 |
Description  |
|
|
Foams are employed for a variety of purposes, especially for heat and sound
insulation in the construction industry. The most commonly used foams are
based on thermoplastics, especially on styrene polymers. These possess
excellent insulating properties and good mechanical properties, but
because of the thermoplastic character of polystyrene their heat
distortion point is low, in particular below 100.degree. C. Foams based on
thermoplastics are also highly flammable. An improvement in this respect
can be achieved by adding halogen-containing flameproofing agents, but
this is not an ideal solution because it affects the processing
characteristics adversely, and because it can lead to the formation of
hydrogen halides. Similar remarks apply to rigid polyurethane foams, which
also have good mechanical properties and insulating properties, but are
flammable and, when they burn, evolve toxic fumes. Thermosetting urea
resins are also used to manufacture foams. These have the advantage of
using a cheap raw material, but are very brittle and prone to crazing, and
also are flammable.
It is true that foams based on melamine-formaldehyde condensates have been
described in various patent publications, but they have not hitherto been
employed industrially for heat or sound insulation in the construction
industry. They are produced by stirring air into an aqueous melamine resin
solution which contains an emulsifier and a curing agent. Such as
described, for example, in German Pat. No. 870,027, have the serious
disadvantage that they are very hard and brittle, and easily break or
crumble when handled. It is alleged in French Pat. No. 1,073,642 that
melamine resin foams can be produced by heating a resin powder in a mold
under reduced pressure. This process, however, does not give useful
resilient foams. U.S. Pat. No. 3,093,600 describes melamine resin foams
which are said to have improved resilience and resistance to crazing as a
result of the incorporation of triols, e.g. trimethylolpropane. However,
it has been found that the resilience, and especially the recovery after
compression, of such foams is inadequate for many applications.
Furthermore, when substantial amounts of triols are incorporated, the
flammability of the foams is substantially increased. U.S. Pat. No.
3,063,953 describes a process for improving the mechanical strength,
resilience and softness of aminoplast resin foams, preferably those based
on urea resins. It is true that by using this process the above properties
of melamine resin foams produced by the prior art can be improved, but
such improvement is not substantial. In particular, the tensile strength
of such foams is far too low.
British Pat. No. 1,161,338 discloses a process for the preparation of foams
based on phenolic, urea or melamine resins, wherein an aqueous resin
solution which contains a blowing agent and a curing catalyst is foamed.
On repeating the tests set forth in the Examples, it is found that the
foams obtained--if any--are brittle, even in the phenolic or urea resins
in the formulations are replaced by melamine resins.
German Laid-Open Application DOS 2,402,441 (equivalent to South African
Patent Application 403/74) describes a process for the preparation of
aminoplast foams, wherein a strong acid is added as a curing agent to an
alkaline aminoplast precondensate containing a blowing agent. The heat
liberated by the neutralization causes the blowing agent to boil and hence
to foam. Since, in this process, curing and foaming take place
simultaneously, relatively brittle foams result.
It is an object of the present invention to provide foams based on
melamine/formaldehyde condensates which, when used as insulating
materials, conform to the standards which the construction industry places
on such materials. In particular, the foams should have good mechanical
properties, i.e. they should be resilient, stable and easily handled. In
addition, they should have acceptable flammability, i.e. they should,
according to the classification of DIN 4,102, have no more than normal
flammability but preferably have low flammability.
Another object of the present invention is to provide a process by means of
which soft and resilient reticulated foams which are based on
melamine/formaldehyde condensates and have a very low flammability are
obtained.
A further object of the present invention is to permit the manufacture of
foams of low density, namely even below 8 g.l.sup.-1 and down to 1.6
g.l.sup.-1, and the attainment of foam heights greater than 60 cm.
