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Description  |
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The invention relates to a filter material which substantially consists of
a sheet structure obtained by coagulation of a polymer solution, and to a
process for producing same.
Laminated and sheet filters have been known for a long time. These
materials substantially consist of woven, knitted or needled fibers of
different length which additionally have been reinforced with an adhesive
or by mechanical means. As fiber material all sorts of materials come into
practical consideration, either of natural or synthetic origin. However,
these filters have the property in common -- as they all have a more or
less porous surface -- of being undesirably clogged by the particles to be
filtered out. Moreover, this effects an initially rapid throughput of
filtrate which is perhaps not yet sufficiently thoroughly filtered, and a
growingly insufficient throughput with increasingly clogged filter area.
Furthermore, the production of sheet structures by coagulation of a polymer
solution has long been known. Moreover, it has been known to produce
microporous sheet structures, e.g. from polyurethane solutions, according
to the coagulation technique; yet, the microporous sheet structures
obtained in this way are permeable only to gaseous phases. as expressly
stated in the published German application No. 1,694,148 which describes
the production of microporous sheet structures from polyurethane.
The difficulties in the manufacture of these microporous sheet structures
reside particularly in the circumstance that it is not possible to produce
sheet structures with smooth top faces. Although this is nearly possible
with thin sheet structures up to a thickness of about 400 microns, this
requires a complicated and lengthy coagulation technique. Normally such a
system must be immersed into a series of coagulating baths with gradually
lesser content of genuine solvent. Frequently even pregelling is necessary
which, in turn, must take place rapidly since otherwise only deficient
microporosity is achieved.
Therefore, it has been an object of the invention to influence and treat
the microporous and/or macroporous sheet structures obtained by
coagulation such that they may be used, on the one hand, as membrane-type
filters and, on the other hand, as genuine filters, e.g. as liquid
filters.
It has been a further object of the invention to provide a process by means
of which polyurethane solutions can be prepared in a very simple and
reproducible way which, by means of the coagulation technique, give
microporous and/or macroporous sheet structures of largely crosslinked
polyurethane elastomers and which, after suitable after-treatment,
constitute the filter materials of the invention.
Further objects of the present invention will become apparent from the
following description.
According to the invention, the first-named object is attained in that at
least one surface of the microporous and/or macroporous sheet structure
obtained by coagulation of a polymer solution is abraded more or less,
depending on the desired pore size. The starting point for this
development was the finding that the sheet structures produced by
coagulation have a special membrane structure, i.e. that extremely thin
poreless external skin layers surround and enclose the porous central
layer proper. The central layer may have randomly arranged pores of
various sizes, or the pores may be aligned predominantly toward the
external layers. Furthermore, they may be interconnected to form a
capillary system, or they may be present as individual pores in more or
less great number.
The second-named object was solved according to the invention by a special
process for preparing the polyurethane solution which results in
microporous sheet structures of largely crosslinked polyurethane
elastomers, which process is both especially simple and nevertheless
precise and reproducible. Moreover, the microporous polyurethane sheet
structures obtained according to this process are especially well suited
for use as filter material, after the abrasion treatment according to the
invention.
Therefore, the subject matter of the invention is a filter material which
consists substantially of a microporous and/or macroporous sheet structure
which is characterized in that at least one surface of the sheet structure
is more or less abraded, depending on the desired pore size.
Furthermore, the invention relates to an especially suitable process for
producing filter material from crosslinked polyurethane elastomers which
is characterized in that
(a) in a manner known per se an NCO pre-adduct is prepared, and said
pre-adduct is dissolved in a suitable solvent, or the NCO-pre-adduct is
directly prepared in a solvent, then
(b) to a previously prepared solution of a suitable solvent and hydrazine
and/or the hydrazine derivatives and/or diamines and/or polyols, with the
proviso that, if said compounds contain only two hydrogen atoms active
according to Zerewitinov, (see The Condensed Chemical Dictionary, 5th
edition 1952; The Merck Index, 9th edition, ONR-97) a compound must be
additionally used which contains at least three hydrogen atoms differently
active according to Zerewitinov, so much of the NCO pre-adduct solution is
continuously admixed over a specific period of time during which the
viscosity is constantly measured, until the viscosity is within a range in
which the addition of lesser and lesser quantities of NCO pre-adduct
solution causes an ever growing increase in viscosity, until finally the
viscosity has reached a level -- and then the addition of NCO pre-adduct
solution must be discontinued at the latest -- at which the addition of
even the minutest amount of NCO pre-adduct solution would result in
instantaneous gelling, the addition of the NCO pre-adduct solution being
effected at a rate such that the so-called "final solution" has a solids
content between 15 and 35% by weight, and
(c) the obtained highly viscous final solution, after shaping on a
subsequently removable support and/or freely supported together with a
reinforcement that will later remain in the filter material, is introduced
into a coagulating bath,
(d) the resulting microporous sheet structure is dried, and
(e) one or both surfaces of the sheet structure are abraded.
