U.S. patent application number 12/600350 was filed with the patent office on 2010-06-17 for xerogels made from aromatic polyureas.
This patent application is currently assigned to BASF SE. Invention is credited to Andreas Emge, Marc Fricke, Vijay Immanuel Raman, Antonio Sanchez-Ferrer, Volker Schadler, Werner Wiegmann.
Application Number | 20100148109 12/600350 |
Document ID | / |
Family ID | 39719023 |
Filed Date | 2010-06-17 |
United States Patent
Application |
20100148109 |
Kind Code |
A1 |
Schadler; Volker ; et
al. |
June 17, 2010 |
XEROGELS MADE FROM AROMATIC POLYUREAS
Abstract
The invention relates to a xerogel comprising from 30 to 90% by
weight of a monomer component (a1) composed of at least one
polyfunctional isocyanate and from 10 to 70% by weight of a monomer
component (a2) composed of at least one polyfunctional aromatic
amine, at least one of which is selected from
4,4'-diaminodiphenylmethane, 2,4'-diaminodiphenylmethane,
2,2'-diaminodiphenylmethane and oligomeric diaminodiphenylmethane,
where the sum of the % by weight of monomer components (a1) and
(a2) adds up to 100% by weight and where the monomer components are
present in polymeric form in the xerogel and the volume-weighted
mean pore diameter of the xerogel is at most 5 .mu.m, The invention
further relates to a process for preparing xerogels, to the
xerogels thus obtainable and to the use of the xerogels as an
insulating material and in vacuum insulation panels.
Inventors: |
Schadler; Volker; (Ann
Arbor, MI) ; Fricke; Marc; (Osnabruck, DE) ;
Wiegmann; Werner; (Rahden-Wehe, DE) ; Emge;
Andreas; (Lemforde, DE) ; Raman; Vijay Immanuel;
(Mannheim, DE) ; Sanchez-Ferrer; Antonio;
(Barcelona, ES) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ LLP
P.O. BOX 2207
WILMINGTON
DE
19899
US
|
Assignee: |
BASF SE
LUDWIGSHAFEN
DE
|
Family ID: |
39719023 |
Appl. No.: |
12/600350 |
Filed: |
May 16, 2008 |
PCT Filed: |
May 16, 2008 |
PCT NO: |
PCT/EP08/56015 |
371 Date: |
November 16, 2009 |
Current U.S.
Class: |
252/62 |
Current CPC
Class: |
C08G 2330/50 20130101;
C08G 2110/0091 20210101; C08G 18/7664 20130101; C08J 2375/02
20130101; E06B 3/6612 20130101; E06B 3/6775 20130101; C08J
2201/0502 20130101; C08J 2205/028 20130101; C08J 9/28 20130101;
C08G 18/3243 20130101 |
Class at
Publication: |
252/62 |
International
Class: |
E04B 1/74 20060101
E04B001/74 |
Foreign Application Data
Date |
Code |
Application Number |
May 16, 2007 |
EP |
07108306.7 |
Claims
1.-15. (canceled)
16. A xerogel comprising: from 30 to 90% by weight of a first
monomer component comprising at least one polyfunctional isocyanate
and from 10 to 70% by weight of a second monomer component
comprising at least one polyfunctional aromatic amine; wherein at
least one of the at least one polyfunctional aromatic amine is
selected from the group consisting of 4,4'-diaminodiphenylmethane,
2,4'-diaminodiphenylmethane, 2,2'-diaminodiphenylmethane and
oligomeric diaminodiphenylmethane; wherein a sum of the % by weight
of the first and second monomer components is about 100% by weight;
and wherein the first and second monomer components are present in
polymeric form in the xerogel and a volume-weighted mean pore
diameter of the xerogel is at most 5 .mu.m.
17. The xerogel of claim 1, wherein the xerogel comprises from 40
to 80% by weight of the first monomer component and from 20 to 60%
by weight of the second monomer component.
18. The xerogel of claim 1, wherein the first monomer component
consists of at least one polyfunctional aromatic amine selected
from the group consisting of: 4,4`-diaminodiphenylmethane,
2,4'-diaminodiphenylmethane, 2,2'-diaminodiphenylmethane and
oligomeric diaminodiphenylmethane.
19. The xerogel of claim 1, wherein the first monomer component
comprises from 50 to 70% by weight of the first monomer component
and from 30 to 50% by weight of the second monomer component.
20. The xerogel of claim 1, wherein the second monomer component
comprises oligomeric diaminodiphenylmethane and has a functionality
of at least 2.5.
21. The xerogel of claim 1, wherein the first monomer component
comprises at least one polyfunctional isocyanate selected from the
group consisting of: diphenylmethane 4,4'-diisocyanate,
diphenylmethane 2,4'-diisocyanate, diphenylmethane
2,2'-diisocyanate and oligomeric diphenylmethane diisocyanate.
22. The xerogel of claim 1, wherein the first monomer component
comprises oligomeric diphenylmethane diisocyanate and has a
functionality of at least 2.5.
23. The xerogel of claim 1, wherein the first monomer component
comprises an oligomeric diphenylmethane diisocyanate and the second
monomer component comprises an oligomeric diaminodiphenylmethane;
and wherein a sum of functionalities of the first monomer component
and the second monomer component is at least 5.5.
24. The xerogel of claim 1, wherein the volume-weighted mean pore
diameter of the xerogel no more than 3 .mu.m.
25. A process for preparing xerogels, comprising providing a gel
precursor comprising first and second monomer components in a
solvent, wherein from 30 to 90% by weight of a first monomer
component comprises at least one polyfunctional isocyanate and from
10 to 70% by weight of a second monomer component comprises at
least one polyfunctional aromatic amine, wherein at least one of
the at least one polyfunctional aromatic amine is selected from the
group consisting of 4,4'-diaminodiphenylmethane,
2,4'-diaminodiphenylmethane, 2,2'-diaminodiphenylmethane and
oligomeric diaminodiphenylmethane; wherein a sum of the % by weight
of the first and second monomer components is about 100% by weight;
and wherein the first and second monomer components are present in
polymeric form in the xerogel and a volume-weighted mean pore
diameter of the xerogel is at most 5 .mu.m; converting the gel
precursor in the presence of the solvent to a gel; and drying the
resulting gel by converting the liquid present in the gel to the
gaseous state at a temperature and a pressure below a critical
temperature and a critical pressure of the liquid present in the
gel.
26. The process for preparing xerogels of claim 25, further
comprising modifying the gel by means of at least one organic
compound which was present neither in the providing step nor the
converting step; wherein the steps are performed in the order
recited.
27. The process for preparing xerogels of claim 25, wherein the
resulting gel is dried by converting the solvent to the gaseous
state at a temperature and a pressure below a critical temperature
and a critical pressure of the solvent.
28. A xerogel obtainable from the process of claims 25.
29. The use of xerogels of claim 1 as an insulating material.
30. The use of xerogels of claim 1 for vacuum insulation panels.
Description
[0001] The invention relates to a xerogel comprising [0002] from 30
to 90% by weight of a monomer component (a1) composed of at least
one polyfunctional isocyanate and [0003] from 10 to 70% by weight
of a monomer component (a2) composed of at least one polyfunctional
aromatic amine, at least one of which is selected from
4,4'-diaminodiphenylmethane, 2,4'-diaminodiphenylmethane,
2,2'-diaminodiphenylmethane and oligomeric diaminodiphenylmethane,
where the sum of the % by weight of monomer components (a1) and
(a2) adds up to 100% by weight and where the monomer components are
present in polymeric form in the xerogel and the volume-weighted
mean pore diameter of the xerogel is at most 5 .mu.m.
[0004] The invention further relates to a process for preparing
xerogels, to the xerogels thus obtainable and to the use of the
xerogels as an insulating material and in vacuum insulation
panels.