Accordingly, the invention relates to resilient reticulated foams based on
a melamine/formaldehyde condensate which contains not less than 50% by
weight, preferably not less than 80% by weight, of melamine and
formaldehyde as condensed units, and up to 50% by weight, preferably up to
20% by weight, of other compounds containing amino, amide, hydroxyl or
carboxyl groups and capable of forming thermosetting resins, and aldehydes
which react therewith to foam such resins, as co-condensed units. The
foams have the following characteristic properties:
(a) the bulk density, measured according to DIN 53,420, is from 4 to 80,
preferably from 6 to 60, [g.l.sup.-1 ];
(b) the heat conductivity, measured according to DIN 52,612, is less than
0.06, preferably less than 0.05 and especially less than 0.04
[W.m.sup.-1..sup.o K.sup.-1 ];
(c) the compressive strength, measured according to DIN 53,577 at 60%
compression, divided by the bulk density, is less than 0.3, preferably
less than 0.2, [N.cm.sup.-2 /g.l.sup.-1 ], and in determining the
compressive strength at 60% compression, the foam must recover to not less
than 70%, preferably not less than 80% and especially not less than 90% of
its original height;
(d) the modulus of elasticity, measured by methods similar to DIN 53,423,
divided by the bulk density is less than 0.25, preferbly less than 0.2 and
especially less than 0.15, [N.mm.sup.-2 /g.l.sup.-1 ];
(e) the deflection on break, measured according to DIN 53,423, is greater
than 6, preferably greater than 9 and especially greater than 12, [mm];
(f) the foams are of not more than normal flammability, when assessed
according to DIN 4,102; and
(g) the tensile strength, measured according to DIN 53,571 is preferably
not less than 0.07, in particular not less than 0.1, [N.mm.sup.-2 ].
The invention for the first time provides industrially useful foams, based
on melamine/formaldehyde condensates, for use in the construction
industry. The foams not only have excellent heat insulating properties,
because of their low density and their low heat conductivity but they are
also good sound insulants because of their open-cell, i.e. reticulated
character. They have the advantage over all conventional organic plastic
foams that even without the addition of flameproofing agents they are of
not more than normal flammability, and in the great majority of cases even
of low flammability, when assessed according to DIN 4,102. In the event of
a fire, the foam carbonizes without melting and dripping. The foam is also
self-extinguishing when the flame is removed. Inasmuch as the starting
materials for the production of melamine resins are CO.sub.2, NH.sub.3 and
CH.sub.3 OH, the foam is made without using petroleum-based raw materials.
On microscopic examination of the foams produced according to the invention
it is found that the foam structure comprises a plurality of mutually
connected three-dimensionally branched webs (see FIG. 3). Melamine resin
foams are sufficiently resilient only if the webs conform to the following
conditions:
1. The mean length:width ratio must be greater than 10:1, preferably
greater than 12:1 and in particular greater than 15:1.
2. The density of the webs must be greater than 1.10, preferably greater
than 1.20, and in particular greater than 1.30 g/cm.sup.3.
Webs which are too short (i.e. in which the l:d ratio is too low) are
obtained if the curing process commences too early, before foaming has
substantially ended. Too low a web density indicates that there are minor
cavities, bubbles and the like in the interior of the webs, resulting from
secondary foaming. Such secondary foaming occurs if the water content of
the melamine resin precondensate is too high. In both cases, brittle foams
are obtained.
The mean l:d ratio is determined microscopically, the length and width of
the webs being determined by a statistical counting method. The web length
is defined as the distance between the centers of two nodes, and the web
width is defined as the narrowest part of a web, in each case measured on
a photomicrograph. To determine the density of the foam webs, the foam is
placed in a suitable liquid, for example isopropanol, with which it
becomes fully impregnated by virtue of its open-cell character. The
density of the webs is then determined by the principle of Archimedes.
The foams according to the invention are melamine/formaldehyde condensates
which in addition to melamine can contain up to 50, preferably up to 20, %
by weight of other compounds which form thermosetting resins, and in
addition to formaldehyde can contain up to 50, preferably up to 20, % by
weight of other aldehydes, as co-condensed units. However, the use of an
unmodified melamine/formaldehyde condensate is particularly preferred.
