CA1324790C

CA1324790C - Process for the preparation of polyisocyanates containing urethane groups and the products obtained

Info

Publication number: CA1324790C
Application number: CA000582791A
Authority: CA
Inventors: Lothar Kahl; Josef Pedain; Wolfgang Wellner
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1987-11-18
Filing date: 1988-11-10
Publication date: 1993-11-30
Anticipated expiration: 2010-11-30
Also published as: EP0316738B1; DE3739261A1; ES2032519T3; EP0316738A2; JPH01167323A; US4910332A; EP0316738A3; DE3870161D1

Abstract

PROCESS FOR THE PREPARATION OF POLYISOCYANATES
CONTAINING URETHANE GROUPS AND THE PRODUCTS OBTAINED
ABSTRACT OF THE DISCLOSURE
The present invention is directed to a process for the preparation of polyisocyanates containing urethane groups in which the content in monomeric start-ing diisocyanate is at most 0.4% by weight by reacting in a urethane-forming reaction a) an excess quantity, based on component b), of a diisocyanate component containing at least one aromatic diisocyanate which is free from urethane groups with b) a polyol component containing at least one poly-hydric alcohol in the molecular weight range of 62 to about 250 followed by distillative removal of the unreacted excess starting diisocyanate a), characterized in that before the distillative removal of the excess starting diisocyanate a) is carried out, c) an aliphatic polyisocyanate containing isocyanurate groups is incorporated in the urethane-containing reaction product of components a) and b) in a quantity of about 0.3 to 10% by weight, based on the undistilled reaction product of components a) and b), and the distillation residue is then optionally dissolved in an inert solvent and up to about 0.5 equivalents of an alcohol, per kg of solvent-free distillation residue on a solvent free basis is added to the resulting solution so that the amount of free starting diisocyanate a) is further reduced and urethane groups are formed.
The present invention is also directed to the polyisocyanates containing urethane groups obtained by this process.

Description

" 1324790 Mo-3110 LeA 25,630 PROCESS FOR THE PREPARATION OF POLYISOCYANATES
CONTAINING URETHANE GROUPS AND THE PRODUCTS OBTAINED
BACKGROUND OF THE INVENTION
.
Field of the Invention This invention relates to a new process for the preparation of urethane group~containing polyisocyanates which contain only a small proportion of monomeric starting diisocyanates and to the products of this process.
10 Description of the Prior Art Aromatic polyisocyanates containing urethane groups, especially those based on diisocyanatotoluene, ~ -are valuable "lacquer polyisocyanates." They are described, for example, in "Lehrbuch der Lacke und 15 Beschichtungen" by H. Kittel, publishers W. A. Colomb, Berlin 1973, Volume I, Part 2. They are generally prepared by the reaction of low molecular weight, poly-hydric alcohols with an excess of diisocyanates followed by removal of the excess diisocyanate by distillation or 20 extraction.
In the process of extraction, the excess diiso- -cyanate can be carefully removed by means of suitable ~-~
solvent mixtures. One important disadvantage of this method is that it requires the use of large quantities 25 of extracting agent which is difficult to subsequently separate, recover and work up such that the process is in most cases uneconomical on a technical scale.
Moreover, extraction removes not only the mon~meric diisocyanates but also a proportion of low 30 molecular weight polyisocyanates from the polyisocyanate mixture, thereby altering the properties of the polymer.
Removal of the excess diisocyanate by distilla-tion i8 most effectively carried out by the method of Mo-3110 ~ ' . ,. "

