Polyurethane hot-melt adhesive having a low content of diisocyanate monomers and good cross-linking speed

Abstract

A moisture-cured hot-melt adhesive can be formulated as a PUR-HM that is R-40 classification-free and stable during storage and processing, having a residual monomer content of less than 1 wt. % and having good cross-linking density and full-curing speed. The hot-melt adhesive is particularly suitable for vehicle construction in industrial manufacture, in particular of automobiles, in the textile or furniture industry or in the packaging industry.

Claims

1. A moisture-curing hotmelt adhesive obtainable by a process comprising A) reacting one or more polyols as component a) with one or more monomeric polyisocyanates as component b), the stoichiometric ratio of isocyanate groups to hydroxyl groups being greater than 1, to give a reaction product comprising an isocyanate-functional polyurethane prepolymer, B) adding at least one mercaptosilane of the formula (I) as component c)
HSR.sup.3Si(X).sub.(3-a)(R.sup.2).sub.a(I) in which R.sup.2 is a linear or branched alkyl group having 1 to 10 C atoms, R.sup.3 is a linear or branched alkylene group having 1 to 10 C atoms, X is a hydrolyzable radical, and a has a value of 0, 1 or 2, to the reaction product and carrying out reaction to give a reaction product modified with the mercaptosilane, and C) adding an isocyanate-functional polyurethane prepolymer having a monomeric polyisocyanate content of below 1 wt % as component d) before, during and/or after the adding of the at least one mercaptosilane, the at least one mercaptosilane being added in an amount of 0.5 to 3.5 wt % and the isocyanate-functional polyurethane prepolymer as component d) being added in an amount of 10 to 50 wt %, based in each case on the total weight of the components a) to d), wherein the isocyanate-functional polyurethane prepolymer comprised in the reaction product of step A) is different from the component d) isocyanate-functional polyurethane prepolymer having a monomeric polyisocyanate content of below 1 wt %.

2. The moisture-curing hotmelt adhesive as claimed in claim 1, wherein the isocyanate-functional polyurethane prepolymer as component d) is added in an amount of 10 to 45 wt % based on the total weight of components a) to d).

3. The moisture-curing hotmelt adhesive as claimed in claim 1, wherein the stoichiometric ratio of isocyanate groups to hydroxyl groups in step A) is in the range from 1.3 to 2.5.

4. The moisture-curing hotmelt adhesive as claimed in claim 1, wherein the at least one mercaptosilane is added in an amount of 1 to 3.3 wt % based on the total weight of components a) to d).

5. The moisture-curing hotmelt adhesive as claimed in claim 1, wherein in step A) the at least one polyol is a diol and/or the polyol used comprises one or more polyether polyols, polyester polyols, polycarbonate polyols or a mixture of these polyols, and component a) comprises at least one liquid polyester diol, at least one amorphous polyester diol or at least one amorphous polycarbonate diol.

6. The moisture-curing hotmelt adhesive as claimed in claim 1, wherein the mercaptosilane is a mercaptosilane of the formula (I) in which X is OR.sup.4, R.sup.4 being a linear or branched alkyl group having 1 to 10 C atoms which optionally contains 1 or 2 ether oxygens.

7. The moisture-curing hotmelt adhesive as claimed in claim 1, wherein the monomeric polyisocyanate as component b) has a molecular weight of not more than 500 g/mol.

8. The moisture-curing hotmelt adhesive as claimed in claim 1, wherein in step B) the mercaptosilane is reacted with the reaction product at a temperature of at least 60 C.

9. The moisture-curing hotmelt adhesive as claimed in claim 1, wherein in step C) the isocyanate-functional polyurethane prepolymer as component d) is a reaction product of at least one polyol and at least one monomeric polyisocyanate, with unconverted monomeric polyisocyanate having been removed by distillation or extraction to a residual content of below 1 wt % in the reaction product.

10. The moisture-curing hotmelt adhesive as claimed in claim 1, wherein the hotmelt adhesive is admixed further with one or more thermoplastic polymers and/or one or more additives, more selected from fillers, catalysts, plasticizers, adhesion promoters, UV absorbers, UV and heat stabilizers, antioxidants, flame retardants, optical brighteners, pigments, dyes, and dryers.

