Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to a laminar prepreg
comprising one or more layers of a laminar carrier,
which carrier is impregnated with an as yet uncured
resin.
l0 Laminar prepregs comprising a plurality of
mutually stacked layers of a-cellulose impregnated with
the reaction product of formaldehyde and melamine are
disclosed, for example, in US-A-3730828. According to
said patent publication, paper is first impregnated with
a melamine-formaldehyde resin. Then a prepreg is made by
precuring layers and thereafter stacking a few layers of
said impregnated paper on top of one another. The resin
is then cured in a press under a pressure of, for
example, 6 MPa and at a temperature of approximately
150°C. The conversion of the reaction between
formaldehyde and melamine in the laminar product thus
obtained is complete (completely cured). The laminar
products described in said patent publication appear to
have good postforming properties. Postforming is
understood as meaning that the laminar product can be
bent at an elevated temperature which is between 160 and
180°C, it being possible to bend the laminar product
along one axis without the laminar product
breaking/cracking.
A disadvantage is that, starting from a
prepreg according to US-A-3730828, it is not possible to
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obtain laminar products which can be bent into complex
shapes along two (or more) mutually intersecting axes
without breaking/cracking. Complex shapes may be
considered to be, for example, a saddle-type pattern, a
small tub, a sickle pattern or a satchel.
The object of this invention is a prepreg
with which more complex shapes can be obtained.
This object is achieved in that the carrier
is a laminar porous polymer and in that the elongation
at break of the prepreg and of the separate impregnated
carrier is greater than 10%.
It has been found that, proceeding from the
prepreg according to the invention, laminar products can
be made in the most diverse shapes without cracks
occurring in the laminar product during the deformation.
Laminar products are now possible which have shapes in
which the prepregs are locally stretched from lo% to
more than 400% during the shaping. A further advantage
is that the further curing of the resin and the
deformation can be performed in one step. This is in
contrast to the method described in the above-mentioned
US-A-3730828 where, proceeding from the prepreg, two
steps are necessary to arrive at a shaped final product.
An additional advantage is that the prepreg can be
processed by a multiplicity of techniques, as a result
of which the most optimum technique can be used for each
final p=oduct.
Proceeding from the prepreg according to the
invention, it is furthermore possible to make laminar
products which are bent into an acute angle, for example
along one axis. Proceeding from the known prepregs based
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on a paper carrier, this is impossible.
JP-A-7002119 discloses a prepreg made up of
a carrier of a polyamide nonwoven material and a carrier
of kraft paper, which carriers are impregnated with a
melamine-formaldehyde resin. An impregnated polyamide
nonwoven carrier has, as a rule, an elongation at break
of more than 10%. However, impregnated kraft paper has
an elongation at break of less than 10%, as a result of
which the elongation at break of the prepreg as a whole
is less than 10%.
A laminar porous polymer carrier is
understood as meaning any polymer which comprises a high
degree of porosity. The porosity of the polymer carrier
is essential for obtaining the advantageous properties
of the prepreg as described above.
As a result of the high porosity, the
laminar polymer carrier can, as it were, be mixed almost
homogeneously with the resin. Preferably, the porosity
is obtained in the form of microscopically small,
mutually communicating cavities and there are preferably
few larger cavities and holes present. Larger holes
result in loss of resin during the further processing of
the prepreg to its final shape. Preferably, the porosity
is sufficiently high for at least 30% by volume of the
final shaped part to consist of resin. The porosity is
the ratio between the density of the porous polymer
carrier with respect to that of the corresponding bulk
polymer (no cavities).
The porous polymer carrier may, for example,
be a nonwoven laminar polymer, a laminar open polymer
foam or a microporous membrane. Preferably, the fibres
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of the nonwoven are smaller than 0.1 mm. Nonwovens
having very small-diameter fibres are also referred to
as open films. This class of nonwovens has, as a result
of the small thread diameter, few larger meshes and many
microscopically small, mutually communicating cavities.
