WO2014172717A2

WO2014172717A2 - Non-wax paper coating

Info

Publication number: WO2014172717A2
Application number: PCT/ZA2014/000005
Authority: WO
Inventors: Douglas Alistair Herbert KNOX; Gonaseelan ANGAMUTHOO
Original assignee: Seal Chemistry (Pty) Ltd
Priority date: 2013-02-28
Filing date: 2014-02-27
Publication date: 2014-10-23
Also published as: WO2014172717A3; ZA201406333B; ZA201405781B

Abstract

This invention relates to a water based composition for coating paper, particularly paper based food and confectionary packaging and to a method of manufacturing the composition. The invention includes paper products coated with the composition, particularly paper food and confectionary packaging adapted to replace hot wax coated packaging. The composition comprises 10 to 30% by volume of an aqueous film forming heat-seal acrylic emulsion and 15 to 40% by volume of an aqueous ethylene acrylic copolymer resin in a blend vehicle constituted by an aqueous polyethylene emulsion together with one or more additives, in which the acrylic emulsion is a styrene acrylic resin and in which the additives include one or more of an emulsion stabiliser, an anti-foaming agent and a lubricant that is formulated to provide slip and anti-blocking characteristics.

Description

MOM -WAX PAPER COATING.

ield of the invention

{001]^' This Invention relates to a water based com position for coating paper., particularly paper based food and confectionary packaging.

[002] ^'In particular, the invention relates to a wafer based coating composition to impart water-, moisture- and grease barrier properties and heat sea! tolerance to. paper, carton board and kra.ft liners that are intended for use in food and confectionary packaging. p>3.) In addition, the invention relates to a coated pape product, particuiariy coated paper food and confectionary packaging to replace hot wax coated packaging.

(004} Further, the invention relates to a method of manufacturing a water based composition for coating paper and a method of coating paper, particularly paper based food and confectionary packaging, using the coated paper of th invention. Background to the Inversion

[005] The process of conversion of paper film or web stock for use in packaging typically involvesthe application of printing, with the printed images normally being printed on the outside of the film, that is on the major face of the film not facing the contents of the package. To protect the printed images from damage due to rubbing,, flexing., abrasion and heat sealing, the printed images are often covered with a suitable barrier coating.

[006] Printing ink contamination of the packaged product Is often a problem and for this reason, the paper is typically coated with a barrier coating, on both the outer and inner major surfaces of the film. This is because ink contamination is possible even If the film is printed on the outside, since most packaging films are supplied in the form, of large rolls and the outside face of the film is in contact with and can contaminate th inside face In the roll which contamination can be transferred to the packaged product.

[007] Paper web stock that is intended for use in food contact application^'s, makes use of food compatible materials and even where printing ink contamination is not a problem, the paper Is typically coated with a barrier on both the outer and inner major surfaces of the film. In these applications, at least the inner coating is formulated to provide release and barrier properties [such as grease- and water resistance).

[008] Existing paper conversion processes tend to be fragmented, with separate printing and coating processes. Printing Is first applied by means of rotary printing processes that operate at extremely high throughput rates, after which the paper is coated in coating processes that run at substantially lower throughput rates. For instance, conventional rotogravure and flexographic printing processes use high printing press speeds, with web throughput velocities of 600 meters per minute being typical, whilst the hot wax coating system typically used for food contact applications runs at a fraction of that rate.

(009] The hot wax system Is the best known paper coating process for food contact applications. If is essentially a two-stage, process, the first stage involving manufacturing of the paper substrate and the second stage involving the -application of hot melted, wax to the paper substrate. The process is typically carried out separately of the conversion line, constituted in most instances by a printing press in which the converter first applies printing inks to the paper, normally in a rotogravure or flexographic printing process in which the outer surface and sometimes the inner surface of the paper is printed. The printed paper is then taken off the conversion Sine and moved to a press where chemicals are added to prepare and supple th paper for hot wax application. In a further separate process, .^'solid blocks of wax are melted in holding tanks in a separate machine. In the holding tanks, the wax is maintained at a relatively high temperature to keep the wax in a molten state. The printed and prepared paperis then passed through the molten wax, which is normally transferred from the holding tank to a heated bath. The wax coated and impregnated paper is subsequently cooled in-line to produce the final wax coated paper product, which is then rewound and transported to the packaging line. (0010] In addition to introducing separate off-fine steps in the conversion process, existing hot wax coating processes are slow compared to the rates at which conventional paper conversion plant operates and wax coating therefore constitutes a process bottleneck that reduces conversion efficiency substantially.