Examples of other compounds capable of forming thermosetting resins are
alkyl-substituted melamine, urea, urethanes, carboxylic acid amides,
dicyandiamide, guanidine, sulfurylamide, sulfonic acid amides, aliphatic
amines, phenol and phenol derivatives. Examples of aldehydes which may be
used are acetaldehyde, trimethylolacetaldehyde, acrolein, benzaldehyde,
furfuraldehyde, glyoxal, phthalaldehyde and terephthalaldehyde. Further
details concerning melamine/formaldehyde condensates are to be found in
Houben-Weyl, Methoden der organischen Chemie, Volume 14/2, 1963, pages 319
to 402. The molar ratio of compound capable of forming a thermosetting
resin to aldehyde can vary within wide limits ranging from 1:1.5 to 1:4.5;
in the case of melamine/formaldehyde condensates it is preferably from
1:2.5 to 1:3.5. The degree of condensation of the precondensate should be
sufficiently low to allow curing accompanied by further condensation. The
mean molecular weight, measured osmometrically, can be from 200 to 1,000,
preferably from 250 to 800. The melamine resins advantageously contain
co-condensed sulfite groups. The sulfite groups may be introduced, for
example, by adding from 1 to 20% by weight of sodium bisulfite during or
after the condensation of the resin. The sulfite groups make the resin
more hydrophilic and hence more compatible with water. Furthermore, higher
degrees of condensation are achieved.
The foams according to the invention can contain up to 50, preferably up to
20, % by weight of conventional additives. Examples are fibrous and
pulverulent inorganic reinforcing agents or fillers, such as glass fibers,
metal powders, metal oxide, metal salts and silicates, e.g. talc, kaolin,
quartz, baryte and chalk; pigments and dyes; flameproofing agents, e.g.
halogen-containing and phosphorus-containing organic compounds;
plasticizers, such as triols, e.g. trimethylolpropane, and acrylates;
water-repellents, e.g. alkylphenols; and agents which lower the toxicity
of cumbustion fumes, e.g. compounds of trivalent boron, divalent copper or
trivalent iron, or which assist carbonization, e.g. sucrose. However, it
is always preferred to dispense with the use of such additives. It is
known, for example, that plasticizers increase the flammability of the
foams. Plasticizers are therefore only added in cases where it is
important to have very flexible foams. In that case it may become
necessary to add flameproofing agents which are normally-especially in the
case of unmodified melamine/formaldehyde condensates-not needed. Since the
foams have an open-pore structure and can therefore absorb water, it may
be necessary, for certain applications, to add from 0.2 to 5% by weight of
a water-repellant.
The foams according to the invention exhibit an extremely low bulk density,
which may be as little as 4 [g.l.sup.-1 ]. When manufactured by the
ultra-high-frequency irradiation method, the foams can exhibit a density
of as little as 1.6 [g.l.sup.-1 ]. The consequence of this low bulk
density is that the foam raw material costs are very low. Higher
densities, of up to 80 [g.l.sup.-1 ], are only of interest for particular
applications, for example in composite materials. Preferably, the bulk
density is from 6 to 60, especially from 8 to 40, [g.l.sup.-1 ]. The heat
conductivity at 10.degree. C. is less than 0.06, preferably less than 0.05
and especially less than 0.04 [W.m.sup.-1..sup.o K.sup.-1 ]. Accordingly,
it is less than in the case of soft polyurethane foams and of the order of
magnitude found with polystyrene foams.