thin lsyer dlstlllat~on b~t ~n~e a polyl~ocy~nate which 18 free from monomer~ h~s a very hlgh 60ftenlng polnt ~nd a hl~h fuslon vi8c081ty~ thl~ d~gtlllstlon ~st be c~rr~ed out ~t te~per~ture~ from 160 to 180 C~ ho~ever, 5 decompo61tion and polymer~zatlon reactlon~ occ~r ~t the6e temperatures Thl- ln turn lead~ to the depo~l-t~o~ of polymers ~nd ~evere mech~nlcal ~traln on t~e th~n layer dl~tlllatlon apparatus ~ ~impliflet method of 6eparat~ng ~onomers from 10 low molecul~r weight aromatlc polyl~ocy8nateg cqntgin~ng urethane group~ ~ descrlbet ln~ 145,353, dated December 3, l980 in whi~h the palyisocyanates Containin~ u~ethane groups are modif~ed with aroma~lc polyi~ocyanaee~ conta~nlng ac~l urea, allophanste or bluret structure~ ~fter remov~l 15 of the exce~- monomerlc dllsocyanate~ by extractl~n or d~stillatlon, the polylsocyanate obtalned ha~ ~ low vlscos~ty and high l~ocyan~tc content Onc ma~or dl6ad-Ysnt~ge of the~e product~, however, l~ thelr h~gh content of monomerlc dll60cyanatc, whlch may be ~6 much 20 ~ 0.6 to 1.95S, ba~ed on ~olld~ Th~ hlgh propo~t~on of free ~t~rtln~ dllsocy~nate severely l~mlt~ th~ use of the~e product~ ln numerou~ fleld~ for re~Qn~ o work-pl~ce hyglene.
St ha~ no~ been found th-t polyl~ocyanatec 25 contalnln~ urcth-ne ~roup~ b~ed on rom~t~c dll-oc7--n-t~-, ln partlcul-r on dl~cocyan-totoluen~, m ~ be obt-lnet w~th ~ lo~ vl~co~lty, ~ h~h ~ocy~n-te cont-nt nd ~ proportlon of fre- ~tartlng dl~ocyan-t- of noe ~or- th~n O.~S, preferabl~ 0 3S by we~ght lf th- d~t~l-30 latlve re~ov~l of exce~ monomerlc ~t-rtln~ dll~oc~anate 1- c~rr~ed out ln the pre~ence of allphatlc polyl-o-cyan~te~ contaln~n~ i~ocyanurat- gro~p~ ~nd the product obtalnet ~ dS~tlllatlon re~ltue l~ fter-treated, ~f ~ecea~-r~ th ~ ~ub-equlvalent quantit~ of monohydrlc ::
Hb-3110 - 2 -' .
, ~ . , ! ', . r , . . , ' ': : ~ ~ . . . . ... .. .. .

~32479~

and/or polyhydric alcohols in the presence of an inert solvent.
SUMMARY OF THE INVENTION
The present invention is directed to a process 5 for the preparation of polyisocyanates containing urethane groups in which the content in monomeric starting diisocyanate is at most 0.4~ by weight by reacting in a urethane-forming reaction a) an excess quantity, based on component b), of a ~^
diisocyanate component containing at least one aromatic diisocyanate which is free from urethane groups with ~-b) a polyol component containing at least one poly-hydric alcohol in the molecular weight range of 62 to about 250 ~ .
followed by distillative removal of the unreacted excess starting dii~ocyanate a), characterized in that before the distillative removal of the excess starting diiso-cyanate a) is carried out, 20 c) an aliphatic polyisocyanate containing isocyanurate groups is incorporated in the urethane-containing reaction product of components a) and b) in a quantity of about 0.3 to 10% by weight, based on the undistilled reaction product of components a) and b), and the distillation residue is then optionally dissolved ln an inert solvent and up to about 0.5 equivalente of an alcohol, per kg of solvent-free :.
distillation re~idue on a solvent free basis i8 added to the resulting solution so that the amount of free starting diisocyanate a) is further reduced ~ -and urethane groups are formed.
The present invention is also directed to the polyisocyanates containing urethane groups obtained by this process.

