WO1999002610A1

WO1999002610A1 - Uv curable compositions

Info

Publication number: WO1999002610A1
Application number: PCT/GB1998/001992
Authority: WO
Inventors: Andrew Trevithick Slark; Michael Stephen Chisholm; Helen Harte
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1997-07-12
Filing date: 1998-07-07
Publication date: 1999-01-21
Anticipated expiration: 2000-01-12
Also published as: AU8232198A; GB9714630D0

Abstract

A UV curable composition is described and a coating formed by curing this composition that can be used in inks, overprint varnishes and the like. The curable composition is substantially free of volatile solvent and comprises a copolymer which has at least one amino functionalised pendant group and a glass transition temperature of at least 10 °C, a non-volatile reactive carrier which is capable of reacting with the amino functionalised copolymer and optionally a photoinitiator.

Description

UV Curable Compositions

The present invention relates to UV curable compositions and in particular UV curable compositions that can be used in inks, overprint varnishes and the like.

Conventional UV curable compositions, that is those compositions that are susceptible to curing by exposure to radiation such as UV light, contain functionalised oligomeric species. Such oligomeric species tend to be based on acrylated-epoxy, acrylated-poiyester or acrylated-urethane resins and to have molecular weights (number average) of from 500 to 2000. The composition is cured by the reaction of the acrylate functional groups. Additionally, such conventional curable compositions may also contain reactive monomeric species such as isobornyl acrylate, hexanediol diacrylate, tripropylene glycol diacrylate and trimethylol propane triacrylate.

These conventional curable compositions often shrink during curing. However, this tendency may be combated by the inclusion of a relatively high molecular weight (number average) copolymer, i.e. greater than 2000. The copolymer is present as a solute and the oligomeric species and (when present) reactive monomeric species act as a solvent. The copolymer is typically a random copolymer formed from two or more of methyl methacrylate, n-butyl methacrylate, ethyl methacrylate and methacrylic acid monomers. However, cure speed and solvent resistance can be impaired. In order to improve the properties of the curable composition, it would be very desirable to cross-link the copolymer with the functional, e.g. acrylate, groups of the oligomeric species and (when present) reactive monomeric species during curing. However, during the preparation of the copolymer through free radical polymerisation, such pendant acrylate groups would participate in the polymerisation process to provide a prematurely cross-linked structure. Conventional UV curable compositions usually contain a photoinitiator. Depending on the mechanism through which the photoinitiator operates, a photoinitiator may be classed as either Type I or Type II. Type I initiators, or direct initiators, absorb UV light and fragment into at least two species which are free radicals some of which are capable of initiating polymerisation. Such fragmentation is commonly called radical cleaving, radical fragmenting or homolytic scission. Type II initiators, or indirect initiators, act through the photochemical excitation of a molecule followed by the abstraction of a hydrogen atom to form a free radical. A typical Type II initiator is benzophenone. Often a Type II initiator is employed with a synergist or co-initiator which enhances its reactivity. Low molecular weight amines can be used to enhance UV curing. For Type I initiators the amine serves to scavenge oxygen which itself can inhibit polymerisation. This can also occur for Type II initiators, but in this case the amine can also have a synergistic effect.

PCT application WO 96/35725 provides an extensive summary of the prior art relating to the preparation and use of functionalised polymers which contain both photoinitiating constituents and co-initiator constituents. The application itself is directed towards pigmented pressure sensitive adhesives comprising a functionalised acrylate copolymer which contains co-initiator constituents and a separate photinitiator. In order to provide copolymers having satisfactory glass transition temperatures for use as pressure sensitive adhesives, the application teaches that the copolymers should contain no more than 15% by weight of alkyl methacrylates (based on the total weight of all monomers) and preferred copolymers should have glass transition temperatures of less than about -5°C. The application also teaches that the amount of crosslinking within the adhesive may be increased by the inclusion of multifunctional acrylates or methacrylates but these should be limited to a maximum of 15 parts per hundred parts (by weight) of the copolymer. US patent US 4532021 is directed towards the preparation of UV curable liquid coating compositions which have improved adhesion to metal substrates. The improved adhesion results from the use of a combination of a copolymer containing a critical proportion of a tertiary amine as synergist, such as a copolymer of methyl methacrylate, 2-ethyl hexyl acrylate and dimethyl aminoethyl methacrylate, with a UV curable ethylenically unsaturated liquid, such as provided by an acrylated-urethane resin and pentaerythritol triacrylate. The necessary soivation of the copolymer into the liquid portion of the coating composition is ensured by the use of an inert volatile solvent. The inert volatile solvent is required to contain a C, to C₄ alkanol and may also contain a co-solvent such as butyl acetate. The inert volatile solvent is present as at least 10%, but more preferably 25 to 50% of the coating composition. The patent teaches that the copolymer is formed by solution polymerisation within the inert volatile solvent.

