EP1114894A1

EP1114894A1 - Process for the production of coated cellulose cardboard, coating composition and coated cellulose cardboard so obtained

Info

Publication number: EP1114894A1
Application number: EP01100339A
Authority: EP
Inventors: Giovanni Dell'aria Burani; Cesare Tocchio; Eugenio Cavallini; Dario Sampietro; Mino Leo Marucci
Original assignee: Reno de Medici SpA
Current assignee: Reno de Medici SpA
Priority date: 2000-01-05
Filing date: 2001-01-04
Publication date: 2001-07-11
Also published as: ITMI20000004A0; ITMI20000004A1; IT1317717B1

Abstract

A process for the production or coated cellulose cardboard comprises the following steps:

providing a substrate of cellulose cardboard,

applying on said substrate a coating composition comprising:

at least an acrylic monomer and/or prepolymer,
a covering/waterproofing means
a solvent/dispersing means
a catalyst/quenching agent
a graft initiating/regenerating system

allowing graft polymerization to occur on the cellulose material forming the cardboard substrate.

Description

The present invention relates to a process for the production of coated cellulose cardboard for different uses, depending on the type of coating applied.
The present invention also relates to the coating composition, as well as the coated cardboard so obtained.
By the term "cardboard" there is intended to indicate in the present invention all the types of paper, board and cardboard of any size and thickness.
As is known, the cardboard intended for containing hygroscopic products or products containing water and/or fats, or products degraded by humidity, is waterproofed and made hydrophobic by coating or covering the same with a film from waterproof material, substantially a protective layer realized from plastic material, such as polyethylene and the like.
Coating is substantially an operation by which the surface of raw cardboard is covered, by way of example by means of rollers and/or metal- or air-blades, with coating products in the form of emulsions or aqueous suspensions, which are so coated on the cardboard in various layers or thicknesses obtained by as many "passages", depending on the intended use. In order to increase the resistance of the cardboard to the passage of humidity and steam, according to the known techniques the cardboard sheet is "coupled" with preformed sheets or films from plastic materials such as polyethylene, vinylidene chloride copolymers and the like, obtaining in this way coated cardboard comprising at least a cardboard layer and one or more layers or preformed sheets from such waterproof materials. In these cases, the bond between the cardboard layer and the coating is only of a physical nature.
Generally, said coated cardboard solves the problem of preventing the passage of humidity satisfactorily, but requires particular working processes carried out by means of delicate and often laborious techniques.
With the transparent protective paints of various type, for instance acrylic, that are applied on the cardboard sheet, one obtains a barrier effect hindering the passage of humidity, which is however lower than that achieved by using the aforementioned coupling technique. Besides, this process is very onerous, as it is necessary to apply several paint layers.
It is also known that if microfractures occur in the protective/barrier layer, the effectiveness of the whole barrier layer is irremediably jeopardized. It suffices, in fact, that steam comes in touch with the underlying cardboard in one single point to cause water to spread in the time on all the cardboard sheet, moistening it.
Object of the present invention is to realize a process for the production of coated cardboard provided with a coating film intimately and irremovably attached to said cardboard.
Another object of the invention is to realize a coated cardboard with a coating film stably attached to said cardboard, that is degradable and suitable for use in containers and packages also for alimentary substances.
Another further object of the invention is to provide compositions for coating cellulose cardboard suitable to realize a coating having excellent waterproofing and hydrophobic characteristics and that is intimately and irremovably attached to said cardboard.
These and still other objects and related advantages that will result from the following description are achieved by a process for the production of coated cardboard, which process, according to the present invention, comprises the following steps:

providing a substrate of cellulose cardboard,
applying on said substrate a coating composition comprising:
- at least an acrylic monomer and/or prepolymer,
- a solvent/dispersing means,
- a catalyst/quenching agent
- a graft initiating/regenerating system
allowing graft polymerization to occur on the cellulose material forming the cardboard substrate. Said graft polymerization occurs spontaneously as a function of the time and/or the catalyst added. Alternatively, graft polymerization may be promoted and/or accelerated by heating at a temperature comprised between 80°C and 150°C or any other suitable means, such as irradiation or the like.