11. The moisture-curing hotmelt adhesive as claimed in claim 1, wherein it has a monomeric polyisocyanate content of not more than 1.0 wt %.

12. A process for preparing a moisture-curing hotmelt adhesive, comprising A) reacting one or more polyols as component a) with one or more monomeric polyisocyanates as component b), the stoichiometric ratio of isocyanate groups to hydroxyl groups being greater than 1, to give a reaction product comprising an isocyanate-functional polyurethane prepolymer, B) adding at least one mercaptosilane of the formula (I) as component c)
HSR.sup.3Si(X).sub.(3-a)(R.sup.2).sub.a(I) in which R.sup.2 is a linear or branched alkyl group having 1 to 10 C atoms, R.sup.3 is a linear or branched alkylene group having 1 to 10 C atoms, X is a hydrolyzable radical, and a has a value of 0, 1 or 2, to the reaction product and carrying out reaction to give a reaction product modified with the mercaptosilane, and C) adding an isocyanate-functional polyurethane prepolymer having a monomeric polyisocyanate content of below 1 wt % as component d) before, during and/or after the adding of the at least one mercaptosilane, the at least one mercaptosilane being added in an amount of 0.5 to 3.5 wt % and the isocyanate-functional polyurethane prepolymer as component d) being added in an amount of 10 to 50 wt %, based in each case on the total weight of the components a) to d), wherein the isocyanate-functional polyurethane prepolymer comprised in the reaction product of step A) is different from the component d) isocyanate-functional polyurethane prepolymer having a monomeric polyisocyanate content of below 1 wt %.

13. A method for adhesively bonding a substrate S1 to a substrate S2, comprising the steps of i) heating a moisture-curing hotmelt adhesive as claimed in claim 1 to a temperature of between 80 C. and 200 C.; ii) applying the heated moisture-curing hotmelt adhesive to a substrate S1; iii) contacting the applied moisture-curing hotmelt adhesive with a second substrate S2; and iv) chemically curing the moisture-curing hotmelt adhesive with water; the substrate S2 consisting of the same material as or different material from the substrate S1, and at least one of the substrates, S1 or S2, being a polymeric film, a textile or glass.

14. An article bonded by a method as claimed in claim 13.

15. The moisture-curing hotmelt adhesive as claimed in claim 1, is applied in at least one of the following: adhesive bonds, vehicle construction, textile, furniture and/or packaging material.

Description

EXAMPLES

(1) Examples follow for further elucidation of the invention, but are not intended to restrict the subject matter of the invention in any way.

(2) Unless otherwise indicated, amounts are given by weight. The examples used the following substances:

(3) TABLE-US-00001 Durez Ter S1151-22 liquid polyester polyol Sumitomo Bakelite Durez Division Dynacoll7250 liquid polyester polyol Evonik DesmodurVP LS NCO-functional polyurethane Bayer Material 2397 based on polypropylene ether Science glycol and MDI, MDI monomer content 0.15 wt %, Mn 1100-1500 g/mol SilquestA-189 mercaptopropyltrimethoxysilane Momentive Performance Material MDI 4,4-diphenylmethane diisocyanate (MDI)
Measurement Methods

(4) The moisture-curing hotmelt adhesives produced were characterized using the following measurement methods.

(5) Residual Monomer

(6) The amount of unconverted monomeric polyisocyanate remaining in the hotmelt adhesive was determined by means of HPLC (detection via photodiode array) and is expressed in wt %, based on the entire hotmelt adhesive analyzed.

(7) Through-Curing

(8) The through-cure rate of the hotmelt adhesive analyzed was tested on a 500 m film of adhesive. For this purpose the adhesive was preheated at 140 C. for 30 minutes. Sicol paper (B700 white, width about 6 cm, length about 60 cm, Laufenberg & Sohn KG) and a bar applicator were preheated on a 150 C. hotplate. 20 g of the preheated adhesive are placed in the bar applicator, and a 500 m film of adhesive with a length of about 60 cm is applied to the Sicol paper on the hotplate. The paper is then removed from the plate. When the adhesive has solidified (start point), the film is housed in a conditioning chamber (23 C., 55% relative humidity). At defined intervals of time, a narrow strip of 10 cm1 cm is cut from the film and placed on a 150 C. hotplate. This is continued until the strip no longer melts on the hotplate, indicating that the adhesive has undergone through-curing. The time interval from the start point to the time interval at which the strip removed does not melt is the time (in hours) for through-curing.