In respect of UV curable compositions for use in inks and overcoat varnishes, in addition to exhibiting low shrinkage and good adhesion to the chosen substrate, the UV curable composition should provide a highly cross-linked structure of high gloss and high resistance to solvent attack. Additionally, there is a desire to reduce the emission of volatile solvents to the environment and the exposure of personnel to such solvents.

It is an object of the present invention to provide a UV curable composition for use in inks and overcoat varnishes which provides a highly crosslinked structure and which is not reliant on the use of volatile solvents for its delivery. Accordingly in a first aspect the present invention provides a curable composition which is substantially free of volatile solvent comprising (a) a functionalised copolymer of general formula (I)

X¹ - (A)_n - (B)_p - X² (I) wherein A is a residue of at least one (meth)acrylate monomeric species, B is a residue of at least one monomeric species copolymerisable with A and which provides at least one amino functionalised pendant group, X¹ and X² are terminal groups which may be the same or different, n and p are both at least one and are chosen such that the copolymer has a glass transition temperature of at least 10 °C; (b) a non-volatile reactive carrier which is capable of reacting with the functionalised copolymer and through which the copolymer remains solvated within the curable composition; and optionally (c) a photoinitiator.

In a second aspect, the present invention provides a coating formed by curing a UV curable composition, which is substantially free of volatile solvent, comprising

(a) a functionalised copolymer of general formula (I) X¹ - (A)_n - (B)_p - X² (I) wherein A is a residue of at least one (meth)acrylate monomeric species, B is a residue of at least one monomeric species copolymerisable with A and which provides at least one amino functionalised pendant group, X¹ and X² are terminal groups which may be the same or different, n and p are both at least one and are chosen such that the copolymer has glass transition temperature of at least 10 °C;

(b) a non-volatile reactive carrier which is capable of reacting with the functionalised copolymer and through which the copolymer remains solvated within the curable composition; and

(c) a photoinitiator.

The curable composition is substantially free of volatile solvent. By substantially free we mean less than 2% by weight of the composition. Normally a volatile solvent would not be added to the curable composition but trace amounts may be present. By volatile solvent we mean a solvent which has a molecular weight (number average) less than 150 and/or a volatilisation rate of greater than 5mg/min. Typical acrylate volatilisation rates are disclosed in "Photogeneration of reactive species for UV curing" by C Roffey, in table 6.1 on page 553. Suitable (meth)acrylate monomeric species which can provide residues A include lower alkyl, i.e. C, to C₂₀ alkyl, (meth)acrylates, e.g. methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, iso-butyl methacrylate, t-butyl methacrylate, 2-ethyl hexyl methacrylate, octyl methacrylate, ethyl acrylate, butyl acrylate. Additionally, cyclic alkyl monomeric species may be used such as cyclohexyl methacrylate and isobornyl methacrylate. Preferably, mixtures of such monomeric species are used in order to optimise the characteristics of the copolymer. In particular, combinations of methyl methacrylate with one or more of n-butyl methacrylate and ethyl methacrylate may be used to good effect.

Suitable monomeric species which can provide residues B include dialkyl aminoalkyl acrylamides such as dimethyl aminoethyl acrylamide and diethyl aminoethyl acrylamide; dialkyl aminoalkyl methacrylamides such as dimethyl aminoethyl methacryiamide, diethyl aminoethyl methacryiamide, dimethyl aminopropyi methacryiamide and diethyl aminopropyl methacryiamide; dialkylamino styrenes such as dimethylamino styrene and diethylamino styrene; dialkylaminoalkyl vinyl ethers such as dimethylaminoethyl vinyl ether and diethylaminoethyl vinyl ether; dialkyl aminoalkyl acrylates such as dimethyl aminoethyl acrylate, diethyl aminoethyl acrylate, dimethyl aminopropyl acrylate and diethyl aminopropyl acrylate; and dialkyl aminoalkyl methacrylates such as dimethyl aminoethyl methacrylate, diethyl aminoethyl methacrylate, dimethyl aminopropyl methacrylate and diethyl aminopropyl methacrylate. Particularly preferred are the dialkyl aminoalkyl acrylates and methacrylates listed supra.

