A process for bonding substrates with an ester group-containing hot melt adhesive

Disclosed herein is a process for bonding substrates comprising applying a hot-melt adhesive to the surface of at least one of the substrates and contacting the surface of said substrate with the surface of the other substrate, said adhesive containing an ester group-containing polyisocyanate which is a solid below 30.degree. C. and liquid above 100.degree. C., corresponding to formula (I) EQU A(OCO-B-NCO).sub.n (I) in which PA0 n stands for a whole number or a fraction (statistical average) of from 2 to 6, PA0 A stands for an n-valent organic group such as is obtained by removal of hydroxyl groups from an n-valent polyhydroxyl compound in the molecular weight (Mn) range of from 800 to 6000 containing ester and/or carbonate groups and PA0 B stands for an aliphatic, cycloaliphatic or aromatic hydrocarbon group having up to 18 carbon atoms.

BACKGROUND OF THE INVENTION 
This invention relates to new hot melt adhesives having a high setting rate 
and a low melt viscosity. 
Reactive, moisture hardening hot melt adhesives from the class of 
isocyanate functional prepolymers are known, for example from DE-OS 2 4OI 
320, EP A 0 107 097 or EP-A-O 125 009. Adhesives of the type mentioned in 
these prior publications are applied to substrates as hot melts and 
solidify on cooling and thus rapidly build up strength so that further 
processing of the substrates can be carried out within a short time. The 
final strength is obtained by the complete reaction of the free isocyanate 
groups still present, for example with atmospheric moisture, to form high 
molecular weight polyurethane polyureas. It is particularly advantageous 
to apply reactive hot melt adhesives without solvents. The disadvantage of 
the adhesives described in the aforesaid publications lies particularly in 
their high melt viscosity. For optimum wetting of a surface with the 
adhesives it is desirable to obtain as low a melt viscosity as possible. 
The adhesives according to U.S. Pat. No. 5,019,638 also require a 
relatively high temperature to reduce the viscosity of the adhesive to a 
sufficiently low value for wetting surfaces. The adhesives are still found 
to have viscosities of from 7000 to 20,000 mPa.s at 130.degree. C. and 
therefore cannot be used for bonding temperature sensitive substrates. 
In view of the very short operating cycles nowadays customary in industry, 
it is necessary for the hot melt adhesives to set very rapidly by 
crystallization after their application so that the bonded substrates are 
immediately ready for further processing. 
German Patent Application P 39 13 406.7 in the name of the present 
Applicants deals with prepolymers of the type which are preferred 
according to the invention, but it only deals with their use for the 
preparation of moisture hardening coating compounds or sealing compounds. 
The use of the prepolymers as hot melt adhesives or for the preparation of 
hot melt adhesives is not described in the Application which is, 
therefore, not a prior publication. 
DE-OS 1 668 069 describes ester group-containing polyisocyanates which are 
said to be suitable as adhesives. However, since the polyisocyanates of 
this prior publication, as can be seen from the examples of embodiments, 
are based on a completely different type of low molecular weight 
polyhydroxyl compounds, they differ fundamentally in their melt 
characteristics from the polyisocyanates used according to the invention. 
This conclusion is justified in that the authors of the said prior 
publication have not taken into account hot melt adhesives of the type to 
be used according to the present invention. 
It was, therefore, an object of the present invention to provide new 
reactive hot melt adhesives which would combine the advantages of a high 
setting speed with a low melt viscosity. 
This problem has been solved by the use, according to the invention, of 
certain polyisocyanates containing ester groups described below. 
SUMMARY OF THE INVENTION 
The invention relates to a process for bonding substrates comprising 
applying a hot-melt adhesive to the surface of at least one of the 
substrates and contacting the surface of said substrate with the surface 
of the other substrate, said adhesive containing an ester group-containing 
polyisocyanate which is a solid below 30.degree. C. and liquid above 
100.degree. C., corresponding to formula (1) 
EQU A(OCO-B-NCO).sub.n (I) 
in which 
n stands for a whole number or a fraction (statistical average) of from 2 
to 6, 
A stands for an n-valent organic group such as is obtained by removal of 
hydroxyl groups from an n-valent polyhydroxyl compound in the molecular 
weight (Mn) range of from 800 to 6000 containing ester and/or carbonate 
groups and 
B stands for an aliphatic, cycloaliphatic or aromatic hydrocarbon group 
having up to 18 carbon atoms. 
