In order to obtain a sufficiently high diffusion rate the first and second mixtures should not have too high a viscosity. The components of the first and second mixtures should not have too high molecular weight. In one embodiment, the viscosity of the first mixture is 10 Pas or lower at the temperature where diffusive mixing occurs. In one embodiment the components of the second mixture should not have too high molecular weight. In one embodiment, the viscosity of the second mixture is 10 Pas or lower at the temperature where diffusive mixing occurs.

In one embodiment, the layer (B) of the first mixture is thinner than the layer (D) of the second layer and in an alternative embodiment the layer (B) of the first mixture is thicker than the layer (D) of the second layer.

The components of the first mixture should in one embodiment be compatible with the components of the second mixture to allow efficient diffusion. Monomers commonly used in UV curing acrylate formulations, including but not limited to HDDA (Hexane diol diacrylate), TMPTA (trimethylolpropane triacrylate), GPTA (propoxylated glycerol triacrylate), TEGDA Triethyleneglycol diacrylate) will in general be good choices since they will be miscible with oligomers and monomers in the subsequent coating. Low viscosity oligomers such as multifunctional acrylates based on polyols are also feasible. Alternatively, the solvating power of the subsequent layer is increased by dilution with a good solvent such as acetone, isopropyl alcohol etc, which facilitates diffusion of the unreacted primer components and evaporates before the cure of the subsequent layer to afford good mechanical properties after cure. Alternatively, heating is used to increase the diffusive mixing rate.

As a final step the applied second mixture is cured. This creates a top coat. Alternatively, further coatings may be applied to form the finished coating.

In one embodiment, the substrate (A) comprises at least one selected from the group consisting of melamine formaldehyde resin, urea formaldehyde resin, poly paraphenylene terephthalamide, polyamide, polyethylene, PMMA, polycarbonate, cyclic olefin copolymers, polyester, polyurethane, acrylonitrile butadiene styrene copolymer (ABS), PVC, polybutadiene, blends of olefins sourced from recycled plastics, a compound comprising at least one abstractable hydrogen atom, and polypropene. As long as there are chemical groups in the substrate (A) and chemical groups on molecules in the first mixture that can react and form a covalent bond, any substrate (A) can be used.

In one embodiment the substrate comprises a polyolefin and at least a part of a surface of the substrate (A) before step b) is treated in the presence of oxygen with at least one method selected from plasma treatment, corona treatment and flame treatment, to obtain a peroxy radical on the surface, wherein the first mixture applied in step b) comprises at least one compound comprising an abstractable hydrogen, so that the peroxy radical on the surface reacts with the at least one compound comprising an abstractable hydrogen to form a peroxide on the surface, wherein step c) has to effect to decompose at least a part of the peroxide on the surface to obtain a surface tethered alkoxy radical and a free hydroxy radical, whereby the surface tethered alkoxy radical reacts with the second type of chemical groups to form the covalent bond (C) by reaction of the surface tethered alkoxy radical and the second type of chemical groups, whereby the layer (B) of the first mixture becomes a layer of a reacted first mixture which excludes the second type of chemical groups which are covalently bound. Without wishing to be bound by any particular scientific theory the inventors believe that as plasma/corona/flame impact the surface, peroxy radicals are quickly formed. In the presence of compounds capable of donating a hydrogen, peroxides are formed on the surface. Peroxides are sensitive to UV light and will decompose into a surface tethered alkoxy radical and a free hydroxy radical when a light quantum with sufficient energy is absorbed by the peroxide. The surface tethered alkoxy radical is not sensitive to atmospheric oxygen and will combine via a radical mediated step with an acrylate, methacrylate, maleate/fumarate, vinyl ether and other reactive groups known to react via peroxide initiation. In the case of a monomer/oligomer with a functionality above 1 the effect is that a reactive group on the monomer is reacted with the surface and the remaining reactive groups are tethered to the surface via the monomer/oligomer backbone. The thus formed surface with tethered reactive groups is very useful for improved adhesion to subsequent layers that have functional groups suitable for reaction with the tethered functional group.

In one embodiment, the first type of chemical groups is a secondary amine and wherein the second type of chemical groups is a carbon-carbon double bond with an electron withdrawing group on at least one side of the carbon-carbon double bond. In one embodiment, wherein the electron withdrawing group is at least one selected from the group consisting of a triflyl group, a trihalide group, a cyano group, a sulfonate group, a nitro group, an ammonium group, an aldehyde group, a keto group, a carboxylic group, an acyl chloride group, an ester group, an amide group, and ether group, and a halide. In one embodiment, the compound comprising at least one carbon-carbon double bond is at least one selected from the group consisting of maleic anhydride and maleimide. In one embodiment, the compound comprising at least one carbon-carbon double bond is at least one selected from the group consisting of a maleate and a fumarate. In one embodiment, the compound comprising at least one carbon-carbon double bond is at least one selected from the group consisting of an acrylate and a methacrylate.

In an alternative embodiment, the first type of chemical groups is an OH-group and wherein the second type of chemical groups is a silane (Si_nH_2n+2). The silane can then react with OH-groups on the surface.

In another alternative embodiment, the first type of chemical groups is an abstractable hydrogen and wherein the second type of chemical groups is a group with the ability to abstract a hydrogen. An abstractable hydrogen is able to participate in a chemical reaction in which a hydrogen atom is abstracted from a substrate (A). The group with the ability to abstract a hydrogen is in one embodiment a radical species itself and may be initiated by UV-radiation. In another embodiment, the photoinitiator is able to form a long lived triplet state capable of abstracting a hydrogen atom. Thus a suitable UV initiator may be used to initiate such a reaction.

