Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~154243
- Reactive hot melt systems containin~ isocyanate ~roups
This invention relates to reactive hot melt systems cont:~ining isocyanate groups which
have a low initial viscosity and enhanced temperature stability comprising
hydroxypolyols having ester and/or ether groupings and special diphenylmethane
diisocyanates, and relates to their use as adhesives in very dirr~lent areas of use.
Joining processes employing solvent-free adhesive systems cont~ining 100% solids are
currently becoming increasingly important, because the use of solvent-containingsystems necessitates costly solvent recovery inct~ tions and the use of aqueous
dispersion or solvent systems necessitates evaporation of the water, which is likewise
very cost-intensive. Melt adhesives, which have long been known, provide a technical
alternative. Their advantages are that they are applied as hot melts which rapidly
solidify on cooling and thereby develop strength. A disadvantage is that the adhesive
bonding of substances which are lelllpelalult-sensitive is made difficult due to the high
melting lelllpelalules, and since the applied adhesive, unless it is further processed
immetli~tçly, transforms due to its rapid solidification into a state in which wetting is
no longer taking place and which can only be thermally activated again under extreme
conditions of lelllpelatul~;. Moreover the bond has a limited hot strength on account
of the thermoplastic character of the hot melt.
One elegant manner of providing the p,ope.ly of melt adhesives, namely of the
development of strength on cooling, and at the same time enabling them to be applied
at low temperatures whilst obtaining adhesive bonds of good hot strength, is a joining
procedure described in principle in DE-OS 2 609 266 which employs reactive hot melt
systems based on isocyanate-containing prepolymers of diisocyanates and polyester
diols having melting ranges above 40C. Due to their low molecular weights, the
products are fluid and workable at temp~,latures a little above the melting range of the
polyester. After a chain-lengthening reaction which proceeds on the substrate they
attain an adequate molecular weight, which together with the recryst~llic~tion of the
crystalline polyester chain segments results in an increased initial strength. The final
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strength is reached by the complete reaction of the free isocyanate groups which are
still present with atmospheric moisture, for example, with the formation of linear, high
molecular weight polyurethane polyureas.
The problem of achieving a balance between the property of the highest possible initial
strength and the lowest possible tel,lpelalulc of application or melting of reactive hot
melt systems has not yet been solved completely satisfactorily. Systems con~ ng
isocyanate groups become increasingly unstable at the elevated tempelalur~;s required
for their viscosity on application, even if atmospheric moisture is excluded. This is
discernible by a sharp increase in viscosity, and in extreme cases by gelling of the
melts. This results in caking in melting vessels and metering devices, which can only
be cleaned again at high cost.
It would therefore be desirable if hot melt systems col)~ ing isocyanate groups were
available which had an enh~ncecl thermal stability, which could be held in the molten
state for a long period at high tempelalul~,s without ill-,~t,~ibly transforming into the
gel state or exhibiting a sharp increase in viscosity.
Surprisingly, it has now been found that when certain polyisocyanates are used hot
melt systems can be obtained which exhibit a significantly enhanced stability of their
viscosity without their other plol)c.lies being impaired.
Accordingly, the present invention relates to reactive hot melt systems cont~ining
isocyanate groups having a low initial viscosity and enhanced telllperalule stability,
comprising
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- 3 -
(i) hydroxypolyols cont~ining ester and/or ether groupings, with a hydroxyl number
of 15 to 150 and an average functionality of 1.95 to 2.2, and
(ii) diphenylmethane diisocyanates
s
with a ratio of isocyanate groups in (ii) to hydroxyl groups in (i) of 1.4:1 to 2.5:1,
which are characterized in that diphenylmethane diisocyanates are used which have a
content of at least 70 weight % of 2,4'-isomers.
The polyols (i) which are used comprise the hydroxyl coll.pounds which are commonly
used in polyurethane chemistry, for example such as polyether polyols which are
preferably based on bifunctional starters, e.g. propylene glycol or 2,2-bis-4-
hydroxyphenylpropane, and propylene oxide which preferably has a hydroxyl numberof 20 to 150. Proportions (5 to 20 weight %, based on (i)) of polyols of higher
functionality may also be used conjointly, such as propylene oxide polyether polyols
started with trimethylolpropane which have a hydroxyl number from 30 to 500, forexample. When polyols of higher functionality are used conjointly, the hydroxyl
number of hydroxypolyols (i) may increase to a value of 180, preferably 160.
