U.S. patent application number 10/594395 was filed with the patent office on 2007-09-27 for coated substrate.
Invention is credited to Jacob Leendert Haas De, Augustinus Eugene Henk Keijzer De, Josef Maria Johannes Mattheij, Rudolfus Antonius Theordorus Maria Van Benthem, Jacobus Adriaan Antonius Vermeulen.
Application Number | 20070224438 10/594395 |
Document ID | / |
Family ID | 34896061 |
Filed Date | 2007-09-27 |
United States Patent
Application |
20070224438 |
Kind Code |
A1 |
Van Benthem; Rudolfus Antonius
Theordorus Maria ; et al. |
September 27, 2007 |
Coated Substrate
Abstract
The invention relates to a method for coating a substrate,
comprising the steps of: a) Applying a layer comprising a
melamine-formaldehyde resin A to a substrate, whereby a coated
substrate is formed; b) Optionally treating the coated substrate
with IR- or NIR-radiation; c) Optionally applying an ink, dye
solution or pigment dispersion to the coated substrate; d)
Optionally applying a layer comprising a melamine-formaldehyde
resin B to the coated substrate from step b) or c); e) Placing the
coated substrate in a press; f) Optionally heating the coated
substrate in the press for a certain amount of time; g) Increasing
the pressure in the press and keeping the coated substrate under
pressure for a certain amount of time. The invention further
relates to a coated substrate as obtainable by the said method, and
its use in a post-forming process.
Inventors: |
Van Benthem; Rudolfus Antonius
Theordorus Maria; (Limbricht, NL) ; Haas De; Jacob
Leendert; (Sittard, NL) ; Mattheij; Josef Maria
Johannes; (Sittard, NL) ; Vermeulen; Jacobus Adriaan
Antonius; (Geleen, NL) ; Keijzer De; Augustinus
Eugene Henk; (Sittard, NL) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Family ID: |
34896061 |
Appl. No.: |
10/594395 |
Filed: |
April 8, 2005 |
PCT Filed: |
April 8, 2005 |
PCT NO: |
PCT/NL05/00272 |
371 Date: |
September 27, 2006 |
Current U.S.
Class: |
428/526 ;
427/595; 525/497 |
Current CPC
Class: |
B05D 3/12 20130101; Y10T
428/31949 20150401; B05D 2203/20 20130101; B05D 3/0263
20130101 |
Class at
Publication: |
428/526 ;
427/595; 525/497 |
International
Class: |
C23C 26/00 20060101
C23C026/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 4, 2004 |
EP |
040760837.0 |
Claims
1. Method for coating a substrate, comprising the steps of: a)
Applying a layer comprising a melamine-formaldehyde resin A to a
substrate, whereby a coated substrate is formed; b) Optionally
treating the coated substrate with IR- or NIR-radiation; c)
Optionally applying an ink, dye solution or pigment dispersion to
the coated substrate; d) Optionally applying a layer comprising a
melamine-formaldehycJe resin B to the coated substrate from step b)
or c); e) Placing the coated substrate in a press; f) Optionally
heating the coated substrate in the press for a certain amount of
time; g) Increasing the pressure in the press and keeping the
coated substrate under pressure for a certain amount of time.
2. Method according to claim 1 wherein the layer comprising
melamine-formaldehyde resin A and the optional layer comprising
melamine-formaldehyde resin B do not comprise a carrier.
3. Method according to claim 1, wherein melamine-formaldehyde resin
A comprises a powderous melamine-formaldehyde resin C and/or a
melamine-formaldehyde resin dispersion D.
4. Method according to claim 1, wherein melamine-formaldehyde resin
A comprises a melamine-formaldehyde resin dispersion D, whereby
said melamine-formaldehyde resin dispersion D comprises a
dispersant, said dispersant comprising a styrene-maleic anhydride
copolymer.
5. Coated substrate obtainable by the method according to claim
1.
6. Use of the coated substrate according to claim 5 in a
post-forming process.
7. Dispersion of liquid or solid melamine-formaldehyde resin
particles in a liquid, whereby the dispersion contains a
dispersant, characterised in that the said dispersant comprises a
styrene maleic anhydride copolymer.
Description
[0001] The invention relates to a method for coating a substrate.
The invention also relates to a coated substrate thus obtainable,
and to the use thereof in a post-forming process.
[0002] A method for coating a substrate is known from U.S. Pat. No.