More particularly, said substrate of cellulose cardboard is comprised of a cardboard sheet in the formation step or a cardboard sheet in the finishing step.
In other words, the coating composition is advantageously applied onto the cardboard during cardboard working without needing to have recourse to a specific coating step on the sheet after the preparation of the mix, in this case preferably by spraying, or on the sheet after drying, in the finishing step by means of a roller or blades, avoiding in this way a whole working step.
Preferably, said monomer and/or prepolymer is an acrylic monomer or prepolymer of the formula
wherein X is -OR or -COOR and R is an alkyl, phenyl group or an alkyl group, having preferably 1 to 10 carbon atoms.
In particular, there has been advantageously used as acrylic monomer or prepolymer an ethylenacrylic acid-based dispersion of prepolymers, such as "Primacer" 4983-48R, produced by Michelman Inc., and a latex comprising carboxylated acrylic prepolimers, such as Hycar 26288 produced by BF Goodrich, Cleveland, Ohio, USA.
Said covering/waterproofing means is constituted of a combination of paraffin and anionic/non ionic emulsion polyolefin wax, such as Michem 70750 Emulsion produced by Michelman Inc., or Aquabead 270 E of the Micro Powders Inc., Tarry Town, New York.
Said solvent/dispersing means is constituted of a monomer substance comprising a mix of trimethylpropane and ethoxylated triacrylate, such as the SR344 Monomer of SR9035 Monomer, produced by Sartomer.
Said graft initiating/regenerating system is constituted of ferrous ammonium sulfide and hydrogen peroxide.
The graft initiator may also consist of the system of metal ions Fe⁺⁺/Fe⁺⁺⁺, Co⁺⁺, Cu⁺⁺, and the peroxide may be selected from among water soluble catalysts such as hydrogen peroxide, urea peroxide, ammonium, and potassium peroxide.
The monomers and prepolimers have collateral functional X groups that can react with one another and the further prepolymers included in the formulation, generating an organic covering made up by graft-crossed atomic bonds. The functional groups of monomers and prepolimers may be comprised of hydroxy groups, carboxy groups, amine secondary and/or tertiary groups. The molecular ratio of the functional groups of the reactive components is so adjusted as not to leave any free group once the reaction has ended.
Substantially, the X group defines the property or properties obtained. In many cases, several monomers are utilized and often more than a property can be altered in one of the process steps. These chains, whose length can be controlled, are permanently bound.
The bond between the grafted polymer and the substrates is of a covalent nature, and therefore the grafted polymer cannot be detached from the substrate. In substance, the chemical graft is a growth of polymer chains on the skeleton of a substrate. The grafted polymer chains are constituted of vinyl monomers or monomers containing suitable functions, for instance, hydroxy, carboxy, epoxy, amine groups, etc.
The "permanent" properties that can be achieved include uninflammability, abrasiveness, abrasion resistance, slidability, stain-repulsion, improvement in ionic-exchange characteristics, protection from UV rays, water absorption power, gas-impermeability, bactericide and antifungal properties, as well as many other ones.
According to the present invention, the coating composition is attached to the cellulose cardboard through a graft-based chemical bond. The so obtained coated cardboard has the following properties:

The product is hydrophobic.
The coating formulation applied to the cellulose cardboard has a uniform thickness.
The coated cardboard is suitable for use in food industry.

As is known, by "chemical graft" there is intended to indicate the process through which a given material is attached to a different material by means of a chemical bond.
The chemical graft is constituted by the activation of the substrate. Once the substrate is activated, chains of monomers connected by carbon-carbon bonds growth on the substrate and become whiskers. These whiskers impart the substrate new, interesting permanent properties without damaging the existing positive characteristics of these materials.
As said, the bond between the grafted polymer and the substrates has a covalent nature and the grafted polymer cannot be detached from the substrate, as it is substantially a growth of polymer chains on said substrate.
As is known, cellulose is a natural carbohydrate polymer (polysaccharide) consisting of anhydrous glucose units bridged by an oxygen bond to form essentially straight molecular chains.
The chemical graft on cellulose can be described as being a process that activates the cellulose molecule, binding the monomers to the reactive centers, followed by chain propagation, such that polymer branches form that are bond to the main cellulose molecule.
According to the present invention, said coating composition has the following composition expressed as percentage by weight:

acrylic monomer and or prepolymer 25 to 50%
covering/waterproofing means 15 to 25%
solvent/dispersing means 10 to 35%
catalyst/quenching agent 0,001 to 0,1%
initiating/regenerating system about 0,0001%

According to the present invention, in the case of a cellulose molecule, the chemical graft is substantially obtained through the extraction of an hydrogen atom from the hydroxy group of the molecule. As indicated above, cellulose molecules have active labile hydrogen atoms (-CH₂ OH) that can be activated in the presence of a graft initiator causing the growth of the free radicals ("X"). The free radicals so produced in the process initiates the graft polymerization.
The initial graft ion spurs the action and the whole process behaves as a self-catalytic one. A minor part of the initial graft ions (10-100 ppm) suffices therefore to develop the graft polymerization process.
All the reactions indicated above occur in the presence of a peroxide that simultaneously regenerates the graft initiator, forming a free radical.
The graft polymerization can be terminated by a radical combination, for instance, between a free radical and one of the substrate polymerized radicals. According to the amount of catalyst, the polymerization reaction proceeds more or less, and it is thus possible to adjust as one wishes the length of the chain and substantially the characteristics of the covering film realized.
The following examples of practical application are reported only by way of illustrative non limiting embodiments of the invention.

EXAMPLE 1

A pre-calculated amount of an acrylic polymer together with paraffin and the emulsified polyethylene waxes are charged in a vessel, followed by the addition of the catalyst, the graft initiator system and the other elements of the composition. The components are taken according to the concentration ratio and the order specified in the formulation of the coating composition. The contents is then mixed until a uniform solution is obtained.
The formulation is then ready to be used in the industrial application.