Isocyanate prepolymers which are comparable in their constitution to the 
isocyanate prepolymers used according to the invention are already 
described in DE-OS 2 120 090. However, according to the teaching of the 
said publication, the isocyanate prepolymers are used for finishing 
textiles containing certain fibers. For this purpose, the isocyanate 
prepolymers are used in the form of highly diluted organic solutions or 
aqueous emulsions. The publication, therefore, gives no indication that 
crystalline isocyanate prepolymers are particularly suitable as hot melt 
adhesives. In particular, it makes no mention of the prepolymers which are 
particularly suitable according to the invention and which are prepared by 
the special process described below.

DESCRIPTION OF THE INVENTION 
The isocyanate prepolymers corresponding to the general formula (I) to be 
used according to the invention are prepared by suitable modification of 
organic polyhydroxyl compounds corresponding to the general formula (V) 
EQU A (OH).sub.n (V) 
This modification may comprise, for example, a reaction of the polyhydric 
alcohols with isocyanato-carboxylic acid chlorides corresponding to the 
general formula (III) 
EQU ClOC--B--NCO (III) 
according to the teaching of DE-OS 2 120 090. 
In these formulae, A, B and n have the meanings already indicated. 
The polyhydroxyl compounds of formula (V) are often mixtures due to the 
method employed for their preparation. Hence, n may have a statistical 
average value of a fraction within the range given above. The same 
applies, of course, to the polyisocyanates of formula (I) used according 
to the invention, which are based on such polyol mixtures. 
The polyhydroxyl compounds of formula (V) used for the preparation of the 
isocyanate prepolymers to be used according to the invention are 
preferably partially crystalline compounds which are solid at temperatures 
below 30.degree. C., preferably below 40.degree. C., and liquid above 
100.degree. C., preferably above 80.degree. C. The isocyanate prepolymers 
of formula (I) based on such polyhydroxyl compounds are, to a large 
extent, similar in their melting properties to the polyhydroxyl compounds 
of formula (V) on which they are based, especially if they have been 
prepared by the preferred method described below. The ester 
group-containing polyisocyanates of formula (I) to be used according to 
the invention are, therefore, also polyisocyanates or polyisocyanate 
mixtures which are solid at temperatures below 30.degree. C., preferably 
below 40.degree. C., and liquid above 100.degree. C., preferably above 
80.degree. C. 
The starting materials of formula (III) or (V) which are preferred for the 
invention and the ester group-containing polyisocyanates of formula (I) 
which are preferred used for the invention are compounds in which n stands 
for a whole number or fraction of from 2 to 3, in particular 2, A stands 
for an n-valent group such as may be obtained by removal of hydroxyl 
groups from an n-valent polyhydroxyl compound in the molecular weight 
range of from 1000 to 5000 containing ester and/or carbonate groups, and B 
stands for an aliphatic hydrocarbon group having 2 to 10, in particular 5 
to 10 carbon atoms. All the information given in connection with the 
molecular weight of the relatively high molecular weight alcohols of 
formula (III) are based on the molecular weight determined by vapor 
pressure osmometry. The average functionality is calculated from the 
hydroxyl group content and from the molecular weight thus determined. 
Ester group-containing polyisocyanates of formula (I) which are 
particularly advantageous to use according to the invention are those 
whose preparation from the starting materials corresponding to formulae 
(III) and (V) have been carried out by a process analogous to that 
described in DE-OS 3 634 248. 
In the said process, the polyhydroxyl compounds are converted in a first 
reaction stage into the corresponding O-silylated compounds corresponding 
to formula (II) 
EQU A[OSiR.sub.3 ].sub.n (II) 
by a method analogous to that described by M. Lalonde and C. H. Chan in 
"synthesis" 1985, pages 817 to 845. For this process, the polyhydroxyl 
compounds are reacted, for example, with chlorosilanes or disilazanes 
corresponding to the general formula (IV) or (VI) 
##STR1## 
optionally with the aid of an auxiliary solvent. The reaction with a 
chlorosilane is generally carried out at a molar ratio of hydroxyl groups 
to chlorosilane of from 1:1 to 1:2 at temperatures from 0.degree. to 
80.degree. C. The reaction, generally, requires the addition of an at 
least equivalent quantity of an organic base, e.g. pyridine or 
triethylamine, for binding the hydrogen chloride formed. The reaction with 
disilazanes is preferably carried out at a molar ratio of hydroxyl groups 
to disilazane of from 1:0.5 to 1:1 at temperatures from 60.degree. C. to 
140.degree. C. until the liberation of ammonia has been completed. It is 
frequently advisable to add a small quantity of an acid catalyst, for 
example a chlorosilane of the type mentioned above. 
Examples of suitable solvents optionally used for this reaction include 
n-hexane, cyclohexane, toluene, xylene, methoxypropyl acetate, ethyl 
acetate, butyl acetate, methyl ethyl ketone and mixtures of such solvents. 
