Note: Descriptions are shown in the official language in which they were submitted.
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PREPARATION OF A CONDENSATION RESIN AND IMPREGNATION PROCESS
The present invention relates to a process for the preparation of a
condensation resin from its monomers in an aqueous medium at elevated
temperature
and pressure.
Processes for the preparation of condensation resins are either
performed batch wise, or in recent days also (semi-) continuous. Examples of
batch
wise preparation are those which are performed in a stirred tank reactor, or
even a
combination of several of stirred tank reactors, all operated in batch mode.
A continuous process has also been considered in the art, an
example whereof being presented in EP-A-355,760. In this publication a
melamine-
formaldehyde precondensate is prepared in a single or double screw extruder.
This has as a consequence that in the extruder (in fact a tubular
reactor with dynamic elements) a lot of mixing energy is consumed, next to the
fact that
only viscous streams can be dealt with.
The present invention recognizes that the use of an extruder is not
appropriate for processes for a preparation of a condensation resin in which
the
process stream has a viscosity well below 50 Pa.s. The present invention also
acknowledges that preparation at such low viscosities at more elevated
pressures than
normally attainable in an extruder are desired, as a result of which the
process can be
performed at a higher temperature. This all being the consequence of the fact
that the
resin preparation is performed in an aqueous medium.
The present inventions present a solution for the above indicated
items, in that the temperature is between 70 and 200 C, the pressure is
between 0.2
and 20 MPa, and in that the monomers are continuously fed to a tubular reactor
which
is provided with static mixer elements.
As a result, the process can be performed with process streams that
have a significant lower viscosity than those which are suitable for an
extruder-
operated process. Generally, the present process can cope with viscosities up
to 10
Pa.s; more preferred the viscosity of the contents of the reactor is at most
1800 mPa.s;
the viscosity being determined at the local conditions in the reactor (i.e. at
the local
pressure and temperature conditions).
The tubular reactor used in the process is, in the inside of the reactor,
provided with one or more static mixer elements. Such a reactor, also named a
static
mixer,
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can be described as a pipe with immovable internal elements that achieve
continuous
multiple splitting and recombination, and/or turbulence of streams of material
passing
through and improve distributive mixing. A short compilation of various types
of static
mixer elements can be found for instance at: http://www.best-
mixer.de/html/stromungs-
mischer.html.
As a result of the presence of static mixer elements in the tubular
reactor a better flow profile of the process stream, with a beneficial
influence on both
mass and heat transfer during the transport of the process stream through the
reactor
is achieved. This beneficial influence is also present in comparison with the
use of an
empty, tubular pipe, as is disclosed in WO 2006/119982 for the preparation of
a
melamine-formaldehyde resin.
The process of the present invention is suitable for the preparation of
any condensation resin, in which said preparation takes place in an aqueous
medium;
or in other words: in all preparations where water is either a solvent or a
dispersion
agent. Other solvents or dispersing liquids may be present, next to water, but
they are
only present in a minor amount compared to water.
A non-limiting list of condensation resins that can be prepared with
the process of the present invention is polyamides, polyacetals, polyesters,
and
adhesives useful in engineered wood, such as condensation resins based on
phenol,
melamine (or more generally triazines), urea, and aldehydes (like (para-
)formaldehyde). In general, here and herein after, a condensation resin is any
class of
polymer formed through a condensation reaction, releasing (or condensing) a
small
molecule by-product such as water or methanol, as opposed to an addition
polymer
which involves the reaction of unsaturated monomers.
Condensation polymerization, a form of step-growth polymerization, is
a process by which two molecules join together, with the loss of a small
molecule which
is often water. The type of end product resulting from a condensation
polymerization is
dependent on the number of functional end groups of the monomer which can
react.
Monomers with only one reactive group terminate a growing chain,
and thus give end products with a lower molecular weight. Linear polymers are
created
using monomers with two reactive end groups; monomers with more than two
reactive
end groups give three dimensional polymers (network polymerization).