Mo-3110 - 3 -DETAILED DESCRIPTION OF THE INVENTION
The starting materials a) may be any aromatic diisocyanates in the molecular weight range of 160 to 300 which are free from urethane groups. 2,4-diiso-5 cyanatotoluene and commercial mixtures thereof with 2,6-diisocyanatotoluene are particularly preferred starting components. The mixture~ preferably contain up to about 35% by weight of 2,6-diisocyanatotoluene, based on the total quantity of the mixture.
The polyol component b) may be any polyhydric aliphatic alcohol in the molecular weight range of 62 to about 250. Examples of 8uch alcohols include ethylene glycol, 1,2- and 1,3-dihydroxypropane, 1,2-, 1,3-, 2,3-and 1,4-dihydroxybutane, 1,6-dihytroxyhexane, tiethylene 15 glycol, triethylene glycol, dipropylene glycol, neopentyl glycol, 2,2,4-trimethyl-1,3-tihydroxy-pentane, glycerol, trimethylolethane, trimethylolpropane and mixtures of such polyhydric alcohols. Component b) preferably is based on mixtures of dihydric alcohols and 20 trihydrlc alcohols which have molecular weights of 62 to about 250, in particular 90 to about 180. Mixtures of such dihydric and trihydric alcohols in which the molar ratio of diol to triol is about 1:2 to 1:0.5 are partic- ~ -ularly preferred. Component b) could, of course, 25 include small quantities of alcohols which are higher than trifunctionsl, for example pentaerythritol.
The polyisocyanates containing urethane groups are prepared in known manner by the reaction of component a) with component b) in proportions which 30 maintain an NCO/OH equivalent ratio above 2. The NCO/OH
equivalent ratio is preferably about 3:1 to 15:1, especially about 3:1 to 7:1. The react~on is generally carried out within a temperature range of about 60~to Mo-3110 - 4 -13247~0 Stsbil~zers may be added to the resction mixture to avold s~de reactlons. Examples of ~ulesble 6tsbilizers include lnor~an~ acids ~nd ino~ganlc and organlc ~cid chlorites ~uch as hydrogen chloride, 5 hydrogen bromide, thlonyl chloride, Bulph~ryl chloride, benzoyl chlorlde, oxalyl chloride, lsophthalgl chlorite, carb~mic acid chlorides and carbamic acid bromide~. If stabilizer~ are used, they are added in quantities of about 100 to 1000 ppm, preferably about 100 to 500 ppm, 10 bssed on the weight of components a) ant b).
The reaction products ob~alnet after the reaction of components 8) and b) are polyisocyanates contalning urethane groups together with exces~ ~arting diisocyanate a~. According to the invention, an 15 aliphatic poly~socyanate containing isocyanurate groups c) i~ added to thl~ mixture, generally in a quantity of about 0.3 to lOZ by weight, preferably about 0.5 to 10~
¦ by weight, based on the weight of the undi~tilled reac-j tion product.
~ particularly ~uitsble aliphatic polyi~o-~ cyanate contsining isocyanurate groups 18 trimer~zed 1 1,6-diisocysnatohe~ane having an isocyanate content of ! about 15 to 25~ by welght, containing not more than 0.5Z
by welght, prefer~bly not more than 0.2~ ~y welght of 25 fre~ 1,6-di~ocyan~tohexane and havlng a v~sco~ity below ~ S000 ~P~.~123-C. Polylsoc~anates of thi~ kind ~re ¦ preferabl~ ,N-tris-(isocyenatohexyl)-lsocyanurate or ¦ mixtures of thl~ trllsocyanate wlth it~ hlgher homologs contalning more than one lsocyanurate ring. The prepar-30 at$on of such polyi60cyanates i8 tescribed, for example, in EP-~-10,589, V.S. Patent 4,324,879 .
The product tesc~ibed ln Example 2 of this publication $8 partlcularly suieable.

.. .
Mo-3110 - 5 -132~79~
The reaction product of components a) and b) to which the component c) has been added is then freed from most of its free starting diisocyanate a) by distil-lation in known manner. This distillative removal of 5 the starting diisocyanate is preferably carried out in a thin layer distillation apparatus within the temperature range of about 150 to 180C, more preferably about 160 to 180C, and preferably at a pressure of about 0.1 to 0.5 mbar.
The virtually complete removal of the starting diisocyanate is made possible by the addition of the above-mentioned component c) which contributes substan-tially to lowering the softening point and the fusion viscosity of the urethane group-containing polyiso-15 cyanate which has been freed from the monomer.
After the distillative treatment, the end products obtained as distillation residue generally contain less than 0.4Z by weight, in most cases less than 0.3Z by weight of free starting diisocyanates a).
20 The end products of the process are normally dissolved in inert solvents such as toluene, xylene, butyl acetate, ethyl acetate, methoxypropyl acetate, methyl ethyl ketone or any mixtures of such solvents to form about 60 to 80Z by weight solutions. The proportion of 25 free starting diisocyanate a) present in these solutions may, if desired, be reduced to less than about 0.2Z by weight by the addition of an alcohol.
Suitable alcohols for this purpose include in particular low molecular weight monohydric and 30 polyhydric alcohols ~uch as methanol; ethanol; the isomeric propanols, butanols, pentanols and hexanols;
and higher valent alcoholfi of the type already mentioned above as examples of component bj. Relatively high molecular weight alcohols such as octyl alcohol or Mo-3110 - 6 - -132~790 lauryl alcohol may, of course, be used for the same purpose instead of the above-mentioned low molecular weight alcohols.
If an after-treatment is found to be advisable, 5 the alcohols are added to the above-mentioned solutions in a quantity of up to about 0.7 equivalents of alcohol per kg of solvent-free distillation residue. The substantially selective urethanization of the remaining quantity of free starting diisocyanate a) after the 10 addition of this alcohol is generally carried out at room temperature or with mild heating of the solutions to temperatures up to about 50C, preferably up to about 40C.
The products obtained according to the inven-15 tion are distinguished by their exceptionally lowcontent in free star~ing diisocyanate, which is less than 0.3~ by weight, preferably less than 0.2~ by weight, and in most cases less than 0.15Z by weight, and the high NC0 content amounting to about 13 to 19% by 20 weight, preferably about 13 to 18% by weight.
The products according to the invention are high quality "lacquer polyisocyanates" and are particu-larly suitable as reaction components for organic poly-hydroxyl compounds in two-component polyurethane 25 lacquers. Such two-component polyurethane lacquers are suitable in particular for coating textiles, leather, plastics, wood, paper or metals.
The invention is further illustrated but is not intended to be limited by the following examples in 30 which all parts and percentages are by weight unless otherwise specified.

