[0001] There are many instances where it is desirable to apply a protective layer over a
substrate formed of a polymeric material. If the protective layer tends to be incompatible
with the substrate it is common practice to apply an organic liquid to the surface
of the substrate, before the protective layer, so as to dissolve or swell the substrate
with the intention of improving adhesion of the protective layer to it. The dissolution
or swelling that occurs is on a gross scale and is generally visible to the naked
eye.
[0002] Whilst this'gives very satisfactory results with many combinations of substrate and
protective layer it is unsatisfactory with some, especially when the protective layer
is very incompatible with the substrate and exhibits a marked tendency to blister
or peel from the substrate. Thus with such combinations it appears that the organic
liquid or the residues of swollen or dissolved polymer between the protective layer
and the remainder of the substrate provide a region of weakness between the main part
of the substrate and the protective layer.
[0003] It is also known to chemically etch surfaces of polymeric substrates so as to improve
the adhesion of subsequently applied protective layers. However it is very difficult
to obtain the right amount of chemical etching, in that with many polymeric compositions
either the degree of etching is so small as to make no difference to adhesion or is
so high that a weak layer of degradation products is formed between the polymeric
substrate and the subsequently applied protective layer. If chemical etching is conducted
on a surface that has previously been swollen or swelled on a macro scale then generally
the etching will result in the formation of such a thick layer of degradation products
that the adhesion between the subsequently applied protective layer and the substrate
will be very poor.
[0004] The need to secure intimate adhesion between the protective layer and the substrate
is particularly serious when the protective layer is a layer of metal or other hard
wearing material and is being applied over an engraved gravure or other intaglio printing
surface. It is often desirable for such printing surfaces to carry such a protective
layer but it is essential that the layer should conform accurately and bond firmly
to the polymeric surface and it.is also very important that it should not blister
away from the surface during use.
[0005] In the invention a surface layer of polymeric material is prepared to receive an
adherent coating by a process comprising penetrating the surface layer only with a
first reactant and then with a second reactant and causing or allowing the second
reactant to react with the first reactant within the surface layer to form pits that
extend from the surface into the surface layer substantially without chemical degradation
of the polymeric material between the pits.
[0006] Since the first and second reactants react to form pits that extend to the surface
it is possible in the invention to avoid the disadvantage of trapping within the surface
layer substantial quantities of a swelling or softening agent. Since the reaction
occurs primarily with the first reactant, with the polymeric material between reaction
sites being left substantially undegraded, it is possible in the invention to avoid
the formation of a surface layer contaminated with large amounts of degraded polymeric
material.
[0007] The first reactant preferably comprises an organic solvent and penetrates the surface
layer during the process substantially without swelling or causing other visible change
in the surface appearance when viewed under a magnification of up to 100. Thus preferably
the organic solvent does not dissolve or swell the structure of the polymeric material
but instead merely penetrates the surface layer while causing substantially no visible
change in the surface appearance. Thus it is generally desirable to avoid the use
of solvents that one would normally consider to be satisfactory for swelling or softening
the polymeric material. For instance when the polymeric material is an epoxy it would
normally be expected that solvents such as tetrahydrofuran, dioxan, aliphatic ketones
and acetates would be useful for swelling the surface, but in fact it is desirable
to avoid such solvents in the method of the invention. If the organic solvent being
used has too powerful an effect,-and thus either swells the surface or penetrates
deeper than is desired, it is possible to moderate this by inclusion in the reactant
of an inert cosolvent or by emulsifying it in water or other inert liquid, or by both
emulsification and the use of a cosolvent. This is described in more detail below.
[0008] The desired depth of the surface layer will depend partly upon the nature of the
adherent coating. Generally it is not more than 5 microns and for many purposes is
less than 1 micron. However it should normally be at least 0.05 microns in thickness,
preferably 0.1 to 0.5 microns. The first reactant preferably penetrates the surface
to the depth desired for the surface layer.
[0009] The organic solvent in the first reactant will be chosen having regard to the nature
of the polymeric material and having regard to the second reactant that is to be used.