[0005] Porous materials, for example polymer foams, with pores in
the size range of significantly below 1 .mu.m and a high porosity
of at least 70% are particularly good thermal insulators on the
basis of theoretical considerations.
[0006] Such porous materials with a small mean pore diameter may be
present, for example, in the form of organic xerogels. In the
literature, the term "xerogel" is not used uniformly throughout. In
general, a xerogel is understood to mean a porous material which
has been prepared by a sol-gel process, the liquid phase having
been removed from the gel by drying below the critical temperature
and below the critical pressure of the liquid phase ("subcritical
conditions"). In contrast, reference is generally made to aerogels
when the removal of the liquid phase from the gel has been
performed under supercritical conditions.
[0007] In the sol-gel process, a sol is first prepared on the basis
of a reactive organic gel precursor, and then the sol is gelated by
a crosslinking reaction to form a gel. In order to obtain a porous
material, for example a xerogel, from the gel, the liquid has to be
removed. This step is referred to hereinafter in a simplifying
manner as drying.
[0008] WO-95/02009 discloses isocyanate-based xerogels which are
suitable especially for applications in the field of vacuum
insulation. The publication additionally discloses a sol-gel-based
process for preparing the xerogels using known polyisocyanates
including aromatic polyisocyanates and an unreactive solvent. As
further compounds with active hydrogen atoms, aliphatic and
aromatic polyamines or polyols are used. The examples disclosed in
the publication comprise those in which a polyisocyanate is reacted
with diaminodiethyltoluene. The xerogels disclosed generally have
mean pore sizes in the region of 50 .mu.m. In one example, a mean
pore diameter of 10 .mu.m is specified.
[0009] The thermal conductivity of the xerogels disclosed is,
however, not sufficient for all applications. For applications in
the region of pressures above the vacuum range, especially in the
pressure range from about 1 to about 100 mbar, the thermal
conductivity is generally unsatisfactorily high. In addition, the
material properties, especially the mechanical stability of the
xerogel and the porosity, are not sufficient for all
applications.
[0010] It was therefore an object of the invention to provide a
porous material which has a low thermal conductivity. Furthermore,
the xerogels should have a low thermal conductivity even at
pressures above the vacuum range, especially in a pressure range
from about 1 mbar to about 100 mbar. This is desirable since a
pressure increase occurs in vacuum panels in the course of time. In
addition, the porous material should have a high porosity and a
sufficiently high mechanical stability. Furthermore, the xerogels
should have a low inflammability and a high thermal stability.
[0011] A further object consisted in providing a process which
makes available a xerogel with low pore size, high porosity and
high mechanical stability. In addition, the process for preparing
the xerogels should provide porous materials with a low thermal
conductivity, especially in the pressure range from 1 to 100
mbar.
[0012] Accordingly, the inventive xerogels and a process for
preparing xerogels have been found.
[0013] Preferred embodiments can be taken from the claims and the
description. Combinations of preferred embodiments do not leave the
scope of this invention.
[0014] Xerogel
[0015] According to the invention, the xerogel comprises from 30 to
90% by weight of a monomer component (a1) composed of at least one
polyfunctional isocyanate and from 10 to 70% by weight of a monomer
component (a2) composed of at least one polyfunctional aromatic
amine, at least one of which is selected from
4,4'-diaminodiphenylmethane, 2,4'-diaminodiphenylmethane,
2,2'-diaminodiphenylmethane and oligomeric diaminodiphenylmethane.
Monomer components (a1) and (a2) are present in polymeric form in
the xerogel. According to the invention, the volume-weighted mean
pore diameter of the xerogel is at most 5 .mu.m.
[0016] The xerogel preferably comprises from 40 to 80% by weight of
monomer component (a1) and from 20 to 70% by weight of monomer
component (a2). The xerogel especially preferably comprises from 50
to 70% by weight of monomer component (a1) and from 30 to 50% by
weight of monomer component (a2).
[0017] In the context of the present invention, a xerogel is
understood to mean a porous material having a porosity of at least
70% by volume and a volume-weighted mean pore size of at most 50
micrometers, which has been prepared by a sol-gel method, the
liquid phase having been removed from the gel by drying below the
critical temperature and below the critical pressure of the liquid
phase ("subcritical conditions").
[0018] In the context of the present invention, functionality of a
compound shall be understood to mean the number of reactive groups
per molecule. In the case of monomer component (a1), the
functionality is the number of isocyanate groups per molecule. In
the case of the amino groups of monomer component (a2), the
functionality is the number of reactive amino groups per molecule.
A polyfunctional compound has a functionality of at least 2.
[0019] If monomer components (a1) or (a2) used are mixtures of
compounds with different functionality, the functionality of the
components is calculated from the number-weighted mean of the
functionality of the individual compounds. A polyfunctional
compound comprises at least two of the abovementioned functional
groups per molecule.
[0020] The mean pore diameter is determined by means of mercury
intrusion measurement to DIN 66133 and is always a volume-weighted
mean value in the context of the present invention. The mercury
intrusion measurement to DIN 66133 is a porosimetry method and is
performed in a porosimeter. In this method, mercury is pressed into
a sample of the porous material. Small pores require a higher
pressure to be filled with the mercury than large pores, and the
corresponding pressure/volume diagram can be used to determine a
pore size distribution and the volume-weighted mean pore
diameter.
[0021] According to the invention, the volume-weighted mean pore
diameter of the xerogel is at most 5 .mu.m. The volume-weighted
mean pore diameter of the xerogel is preferably at most 3.5 .mu.m,
more preferably at most 3 .mu.m and especially at most 2.5
.mu.m.
[0022] A minimum pore size with high porosity is desirable from the
point of view of low thermal conductivity. However, for production
reasons and in order to obtain a sufficiently mechanically stable
xerogel, a practical lower limit in the volume-weighted mean pore
diameter arises. In general, the volume-weighted mean pore diameter
is at least 200 nm, preferably at least 400 nm. In many cases, the
volume-weighted mean pore diameter is at least 500 nm, especially
at least 1 micrometer.
[0023] The inventive xerogel preferably has a porosity of at least
70% by volume, especially from 70 to 99% by volume, more preferably
at least 80% by volume, most preferably at least 85% by volume,
especially from 85 to 95% by volume. The porosity in % by volume
means that the reported proportion of the total volume of the
xerogel consists of pores. Although a maximum porosity is usually
desirable from the point of view of minimal thermal conductivity,
the upper limit in the porosity arises through the mechanical
properties and the processability of the xerogel.
[0024] According to the invention, monomer components (a1) and
(a2), referred to hereinafter as components (a1) and (a2), are
present in polymeric form in the xerogel. Owing to the inventive
composition, components (a1) and (a2) are present in the xerogel
bonded predominantly via urea linkages. A further possible linkage
in the xerogel is that of isocyanurate linkages, which arise
through trimerization of isocyanate groups of component (a1). When
the xerogel comprises further monomer components, further possible
linkages are, for example, urethane groups which are formed by
reaction of isocyanate groups with alcohols or phenols.
[0025] Components (a1) and (a2) are preferably present in the
xerogel linked by urea groups --NH--CO--NH-- to an extent of at
least 50 mol %. Components (a1) and (a2) are preferably present in
the xerogel from 50 to 100 mol % linked by urea groups, especially
from 60 to 100 mol %, even more preferably from 70 to 100 mol %,
especially from 80 to 100 mol %, for example from 90 to 100 mol
%.
[0026] The molar % lacking from 100 mol % are present in the form
of further linkages, especially as isocyanurate linkages. The
further linkages may, however, also be present in the form of other
linkages of isocyanate polymers known to those skilled in the art.
Examples include ester, urea, biuret, allophanate, carbodiimide,
isocyanurate, uretdione and/or urethane groups.
[0027] The molar % of the linkages of the monomer components in the
xerogel are determined by means of NMR spectroscopy (nuclear spin
resonance) in the solid or in the swollen state. Suitable
determination methods are known to those skilled in the art.
