Note: Descriptions are shown in the official language in which they were submitted.
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Polyurethane casting elastomers made of NCO prepolymers based on 2,4'-MDI,
a process for their preparation and their use
The present invention relates to novel polyurethane (PUR) casting elastomers
made of
NCO-functional prepolymers based on 2,4'-MDI and amine-based chain extenders
and/or crosslinking agents, to a process for their preparation and to their
use.
MDI (diphenylmethane diisocyanate) is a technically important group of poly-
isocyanates; it has a very heterogeneous composition in terms of its structure
and
comprises monomer grades characterized in that they have two aromatic
structural
elements bonded via a single methylene bridge, and higher oligomers having
more
than two aromatic structural elements and possessing more than one methylene
bridge, which are referred to as polymeric MDI.
Monomeric MDI contains predominantly the 4,4' and 2,4' isomers as a
consequence
of its synthesis. The 2,2' isomer also occurs to a lesser extent, but is
largely of no
technical value.
The ratio of monomeric MDI to polymeric MDI, and the proportions of the 2,4'
and
4,4' isomers in monomeric MDI, can be varied within wide limits by varying the
conditions of synthesis of the precursor.
The crude MDI obtained in the MDI synthesis is separated substantially by
distillation, it being possible, depending on technical expenditure, to
separate off
either almost isomerically pure fractions with proportions of 4,4'-MDI, for
example,
of more than 97.5 wt.%, or isomer mixtures with proportions of 4,4'-MDI and
2,4'-
MDI of about 50 wt.% in each case.
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In the past, because of technical conditions, pure 2,4' isomer was
commercially
available only in very limited quantities, if at all. Recently, however, more
effort has
been devoted to making this isomer available in high purity as well.
A basic reason for this effort is the differences in reactivity of the 2- and
4'-NCO
groups of 2,4'-MDI, in a similar way to the differences in reactivity of the 2-
and
4-NCO groups of 2,4-toluylene diisocyanate (TDI).
These differences in reactivity allow or facilitate the synthesis of monomer-
poor NCO
prepolymers. NCO prepolymers are polyols with terminal NCO groups which are
obtained by reacting a polyol with a polyisocyanate using a molar excess of
NCO,
based on the NCO-reactive groups, at room temperature to about 100 C.
Depending
on the initial molar proportions, NCO prepolymers prepared in this way always
contain free monomeric diisocyanate.
In the case of 2,4-TDI, the driving force behind the preparation of monomer-
poor to
practically monomer-free NCO prepolymers is justified by its high vapour
pressure
and the resulting health hazards. NCO prepolymers based on aliphatic
diisocyanates,
e.g. hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI), are
to be
regarded as even more critical in this context. This aspect is also relevant
to MDI,
although to a markedly reduced extent because its vapour pressure is lower
than that
of TDI. Moreover, reducing the monomer content of the prepolymer results in
polyurethanes that are softer than those prepared from monomer-containing NCO
prepolymers.
Monomer-poor NCO prepolymers can be prepared in several different ways:
a.) Removal of the free monomeric diisocyanate by technically expensive film
evaporation or short-path evaporation. This is independent of whether the
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diisocyanates used have NCO groups of the same or different reactivity.
Entraining agents, for example, can also be used for this purpose.
b.) Use of diisocyanates with NCO groups of different reactivity or NCO groups
of the same reactivity, and specially chosen stoichiometric proportions, e.g.
molar proportions of NCO to NCO-reactive groups of less than 2:1, and/or
optionally under special catalysis.
c.) Combinations of both processes, e.g. in such a way that the proportion of
free
monomeric diisocyanate is initially limited to a certain extent by process b.)
and then minimized further by process a.).
Such combinations can be useful when the viscosity of the prepolymers is to be
minimized. The disadvantage of process b.) is basically that reactions with
stoichio-
metric proportions particularly of less than 2:1 lead to increased pre-
extension,
inherently resulting in a marked increase in the viscosity of the reaction
product.
WO 01 /40340 A2 (Crompton Corp.) gives examples of such combinations wherein,
in
a first step, the diisocyanate is converted to an NCO prepolymer with the
concomitant
use of a selectivity-increasing catalyst, and said prepolymer is then freed of
excess
monomer by film evaporation.