When, as is preferred, the second reactant is an oxidising agent it is desirable to
use as the first reactant an organic liquid that will react with such oxidising agent
with a redox reaction that forms gaseous products. Suitable reactants include heterocyclic
compounds and in particular derivatives of pyridine or pyrrole. Preferred reactants
are keto derivatives of pyrrole, including keto derivatives of partially or fully
saturated pyrroles, that is to say pyrrolidones and pyrrolinones. Preferred reactants
comprise N-methyl-3-pyrrolin-2-one and N-methyl-2-pyrrolidone. Very satisfactory results
can be obtained using Russian White turpentine.
[0010] Some solvents used as part or all of the first reactant may have a swelling or dissolving
effect on the polymeric material that is too powerful or may attack apparatus in which
the process is to be conducted or may have an unpleasant odour. For instance when
using the preferred solvents identified above they can give handling difficulties
due to their odour and the risk of them attacking the apparatus. It is therefore desirable
to use the solvent diluted with an organic diluent or emulsified in water or both.
Any inert compatible organic diluent can be used but alkyl ethers of diethylene glycol
or ethylene glycol are preferred, especially the mono-n-butyl ether of ethylene glycol.
[0011] The proportion of the chosen heterocyclic liquid to any other organic liquid is generally
from 0.5:1 to 5:1, most usually about 1:1 to 3:1 (parts by volume). Preferably the
organic liquid or liquids are used in the form of an emulsion in water. The concentration
of organic liquid or liquids emulsified in water may be, for instance, from 40 to
70% and the emulsion may be stabilised by an appropriate emulsifying agent, such as
ethoxylated castor oil.
[0012] After application of the first reactant at a temperature and for a duration such
that the desired depth of penetration is achieved, excess of the first reactant is
generally then removed, for instance by rinsing, and the second reactant may then
be applied.
[0013] The second reactant generally comprises a chemical etchant for the polymeric material
and thus is capable, when used under appropriate conditions (e.g. temperature and
duration), of etching and degrading the polymeric material even in the absence of
first reactant. Preferably it is applied under conditions such that if it were used
alone it would cause substantially no visible change in the surface appearance when
viewed under a magnification of up to 100. However under the actual conditions of
use as a result of the presence of the first reactant within the surface layer the
etchant reacts preferentially with that reactant, and possibly also with the polymeric
material in localised positions containing first reactant, to form pits in the surface
layer while leaving the polymeric material between the pits substantially unreacted
and undegraded. The etchant is preferably one that reacts relatively violently with
the first reactant when contacted in bulk. It may be an oxidising agent that will
react with the first reactant with a redox reaction that forms gaseous products. The
oxidising agent or other etchant is usually an oxidising acid or mixture of acids
and may be based on,. for instance, nitric acid, sulphuric acid, phosphoric acid or
chromic acid. Preferably however it comprises chromic acid both because of the desirable
reactions that occur between chromic acid and the preferred first reactants and because
of the strong penetrating properties of chromic acid. The chromic acid is preferably
introduced as an aqueous solution of from 200 to 600 g/1 generally 350 to 500 g/l.
The second reactant is normally an aqueous solution. It may include surfactant.
[0014] The penetration of the surface layer with the first reactant is achieved by applying
the first reactant by immersion, spray, brush or any other convenient technique for
an appropriate duration and temperature.
[0015] The temperature is generally from ambient to 80°C. Generally excess first reactant
is applied to the surface and is left in contact with the surface for from 0.5 to
10 minutes and excess solvent is then removed and the surface rinsed with hot water.
Alternatively contact may be achieved by submerging the surface for a shorter period,
e.g. up to 1 minute, in the first reactant. After removal of the excess solvent the
surface may feel substantially dry to the touch.
[0016] Penetration of the second reactant into the surface layer followed by reaction of
the reactants may be achieved by application of the second reactant by any convenient
application technique, such as those discussed above, and maintaining the reactant
in contact with the surface for from 2 to 20 minutes at from 40 to 80°C although shorter
times are satisfactory if contact is by prolonged submersion, e.g. for up to 2 minutes.
The surface may then be rinsed with water and dried.