The use of crystalline or amorphous hydroxylpolyesters as polyols (i) is plef~ ,d.
Crystalline polyester polyols are be understood as those which exhibit an endothermic
m~ximnm above room te",pe,alu,e on differential thermal analysis (DTA). Amorphous
polyester polyols are to be understood as those which exhibit no endothermic
maximum above room telllpelalulG on dirre.~,.-lial thermal analysis.
Esterification products of adipic acid or dodecane diacid, of ortho-. iso- or terephthalic
acid, of carbonic acid or of dimeric fatty acid (polycarboxylic acids obtained by the
dimerization of unsaturated fatty acids (e.g. oleic acid)) with glycols, such as ethylene
glycol, 1,4-butanediol or 1,6-hexanediol, for example, are preferably used as the
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polyester polyols having hydroxyl numbers from 15 to 100, preferably 20 to 75.
Mixtures of acids andtor glycols may also of course be used. The plol)ollional use in
conjunction of trimethylolpropane, for example, is also possible. In addition to the
above-mentioned dicarboxylic acids, the use of hydroxycaproic acid or caprolactone
S is also possible.
The prepa~alion of polyether- and polyester polyols (i) is known and is described, for
example, in Ullm~nn~ Encyklopadie der technischen Chemie [Ullmann's Encyclopediaof Tnrlustri~l Chemistry], "Polyesters", Fourth Edition, Verlag Chemie, Weinheim,
1980.
Di-, tri- andlor tetraethylene glycol, 1,4-dimethylolcyclohexane or trimethylolpropane
or reaction products of 4,4'-hydroxyphenylpropane with ethylene- and/or propylene
oxide should be cited as low molecular weight hydroxyl compounds which are
optionally to be used conjointly with coll.ponent (i). Diols cont~ining ions or
components which form ionic groups, such as dimethylolpropionic acid, N-methyl
diethanolamine andlor reaction products of sodium bisulphite with propoxylated 1,4-
butenediol, for example, may also of course be used for special effects. The amount
of low molecular weight hydroxyl compounds is preferably 0.01 to 0.5 moles per mole
of component (i).
Polyester polyols and polyether polyols (i) preferably have an average functionality of
1.95 to 2.05.
According to the invention, diphenylmethane diisocyanates (ii) are used which have
a content of at least 70 weight %, preferably of at least 85 weight ~o, of 2,4'-isomers.
Diphenylmethane diisocyanates (ii) such as these may be prepared by various methods,
such as the ~ till~tion of an industrial diphenylmethane diisocyanate mixture, for
example.
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The ratio of hydroxyl groups in polyol components (i) to isocyanate groups in
diisocyanates (ii) is preferably 1:1.5 to 1:2Ø
The hot melt systems cont~ining isocyanate groups are pl~arcd, for example, by
mixing the liquid polyols with an excess of polyisocyanates, whereupon the
homogeneous mixture is drawn off or is stirred until a constant NCO value is reached,
which is mostly achieved after two hours, and is then drawn off. A temperature of 60
to 150C, preferably 65 to 110C, is selected as the reaction ~ pe,alule. The
prep~alion of the reactive hot melt may also of course be carried out continuously in
a c~ de of stirred vessels or in suitable mixing units, such as high-speed mixers
based on the rotor-stator principle, for example.
It is of course possible to modify the polyester- and/or polyether polyols or part of the
same with a deficit of diisocyanates, preferably hexamethylene diisocyanate, and to
react the polyols cont~ining urethane groups, after this reaction is complete, with an
excess of diisocyanates to form a hot melt co~ ling isocyanate groups.
lt is also possible to conduct the reaction of the polyols with the diisocyanates in the
presence of up to 5 weight % of trimers of aliphatic diisocyanates, for example, such
as hexamethylene diisocyanate for example, or to add trimers such as these after the
completion of prepolymerization.