Suitable chlorosilanes and disilazanes for the reaction are in particular 
those corresponding to the above mentioned formulae in which R stands for 
an alkyl group having 1 to 4 carbon atoms or a phenyl group, preferably a 
methyl group. 
The O-silylated polyhydroxyl compounds corresponding to formula (II) are 
subsequently reacted with isocyanato carboxylic acid chlorides 
corresponding to the above mentioned general formula (III). The quantity 
of reactants is, generally, chosen to provide from 0.8 to 1.2 mol of 
silylated hydroxyl groups for each mol of chloro-carbonyl group. 
The process is, generally, carried out with equimolar quantities of 
starting materials. The reaction of silylated hydroxyl compounds with 
isocyanato acid chlorides is generally carried out at temperatures from 
50.degree. to 150.degree. C., optionally with the addition of known 
catalysts for this reaction, such as quinoline or pyridine. 
A trialkyl or triaryl chlorosilane is formed as by-product of the reaction 
and can easily be removed from the reaction mixture by distillation. 
This reaction may also be carried out in the presence or absence of a 
solvent exemplified above. Solvents may generally be omitted if the 
silylated hydroxyl compounds used have a sufficiently low viscosity for 
the process at the reaction temperature. 
In accordance with the requirement of preparing solvent-free hot melt 
adhesives, the ester group-containing polyisocyanates of formula (I) are 
preferably prepared without the use of inert solvents of the type 
exemplified. If necessary, volatile substances, including any solvents 
used, may be removed from the ester group-containing polyisocyantes by 
distillation, e.g. by thin layer distillation. 
Both in the preferred method of preparing the ester group-containing 
polyester isocyanates of formula (I) and in the direct reaction of the 
polyhydroxyl compounds with the chlorocarbonyl isocyanates (III) according 
to DE-OS 2 120 090, small quantities of dicarboxylic acid dichlorides may 
be used for obtaining particular properties. Examples of such dicarboxylic 
acid dichlorides include phthalic acid dichloride, isophthalic acid 
dichloride and terephthalic acid dichloride. In these cases also, the 
quantity of reactants is chosen to provide from 0.8 to 1.2 moles, 
preferably 1 mole of optionally silylated hydroxyl groups for each mole of 
chlorocarbonyl group. 
The polyhydroxyl compounds corresponding to formula (V) are ester 
group-containing and/or carbonate group-containing polyhydroxyl compounds 
in the molecular weight range mentioned above and with the melt 
characteristics mentioned above, such as are known in principle from 
polyurethane chemistry. Partially crystalline polyester polyols carrying 
terminal hydroxyl groups such as are obtained in known manner by the 
reaction of polybasic carboxylic acids with excess quantities of 
polyhydric alcohols are particularly suitable. 
The polybasic carboxylic acids used may in particular be aliphatic 
dicarboxylic acids having 6 to 14 carbon atoms, such as adipic acid, 
sebacic acid, azelaic acid, suberic acid, dodecane dicarboxylic acid or 
tetradecane dicarboxylic acid, and the acid component may also contain 
small quantities of cycloaliphatic or aromatic dicarboxylic acids or 
dicarboxylic acid anhydrides. 
The polyhydric alcohols used for the preparation of the polyester polyols 
are preferably diols having 2 to 12 carbon atoms, e.g. ethylene glycol, 
propylene glycol, 1,4-dihydroxybutane, 1,6-dihydroxyhexane or 
1,12-dihydroxy-dodecane, but the alcohol component may also contain 
polyhydric alcohols such as trimethylolpropane or glycerol so that the, 
compounds finally obtained are branched polyester polyols of formula (V) 
in which n stands for a whole number or fraction above 2 within the limits 
mentioned above. It is particularly preferred, however, to prepare the 
polyester polyols exclusively from difunctional starting materials so that 
the polyester polyols obtained consist essentially of polyester diols. 
Polyesters of lactones such as .epsilon.-caprolactone or of 
hydroxycarboxylic acids such as .OMEGA.-hydroxycarboxylic acid may also be 
used. 
The hydroxyl group-containing polycarbonates used may be of known type, 
e.g. those prepared by the reaction of diols such as propane-1,3-diol, 
butane-1,4-diol and/or hexane-1,6-diol, diethylene glycol, triethylene 
glycol, tetraethylene glycol or thiodiglycol with diaryl carbonates, e.g. 
diphenyl carbonate, or phosgene (DE Auslegeschriften Nos. 1 694 080, 1 915 
908 and 2 221 751; DE-Offenlegungsschrift 2 605 024). 