3,730,828, which publication describes that a post-formable
substrate is coated with a decorative barrier sheet consisting of
ordinary a-cellulose wood fibre impregnated with a fully cured,
unplasticized melamine-formaldehyde resin with a narrow range for
the mole ratio between the formaldehyde and the melamine. A
laminate is prepared by assembling various layers of kraft paper
and a top, barrier, sheet with the unplasticized
melamine-formaldehyde resin. All these carrier layers are formed
into a laminate by placing the stacked layers in a laminating press
under high pressure and at high temperatures during a certain
amount of time. A disadvantage of the laminates thus obtained is
that they cannot be bent into complex shapes along two (or more)
mutually intersecting axes without breaking and/or cracking.
[0003] In WO 99/13000, a laminar prepreg is described which prepreg
comprises one or more layers of a laminar carrier that is
impregnated with an as yet uncured resin. The carrier is a laminar
porous polymer. The laminar porous polymer may be a non-woven
laminar polymer, a laminar open polymer foam or a microporous
membrane. The laminar porous polymer is impregnated with the resin
upon which drying is necessary at a temperature between 100 and
160.degree. C. The so obtained carrier is stacked after drying. The
prepreg can subsequently be processed into a shaped final product
by first deforming the prepreg and then curing the shaped
intermediate product at elevated temperature or by combining the
deformation and the curing step in one step. A disadvantage of the
prepreg according to WO 99/13000 is that the laminar prepreg
comprises at least two components that first must be obtained in
separate steps: a laminar porous polymer and an (at the prepreg
stage) uncured resin.
[0004] In WO00/53666 a prepreg is described that contains one or
more layers of a porous carrier sheet, which porous carrier sheet
has been impregnated with an as yet uncured resin, the carrier
containing a meltable polymer mixed with cellulose or regenerated
cellulose or mixtures there from. In WO 00/53667 a prepreg is
described that contains one or more layers of a porous carrier
sheet, which porous carrier sheet has been impregnated with an as
yet uncured resin, the carrier being a porous carrier on the basis
of wholly or partly regenerated cellulose. In WO 00/53688 a prepreg
is described that contains one or more layers of a porous carrier
sheet, which porous carrier sheet contains at least one fibrous
cellulose ester. A disadvantage of these systems that are based on
a porous polymer as a carrier and an (at the prepreg stage) uncured
resin is that at least two components are necessary in the initial
stages of the prepreg stage.
[0005] It is an object of the invention to overcome the
above-mentioned disadvantages and to make available a method to
coat a substrate with a melamine-formaldehyde resin which is less
complicated than the methods in the prior art and still results in
a coated substrate with good mechanical properties. Good mechanical
properties are for example required when the coated substrate is
afterwards subjected to post-forming.
[0006] The object is reached by a method that comprises the
following steps: [0007] a) Applying a layer comprising a
melamine-formaldehyde resin A to a substrate, whereby a coated
substrate is formed; [0008] b) Optionally treating the coated
substrate with IR- or NIR-radiation; [0009] c) Optionally applying
an ink, dye solution or pigment dispersion to the coated substrate;
[0010] d) Optionally applying a layer comprising a
melamine-formaldehyde resin B to the coated substrate from step b)
or c); [0011] e) Placing the coated substrate in a press; [0012] f)
Optionally heating the coated substrate in the press for a certain
amount of time; [0013] g) Increasing the pressure in the press and
keeping the coated substrate under pressure for a certain amount of
time.
[0014] In the method according to the invention, it is not
necessary to impregnate and dry a carrier such as a paper or other
cellulose-based material or a porous polymer. Indeed, in an
embodiment of the invention that the layer comprising
melamine-formaldehyde resin A does not comprise such a carrier.
Similarly, it is preferred that the optional layer comprising
melamine-formaldehyde resin B does not comprise a carrier. In such
cases the space and apparatus necessary for the process of coating
a substrate can be reduced compared to the prior art where
impregnating and drying of a carrier are done. Another advantage of
the method according to the invention is that separate stacking of
the various layers is not necessary anymore. A further advantage of
the method according to the present invention is that the coating
is applied directly onto the substrate that needs to be coated for
further use, as for example worktops cupboards and fronts of
kitchen cupboards. In this method a separate "prepreg stage" is
absent.
[0015] If the layer(s) on the coated substrate do not comprise a
carrier, it is a further advantage of the method according to the
invention that the coated substrate so obtainable can be subjected
to a post-forming step in which it is bent along two or more axes;
such a post-forming step is also called 3D-forming. Moreover, the
preferred absence of a carrier reduces the amount of waste as
generated during the preparation of the coated substrate.