The advantageous mechanical properties are expressed in terms of the
compressive strength, the modulus of elasticity, the deflection on break
and the tensile strength. The modulus of elasticity is determined by a
method based on DIN 53,423, the test speed being 5 [mm.min.sup.-1 ] and
the measurement being taken to a maximum deflection of 0.5 mm. The
compressive strength and the modulus of elasticity are found to vary about
proportionally to the density, so that the quotient of compressive
strength to bulk density, or of modulus of elasticity to bulk density, can
be employed to characterize the materials. A very important performance
characteristic of the foams according to the invention is their recovery
after compression. It is measured when determining the compressive
strength and at 60% compression is found to be not less than 70%,
preferably not less than 80% and especially not less than 90%. It is
noteworthy that with freshly produced foam the first compression differs
substantially from subsequent compressions. For the first compression, a
somewhat greater force is required, evidently attributable to the fact
that residual hard zones in the cell structure are destroyed. These hard
zones can be destroyed by milling the foams. A further very important
property is the flammability. When assessed according to DIN 4,102, the
foams are of not more than normal flammability and preferably of low
flammability. Normal flammability is confined to exceptional cases where,
for example, a plasticizer has been added or a modifier (in the nature of
a compound which forms aminoplasts), e.g. urea, has been co-condensed.
The foams according to the invention can be prepared by foaming an aqueous
solution or dispersion which contains a melamine/formaldehyde
precondensate, an emulsifier, a blowing agent and a curing agent, with or
without conventional additives, and then curing the foam. We have found
that particularly resilient foams are obtained if relatively highly
concentrated solutions or dispersions, preferably of more than 68 percent
strength by weight and especially of more than 72 percent strength by
weight, are employed (the concentrations being based on the mixture of
resin and water, without additive), and if foaming is carried out under
conditions such that initially there is only a slight rise in viscosity
and the process of curing, with substantial increase in viscosity, only
commences when the foaming process has been substantially completed.
A preferred process for the preparation of the resilient foam based on a
melamine-formaldehyde condensate comprises the following steps:
(a) the concentration of the precondensate in the mixture of precondensate
and water (without additives) is selected to be above the salient point of
the 1st derivative of the curve which is obtained when, keeping all other
conditions constant, the amount of water in the mixture of precondensate
and water is varied and the viscosity of the mixture (measured at the
boiling point of the blowing agent under the conditions prevailing at the
start of foaming) is plotted against the concentration of the
precondensate, which concentration must however not be higher than the
value which in the curve described corresponds to a viscosity of 5,000
dPas, preferably 2,000 dPas and especially 1,000 dPas;
(b) during the foaming process, up to the time at which the foam has
reached 80% of the maximum attainable rise height, the viscosity of the
aqueous solution or dispersion must not fall below the value which, in the
curve described under (a), corresponds to the minimum concentration
defined there, but must not exceed 6,000 dPas, preferably 2,500 dPas and
especially 1,200 dPas; and
(c) after reaching the time defined under (b), the viscosity exceeds a
value of 10,000 dPas, due to curing of the precondensate, within 8
minutes, the viscosities referred to in (b) and (c) being measured, in
each case, on a parallel system which is free from blowing agent.
This process surprisingly gives resilient, soft reticulated foams which
when used as insulating materials meet the standards placed by the
building trade on such materials, especially with respect to their
heat-insulating and sound-insulating properties, their mechanical
properties and their behavior on exposure to fire. The foaming of very
concentrated solutions or dispersions must be regarded as a bold step into
technologically new territory, since the art has always avoided using
melamine resin concentrations which are so high that, in particular, the
dispersions are not stable on storage.
