Process for the production of strongly adherent...

Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate

Reexamination Certificate

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C427S535000, C427S536000, C427S517000, C427S407100

Reexamination Certificate

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06733847

ABSTRACT:

The invention relates to a process for the production of strongly adhering free-radical-curable coatings on inorganic or organic substrates by the deposition of electron- or H-donors having at least one ethylenically unsaturated group on a plasma-activated substrate. The invention relates also to the use of electron- or H-donors, for example amines, thioethers or thiols, having at least one ethylenically unsaturated group in the production of such layers and to the strongly adherent coatings themselves.
The adhesion of finishes, coatings or metallic layers on inorganic or organic substrates, especially on non-polar substrates such as polyethylene, polypropylene or fluorine-containing polyolefins, such as those known by the trade name Teflon®, is frequently inadequate, so that additional coating steps have to be taken in order to achieve satisfactory results. One possibility lies in first applying special priming coatings, so-called primers, and only then applying the desired coating thereto.
A further possibility lies in subjecting the substrates to be coated to a plasma or corona treatment and then coating them, it being possible for a grafting process with e.g. acrylate monomers to be carried out between those two operations (J. Polym. Sci., Part A: Polym. Chem. 31, 1307-1314 (1993)).
International Patent Application WO 00/24527 likewise describes a process for the production of strongly adherent coatings, in which in a first step the surface is pretreated, for example with a low-temperature plasma, and then a photoinitiator containing at least one ethylenically unsaturated bond is grafted onto the surface. The surface so pretreated is then provided with a UV-curable layer.
The production of low-temperature plasmas and the plasma-assisted deposition of thin organic or inorganic layers have been known for some time and are described, for example, by A. T. Bell, “Fundamentals of Plasma Chemistry” in “Technology and Application of Plasma Chemistry”, published by J. R. Holahan and A. T. Bell, Wiley, New York (1974) or by H. Suhr, Plasma Chem. Plasma Process 3(1), 1, (1983).
It is also known that, for example, plastics surfaces can be subjected to a plasma treatment and as a result the subsequent finish exhibits improved adhesion to the plastics substrate. This is described by H. J. Jacobasch et al. in Farbe+Lack 99(7), 602-607 (1993) for low-temperature plasmas under vacuum conditions and by J. Friedrich et al. in Surf. Coat. Technol. 59, 371-6(1993) for plasmas ranging from in vacuo up to normal pressure conditions, the low-temperature plasma changing into a corona discharge over that range.
It has now been found that coatings having especially good adhesion can be achieved with free-radical-curable compositions by grafting onto the substrate to be coated at least one electron- or H-donor, e.g. a primary, secondary or tertiary amine, a thiol or a thioether or a mixture thereof, each having at least one ethylenically unsaturated group, then providing the substrate so grafted with a free-radical-curable composition which additionally comprises either a photoinitiator or a thermally activatable initiator, e.g. an organic peroxide, and curing that composition under the action of electromagnetic radiation (UV, visible or IR light) or the application of heat. The resulting coatings exhibit surprisingly good adhesion which does not suffer any appreciable impairment even after several days' storage in sunlight.
The process is simple to carry out and allows a high throughput per unit of time, since lengthy drying steps and slow crosslinking reactions are not required. The process is especially well suited to workpieces that are composed of different plastics and/or metals or types of glass and that without the pretreatment would therefore exhibit different degrees of adhesion to the different components or that in the case of a conventional primer treatment exhibit different affinities for the primer.
The improvement in adhesion can be extraordinarily great, since a covalent bond to the substrate can be formed. The substrate suffers virtually no temperature-related damage, since the process can be carried out in such a manner that the temperatures that occur are at most briefly slightly higher than room temperature. Since the first process step is preferably carried out in vacuo, even substrates on which extraordinarily high purity demands are made can be so treated. Examples thereof can be found in the field of electronics, medicine and food packaging.
In principle, areas that are structured imagewise can also be rendered strongly adherent by the process, for example by pretreating the substrate with the plasma through a mask and thus modifying only certain areas. Structuring can also be carried out subsequently, however, if the free-radical-curable composition contains a photoinitiator and the irradiation is carried out e.g. through a mask.
The invention relates to a process for the production of strongly adherent coatings on an inorganic or organic substrate, in which process in a first step
a) the inorganic or organic substrate is subjected to the action of a low-temperature plasma discharge, a corona discharge, high-energy UV radiation or electron radiation, the radiation or discharge is then discontinued, in a further step
b) at least one electron- or H-donor, each containing at least one ethylenically unsaturated group, is applied to the inorganic or organic substrate in vacuo or at normal pressure and reacted with the free radicals formed there, and
c1) the substrate so precoated with coinitiator is coated with a composition comprising at least one ethylenically unsaturated monomer or oligomer and a photoinitiator, and the coating is cured by means of electromagnetic and/or ionising radiation; or
c2) the substrate so precoated with coinitiator is coated with a composition comprising at least one ethylenically unsaturated monomer or oligomer and one or more thermally activatable initiators, and the coating is cured thermally.
Possible ways of obtaining plasmas under vacuum conditions have been described frequently in the literature. The electrical energy can be coupled in by inductive or capacitive means. It may be direct current or alternating current; the frequency of the alternating current may vary from a few kHz up into the MHz range. A power supply in the microwave range (GHz) is also possible.
The principles of plasma production and maintenance are described, for example, in the review articles of A. T. Bell and H. Suhr mentioned above.
As primary plasma gases it is possible to use, for example, He, argon, xenon, N
2
, O
2
, CO
2
, NO, steam or air.
The process according to the invention is not sensitive per se in respect of the coupling-in of the electrical energy.
The process can be carried out batchwise, for example in a rotating drum, or continuously in the case of films, fibres or woven fabrics. Those processes are known and are described in the prior art.
The process can also be carried out under corona discharge conditions. Corona discharges are produced under normal pressure conditions, the ionised gas used most frequently being air. In principle, however, other gases are also possible, in which case the operation should be carried out in a closed system in order to exclude atmospheric air. Another possibility is to use air as ionisation gas in corona discharges, so that the operation can be carried out in an apparatus open to the outside and, for example, a film can be drawn through continuously between the discharge electrodes. Such process arrangements are known and are described, for example, in J. Adhesion Sci. Technol. Vol 7, No. 10, 1091-1127, (1993).
When a corona discharge in an open apparatus is used, it is preferable to carry out the operation with the exclusion of oxygen, which can be achieved by a sufficiently large flow of inert gas.
The process can also be carried out using high-energy electromagnetic radiation for the treatment of substrates in vacuo or with the exclusion of oxygen. High-energy electromagnetic radiation that comes into co

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