The hot melt systems have an almost unlimited storage life when they are stored at
room temperatures up to 40C with the exclusion of moisture. They can be modified
in the usual manner with inorganic or organic fillers, colorants, resins and/or extender
oils and constitute excellent adhesives.
They are applied at an elevated temperature, wherein the hot melt systems are melted
continuously or batch-wise at temperatures of 80 to 160C and the melts are brought
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into contact with the substrates to be adhesively bonded. Bonding is firstly effected
physically due to the increase in viscosity or by recryst~lli7~tion of the polyester
segm~-nt~, and is effected later by chemical reaction of the isocyanate groups with
moisture or other Zerewitinoff-active groups, e.g. with glycols.
s
The substrate and atmospheric moisture are normally sufficient for bonding, but the
reaction may of course be accelerated by providing an additional misting with media
cont~ining water, glycols or catalysts, such as water vapour containing triethylamine.
Co",paled with systems based on conventional diphenylmethane diisocyanates of
co~ ble molar compositions, hot melt systems of the type claimed are distinguished
by a lower viscosity and, particularly when crystalline polyester polyols are used, by
their more rapid cryst~lli7~tion and thus by their more rapid initial solidification or
shorter "open time", as described in DE 3 931 845 for example. In particular,
however, the thPrm~l stability of the systems is significantly improved and thusconstitutes a technical advance which makes the class comprising these hot melt
systems even more attractive. Moreover it is now possible to produce hot melts
cont~inin~ isocyanate groups based on very viscous polyester polyols, which can be
used, despite the requisite high proces~ing te"lp~,~lules, without the very
disadvantageous sharp increase in viscosity which occurred hitherto during proces.~ing
In particular, considerably improved reactive hot melt adhesives are thus accessible for
the adhesive bonding of valve bags. They solve the problem which existed hitherto,
which consists of the combination of properties which are actually m~ lly exclusive
which is required for trouble-free production, namely a very high viscosity or abrasion-
resistance of the applied adhesive film for compounds which provide very good
stability of viscosity of the adhesive which is present at very high lelllpel~lul~s in the
melt application vessel and in the applicator unit.
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In the same manner, the production of technically improved reactive hot melt
adhesives for composite film manufacture is also possible.
The hot melt systems according to the invention can be used as adhesives for a
S diversity of applications, for example as a mounting adhesive for the preliminary fixing
of components, as a bookbinding adhesive, or as adhesives for the manufacture ofvalve bags, composite films or l~rnin~te.
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Examples
Polyester polyols
A-l A crystalline hydroxyl polyester of adipic acid and 1,6-hexanediol: hydroxylS number 28.0, acid number 0.6, average functionality 2.0
A-2 An amorphous hydroxyl polyester of neopentyl glycol and a mixture of adipic
acid and isophthalic acid in a 6:4 molar ratio:
hydroxyl number 56.2, acid number 0.7, average functionality 2.00
A-3 An amorphous hydroxyl polyester, Dynacoll 7210 manufactured by Huls AG,
with a hydroxyl number of 33 and an average functionality of 2.0
A-4 An amorphous hydroxyl polyester, Dynacoll 7110 manufactured by Huls AG,
with a hydroxyl number of 52 and an average functionality of 2.0
Diisocyanates
C-l A diisocyanatodiphenylmethane mixture conlainillg 87.1 % 2,4'-, 11.2 %
4,4'- and 1.7 % 2,2'-isomers
C-lA A diisocy~n~todiphenylmethane ~ lule containing 77.0 % 2,4'-, 22 % 4,4'- and 1 % 2,2'-isomers
C-2 4,4'-diisocyanatodiphenylmethane (100 %).
Preparation of prepolymers
The molten polyols were dehydrated for 60 minutes at 100C and at about 20 mbar
with stirring.
The diisocyanate was added at about 80 to 90C and stirred under nitrogen until a
constant isocyanate content was obtained.
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The prepolymer was introduced into cartridges.
Investi~tinn of the hot melt systems
After storage for two weeks at room telllpel~lule the cartridges were melted at 130 to
140C for 60 minutes in an oven.