[0016] In step a) of the method according to the invention, a layer
is applied to a substrate, whereby the layer comprises a
melamine-formaldehyde resin A. As a result of the application of
the layer, a coated substrate is formed. The melamine-formaldehyde
resin A is understood to be a resin with as main building blocks
melamine and formaldehyde. The melamine-formaldehyde resin A can
additionally contain other building blocks, for example urea;
flexibilizers as for example diethylene glycol and sugars; and
other compounds as known to the skilled person, such as a catalyst.
Furthermore, formaldehyde may according to the invention be partly
or wholly replaced by another suitable compound; examples of such a
compound are alkanol hemacetals such as methylglyoxylate methanol
hemiacetal (GMHA) and other hemiacetals as discosed on page 3 of WO
03/101973 A2. The melamine-formaldehyde resin A can comprise or
even consist essentially of a resin in powder form
(melamine-formaldehyde resin C), in dispersion form
(melamine-formaldehyde resin D), in liquid form
(melamine-formaldehyde resin E), or any suitable combination of
these. In a preferred embodiment, the melamine-formaldehyde resin A
comprises at least 40, 50, 60, 70, or even 80, 90 or essentially
100 wt. % of powderous melamine-formaldehyde resin C and/or
melamine-formaldehyde resin dispersion D. This has the advantage
that the risk that the resin will migrate into the substrate is
reduced. Preferably, no carrier is used; as less or even no drying
of an impregnated carrier at increased temperatures is necessary in
the present invention the method is advantageous from an economical
and ecological point of view. The weight ratio between powderous
melamine-formaldehyde resin C and melamine-formaldehyde resin
dispersion D--as expressed in weight percentages relative to each
other--may vary between a wide range, for example between 1%:99%
and 99%:1%, more preferably between 10%:90% and 90%:10%, between
25%:75% and 75%:25%, or between 40%:60% and 60%:40%. More
preferably, the melamine-formaldehyde resin A consists essentially
of powderous melamine-formaldehyde resin C. This has the advantage
that a pattern as brought on via an ink, dye solution of pigment
dispersion--as may be done in optional step c) to be discussed
hereafter--will remain intact to the greatest possible extent. In
this preferred embodiment of the method according to the invention,
a powderous melamine-formaldehyde resin C is applied onto the
surface of the substrate so as to form a layer. With powder is here
and hereinafter meant a solid consisting of small particles
generally with a particle size smaller than 250 .mu.m, preferably
below 100 .mu.m. With very large particles an even distribution
over the surface of the substrate is difficult, further the
resolution of an applied pattern that is formed by the ink, dye
solution or pigment dispersion in optional step c) is worse than
when smaller particles are used. The thickness of the layer is not
particularly critical and can be chosen between wide ranges, for
example between 20 and 500 .mu.m. A preferred layer has a thickness
between 50 and 250 .mu.m. A balance should be found between a thick
layer that is advantageous for hiding imperfections in the
substrate surface, and a thin layer that is more advantageous for
post-forming. It is known to the man skilled in the art how powders
can be obtained from melamine-formaldehyde resins. Reference can
for example be made to "Kunststoff Handbuch, 10-Duroplaste" by W.
Becker, D. Braun, 1988 Carl Hanser Verlag; more specifically to the
chapter "Melaminharze", page 41 and further. The powderous
melamine-formaldehyde resin C that may be used in step a) should
have a glass transition temperature (T.sub.g) high enough to be
stable at room temperature as a powder for an extended period of
time. In case the T.sub.g is not high enough the resin particles
will coagulate and the powder will lose its form and stability.
Suitable values for the T.sub.g are at least 30.degree. C.,
preferably 40.degree. C., with more preference between 60 and
90.degree. C. The T.sub.g should be below the temperature at which
steps f) and/or g) in the method are performed; as when the T.sub.g
is higher than the temperature in step f) the resin will not melt
and thus it will not flow. Generally the T.sub.g should therefore
be below 140.degree. C., preferably lower than 120.degree. C.
[0017] The melamine-formaldehyde resin A may optionally comprise a
melamine-formaldehyde resin dispersion D. Within the context of the
present invention, a melamine-formaldehyde resin dispersion D is
understood to mean a system where a liquid such as for example
water or an alcohol is the continuous phase, and whereby the
continuous phase comprises small non-dissolved particles comprising
a melamine-formaldehyde resin in uncured, partially cured or fully
cured state. The preparation of such dispersions is described in
a.o. WO 97/07152, EP 1 099 726 A2 and U.S. Pat. No. 6,245,853 B1.