Mo-3110 - 7 -l3247sa EXAMPLES
Example 1 1287 g of a mixture of 65 parts by weight of 2,4~diisocyanatotoluene and 35 parts by weight of 5 2,6-diisocyanatotoluene were stabilized with 0.3 g of isophthaloyl chloride and heated to 80C. A mixture of 83.1 g of trimethylolpropane and 40.3 g of diethylene glycol was added within one hour. The reaction mixture was stirred for 3 hours at this temperature, cooled to lO room temperature and mixed with 64.3 g of a polyiso-cyanate containing isocyanurate groups based on 1,6-diisocyanatohexane, having an isocyanate content of 21.5% by weight, containing 0.2Z by weight of free 1,6-diisocyanatohexane and prepared according to 15 Example 2 of EP-A-10,589. The resulting mixture was freed from the volatile constituents in a thin layer evaporator at a temperature of 160C and a pressure of 0.4 mbar. After dilution with 215 g of ethyl acetate, 860 g of a 75~ solution of the polyisocyanate in ethyl 20 acetate was obtained having a free diisocyanatotoluene content of 0.2~ by weight, based on the quantity of solution. 7.5 g of methanol were then added to the solution and the solution was gently stirred at 30C for 10 hours.
A solution having a viscosity of 2750 mPa.s/23C was obtained. The solution had an NC0 content of 12.5Z and contained 0.05~ of free diisocya-natotoluene.
Example 2 I

