was less than 50 grams per 2.54 cm (linear inch).

Another extrusion coating composition was prepared and tested in the same manner except that -7-

15% tackifier (Foral 105) and 85% crystalline poly¬ propylene were used. The composition could not be extrusion coated satisfactorily as the coating had excessive edgeweave and coated poorly. Another extrusion coating composition was prepared and tested in the same manner except that 20% polyethylene and 80% crystalline polypropylene were used. The composition was extrusion coated to unprimed aluminum foil. The bond strength of coating to foil was less than 20 grams per 2.54 cm. (linear inch) and the coating delaminated from the foil on handling, showing that, without the particular tackifying resin being present, the coating has substantially no adhesion to the unprimed aluminum foil.

Another extrusion coating composition was prepared and tested in the same manner except that the amount of tackifier (Foral 105) was reduced to 4% by weight. The composition was extrusion coated to unprimed aluminum foil. The bond strength of coating to foil was less than 50 grams per 2.54 cm (linear inch) , showing that more than 4% of the tackifying resin is needed to provide useful coatings on unprimed aluminum foil. EXAMPLE 2

An extrusion coating composition was prepared and tested according to Example 1 except that the composition contained 25% polyethylene, 15% tackifier (Foral 105) and 65% crystalline poly- propylene. The composition was extrusion coated to unprimed aluminum foil satisfactorily. The bond strength of the coating to the foil was 130 grams per 2.54 cm (linear inch), showing that using the high level of tackifier and less than the preferred amount of polypropylene lowers the coating to foil bond strength. EXAMPLE 3

An extrusion coating composition was prepared and tested according to Example 1 except that the composition contained 20% polyethylene, 7.5% tackifier (Foral 105) and 72.5% crystalline polypropylene. The composition was extrusion coated to unprimed aluminum foil satisfactorily. The bond strength of the coating to the foil was 250 grams per 2.54 cm (linear inch).

Melt flow rates were determined by ASTM Procedure ASTM-D-1238; density was determined by ASTM-D-1505-57D; and melt index and melt index recovery were determined by ASTM-D-1238-62T.

Retortable pouches are packages for preserving and storing food. The food is usually frozen and it is heated for serving by placing the packaged food in boiling water.

Claims

-9-Claims:

1. Extrusion coating compositions which provide coatings on unprimed aluminum foils having a bond strength of at least 100 grams per 2.54 cm

•5 (linear inch) containing: (1) 50 to 90 percent by weight of crystalline polypropylene or crystalline propylene copolymer having a melt flow rate of 40 to 100 dg/m at 230°C, (2) 5 to 35 percent by weight of polyethylene having a density of 0.916 to 0.921, a

¹⁰ melt index at 190°C. of 2.5 to 4.5 and a melt index recovery of greater than 50 and (3) 5 to 15% by weight of a rosin ester tackifying resin.

2. Extrusion coating compositions according to claim 1 containing: (1) 70 percent by weight of 5 crystalline polypropylene having a melt flow rate of 40 to 100 dg/m at 230°C, (2) 20 percent by weight of polyethylene having a density of 0.916 to 0.921, a melt index at 190°C. of 2.5 to 4.5 and a melt index recovery of greater than 50 and (3) 10% by weight of 0 _a rosin ester tackifying resin.

3. Extrusion coating composition according to claim 2 wherein said rosin ester tackifying resin is pentaerythritol rosin ester having a softening point of 102-110°C, an acid number of 7-16 and a -5 specific gravity of about 1.07.