Depending on the coagulation technique employed, one obtains sheet
structures with symmetrical or asymmetrical pore configuration in the
central stratum enclosed by extremely thin poreless outer skin layers.
When the polymer solution applied by doctor blade on a smooth continuous
substrate is coagulated, this results in the formation of an asymmetrical
pore configuration, i.e. pores will form which are shaped like funnels. On
the other hand, when the polymer solution applied by doctor blade onto a
reinforcing grid such that the reinforcement is substantially embedded
therein is subjected to coagulation in freely supporting relationship, the
liquid of the coagulating bath can attack the sheet structure to be
coagulated from both sides so that a symmetrical pore configuration is
obtained, i.e. numerous tubular pores are formed that are aligned
perpendicularly to the surface. The symmetrical and the asymmetrical pore
configurations are illustrated by the attached drawings in which
FIG. 1 shows schematically an enlarged cross section through a microporous
sheet with asymmetrical pore configuration which has not yet been
subjected to the abrasion treatment of the invention, and
FIG. 2 shows schematically an enlarged cross section through a microporous
sheet with symmetrical pore configuration which has not yet been subjected
to the abrasion treatment of the invention.
In FIG. 1 the poreless skin layer A denotes the layer which has been
exposed to the influence of the coagulating liquid. The poreless external
layer B constitutes the side that was in direct contact with the smooth
substrate so that from this side the coagulating liquid could not affect
the layer to be coagulated.
In FIG. 2 the poreless skin layers A are substantially identical since both
were equally exposed to the influence of the coagulating liquid. The
circles C denote the reinforcing material.
According to the invention, the extremely thin non-porous skin layers A and
B are abraded. Depending on the ultimate purpose of the filter material,
either one or both faces are subjected to abrasion. The question how much
of the surface on the sheet structures is abraded depends on what filter
efficiency is to be achieved with the individual filter material. Hence,
the pore size of the filter material may also be influenced by the degree
of abrasion. Abrasion is effected in a manner known per se, preferably
according to the wet grinding technique. Otherwise the pore size is
controlled in a manner known per se, e.g. by the type of coagulation bath
or the applied coagulation technique and by the addition of specific
pore-regulating substances, e.g. organic acid, for instance formic acid.
The filter material with asymmetrical pore configuration (i.e. without
reinforcement) manufactures with the process preferred according to this
invention can be cleft in case filter material of even lesser thickness is
desired. In such cases splitting is effected after the abrasion operation.
Hereafter the preferred process of the invention for the production of the
polyurethane filter materials will be described by means of which it is
possible without high investment of equipment to prepare crosslinked
polyurethane elastomers which have such a high degree of crosslinkage that
after their preparation they are no longer completely soluble in the
previously used solvent.
The NCO pre-adducts used according to the invention are higher molecular
weight compounds with two terminal NCO groups, which preferably possess a
molecular weight of 100 to 10,000, especially between 800 and 2,500 .
Preferably the NCO pre-adducts have a content of NCO groups of 1.5 to 5%.
The preparation of these NCO pre-adducts is carried out in a known manner
by reacting higher molecular weight compounds containing OH groups with an
excess of polyisocyanate. The preparation of such NCO pre-adducts is
described, for example, in Angewandte Chemie 64, 523 to 531 (1952);
Kunststoffe 42, 303 to 310 (1952); German Pat. No. 831,772; German Pat.
No. 897,014; German Pat. No. 929,507 ; and U.S. Pat. No. 3,000,757.