The isocyanate content of the liquid prepolymer was determined by titration withdibutylamine.
The liquid hot melt was applied to beechwood test pieces and its curing characteristics
were determined using a "CUREM" (see DE 3 931 845), divided into a wetting phaseA (seconds) and a cryst~lli7~tion phase B (seconds). For this purpose the adhesive in
the joint gap of the m~teri~l~ to be adhesively bonded was subjected to sinusoidally
lS alternating shear loading, and the shear forces thus arising were continuously
measured. The measured shear forces were displayed as a function of time (in
seconds). The shear was produced via a frequency-controlled thrust motor and a fine
threaded spindle. The displacement was 20011m, and the loading frequency was 1 Hz.
The joint gap measured 0.2 mm and the telllpeldluie of the test piece clamping jaws
was set at 20+2C. McasulGlllents were recorded for 10 minutes.
The cohesion strength was determined on the beechwood test pieces (after 24 hours
and after 7 days) [see Table 2].
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- 10 -
Testin~ the stora~e stability (Table 3)
The hot melt systems were melted at 120C in aluminium cartridges and stored for 24
hours or 72 hours. Viscosities were determined at 120C at a shear rate of D = 50 Vs
using an MC-10 cone/plate viscometer (manufactured by Physika).
Table 1
(moles) Pol-A moles TMP* (moles) diiso. C NCO/OH % NCO
1 1.0 A-1 / 1.9 C-1 1.9 1.69
1 A 1.0 A-1 / 1.9 C-2 1.9 1.59
2 1.0 A-1 0.25 2.61 C-1 1.9 1.99
2 A 1.0 A-1 0.25 2.61 C-2 1.9 2.00
3 1.0 A-1 0.1 2.12 C-1 1.85 1.53
3 A 1.0 A-1 0.1 2.12 C-2 1.85 1.51
4 1.0 A-2 / 2.0 C-1 2.0 3.59
4 A 1.0 A-2 / 2.0 C-2 2.0 3.54
1.0 A-1 0.1 2.19 C-1 1.9 1.74
5 A 1.0 A-1 0.1 2.19 C-2 1.9 1.69
6 1.0 A-1 0.25 2.47 C-1 1.8 1.72
6 A 1.0 A-1 0.25 2.47 C-2 1.8 1.62
7 1.0 A-3 / 2.0 C-l A 2.0 2.06
7 A 1.0 A-3 / 2.0 C-2 2.0 2.19
8 1.0 A-4 / 2.0 C-1 A 2.0 3.24
8 A 1.0 A-4 / 2.0 C-2 2.0 3.29
*TMP = trimethylolpropane
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Table 2
Wetting Crysr~ion Strength in N/mm'
(sec.) (sec.)after 24 hours 7 days
5.0 12.6
lA 65 120 8.1 15.0
2 60 85 6.0 14.6
- 2 A 100 150 8.3 15.1
3 45 70
3 A 70 115
1 1 5.0 13.0
5 A / / 8.9 14.8
7 1 1 0.2 3.4
7 A I 1 1.8 3.8
8 1 1 2.0 3.1
8 A I 1 1.8 8.2
The tests denoted by A were each comparative tests. The more rapid initial
20 solidificationofthehotmeltsystems based on crystalline polyesters (1-3)
according to the invention can clearly be seen.
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Table 3
Adhesive Viscosity (Pas) at 120C after s~orage for
0 hours24 hours 72 hours
1 9.0
I A 22.0
2 S.0 - 16.0 28.0
2 A 15.0 145.0 596.0
3 13.0
3 A 35.0
4 2.8 6.1 13.4
4 A 4.4 13.3 67.9
6.0 12.0 20.0
5 A 19.0 80.0 242.0
6 6.8 17.5 72.0
6 A 24.0 240.0 crocclinl~Pd
7 5.0 7.6 12.0
7 A 7.8 18.0 58.0
8 6.7 17.0 64.0
~0 8 A 8.5 16.0 crosslin~d
The lower initial viscosity and the reduced increase in viscosity at elevated temperature
after the period of storage can clearly be seen for the hot melt systems according to the
25 invention.
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