Particularly critical applications, for instance in the food sector, are
affected by the
matter of industrial hygiene, which applies to a high degree to TDI and also
to MDI.
This is indicated by numerous patents dealing even with monomer-poor MDI
prepolymers, e.g. WO 03/006521 (Henkel KGaA), WO 03/033562 (Henkel KGaA),
WO 03/055929 (Henkel KGaA), WO 03/051951 (Henkel KGaA), WO 93/09158
(Bayer AG) and EP 0 693 511 A l(Bayer AG).
The object of the present invention was therefore to provide polyurethanes
based on
2,4'-MDI which have processing advantages compared with the state of the art,
for
instance in the form of longer casting times and lower prepolymer viscosities,
and at
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the same time are at least equal to the state of the art in terms of their
mechanical
properties.
Surprisingly, it has now been found that, in terms of mechanical properties
(e.g.
abrasion, ultimate strength, tear propagation resistance, elongation at
break), valuable
PUR are obtained from NCO prepolymers based on 2,4'-MDI with a 2,4' isomer
content of at least 85 wt.% and a proportion of free monomeric MDI in the
prepolymer of at least 1 wt.% to 20 wt.%, preferably of at least 2 wt.% to 18
wt.% and
particularly preferably of 3 to 15 wt.%, based on the prepolymer. The low
viscosity of
the NCO prepolymers is a further advantage.
NCO prepolymers are understood hereafter as meaning NCO prepolymers which have
been prepared from pure 2,4'-MDI, contain at least I wt.% and max. 20 wt.% of
free
monomeric diisocyanate, based on the prepolymer, and have not been extracted
or
distilled.
Pure 2,4'-MDI is understood hereafter as meaning MDI grades which have a 2,4'
isomer content of at least 85 wt.%, preferably of at least 90 wt.%,
particularly
preferably of at least 95 wt.% and very particularly preferably of at least
97.5 wt.%.
The present invention provides polyurethane elastomers obtainable (by the
casting
process) from
a) NCO prepolymers based on diphenylmethane diisocyanate with a 2,4' isomer
content of at least 85 wt.%, preferably of at least 90 wt.%, particularly
preferably of at least 95 wt.% and very particularly preferably of at least
97.5
wt.%, the proportion of free monomeric 2,4'-MDI being at least I wt.% to 20
wt.%, preferably 2 to 18 wt.% and particularly preferably 3 to 15 wt.%, based
on the NCO prepolymer, and polyols having OH numbers of 20 to 200 mg
KOH/g and functionalities of 1.95 to 2.40, preferably of 1.96 to 2.20,
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b) amine-based chain extenders and/or crosslinking agents, preferably aromatic
amine-based chain extenders and/or crosslinking agents, and
c) optionally auxiliary substances and additives.
The polyurethanes according to the invention are superior to the state of the
art
because they have particularly favourable combinations of advantageous
properties in
respect of prepolymer viscosity, casting time and mechanical and mechanico-
dynamic
properties.
The invention also provides a casting process for the preparation of the
polyurethane
elastomers according to the invention, said process being characterized in
that
A) diphenylmethane diisocyanate (MDI) with a 2,4' isomer content of at least
85
wt.%, preferably of at least 90 wt.%, particularly preferably of at least 95
wt.% and very particularly preferably of at least 97.5 wt.% is reacted with
polyols having OH numbers of 20 to 200 mg KOH/g and functionalities of
1.95 to 2.40 to give NCO prepolymers with a proportion of free monomeric
2,4'-MDI of I wt.% to 20 wt.%, preferably of 2 to 18 wt.% and particularly
preferably of 3 to 15 wt.%, based on the NCO prepolymer, and
B) amine-based chain extenders and/or crosslinking agents and optionally
auxiliary substances and additives are added to the prepolymer from A) in
order to prepare the elastomer.
The preparation of elastomers by the casting process is a generally important
use of
NCO-terminated prepolymers, the NCO prepolymers either being reacted with a
chain
extender directly after their preparation or being cooled to a lower
temperature
(storage temperature) and stored for the purpose of chain extension at a later
stage.
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The synthetic route via prepolymers is favourable in that part of the heat of
reaction is
already produced during the synthesis of the prepolymer, thereby reducing the
exothermicity of the actual polymer synthesis. This has a favourable effect on
the rate
of molecular weight build-up and allows longer casting times, representing a
processing advantage.
