This invention is concerned with improvements in and relating to printing and, more particularly, to the printing of materials, such as printed packaging or wrapping materials, intended for the packaging of foodstuffs and whose printed indicia (or components therefrom) may come into indirect contact with the foodstuff.

Foodstuffs are commonly packaged in a wrapping or packing formed of synthetic plastics materials such as polyethylene, polypropylene, polyesters, or polyamides, or combinations of these with other packaging materials such as paper, board and/or metal foil. It is frequently desired to print upon such packaging to provide labelling, information, decoration, etc. Since the printed indicia (or components therefrom) may come into contact with the foodstuff it is most desirable that the printed indicia be as inert as possible to the foodstuff and that no components from the printed indicia be extracted or migrate into the foodstuff. Such migratable or extractable components are undesirable as there is the strong possibility that they may impart taint or odour to the packaged foodstuff.

It has now been found, in accordance with the present invention, that printed indicia having remarkably low contents of extractable components may be produced by the electron beam curing of certain compositions, as hereinafter more particularly defined, containing ethylenically unsaturated components and an ether component.

According to the invention, therefore, there is provided a process of forming printed indicia upon a substrate intended as a packaging material for foodstuffs, which process comprises forming patterned indicia upon the substrate of a composition comprising:

(i) one or more ethylenically unsaturated monomers; and

(ii) a polyether-containing compound;

and curing the applied indicia by exposure to ionising radiation; e.g. nuclear radiation, pile radiation, α- or β-radiation and, especially, a beam of electrons.

The principal polymerisable (curable) portion of the composition used in accordance with the invention comprises one or more ethylenically unsaturated monomers, preferably one or more polyethylenically unsaturated (meth) acrylate monomers, that is acrylic or

methacryl ^' ic acid esters of alcohols, especially aliphatic alcohols, containing two or more hydroxyl groups. Specific examples of such poly(meth)acrylate monomers include propoxylated glycerol triacrylate (GPTA) , trimethylolpropane triacrylate (T PTA) , tripropylene glycol triacrylate (TPGDA) and hexanediol diacrylate (HDDA) . Other ethylenically unsaturated monomers which may be used include vinyl monomers such as N-vinyl pyrrolidone. The ethylenically unsaturated component of the composition is suitably present in the composition in an amount of from 0.1 to 90% by weight, preferably from 10 to 80% by weight, based on the total amount of components (i) and (ii) .

The polyether-containing compound is suitably one containing repeating units of the formula: -

RO -

in which R is a straight or branched C--C. alkylene group.

The polyether-containing compound may suitably be a polyalkylene glycol, such as polyethylene glycol (PEG) , polypropyleneglycol (PPG) or polytetramethyleneglycol, which glycol may further be esterified or etherified. In accordance with a preferred feature of the invention the polyether grouping is linked to an ethylenically

unsaturated group. For instance, this is conveniently achieved by reacting a polyether glycol with a urethane acrylate, that is the reaction product of a polyisocyanate and a hydroxyl group-containing (meth)acrylate, which may optionally contain polyether moieties.

The urethane acrylate component is generally the simple reaction product of an acrylate functional alcohol, preferably containing one hydroxyl group and one or more ethylenically unsaturated groups, e.g. a hydroxyalkyl (meth)acrylate, with an aliphatic polyisocyanate, especially an aliphatic diisocyanate such as, for example isophorone diisocyanate (IPDI) , tetramethylxylene diisocyanate (T XDI) , hexamethylene diisocyanate (HMDI) , bis- (4,4' -isocyanato-cyclohexyl) - methane (H _MDI) 2,2,4-trimethylhexa- methylene diisocyanate (TMDI) and derivatives thereof such as the biuret or isocyanurate trimer of HMDI. The polyether component is suitably a polyalkyleneoxy ether or derivative thereof, such as a polyethylene glycol or polypropylene glycol or polytetramethylene glycol, suitably having a molecular weight in the range 100 to 10,000, preferably 650 to 2000.

The combined urethane acrylate polyether species is preferably the reaction product of an aliphatic polyisocyanate (e.g. an aliphatic diisocyanate as noted above) , a polyether polyol (e.g. polyethylene glycol or

polypropylene glycol as noted above) and a hydroxyalkyl (meth)acrylate. In accordance with one procedure the polyisocyanate and diol are first reacted together, using a stoichiometric excess of isocyanate, to provide an isocyanate group-containing oligomer which is subsequently reacted with the hydroxyalkyl (meth)acrylate. In an alternative procedure the diisocyanate and hydroxyalkyl (meth)acrylate are first reacted together in equimolar proportions and the resultant product then reacted with the polyalkylene glycol.