(9) Heat Stability

(10) The test specimens used for this measurement are two wooden specimens (100 mm25 mm5 mm). The area to be bonded is 25 mm25 mm, and the thickness of adhesive is 1 mm, ensured by means of a spacer 1 mm thick.

(11) The adhesive is melted in a sealed tube at 140 C. for 20 minutes in a heating cabinet. The adhesive is then applied to one side of the first wooden specimen; by means of gentle pressure, it is pressed onto the second specimen, and the bond is fixed with a 500 g weight. The heat stability after full curing (around 1 week) is measured in a heating chamber, beginning at 40 C. The test assembly is suspended in the heating chamber, with a weight of 500 g fastened to the lower specimen. The temperature is raised by 10 K per hour. A record is made of the last temperature at which the bond has not yet failed.

(12) Heat Stability by Canvas Cloth Method

(13) For the determination of the heat stability at 80 C., two canvas cloths each with an area of 15050 mm are bonded so as to overlap with a bonded area of 10050 mm. The adhesive is processed by melting in a heating chamber at 140 C. for 30 minutes. A strip of Sicol paper (B700 white, width about 6 cm, length about 50 cm, Laufenberg & Sohn KG) and a bar applicator are preheated and dried on a 150 C. hotplate. About 20 g of the melted adhesive are placed into the bar applicator, and a 100 m film of adhesive with a length of 50 cm is drawn down onto the Sicol paper on the hotplate. Subsequently, in a transfer process, the film of adhesive is transferred from the Sicol paper to the canvas cloth, with a bond area of 10050 mm and a thickness of 0.1 mm. It may be necessary to reactivate the dried film of adhesive on the Sicol paper on the hotplate to allow it to be transferred to the canvas cloth. The canvas cloth on which the adhesive is located is placed overlappingly onto a second canvas cloth. The adhesive in the canvas cloth assembly is then reactivated on a 150 C. hotplate, and the two canvas cloths are bonded using a 5 kg pressure roller. The canvas cloth assembly is stored at 23 C. and 550% relative humidity for 7 days for the adhesive to cure. A total of 3 such test specimens are prepared for the test.

(14) The 3 test specimens are then suspended in an 80 C. heating chamber. The 5050 mm unbounded portion of one canvas cloth is suspended from a hook, and a weight of 500 g is hung from the unbounded portion of the other canvas cloth. The test specimens spend 30 minutes at 80 C. in the heating chamber. After this time, the test specimens are removed from the heating chamber, and any slippage of the bonded area of canvas cloth is measured and recorded. Canvas cloth heat stability is attained when the bonded area of canvas cloth exhibits no slippage (satisfactory, OK). If complete or very expensive slippage (>8 cm) is found, the adhesive bond lacks sufficient heat stability (unsatisfactory, not OK).

(15) Tensile Strength and Elongation at Break

(16) In a method based on DIN 53504, five rectangular test specimens measuring 2.510 cm were cut from a cured film 500 m thick (cured for 7 days at 23 C./50% RH) of the sample. These specimens were clamped into the tensile testing machine (Zwick Z 020) and pulled apart with a speed of 100 mm/min (test conditions 23 C./50% RH). The parameter measured was the tensile force accepted by the sample at maximum. This was used to determine tensile strength and elongation at break, with the average value being determined in each case from the five specimens.

(17) Moisture-curing hotmelt adhesives were prepared. Table 1 reports the fractions used of the components for each of the examples in wt %, and also the properties, determined by the measurement methods above, of the hotmelt adhesives prepared in the examples.

Example 1 (Comparative)

(18) A polyester mixture of Durez Ter S1151-22 and Dynacoll 7250 in the quantities reported in table 1 was charged to a 1 liter reaction vessel with nonstick coating and a 4-neck lid with ground glass joints. This initial charge was melted in an oil bath with temperature control at 120 C. for 4 hours.

(19) The liquid polyol mixture thus obtained was dewatered in a high vacuum with stirring for an hour, with the temperature maintained.