The aqueous solution or dispersion of the melamine resin contains an
emulsifier, preferably in an amount of from 0.5 to 5% by weight, and
especially from 1.0 to 3.0% by weight, based on resin. The purpose of the
emulsifier is to disperse the organic blowing agent homogeneously in the
aqueous solution or dispersion. Accordingly, the emulsifier ensures the
stability of the system and prevents phase separation during foaming; such
phase separation would result in an inhomogeneous foam. The higher the
foaming temperature, the more effective the emulsifier must be, and the
higher should be the concentration used. The emulsifier furthermore acts
as a nucleating agent for the foaming process. Suitable materials are
anionic compounds, especially metal alkylsulfonates and
alkylarylsulfonates, where alkyl is of 8 to 20 carbon atoms, the metal
preferably being sodium; metal salts of sulfosuccinic acid esters,
sulfonated castor oils, alkylnaphthalenesulfonic acids, phenolsulfonic
acids and sulfuric acid esters, for example C.sub.12 -C.sub.18 -alkyl
hydrogen sulfates and C.sub.16 -C.sub.18 -fatty alcohol hydrogen sulfates,
are also suitable, as are cationic compounds, e.g. oleic acid
triethanolamine ester and laurylpyridinium chloride, and non-ionic
compounds, e.g. oxyethylated caster oil, oxyethylated tallow alcohols,
oxyethylated stearic acid or oleic acid, and oxyethylated nonylphenol.
The aqueous solution or dispersion additionally contains a volatile blowing
agent, preferably boiling at from -20.degree. to 100.degree. C.,
especially from +20.degree. to +80.degree. C. Examples are hydrocarbons,
halohydrocarbons, alcohols, ketones, ethers and esters. Preferred blowing
agents are pentane, hexane, trichlorofluoromethane and
trichlorotrifluoroethane. The amount of blowing agent depends on the
desired density of the foam and can be from 1 to 50% by weight, preferably
from 5 to 40% by weight, based on resin. These blowing agents are
uniformly distributed in the aqueous solution or dispersion. Such agents
vaporize on heating, and cause the materials to foam. It is also possible
to force gases under pressure into the aqueous solution or dispersion and
bring about the foaming process by releasing the pressure. Finally, it is
also possible to introduce into the mixture chemical compounds which
react, or decompose, with the formation of a gas, and to bring about the
reaction or decomposition by heating.
The curing agents employed are compounds which under the reaction
conditions split off, or form, protons, which then catalyze the further
condensation of the melamine resin. The amount used is from 0.01 to 20,
preferably from 0.05 to 5, % by weight, based on the resin. Examples of
suitable compounds are inorganic and organic acids, e.g. hydrochloric
acid, sulfuric acid, phosphoric acid, formic acid, acetic acid, oxalic
acid, lactic acid and amino acids, and latent curing agents, e.g. salts of
halocarboxylic acids, chloroacetamide, hydrogen phosphates, acid
anhydrides and ammonium salts. Formaldehyde itself can also undergo
disproportionation at elevated temperatures, to form formic acid, and can
thus act as a curing agent.
The aqueous solution or dispersion is preferably free from additives. For
some purposes, however, it can be advantageous to add up to 20% by weight,
but preferably less than 10% by weight, based on resin, of conventional
additives, such as fibrous or pulverulent inorganic reinforcing agents or
fillers, pigments, dyes, flameproofing agents, plasticizers, agents for
reducing the toxicity of the gas evolved on combustion or agents to
promote carbonization. Since the foams have an open-cell structure and can
absorb water, it may be necessary, for some purposes, to add from 0.2 to
5% by weight of a water-repellent agent. Examples are alkylphenols, where
alkyl is of 5 to 15 carbon atoms, silicons and paraffins.
The additives are mixed homogeneously with the aqueous solution or
dispersion of the melamine resin, during which mixing the blowing agent
can be forced in, where appropriate under pressure. However, it is also
possible to start from a solid, for example spray-dried, melamine resin
and to mix this with the blowing agent and with an aqueous solution of the
emulsifier and of the curing agent. The sequence of addition of the
components depends on the particular mixing process. The mixture is
brought to the boiling point of the blowing agent in the solution or
dispersion at the prevailing pressure. This may be done by heating, for
example with hot air, steam or high frequency radiation, or by utilizing
the heat of reaction. On being brought to this temperature, the blowing
agent is transformed to a gas and is thus able to cause the mixture to
foam. During the isothermal foaming, the aqueous solution or dispersion
assumes the boiling point of the blowing agent at the prevailing pressure.