860 g of a polyisocyanate solution prepared as in Example 1 were ~ubjected to thin layer distillation and dissolved in ethyl acetate. 8.3 g of diethylene glycol were then added and the mixture was stirred at 50C for 6 hours.
A product having a viscosity of 3950 mPa.s/23C
and an NC0 content of 12.5% was obtained. The free TDI
content was 0.06%.
Mo-3110 - 8 - -''' : "
' ' ' ~324~90 Example 3 1287 g of 2,4-diisocyanatotoluene were heated to 80C and a mixture of 83.1 g of trimethylolpropane and 40.3 g of diethylene glycol was added in the course 5 of one hour. The reaction mixture was stirred for 3 hours at this temperature ancl then mixed with 64.3 g -of the polyisocyanate containing isocyanurate group~
mentioned in Example 1.
The excess diisocyanatotoluene was then 10 distilled off in a thin layer evaporator a~ a tempera-ture of 160C and a pressure of 0.4 mbar. After dilu- -tion with 215 g of ethyl acetate, 860 g of a 75Z solu-tion of the polyisocyanate in ethyl acetate wa6 obtained. The solution contained 0.22% of free diiso-15 cyanatotoluene. After the addition of 6.1 g of methanol and stirring of the reaction mixture for 12 hours at 30C, a solution containing O.lOZ of free diiso-cyanatotoluene was obtained. The viscosity of the -solution was 1700 mPa.s/23C and its isocyanate content 20 was 13.0Z.
.xample 4 860 g of a polyisocyanate solution prepared by a method analogous to that of Example 3 was subjected to thin layer distillation, dissolved in ethyl acetate and 25 then reacted with 12.2 g of methanol for 12 hours at A product having a viscosity of 2600 mPa.s/23C
and an isocyanate content of 11.83% was obtained. The free TDI content was 0.05Z.
30 Example 5 1287 g of a mixture of 65 parts by weight of 2,4-diisocyanatotoluene and 35 parts by weight of 2,6-diisocyanatotoluene were stabilized with 0.3 g of benzoyl chloride and heated to 90C. A mixture of 35 73.7 g of trimethylpropane and 50.9 g of diethylene glycol was added in the course of one hour.
Mo-3110 - 9 -132479~ ~
The reaction mixture was stirred at this temperature for 2-1/4 hours, cooled to room temperature and mixed with 30.5 g of the polyisocyanate containing isocyanurate groups mentioned in Example 1. The 5 volatile constituents of this mixture were distilled off in a thin layer evaporator at a temperature of 160C and a pressure of 0.35 mbar.
After dilution with 203 g of ethyl acetate 810 g of a 75Z solution of the polyisocyanate in ethyl 10 acetate was obtained having a free diisocyanatotoluene content of 0.24% by weight, based on the quantity of solution.
Reaction of this solution with a mixture of 9.7 g of dipropyleneglycol and 24.2 g of lauryl alcohol 15 at 30C for 20 hours yielded a product having a viscosity of 3500 mPa.s/23C and an isocyanate content of ll.5Z. The free TDI content was 0.12%.
~xamPle 6 830 parts by weight of trimethylolpropane and 20 509 parts by weight of dipropylene glycol were added at 50 to 60C to 1287 parts by weight of 2,4-diisocyanato-toluene. The temperature rose to 80C during this addition and was kept at this level for 5 hours with vigorous stirring. At the end of this time, the isocya-25 nate content of the clear liquid obtained was 35.8X(calculated 36Z). lOZ by weight of this substance was separated off and worked up separately. To the major portion of the liquid were added 53 parts by weight of the polyisocyanate containing isocyanurate groups from 30 Example 1.
Excess 2,4-diisocyanatotoluene was then removed using a molecular evaporator with pre-evaporator at 160C and 0.3 mbar. A solid, clear resin was obtained and dissolved in ethyl acetate to form a 75% solution.
35 The isocyanate content of the solution was 13.5X and the free diisocyanate content was 0.12X (solution 6 a).
Mo-3110 - 10 -~324790 The separated portion of liquid to which no polyisocyanate from Example 1 had been added was then also distilled under the same conditions and dissolved in ethyl acetate to form a 75% solution. This solution 5 had an isocyanate content of 13.lZ and a free diiso-cyanate content of 0.4~ by weight (solution 6 b).
Various hydroxyl compounds were now added to -100 g portions of the solutions, vigorously mixed and left to stand for 24 hours at about 30C. The free TDI
10 contents were then again determined. The results are summarized in the following Table.
100 g of 100 g of Solution 6a Solution 6b 15Iso Mono Vis- I80- Vis-cya- meric cos- cya- Free cos-nate TDI ity nate TDI ity con- con- mPas/ con- con- mPas/
tent tent 23C tent tent 23C
20 + 0.37 g isobutanol 13.2~ 0.10~ 1200 12.6~ 0.32Z 1600 + G.3 g isopropanol 13.1Z 0.05Z 1000 12.7~ 0.28Z 1500 + 0.335 g 25 dipropylene glycol 12.8% 0.07Z 1300 12.lZ 0.32% 2100 Although the invention has been described in detail in the foregoing for the purpose of illustration, 30 it is to be understood that such detail is ~olely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.
.

Mo-3110 ... .

Claims

1. A process for the preparation of a urethane group-containing polyisocyanate which contains not more than 0.4% of monomeric starting diisocyanate, based on the weight of said polyisocyanate, which comprises i) forming a urethane group-containing reaction product by reacting a) an excess quantity, based on the equivalents of component (b), of a diisocyanate component comprising at least one aromatic diisocyanate which is free from urethane groups with b) a polyol component comprising at least one polyhydric alcohol having a molecular weight of 62 to 250, ii) adding about 0.3 to 10% by weight, based on the weight of said urethane group-containing reaction product, of c) an aliphatic polyisocyanate containing isocyanurate groups, iii) distilling the mixture formed in accordance with step (ii) to remove monomeric diiso-cyanate and iv) optionally adding up to about 0.5 equivalents of an alcohol per kg of solvent-free distillation residue to further reduce the amount of monomeric diisocyanate.

2. The process of Claim 1 wherein component (a) comprises 2,4-diisocyanatotoluene or mixtures with 2,6-diisocyanatotoluene.

3. The process of Claim 1 wherein component (b) comprises a mixture of at least one aliphatic diol and at least one aliphatic triol in a molar ratio of about 1:2 to 1:0.5.

4. The process of Claim 2 wherein component (b) comprises a mixture of at least one aliphatic diol and at least one aliphatic triol in a molar ratio of about 1:2 to 1:0.5.

5. The process of Claim 1 wherein component (c) comprises N,N,N-tris-(isocyanatohexyl)-isocyanurate.

6. The process of Claim 2 wherein component (c) comprises N,N,N-tris-(isocyanatohexyl)-isocyanurate.

7, The process of Claim 3 wherein component (c) comprises N,N,N-tris-(isocyanatohexyl)-isocyanurate.

8. The process of Claim 4 wherein component (c) comprises N,N,N-tris-(isocyanatohexyl)-isocyanurate.