Note: Descriptions are shown in the official language in which they were submitted.
2764~
~C3 62596
This invention relates to the preparation of a laminate by applying
a fibrous sheet material, e.g. paper, impregnated with an aminoplastic resin
solution onto a substrate at an elevated temperature and pressure.
Laminates can be prepared by such a process by depositing an
impregnated and dried paper on a suitable substrate, generally chipboard,
inserting the assembly into a press, and operating the press at a tempera-
ture between 125 and 175 C under a pressure between 10 and 50 kg/cm e.g.
for 3 to 6 minutes, after which the laminated assembly is allowed to cool
in the press under pressure, According to the recently developed Kurztakt
technique the assembly of substrate and impregnated paper is inserted into a
heated press and compressed for 0.5 to 2 minutes at 125 to 200 C and 15 to
50 kg/cm pressure, the laminated assembly removed from the press whilst
still hot and allowed to cool without pressure being maintained thereon.
The aminoplast resins used in such laminating processes are
generally melamine-formaldehyde resins, which may contain one or more
modiiiers to improve storage stability of the resin solution and/or the sur-
face properties of the finished laminate. The resin solutions may contain
if desired a small proportion of a water-soluble inorganic silicate, which
serves to improve the stability of the resin solution and when present in
sufficient amount, also effects improvement of the surface properties of the
laminate, particularly when obtained by the Kurztakt process. It is desirable
however that such solutions should be used in the form of a stabilized
highly acid silicate solution. Furthermore they should be incorporated in
the resin sglution very carefully in order to avoid flaking out of the sili-
cate in the resin solution.
A disadvantage of melamine-formaldehyde resins is their high cost.
The less expensive urea-formaldehyde resins and urea melamine formaldehyde
resins cannot be used for the preparation of laminates with a high-quality
surface, and furthermore the laminates obtained have a low chemical resis-
tance.
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The invention provides a process for preparing a laminate comprising
impregnating a fibrous sheet material with a urea-modified melamine formal-
dehyde resin solution, the said solution
(a) having a urea content between 4 % and 50 % by weight based on the weight
of urea plus melamine,
(b) a molar ratio of formaldehyde to the three NH2 groups originating from
urea or melamine of between 1 : 1 and 2.5 : 1, and
(c) an inorganic water-soluble silicate in an amount corresponding to 0.2
to 1.4 % by weight of SiO2 based on the total weight of the said solution,
and drying the impregnated fibrous sheet and applying the dried sheet to a
substrate at an elevated temperature and pressure to form a laminate there-
with.
By the practice of the invention laminates having a high-quality
surface can be obtained using the relatively inexpensive urea-modified
melamine-formaldehyde resin solutions which hitherto have been completely
unsuitable for the preparation of laminates.
Furthermore the water-soluble silicate may readily be added to the
urea-modified melamine-formaldehyde resin without special precautions.
Thirdly, the storage stability of 'pot-life' of the catalyzed silicate-
containing urea-modified resin is greater than that of a silicate-containing
pure melamine-formaldehyde resin.
The presence of the silicate in effecting the invention is essential
for the production of a laminate having a surface of high quality with the
urea-modified melamine-formaldehyde resins used in the process of the
invention.
Preferably the fibrous sheet material used is paper.
The said urea-modified melamine-formaldehyde resin solution
preferably has a urea content between 25 % and 40 % based on the weight of
urea plus melamine and a said molar ratio between 1 : 1 and 2.5 : 1, more
preferably between 1.2 : 1 and 2.0 : 1 particularly between 1.5 : 1 and
1.8 : 1 and has said silicate content between 0.4 % and 0~8 % based on the
total weight of solution.
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The pH value of such a non-catalyzed resin solution may lie between
~.5 and 10.0 preferably between 9.0 and 9.5. A resin solution of this kind
generally has a storage stability of a few weeks. The calculated solid
content is preferably between 53 % and sB % by weight. If desired the resin
solution may also contain conventional modifiers e.g. caprolactam, sugars,
toluenesulphonamide, glycols and glycol ethers.