[OOi i] Hotwax coatings fend to suffer from re-melting during the packaging process where heat is often applied by way of the heated product being deposited in the film package or by way of folding, gripping and heat sealing equipment forming part of the packaging: plant. Wax coatings have a relatively high initial lustr and shine but tend to scuff and smear relatively- easily during handling, resulting in a flat "smeared" appearance that detracts from the aesthetics of the final packaged food product.

(0012). Wax coated wrapping paper is detrimental to the environment since the wax coating renders the paper Incapable of re-pulping and recycling and the used wrapper, once disposed of, will not biodegrade in iandfili.

TOO 13] If is an object of this invention to address these problems. Summary of the Invention

[0014] According to this invention an aqueous paper coating composition comprises:

10 to 30% by volume of an aqueous film forming heat-seal acrylic emulsion; and 15 to 40%. by volume of an aqueous ethylene acrylic copolymer resin; in a blend vehicle constituted by an aqueous polyethylene emulsion; together with one or more additives;

In which the acrylic emulsion is a sfvrene acrylic resin and in which the additives include one or more of an emulsion stabiliser, an anti-foaming agent and a lubricant that is formulated to provide slip and anti-blocking characteristics.

§0015] The invention Includes, a paper web, which includes paper film ^'or sheet coated with the composition of the invention.

|Ό016| The paper we is preferably a paper based food and confectionary packaging filr or web for wrapping food products. i^'OO s ?] The invention includes a method of manufacturing a wafer based composition for coating paper, the method including the steps of preparing an aqueous paper coating composition comprising 10 to 30% by volume of an aqueous film forming heaf-sea! acrylic emulsion and 15 to 40% by volume of an aqueous ethylene acrylic copolymer resin in a blend vehicle constituted by an aqueous polyethylene emulsion, together with one or more additives, in which the acrylic emulsion is a sfvrene acrylic resin and in which the additives include one or more of an emulsion stabiliser, an anti-foaming agent and a lubricant that is formulated to provide slip and anti-blocking characteristics. [0018] The invention inc!udes, further, a method of coating paper, particularly paper based food and confectionary packaging, comprising the steps of preparing an aqueous pape coating composition comprising 10 to 30% by volume of an aqueous film forming heat-seal acrylic emulsion and 15 to 40% by volume of an aqueous ethylene acrylic copolymer resin in a blend vehicle constituted by an aqueous polyethylene emulsion, together with one or more additives, in which the acrylic emulsion is a styrene acrylic resin and. in which the additives include one or more of an emulsion stabiliser, an anfi-faaming agent and a lubricant that is formulated to provide slip and anti-biocking characteristics, coating a paper film or web with the aqueous coating so prepared and drying the paper for use.

[001 ] in the preferred form of the invention, this method includes the steps of formulating the aqueous composition for application by means of continuous web printing equipment and coating the paper by means of the continuous web printing equipment.

[0020] The printing equipment is preferably configured to apply the aqueous composition of the invention to the paper web on both major surfaces thereof in a continuous print application process, the method including the prior step of formulating the aqueous composition to include inner and outer surface coating formulations with different characteristics.

Description of em odime ts of the Invention

[Q025 ] The invention provides a wafer-based or aqueous composition to impart water-, moisture- and grease barrier properties as well as heat sea! tolerance to paper films and webs intended for use as food packaging, particularly paper, cartonboard and kraff liners that are intended for use in food and confectionary packaging.

[0022] The invention will be described, by way of illustration, with reference to the example of a high solids, water based paper coating specifically formulated for surface application onto paper confectionary wrappers as an alternative to wax impregnated wrappers, An example is included of an outer surface coating composition formulated to impart heat seal tolerance, water-, moisture- and grease barrier characteristics to paper, cartonboard and lcraff liners. An example is also given of an inner surface coating for use in combination with the outer coating. If will be appreciated that none of these exampies are intended to limit the scope of the invention to such specific exampies.