Preferably the formation of the NCO pre-adduct is controlled such that
linear NCO pre-adducts having a narrow molecular weight distribution are
obtained. The question which NCO pre-adducts to use in the individual case
depends on the individual systems, because the reactivity of the NCO
pre-adduct relative to the chain propagation and/or crosslinking agent
largely determines the relationship between linear molecular weight and
the degree of crosslinkage.
Higher molecular weight compounds containing OH groups which are suitable
for the production of the NCO pre-adducts are, for example, polyesters,
polyethers, polyesteramides, polythioethers and polyacetals.
As polyols for the preparation of the NCO pre-adducts one may use, for
example, linear hydroxylpolyesters which contain primary and/or secondary
and/or tertiary hydroxyl groups and which have been obtained either by
polycondensation of .epsilon.-caprolactone or 6-hydroxycaproic acid or by
copolymerisation of .epsilon.-caprolactone with dihydric alcohols or by
polycondensation of dicarboxylic acids with dihydric alcohols.
The hydroxylpolyesters used for the production of the NCO pre-adducts can
also be produced from dicarboxylic acids or mixtures of dicarboxylic acids
and dihydric alcohols. Suitable dicarboxylic acids include, for example,
adipic acid, succinic acid, suberic acid, sebacic acid, oxalic acid,
methyladipic acid, glutaric acid, pimelic acid, azelaic acid, phthalic
acid, terephthalic acid, isophthalic acid, maleic acid, fumaric acid,
citraconic acid, itaconic acid. Suitable dihydric alcohols or mixtures
thereof which are reacted with the dicarboxylic acids or
.epsilon.-caprolactone to form the desired hydroxypolyesters include, for
example, ethylene glycol, propylene glycol, butylene glycols, for example,
1,4-butanediol; butenediol, butindiol, bis-(hydroxymethylcyclohexane),
diethylene glycol, 2,2-dimethylpropylene glycol, 1,3-propylene glycol.
However, the above mentioned diols may also be used all by themselves.
This also applies to diamines or other low molecular weight compounds
containing two hydrogen atoms with Zerewitinov activity.
Suitable polyalkylene ethers possessing primary and/or secondary and/or
tertiary hydroxyl groups which can be used according to the invention are
obtained by reacting an alkylene oxide with a small quantity of a compound
containing active hydrogen, such as water, ethylene glycol, propylene
glycol, amylene glycol. It is also possible to use alkylene oxide
condensates of ethylene oxide, propylene oxide, butylene oxide, amylene
oxide, styrene oxide and mixtures thereof. It is also possible to use the
polyalkylene ethers which can be produced from tetrahydrofuran.
According to the invention every suitable polyester amide can be used for
the preparation of the NCO pre-adducts, for example the reaction product
of an amine and/or amino-alcohol with a dicarboxylic acid. Suitable amines
are, for example, ethylenediamine, propylenediamine; suitable
amino-alcohols are, for example, 1-hydroxy-2-amino-ethylene. Any suitable
polycarboxylic acid can be used, for example those which have already been
mentioned for the production of the hydroxypolyesters. Furthermore it is
possible to use a mixture of a glycol and of an aminoalcohol or polyamine.
Each of the glycols already mentioned for the production of the
hydroxypolyesters can also be used for the production of the
hydroxypolyester amides.
According to the invention it is also possible to use for the preparation
of the NCO pre-adducts those polyols which can be referred to as
polyetherester polyols, in which there occur alternating ester bonds and
ether bonds. These polyetherester polyols are described in Canadian Patent
Specification No. 783,646.
Polyols preferably used for the preparation of the NCO pre-adducts include
polyesters on the basis of adipic acid, 1,6-hexanediol and neopentyl
glycol with an average molecular weight of approximately 2,000 (Polyol
2,002 manufactured by Polyol Chemie of Osnabruck, hydroxyl number 56, acid
number 1), polyesters on a polycaprolactone basis with an average
molecular weight of 2,000 (Niax Polyol D 560 manufactured by Union Carbide
Corporation) and polyethers with the trade name "Polyol PTMG" of BASF with
an average molecular weight of 2,000.