[0017] The adherent coating may be a metal coating, for instance of copper, nickel or chromium,
and is preferably applied in an electroless manner. Thus plating may be conducted
by depositing a colloidal palladium based solution, preferably after altering the
charge on the surface by subjecting it to a cationic surfactant solution, followed
by electroless deposition of copper, nickel, or chromium in conventional manner. Suitable
plating methods are described in British Patent Specification No. 1,524,717.
[0018] The process of the invention is preferably carried out on a gravure printing surface
that has been engraved to form the desired gravure cells, which may be discrete cells
or, when the surface is a cylinder, may be a continuous spiral and may be at least
15 microns deep. This engraving is preferably carried out using an electron, ion or
laser beam. Accordingly the polymeric material is preferably of a composition that,
when struck by an electron, ion or laser beam in an area, is converted to volatile
products and volatilises throughout the entire area while remaining as a rigid solid
in the zone adjacent the area where it volatilises, as described in our British Patent
Specification No. 2034636. Preferably it is of an epoxy material that is substantially
unfilled, preferably having a total filler content of below 201 by weight. Preferably
the epoxy or other polymeric material includes carbon black and will generally have
been mechanically polished, for example by diamond turning, before engraving so as
to give it non-print characteristics before being engraved, i.e. between the engraved
cells. Other polymeric materials that may be prepared in accordance with the invention
for subsequent coating with metal or other coatings include, for instance, polypropylene.
The first reactant for this may consist solely of one of the specified pyrrole derivatives
and then the second reactant may be chromic acid or a chromic acid-sulphuric acid
mixture.
[0019] The invention includes not only the described processes but also the novel solvent
compositions,and in particular aquoeus emulsions of a mixture of a pyrrole derivative
with cosolvent, as described above.
[0020] The following are examples of the invention.
Example 1
[0021] A cylindrical substrate is coated with an epoxy resin composition containing about
5% to 6% carbon and 1% molybdenum sulphide but free of substantially all other fillers
by a powder coating technique followed by fusion of the surface. It is then diamond
turned until it is smooth (even when viewed under 100 magnification) and then laser
engraved to form the desired gravure cells.
[0022] A mixture is formed of one part by volume n-butyl ether of ethylene glycol (n-butyl
Cellosolve, Cellosolve being a trade mark) and 2 parts by volume N-methyl-3-pyrrolin-2-one
and this mixture is emulsified in water to form a 60% emulsion of the solvent mixture
in water. 10% by weight of ethoxylated castor oil is included in the water as an emulsifying
agent.
[0023] Excess of this emulsion is then applied to the engraved epoxy resin surface layer
and contact is maintained between the emulsion and the surface for from 2 to 6 minutes.
The temperature of the surface throughout this time is between 30 and 60°C. The appearance
of the surface is unchanged by this treatment.
[0024] The surface is then water rinsed and then has applied to it aqueous chromic acid
having a concentration of 400 to 480 g/1 and containing 5% to 10% by volume surfactant,
for 5 to 10 minutes at 55 to 70°C. The surface is then given a hot water rinse at
about 65
0C. The surface is highly pitted (as viewed under.a magnification of 100), with the
pits extending less than 0.5 microns deep.
[0025] The surface is then contacted for from 5 to 10 minutes at 40 to 60°C with a solution
of stannous chloride and 30% hydrochloric acid, in order to neutralise the surface.
It is then rinsed and charged transfer is effected by immersing the surface for 2
minutes at 30°C in a 3% by volume aqueous solution of a cationic surfactant.
[0026] The surface is then rinsed in water again and is conditioned by contact with a 10%
by volume solution of colloidal palladium in hydrochloric acid at 25 to 30°C for from
3 to 5 minutes followed by rinsing and reduction of the colloidal palladium by treatment
with a 10% by volume solution of hydrochloric acid containing a small amount of stannous
chloride for 3 minutes at 40 to 50°C.
[0027] The surface is then again rinsed with water and is then subjected to electroless
nickel deposition using an autocatalytic proprietory solution that deposits on the
cylinder at 80 to 90°C at a rate of approximately 12 to 14 microns per hour.