(20) Then MDI in an NCO/OH molar ratio of 2:1 was added to the polyol in the reaction vessel, and reaction took place with stirring for 1 hour, with the temperature maintained. The reaction product formed was kept at room temperature in the absence of moisture.

Example 2 (Comparative)

(21) The prepolymer preparation procedure described in example 1 was carried out identically with the components reported in table 1 and quantities for example 2, using Desmodur VP LS 2397 instead of MDI. The reaction product formed was kept at room temperature in the absence of moisture.

Example 2b (Comparative)

(22) A prepolymer was prepared with the quantities reported in table 1, in the manner described in example 1, and the amount of Desmodur VP LS 2397 listed in table 1 was mixed homogeneously at a temperature of 120 C. for 30 minutes. The reaction product formed was kept at room temperature in the absence of moisture.

Examples 3 and 3b (Comparative)

(23) A prepolymer was prepared with the quantities reported in table 1, in the manner described in example 1, and the amounts of Silquest.sup.eA-189 listed in table 1 were mixed homogeneously at a temperature of 120 C. for 1 hour. The reaction product formed was kept at room temperature in the absence of moisture.

Examples 4 to 8 (Examples 5 and 8 are Comparative Examples)

(24) A prepolymer was prepared with the quantities reported in table 1, in the manner described in example 1, and the amounts of Desmodur VP LS 2397 listed in table 1 were mixed homogeneously at a temperature of 120 C. for 30 minutes.

(25) Thereafter the amount of SilquestA-189 listed in table 1 was mixed homogeneously for 1 hour with the temperature maintained. The reaction product formed was kept at room temperature in the absence of moisture.

(26) TABLE-US-00002 TABLE 1* (amounts in wt %) Example 1 2 2b 3 3b 4 5 6 7 8 Formula Durez Ter 52.1% 32.6% 45.6% 51.3% 52% 44.6% 43.7% 45% 28.1% 48.3% S1151-22 Dynacoll 7250 34.7% 21.7% 30.4% 34.2% 34.7% 29.7% 29% 30% 18.8% 32.2% Desmodur VP LS 45.7% 14% 14% 13.7% 14.1% 45% 7% 2397 Silquest A-189 3.3% 1.9% 1.9% 4% 1% 1.9% 1.9% MDI 13.2% 10% 11.2% 11.4% 9.8% 9.6% 9.9% 6.2% 10.6% Results of measurement Residual 2.3% <0.1% 1.1% 0.57% 1.1% 0.46% 0.49% 0.80% 0.46% 0.88% monomer Through-curing 19 33 21 42 35 32 51 22 32 22 55% rel. humidity [hours] Heat stability >200 C. 140 C. 130 C. 90 C. 160 C. 160 C. 90 C. 140 C. 140 C. 130 C. Heat stability by OK OK OK not OK not OK OK not OK OK OK not OK canvas cloth method, 7 d, 80 C. Tensile strength 11.2 5.8 8.2 7.6 10.5 9.1 9.5 7.9 8.7 8.3 [MPa] Elongation at 405 467 400 378 444 370 331 357 330 386 break [%] *Examples 4, 6, and 7 are inventive examples; all others are comparative examples

(27) Storage Stability

(28) The adhesives for examples 1, 2, 3, and 4 were investigated for their storage stability. The test here is of the storage stability in the application equipment under typical processing conditions, i.e., at elevated temperature. For this purpose, the compositions were stored at 140 C. and the viscosity was determined at defined intervals of time. Viscosity measurement was carried out continuously at 140 C.

(29) For the measurement of the storage stability (stability of viscosity), the viscosity [mPas] of the respective composition was measured by means of a Rheomat (Brookfield, Thermosel, spindle 27, shear rate 1 min.sup.1) after a storage time t at 140 C. The percentage increase in viscosity in the course of storage was determined relative to the viscosity after 30 minutes of storage.

(30) From the viscosity values determined at each of the storage times, a determination was made of the percentage increase in viscosity over time, which is plotted as a graph in the figure. It is clearly evident that examples 2 and 3 exhibit relatively poor thermal stability, while the monomer-rich example 1 and the inventive, low-monomer-content example 4 display good thermal stability.