Examples of suitable isocyanato carboxylic acid chlorides of formula (III) 
for the preparation of the isocyanate prepolymers include 
3-isocyanatopropionic acid chloride, 4-isocyanatobutyric acid chloride, 
6-isocyanatocaproic acid chloride, 11-isocyanatoundecanoic acid chloride, 
4-isocyanatobenzoic acid chloride and 4-isocyanatocyclohexane carboxylic 
acid chloride. 6-isocyanatocaproic acid chloride is particularly 
preferred. 
The ester group-containing polyisocyanates of formula (I) prepared by the 
described process from the starting materials exemplified above, and used 
according to the invention may be used as hot melt adhesives without 
further additives or they may be used as hot melt adhesives in a modified 
form. 
The hot melt adhesives are applied solvent-free. They may be adjusted to 
the required viscosity for coating (2000 to 10,000 mPa.s) by heating them 
to a relatively low temperature, e.g. to 80.degree.-120.degree. C. At this 
temperature, the hot melt adhesives according to the invention have a 
surprisingly low viscosity and therefore wet the substrate very 
efficiently. 
The property of the hot melt adhesives according to the invention of 
setting immediately by crystallization on cooling after their application 
results in a high initial bond strength similar to that of conventional 
hot melt adhesives since the adhesive spontaneously manifests a high 
cohesive force on cooling to a temperature below the melting point. Since 
the adhesives can be employed at comparatively very low temperatures due 
to their low melt viscosity, the substrate can be bonded together 
immediately after they have been coated. Owing to only the slight 
difference between the temperature of application and the crystallization 
temperature, the hot melt adhesives according to the invention provide the 
possibility of an extremely rational operating procedure, for example for 
the series production of bonded articles. 
The hot melt adhesives according to the invention may be applied by methods 
already known for conventional hot melt adhesives. For example, the 
substrates which are to be bonded may be coated with the adhesives by 
roller application, by casting or, by application with extruders or by 
spraying. 
The hot melt adhesives according to the invention have a virtually 
unlimited shelf life if they are stored with the exclusion of moisture at 
temperatures from room temperature to 50.degree. C. They may be modified 
with fillers such as chalk, heavy spar or polymer powder such as PVC or 
CBS powder, dyes, resins and/or extender oils. 
For building up the final strength within a sufficiently short time when 
using the preferred ester group-containing polyisocyanates (I) having 
aliphatically bound isocyanate groups, it is advisable to add reaction 
accelerators. These are catalysts known from polyurethane chemistry, e.g. 
tertiary amines such as triethylamine, pyridine, methyl pyridine, benzyl 
dimethylamine, N,N'-dimethylaminocyclohexane, N-methylpiperidine, 
pentamethy-diethylenetriamine, N,N'-dimethylpiperazine, etc. and metal 
salts such as iron(III) chloride, zinc chloride, zinc-2-ethyl caproate, 
tin(II)-2-ethylcaproate, dibutyl tin(II) dilaurate or molybdenum 
glycolate. 
The moisture of the substrate and of the atmosphere is normally sufficient 
for final curing but the reaction may, of course, be accelerated by 
spraying with water or with media containing glycols and/or catalysts. 
The hot melt adhesives according to the invention are suitable for bonding 
a wide variety of substrates such as metals, wood, products containing 
wood, paper, glass, ceramics, leather or plastics in a solid form or as 
foams, such as PVC, PUR, ABS, polyethylene or polypropylene, and each of 
these materials may be bonded to an identical material or to the other 
materials mentioned. The adhesives may be used for a wide variety of 
applications, for example as an assembly adhesive for temporarily fixing 
structural parts, as adhesive for book-binding, in which an early build-up 
of strength is obtained by the rapid crystallization so that short cycle 
times can be obtained in conventional bookbinding machines; for the 
rational production of bonded flat materials which are immediately 
subjected to high tensions after they have been bonded and therefore 
require an adhesive with a high initial strength which builds up rapidly, 
as for example for the bonding of shoe soles; for the manufacture of 
furniture; for the production of hat racks; for the production of internal 
parts of motor vehicles. 
The percentages given in the following Examples are all percentages by 
weight. 
EXAMPLES 
General method of preparation for the silylation of the polyhydroxyl 
compounds 
1 OH Equivalent of a polyhydroxyl compound and 0.7 mol of 
hexamethylsilazane are stirred together with the addition of 2 ml of 
trimethylchlorosilane at 80.degree. to 120.degree. C. until no more 
ammonia is split off. The reaction may be followed IR spectroscopically 
from the reduction in the OH band. As soon as no OH band is visible any 
more, excess disilazane is removed by distillation at reduced pressure. 