Example 2
[0028] The process of Example 1 is repeated except that the N-methyl-3-pyrrolin-2-one is
replaced by an equal volume of N-methyl-2-pyrrolidone. Similar results are achieved.
Example 3
[0029] The process of Example 1'is repeated except that the N-methyl-3-pyrrolin-2-one is
replaced with an equal volume of Russian White terpentine.
Example 4
[0030] A flat substrate formed of the same epoxy composition as in example 1 is immersed
in the same solvent composition for 1 minute at 42°C, while another piece is immersed
in the chromic acid solution at 68°C for 30 seconds. Both pieces are washed and dried
and examined under 100 magnification. Neither treatment results in any noticeable
change in the surface appearance.
[0031] The piece that has been solvent treated is then immersed in the same chromic acid
solution at 68°C for 30 seconds and then rinsed. When viewed under 100 magnification
the surface appears highly pitted. The pits extend to a depth of less than 1 micron.
The pitted surface can then be neutralised and plated as described in Example 1.
1. A process in which a surface layer of a polymeric material is prepared to receive
an adherent coating by application of a reactant, characterised in that the process
comprises penetrating the surface layer only with a first reactant and then with a
second reactant and causing or allowing the second reactant to react with the first
reactant within the surface layer to form pits that extend from the surface into the
surface layer substantially without chemical degradation of the polymeric material
between the pits.
2. A process according to claim 1 characterised in that the first reactant comprises
an organic solvent and penetrates the surface layer substanitally without swelling
or causing other visible changes in the surface appearance when viewed under a magnification
of up to 100.
3. A process according to claim 2 characterised in that the first reactant comprises
a keto derivative of a pyrrole that may be partially or fully saturated.
4. A process according to claim 3 characterised in that the first reactant comprises
N-methyl-2-pyrrolidone or N-methyl-3-pyrrolin-2-one.
5. A process according to claim 3 characterised in that the first reactant comprises
Russian White turpentine.
6. A process according to any preceding claim characterised in that the second reactant
comprises a chemical etchant that is applied at a temperature and duration such that,
if used without the first reactant, it causes substantially no visible change in the
surface appearance when viewed under a magnification of up to 100.
7. A process according to any preceding claim. characterised in that the second reactant
comprises an oxidising agent that reacts with the first solvent with a redox reaction
to form gaseous products.
8. A process according to any preceding claim characterised in that the second. reactant
comprises chromic acid.
9. A process according to any preceding claim characterised in that the surface layer
is less than 5 microns thick.
10. A process according to claim. 9 characterised in that the surface layer is from
0.05 to 0.5 microns thick.
11. A process according to any preceding claim characterised in that the penetration
of the first reactant is controlled by providing it in the form of a solution in an
inert cosolvent or in an emulsion in water or both.
12. A process according to claim 11 characterised in that the first reactant is provided
as a 40 to 70% by volume emulsion in water of a mixture of 0.5 to 5 parts by volume
of a keto derivative of a pyrrole that may be partially or fully saturated or Russian
White turpentine with one part by volume inert cosolvent. -
13. A process according to claim 11 or claim 12 characterised in that the cosolvent
is an alkyl ether of ethylene glycol or of diethylene glycol.
14. A process according to any preceding claim characterised in that the polymeric
material is an epoxy resin containing less than 20% filler.
15. A process according to any preceding claim characterised in that the surface layer
is the surface layer of a printing member that is engraved with gravure cells and
that has substantially non-print characteristics between the cells.
16. A process according to any preceding claim characterised in that an adherent coating
of metal is subsequently applied to the pitted surface by electroless metal deposition.
17. A composition suitable for use in the process of claim 12 and comprising a reactant
that is a keto derivative of a pyrrole that may be partially or fully saturated or
is Russian White turpentine characterised in that the reactant is provided as a 40
to 70% by volume emulsion in water of a mixture of 0.5 to 5 parts by volume of the
reactant with one part by volume of an inert cosolvent.
18. A composition according to claim 17 characterised in that the cosolvent is an
alkyl ether of ethylene glycol or of diethylene glycol.