[0028] The use ratio (equivalence ratio) of NCO groups of monomer
components (a1) to amino groups of monomer component (a2) is
preferably from 0.9:1 to 1.3:1. The equivalence ratio of NCO groups
of monomer component (a1) to amino groups of monomer component (a2)
is more preferably from 0.95:1 to 1.2:1, especially from 1:1 to
1.1:1.
[0029] According to the invention, the xerogel comprises from 40 to
80% by weight of at least one polyfunctional isocyanate in
polymeric form. Useful polyfunctional isocyanates include aromatic,
aliphatic, cycloaliphatic and/or araliphatic isocyanates. Such
polyfunctional isocyanates are known per se or can be prepared by
methods known per se. The polyfunctional isocyanates can especially
also be used in the form of mixtures, such that component (a1) in
this case comprises different polyfunctional isocyanates.
Polyfunctional isocyanates useful as a constituent of component
(a1) have two (referred to hereinafter as diisocyanates) or more
than two isocyanate groups per molecule of the monomer
component.
[0030] Especially suitable are diphenylmethane 2,2'-, and/or
4,4'-diisocyanate (MDI), naphthylene 1,5-diisocyanate (NDI),
tolylene 2,4- and/or 2,6-diisocyanate (TDI), dimethyldiphenyl
3,3'-diisocyanate, diphenylethane 1,2-diisocyanate and/or
p-phenylene diisocyanate (PPDI), tri-, tetra-, penta-, hexa-,
hepta- and/or octamethylene diisocyanate, 2-methylpentamethylene
1,5-diisocyanate, 2-ethylbutylene 1,4-diisocyanate, pentamethylene
1,5-diisocyanate, butylene 1,4-diisocyanate,
1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane
(isophorone diisocyanate, IPDI), 1,4- and/or
1,3-bis(isocyanatomethyl)cyclohexane (HXDI), 1,4-cyclohexane
diisocyanate, 1-methylcyclohexane 2,4- and/or 2,6-diisocyanate
and/or dicyclohexylmethane 4,4'-, 2,4'- and 2,2'-diisocyanate.
[0031] Preferred polyfunctional isocyanates of component (a1) are
aromatic isocyanates. Particularly preferred polyfunctional
isocyanates of component (a1) have the following embodiments:
[0032] i) polyfunctional isocyanates based on tolylene diisocyanate
(TDI), especially 2,4-TDI or 2,6-TDI or mixtures of 2,4- and
2,6-TDI; [0033] ii) polyfunctional isocyanates based on
diphenylmethane diisocyanate (MDI), especially 2,2'-MDI or 2,4'-MDI
or 4,4'-MDI or oligomeric MDI, which is also referred to as
polyphenylpolymethylene isocyanate, or mixtures of two or three of
the aforementioned diphenylmethane diisocyanates, or crude MDI
which is obtained in the preparation of MDI, or mixtures of at
least one oligomer of MDI and at least one of the aforementioned
low molecular weight MDI derivatives; [0034] iii) mixtures of at
least one aromatic isocyanate according to embodiment I) and at
least one aromatic isocyanate according to embodiment ii).
[0035] As a polyfunctional isocyanate, particular preference is
given to oligomeric diphenylmethane diisocyanate. Oligomeric
diphenylmethane diisocyanate (referred to hereinafter as oligomeric
MDI) is one oligomeric condensation product or a mixture of a
plurality of oligomeric condensation products and hence derivatives
of diphenylmethane diisocyanate (MDI). The polyfunctional
isocyanates may preferably also be formed from mixtures of
monomeric aromatic diisocyanates and oligomeric MDI.
[0036] Oligomeric MDI comprises one or more polycyclic condensation
products of MDI having a functionality of more than 2, especially 3
or 4 or 5. Oligomeric MDI is known and is frequently referred to as
polyphenylpolymethylene isocyanate. Oligomeric MDI is typically
formed from a mixture of MDI-based isocyanates with different
functionality. Typically, oligomeric MDI is used in a mixture with
monomeric MDI.
[0037] The (mean) functionality of an isocyanate which comprises
oligomeric MDI may vary in the range from about 2.3 to about 5,
especially from 2.5 to 3,5, especially from 2.5 to 3. Such a
mixture of MDI-based polyfunctional isocyanates with different
functionalities is especially crude MDI, which is obtained in the
preparation of MDI.
[0038] Polyfunctional isocyanates or mixtures of a plurality of
polyfunctional isocyanates based on MDI are known and are sold, for
example, by Elastogran GmbH under the name Lupranat.RTM..
[0039] The functionality of component (a1) is preferably at least
two, especially at least 2.2 and more preferably at least 2.5. The
functionality of component (a1) is preferably from 2.5 to 4 and
more preferably from 2.5 to 3.
[0040] The content of isocyanate groups in component (a1) is
preferably from 5 to 10 mmol/g, especially from 6 to 9 mmol/g, more
preferably from 7 to 8.5 mmol/g. It is known to those skilled in
the art that the content of isocyanate groups in mmol/g and the
so-called equivalence weight in g/equivalent are in a reciprocal
ratio. The content of isocyanate groups in mmol/g is calculated
from the content in % by weight to ASTM D-5155-96 A.
[0041] In a particularly preferred embodiment, monomer component
(a1) consists of at least one polyfunctional isocyanate selected
from diphenylmethane 4,4'-diisocyanate, diphenylmethane
2,4'-diisocyanate, diphenylmethane 2,2'-diisocyanate and oligomeric
diphenylmethane diisocyanate.
[0042] In this particularly preferred embodiment, component (a1)
most preferably comprises oligomeric diphenylmethane diisocyanate
and has a functionality of at least 2.5.
[0043] According to the invention, the monomer component (a2)
consists of at least one polyfunctional aromatic amine, at least
one of which is selected from 4,4'-diaminodiphenylmethane,
2,4'-diaminodiphenylmethane, 2,2'-diaminodiphenylmethane and
oligomeric diaminodiphenylmethane.
[0044] When the monomer component (a2) used is
4,4'-diaminodiphenylmethane, 2,4'-diaminodiphenylmethane,
2,2'-diaminodiphenylmethane and/or oligomeric
diaminodiphenylmethane in a mixture with a further polyfunctional
aromatic amine, useful further polyfunctional aromatic amines are
preferably toluenediamine, especially toluene-2,4-diamine and/or
toluene-2,6-diamine and diethyltoluenediamine, especially
3,5-diethyltoluene-2,4-diamine and/or
3,5-diethyltoluene-2,6-diamine.
[0045] Preferably, monomer component (a2) consists of at least one
polyfunctional aromatic amine selected from
4,4'-diaminodiphenylmethane, 2,4'-diaminodiphenylmethane,
2,2'-diaminodiphenylmethane and oligomeric
diaminodiphenylmethane.
[0046] Oligomeric diaminodiphenylmethane comprises one or more
polycyclic methylene-bridged condensation products of aniline and
formaldehyde. Oligomeric MDA comprises at least one oligomer of
MDA, but generally a plurality of oligomers of MDA, having a
functionality of more than 2, especially 3 or 4 or 5. Oligomeric
MDA is known or can be prepared by methods known per se. Typically,
oligomeric MDA is used in the form of mixtures with monomeric
MDA.
[0047] The (mean) functionality of a polyfunctional amine which
comprises oligomeric MDA can vary in the range from about 2.3 to
about 5, especially from 2.5 to 3.5 and especially from 2.5 to 3.
Such a mixture of MDA-based polyfunctional amines with different
functionalities is especially crude MDA which is formed especially
in the condensation of aniline with formaldehyde, typically
catalyzed by hydrochloric acid, as an intermediate of the
preparation of crude MDI. Monomer component (a2) preferably
comprises oligomeric diaminodiphenylmethane and has a functionality
of at least 2.5.