Furthermore, higher molecular weight compounds with terminal carboxyl,
amino and mercapto groups are suitable. Polysiloxanes which have groups
which are reactive with isocyanates should also be mentioned. Further
utilisable compounds are described, for example, in J. H. Saunders, K. C.
Frisch "Polyurethanes" Part 1, New York, 1962, pages 33 to 61 and in the
literature cited here.
For the preparation of the NCO pre-adducts it is possible to use any
suitable organic diisocyanate, for example aliphatic diisocyanates,
aromatic diisocyanates, alicyclic diisocyanates and heterocyclic
diisocyanates, for example methylene diisocyanate, ethylene diisocyanate,
propylene diisocyanate, butylene diisocyanate, cyclohexylene
1,4-diisocyanate, cyclohexylene 1,2-diisocyanate, tetra- or hexamethylene
diisocyanate, arylene diisocyanates or their alkylation products, such as
phenylene diisocyanates, naphthylene diisocyanates, diphenylmethane
diisocyanates, toluylene diisocyanates, di- or triisopropylbenzene
diisocyanates; aralkyl diisocyanates such as xylylene diisocyanates,
fluoro-substituted isocyanates, ethyleneglycol
diphenylether-2,2'-diisocyanate, naphthalene-1,4-diisocyanate,
naphthalene-1,1'-diisocyanate, biphenyl-2,4'-diisocyanate,
biphenyl-4,4'-diisocyanate, benzophenone-3,3-diisocyanate,
fluorene-2,7-diisocyanate, anthraquinone-2,6-diisocyanate,
pyrene-3,8-diisocyanate, chrysene-2,8-diisocyanate, 3'-methoxyhexane
diisocyanate, octane diisocyanate,
.omega.,.omega.'-diisocyanate-1,4-diethylbenzene-.omega.,.omega.'-diisocya
nate-1,4-dimethylnaphthalene, cyclohexane-1,3-diisocyanate,
1-isopropylbenzene-2,4-diisocyanate, 1-chlorobenzene-2,4-diisocyanate,
1-fluorobenzene-2,4-diisocyanate, 1-nitrobenzene-2,4-diisocyanate,
1-chloro-4-methoxybenzene-2,5-diisocyanate,
benzene-azonaphthalene-4,4'-diisocyanate, diphenylether-2,4-diisocyanate,
diphenylether-4,4-diisocyanate, as well as oliisocyanates containing
isocyanurate groups.
Diisocyanates which are preferably used according to the invention are:
4,4'-diphenylmethane diisocyanate and/or its 2,4- and/or its 2,2'-isomers,
1,6-hexamethylene diisocyanate, 2,4-toluylene and/or 2,5-toluylene
diisocyanate and m-xylylene diisocyanate.
The chain propagating/crosslinking agents are preferably substances
containing highly active hydrogen atoms of different activity, as present,
for instance, in the NH.sub.2 group of hydrazines. The use of hydrazine
compounds, especially of hydrazine itself, is therefore preferred.
Preferably the chain propagation takes place such that about 60% of said
NH.sub.2 groups are used for chain extension and the remaining 40% for
chain crosslinkage. The reaction must take place spontaneously so that
addition of pre-adduct and rise in viscosity occur in proportion. Systems
with chain propagating/crosslinking mixtures of lesser activity (than
hydrazine, for instance) must be catalyzed such that the 60 to 40%
relation is maintained, to thereby utilize in an especially favorable way
the principles of the Flory equation which determines the degree of
polymerization in a solvent with the number of crosslinkage sites.
In this connection it is pointed out that according to Saunders and Frisch
in "polyurethanes: Chemistry and Technology, II." Technology, page 319,
the preparation of polyurethane elastomers is especially difficult if
diamines are used as chain propagating agents, since they easily lead to
non-homogeneous products due to their high reactivity. However, the
invention makes use of this very circumstance in order to obtain uniform
products. Thus, the high reactivity of hydrazine is deliberately utilized
to achieve chain propagation and a self-controlling degree of crosslinkage
such that this takes place in solution and the final solution remains
castable for at least 24 hours when properly stored.
In order to achieve additional crosslinkage, the cross-linking reactions
familiar in the polyurethane chemistry may be utilized. Moreover,
formaldehyde in polymeric form may be added to the crosslinking agent
solution. When formaldehyde in dimethylformamide and hydrazine hydrate are
introduced first into the reactor, the first step of a Wolff-Kishner
reaction is initiated in which the hydrazone is formed besides water.