EXAMPLE 1 
Prepolymer based on a silylated polyester: 
2073 g of a bis-trimethylsiloxy polyester (base polyester of adipic acid 
and hexanediol, OH number 28, melting point 55.degree. to 58.degree. C.) 
and 175.5 g of 6-isocyanato caproic acid chloride are stirred together at 
80.degree. to 100.degree. C. with the addition of 1 ml of pyridine until 
no more acid chloride bands are seen in the IR spectrum. 
Trimethylchlorosilane formed during the reaction is continuously distilled 
off. 
0.05% by weight of dibutyl tin dilaurate are added after removal of 
residues of volatile constituents by thin layer distillation at 
140.degree. C./0.05 mbar. The isocyanate prepolymer obtained has the 
following characteristic data: 
NCO content: 1.75% 
Melting point: 55.degree. to 59.degree. C. 
Viscosity: 600 mPa.s/120.degree. C. 
EXAMPLE 2 
Prepolymer based on a silylated polyester: 
1573 g of a bis-trimethylsiloxypolyester (base polyester of adipic acid and 
hexane diol, OH number 37.3, melting point 50.degree. to 53.degree. C.) 
and 175.5 g of 6-isocyanato-caproic acid chloride are reacted as in 
Example 1 with the addition of 1 ml of pyridine. Trimethylchlorosilane is 
distilled off during the reaction with constant reduction in pressure. 
The reaction mixture is stirred for 4 hours at 90.degree. C./1 mbar after 
the IR spectrum ceases to show an acid chloride band and 0.05% by weight 
of dibutyl tin dilaurate are added thereafter. The isocyanate prepolymer 
obtained has the following characteristic data: 
NCO content: 2.49% 
Melting point: 52.degree. to 55.degree. C. 
Viscosity: 5400 mPa.s/120.degree. C. 
EXAMPLE 3 
Prepolymer based on a silylated polyester: 
1573 g of the bis-trimethylsiloxypolyester from Example 2 are reacted as in 
Example 2 with 30 g of terephthalic acid dichloride and 123 g of 
6-isocyanato-caproic acid chloride with the addition of 1 ml of pyridine. 
After the addition of 0.05% by weight of dibutyl tin dilaurate, the 
isocyanate prepolymer has the following characteristic data: 
NCO content: 1.78% 
Melting point: 53.degree. to 55.degree. C. 
Viscosity: 5800 mPa.s/120.degree. C. 
EXAMPLE 4 
Prepolymer based on a silylated polycarbonate: 
1073 g of a bis-trimethylsiloxy polycarbonate (based on a polycarbonate of 
hexanediol and diphenyl carbonate, OH number 56, melting point 49.degree. 
to 52.degree. C.) and 175.5 g of 6-isocyanatocaproic acid chloride are 
reacted as in Example 1 with the addition of 1 ml of pyridine. 0.05% by 
weight of dibutyl tin dilaurate are added after the removal of residues of 
volatile constituents by thin layer distillation at 140.degree. C./0.05 
mbar. The isocyanate prepolymer obtained has the following characteristic 
data: 
NCO content: 3.35% 
Melting point: 45.degree. to 49.degree. C. 
Viscosity: 1400 mPa.s/120.degree. C. 
Use According To The Invention 
The prepolymers heated to 120.degree. C. are applied to beechwood samples 
measuring 120.times.25.times.4 mm by means of a hot melt spray gun with 
heatable cartridge (Beyer & Otto, Kleinostheim). 
Immediately after application of the adhesive, the sample is bonded to a 
second, similar sample of beechwood with the two samples overlapping over 
an area of 25.times.25 mm. The joint is adjusted to a thickness of about 
0.2 mm by means of a spacer. 
The test samples thus bonded together are tested for their combined tension 
and shear resistance after 5 minutes, and after 7 days storage at 
23.degree. C. and 50% relative humidity in accordance with DIN 53 283. The 
results given are average values obtained from five measurements. 
The setting time is the time within which the freshly bonded beechwood 
samples can still be displaced in relation to one another by firm finger 
pressure. 
______________________________________ 
Tension and Tension and 
Setting shear resistance 
shear resistance 
Example time after 5 min. 
after 7 days 
______________________________________ 
1 2 sec 3.8 N/mm.sup.2 
8.9 N/mm.sup.2 
2 4 sec 3.7 N/mm.sup.2 
5.8 N/mm.sup.2 
3 6 sec 0.9 N/mm.sup.2 
6.0 N/mm.sup.2 
4 7 sec 2.9 N/mm.sup.2 
8.2 N/mm.sup.2 
______________________________________