In one particularly preferred embodiment of the preparation of the PUR
elastomers by
the prepolymer process, the prepolymers are first degassed by the application
of a
reduced pressure at room temperature or elevated temperature, and then stirred
with a
chain extender, usually at elevated temperature.
In the process according to the invention, the prepolymer is preferably heated
to a
temperature of 60 C to 1] 0 C and degassed under vacuum, with stirring. The
chain
extender and/or crosslinking agent is then added in liquid form, optionally
after
having been heated to temperatures typically of at least 5 C above its melting
point.
The reaction mixture is cast into preheated moulds (preferably 90 C to 120 C)
and
cured at 90 C to 140 C for about 24 hours.
Polyols which can be used are polyether-, polyester-, polycarbonate- and
polyetheresterpolyols having hydroxyl numbers of 20 to 200 mg KOH/g,
preferably of
27 to 150 and particularly preferably of 27 to 120.
Polyetherpolyols are prepared from an initiator molecule and epoxides,
preferably
ethylene oxide and/or propylene oxide, by means of either alkaline catalysis
or double
metal cyanide catalysis, or optionally by means of alkaline catalysis and
double metal
cyanide catalysis in a stepwise reaction, and have terminal hydroxyl groups.
Initiators
which can be used here are the compounds with hydroxyl and/or amino groups
known
to those skilled in the art, and water. The functionality of the initiators is
at least 2
and at most 4. Of course, it is also possible to use mixtures of several
initiators.
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Mixtures of several polyetherpolyols can also be used. Polyetherpolyols can be
tetrahydrofuran oligomers with terminal hydroxyl groups.
Polyesterpolyols are prepared in a manner known per se by the polycondensation
of
aliphatic and/or aromatic polycarboxylic acids having 4 to 16 carbon atoms,
optionally
their anhydrides and optionally their low-molecular esters, including cyclic
esters, the
reaction component used being predominantly low-molecular polyols having 2 to
12
carbon atoms. The functionality of the structural components for
polyesterpolyols is
preferably 2, but can also be greater than 2 in individual cases, the
components having
functionalities greater than 2 only being used in small amounts so that the
arithmetic
number-average functionality of the polyesterpolyols ranges from 2 to 2.5,
preferably
from2to2.1.
Polyetheresterpolyols are prepared by the concomitant use of polyetherpolyols
in the
synthesis of polyesterpolyols.
Polycarbonatepolyols are obtained according to the state of the art by the
polycondensation of carbonic acid derivatives, e.g. dimethyl or diphenyl
carbonate or
phosgene, and polyols.
Preferred chain extenders are aromatic amine-based chain extenders, e.g.
diethyl-
toluenediamine (DETDA), 3,3'-dichloro-4,4'-diaminodiphenylmethane (MBOCA),
isobutyl 3,5-diamino-4-chlorobenzoate, 4-methyl-2,6-bis(methylthio)-1,3-
diamino-
benzene (Ethacure 300), trimethylene glycol di-p-aminobenzoate (Polacure 740M)
and 4,4'-diamino-2,2'-dichloro-5,5'-diethyldiphenylmethane (MCDEA). MBOCA
and isobutyl 3,5-diamino-4-chlorobenzoate are particularly preferred.
Aliphatic
amine-based chain extenders can likewise be used (concomitantly).
It is also possible to use auxiliary substances and additives, for instance
catalysts,
stabilizers, UV stabilizers, hydrolysis stabilizers, emulsifiers, and
dyestuffs and
coloured pigments that are preferably capable of incorporation.
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Examples of catalysts are trialkylamines, diazabicyclooctane, tin dioctanoate,
dibutyltin dilaurate, N-alkylmorpholine, lead, zinc, calcium or magnesium
octanoate
and the corresponding naphthenates and p-nitrophenate.
Examples of stabilizers are Broensted and Lewis acids, for instance
hydrochloric acid,
benzoyl chloride, organomineral acids, e.g. dibutyl phosphate, and also adipic
acid,
malic acid, succinic acid, tartaric acid or citric acid.
Examples of UV stabilizers and hydrolysis stabilizers are 2,6-dibutyl-4-
methylphenol
and sterically hindered carbodiimides.