The process of the present invention is performed at elevated
pressure and temperature, which can be selected for the preparation of the
desired
resin, within the boundaries of the conditions needed for said preparation.
The
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temperature is between 70 and 200 C; the pressure is between 0.2 and 20 MPa.
Preferably, the temperature is between 100 and 150 C, at a pressure which is
at least
the corresponding vapor pressure. Of course the skilled man can decide to
select his
own pressure, deviating from the vapor pressure, for instance by using a
pressure
control. The dimensions of the tubular reactor can be chosen freely, depending
on the
desired throughput. Preferably the tubular reactor has a circular cross-
section. The
diameter of the tube will in general be at least 5 mm, and will in general not
exceed 500
mm. The length will in general be at least 25 mm, and will in general not
exceed 100 m.
The skilled man is able to select the material of the tubular reactor wall and
of the static
mixer elements, based on the materials (like monomers/polymer, other added
ingredients) to be processed in the reactor.
In general the process is be applied using an aqueous medium, in
which the solids content of the resin is between 20 and 85 wt.%; more
preferred this
content is between 45 and 75 wt.%.
The process of the present invention is very suitable for the
preparation of a condensation resin, wherein said resin is prepared from an
aldehyde
(preferably (para-)formaldehyde), a triazine (preferably melamine), an
aromatic alcohol
(preferably phenol), or urea. Next to the individual ingredients, also
mixtures of said
ingredients can be used (like a mixture of melamine, urea and formaldehyde,
resulting
in a so-called MUF-resin, or a mixture of melamine, urea, phenol, and
formaldehyde,
resulting in a MUPF-resin). The preparation of the resin according to the
present
invention can also start with a so-called precondensate, which is a low-
molecular
precursor of the desired resin, but in which already some degree of
condensation
between the constituting monomers has taken place. Preferably, the
condensation
resin to be prepared according to the process of the present invention is an
aminoplast
(a condensation resin based on a triazine or urea, and an aldehyde) or a
phenolic resin
(a condensation resin based on an aromatic alcohol, and an aldehyde). The
resulting
resin is a so-called network polymer, in contrast to a linear or branched
polymer.
In the case of an aminoresin, the triazine is preferably melamine; the
aldehyde is preferably (para-)formaldehyde. In the case of a phenolic resin,
the
aromatic alcohol is preferably phenol; the aldehyde is preferably (para-
)formaldehyde,.
Even more preferred is a condensation resin, based on
formaldehyde, melamine and urea.
In case a condensation resin is prepared having melamine and
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(para-)formaldehyde as the at least two constituting ingredients of the final
resin, the
F/M ratio (being the molar ratio between the formaldehyde (F) and the melamine
(M) in
the condensation resin) is generally between 0.5 and 4.0; preferably this
ratio is
between 0.75 and 1.8. In the present process, all the melamine-formaldehyde
resins as
disclosed in WO 2006/119982 can be prepared.
The ingredients necessary for the preparation of the condensation
resin can be metered to the reactor in any desired way: pre-mixed at a low
temperature; or at different locations over the length of the reactor. One can
also
choose to feed one or more of the ingredients at multiple locations along the
length of
the reactor. All of these possibilities are available to the skilled man in
order to fine-tune
the preparation process.
The ingredients can also, and preferably, be metered individually to
the reactor, in order to better control the dosage of each ingredient.
In case the ingredients are premixed before entering the tubular
reactor, this tubular reactor is preceded with equipment in which the
ingredients for the
preparation of the condensation resin are premixed. This can be done in a
stirred tank,
or in a tube, coupled at the front end of the reactor. As a result, a well
mixed
feedstream of the ingredients in water can be achieved, before the reaction
takes place
in the tubular reactor. The premixer can also be used to (partially) preheat
the mixture
from room temperature. Premature polymerization in the premixer should be
avoided
as much as possible.
Of course also a combination of the above can be used. Reference
can be given to a situation in which part of one or more of the ingredients
needed for
the condensation resin is fed as a premix to the tubular reactor, and in which
the
remaining amount(s) are directly fed to the reactor, possibly at multiple
locations along
the length of the reactor. All this is within the skills of the man skilled in
the art of
process technology.