In one embodiment, the reaction in step c) is initiated with actinic radiation.

In one embodiment, the substrate (A) is an olefin, where examples of suitable olefins include but are not limited to polypropylene and polyethylene. The primer, i.e. the first mixture in such an embodiment comprises a compound capable of abstracting a hydrogen from the substrate (A) causing an unpaired electron (radical) to be located on the olefinic chain. The radical subsequently reacts or is paired with components in the primer, for example a (meth)acrylate or a persistent or latent source with a radical character such as atmospheric oxygen resulting in a covalent bond between the substrate (A) and primer components either directly or in subsequent reactions potentially initiated by components in the subsequently applied layer.

In one embodiment, the viscosity of the first mixture is 10 Pas or lower measured at the temperature where diffusive mixing occurs. In one embodiment, the viscosity of the second mixture is 10 Pas or lower measured at the temperature where diffusive mixing occurs. The lower viscosity facilitates the diffusive mixing.

In one embodiment, the first mixture and the second mixture have different viscosities.

In one embodiment, the first mixture and the second mixture are the same and wherein one of the applied layers is thinner than the other. In such an embodiment, oxygen in the ambient air may be utilized to inhibit an initiator in the thinner layer so that the reaction occurs to a desired extent.

In one embodiment at least one additional layer (G) is added, which additional layer (G) is able to form a solid coating. Thus several coating layers can be added to form a finished coated substrate.

In one embodiment at least one intermediate layer (H) is added after step c) and before step d). This intermediate layer can for instance be a solvent or another compound or mixture. Subsequently in the process the intermediate layer (H) mixes with the first layer and the second layer so as to form a resulting mixture (E), which can be cured.

In one embodiment each molecule of the compound in the first mixture comprises at least two chemical groups of the second type. Thus in one embodiment the compound in the first mixture is difunctional with respect to the second type of chemical groups.

In one embodiment only a fraction of the chemical groups of the second type are reacted in step c). The bulk of the layer (B) of the first mixture is in one embodiment essentially unreacted during step c).

In a second aspect there is provided a substrate (A) coated according to the method as outlined above.

There is the possibility to form patterns on the substrate (A) by only applying the mixtures on parts of the surface. In the case of irradiation curing, the possibility to form patterns on the substrate (A) can also be achieved by only irradiating parts of the surface, for example by UV photo patterning.

Other features and uses of the invention and their associated advantages will be evident to a person skilled in the art upon reading the description and the examples.

It is to be understood that this invention is not limited to the particular embodiments shown here. The embodiments are provided for illustrative purposes and are not intended to limit the scope of the invention since the scope of the present invention is limited only by the appended claims and equivalents thereof.

EXAMPLES
Example 1

One of the following primers were applied on a black melamine-formaldehyde substrate in a 6 μm thick layer with a wire rod. The primers were applied in a first mixture diluted 1:4 in acetone. After curing this gave about 1 g/m².

The primer was cured using UV in room temperature.

After curing of the primer a top-coat (BONA UE 1664) was applied. The top-coat was applied in an amount of 6 g/m².

For the different samples the X-hatch and Hamberger Kobel values were measured. Hamberger Industriewerke has developed a testing device called the “Hamberger Planer”, which can be used to conduct a “coin test” under defined conditions. A piece of metal with a coin-like edge is pushed across the covered surface at a pre-definable pressure. The test result will be the maximum force to be applied until arriving at the appearance of first white marks. It is indicated/expressed in Newton.

The x-hatch measured the coating adhesion according to ISO 2409. According to ISO 2409, the scale is 0-5 where “0” represents the best, and “5” represent the worst adhesion performance.

Before application the viscosity of the primer, i.e. the first mixture was measured.

The measurement results are shown in the table below.

Viscos-

ity
X-

Oligomer
Type
M.W.
mPas
hatch
Hamb.

Laromer ® PE56F
Polyester

20-40
“5”
<10N

(BASF)
acrylate

Laromer ® PE56F
Polyester

20-40
“5”
<10N

(BASF), with 30
acrylate

min duration

between top

application and

curing

EBECRYL ® 2221
Urethane
1200
21
“5”
<10N

(ALLNEX)
acrylate

CN2601 (ARKEMA)
Polyester

20-30
“5”
15N

acrylate

EBECRYL ® 4101
Urethane

7
“1”
30-35N

(ALLNEX)
acrylate

CN736 (ARKEMA)
Chlorinated

1.5
“0”
35N

Polyester

acrylate in

40%

TMPTA

UCECOAT ® 2501
Aliphatic

“0”
20N

(ALLNEX)
polyurethane

acrylate

dispersion

Laromer® PE56f is a liquid polyester modified acrylic resin intended for the formulation of energy curable coatings.

EBECRYL® is an Aromatic urethane acrylate with molecular weight Mn 1200.

CN2610 by Sartomer Arkema Group is a polyester acrylate oligomer. It is difunctional and intended for use in ultra violet and electron beam curing compositions.

EBECRYL® 4101 is an undiluted, unsaturated aliphatic urethane acrylate designed for coatings.

CN736 is a chlorinated polyester acrylate oligomer.

UCECOAT® 2501 is an Aliphatic polyurethane acrylate dispersion.

It can be concluded that for the primer (first mixture) with low viscosity the unreacted parts of the first mixture could diffuse into the top-coat and thereby avoid the weak layer. The more viscous primers (first mixtures) did not diffuse into the top-coat (second mixture) to the same extent and thereby created a weak layer of unreacted primer or a weak layer due to an unfavorable monomer/oligomer composition with respect to mechanical properties.