[00:23] As indicated above, the processing of printed paper web stock to apply barrier coatings thereto typically involves the use of ά hot wax coating process that introduces a number of separate off-line process steps that are slow compared to the throughput rates of the printing plant forming the first pad of the conversion process,

[0024] Gravure or rotogravure printing presses use printing cylinders which transfer their ink directly onto the substrate (in this case a paper film or web) to be printed by a combination of pressure and capillary action, so producing the printed image. Handling process equipment normally forming a part of the press transports the printed web through converting equipment (such as die-cut, creasing and embossing units} to a sheeter or web rewinder.The layout of a gravure printing and converting press follows an in-line arrangement where the number of printing and converting units or print stations required to print the requisite number of colours on the substrate, are installed in a substantially horizontal line. Each print station comprises of a printing cylinder that carries the printing ink onto the substrate; a doctor blade that removes excess Ink from the printing cylinder; an impression roller thai presses the substrate against the printing cylinder; a inking system consisting of an ink pan, ink holding tank and ink pump with ink delivery and return lines; and a drierwhic dries the ink on the substrate prior to the inked portion reaching the next print station. After each color has bee printed and any final coating has been applied, the web travels on for conversion which may include die-cutting, punching, creasing/scoring, embosslng de-bossing, and piling, sheeting or rewinding.

[0025] Fiexography (often abbreviated to flexo) is a printing process which utilises a flexible relief plate. If is essentially a modern version of letterpress which can be used for printing on almost any type of substrate, including plastic, metallic films, cellophane, and paper, in a flexo press, the print stations are also single color stations and each print station includes a roller with a flexographie printing plate mounted thereon; an anilox roller; an ink fountain from which a layer of ink is deposited on the anilox roller: and a doctor blade that is used to scrape excess ink from the surface of the anilox roller, leaving a measured amount of ink in the cells of the roller. The anilox roller rotates into contact with the flexible relief (printing) plate which receives the ink from anilox roller the cells and transfers the ink to the substrate, in this case a paper web,, which is transported into contact with the relief plate by means of an impression cylinder. The print stations are typically positioned in a cylindrical arrangement around a central impression cylinder and a between-colour drier is located between print stations, with one or more driers are iocated downstream of the last print station to dryfhe web before conversion which may include die-cutting, punching, creasing/scoring., embossing/de-bossing., and piling, sheeting or rewinding.

[0026} in addition to providing a coating composition as outlined above, this invention seeks to provide a composition that can be used within existing paper and film conversion process equipment as an in-line part of the process, operating at the same speed as th conversion process.

[0027] In Its most basic form, the paper coating composition of the invention -comprises! 0 to 30% by volume of an aqueous film forming heat-seal acrylic emuisiondnd iS to 50% by volume of an aqueous ethylene acrylic copolymer resinin a blend vehicle constituted by an aqueous polyethylene emulsion, together with one or more additives.

[0028] Th aqueous film forming heat-seal acrylic emulsion is preferably a styrene acrylic resin that, in the preferred form of the invention,, is present in an amount of approximately 167% by volume.

[0029] The styrene acrylic resin is used primarily as a coating component thai reduces the amount of binder required fa make the coating composition of the Invention. Plus, it serves, in the composition, to improve flow characteristics, gloss, processability, pnntability and viscosity. The styrene acrylic resin is preferably selected for lowpo!ydispersify with a view ^'to improving uniformity, decreasing viscosity and decreasing volatile organic compounds in the composition.

[0030] Ln applications where a relatively more resistant coating is required, such as an outer web surface coating, the aqueous ethylene acrylic copolymer resin is preferably present in an amount of 30 to 40% and preferably 34,3% by volume,

[0031] in applications where a relatively less resistant coating is required, such as an inner web surface coating, the aqueous ethylene acrylic copolymer resin is preferably present in an amount of 30 to 50% and preferably S 3% by volume.

[0032] The aqueous ethylene acrylic copolymer resin Is used primarily to improve adhesion of the composition to polar substrates such as paper without the requirement for a primer coating, in addition, the aqueous ethylene acrylic copolymer resin serves as a binder or modifier for the water-based heat-seal lacquer constituted by the styrene acrylic resin.

[0033] For inner web coating applications, the composition may include 40 to 80% and preferably 57.43 by volume of a glycol ether-free acrylic emulsion. f003 ] The glycol ether-free acrylic emulsion is preferably a soft (less resistant) film forming, rheology controiied acrylic emulsion suitable for packaging coating applications.