The said particles may be themselves liquid or solid. Although the
size of the dispersed particles may vary within wide limits, it is
preferred that the weight-size lies between 0.1 .mu.m and 100
.mu.m, more preferably between 0.5 and 75 .mu.m, between 1 and 50
.mu.m, between 1.5 and 25 .mu.m, or even between 1.75 .mu.m and 15
.mu.m or between 2 .mu.m and 10 or 5 .mu.m. Such dispersions are as
such known; as is also known, it is often helpful or even necessary
to use a dispersant/protective colloid in order to achieve a stable
dispersion. Within the context of the present invention, the term
`stable` can have two meanings: either that the dispersed particles
do not separate out, coagulate or settle for at least 30 minutes
(or even at least 1 hour or preferably at least 24 hours) after
dispersion preparation; or that the dispersed particles can be
easily re-dispersed through agitation in case they have separated
out, coagulated or settled.
[0018] As a general recommendation, it is noted that it is
preferably avoided to have a degree of condensation in the
dispersed particles that is so high that it becomes impossible to
let these particles undergo a curing in the method according to the
invention in such a fashion that the particles fuse with other
particles. Similarly, it is noted that it is preferably avoided to
have a degree of curing in the dispersed particles that is so high
that it becomes impossible to let these particles undergo a further
curing in the method according to the invention in such a fashion
that the particles fuse with other particles.
[0019] As is disclosed in the cited disclosures, various suitable
dispersant for use in the preparation of a stable
melamine-formaldehyde resin dispersion D have been identified. In
general, dispersants are often derived from long-chained polymers
in the solid or liquid state, whereby the said polymers are
converted into a form in which they are soluble in the liquid that
is the continuous phase in the dispersion; such conversions are
known and may comprise a treatment at elevated temperature in the
said liquid, possibly aided by other compounds such as acids or
bases. It may hereby be beneficial if the said dispersant, once
prepared in liquid form, has a pH below 9, more preferable below 8
or even 7; this can have the advantage that the dispersant does
not--in the later to be discussed step g)--slow the resin
condensation and/or curing reactions down, and may even accelerate
them advantageously.
[0020] The known suitable dispersants, however, have as
disadvantage that the solids content of the dispersion is rather
low; in WO 97/07521, for example, the solids content is about 25%.
Solids content is defined as the cumulated weight percentage of all
compounds except water, as calculated from the raw materials as
used in preparation. It was found, surprisingly, that dispersants
containing styrene maleic anhydride copolymers or compounds derived
therefrom are particularly suitable. More in particular, aqueous
solutions of styrene maleic anhydride copolymers having a molecular
weight higher than 1,500 are preferred. The invention thus also
relates to a dispersion of liquid or solid melamine-formaldehyde
resin particles in a liquid, preferably water or an alcohol,
whereby the dispersion contains a dispersant, whereby the said
dispersant comprises a styrene maleic anhydride copolymer, said
copolymer preferably having been treated so as to render it soluble
in the continuous phase of the dispersion. Within the context of
the present invention, the term styrene maleic anhydride copolymer
can refer to the copolymer as such or to the copolymer after it has
been treated so as to render it soluble in the continuous phase of
the dispersion. An advantage of using the said copolymers is that
it is possible to obtain a stable melamine-formaldehyde resin
dispersion D having a high solids content, i.e. higher than 30% and
preferably between 35% and 65%. It is preferred that the
weight-averaged molecular weight (Mw) of the styrene maleic
anhydride copolymers is higher than 1,500, 3000, 10,000, 50,000 or
even 100,000; preferably, the said molecular weight Mw is at most
3,000,000 or 2,000,000, more preferably at most 1,000,000. The
molar ratio of styrene to maleic anhydride in the copolymer may
according to the invention vary between wide limits, preferably
between 1:0.1 and 1:1, more preferably between 1:0.5 and 1:1. As is
known, aqueous solutions of styrene maleic anhydride copolymers may
be prepared by a treatment of the copolymer in water with a base at
an elevated temperature. An example of a suitable styrene maleic
anhydride copolymer is Scripset.RTM. 520 (supplier: Hercules;
molecular weight about 350,000, molar styrene to maleic anhydride
ratio 1:1). In a separate aspect of the invention, the styrene
maleic anhydride copolymer may also be used to create a stable
dispersion of melamine particles in a liquid such as water or an
alcohol.