Next to the ingredients necessary for the preparation of the
condensation resin, additives can also be present in the final resin. These
additives,
the nature and function of which are known to the skilled man, are also fed to
the
reactor, or added to and in the premixing step, and are for instance catalyst,
fillers,
emulgators, etc. The reader is referred to the literature hereon, including
the teachings
of the EP-A-355,760.
The product resulting from the process of the present invention is a
condensation resin in an aqueous medium, at elevated temperature and pressure.
This
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makes this product very suitable for use in an impregnation process, in which
a
substrate (like paper, wool, etc.) is impregnated with the resin, especially
when the
impregnation process is also performed under elevated pressure (in order to
improve
the degree and/or speed of impregnation).
This effect is the more suitable, when the pressure in the
impregnation process is essentially the same or lower, than the pressure in
the resin
preparation. Then the process for the preparation of the condensation resin
and the
impregnation process can be directly coupled, so as to avoid for instance
storage of the
resin, and reheating and repressurizing the stored resin to the impregnation
conditions.
Preferably, also the impregnation process is performed at a temperature
essentially the
same as the temperature in the resin preparation. As a result, an inline
combination of
condensation resin preparation and impregnation is created. In the context of
the
present specification, the term "essentially" is meaning that the pressure or
temperature in the impregnation do not differ more than 15 % of the value(s)
thereof in
the tubular reactor. Before the impregnation process, and at the exit of the
tubular
reactor, the usual ingredients used in an impregnation process (like hardener,
wetting
agent, release agent) are added. The skilled man is aware of these
impregnation
ingredients and how to supply them. Some of them may also be present in the
feed to
the tubular reactor, in order to achieve an intimate mix of these ingredients
with the
resin.
The invention will be elucidated with the following Examples and
comparative experiment, which are intended to show the benefits of the
invention, but
not to restrict it.
The Examples and experiment were performed in a heated steel
tubular reactor with an internal diameter of 10 mm and a length of 2.0 m. The
reactor
was provided with 96 SM X L static elements of Sulzer having a diameter of 10
mm.
The result of the Examples was determined with respect to the so-called water-
tolerance (W.T.) of the obtained resin. This W.T. is the amount of resin that
can be
dissolved in water at room temperature (dimension: gram/gram).
Example I
Formaline (with 30 wt. % formaldehyde (F), melamine (M), di-
ethylene glycol (DEG) and caprolactam were mixed with water to obtain a
dispersion
with an F/M-molar ratio of 1.65. Table 1 gives the used recipe (in wt. %). The
ingredients were mixed in a storage tank, provided with a circulation pump.
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Table 1
Formaline Melamine DEG Caprolactam Water Solids
56.0 35.3 1.5 1.5 5.8 55.0
The flow through the reactor was varied. The temperature of the mixture
entering the
reactor was 120 C. At the exit of the reactor the temperature was 140 C;
thereafter
the mixture was cooled via a water bath to room temperature. The pressure in
the
reactor was set at 1 MPa. Table 2 gives the realized water tolerances (W.T.)
of the
produced resin, as a function of the flow through the reactor.
Table 2
Flow (kg/h) 5.4 4.8 4.6 4.2
W.T. (g/g) 4.8 2.2 1.8 1.2
Example II
Example I was repeated, but now with an F/M molar ratio of 1.4.
Table 3 gives the recipe (in wt. %).
Table 3
Formaline Melamine DEG Caprolactam Water Solids
56.0 35.3 1.5 1.5 5.8 60.0
The flow was set at 5.6 kg/h; it resulted in a water tolerance of 0.6.
Comparative experiment A
Example I was repeated, but now in absence of the static mixer
elements in the reactor (i.e. with the use of a non-filled tube). After 4
hours of
experimentation, the tube appeared to be plugged internally due to the
formation of
polymer on the internal wall of the tube.