Preferably, the process is carried out under atmospheric pressure, with
the material at from 20.degree. to 80.degree. C., though the temperature
of the surroundings may be substantially higher.
A critical feature (a) of the preferred process is the concentration of the
precondensate in its mixture with water (without additives). The optimum
concentration is different for every foaming temperature, i.e. it depends
on the nature of the blowing agent. In this preferred process, the minimum
concentration must conform to the condition that it should lie above the
salient point of the 1st derivative of the curve which is obtained when,
keeping all other conditions constant, the amount of water in the mixture
of precondensate and water is varied and the viscosity of the mixture
(measured at the boiling point of the blowing agent under the conditions
prevailing at the start of foaming) is plotted against the concentration
of the precondensate. In practice, the minimum concentration is determined
by preparing mixtures of precondensate and water, containing various
amounts of the latter, and then heating these mixtures to a temperature at
which the blowing agent would boil under the pressure envisaged for the
start of foaming. The corresponding viscosity for each concentration of
the melamine resin is then measured under these conditions. Thereafter,
the measured viscosity is plotted against the particular concentration.
The initial part of the resulting curve has the shape of a straight line
of only slight slope, but then rises progressively more rapidly and
ultimately assumes a paraboloid course. The 1st derivative of this curve
is obtained by graphical methods. It is initially in the form of a
horizontal straight line, followed by a curved salient region, and
ultimately becomes a steep straight line. The salient region in general
extends over a range of at most about 1% difference in concentration of
the precondensate. This region represents the minimum concentration of the
melamine resin. If it is desired to locate the salient point even more
precisely, the straight line portions of the curve representing the 1st
derivative are extrapolated and their point of intersection is determined.
The upper limit of the melamine resin concentration must conform to the
following condition: it must not exceed the value which, in the curve
described, corresponds to a viscosity of 5,000 dPas, preferably 2,000 dPas
and especially 1,000 dPas.
For the preferred blowing agents, the preferred resin concentrations, which
lie within the range defined above, are as follows:
n-pentane: from 70 to 80, preferably from 72 to 79, especially from 73 to
78.5, % by weight;
n-hexane: from 73 to 85, preferably from 74 to 84, and especially from 78
to 83, % by weight;
trichlorofluoromethane: from 68 to 78, preferably from 69 to 77, especially
from 70 to 76, % by weight;
trichlorotrifluoroethane: from 72 to 82, preferably from 74 to 80, % by
weight.
The above concentrations are again based on the mixture of precondensate
and water, without additives.
The second critical feature (b) of the preferred process is that during
foaming, up to the time at which the foam has reached 80% of the maximum
attainable rise height, the viscosity of the aqueous solution or
dispersion must not fall below the value which corresponds, on the curve
described under (a), to the minimum concentration defined under (a), but
must not exceed 6,000 dPas.
The third critical characteristic (c) means that after reaching the time
defined under (b) the viscosity must, due to curing of the precondensate,
exceed a value of 10,000 dPas within 8 minutes, preferably within 6
minutes, especially within 4 minutes, i.e. the resin must have cured
within this time. The two conditions (b) and (c) ensure that foaming and
curing are correctly balanced; for a given blowing agent and hence a given
foaming temperature, the two conditions can be met if the nature and
amount of the curing agent are correctly chosen. In practice, two aqueous
solutions or dispersions are prepared, of which one contains the melamine
resin in the concentration determined according to (a), the emulsifier,
the curing agent, the blowing agent and the additives, if any, while the
other contains the same constituents, but without blowing agent. Both
systems are then brought to the foaming temperature. The parallel batch
comprising the solution or dispersion which is free from blowing agent
must be prepared since the system containing blowing agent foams under
these conditions and hence its viscosity cannot be measured. For the first
system, the rise height of the foam is plotted against time in order to
determine the time at which the foam has reached 80% of the maximum
attainable rise height. Using the second system, the viscosity is measured
as a function of time. The results are then checked as to whether the
requirements stated under (b) and (c) are met. Should this not be the
case, the curing conditions must be varied, and this can best be achieved
by varying the nature and amount of the curing agent and, where necessary,
also by selecting a different blowing agent and hence a different foaming
temperature. In the latter case, however, the optimum concentration of the
melamine resin would have to be determined afresh, in accordance with (a).