[0021] The dispersant may be added to the
melamine-formaldehyde/liquid system prior to, during or subsequent
to the melamine-formaldehyde resin- and/or particle forming
reactions. The amount to be added may vary within wide limits,
depending a.o. on the precise nature of the dispersant, the liquid,
and also depending on the desired particle size to be achieved.
Preferably, the melamine-formaldehyde dispersion D contains between
0.01 wt. % and 10 wt. % dispersant, more preferably between 0.05
wt. % and 7.5 wt. %, between 0.2 wt. % and 5 wt. % or between 1 wt.
% and 3 wt. %.
[0022] The melamine-formaldehyde resin A may optionally comprise a
liquid melamine-formaldehyde resin E. The preparation of a liquid
melamine-formaldeyde resin E is known to the person skilled in the
art, e.g. from the abovementioned reference "Kunststoff Handbuch,
10-Duroplaste" ("Melaminharze" chapter). In a preferred embodiment,
melamine-formaldehyde resin A comprises both a liquid
melamine-formaldehyde resin E and a melamine-formaldehyde resin
dispersion D. This has the advantage that, compared to a coated
substrate wherein melamine-formaldehyde resin A contains the same
amount--expressed in solids content--of only a liquid
melamine-formaldehyde resin E, the gloss of the coated substrate
after curing (i.e. after completion of step g), to be discussed
below) is higher. The weight ratio between liquid
melamine-formaldehyde resin E and melamine-formaldehyde resin
dispersion D--as expressed in weight percentages relative to each
other--may vary between a wide range, for example between 1%:99%
and 99%:1%, more preferably between 10%:90% and 90%:10%, between
25%:75% and 75%:25%, or between 40%:60% and 60%:40%. In particular,
the combinations lying between 98%:2% and 85%:15% of liquid
melamine-formaldehyde resin E and melamine-formaldehyde resin
dispersion D are preferred for gloss enhancement. The said gloss
enhancement is observed irrespective of whether the layer
comprising melamine-formaldehyde resin A as applied to the
substrate comprises a carrier such as a decor paper or not. In one
embodiment of this aspect of the invention, step d) is not executed
in order to fully benefit from the gloss enhancement.
[0023] It may be that where a melamine-formaldehyde resin A
contains both a liquid melamine-formaldehyde resin E and a
melamine-formaldehyde resin dispersion D, the resin A will have a
paste-like form. This is advantageous in that it may be applied in
an easy and controllable fashion.
[0024] The melamine-formaldehyde resin A is preferably able to form
a non-porous layer after curing in step g) so as to prevent
diffraction of light by included components as for example water or
air. Diffraction would lead to a "blurred" pattern. This
requirement can advantageously be reached by a
formaldehyde-melamine ratio in the resin A between 1 and 3 (on mol
basis). Optionally the melamine-formaldehyde resin A can be
partially crosslinked before it is applied onto the substrate.
Depending on the precise composition of the melamine-formaldehyde
resin A, the rate of pre-crosslinking influences the ability to
flow. When the pre-crosslinking has proceeded too far, the resin A
will not flow sufficiently and it will be impossible to remove all
included components such as for example air by pressing. The man
skilled in the art can easily determine the desired rate of
pre-crosslinking.
[0025] Another method to increase the possibility that a non-porous
layer is formed is by chosing melamine-formaldehyde resin A such
that it comprises a mixture of powderous melamine-formaldehyde
resin C and melamine-formaldehyde resin dispersion D, as indicated
above.
[0026] The melamine-formaldehyde resin A can additionally contain a
colour-inducing substance such as a pigment. In that case the
coating that is finally obtained typically has a more or less
uniform colour, with no special recognizable pattern. In case that
such a uniform coloured layer is required the optional step c)
could be left out but it may still be desirable to add in step d) a
transparent topcoat layer on top of layer A to obtain a better
appearance and durability. When a decor pattern in the coating is
desired steps c) and d) are required.