The fibres of said nonwovens lie, as a rule, parallel to
the plane of the laminar porous polymer. Polymer foams
inherently have a reasonably definable and uniform size
of the mutually communicating cavities. The cavities
l0 preferably have a diameter of less than 1 mm. Larger
cavities may occasionally be present in the foam
provided more than 80% by volume of the cavities have
said smaller diameters.
The porous polymer carrier may in principle
be any porous polymer which meets the requirement that
the impregnated carrier has an elongation at break of
more than 10%. Examples of porous polymers are porous
polyethylene, porous polypropylene, porous polystyrene,
porous ethylene/propylene copolymers, porous EPDM,
porous polyamides, for example, porous nylon 6,6, porous
nylon 6, porous ethylcellulose, porous cellulose
acetates, for example porous cellulose diacetate, porous
SMA/SAN, porous polyesters, for example porous
polyethylene terephthalate (PET), porous polybutylene
terephthalate (PBT), porous polyethers. Also possible
are combinations of different porous polymers or
combinations of porous polymers with paper. As porous
polymer carrier, the above-mentioned polymers can be
used as nonwoven, open film or open polymer foam.
Examples of an open-film polyethylene are
the polyethylene types which are known under the name of
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Solupor (Solupor is a DSM N.V. brand name). Nonwovens
based on various polymers are suitable to be used as
carrier. An example of a nonwoven polyethylene is Tyvek
(Tyvek~ is a DuPont de Nemours brand name). An example
of a nonwoven polypropylene is Lutrasil~ (Lutrasil~ is a
Freudenberg brand name). An example of a nonwoven
polyester is Viledori H1206. An example of a nonwoven
polyamide is Viledon FS2118 (Viledon is a Freudenberg
brand name). An example of an open polymer foam is
Calligan open-cell foam standard 1.17 polyether
(Calligari is a Calligan Europe B.V. brand name). An
example of a cellulose filled nonwoven polypropylene is
Workhouses (Workhouses is a Kimberly-Clark brand name).
An example of a nonwoven cellulose acetate is TAT 2121
which is a product of Freudenberg.
The polymer may have either polar or apolar
properties. As a result of making use of suitable
wetting agents, the desired degree of impregnation of
the polymer sheet by the resin can be obtained. As a
rule, all the wetting agents known to the person skilled
in the art can be used. Examples of wetting agents are
PAT 523W, PAT 959 (PAT is a Wiirtz brand name),
Nonidet~ P40 (Nonidet~ P40 is a Sigma Chemie brand name)
and Aminol~ N (Aminol~ is a Chem-Y brand name).
The resin may in principle be any known
thermosetting resin. Examples are aminoplast
formaldehyde resin, phenol-formaldehyde resin (PF
resin), epoxy resin or unsaturated polyester resin.
Preferably, an aminoplast-formaldehyde resin
is used as resin. Urea, melamine or benzoguanamine, for
example, can be used as aminoplast. Preferably, melamine
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is used because of its superior mechanical properties.
The resin can be prepared in a process known to the
person skilled in the art by reacting aminoplast and
formaldehyde in water. The ratio of, for example,
formaldehyde:melamine is normally speaking between 1:1
and 6:1, preferably between 1.2:1 and 2:1. Optionally,
the aminoplast can be partially replaced by, for
example, phenol, but this may have adverse effects on
the colour. Modifiers, such as sorbitol, s-caprolactam,
l0 ethylene glycol, trioxitol, toluenesulphonamide, and
benzo- and acetoguanamine can also be added.
Mechanical properties which can be used well
in practice are achieved if 10-70% by weight of porous
polymer and 90-30% by weight of resin are used. The
resin may contain the known fillers such as lime, clay,
glass, carbon, silica or metal particles. It has been
found, however, that the best results are achieved in
the absence of fillers or at any rate with less filler
than has been usual hitherto. The filler and resin
weight ratio is preferably between 0:1 and 0.5:1. These
ratios relate to the cured, final laminar product.