Dyestuffs capable of incorporation are those which possess Zerewitinoff-active
hydrogen atoms, i.e. which can react with NCO groups.
Other auxiliary substances and additives include emulsifiers, foam
stabilizers, cell
regulators and fillers. A survey can be found in G. Oertel, Polyurethane
Handbook,
2nd edition, Carl Hanser Verlag, Munich, 1994, chap. 3.4.
The polyurethane elastomers according to the invention can be used in a very
wide
variety of applications, e.g. as elastic mouldings produced by the casting
process, as
well as in coatings and adhesive bonds produced by a spraying process, as e.g.
in
parking deck coating systems, concrete repairs and corrosion protection.
The invention will be illustrated in greater detail with the aid of the
Examples which
follow.
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Examples
Methods of measurement used:
Property Dimensions DIN standard ISO/ASTM standard
Hardness [Shore] DIN 53505 ISO 868
Stress [MPa] DIN 53504 ISO 527
Ultimate strength [MPa] DIN 53504 ISO 527
Elongation at break [%] DIN 53504 ISO 527
Tear propagation resistance [kN/m] DIN 53515 ISO 527
Abrasion [mm3] DIN 53516 ASTM D 1242
Density [g/mm3] DIN 53420 ISO 1183
Permanent set, PS [%] DIN 53517 DIN ISO 815
Chemicals used:
Polyesterpolyol 1: poly(ethylene-co-butylene) adipate having an OH number of
56
mg KOH/g from Bayer MaterialScience AG; nominal
functionality 2.0;
4,4'-MDI: 4,4'-diphenylmethane diisocyanate, Desmodur 44M from
Bayer MaterialScience AG; 98.5 wt.% of 4,4' isomer;
2,4'-MDI: 2,4'-diphenylmethane diisocyanate (laboratory product) from
Bayer MaterialScience AG; 98.5 wt.% of 2,4' isomer;
Isobutyl 3,5-diamino-4-chlorobenzoate: RC-Crosslinker 1604 from Rheinchemie,
Rheinau.
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Example 1: Preparation of MDI-based ester prepolymers
Instructions for the preparation of prepolymers using prepolymer 2 as an
example (Table 1):
25 parts by weight of 2,4'-MDI were heated to 70 C in a stirred flask under
nitrogen
and stirred rapidly with 100 parts by weight of dehydrated polyesterpolyol 1
heated to
70 C. The reaction was allowed to proceed for 2 hours and the physical
properties
were determined (cf. Table 1).
Table 1: Formulations of MDI-based ester prepolymers (according to the
invention
and Comparative Examples)
1C 2 3C 4C 5 6C
Polyesterpolyol I [parts by weight] 100 100
Prepolymer I [parts by weight] 100 100
Prepolymer 2 [parts by weight] l00 100
4,4'-MDI [parts by weight] 25 10 10
2,4'-MDI [parts by weight] 25 10 10
NCO (theoret.) [wt.% of NCO] 3.36 3.36 6.1 6.1 6.1 6.1
NCO (exp.) [wt.% of NCO] 3.4 3.44 6.2 6.1 6.17 6.15
Free MDI [wt.%] 4.8 3.1 11.9 13.4 11.9 13.4
Viscosity at 70 C [mPas] 10,600 4800 2900 6200 2900 6400
C: Comparison
Prepolymer 1: from 100 parts by weight of polyesterpolyol I and 25 parts by
weight
of 4,4'-MDI
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Prepolymer 2: from 100 parts by weight of polyesterpolyol I and 25 parts by
weight
of 2,4'-MDI
Comparison of the viscosity values for MDI prepolymers with a theoretical NCO
content of 3.36 wt.% shows the advantages of the prepolymer based on 2,4'-MDI
(prepolymer 2, according to the invention) over the 4,4' analogue (prepolymer
1 C, not
according to the invention).
Mixing of these two prepolymers with additional MDI to NCO contents of 6.1
wt.%
of NCO (theoret.) obviously gives in all cases prepolymers with lower
viscosities than
the starting prepolymers (prepolymers 3 C, 4 C, 5 and 6 C in Table 1). It is
further
seen that the equally low viscosity of prepolymers 3 C and 5 (in each case
2900 mPas
at 70 C) is not sufficient for advantageous processing (e.g. casting time) to
casting
elastomers. Only prepolymer 5 could advantageously be processed further to an
elastomer (cf. Tables 2 and 3).