[0048] Process for Preparing Xerogels
[0049] The process according to the invention comprises the
following steps: [0050] (a) providing a gel precursor (A)
comprising monomer components (a1) and (a2) in a solvent (C);
[0051] (b) converting the gel precursor (A) in the presence of the
solvent (C) to a gel; [0052] (d) drying the gel obtained in the
previous step by converting the liquid present in the gel to the
gaseous state at a temperature and a pressure below the critical
temperature and the critical pressure of the liquid present in the
gel.
[0053] In a preferred embodiment, monomer components (a1) and (a2)
are first provided separately in one solvent (C) each and finally
combined at the start of step (b). The process according to the
invention accordingly preferably comprises the following steps:
[0054] (a-1) providing monomer components (a1) and (a2) separately
in one solvent (C) each; [0055] (a-2) providing a gel precursor (A)
comprising monomer components (a1) and (a2) in a solvent (C) by
combining the monomer components provided in step (a-1); [0056] (b)
converting the gel precursor (A) in the presence of the solvent (C)
to a gel; [0057] (d) drying the gel obtained in the previous step
by converting the liquid present in the gel to the gaseous state at
a temperature and a pressure below the critical temperature and the
critical pressure of the liquid present in the gel.
[0058] The process according to the invention preferably further
comprises the following step, the steps being performed in the
sequence a-b-c-d: [0059] (c) modifying the resulting gel by means
of at least one organic compound (D) which was present neither in
step (a) nor in step (b).
[0060] Step (a)
[0061] According to the invention, in step (a), a gel precursor (A)
comprising monomer components (a1) and (a2) is prepared in a
solvent (C). The gel precursor (A) thus comprises the monomer
components (a1) and (a2) described above under xerogel in the
proportions likewise described above.
[0062] Monomer components (a1) and (a2) are present in the gel
precursor (A) in monomeric form or have been converted beforehand
by partial or nonequimolar reaction of isocyanate and amino groups
to a prepolymer which forms the gel precursor (A), if appropriate
with further monomer components (a1) or (a2). The gel precursor (A)
is thus gelable, i.e. it can be converted to a gel by crosslinking.
The proportions of monomer components (a1) and (a2) in the xerogel,
in which they are present in polymeric form, correspond to the
proportions of monomer components (a1) and (a2) in the gel
precursor (A) in which they are present in as yet unconverted
monomeric form.
[0063] The viscosity of component (a1) used may vary within a wide
range. Component (a1) used in step (a) of the process according to
the invention preferably has a viscosity from 100 to 3000 mPas,
more preferably from 200 to 2500 mPas.
[0064] The term "gel precursor (A)" indicates the gelable mixture
of components (a1) and (a2). The gel precursor (A) is subsequently
converted in step (b), in the presence of the solvent (C), to a
gel, a crosslinked polymer.
[0065] In step (a) of the process according to the invention, a
mixture comprising the gel precursor (A) in a liquid diluent is
thus provided. In the context of the present invention, the term
"solvent (C)" comprises liquid diluents, i.e. both solvents in the
narrower sense and dispersants. The mixture may especially be a
true solution, a colloidal solution or a dispersion, for example an
emulsion or suspension. The mixture is preferably a true solution.
The solvent (C) is a compound which is liquid under the conditions
of step (a), preferably an organic solvent.
[0066] It is known to those skilled in the art that aromatic
amines, especially diamines, are formed when aromatic isocyanates,
especially diisocyanates, are reacted with water. Accordingly, it
is possible, instead of polyfunctional aromatic amines, to use
corresponding aromatic polyfunctional isocyanates and an equivalent
amount of water as component (a2), such that the desired amount of
polyfunctional aromatic amine is formed in situ or in a preliminary
reaction. In the case of an excess of component (a1) and
simultaneous addition of water, component (a1) can be converted in
situ partly to component (a2), which then reacts immediately with
the remaining component (a1) to form urea linkages.
[0067] However, the polyfunctional amine is preferably not obtained
from component (a2) in the presence of monomer component (a1) in
the solvent (C), but rather is added separately as component (a2).
Accordingly, the mixture provided in step (a) preferably does not
comprise any water.
[0068] Useful solvents (C) include in principle one compound or a
mixture of a plurality of compounds, the solvent (C) being liquid
under the pressure and temperature conditions under which the
mixture is provided in step (a) (dissolution conditions for short).
The composition of the solvent (C) is selected such that it is
capable of dissolving or dispersing the organic gel precursor,
preferably of dissolving it. Preferred solvents (C) are those which
are a solvent for the organic gel precursor (A), i.e. those which
dissolve the organic gel precursor (A) completely under reaction
conditions.
[0069] The reaction product from step (b) is a gel, i.e. a
viscoelastic chemical network which is swollen by the solvent (C).
A solvent (C) which is a good swelling agent for the network formed
in step (b) generally leads to a network with fine pores and small
mean pore diameter, whereas a solvent (C) which is a poor swelling
agent for the gel resulting from step (b) leads generally to a
coarse-pore network with large mean pore diameter.
[0070] The selection of the solvent (C) thus influences the desired
pore size distribution and the desired porosity. The solvent (C) is
generally additionally selected such that precipitation or
flocculation as a result of formation of a precipitated reaction
product very substantially does not occur during or after step (b)
of the process according to the invention.
[0071] In the case of selection of a suitable solvent (C), the
proportion of precipitated reaction product is typically less than
1% by weight based on the total weight of the mixture. The amount
of precipitated product formed in a particular solvent (C) can be
determined gravimetrically by filtering the reaction mixture
through a suitable filter before the gel point.
[0072] Useful solvents (C) include the solvents known from the
prior art for isocyanate-based polymers. Preferred solvents are
those which are a solvent for both components, (a1) and (a2), i.e.
those which dissolve components (a1) and (a2) substantially
completely under reaction conditions, such that the content of the
organic gel precursor (A) in the overall mixture provided in step
(a) including the solvent (C) is preferably at least 5% by weight.
The solvent (C) is preferably inert, i.e. unreactive, toward
component (a1).
[0073] Useful solvents (C) include, for example, dialkyl ethers,
cyclic ethers, ketones, alkyl alkanoates, amides such as formamide
and N-methylpyrrolidone, sulfoxides such as dimethyl sulfoxide,
aliphatic and cycloaliphatic halogenated hydrocarbons, halogenated
aromatic compounds and fluorinated ethers. Likewise useful are
mixtures of two or more of the aforementioned compounds.
[0074] Additionally useful as solvents (C) are acetals, especially
diethoxymethane, dimethoxymethane and 1,3-dioxolane.
[0075] Dialkyl ethers and cyclic ethers are preferred as solvents
(C). Preferred dialkyl ethers are especially those having from 2 to
6 carbon atoms, especially methyl ethyl ether, diethyl ether,
methyl propyl ether, methyl isopropyl ether, propyl ethyl ether,
ethyl isopropyl ether, dipropyl ether, propyl isopropyl ether,
diisopropyl ether, methyl butyl ether, methyl isobutyl ether,
methyl t-butyl ether, ethyl n-butyl ether, ethyl isobutyl ether and
ethyl t-butyl ether. Preferred cyclic ethers are especially
tetrahydrofuran, dioxane and tetrahydropyran.
[0076] Ketones having alkyl groups having up to 3 carbon atoms per
substituent are likewise preferred as solvents (C). Particularly
preferred solvents (C) are the following ketones: acetone,
cyclohexanone, methyl t-butyl ketone and methyl ethyl ketone.
[0077] Also preferred as solvents (C) are alkyl alkanoates,
especially methyl formate, methyl acetate, ethyl formate, butyl
acetate and ethyl acetate. Preferred halogenated solvents are
described in WO 00/24799, page 4 line 12 to page 5 line 4.
[0078] Dialkyl ethers, cyclic ethers, ketones and esters are very
particularly preferred as solvents (C).
[0079] In many cases, particularly suitable solvents (C) arise by
using two or more compounds which are completely miscible with one
another and are selected from the aforementioned solvents in the
form of a mixture.