The terminal groups X¹ and X² are determined by the monomeric species used and also the reagents used in the polymerisation of the monomers. Additionally, it may be possible to further functionalise the copolymer so that one or other or both have amino functionality.

The parameters n and p are both at least 1 and are chosen such that the glass transition temperature of the copolymer is at least 10 °C, preferably from 20 to 120°C and especially from 30 to 110°C. It is preferred that n and p are chosen such that the number average molecular weight of the copolymer is more than 2000, preferably from 2000 to 100000 and particularly from 3000 to 40000. It is also preferred that the parameters n and p are chosen such that the ratio of n:p is from 1 :1 to 500:1, particularly from 3:1 to 100:1 and especially from 3:1 to 50:1. Where the at least (meth)acrylate monomeric species is a mixture of monomers then for each monomer there will be an individual value of n and the sum of the individual values of n should be used in the foregoing relationships between n and p. Similar allowances should be made where a mixture of monomeric species provide the residues B.

The copolymer may be prepared using conventional free radical polymerisation techniques such as those used in the suspension, solution, emulsion and bulk polymerisation of (meth)acryiate polymers. Preferably, the copolymer is prepared by suspension polymerisation. The copolymer is preferably a random copolymer.

The non-volatile reactive carrier is capable of reacting with the functionalised copolymer and ensures that the copolymer remains solvated within the curable composition. Suitable non-volatile reactive carriers include poly functional acrylates such as hexanediol 5 diacrylate, tripropylene glycol diacrylate and trimethylol propane triacrylate, butanedioi diacrylate, neopentyl glycol diacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate, dipropylene glycol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate; methacrylate analogues of the aforementioned poiyfunctional acrylates; and reactive oligomeric species such as acrylated-epoxy, acrylated-polyester and acrylated-urethane resins o having molecular weights (number average) of from 500 to 2000.

The copolymer (I) may be up to 70% by weight of total weight of copolymer (I) and non-volatile reactive carrier, preferably the copolymer (I) represents up to 50% by weight.

The photoinitiators capable of use in the present invention may be either Type I or Type II initiators or a mixture of both. Type I initiators include benzoin, benzoin alkyl ethers, 5 acetophenone, acyl phosphine oxides, ketosulphides and their derivatives, hydroxyalkylphenone, aminoalkylphenone, benzilketals. Type II initiators include benzophenone, thiaoxanthone, aromatic diketones and their derivatives.

When used in UV curable inks, such inks usually include plasticisers, dyes, pigments and photoinitiators. Typically, the amount of copolymer (I) used in the UV curable ink will be 0 from 1 to 50% by weight and preferably from 2 to 25% by weight of the ink. Similar concentrations of copolymer (I) may be used in respect of overprint varnishes, which by their nature do not contain dyes or pigments. In these application areas it is important that the T_g of the finished product surface coating should be much greater than room temperature. This is so the product itself is non-tacky, can be handled easily, will not pick up dust and will not stick to 5 other media placed on top of it. Therefore it is important that the copolymer (I) itself has a T_g of at least 10°C, preferably from 20 to 120°C and especially from 30 to 110°C.

The compositions of the present invention may also contain small amounts of other additives such as relatively non-volatile monofunctional acrylates such as isobornyl acrylate, 2-ethyi hexyl acrylate and octyl acrylate. 0 The invention is further illustrated by reference to the following examples.

Example 1 Suspension Polymerisation to Produce Copolymer (I)

Amino functionalised copolymers were prepared by suspension polymerisation of the monomers methyl methacrylate (MMA), butyl methacrylate (BMA) and dimethylamino ethyl methacrylate (DMAEMA) in the presence of indicated amounts of a chain transfer agent dodecyl mercaptan (DDM), a dispersant (hydroxy ethyl cellulose), an initiator azobisisobutyronitrile (AIBN) in deionised water. The polymerisations were carried out under a blanket of nitrogen with high speed agitation. The resulting copolymers were then centrifuged and washed twice with deionised water before being dried in a fluid bed drier.