[0027] If the melamine-formaldehyde resin A comprises or even
consists essentially of the powderous melamine-formaldehyde resin
C, the said powderous melamine-formaldehyde resin C should
preferably display such a combination of properties that it can
form a porous layer after the melting in optional step b). When the
resin particles melt, they flow towards each other. Depending on
the circumstances during the flow phase, and given enough time, the
molten particles will eventually touch each other completely,
thereby eventually giving rise to a non-porous layer. However for
the optional step c) it is preferred or even necessary to have a
layer that is still porous to some level; this has the advantage
that the pattern as applied via an ink, dye solution of pigment
dispersion will remain intact to a greater extend during the later
steps of the method according to the invention. To keep the layer
porous, the temperature during the flow-phase shouldn't be too high
when the time for the flow phase is moderate. When the time
available for flowing is only short, the temperature should be
higher. Therefore a balance should be found between these
parameters that determine the amount of flow. The man skilled in
the art can easily determine the suitable conditions by routine
experimentation.
[0028] The kind of substrate depends on the final use of the coated
substrate and can be for example wood or wood-based material,
paper, metal, glass or plastic. Examples of wood-based materials
are MDF (Medium Density Fibreboard) or HDF (High Density
Fibreboard), OSB (oriented strand board), particle board, plywood.
The coated substrates can be used in a large number of
applications, for example serving trays, washing-up basins,
crockery, doors, kitchen worktops, furniture and wall panels,
kitchen cupboards, window frames, laminated flooring.
[0029] Prior to executing step b) it may, depending on the nature
of the melamine-formaldehyde resin A and on the demands as placed
on the coated substrate, be useful, desirable or even necessary to
implement a drying step a1). Such a drying step as such is known
may be executed according to the guidelines available to the
skilled person.
[0030] In optional step b) of the method according to the invention
the applied resin is being treated by infrared (IR-) or
near-infrared (NIR-)radiation. Radiation within the near-infrared
range refers to radiation with a wavelength of between 0.8 .mu.m
and 1.5 .mu.m. It is especially preferred to implement step b) if
the melamine-formaldehyde resin A comprises or even consists
essentially of powderous melamine-formaldehyde resin C; this has
the advantage that the layer becomes more coherent. The type of
infrared radiation can in principle be chosen freely; it is
preferred to use regular IR-radiation for thinner layers, for
example up to 5 .mu.m thickness, while NIR is preferably used for
thicker layers for example up to 0.5 mm to ensure a favourable
temperature profile across the layer thickness. The duration of the
radiation depends on the intensity of the radiation and the
characteristics of the resin to be treated. If the
melamine-formaldehyde resin A comprises or even consists
essentially of powderous melamine-formaldehyde resin C, and in
particular if optional step c) is applied, the duration and
intensity of the radiation should be such that a coherent layer is
obtained, that still has a maximum porosity. This can easily be
determined by routine experimentation by the man skilled in the
art.
[0031] In optional step c) a decor pattern is applied to the coated
substrate. The coloured material for forming such a pattern can be
an ink, or a dye solution in water, solvent or in a polymer, or a
pigment dispersion in water, solvent or in a polymer. The coloured
material can be a solid or a liquid and can be applied by any kind
of imaging technique, such as off-set and roller printing, ink-jet
printing, heat-transfer printing, toner printing etc as described
in "Handbook of Imaging Materials" (Arthur S. Diamond ed., Marcel
Dekker, 1991).
[0032] In optional step d) a layer comprising a
melamine-formaldehyde resin B is applied to the coated substrate
obtained after step a), b) or c). The melamine-formaldehyde resin B
can be in the form of a liquid, a dispersion, a powder, or any
combination of these. In a preferred embodiment,
melamine-formaldehyde resin B is essentially in powderous form;
this has the advantage that the risk is reduced that the resin will
penetrate into the underlying layer(s) of the coated substrate. The
nature of the melamine-formaldehyde resin B can vary, depending on
the required properties of the final coating. Preferably the resin
B is in powderous form with a T.sub.g in the same ranges as
indicated for powderous melamine-formaldehyde resin C; it
preferably has a good flow at the temperature in subsequent steps
f) and g) and cures into a transparent and scratch-resistant
topcoat so as to make any underlying layer or pattern visible,
while also giving an excellent adhesion with the underlying layer
by co-reaction in the subsequent step or steps f) and g). In order
to protect the underlying layer(s) from environmental influences
and degradation, stabilizers for example Hindered Amine Light
Stabilizers (HALS), and UV absorbers can be added to
melamine-formaldehyde resin B. In addition, to ensure a better
scratch resistance inorganic fillers for example clay, silica and
corundum can be added to the resin. Preferably, for the purpose of
maintaining the optical transparency, filler particles of less than
300 nanometer are used.