[0080] In order to obtain, in step (b), a sufficiently stable gel
which does not shrink greatly in the course of drying in step (d),
the proportion of the gel precursor (A) in the overall mixture
provided in step (a) of the process according to the invention
generally must not be less than 5% by weight. The proportion of the
gel precursor (A) in the overall mixture provided in step (a) of
the process according to the invention including the solvent (C) is
preferably at least 8% by weight, more preferably at least 10% by
weight, especially at least 12% by weight.
[0081] On the other hand, the concentration of the monomer
components in the mixture provided must not be selected at too high
a level, since a xerogel with favorable properties is otherwise not
obtained. In general, the proportion of the gel precursor (A) in
the overall mixture provided in step (a) of the process according
to the invention is at most 40% by weight. The proportion of the
gel precursor (A) in the overall mixture provided in step (a) of
the process according to the invention including the solvent (C) is
preferably at most 30% by weight, more preferably at most 20% by
weight, especially at most 15% by weight.
[0082] Optionally, the mixture provided in step (a) comprises, as a
further component (B), also at least one catalyst (b1). However,
preference is given to performing the conversion of the gel
precursor (A) without the presence of a catalyst.
[0083] When a catalyst (b1) is used, typically trimerization
catalysts which catalyze the formation of isocyanurates are used.
Such trimerization catalysts used may, for example, be catalysts
widely known to those skilled in the art, for example those listed
below.
[0084] When trimerization catalysts are used as component (b1),
known catalysts such as quaternary ammonium hydroxides, alkali
metal and alkaline earth metal hydroxides, alkali metal and
alkaline earth metal alkoxides, and alkali metal and alkaline earth
metal carboxylates, e.g. potassium acetate and potassium
2-ethylhexanoate, particular tertiary amines and nonbasic metal
carboxylates, e.g. lead octoate and triazine derivatives,
especially symmetrical triazine derivatives, are suitable. Triazine
derivatives are particularly suitable as trimerization
catalysts.
[0085] Components (a1) and (a2) are preferably used such that the
gel precursor (A) comprises from 30 to 90% by weight of component
(a1) and from 10 to 70% by weight of component (a2). The gel
precursor (A) preferably comprises from 40 to 80% by weight of
component (a1) and from 20 to 60% by weight of component (a2). The
gel precursor especially preferably comprises from 50 to 70% by
weight of component (a1) and from 30 to 50% by weight of component
(a2).
[0086] The use ratio (equivalence ratio) of components (a1) and
(a2) of the organic gel precursor (A) is, in a preferred
embodiment, selected such that, on completion of gelation in step
(b), the gel still has reactive groups which can be converted in
step (c) by chemical reaction with an organic compound (D) which
was present neither in step (a) nor in step (b). For example, the
organic gel precursor (A) may comprise reactive groups which do not
react until they do so with compound (D). In a particularly
preferred embodiment, the organic compound (D) reacts with reactive
groups which were already present in the organic gel precursor (A)
and did not react completely in the course of the conversion to a
gel in step (b). A reactive group shall be understood to mean a
functional group or a reactive site in a molecule, for example a
position in an aromatic ring, which is reactive toward the compound
(D).
[0087] In one embodiment, the organic gel precursor (A) comprises
components (a1) and (a2) in a nonstoichiometric ratio of the
reactive functional groups, such that, at the start of step (c) of
the process according to the invention, the reactive functional
groups of one of the two components (a1) or (a2) are present in a
molar excess relative to the reactive functional groups of the
other component in each case in unconverted form.
[0088] The molar excess of reactive functional groups of one of the
two components (a1) or (a2) at the start of step (c) of the process
according to the invention relative to the reactive functional
groups of the other component in each case is preferably at least 5
mol %, for example from 5 to 15 mol %, especially from 6 to 12 mol
%. The upper limit in the excess of the reactive groups in one
component in each case arises through practical considerations,
since a gel must form in step (b) of the process according to the
invention.
[0089] In a particularly preferred embodiment, component (a1) is
used in relation to component (a2) such that the excess of
isocyanate groups at the start of step (c) of the process according
to the invention is at least 5 mol %, especially from 5 to 15 mol
%, most preferably from 6 to 12 mol %.
[0090] In a further preferred embodiment, component (a1) is used in
relation to component (a2) such that the excess of groups in
component (a2) reactive toward isocyanate groups, at the start of
step (c) of the process according to the invention, is at least 5
mol %, especially from 5 to 15 mol %, most preferably from 6 to 12
mol %.
[0091] The mixture provided in step (a) may also comprise typical
assistants known to those skilled in the art as further
constituents (B). Examples include surface-active substances, flame
retardants, nucleating agents, oxidation stabilizers, lubricating
and demolding aids, dyes and pigments, stabilizers, for example
against hydrolysis, light, heat or discoloration, inorganic and/or
organic fillers, reinforcing agents and biocides.
[0092] Further details of the assistants and additives mentioned
above can be taken from the technical literature, for example from
Plastics Additive Handbook, 5th edition, H. Zweifel, ed, Hanser
Publishers, Munich, 2001.
[0093] The mixture can be provided in step (a) of the process
according to the invention in a typical manner. For this purpose, a
stirrer or another mixing apparatus is preferably used to achieve
good mixing. The other mixing conditions are generally uncritical;
for example, it is possible to mix at from 0 to 100.degree. C. and
from 0.1 to 10 bar (absolute), especially, for example, at room
temperature and atmospheric pressure.
[0094] The mixture provided in step (a) can also be referred to as
a sol. A sol shall be understood to mean either a colloidal
solution in which the organic gel precursor (A) is dispersed
ultrafinely in a solvent as a dispersion medium, or a true solution
of the organic gel precursor (A) in a solvent.
[0095] Step (b)
[0096] According to the invention, in step (b), the gel precursor
(A) is converted to a gel in the presence of the solvent (C). In
step (b) of the process according to the invention, the organic gel
precursor (A) is thus converted to a gel in a gelation reaction.
The gelation reaction is a polyaddition reaction, especially a
polyaddition of isocyanate groups and amino groups.
[0097] A gel shall be understood to mean a crosslinked system based
on a polymer which is present in contact with a liquid (so-called
solvogel or lyogel, or with water as a liquid: aquagel or
hydrogel). In this case, the polymer phase forms a continuous
three-dimensional network.
[0098] In step (b) of the process according to the invention, the
gel forms typically by being left to stand, for example by simply
leaving the vessel, reaction vessel or reactor in which the mixture
is present to stand (referred to hereinafter as gelation
apparatus). During the gelation (gel formation), the mixture is
preferably not stirred or mixed because this might hinder the
formation of the gel. It has been found to be advantageous to cover
the mixture during the gelation or to close the gelation
apparatus.
[0099] The duration of the gelation varies according to the type
and amount of components used and the temperature and may be
several days. It is typically from 1 minute to 10 days, preferably
less than 1 day, especially from 5 minutes to 12 hours, more
preferably at most 1 hour, especially from 5 minutes to 1 hour.
[0100] The gelation can be performed without supplying heat at a
temperature in the region of room temperature, especially from 15
to 25.degree. C., or at a temperature elevated relative to room
temperature which is 20.degree. C. or more, especially from
25.degree. C. to 80.degree. C. Typically, a higher gelation
temperature shortens the duration of gelation. However, a higher
gelation temperature is not advantageous in all cases, since an
elevated gelation temperature can lead to gels with inadequate
mechanical properties. Preference is given to performing the
gelation at a temperature in the region of room temperature,
especially from 15.degree. C. to 25.degree. C.
[0101] The pressure in the course of gelation can vary within a
wide range and is generally not critical. It may, for example, be
from 0.1 bar to 10 bar, preferably from 0.5 bar to 8 bar and
especially from 0.9 to 5 bar (in each case absolute). In
particular, it is possible to allow aqueous mixtures to gel at room
temperature and atmospheric pressure.