However, under the selected reaction conditions this reaction is largely
suppressed and the following crosslinking reaction is promoted:
##STR1##
The amount of added aldehyde depends on the later use of the product. The
upper limit is the stoichiometrical point, based on NH groups of the
elastomer. However, this crosslinkage is different, since it takes place
later at elevated temperatures such as those occurring during the removal
of the solvent.
Additional crosslinkage is also achieved when unsaturated systems are
employed, and these double bonds are broken up by electron bombardment
thereby initiating localized branching reactions.
As mentioned before, suitable substances for the purposes of the invention
are particularly hydrazine compounds such as hydrazine hydrate,
carbohydrazide, carbodihydrazide, semicarbazide, carbazone, oxalic acid
dihydrazide, terephthalic acid dihydrazide and dihydrazides of longer
chain dicarboxylic acids and also dihydrazine compounds of the general
formula
H.sub.2 N--NH--X--NH--NH.sub.2,
in which X signifies CO, CS, P(O)OR, P(O)NR.sub.2, BOR or SiO.sub.2, in
which R stands for an aliphatic or aromatic radical, as well as compounds
which have the piperazine structure
##STR2##
and two or more terminal amino groups.
Preferably the radicals R represent alkyl or aryl radicals. Of course, in
the process according to the invention one can also use the corresponding
hydrate forms, which is even preferred in the case of hydrazine in view of
the lesser handling hazard.
By the polyaddition of the above-described NCO prepolymers with hydrazine
or dihydrazine compounds one obtains, for example, polycarbohydrazides or
polycarbodihydrazides or mixtures thereof, with recurring units partially
cross-linked via the "--NH--" and "--NH--NH--" groups.
##STR3##
wherein "V" represents a possible and controlled site of linkage with an
adjacent polymer chain.
In these formulae the abbreviation PE signifies polyester, polyether,
polyamides, polythioether, polyacetals and X signifies a carbonyl,
thiocarbonyl, sulpho, SiO.sub.2, BOR, P(O)OR or P(O)NH.sub.2 group and R
stands for an aliphatic or aromatic radical. n signifies that the final
polyurethane contains plurality or multiplicity of the units mentioned
before.
Suitable diamines which can be used according to the present invention
include, for example, ethylenediamine, propylenediamine, toluylenediamine,
xylylenediamine, piperazine or piperazine hexahydrate as well as
1,4-diaminopiperazine.
As already mentioned, according to the invention one uses the hydrazines,
dihydrazine compounds and/or diamines preferably either in a deficient
quantity or in excess. When one uses a deficient quantity one adds to the
prepared component solution further substances which contain at least two
groups possessing active hydrogen atoms which react, optionally
differently, with isocyanates, and which can act either as chain
propagating and/or crosslinking agents, these substances occurring in
excess after the stoichiometrical reaction has taken place.
In both cases the excess can amount up to 30%. Suitable compounds of this
type include all the other chain propogating agents or crosslinking agent
generally employed in polyurethane chemistry, for example diols, e.g.
ethyleneglycol, propyleneglycol butyleneglycols, 1,4-butanediol,
butenediol, butindiol, xylyleneglycols, amyleneglycol,
1,4-phenylene-bis-.beta.-hydroxyethyl ether,
1,3-phenylene-bis-.beta.-hydroxyethyl ether,
bis-(hydroxymethylcyclohexane), hexanediol, and alkanolamines, for example
ethanolamine, aminopropyl alcohol, 2,2-dimethylpropanolamine,
3-amino-cyclohexyl alcohol, .rho.-aminobenzyl alcohol, trimethylolpropane,
glycerol or N,N,N',N'-tetrakis-(2-hydroxypropyl)ethylenediamine. Of all
these substances one preferably uses glycerol. Of course, several chain
lengthening and/or crosslinking agents can be used at the same time.
If desired, it is possible to add to the above-described solution
containing hydrazines dihydrazine compounds and/or diamines chain
terminating agents and optionally additional gelling agents either besides
or instead of the added chain-lengthening and/or crosslinking agents.