The prepreg comprises one or more stacked
sheets of the impregnated porous polymer carrier. The
number of laminar carriers is, as a rule, 10 or lower,
and preferably 5 or lower. The thickness of the prepreg
can vary from 100 ~m to approximately 3 cm. The degree
to which the resin is cured in the prepreg can be
determined for melamine-formaldehyde (MF) and PF resins
by determining the residual volatility. Said residual
volatility is the loss in mass of the prepreg during 7
minutes at 160°C. The residual volatility of the prepreg
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is, as a rule, between 2 and 20%. The elongation at
break of the prepreg as used in the description is
defined as the elongation at break for a residual
volatility of approximately 5%.
The prepreg is made under the conditions
which are already known for making prepregs based on a
paper carrier, as described, for example, in the above-
mentioned US-A-3730828 or JP-A-7002119. The resin is
preferably dissolved in water and is water-fluid , the
resin preferably having a viscosity of 1-1000 Pa.s. The
viscosity can be influenced by varying the amount of
solvent. The solvent will vary depending on the chosen
resin. Still uncured aminoplast-formaldehyde is
preferably dissolved in water. Preferably solvents are
used in which the polymer carrier does not dissolve or
swell. The temperature during the impregnation may be
between 15 and 60°C and, for practical reasons, is often
room temperature. Higher temperatures are less practical
because the resin partially cures during the
impregnation. The pressure during the impregnation is
not critical and, for practical reasons is, as a rule,
atmospheric.
The carriers impregnated in this way are
dried until the residual volatility is reached, as
described above for the prepreg. The drying preferably
takes place at a temperature of 100-160°C. Higher
temperatures are less practical because the drying times
then become too short, resulting in a less controllable
process. The temperature will in practice also be
determined by the type of oven. Preferably, the carrier
is stacked after the drying.
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_ _ g _
The elongation at break of the separate
impregnated carrier (residual volatility = approximately
5%) and of the prepreg obtained is greater than 10% and
preferably greater than 50%. The prepreg may optionally
also comprise layers of a non-porous polymer in addition
to the porous polymer carriers provided these polymers
also have an elongation at break which is greater than
or equal to that of the prepreg.
The prepreg can 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 in one step.
The deformation, optionally in combination
with the curing, can be performed by means of bending,
embossing, stamping, pneumatically stretching or
mechanically stretching.
The temperature during the deformation will
depend on the yield stress of the prepreg. The yield
stress is the stress at which the material begins to
flow. Said yield stress is determined by the resin
composition, resin content, polymer type and water
content. In principle, said temperature may be between
room temperature and 200°C. At temperatures higher than
100°C, the resin will already (partially) cure during
the deformation, as a result of which the deformation
and curing of the resin will take place simultaneously.
The shaped product obtained in this way can
be used as final product or as protective layer around
an object having a core material of, for example, wood,
metal, glass or plastic, for example polyethylene,
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polypropylene, ABS, polyamide and MF resins, PF resins
and epoxy resins.
Examples of final products of the shaped
product are serving trays, washing-up basins, crockery,
doors, kitchen worktops, furniture, wall panels.
Examples of end products where the shaped product is
used as protective layer for a wooden core are worktops
having, for example, an acute angle, (kitchen) cupboards
particularly the fronts of (kitchen) cupboards
consisting of for example milled MDF (= medium density
fibre board) or window frames. Examples of articles in
which the shaped product serves as protective layer for
a plastic core are bumpers, petrol tanks, garden
furniture, worktops or car bodywork components.
The protective layer may be glued to the
core material. Another possibility is that the shaped
product is applied to the core while the resin is still
incompletely cured. The resin then serves as glue joint
when it is subsequently cured. Then curing and
deformation takes place in one process step.
The invention will be described by means of
the following, non-limiting examples.
Examble 1
Preparation of resin
In a reactor, 24 parts of water and 135
parts of formaldehyde (30% formaldehyde in water
adjusted to a pH of 9.3 using 50% NaOH) were added to
100 parts of melamine. The F/M ratio of the resin was
1.7 (F/M ratio is the formaldehyde-melamine molecular
ratio). The condensation reaction was performed at 95°C
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until the water dilutability of the resin at 2D°C was
1.5 g of resin per g of water. The water dilutability is
the amount (g) of water which can be added to a resin
solution (g) at 20°C before the solution becomes turbid.