[0102] During the gelation, the mixture solidifies to a more or
less dimensionally stable gel. Gel formation can therefore be
recognized in a simple manner by the contents of the gelation
apparatus no longer moving when the gelation apparatus or a vessel
with which a sample has been taken is tilted slowly. Moreover, the
acoustic properties of the mixture change in the course of
gelation: when the outer wall of the gelation apparatus is tapped,
the gelled mixture gives a different ringing sound from the as yet
ungelled mixture (so-called ringing gel).
[0103] In a preferred embodiment, the gel obtained in the gelation
in step (b), before step (c) is performed or, when step (c) is not
performed, before step (d), is subjected to a so-called aging in
which the formation of the gel is completed. The aging is effected
especially by exposing the gel to a higher temperature than in the
preceding gelation for a certain time. To this end, for example, a
heating bath or a heating cabinet can be used, or the apparatus or
environment in which the gel is present can be heated in a suitable
manner.
[0104] The temperature in the course of aging can vary within a
wide range and is not critical per se. In general, aging is
effected at temperatures of from 30.degree. C. to 150.degree. C.,
preferably from 40.degree. C. to 100.degree. C. The aging
temperature should be in the range from 10.degree. C. to
100.degree. C., especially from 20.degree. C. to 80.degree. C.,
above the gelation temperature. When gelation has been effected at
room temperature, it is possible to effect aging especially at
temperatures of from 40.degree. C. to 80.degree. C., preferably at
about 60.degree. C. The pressure in the course of aging is
uncritical and is typically from 0.9 to 5 bar (absolute).
[0105] The duration of the aging depends on the type of the gel and
may be a few minutes, but may also take a long time. The duration
of the aging may, for example, be up to 30 days. Typically, the
duration of the aging is from 10 minutes to 12 hours, preferably
from 20 minutes to 6 hours and more preferably from 30 minutes to 5
hours.
[0106] According to the type and composition, the gel may shrink
slightly during the aging and become detached from the wall of the
gelation apparatus. Advantageously, the gel is covered during the
aging, or the gelation apparatus in which the gel is present is
closed.
[0107] Step (c)
[0108] In an optional preferred embodiment, in step (c) of the
process according to the invention, the gel obtained in step (b) is
modified by means of at least one organic compound (D) which was
present neither in step (a) nor in step (b).
[0109] The organic compound (ID) may either be formed exclusively
from nonmetals or comprise semimetals or metals. The organic
compounds (D), however, preferably do not comprise any metals or
any semimetals such as silicon. The organic compound (D) preferably
comprises reactive functional groups which are reactive toward the
gel obtained in step (b).
[0110] The modification preferably reduces the compatibility of the
resulting gel, especially the compatibility of the pore surface of
the gel, with the solvent (C). The compound (D) is thus preferably
a compound which comes into contact with the pore surface of the
gel from step (b) and remains there, which reduces the
compatibility of the resulting gel with the solvent (C). Reduction
of the compatibility is understood to mean that the attractive
interaction between the gel and the liquid phase in contact with
the gel is reduced. In the context of the present invention, the
compatibility is thus a thermodynamic compatibility, decreasing
compatibility being accompanied by increasing microscopic
separation, i.e. the components have, in the case of a reduced
compatibility, a reduced tendency to penetrate at the molecular
level, especially in the form of swelling of the gel by the solvent
(C). Compatibility of gel and solvent (C) is understood to mean the
strength of the physicochemical interaction between the pore
surface of the gel and the solvent. It is determined by
physicochemical interactions, for example the interaction between
apolar compounds or dipole-dipole interactions or hydrogen
bonds.
[0111] Preferred compounds (D) in the process according to the
invention are in principle those organic compounds which are
unreactive toward the solvent (C). Unreactive toward the solvent
(C) means that the compound (D) does not enter into a chemical
reaction with the solvent and is especially not hydrolyzed and not
solvolyzed by the solvent (C).
[0112] Modification of the gel shall be understood to mean any
measure in which the pore surface of the gel is modified by at
least one compound (D). Preference is given to effecting the
modification by a chemical reaction between the compound (D) and
the gel (referred to hereinafter as chemical modification),
especially in the region of the pore surface of the gel. In
principle, the modification can also be effected by physicochemical
interactions which do not arise through a chemical reaction in the
actual sense, especially through hydrogen bonds or other
intermolecular interactions, for example ionic interactions and
donor-acceptor interactions, such that no chemical modification is
present here. The physicochemical interactions must at least be
sufficiently great that the compatibility of the gel thus modified
with the solvent is altered. However, preference is given to
chemically modifying the resulting gel in step (c).
[0113] A pore surface is considered to be that region of the gel
which is accessible by the compound (D), i.e. is either on the
interface between gel and liquid or can be reached by the liquid
present in the pores of the gel, especially as a result of
swelling.
[0114] In this preferred embodiment, the compatibility is
preferably determined by contacting the gel with the solvent (C)
until the time at which the end point of the swelling is attained
and determining the swelling capacity. The reduction in the
swelling capacity serves to characterize the reduction in the
compatibility of the gel with the solvent (C). The inventive
modification of the resulting gel by means of at least one organic
compound (D) in step (c) of the process according to the invention
preferably reduces the swelling capacity of the gel in the solvent
(C). Reduction in the swelling capacity (SC) is understood to mean
SC=(V.sub.ps-V.sub.0)/V.sub.0 where V.sub.ps is the partial
specific volume of the polymer in the gel under swollen conditions
and V.sub.0 is the specific volume in the unswollen dry state. The
specific volumes can be determined, for example, by pycnometry to
DIN 66137.
[0115] As a result of the reduced compatibility, the phase
separation of polymer and solvent is enhanced. This results
generally in an increased pore volume and an elevated porosity
after the drying of the modified gel compared to the unmodified
gel.
[0116] The swelling capacity can be used to compare the
compatibility between gels obtainable under otherwise identical
conditions in order thus to determine the influence of the
modification of the gel in step (c) of the process according to the
invention. The gel is modified preferably by reaction with chemical
groups in the region of the pore surface of the gel (chemical
modification). In the case of a chemical modification of the gel,
crosslinking can simultaneously be effected in the region of the
pore surface if a compound (D) including more than one reactive
functional group is used.
[0117] When the resulting gel still comprises reactive isocyanate
groups, useful compounds (D) are preferably amines which are
reactive toward isocyanates and whose reaction with the gel results
in reduced compatibility of the resulting gel with the solvent. In
this case, useful compounds (D) are preferably amines which have a
polarity opposite to that of the solvent and are reactive toward
isocyanates.
[0118] Opposite polarity shall be understood to mean an opposite
direction of the polarity and not an absolute magnitude. The
modification with at least one compound (D) having opposite
polarity in step (c) of the process according to the invention
lowers the compatibility with the solvent. In the case of a polar
solvent (C), "opposite" means an isocyanate which lowers the
polarity of the pore surface, and, in the case of a (less
preferred) nonpolar solvent (C), an isocyanate which increases the
polarity of the pore surface. In the case of a moderately polar
solvent, there are two possibilities for opposite polarity, an
isocyanate which modifies the pore surface in a polar manner
or--which is preferred--an isocyanate which lowers the polarity of
the pore surface by modification to such an extent that the
compatibility with the moderately polar solvent is reduced.
[0119] In this preferred embodiment, the amines react with excess
isocyanate groups of the gel at the pore surface to form urea
groups. This preferably reduces the compatibility of the gel with
the solvent (C) used.
[0120] Preferred aromatic amines used as compound (D) are
especially diphenyl sulfones having at least two amino groups,
especially diaminodiphenyl sulfones (DADPS), as the organic
compound (D), very particular preference being given to
4,4'-DADPS.
[0121] When the resulting gel still comprises reactive amino groups
or other groups reactive toward isocyanates, useful compounds (D)
are especially compounds which are reactive toward component (a2)
and whose reaction with the gel results in a reduced compatibility
of the resulting gel with the solvent (C). In particular, the
compounds (D) reactive toward component (a2) may have a polarity
opposite to that of the solvent.