Suitable chain terminating agents include, for example, monohydric
alcohols, for example methyl alcohol, ethyl alcohol, propyl alcohol or
butyl alcohol, or substances with an amino group such as ethylamine.
The above described solutions containing hydrazines, dihydrazine compounds
and/or diamines can have added to them prior to their reaction with the
NCO pre-adducts fillers, organic or inorganic pigments, dyestuffs, optical
brighteners, ultra violet absorbers, anti-oxidants and/or additional
crosslinking substances, especially substances which effect crosslinkage
only after coagulation at elevated temperatures. Sometimes, however, it is
more advantageous to add the above mentioned additives to the final
polyurethane solution, optionally just before its use for the production
of the desired products, rather than to the solution containing the
hydrazines, dihydrazine compounds and/or diamines.
Advantageously the dyestuffs, which should be soluble in the solvent used,
are added shortly prior to shaping, because a few types of dyestuff can
exert an undesirable catalytic action on the NCO pre-adduct. A
disadvantage of these dyestuffs is that many of them bleach as a result of
the action of light. It is therefore more favorable in certain cases to
use the above-mentioned pigments. It is true that generally speaking these
do not give such bright tones as do the soluble dyestuffs, but they are
characterized by good covering power.
It has been additionally found that, contrary to expectations, pigmentation
enhances the microporous structure of the sheet material. This has a
favorable effect on the permeability.
If the pigments are properly chosen, the quantity used can be kept small.
There is therefore no fear of the elastic properties of the system being
adversely affected afterwards to any appreciable extent.
In the case of carbon black pigments it is even possible, if suitable
products are chosen which possess a certain number of OH groups, to
incorporate these firmly in the pre-adduct. It should also be pointed out
that carbon black pigments are the best stabilizers against hydrolysis of
such polyurethane systems.
The above mentioned flotation of other pigments can be avoided by the
addition of so-called anti-floating agents.
Pure fillers are available in large numbers. Generally speaking it can be
said that all non-reactive powdered or fibrous materials whose individual
fibre length is below the thickness of the film can be embedded. In this
way one can ensure that thinner coatings than usual will give more uniform
surfaces on a substrate material.
Of particular interest is the admixture of microporous silicas by means of
which the permeability of the material can be precisely controlled.
Moreover, these porous substances, being first porous centers in the not
yet coagulated film, offer valuable assistance for the later exchange of
non-solvent.
They have both a supporting function for the incoming non-solvent and a
receiving function for the displayed solvent. This effects more rapid
coagulation and more uniform microporosity.
However, it is also possible to operate with reactive additives. If one
chooses, for example, a substance containing OH groups, e.g. cellulose
powder or fibers, it is possible to some extent to incorporate these
firmly as fillers. These substances are particularly suited to improve the
initial tear strength. Also the nature of the surfaces of the microporous
sheet structure can be varied depending on the fiber length.
Moreover, it is possible, if desired, to incorporate flavoring substances,
e.g. coffee extract.
Suitable solvents for the reaction components include, according to the
invention, preferably organic solvents, especially highly polar solvents.
Examples for such solvents are aromatic hydrocarbons such as benzene,
toluene, xylene, tetraline, decaline; chlorinated hydrocarbons such as
methylene chloride, chloroform, trichloroethylene, tetrachloroethane,
dichloropropane, chlorobenzene; esters such as ethyl acetate, propyl
acetate, butyl acetate, diethyl carbonate; ketones such as acetone,
butanone-2, pentanone-2, cyclohexanone; ethers such as furan,
tetrahydrofuran, dioxan, anisol, phenetol, dialkoxyethanes and
ether-esters of glycol; acid amides such as formamide, dimethylformamide,
dimethylacetamide; and sulfoxides such as dimethylsulfoxide. The
especially preferable employed solvents include the acid amides, e.g.
formamide, dimethylformamide and N,N-dimethylacetamide; sulfoxides, e.g.
dimethylformamide and N,N-dimethylacetamide, sulfoxides, e.g.
dimethylsulfoxide, dioxane, tetrahydrofuran or mixtures thereof. Although
the NCO pre-adducts may be dissoled in a solvent other than hydrazines,
dihydrazine coumpounds and/or diamines, the same solvent or solvent
mixture is prefeably used in the process of the invention for both
reaction components.