The relative proportions of the monomers were varied to give three copolymers having different T_B. Copolymer 1 was prepared using (by weight) 1 % dispersant, 1% AIBN and 0.3% DDM. Copolymer 2 was prepared using (by weight) 1 % dispersant, 0.3% AIBN and 0% DDM. Copolymer 3 was prepared using (by weight) 1 % dispersant, 0.3% AIBN and 0.5% DDM.

1. 39% MMA ; 59% BMA ; 2% DMAEMA (T_g 55°C)

2. 38% MMA ; 57% BMA ; 5% DMAEMA (T_g 70°C)

3. 36% MMA; 54% BMA: 10% DMAEMA (T_g 65°C)

For comparison purposes a hydroxy functionalised copolymer , Copolymer 4, was produced wherein the DMAEMA was replaced by hydroxy ethyl methacrylate (HEMA). Also for comparison purposes Copolymer 5 was produced, wherein the DMAEMA was replaced by methacrylic acid (MAA).

4. 36% MMA; 54% BMA: 10% HEMA (T_g 67°C)

5. 39% MMA; 60% BMA: 1 % MAA (T_g 65°C)

Example 2

Formulation into a Curable Coating Composition

The following formulations (expressed as weight percent) were prepared using the copolymers 1 , 2, and 4 of Example 1 , tripropylene glycol diacrylate (monomer) as the non-volatile reactive carrier, benzophenone (Type II) and 1-hydroxycyclohexylphenylketone (Type I) (present in equal quantities sold as Irgacure 500 supplied by Ciba-Geigy) as photoinitiator and P115 supplied by UCB as a further synergist.

% Copolymer 9

Monomer 81 Photoinitiator 5

Synergist 5

The formulations were then coated on to paper substrates to a thickness of 12 microns. The coatings were then cured using a Primarc UV curing unit with a high-pressure mercury lamp light source at a power of 80 W.cm ². The coating was cured by a single pass at a rate of 16 metres per minute.

The cured coatings were then tested for solvent resistance and gloss. Solvent resistance was determined in respect of methyl ethyl ketone (MEK). The cured coating was rubbed with a cloth saturated with MEK and the number of double rubs recorded at the failure of the coating. Gloss was determined using a reflectometer, after calibration with a highly polished glass plate having a refractive index of 1.567. Gloss was measured at an angle of 60°.

The results of the tests are as follows: Copolymer Solvent Resistance Gloss

1 50 87.6

2 70 89.6 4 (Comparative) 18 89.8 None 115 71.7 This illustrates that copolymers of the present invention have both good solvent resistance and gloss properties. Example 3 Formulation into a Curable Coating Composition

Example 2 was repeated using the following formulation with the addition of an oligomeric acrylated-epoxy resin (EB605 from UCB)(oligomer) as additional non-volatile reactive carrier.

% Copolymer 10

Carriers Monomer 40

Oligomer 40

Photoinitiator 5

Synergist 5

As before, the formulations were coated on to paper substrates to a thickness of 12 microns. The coatings were then cured using a Primarc UV curing unit with a high-pressure mercury lamp light source at a power of 80 W.cm ². The coating was cured by a single pass at a rate of 34 metres per minute.

In addition to solvent resistance and gloss, the coatings were also tested for shrinkage on curing. Shrinkage on curing was determined by the degree of curl of the paper substrate after the coating had been cured. Zero shrinkage indicates that the substrate remained flat whereas a shrinkage of 100 indicates that the substrate curled up upon itself to the extent that the opposing edges touched. The results of the tests were as follows: Copolymer Solvent Resistance Gloss Shrinkage

2 115 88.1 0.03

4 46 93.9 0

None >120 94.0 0.26

This illustrates that the copolymer of the present invention has good solvent resistance, gloss and shrinkage properties. Example 4

Example 3 was repeated except that the oligomeric acrylated-epoxy resin was replaced by an oligomeric acrylated-polyester resin (EB 81 from UCB). The coating was cured at a rate of 5 metres per minute using a single pass. The results of the tests were as follows:

Copolymer Solvent Resistance Gloss Shrinkage

2 49 89.0 0

4 (Comparative) 25 89.8 55

None 95 90.9 100 This illustrates that the copolymer of the present invention has good solvent resistance, gloss and shrinkage properties. Example 5

The following formulations (expressed as weight percent) were prepared using the Copolymers 3 and 5 of Example 1 and all the other ingredients of Example 3: %