Such resin solutions can be obtained by $irst allowing melamine and
formaldehyde to react in an aqueous solution in conventional manner,
optionally in the presence of one or more modifiers. The formaldehyde to
melamine molar ratio should then be smaller than 3.5 : l, preferably between
2 : 1 and 3 : l.
If the condensation has proceeded sufficiently far to a water
compatibility between 2 and 6, urea is added, whereupon the condensation
reaction is allowed to proceed for a short time or the resin solution cooled.
The pH of the resin solution thus obtained is generally between 9.0 and 9.5.
The resin solution may also be obtained by the mixing of a melamine-
formaldehyde resin solution with a urea-formaldehyde resin solution.
The impregnating resin solution used according to the invention may
contain between 4 % and 50 % by weight of urea based on the weight of urea
plus melamine. At high urea contents the laminates may have a slightly poorer
surface. The urea content may be very low at the expense of cost advantage,
and thus it is preferable to use a resin solution with a urea content of
between 25 % and 40 % by weight.
In some cases, it is desirable that laminates after curing may
be reshaped or bent whilst being hsated, without deterioration of the surface
properties. If laminates are prepared according to the invention using a
resin solution containing between 0.2 and 1.4 % by weight SiO2 based on the
total weight of the solution and a F/3 NH3 ratio of between l : 1 and 1.5 : l,
reshapeable laminates with satisfactory surface characteristics are obtained.
The silicate is responsible for improving the suriace characteristics both
before and after reshaping (post-forming), without diminishing the reshape-
ability.
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Since it is difficult to prepare melamine-formaldehyde resin solutions
with a formaldehyde/3NH2 ratio which is substantially lower than 1,5, a urea
content of at least 4 %, based on uroa plus melamine, is required to bring
the formaldehyde/3NH3 ratio below 1,5 : 1, while larger proportions of urea
make it possible to commence with a higher formaldehyde/melamine ratio and/or
to achieve a lower final value for the formaldehyde/3NH~ ratio, The cost
price of the resin decreases and the reshapeability of laminates obtained
using said resin solutions increases with increasing urea content, but the
surface properties tend to deteriorate at an increasing urea content
notwithstanding the presence of the silicate.
A particularly useful urea-modified melamine-formaldehyde impregna-
ting resin solution for use according to the invention in the preparation of
post-formable laminates comprises a molar ratio of formaldehyde/3N~2 groups
originating from urea or melamine of between 1 : 1 and 1,5 : 1, a urea
content of 4 to 20 % by weight based on the weight of urea plus melamine, and
a water-soluble inorganic silicate content corresponding with an SiO2 content
of the solution of between 0.2 and 1.4 % by weight.
~eduction of the formaldehyde/3NH2 iatio improves the reshapeability
but causes the surface properties to deteriorate owing to enlargement of the
amount of urea used, The optimum formaldehyde/3NH2 ratio lies between 1,2 : 1
and 1,4 : 1,
The imprGgnating resin solution is prepared by first condensing
melamine and formaldehyde in the usual way at a formaldehyde/3NH2 ratio of
between approximately 2 : 1 and 1,5 : 1 and subsequently adding urea, and if
desired continuing the condensation for a short time.
Since the silicate-containing resin solution having a silicate
content of more than about 0.2 % SiO2 has only a limited storage stability,
the silicate is added to the resin solution shortly before the impregnation
step is commenced. The silicate may be added in the form of an aqueous
solution to the resin solution.
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If a concentrated silicate solution is used the formation of gel-
like flakes may take place, so that preferably a silicate solution is used
containing i'rom 10 % to 20 % by weight of SiO2.
The pH value of the resin solution may rise to about 11 when the
silicate is added. Before, during or after the addition of silicate the resin
solution is usually catalyzed by setting the pH between 7 and 9, particularly
between 8.5 and 9Ø These catalyzed resin solutions have a storage stability
o~ at least 24 hours at 20 C. The acid catalyst may be added to the resin
solution bef'ore, during or after the addition of the silicate solution.