For the chain propagation and crosslinkage reactions proper both of which
take place substantially simultaneously the NCO pre-adduct solution is
added to the previously introduced chain propagation/crosslinking agent
solution with continuous stirring. The polyurethane formation reaction
takes place exothermically and is rapidly terminated. Both the chain
propagation and the crosslinkage effect the rise in viscosity. The
viscosity abruptly increases rapidly, after an initial slight increase.
During this stage of the process the NCO pre-adduct solution must be
carefully added to the other component, because after the addition of a
certain amount even minutest additions of further NCO pre-adduct effect a
high increase in viscosity so that a certain point of the process the
reaction solution suddenly gels. According to the invention, it has been
surprisingly found that excellently suitable polyurethane solutions are
obtained when the addition of NCO pre-adduct is discontinued when the
viscosity of the solution has reached such a level. This viscosity level
lis between 6000 and 40,000 cps. in the normally employed systems, i.e.
when the reaction solution or the final solution has a honey-like
consistency.
In the practical operation of a preferred embodiment of the process of the
invention it has further been found that the NCO pre-adduct solution,
which preferably has a solids content of 60 to 80% by weight, especially
of 70% by weight, and a content of free isocyanate groups of from 1.5 to
5%, is continuously added with stirring to the other component solution
containing the hydrazines, dihydrazine compounds and/or diamines at a
concentration of 0.02 to 0.05 mole percent, at a rate such that in the
instant where rapid increase in viscosity takes place the reaction
solution has a solids content between 15 and 35% by weight. As the
quantity of the NCO pre-adduct added depends on many factors (temperature,
molecular weight of the polyester or polyether used for the production of
the pre-adduct, solids content of NCO groups, age of the pre-adduct), it
is not possible to calculate this exactly; it is therefore necessary to
operate empirically. The safest way is to proceed by determining in a
small preliminary test the quantity of pre-adduct approximately required
and then, in the actual preparation of the final polyurethane solution, to
rely upon the increase in viscosity. In order to produce larger quantities
of a useful final solution or reaction solution, it is advisable to use a
flow-through gauge which can reproduce the figure found during the
preliminary test. However, according to the invention the precision
adjustment is then made by mens of a built-in viscosimeter. The simplest
apparatus recommended for measuring the viscosity is a falling ball
viscosimeter, because the accuracy is sufficient and cleaning is easy,
although this instrument can only be used for unpigmented systems. If one
wishes to stain the product, this must be done after the viscosity has
been adjusted. Another possibility to determine the viscosity is the
measurement of the power output of the stirrer motor by means of a
suitable ammeter.
According to a special embodiment of the invention, one preferably uses as
NCO pre-adduct one which has been obtained by reacting a polyester
(produced from adipic acid and hexanediol-1,6) with 4,4'-diphenylmethane
diisocyanate in dimethylformamide. This NCO pre-adduct solution is added
to a solution of a deficient quantity of hydrazine hydrate in
dimethylformamide, to which one has added an excess of glycerol. During
the addition and the reaction one maintains the reaction temperature
between 20.degree. and 45.degree. C., preferably between 25.degree. and
40.degree. C.
Generally one controls the preparation of the final polyurethane solution
in such a way that prior to shaping all the polyurethanes are dissolved in
the reaction solution, but after shaping a polymer structure is obtained
which is no longer soluble to the extent of more than 50% by weight in
cold dimethylformamide and is no longer soluble to the extent of more than
60 percent by weight in boiling dimethylformamide.
The solution thus obtained is highly viscous, uniformly pregelled may be
stored for days. When the indicated reaction conditions (temperature,
solids content, time and the proper sequence of the individual reaction
steps) are observed, a final solution is reproducibly obtained which is
within the borderline state between solution and gel and which
surprisingly results in the subsequently described elastomeric products.
By the selection of suitable raw materials the physical properties can be
tailored to a specific end use. The optical properties may be determined
during the formation of the polymer structure without any additional
processing.
Suitable other gelling agents include, for instance, solvents miscible with
the solvent or the solvent mixture of the final reaction solution but
constituting a non-solvent for the resulting crosslinked polyurethanes.
Such non-solvents i | | |