The urethane oligomers can be readily prepared from the above materials by methods well known to those skilled in the art. In particular, the reaction can be carried out in the absence or presence of a catalyst. The use of catalyst reduces reaction times and the temperatures required to complete the synthesis. Examples of the catalyst employed are well known to those skilled in the art and may be illustrated by the organometallic salts such as dibutyltin dilaurate, stannous octoate, zinc octanoate and the like. .Another class of compounds which display catalytic activity in the urethanation process are tertiary amines such as diazabicyclo- [2.2.2.] -octane, triethylamine and the like. Especially preferable in cases where the level of low molecular weight components are to be minimised are hydroxy functional tertiary amines such as N,N-dimethyl

ethanolamine, which are bound into the oligomer by reaction with the isocyanate group. The level of catalyst used ranges from 0.001% to 0.1% by weight of the oligomer.

One particular class of oligomers comprises those derived from the reaction of an aliphatic diisocyanate with a polyetherdiol and a hydroxyalkyl acrylate and which may be represented by the illustrative formula:

in which H represents the residue of a hydroxyalkyl acrylate group, I represents the residue of an aliphatic diisocyanate, P represents the residue of a polyether diol and n is an integer from 1 to 20, preferably from 1 to 5.

The polyether component preferably forms from 0.1 to 90% by weight of the total composition, more especially 5 to 60% by weight thereof.

In addition to the basic components (i) and (ii) , the compositions used in the process of the invention can, and most usually will, contain other ingredients, especially colourants such as dyestuffs or pigments. These should be present in amounts sufficient to afford the desired level of colouration to the cured

composition, e.g. in amounts of upto 10% by weight in the case of dyestuffs and amounts of up to 60% by weight in the case of pigments, preferably 5-30% by weight. Other components which may be present in the compositions include fillers and extenders and waxes, silicones, surfactants, rheology modifiers, stabilisers, adhesion promoters and slip agents. Such other components are discussed in more detail in "The Printing Ink Manual", 5th Edition, Leach & Pierce (Eds) , Blueprint 1993, especially at Chapter 4.

The composition used in accordance with the invention is printed onto the substrate by any suitable process such as offset lithography, dry offset, letterpress, flexography, rotogravure, screen printing, roller printing, spray coating, dip coating and curtain coating. After having been printed upon the substrate, the indicia should, of course, be cured and this is affected, in accordance with the invention, by ionising radiation e.g. by exposure to an electron beam. The exposure dose will generally be such as is conventionally used for electron beam curing of printed indicia, e.g. 0.1 to 10 Mrad, preferably 1-3 Mrad, and will suitably be carried out in an atmosphere of reduced oxygen content, e.g. having less than 100 ppm of oxygen.

In order that the invention may be well understood the following Examples are given by way of illustration only.

Preparative Example I (INTERMEDIATE I)

222g Isophorone diisocyanate (1 mole) and 0.3g (1000 ppm) 2, 6-di-.t-butyl-4-methylphenol (BHT) were placed in a four-necked, glass reaction vessel equipped with a stirrer, thermostat, thermometer, reflux condenser and air sparge. This was heated to 40°C, then 0.lg (300 ppm) of dibutyltin dilaurate catalyst was added. 104.4g of 2-hydroxy ethyl acrylate (0.9 mole) were added slowly over 1 1/2 hours, the temperature being allowed to rise to 50-60°C at the end of the addition. The reaction was continued for three hours when the isocyanate value was

_3 determined to be 3.49 x 10 eq/g (theoretical value

_ ₃ 3.37 x 10 eq/g) . Intermediate I (326.4g) is obtained as a low viscosity clear liquid.

EXAMPLE 1

85.Og (0.297 eq. ) of intermediate I (was charged into the reaction vessel, together with O.lg (1000 ppm) dibutyl tin dilaurate. The mixture was heated to 40°C and 34.5g hydroxyethyl acrylate (0.297 mole) was added over two hours. The reaction mixture was held at 60°C for 7 hours until there was no isocyanate remaining as determined by infrared spectroscopy. Oligomer I (117.8g) was obtained as a clear, highly viscous liquid.