[0033] Prior to executing step e) and if one of the optional steps
b), c) and d) have been implemented it may, depending on the nature
of the melamine-formaldehyde resin A and on the demands as placed
on the coated substrate, be useful, desirable or even necessary to
implement a drying step d'). Such a drying step as such is known
may be executed according to the guidelines available to the
skilled person.
[0034] Subsequent to step a) and optionally steps b), c), d), and
d'), the coated substrate is, in step e), placed in a press.
Presses are as such known to the skilled person; an example of a
known press is a laminating press. Should the step preceding to
step e) of the method according to the invention be already
executed in a press, then step e) may be interpreted as simply
leaving the coated substrate in the press.
[0035] Once the coated substrate has been placed in a press, it
is--in step f)--optionally heated in the press for a certain amount
of time. The absolute duration is not critical. Time and
temperature are interdependent, it means that the result obtained
counts but that the result can be obtained either by a high
temperature and a relatively short time or by a somewhat lower
temperature and a longer time. For example the time can vary
between 1 and 3 minutes and the temperature can than vary between
100 and 140.degree. C. The balance between time and temperature
should be chosen so as to result in a level of cure to give
acceptable properties regarding the gloss and Kiton test for
laminates.
[0036] Advantageously the coated substrate is pre-cured in step f)
before applying pressure in subsequent step g). This pre-curing is
done to reduce or even prevent flow and absorption of the resin
into the substrate or to the side of it under the influence of
pressure, which would negatively influence the mechanical and/or
optical characteristics of the coated substrate. For example, the
risk of an image--if brought onto the coated substrate in optional
step c)--getting blurred is reduced by the pre-curing according to
the invention. For this purpose, the press can for example been
pre-heated before the coated substrate is fed to it. Alternatively
the coated substrate can be pre-treated in a separate step so as to
result in a partially cured coating, for example by using IR or NIR
radiation. Both alternatives result in a better resolution of any
pattern when a pigment, ink or dye is used in step c). It has also
been found that the pre-heating or pre-treating results in a better
topcoat.
[0037] In step g) the coated substrate is subjected to increased
pressure and kept under pressure for a certain amount of time. The
object of step g) is to achieve an at least partial but preferably
full curing of the layer(s) in the coated substrate. The conditions
of pressure, time and temperature are as generally used for curing
in the field of melamine-containing laminates.
[0038] The invention also relates to the melamine-formaldehyde
coated substrate that is obtainable with the method according to
the invention. It has good mechanical properties. Depending on its
further use, requirements need to be met in view of for example
scratch resistance, flexibility, durability, chemical resistance,
abrasion resistance, cold check (that means the crack resistance at
a sharp temperature change). Depending on its final use some of the
above-mentioned properties are more important than others. Also
depending on its further use of the coated substrate, requirements
need sometimes to be met in regard of its appearance.
[0039] The melamine-formaldehyde coated substrate that is
obtainable according to the invention shows improved adhesion
between the applied layer(s) and the substrate compared to the
prior art laminates in which first a prepreg is made where after
the prepreg is pressed onto the substrate.
[0040] The invention further relates to the use of the coated
substrate that is obtainable according to the method of the
invention in a post-forming process; this has the advantage that
the risk of sustaining structural damage to the coated
substrate--and in particular to its layer(s)--is greatly reduced or
even eliminated.
[0041] The invention also relates to the use of a
melamine-formaldehyde resin with a molar formaldehyde to melamine
ratio between 1 and 3 in a method according to the invention.
[0042] The invention is further elucidated by means of the
following example, without being limited thereto.
EXAMPLE 1
[0043] A powderous melamine-formaldehyde resin C was obtained by
spray-drying of a 65% solid content melamine-formaldehyde resin,
formed from a commercially available resin (Madurit MW 909) having
a molar formaldehyde to melamine ratio of 1.7, and catalysed with 3
wt. % ammonium sulphate. 0.018 grams of the spray-dried resin C was
applied to a wood panel; the coated surface area was 2 cm by 6 cm.
Thus the melamine-formaldehyde resin A consisted for 100% of
powderous melamine-formaldehyde resin C. The panel was pressed with
a pressure of 69 bar during 4 minutes; the temperature during
pressing was 140.degree. C. As a result, the coated substrate had a
high gloss and hard surface, comparable to a known laminate as
produced through an impregnated carrier.