[0122] In particular, useful compounds (D) in this second preferred
embodiment include the polyfunctional isocyanates (a1) discussed
above, in which case, in accordance with the invention, the
compound (D) was not part of the organic gel precursor (A) in steps
(a) and (b) of the process according to the invention and,
additionally preferably, the polarity of the pore surface is
modified in the opposite direction.
[0123] Step (d)
[0124] According to the invention, in step (d), the gel obtained in
the previous step is dried by converting the liquid present in the
gel to the gaseous state at a temperature and a pressure below the
critical temperature and the critical pressure of the liquid
present in the gel.
[0125] Preference is given to drying the resulting gel by
converting the solvent (C) to the gaseous state at a temperature
and a pressure below the critical temperature and the critical
pressure of the solvent (C). Accordingly, preference is given to
effecting the drying by removing the solvent (C) which was present
in the reaction without preceding exchange for a further
solvent.
[0126] Consequently, after step (c) or step (b) and before step
(d), the gel is preferably not contacted with an organic liquid in
order to exchange the solvent (C) present in the gel, especially in
the pores of the gel, for this organic liquid. This is true
irrespective of whether the gel is aged or not. When a solvent
exchange is omitted, the process can be performed in a particularly
simple and inexpensive manner. When, however, a solvent exchange is
performed, it is preferred to exchange the solvent (C) for a
nonpolar solvent, especially for hydrocarbons such as pentane.
[0127] For the drying by conversion of the liquid present in the
gel, preferably the solvent (C), to the gaseous state, useful
methods are in principle both vaporization and evaporation, but not
sublimation. Drying by vaporization or evaporation includes
especially drying under atmospheric pressure, drying under reduced
pressure, drying at room temperature and drying at elevated
temperature, but not freeze-drying. According to the invention,
drying is effected at a pressure and a temperature which are below
the critical pressure and below the critical temperature of the
liquid present in the gel. In step (d) of the process according to
the invention, the solvent-containing gel is thus dried to form the
organic xerogel as the process product.
[0128] To dry the gel, the gelation apparatus is typically opened
and the gel is kept under the stated pressure and temperature
conditions until the liquid phase has been removed by conversion to
the gaseous state, i.e. the liquid phase is vaporized or
evaporated. In order to accelerate the vaporization, it is
frequently advantageous to remove the gel from the vessel. In this
way, the gel/ambient air phase interface over which the
vaporization and/or evaporation takes place is enlarged. For
example, the gel can be placed onto a flat underlay or a sieve for
drying. Useful drying processes are also the drying processes
familiar to those skilled in the art, such as convection drying,
microwave drying, vacuum drying cabinets or combinations of these
processes,
[0129] The gel can be dried under air or, if it is
oxygen-sensitive, also under other gases such as nitrogen or noble
gases, and it is possible for this purpose, if appropriate, to use
a drying cabinet or other suitable apparatus in which the pressure,
the temperature and the solvent content of the environment can be
controlled.
[0130] The temperature and pressure conditions to be selected in
the course of drying depend upon factors including the nature of
the liquid present in the gel. According to the invention, drying
is effected at a pressure which is below the critical pressure
p.sub.crit of the liquid present in the gel, preferably the solvent
(C), and at a temperature which is below the critical temperature
T.sub.crit. Accordingly, drying is effected under subcritical
conditions. In this context, critical means: at the critical
pressure and the critical temperature, the density of the liquid
phase is equal to the density of the gas phase (so-called critical
density), and, at temperatures above T.sub.crit, the fluid phase
can no longer be liquefied even in the case of application of ultra
high pressures.
[0131] When acetone is used as the solvent, drying is effected at
temperatures of from 0.degree. C. to 150.degree. C., preferably
from 10.degree. C. to 100.degree. C. and more preferably from
15.degree. C. to 80.degree. C., and at pressures from high vacuum,
for example from 10.sup.-3 mbar, to 5 bar, preferably from 1 mbar
to 3 bar and especially from 10 mbar to about 1 bar (absolute). For
example, drying can be effected at atmospheric pressure and from
0.degree. C. to 80.degree. C., especially at room temperature.
Particular preference is given to drying the gel in step (d) at a
pressure of from 0,5 to 2 bar (absolute) and at a temperature of
from 0 to 100.degree. C.
[0132] Other liquids present in the gel, especially solvents (C)
other than acetone, require adjustments to the drying conditions
(pressure, temperature, time) which can be determined by the person
skilled in the art by simple tests.
[0133] The drying can be accelerated or completed by applying a
vacuum. In order to further improve the drying action, this vacuum
drying can be undertaken at a higher temperature than the drying at
customary pressure. For example, the majority of the solvent (C)
can first be removed at room temperature and atmospheric pressure
within from 30 min to 3 hours, and then the gel can be dried at
from 40 to 80.degree. C. under a reduced pressure of from 1 to 100
mbar, especially from 5 to 30 mbar, within from 10 min to 6 hours.
It will be appreciated that longer drying times are also possible,
for example from 1 to 5 days. However, preference is frequently
given to drying times of below 12 hours.
[0134] Instead of such a stepwise drying, the pressure can also be
lowered continuously, for example in a linear or exponential
manner, during the drying, or the temperature can be increased in
such a manner, i.e. according to a pressure or temperature program.
By its nature, the lower the moisture content of the air, the more
rapidly the gel dries. The same applies mutatis mutandis to liquid
phases other than water and to gases other than air.
[0135] The preferred drying conditions depend not only on the
solvent but also on the nature of the gel, especially the stability
of the network in relation to the capillary forces acting in the
course of drying.
[0136] In the course of drying in step (d), the liquid phase is
generally removed completely or down to a residual content of from
0.01 to 1% by weight based on the resulting xerogel.
[0137] Properties of the Xerogels and Use
[0138] The xerogels obtainable by the process according to the
invention have a volume-averaged mean pore diameter of at most 5
.mu.m. The volume-averaged mean pore diameter of the xerogels
obtainable by the process according to the invention is preferably
from 200 nm to 5 .mu.m.
[0139] The particularly preferred volume-weighted mean pore
diameter of the xerogels obtainable by the process according to the
invention is at most 5 .mu.m, especially at most 3.5 .mu.m, most
preferably at most 2.5 .mu.m.
[0140] Although a minimum mean pore diameter with simultaneously
high porosity is in principle desired from the point of view of
reduced thermal conductivity, the lower limit in the mean pore
diameter arises through the worsening of mechanical properties of
the xerogel, especially its stability and processability, by
practical considerations. In general, the volume-weighted mean pore
diameter is at least 200 nm, preferably at least 400 nm. In many
cases, the volume-weighted mean pore diameter is at least 500 nm,
especially at least 1 micrometer.
[0141] The xerogels obtainable by the process according to the
invention preferably have a porosity of at least 70% by volume,
especially from 70 to 99% by volume, more preferably at least 80%
by volume, most preferably at least 85% by volume, especially from
85 to 95% by volume. The porosity in % by volume means that the
stated proportion of the total volume of the xerogel consists of
pores. Although a maximum porosity is usually desired from the
point of view of minimal thermal conductivity, the upper limit in
the porosity arises through the mechanical properties and the
processability of the xerogel.
[0142] The density of the organic xerogels obtainable by the
process according to the invention is typically from 20 to 600 g/l,
preferably from 50 to 500 g/l and more preferably from 100 to 400
g/l.
[0143] The process according to the invention gives rise to a
coherent porous material and not just a polymer powder or polymer
particles. The three-dimensional shape of the resulting xerogel is
determined by the shape of the gel, which is determined in turn by
the shape of the gelation apparatus. For example, a cylindrical
gelation vessel typically gives rise to an approximately
cylindrical gel which is then dried to a xerogel in cylinder
form.