EXAMPLE 2

110.3g (0.385 eq.NCO) of intermediate I, 0.lg dibutyl tin dilaurate (300 ppm) and 0.lg (300 ppm) 2,6-di-t. -butyl-4-methylphenol were charged into a reaction vessel and heated to 40°C under air sparge. 200g (0.390 eq.OH) of polypropylene glycol (MW 1000) were slowly added over two hours. At the end of the addition, the temperature was raised to 60°C and held for 3.5 hours, until no isocyanate peak was visible by infrared spectroscopy. Oligomer II (310g) was obtained as a clear, water white viscous liquid.

EXAMPLE 3

Intermediate I was prepared as described above

. ₃ having an isocyanate value of 3.27 x 10 eq.NCO/g.

80g of this intermediate I (0.262 eq.) was charged into the reaction vessel together with 0.075g (200 ppm) 2,6-di-t-butyl-4-methylphenol and 0.075g (200 ppm) dibutyl tin dilaurate, and the mixture heated to 50°C. 93.5g (0.522 OH equivalents) of glycerol monostearate was added slowly and reacted until no isocyanate remained. IPDI (60g, 0.540 NCO eq.) was added and the reaction continued at 60°C until the isocyanate value had decreased to 1.42 x 10 eq. NCO/g. 143.5g (0.280 eq. OH) of PPG 1000 was slowly added and the reaction

10 continued until no isocyanate peak was detectable by infrared spectroscopy. Oligomer III was obtained as a highly viscous, amber liquid.

COATINGS PREPARATION

Coatings of the above oligomers were prepared along with comparative oligomers, to investigate EB curing properties.

Coatings I α πi IV V VI Ingredient

Oligomer I 50 - - - - 18.2

Oligomer Q - so - - - -

Oligomer HI - - 50 - - -

Urethane acrylate - - - 50 - - Epoxy acrylate 2) - - - - 50 -

PPG 1025 - - - - - 31.8

GPTA " 50 50 50 50 50 50

lOOg 100g "»g 100g "»B lOO.Og

Where necessary, the mixtures were heated gently at 60°C to aid dissolution of the oligomer in the monomer.

1) A commercially available polyester urethane acrylate.

2) A commercially available Bisphenol A epoxy acrylate.

3) A commercially available propoxylated glycerol triacrylate.

Coating Dose /Mrad Extractable GPTA/ppm

I 3 11-500 π 3 45 in 3 65

IV 3 900

V 3 15,830

VI 3 400

APPLICATION AND CURING

Coatings were applied using a No . 1 K-bar (notional

2 wet film weight 10 g/m ) onto corona treated LDPE o

(Brithene BLA) . The prints were exposed to varying doses of electron beam radiation using an ESI Electrocurtain LB80 curing unit. All films were exposed in an atmosphere containing < 100 ppm oxygen.

EXTRACTION

The quantity of unreacted monomer (GPTA) in the films was determined using solvent extraction. A known quantity of coating is subjected to a 16 hour cold soaking in dichloromethane.

The extract is concentrated, an internal standard added and the quantity of extracted GPTA present is assayed using a gas chromatograph with a mass selective detector (gc/ms) .

The quantity of extractable GPTA is expressed as a fraction of the original coating weight and is quoted in ppm.

Example 4

A polyether urethane (oligomer IV) was prepared by reacting 185.92g of Desmodur N3300 (an isocyanurate trimer of HMDI, NCO content = 21.5%) with a blend of 293.14g of Bisomer PPA6 (a hexapropylene glycol- monoacrylate) and 120.04g of Tone M-100 (an oligocaprolactone monoacrylate) in the presence of 0.6g BHT and 0.3g dibutyl tin dilaurate. The reaction was carried out in the same manner as the previous examples and continued until no isocyanate peak was visible by infra-red spectroscopy.

Oligomer IV was obtained as a low viscosity, clear liquid.

Preparation of Offset Inks (Coatings VII and VIII)

Offset inks were prepared according to the following formulae

Phthalocyanine blue pigment 20.00 20.00

Calcium carbonate 3.00 3.00

Talc 2.40 2.40

Oliomer IV 54.64 -

Polyester oligomer* - 55.25

GPTA 19.20 18.59

Stabiliser 0.76 0.76

100.00 100.00

*A commercially available polyester hexacrylate.

The inks were prepared by grinding the pigment, calcium carbonate and talc into the other ingredients using a three-roll mill.

₂ Inks were printed at a film weight of 1.9-2.0g/m onto Melinex 0 (polyethylene terephthalate film) and cured using an electron beam pilot line with a dose of 3Mrad and

<100ppm of oxygen.

The cured films were then extracted and assayed for residual GPTA as previously described.

Results

Dose/Mrad Extractable GPTA/ppm

Coating VII 3 8,140 Coating VIII 3 29,760