In the laboratory, the formulation was coated on the cardboard by means of a Meyer rod. However, in the industrial use, the formulation is coated with paper production machines. The coated cardboard was then submitted to a 100°C treatment for 10 minutes. Formulation of the coating composition:

	PARTS BY WEIGHT
(Michelman Inc.)
Dispersion of
acrylethylene acid	52,00
copolimers
Primacer 4983-48R
(Michelman Inc.)
Michem 70750 emulsion	26,00
Deionized water	16,00
Isopropyl alcohol	18,4
solvent
Deionized water	27,6
Monomer SR344	0,01
Graft initiator
Ferrous ammonium sulfide
(1% solution)	0,001
Hydrogen peroxide	0,001

EXAMPLE 2

The modalities are those indicated in Example 1. Formulation of the coating composition:

	PARTS BY WEIGHT
(B.F. Goodrich)
Acrylic copolymer	306,00
Hycar 26288 (latex)
(Micropowders Inc.)
Aquabead 270 E
(Wax emulsion)	234,00
Water	153,00
Isopropyl alcohol	45,00
(Sartomer)
Monomer SR 9035	0,40
Graft initiator
Ferrous ammonium sulfide
(1% solution)	0,001
Hydrogen peroxide (28%)	0,001

EXAMPLE 3

	PARTS BY WEIGHT
(B.F. Goodrich)
Acrylic copolymer	374,0
Hycar 26288
(Micropowders Inc.)
Aquabead 270 E	286,00
Deionized water	44,00
Isopropyl alcohol	44,00
solvent
(Sartomer)
Monomer SR 9035	0,400
Ferrous ammonium sulfide
(1% solution)	0,001
Hydrogen peroxide (28%)	0,001

EXAMPLE 4

	PARTS BY WEIGHT
(B.F. Goodrich)
Acrylic copolymer	270,00
Hycar 26288
(Micropowders Inc.)
Aquabead 270 E	270,00
Deionized water	153,00
Isopropyl alcohol	45,00
(Sartomer)
Monomer SR 344	0,40
Graft initiator
Ferrous ammonium sulfide
(1% solution)	0,001
Hydrogen peroxide (28%)	0,001

EXAMPLE 5

	PARTS BY WEIGHT
(B.F. Goodrich)
Acrylic copolymer	234,00
Hycar 26288
(Micropowders Inc.)
Aquabead 270 E	307,00
Deionized water	153,00
Isopropyl alcohol	45,00
(Sartomer)
Monomer SR 344	0,40
Graft initiator
Ferrous ammonium sulfide
(1% solution)	0,001
Hydrogen peroxide (28%)	0,001

Claims

A process for the production of coated cardboard, characterized in that it comprises the following steps:

providing a substrate of cellulose cardboard,

applying on said substrate a coating composition comprising:

at least an acrylic monomer and/or prepolymer,

a covering/waterproofing means

a solvent/dispersing means

a catalyst/quenching agent

a graft initiating/regenerating system

allowing graft polymerization to occur on the cellulose material forming the cardboard substrate.
The process according to claim 1, characterized in that said polymerization is carried out by heating at a temperature comprised between 80°C and 150°C, or other suitable means, such as irradiation or the like.
The process according to claim 1, characterized in that said substrate from cellulose cardboard is constituted of a cardboard sheet in the formation step, or a cardboard sheet in the finishing step.
The process according to claim 1, characterized in that said monomer and/or prepolymer is an acrylic monomer or prepolymer
wherein X is -OR or -COOR and R is an allyl, phenyl group or an alkyl group, having preferably 1 to 10 carbon atoms.
The process according to claim 4, characterized in that said acrylic monomer or prepolymer is a dispersion of ethylenacrylic acid copolymers.
The process according to claim 4, characterized in that said acrylic monomer or prepolymer is a latex comprising carboxylated acrylic copolymers.
The process according to claim 1, characterized in that said covering/waterproofing means . is constituted of a combination of paraffin and emulsion anionic/non ionic polyolefin wax.
The process according to claim 1, characterized in that said solvent/dispersing means is constituted of a mix of isopropyl alcohol and deionized water.
The process according to claim 1, characterized in that said catalyst/quenching means is constituted of a monomer substance constituted of a mix of trimethylpropane and ethoxylated triacrylate.
The process according to claim 1, characterized in that said graft initiator/regenerator is constituted of ferrous ammonium sulfide and hydrogen peroxide.
A coating composition according to claims 1 and 4, characterized in that it has the following composition expressed in parts by weight:

acrylic monomer and or prepolymer 25 to 50%

covering/waterproofing means 15 to 25%

solvent/dispersing means 10 to 35%

catalyst/quenching agent 0,001 to 0,1%

initiating/regenerating system about 0,0001%
A coated cardboard according to claim 1, characterized in that it comprises a substrate from cellulose cardboard and a film from polymer material attached to said substrate by means of chemical graft.