[0035] The polyethylene ernuision is preferably present in an amount of 10.24 before the addition of wafer to the composition and serves primarily as a blend vehicle and binder in the composition.

[0036] As is described in the more specific examples below, deionised (purified] wafer is added to the polyethylene emulsion to constitute the eventual blend vehicle.

[0037] The additives used in the composition of the invention may include an emulsion stabiliser constituted by an inorganic base suc as ammonia (preferably ammonia solution 23%), preferably in an amount of 4.45% by volume, to maintain the aqueous emulsion in emulsion form.

10038] in embodiments of the invention, the additives further comprise one or more defoaming--, lubricating-, and anti-blocking agents.

[0Q39] Defoqmers in wafer-based printing Inks aid in preventing the foaming of the printing ink under the adverse conditions of high temperature and shearing force encountered in fiexogrqphic and rotogravure printing processes as well as in the composition m anuf actu rin g process .

[0040] Blocking is the adhesion of two adjacent layers of film, often arising from the inherent friction and tackiness of polymers and polymer- based coatings. Blocking between adjacent layers results in increased friction (resistance to motion) and the addition of an anfibiack generally contributes to a reduction in the film-to-film coefficient^' of ^'friction (COF) . COF is a measure of the relative difficuity with which one surface wiii slide over an adjoining surface.

[0041 } The most effective method for combating these handling issues is to add an antiblock additive that minimises film-to-film COF, thereby minimising blocking. Anti-block additives typically consist of a component that creates asperities (little bumps in the resin which minimise fi!m-to-fiim surface contact). In addition, these agents often include a lubricating agent that serves to reduce friction (slip effect) In addition to combating adhesion forces fanfibloek effect),

[0042] The preferred defoamers for use in the present invention include polyefher siioxane copolymer, which is preferably present in an amount of 0.3 to 0.8% and preferably 0.57% by volume.

[0043]: In the preferred form of the Invention, the composition may also include trace amounts (0.00003 to 0.00008% and preferably 0.0000512% by volume) dimethyl palysiloxane emulsion, which has the added benefit of providing, in addition to antifoam characteristics,; lubricant, slip and anti-blocking characteristics as well as gloss, and durability characteristics .

[0044] Deionised water lacks metal ions and other contaminants that may be found in tap water and whilst the composition of the invention does not have an absolute requirement for the use of deionised water, this is preferred over tap wafer.

[0045] The invention comprises a method for manufacturing the composition disclosed herein.

[00 6] In one embodiment of the method of the invention, the method comprises the steps of blending the blend vehicle of wafer and polyethylene emulsion , a drying refarder, a defo_.amer, and a coalescing agent; adding a dextrin gradually while blending continues; introducing heat slowly upon the addition of a dextrin while blending continues; and, optionally, adding water or deionized water while blending continues to complete the base material. Agitation with mechanical mixing, preferably in concert with a blade that imparts shear, is appropriate for use in this method. It is understood by those of ordinary skill in the art that a shearing-type blending blade is particularly appropriate for use in this method. If may be desirable to add pigment to the base material, and it may additionally be desirable to adjust for pH and viscosity after the addition of the pigment through methods known by those of ordinary skill in the art, in order to create a printing ink from the base material.

Example - process of manufacture ~~ outer codtlrsg mix - part 1

references to "parts'* are percentages by volume

[0047] Using a high speed Silverson mixer, a dedicated stainless steel mixing vessel, premix and pre-weighing virgin containers and Industrial weighing equipment: add 30.7419510 ports wafer to the mixing vessel add, 4.454400 parts ammonia solution 23% to the mixing vessel add 0.0000512 parts dimethyl porysi oxane emulsion to the mixing vessel add 0.1024 parts polyether siloxane copolymer to. the mixing vessel star t up mixer add 16.6912 parts ethylene acrylic copolymer resin to the mixing vessel allow mixing for 4 hours or until dissolved allow mix to stand for 24 hours mix - part 2 add 1 .0 port wafer to 10.24 parts polyethylene emulsion in clean mixing vessel start up mixer premix separately 1 part wafer with 51 .99002 parts part 1 , add to mixing vessel premix separately 0.5 part wafer with 34.30 part ethylene acryfic copolymer resin., add to mixing vessel premix separately 0.5 ports water with 0.47 ports polyether siioxane copolymer, add to mixing vessel