Copolymer 15

Carriers

Monomer 35

Oligomer 40 Photoinitiator 5

Synergist 5

The formulations were then coated on to paper substrates to a thickness of 12 microns. The coatings were then cured using a Primarc UV curing unit with a high-pressure mercury lamp light source at a power of 80 W.cm². The maximum cure rate, for a single pass curing, that could be tolerated to obtain a non-tacky coating is illustrated below

Copolymer Solvent Resistance Maximum Cure Rate (metres per minute)

3 60 52 5 (Comparative) 38 43

Example 6

Example 5 was repeated where various Type I initiators were used in combination with Type benzophenone as follows

%

Copolymer 15

Carriers

Monomer 41

Oligomer 35

Photoinitiator

Benzophenone 3

Synergist 3

Photoinitiator

(Type I) 3

The maximum cure rates (metres per minute) for various Type I photoinitiators were Photoinitiator No Copolymer Copolymer 3 Copolymer 5

(Type I)

Irgacure 184 43 52 48

Lucirin TPO 28 43 28

Darocur 1173 25 43 32

where Irgacure 184 is 1-hydroxycyclohexylphenylketone, Darocur 1173 is

2-hydroxy-2-methyl-1-phenyl-propan-1-one both supplied by Ciba-Geigy. Lucirin TPO is 2,4,6-trimethylbenzoyldiphenylphosphine oxide supplied by BASF. Therefore the cure rate was found to be dependant on the Type I photoinitiator.

Claims

1. A curable composition which is substantially free of volatile solvent comprising (a) a functionalised copolymer of general formula (I)

2. A curable composition as claimed in claim 1 wherein the at least one (meth)acrylate monomeric species is at least one of C, to C₂₀ alkyl (meth)acrylates.

3. A curable composition as claimed in claim 2 wherein the at least one (meth)acrylate monomeric species is a combination of methyl methacrylate with one or more of n-butyl methacrylate and ethyl methacrylate.

4. A curable composition as claimed in any one of claims 1 to 3 wherein the monomeric species which can provide residue B is selected from a dialkyl aminoalkyl acrylamide, a dialkyl aminoalkyl methacryiamide, a dialkylamino styrene, a dialkylaminoalkyl vinyl ether, a dialkyl aminoalkyl acrylates or a dialkyl aminoalkyl methacrylate.

5. A curable composition as claimed in claim 4 wherein the monomeric species which can provide residue B is selected from dimethyl aminoethyl acrylate, diethyl aminoethyl 5 acrylate, dimethyl aminopropyl acrylate, diethyl aminopropyl acrylate, dimethyl aminoethyl methacrylate, diethyl aminoethyl methacrylate, dimethyl aminopropyl methacrylate or diethyl aminopropyl methacrylate.

6. A curable composition as claimed in any one of claims 1 to 5 wherein the parameters n and p are chosen such that the glass transition temperature of the copolymer is θ from 20 to 120°C.

7. A curable composition as claimed in any one of claims 1 to 6 wherein the non-volatile reactive carrier is selected from a polyfunctional acrylate, a polyfunctional methacrylate or a reactive monomeric species.

8. A curable composition as claimed in claim 7 wherein the non-volatile reactive carrier is selected from hexanediol diacrylate, tripropylene glycol diacrylate and trimethylol propane triacrylate, butanediol diacrylate, neopentyl glycol diacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate, dipropylene glycol diacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate or their methacrylate analogues or acrylated-epoxy, 5 acrylated-polyester or acrylated-urethane resins having molecular weights (number average) of from 500 to 2000.

9. A curable composition as claimed in any one of claims 1 to 8 wherein the copolymer (I) is up to 70% by weight of total weight of copolymer (I) and non-volatile reactive carrier.

10. A curable composition as claimed in any one of claims 1 to 9 containing o photoinitiator.

11. A coating formed by curing a UV curable composition which is substantially free of volatile solvent comprising

(a) a functionalised copolymer of general formula (I)

X¹ - (A)_n - (B)_p - X² (I) 5 wherein A is a residue of at least one (meth)acryiate monomeric species, B is a residue of at least one monomeric species copolymerisable with A and which provides at least one amino functionalised pendant group, X¹ and X² are terminal groups which may be the same or different, n and p are both at least one and are chosen such that the copolymer has a glass transition temperature of at least 10 °C; 0 (b) a non-volatile reactive carrier which is capable of reacting with the functionalised copolymer and through which the copolymer remains solvated within the curable composition; and

(c) a photoinitiator.