Example 2: Preparation of casting elastomers according to the invention from
prepolymers 2 and 5 of Example l
Instructions for the preparation of casting elastomers using casting elastomer
A
as an example:
100 parts of prepolymer 2 were degassed at 90 C under vacuum, with slow
stirring,
until free of bubbles. This was then stirred with 9.05 parts of isobutyl 3,5-
diamino-4-
chlorobenzoate preheated to 100 C, and the reacting homogeneous melt was cast
into
moulds preheated to 110 C, having dimensions corresponding to the testing
standards.
The melt was then heated for 24 hours at 1 l0 C and the mechanical properties
listed
in Table 2 were determined.
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Table 2: Formulations, preparation and properties of the casting elastomers
according to the invention
Casting elastomer No. A B C D E
Formulation and preparation:
No. 2 2 2 2 5
Prepolymer [parts by 100 80 60 40 100
weight]
No. 5 5 5
Prepolymer [parts by 20 40 60
weight]
NCO (theoret.) [wt.% of NCO] 3.36 3.9 4.46 5.04 6.1
Prepolymer temperature [ C] 90 90 90 90 85
Viscosity of prepolymer [mPas] 2030 1940 1750 1600 1200
mixture, 90 C
Isobutyl 3,5-diamino-4- [parts by 9.05 10.5 12.0 13.6 16.4
chlorobenzoate weight]
Temperature of [ C] 100 100 100 100 100
crosslinking agent
Index (theoret.) 107 107 107 107 107
Casting time [s] 225 165 105 105 60
Peeling time [min] 8 8 7 7 5
Mould temperature [ C] 110 ]10 110 110 110
Post-heating temperature [ C] 110 110 110 110 110
Post-heating time [h] 24 24 24 24 24
Mechanical properties:
Hardness [Shore A] 91 92 93 97 99
[Shore D] 37 49
Stress 10% [MPa] 3.61 4.22 5.26 6.45 9.23
Stress 100% [MPa] 6.5 6.9 7.5 8.3 10.0
Stress 300% [MPa] 9.9 10.0 11.4 12.0 14.3
Ultimate strength [MPa] 43.31 36.3 44.4 42.6 46.0
Elongation at break [%] 683 607 591 616 609
Tear propagation [kN/m] 62.8 67.3 71.6 83 99.2
resistance, Graves
Impact resilience [%] 47 47
Abrasion (DIN) [mm3] 59 57 62 52
Density [g/mm3] 1.214 1.218 1.224 1.214
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PS 22 C [%] 25.4 64 36.7
PS 70 C [%] 47.4 61 56.4
at 0 C 36.0 48.4 70.6 86.5 139
Storage modulus G' at 20 C 28.2 36.9 53.3 65.9 108
[MPa] at 50 C 24.6 31.2 43.9 53.1 84.8
at 80 C 24.3 29.4 41.0 47.4 74.9
at 110 C 25.5 29.6 40.7 45.5 70.2
at 0 C 0.1302 0.1246 0.1170 0.1045 0.0903
Loss factor, tan S at 20 C 0.0768 0.0789 0.0756 0.0734 0.0690
at 50 C 0.0484 0.0494 0.0497 0.0542 0.0543
at 80 C 0.0302 0.0318 0.0318 0.0392 0.0389
at 110 C 0.0177 0.0193 0.0193 0.0259 0.0270
tan S max. -36 -36 -36 -36 -36
tan 5 min. 130 130 130 130 130
at 0 C 4.69 6.0 8.26 9.04 12.5
at 20 C 2.16 2.9 4.03 4.84 7.46
Loss modulus G" [MPa] at 50 C 1.19 1.5 2.18 2.88 4.61
at 80 C 0.74 0.9 1.30 1.86 2.91
at 110 C 0.45 0.6 0.79 1.18 1.89
Softening point [ C] 190 195 195 210 195
Example 3: Preparation of casting elastomers not according to the invention
from
prepolymers I C, 3 C, 4 C and 6 C of Example I
The preparation was carried out as described under Example 2.