Suitable acidic catalysts are ior instance formic acid, iormamidinesulf'inic
acid, toluenesulphonic acid, oxalic acid, acetic acid, phtalic acid,
ammonium chloride or diammoniumphosphate.
If silicate is added to a pure melamine-formaldehyde resin solution
the risk of flaking-out increases to such an extent that the silicate is added
in the ~orm of' a stabilized acid solution. The storage stability of catalyzed
silicate-containing pure melamine-f'ormaldehyde resin solutions is significantly
less than the storage stability of catalyzed urea modified melamine-formal-
dehyde resins with the same silicate content.
Particular inorganic water-soluble silicates incorporated in the
resin solution for use according to the invention are lithium silicate, sodium
silicates, potassium silicates and quaternary ammonium silicates.
Particularly good results are achieved using aqueous solutions of
sodium silicate..Generally, the silicate solution is added in a quantity so
that the SiO2 content of the resin solution is between 0.2 % and 1.2 % by
weight, preferably between 0.4 % and 1.0 % by weight, and particularly
between 0.4 % and 0.8 % by weight.
The resin solutions for use according to the invention are applied
f'or impregnation of paper or other fibrous material, followed by drying of
the impregnated material and its processing into laminates. The impregnation
is e~fected in the usual manner. The resin content of' the dried and impreg-
nated material may be between 50 ~ and 60 % by weight. The percentage of
volatile material may be between 5 % and 7.5 % by weight, preferably between
6 % and 6.5 % by weight
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The impregnated and dried material may in the usual way be processed
to form a laminate, or by the so-called Kurztakt process in which the material
is compressed for a short time and cools outsicle the press, or by the con-
ventional process according to which the laminate cools in the press.
The following Examples of the invention are provided:
Example I
A urea-modified resin solution was prepared by heating 52.4 kg of
30 % formalin having a pH value of 9.3, 23.6 kg of melamine, 8.7 kg of water
and 0.6 kg of aqueous sodium silicate solution (SiO2/Na20 = 3.3) at 95 - 96 C
and by adding 12.8 kg oi' urea at the end oi the condensation reaction.
After cooling, the pH value of the resin solution was brought to approximately
9.5 by addition of dimethylaminoethanol.
Said resin solution was divided into three portions each of which
was catalyzed and modified in the way indicated below. With the aid o~ the
resin solutions obtained in this manner white decorative paper (90 g/m grade)
was then impregnated. When dry, the decorative paper was pressed onto chipboard
according to the Kur7takt process, the compression time amounting to 60 seconds
and the temperature of the press plate to about 155 C. The surface of the
laminates was judged for curing by means of the 'Kiton' colouring test, for
surface compactness by the shoe wax test, and for crackle resistance by
heating at 100 C in the air for 16 hours. The results are compiled in the
table.
A. Starting from the resin solution described above, resin solution A
was prepared by adding 19 g of triethyleneglycolmonoethylether to every kg
of the resin solution and by setting the B-time with toluenesulphonic acid
at 99 sec. The B-time is an indication of the reactivity of the resin solution
and represents the time required by a resin solution in a tube which is seal-
melted and heated at 140 C to become turbid. Resin solution A has a total
SiO2 content of 0.05 % and is not covered by the invention.
B. Resin solution B was prepared by adding 19 g oi' triethylene-
glycolmonoethylether and 40 g of 18 % sodium silicate solution
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(SiO2/Na20 = 3.3) per kg of resin solution, whilst stirring, and by subse-
quently setting the B-time at 99 seconds with toluenesulphonic acid. The total
SiO2 content amounted to 0.6 %.