A resilient, stable foam of low density is only obtained if during foaming
and curing the conditions specified under (a), (b) and (c) are observed.
If the initial concentration of melamine resin is too low, or if the
viscosity drops below the defined initial viscosity before the foam has
reached 80% of its maximum rise height, brittle foams are obtained, as
with the prior art processes. If solutions or dispersions which are
initially too viscous are employed, or if the viscosity increases above
the defined permissible limit before 80% of the maximum foam rise height
has been reached, the foaming pressure no longer suffices for satisfactory
foaming, and the foams obtained are too dense and insufficiently
resilient. If, after reaching 80% of the maximum rise height of the foam,
the viscosity increases too slowly, i.e. if the foam does not cure
sufficiently rapidly, it collapses and a brittle, non-homogeneous foam
having too high a density is produced.
Preferably, the pressure in the foaming apparatus, and hence the material
temperature during foaming, are kept constant. However, these conditions
can be varied in the course of the foaming process. The foaming process in
general requires from 20 seconds to 20 minutes, preferably from 30 seconds
to 10 minutes, depending on the nature and intensity of heating employed.
The foaming process is considered to have been completed when the resin
has finished foaming and has cured to the point that it retains its shape.
The manufacture of the resilient foam can be carried out batchwise or
continuously. In continuous operation, which is preferred, the aqueous
solution or dispersion is advantageously applied to a continuously moving,
preferably heated, metal belt, on which it is spread uniformly and then
foamed and cured in a heated tunnel. To prevent the formation of a brittle
skin on the surface of the foam, the foaming process can advantageously be
carried out between two plastic films travelling synchronously with the
metal belt. This foaming can be followed directly by the heat treatment
and/or milling treatment.
In a special embodiment of the invention, a resilient foam of very low
density is manufactured by foaming an aqueous or alcoholic solution or
dispersion which contains a melamine-formaldehyde precondensate, an
emulsifier, a blowing agent and a hardener, with or without conventional
additives, and then crosslinking the precondensate, wherein, to effect
foaming and crosslinking, the solution or dispersion is heated by
ultra-high-frequency irradiation in such a manner that the power uptake by
the solution or dispersion is from 5 to 200 KW per kg of water or alcohol
in the solution or dispersion.
Using this process, it is possible to produce foams, based on
melamine-formaldehyde condensates, which have a density of less than 8
g.l.sup.-1 and a foam height in excess of 60 cm.
We have found, surprisingly, that in this process the volume of the foam is
substantially greater than the gas volume of the blowing agent employed.
In thermal foaming, foam formation is virtually only due to the blowing
agent which is employed and which on heating forms a gas, i.e. one mole of
blowing agent gives at most 22.4 liters of foam under standard conditions
of temperature and pressure. In the process according to the special
embodiment of the invention, surprisingly, substantially more foam, for
example five times as much, is formed. This phenomenon is attributable to
the fact that the ultra-high-frequency irradiation of the aqueous melamine
resin solution or dispersion causes not only volatilization of the blowing
agent employed, but also of water, which acts as an additional blowing
agent. This effect is of great industrial importance, inasmuch as in this
way much less of the volatile blowing agent need be employed and
volatilized and accordingly the process causes substantially less
pollution of the environment than the prior art process, or substantially
smaller amounts of volatilized blow | | |