Example - process of manufacture - Inner coating mix - part ! references to "parts" are percentages by volume

[0048] using a high speed Silverson mixer, a dedicated stainless steel mixing vessel premix and pre-weighing virgin containers and industrial weighing equipment: add 30.7419490 parts water to mixing vessel add 4.454400 parts ammonia solution 23% to mixing vessel add 0.0000512 parts dimethyl pol siioxane emulsion to mixing vessel add 0, 1024 parts polyether siloxane copolymer to mixing vessel start up mixer add 16.691200 parts ethylene acrylic copolymer resin to mixing vessel mix for 4 hours or until dissolved allow mix to stand for mi - part 2 add 1.0 part wafer to 10.24 parts polyethylene emulsion in clean mixing vessel start up mixer premix separately 1.0 part water with 51.99002 parts of part 1 , add to mixing vessel premix separately 0.5 parts water with 34.30 parts ethylene acrylic copolymer resin, add to mixing vessel premix separately 0.5 parts water with 0.47 parts poiyether si!oxane copolymer, add to mixing vessel 1.

[00 9j The compositions outlined immediately above are high solids., wafer based confectionary wrap coatings, specifically formulated for surface application onto the outer and inner surfaces respectively of paper confectionary wrappers as an alternative to wax impregnated wrappers. The outer coating is formulated to impart heat seal tolerance, wafer, moisture and grease barrier onto papers, cartonboard and kraft liners. The inner coating is formulated for use in combination with the outer Goating and to impart heat sea! tolerance, water,, moisture and grease barrier onto papers, cartonboard and kraft liners.

[0050] An application of the coatings of the invention will be described below with reference to the use and application thereof by means of gravure and fJexo printing processes, it will be appreciated that this is done purely to ^'illustrate the invention and it is not intended thereby to limit the scope of the invention to these specific processes.

[005]] The composition of the invention, as exemplified above, is formulated for in-line gravure conversion over gravure printed inks onto confectionary wrapping papers.The composition of the invention is loaded onto an end-of-line printing station on a rotogravure press from which the composition is transferred, using the conventional rotogravure process, from recessedcell engravings on a metal rotating gravure cylinder onto the paper web substrate. T ecells are flooded, the excess coating is wiped or scraped away with a doctor blade and the cell contents are transferred onto the substrate at normal rotogravure process speeds. The gravure cylinder cells are preferably engraved with a cell line count per area, ceil cut angles and cell depths (in accordance with conventional gravure print medium transfer principles)!© allow the application of the composition of the invention at the desired coverage, typically 2.5 to 3,Qgrams m²of substrate. The coated paper substrate is then dried in-line as part of the conventional gravure press drying and exhaustion process.

[0052] Once dried., the paper web is rewound at th end. of the gravure press Ii e, with or without slitting and ready fo use in a confectionary wrapping line.

[0053] important technical advantages of this invention include., but are not limited fa the advantages set out below.

[0054] The exemplary composition of the invention described abov is specifically developed to exhibit an aesthetic gloss finish whic is retained after processing through the wrapping unit, compared to Hot wax wrapper coatings whic evidence melting of the wax resulting i a smeared matt wrapper finish.

[0055] The composition has good outer to outer and outer to inner heat seal integrity at lower sealing temperatures than those conventionally used in wax wrapping processes. Paper coated with the composition of the invention has outer to outer, outer to inner and inner to inner heat sealability of heat sealing temperatures of 170°C to ] 90°Ο, compared to wax coated wrappers which require temperatures up to 240°C to achieve paper heat seal integrity.

[0056] Thefempera!ures used in the wax coating process, besides resulting i higher energy consumption, result in re-solubiltsing, melting and burning of the wax., thereby causing vapour emissions and wax decomposition products which may introduce long term human health risks,! n contrast, the coating of the invention releases no decom osition products during standard processing, either during manufacture or during the wrapping process.