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Table 3: Formulations, preparation and properties of the casting elastomers
not
according to the invention
Casting elastomer No. F G H 1
Formulation and preparation:
Prepolymer No. 1 C 3 C 4 C 6 C
[parts by 100 100 100 100
weight]
NCO (theoret.) [wt.% of NCO] 3.36 6.1 6.1 6.1
Prepolymer temperature [ C] l00 90 90 90
Viscosity of prepolymer, [mPas] 4530 1200 2710 2720
90 C
Isobutyl 3,5-diamino-4- [parts by 9.05 16.4 16.4 16.4
chlorobenzoate weight]
Temperature of [ C] 100 100 100 100
crosslinking agent
Index (theoret.) 107 107 107 107
Casting time [s] 75 30 60 60
Peeling time [min] 9 4 3 4
Mould temperature [ C] 110 110 110 110
Post-heating temperature [ C] 110 110 110 110
Post-heating time [h] 24 24 24 24
Mechanical properties:
Hardness [Shore A] 83 99 99 99
[Shore D] 31 49 48 48
Stress 10% [MPa] 1.92 9.91 9.09 8.45
Stress 100% [MPa] 4.0 10.2 9.06 8.6
Stress 300% [MPa] 8.8 16.0 13.8 12.3
Ultimate strength [MPa] 10.3 51.5 35.1 33
Elongation at break [%] 325 538 543 589
Tear propagation [kN/m] 14.9 89.6 89.5 87.8
resistance, Graves
Impact resilience [%] 50 48 47 46
Abrasion (DIN) [mm3] 101 69 46 55
Density [g/mm3] 1.205 1.228 1.228 1.228
PS 22 C [%] 8.5 45.9 45.1 47
PS 70 C [%] 16.2 66.8 57.8 67.4
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at 0 C 16.6 177 140.4 138.8
Storage modulus G' at 20 C 14.6 137 110.5 101.8
[MPa] at 50 C 15.0 107 87.6 78.0
at 80 C 15.8 94.0 78.7 66.2
at 110 C 16.2 87.2 74.8 60.8
at 0 C 0.1295 0.0870 0.0807 0.0976
at 20 C 0.0428 0.0665 0.0605 0.0735
Loss factor, tan S at 50 C 0.0169 0.0544 0.0468 0.0616
at 80 C 0.0097 0.0417 0.0358 0.0488
at 110 C 0.0075 0.0309 0.0231 0.0352
tan S max. -33 -36 -36 -33
tan S min. 110 130 190 140
at 0 C 2.15 15 11.32 13.55
at 20 C 0.63 9.08 6.68 7.48
Loss modulus G" [MPa] at 50 C 0.25 5.81 4.10 4.81
at 80 C 0.15 3.92 2.82 3.23
at 110 C 0.12 2.69 1.73 2.14
Softening point [ C] 165 195 230 200
The advantages of the systems according to the invention are made clear by
comparing Tables 2 and 3:
At comparable prepolymer temperatures (starting temperature) and comparable
NCO
contents, i.e. comparable formulations, the casting times of the prepolymers
according
to the invention (Table 2) are up to 3 times longer than those of the systems
not
according to the invention (Table 3), which represents a clear processing
advantage.
The particularly favourable combinations of the properties of "long casting
time" and
"low prepolymer viscosity" are only achieved by the systems according to the
invention.
The casting elastomers according to the invention also exhibit advantages in
respect of
their mechanical properties:
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If, for example, the PUR prepared from prepolymer 2 (casting elastomer A,
Table 2)
is compared with a PUR prepared from prepolymer I C (casting elastomer F,
Table 3)
- both prepolymers having the same NCO value of 3.36 wt.% of NCO - the system
according to the invention has a better ultimate strength, elongation at
break, tear
propagation resistance and abrasion.
If the PUR prepared from prepolymer 5 (casting elastomer E, Table 2) is
compared
with a PUR prepared from prepolymers 3 C, 4 C and 6 C (casting elastomers G, H
and
I, Table 3) - all the prepolymers having the same NCO value of 6.1 wt.% of NCO
-
the system according to the invention has a comparably good ultimate strength,
elongation at break, tear propagation resistance, abrasion and permanent set,
within
the limits of experimental error.
The same also applies in terms of the mechanico-dynamic properties (storage
and loss
moduli and loss factor).
The systems according to the invention exhibit a unique combination of
advantageous
properties in respect of prepolymer viscosity, casting time and mechanical and
mechanico-dynamic properties.