The resin was made reactive (catalyzed) with a 50% (by
weight) p-toluenesulphonic acid solution to a pH of 8.1.
Four 10 by 10 cm sheets of Solupor (type
16P03; DSM) were impregnated at room temperature with
the above-mentioned resin solution, to which 1% (m/m) of
Nonidet~ P40 (Sigma Chemie) had been added as wetting
agent. After a few minutes, the sheets impregnated with
resin were removed from the resin set-up and the excess
resin was removed with the aid of a "wringer". The
sheets were dried in a ventilated oven for 4 minutes at
100°C.
Resin content: the resin content of the
prepreg was determined by weighing the prepreg and the
polymer carrier and was 440%. The resin content is
defined as:
(g (prepreg) -g (polym) ) /g (polym) ,
g(prepreg) and g(polym) are the weights of the prepreg
and of the Solupor polymer carrier, respectively.
Residual volatility content: the residual
volatility content was determined by measuring the
weight loss after drying and curing the prepreg further
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for 7 minutes in an oven at 160°C and was 3.5%. The
residual volatility content is defined as:
(g (before) -g (after) ) /g (before) ,
g (before) and g (after) are the weights of the prepreg
before and after treatment at 16o°C, respectively.
Resin content: the resin content of the
laminate was determined by weighing the laminate and the
starting polymers and was 406%. The resin content is
defined as:
(g (laminate) -g (polym) ) /g (polym) ,
g(laminate) and g(polym) are the weights of the laminate
and of the polymer carrier, respectively.
Elongation at break: the elongation at break was
measured on test pieces (dimensions 4.0*50.0*0.07 mm) at
140°C with the aid of a standard Zwick tensile test
bench in accordance with ISO 37 type 3 and was more than
400%, the maximum value measurable with said equipment.
The tensile strength was 5.4 MPa. The rate of
deformation was 100 mm/min.
- 2D pressing:
Four 10*10 cm sheets of the prepreg were pressed into a
laminate by the following high-pressure laminate
pressing cycle: the sheets are placed in the press (of
the type Fontijnen SRA 100) at 60°C and the press is
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brought to a pressure of 80 bar and the press is further
heated in approximately 15 minutes to 130°C, said
temperature is maintained for 30 minutes and then
cooling to 60°C takes place in approximately 15 minutes,
after which the pressure is let down.
- 3D pressing:
A few sheets of Solupor prepreg were pressed on a high-
density polyethylene core material in an S-shaped mould
at 140°C for 20 minutes. The mould clamping force was 9
MPa. Cooling was then carried out for 10 minutes to
40°C. The results are further reported in Tables 1 and
2.
Example 2
The process of Example 1 was repeated, but
Tyvek L1058D (DuPont) was used instead of Solupor.
Table 1 shows the resin content, the residual volatility
content, the pressing conditions of the prepreg and the
resin content of the laminate. Table 2 shows the
elongation at break and the tensile strength of the
prepreg at the stated test temperature.
The process of Example 1 was repeated, but
Lutrasil~ 3450 (Freudenberg) was used instead of
Solupor. See Tables 1 and 2 for further conditions and
results.
3 0 Examp; a 4
The process of Example 1 was repeated, but
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cellulose diacetate (CD) nonwoven was used instead of
Solupor. See Tables 1 and 2 for further conditions and
results.
Example 5
The process of Example 1 was repeated,
Viledon FS2118 being used instead of Solupor~. See
Tables 1 and 2 for further conditions and results.
Example 6
The process of Example 1 was repeated,
Viledon H1206 being used instead of Solupo= . See Tables
1 and 2 for further conditions and results.
Co~arison Experiment A
The same process as in Example 1 was used,
but a decorative paper (80 g/m2) was used instead of
Solupor. See Tables 1 and 2 for further conditions and
results.
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