[0144] The inventive xerogels and the xerogels obtainable by the
process according to the invention have a low thermal conductivity,
a high porosity and a low density. According to the invention, the
xerogels have a low mean pore size. The combination of the
aforementioned properties allows use as an insulating material in
the field of thermal insulation, especially for applications in the
vacuum sector where a minimum thickness of vacuum panels is
preferred, for example in cooling units or in buildings. For
instance, preference is given to use in vacuum insulation panels,
especially as a core material for vacuum insulation panels.
Preference is also given to the use of the inventive xerogels as an
insulating material.
[0145] Furthermore, the low thermal conductivity of the inventive
xerogels enables applications at pressures of from 1 to 100 mbar
and especially from 10 mbar to 100 mbar. The property profile of
the inventive xerogels opens up especially applications in which a
long lifetime of the vacuum panels is desired and which have a low
thermal conductivity even in the case of a pressure increase of
about 2 mbar per year even after many years, for example at a
pressure of 100 mbar. The inventive xerogels and the xerogels
obtainable by the process according to the invention have favorable
thermal properties on the one hand, and favorable material
properties such as simple processability and high mechanical
stability, for example low brittleness, on the other hand.
EXAMPLES
[0146] The pore volume in ml per g of sample and the mean pore size
of the materials were determined by means of mercury porosimetry to
DIN 66133 (1993) at room temperature. In the context of this
invention, the mean pore size can be equated to the mean pore
diameter. The volume-weighted mean pore diameter is determined by
calculation from the pore size distribution determined according to
the abovementioned standard.
[0147] The porosity in the unit % by volume was determined by the
formula P=(V.sub.i/(V.sub.i+V.sub.s))*100% by volume, where P is
the porosity, V.sub.i is the Hg intrusion volume to DIN 66133 in
ml/g and V.sub.s is the specific volume in ml/g of the
specimen.
[0148] The density .rho. of the porous material in the unit g/ml
was calculated by the formula .rho.=1/(V.sub.i+V.sub.s). The
specific volume used for porous materials based on melamine and
formaldehyde was the value 1/V.sub.s=1.68 g/ml, and the specific
volume used for porous materials based on isocyanate was the value
1/V.sub.s=1.38 g/ml. Both values were determined by He
pycnometry.
[0149] The thermal conductivity .lamda. was determined by means of
the dynamic hot wire method. In the hot wire method, a thin wire is
embedded in the sample to be analyzed, which serves simultaneously
as the heating element and temperature sensor. The wire material
used was a platinum wire with a diameter of 100 micrometers and a
length of 40 mm, which was embedded between two halves of the
particular specimen. The test setup composed of sample and hot wire
was prepared in an evacuable recipient in which, after the
evacuation, the desired pressure was established by admitting
gaseous nitrogen.
[0150] During the experiment, the wire was heated at constant
power. The temperature was 25.degree. C. The evolution with time of
the resulting temperature rise at the site of the heating wire was
recorded by measuring the resistance. The thermal conductivity was
determined by fitting an analytical solution to the evolution of
temperature with time, taking account of a thermal contact
resistance between sample and wire, and axial heat losses,
according to H.-P. Ebert et al., High Temp.-High. Press, 1993, 25,
391-401. The gas pressure was determined with two capacitative
pressure sensors with different measurement ranges (0.1 to 1000
mbar and 0.001 to 10 mbar).
Example 1
[0151] a) 1.9 g of oligomeric MDI (Lupranat.RTM. M200 R) with an
NCO content of 30.9 g per 100 g to ASTM D-5155-96 A, a
functionality in the region of three and a viscosity of 2100 mPas
at 25.degree. C. to DIN 53018 were dissolved in 10.6 g of acetone
in a beaker at 20.degree. C. with stirring. 1.26 g of
4,4'-diaminodiphenylmethane were dissolved in 10.9 g of acetone in
a second beaker. [0152] b) The two solutions from step (a) were
mixed. A clear low-viscosity mixture was obtained. The mixture was
left to stand at room temperature for 24 hours for curing. [0153]
d) Subsequently, the gel was removed from the beaker and the liquid
(acetone) was removed by drying at 20.degree. C. for 7 days.
[0154] The resulting material had a pore volume of 5.1 ml/g and an
average pore diameter of 2.9 .mu.m. The porosity was 87% by volume
with a corresponding density of 170 g/l. The thermal conductivity
.lamda. of the resulting material can be seen in table 1.
TABLE-US-00001 TABLE 1 Thermal conductivity .lamda. (example 1)
Pressure [mbar] .lamda. [mW/m*K] 1 6.5 2.1 7.8 3.1 8.9 7 12.2 14 14
41 20.1 71 26.3 100 28.8 299 34 701 36.7 1004 37.6
Example 2
[0155] a) 1.9 g of oligomeric MDI (Lupranat.RTM. M200 R) with an
NCO content of 30.9 g per 100 g to ASTM D-5155-96 A, a
functionality in the region of three and a viscosity of 2100 mPas
at 25.degree. C. to DIN 53018 were dissolved in 14 g of dioxane in
a beaker at 20.degree. C. with stirring. 1.24 g of
4,4'-diaminodiphenylmethane were dissolved in 14.4 g of dioxane in
a second beaker. [0156] b) The two solutions from step (a) were
mixed. A clear low-viscosity mixture was obtained. The mixture was
left to stand at room temperature for 24 hours for curing. [0157]
d) Subsequently, the gel was removed from the beaker and the liquid
(dioxane) was removed by drying at 20.degree. C. for 7 days.
[0158] The resulting material had a pore volume of 4.1 ml/g and an
average pore diameter of 2.5 .mu.m. The porosity was 85% by volume
with a corresponding density of 207 g/l. The resulting material had
a thermal conductivity .lamda. of 5.7 mW/m*K at a pressure of 1.53
mbar.
Comparative Example 3C
[0159] a) 4.19 g of oligomeric MDI (Lupranat.RTM. M20 S) having an
NCO content of 31.8 g per 100 g to ASTM D-5155-96 A, a viscosity of
220 mPas at 25.degree. C. and a functionality of about 2.7 were
dissolved in 50 g of acetone in a beaker at 20.degree. C. with
stirring. 2.66 g of diethyltoluenediamine (Ethacure.RTM. 100, a
mixture of 3,5-diethyltoluene-2,4-diamine and
3,5-diethyltoluene-2,6-diamine) were dissolved in 50 g of acetone
in a second beaker. [0160] b) The two solutions from step (a) were
mixed. A clear low-viscosity mixture was obtained. The mixture was
left to stand at room temperature for 24 hours for curing. [0161]
d) Subsequently, the gel was removed from the beaker and the liquid
(acetone) was removed by drying at 20.degree. C. for 7 days.
[0162] The gel body obtained in step (b) had a greasy consistency
and exhibited a low mechanical stability. The product obtained in
d) had a significantly shrunken form compared to examples 1 and
2.
Example 4C
[0163] a) 1.56 g of oligomeric MDI (Lupranat.RTM.-M200 R) with an
NCO content of 30.9 g per 100 g to ASTM D-5155-96 A, a
functionality in the region of three and a viscosity of 2100 mPas
at 25.degree. C. to DIN 53018 and 0.8 g of diethyltoluenediamine
(Ethacure.RTM. 100, a mixture of 3,5-diethyltoluene-2,4-diamine and
3,5-diethyl-toluene-2,6-diamine) were dissolved in 34 g of acetone
in a beaker at 20.degree. C. with stirring. [0164] b) The mixture
was left to stand at room temperature for 24 hours for curing.
[0165] d) Subsequently, the gel was taken out of the beaker and the
liquid (acetone) was removed by drying at 20.degree. C. for 7
days.
[0166] The gel body obtained in step (b) had a greasy consistency
and exhibited a low mechanical stability. The product obtained in
d) had a significantly shrunken form compared to examples 1 and
2.
* * * * *