EXAMPLE 2
[0044] A 65% solids content resin was made from a commercially
available aqueous melamine-formaldehyde resin (Madurit MW 909) and
was catalysed with 3 wt. % ammonium sulphate so as to obtain a
liquid melamine-formaldehyde resin E. The melamine-formaldehyde
resin A consisted in this example for 100% of the said liquid
melamine-formaldehyde resin E. The resin A was applied to an MDF
panel; the layer thickness was 120 .mu.m; the coated surface area
was 10 cm by 15 cm. The panel was pressed with a pressure of 26 bar
during 4 minutes; the temperature during pressing was 140.degree.
C. As a result, the coated substrate had a high-gloss and hard
surface.
EXAMPLE 3
Preparation of a Melamine-Formaldehyde Resin Dispersion D
[0045] Preparation of a dispersant: an aqueous solution of a
styrene maleic anhydride copolymer (Scripset.RTM. 520, supplier:
Hercules) was prepared. 61 gram of Scripset 520 was charged slowly
to 455 grams of stirred water. After a reasonable lump-free slurry
was achieved, 50 grams of 25 (wt. %) NaOH solution in water was
added. An exothermal reaction to about 40.degree. C. occured. The
batch was then heated to 82.degree. C. under stirring and held for
45 minutes. The pH was adjusted (upwards only) to 6.5 if
needed.
[0046] Synthesis of an MF dispersion: the pH of 406 grams of a 38.5
wt. % aqueous solution of formaldehyde, to which 199 grams of water
was added, was adjusted to 9.0 with 2 M NaOH. 394 grams of melamine
were added; the mixture was then heated to reflux. After the
melamine dissolved and a clear solution was obtained, the mixture
was cooled down to 82.degree. C. Then 145 grams of the dispersant
solution as prepared above was added, and the pH of the mixture was
adjusted to 7 with HNO.sub.3, while at the same time the solution
is vigorously stirred. After about 15 minutes a sharp turning
point--i.e. a whitening--of the solution is seen. The solution
turns from milky to a clear white turbid dispersion. The
condensation reaction of the dispersion is continued for an
additional 9 minutes after the turning point and then cooled down
to 20.degree. C. One minute after starting cooling down the pH of
the dispersion is adjusted to pH=8.6 with 5M NaOH. About 8 grams is
needed. While stirring the solution is cooled down to 20.degree. C.
and stored in plastic bottle. The resulting dispersion had a solids
content of 55% and a molar formaldehyde to melamine ratio of 1.65.
The dispersion was stable for several days.
Preparation of a Coated Substrate
[0047] The synthesized resin dispersion D was applied to a beech
veneer by means of a small doctor roll. Thus the
melamine-formaldehyde resin A consisted in this example for 100% of
resin dispersion D. After applying the wet dispersion the coating
looks white. After drying at room temperature the coating turned
into a white surface. This dried coated substrate is pressed at
150.degree. C. for 3 minutes at 30 bar in a press. After pressing a
substrate having a clear transparent coating was obtained.
EXAMPLE 4
[0048] A melamine-formaldehyde resin A comprising both a liquid
melamine-formaldehyde resin E and a melamine-formaldehyde
dispersion D was prepared by combining a liquid
melamine-formaldehyde resin E having a molar formaldehyde to
melamine ratio of 1.7 with a melamine-formaldehyde dispersion D as
prepared in Example 3. The solids content of the
melamine-formaldehyde resin A was 58%, whereby 55% originated from
the liquid melamine-formaldehyde resin E and 3% originated from the
melamine-formaldehyde dispersion D. Furthermore,
melamine-formaldehyde resin A contained 0.2 wt % of wetting agent
Netzmittl PAT959/9 and 0.2 wt. % of a release agent PAT-2523.
[0049] A 20 cm.times.20 cm piece of a carrier in the form of a
Munksjo Decor Paper (80 g/m.sup.2) was impregnated once with the
melamine-formaldehyde resin A, then dried for 420 seconds at
100.degree. C. The impregnated paper carrier was then laminated on
an MDF board at 100 kN and 190.degree. C. for 50 seconds. As the
skilled person knows, these conditions are typical conditions for
preparing a so-called LPL, i.e. a `low-pressure` laminate.
Surprisingly, the gloss of the laminate as obtained as measured at
20.degree. was 120, about the same as that of a HPL (high-pressure
laminate). As the skilled person knows, known LPL's made with
once-impregnated carriers have a lower gloss than a HPL, typically
95-100 when measured at 20.degree..
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