[0057] The polymers and lubricants used In the coating of the invention provide the coating with improved hot shear release during high speed movement of the wrapper and fully or partially wrapped confectionaries through the heated sealing chutes and jaws of the wrapping machinery compared to wax coated papers. The coating has high sli characteristics to facilitate efficient glide-through of the wrapping machine chutes and conveyors, in addition, the coating has excellent deadfoid characteristics, an essential requirement for efficient wrapping machine grab- grip- and wrapping conversion. As result, wrappers coated with the composition of the invention fold flat and exhibit no curl. These characteristics combined lead to high wrapping speeds and improved wrapping performance compared to hot wax wrappers.

[0058] The exemplary inner coating described above is specifically formulated to exhibit excellent water and moisture barrie characteristics and to have a low coefficient of friction to provide efficient release., especial y from the confectionary product.

[0059] The composition of the invention is applied in-line as an overprint process on existing conversion and printing lines, over the outer and inner sides of the pape in a single process at 2.5 to 3,Qgrarns/m², compared to the hot wax wrapper coating process which requires th printed paper to be processed through two additional stages, including suppling of the paper and subsequently hot wax dipping of the suppled paper.

[0060] The -composition of the Invention Is formulated to impart aesthetic gloss which is maintained through all stages of print conversion and hot wrapping., compared to wax coated wrappers- which evidence re-nielting and smearing during the: wrapping process, in addition, the dead fold ^'integrity of paper coated with the Composition of the invention ensures paper flatness during the handling and folding process which ensure efficient confectionary wrapping integrity. The coating of the Invention further increases paper rigidity and hardness thereby eliminating the problems associated with wax cracking: and flaking during the wrapper forming process, in which free wax bits contaminate the confectionary and introduce potential human consumption risks,

[0061 ] Finally, the constituents of the composition of the invention are selected to be readily biodegradable thereby rendering wrappers coated with the composition re-puipable and recyclable using conventional methods. in addition, wrappers coated with the composition will biodegrade i landfill. Hot wax coated wrappers are neither re-eyciobie norbiodegradable.

[00621 Without departing from the spirit and scope of this invention, a person of ordinary skill !n the art can make various changes and modifications to the invention to adapt it to various usages and conditions, including those not specifically laid out in this specification. As such,, those changes and modifications are intended to be within the full range of equivalents of the Invention disclosed in this specification.

Claims

An aqueous paper coating composition comprising:

10 to 30% by volume of an aqueous film forming heat-seal acrylic emulsion; and

15 to 40% by volume of an aqueous ethylene acrylic copolymer resin; in a blend vehicle¹ constituted fay an aqueous polyethylene emulsion; together with one or more add if Ives; in which the acrylic emulsion is a styrene acrylic resin and in which the additives include one or more of an emulsion stabiliser, an anfi-foaming agent and a lubricant that is formulated to provide slip and anti-blocking characteristics.

A paper web coated with the composition of claim .

A paper based food or confectionary packaging film or web for wrapping food products coated with the composition of claim 1 .

A method of manufacturing a wafer based composition for coating paper, the method inducting the steps of preparing an aqueous paper coating composition comprising 10 to 30% by voiume of an aqueous film forming heat-seat acrylic emulsion and ] S to 40% by voiume of an aqueous ethylene acrylic copolymer resin in a blend vehicle constituted by an aqueous polyethylene emulsion,, together with one or more additives, in which the acrylic emulsion is a s!yrene acrylic resin and in which the additives include one or more of an emulsion stabiliser,, an a ti-foaming agent and a lubricant that is formulated to provide slip and antiblocking- characteristics .

A method of coating paper comprising the steps of preparing an aqueous paper coating composition comprising ?0 to 30% by volume of an aqueous film forming heat-seal acrylic emulsion and I S to 40% by volume of an aqueous ethylene acrylic copolymer resi in a blend vehicle constituted by an aqueous polyethylene emulsion, together with one or more additives, in which the acrylic emulsion is a styrene acrylic resin and in which the additives include one or more of an emulsion stabiliser, an anfi-foamfng agent and a lubricant that is formulated to provide slip and antiblocking characteristics, coating a paper film or web with the aqueous coating so prepared and drying the paper for use.

The method of claim 5 Including the steps of formulating the aqueous composition for application by means of continuous web printing equipment and coating the paper by means of the continuous web printing equipment.

The method of claim 6 including the steps of configuring the printing equipment to apply the aqueous composition to the paper web on both major surfaces thereof in a continuous print application process, the method including the prior step of formulating the aqueous composition to include inner and outer surface coating formulations with different characteristics.