C. Resin solution C was prepared by adding, with stirring, an acid,
stabilized silicate solution to the resin solution to a total SiO2 content
of 0.6 % and by setting the B-time at 95 seconds. The solution added contained
toluenesulphonic acid, triethyleneglycolmonoethylether and sodium silicate
(SiO2/Na20 = 3.3) and was obtained in the way described in the Netherlands
Patent Application 72.10.402.
The properties of the laminates obtained with spplication of the
resins A, B and C are listed below:
resin % of volatile 'Kiton' surface crackle
solution matter in test compactness resistance
impregnated
paper
A ) 6.7 4 ) 4 _ 5
B 6.9 2 - 3 1 - 2
C 7.2 2 - 3 1 - 2
Scale of judgment: 1 = excellent, 2 = good, 3 = fair~ 4 = poor, 5 = very poor.
)The 'Kiton' test continues to show strong discolouration also at longer
compression times; the chemical resistance of the surface is too poor to allow
of the curing degree being judged on the basis of this test.
)Not according to the invention.
Laminates obtained by compression in the conventional way have,
virtually, the same properties.
Example II
A urea-modified resin solution was prepared by heating 29.1 kg of 30 %
formalin, which had been given a pH value of 9.3 with 0.2 kg of aqueous sodium
silicate solution (18 weight % SiO2~ SiO2/Na20 = 3.3), together with 9.0 kg of
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water and 25.3 kg of melamine to approximately 95 C until the capability of
the formed resin solution of being diluted with water had attained a value
of about 2.0 (at 20 C). At the end of the condensation reaction 1.3 kg of
urea were added. In this way, resin solution A was obtained, F/3 NH2 = 1.35,
containing 4.9 % by weight of urea referred to melamine plus urea.
In the same way a urea-modified melamine-formaldehyde solution was
prepared, starting from 22.4 kg of melamine with addition of 3.4 kg of urea.
The solution (solution B) so obtained has an F/3 NH2 ratio of 1.35 and contains
13.1 % by weight of urea.
To each of the solutions A and B 38 g of sodium silicate solution
(18 % by weight of SiO2, SiO2/Na20 = 3.3) per kg of resin solution were added
with stirring, whereupon the B-time was set at about 100 seconds through
addition of toluenesulphonic acid, the pH value then amounting to approximately
7.7 (resin solutions A' and B' respectively). The B-time is an indication of
the reactivity of the resin solution and is the time a resin solution, heated
at 140 C, in a seal-melted tube requires to become turbid.
For comparison, part of solution B, to which no silicate had been
added, was in the same way given a B-time of about 100 sec. (resin solution C)
by addition of toluene sulphonic acid.
Again for comparison, also a recipe was followed which is much
applied for preparation of resin solutions used for reshapable laminates.
According to this recipe a melamine-formaldehyde resin solution (F/3 NH2 = 1.8),
modified with a condensate of toluenesulphonic amide and formaldehyde, was
set at B-time of 240 sec. with toluenesulphonic acid (pH = 9.0) (resin solu-
tion D).
Each of the catalyzed resin solutions was used for impregnation of
white decorative paper (90 gm/m2 grade) which, when having been dried in the
usual way, with a laminate temperature of at most 148 C and cooling in the
press, was pressed onto 6 layers of standard-quality, reshapable core paper
which was impregnated with a phenol resin.
The laminates so obtained were then examined. The degree of curing
.md the chemical resistance were examined by application of the well-known
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'Kiton' colouring test. The reshapability was examined by locally heating the
laminate with infra-red radiation to about 140 C and by bending it to a radius
of 9 mm, whereupon the bending-face was judged for crack formation and loss
of gloss
The results are summarized in the table, in which the scale oi
judgment ranges from 1 (excellent, no colouration, no cracks) to 5 (very poor).
resin B-time % of volatile 'Kiton' gloss bending-face
solution matter in test crack formation
impregnated
paper
:
A' 100 7.1 2 1 - 2
B' 107 5.5 1 - 2 1 - 2
C ) 96 6.0 3 - 4 2 - 3
D ) 240 5.5 2 1 - 2 2 - 3
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)not according to the invention.
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