Process for the Production of Hot Melt Adhesives Having a Low Emission of Monomeric Isocyanates

Abstract

The present invention refers to a process for the production of hot melt adhesives having a low emission of monomeric isocyanates upon heating wherein in a first step a polyurethane prepolymer is generated which is subsequently reacted with a chain extender. Further, the present invention refers to hot melt adhesives obtained by the inventive process.

Claims

1. A process for the production of a hot melt adhesive including a polyurethane prepolymer, comprising: reacting the polyurethane prepolymer with a chain extender wherein the polyurethane prepolymer is obtained by; (i) reacting 2,4′-toluene diisocyanate (2,4′-TDI) with a polyol and in a subsequent step (ii) reacting methylene diphenyldiisocyanate (MDI) to the reaction mixture of step (i); wherein the hot melt adhesive has an application temperature of 80 to 160° C. and the chain extender is selected from the group consisting of 1,3-butane diol, 1,2-propane diol, 2-ethyl-1-hexanol, diethylene glycol, tripropylene glycol, isosorbide, resorcinol, 1,4-dimethylol cyclohexane, and mixtures thereof.

2. The process according to claim 1 wherein step (i) and (ii) of the process are conducted in the presence of a catalyst selected from the group consisting of metal organic compounds and amine-containing organic compounds.

3. The process according to claim 1 wherein the polyol employed in step (i) has an average number molecular weight M.sub.n of 100 to 10000 g/mol, determined by GPC.

4. The process according to claim 1 wherein polyetherpolyols and/or polyesterpolyols are employed as the polyol in step (i).

5. The process according to claim 1 wherein the polyol has a hydroxyl value (OH value) of 50 to 1500 mg KOH/g.

6. The process according to claim 1 wherein the ratio of isocyanate groups to hydroxyl groups (NCO:OH) in step (i) and/or step (ii) is 1.1:1 to 4:1.

7. The process according to claim 1 wherein the content of monomeric toluene diisocyanate in the hot melt adhesive is less than 0.02 wt.-%, based on the total weight of the hot-melt adhesive.

8. The process according to claim 1 wherein the content of monomeric methylene diphenyl diisocyanate in the hot melt adhesive is less than 0.1 wt.-%, based on the total weight of the hot-melt adhesive.

9. The process according to claim 1 wherein the hot melt adhesive has a softening point of more than 40° C., determined according to the method of ring & ball.

10. A hot melt adhesive obtained by the process according to claim 1, wherein the content of monomeric toluene diisocyanate in the hot melt adhesive is less than 0.02 wt.-%, based on the total weight of the hot-melt adhesive.

11. The hot melt adhesive according to claim 10 wherein the content of monomeric methylene diphenyl diisocyanate in the hot melt adhesive is less than 0.1wt.-%, based on the total weight of the hot-melt adhesive.

12. The hot melt adhesive according to claim 10, wherein the hot melt adhesive shows an increase in viscosity while being kept at application temperature for a period of 2 hours of no more than 20%.

13. The hot melt adhesive according to claim 10, wherein an emission rate of toluene diisocyanate is less than 2 ppb.

14. A hot melt adhesive comprising a chain extended polyurethane prepolymer, wherein the chain extended polyurethane prepolymer is the reaction product of a polyurethane prepolymer and a chain extender, wherein: the polyurethane prepolymer is the reaction product of 2,4′-toluene diisocyanate (2,4′-TDI), a polyol and methylene diphenyl diisocyanate (MDI); the chain extender is selected from the group consisting of 1,3-butane diol, 1,2-propane diol, 2-ethyl-1-hexanol, diethylene glycol, tripropylene glycol, isosorbide, resorcinol, 1,4-dimethylol cyclohexane, and mixture thereof; and the hot melt adhesive has an application temperature of 80 to 160° C., the content of monomeric toluene diisocyanate is less than 0.02 wt.-%, based on the total weight of the hot-melt adhesive and the content of monomeric methylene diphenyl diisocyanate in the hot melt adhesive is less than 0.1 wt.-%, based on the total weight of the hot-melt adhesive.

15. The hot melt adhesive of claim 14 wherein the polyurethane prepolymer is the reaction product of a sequence of reaction steps comprising (i) a reaction of the 2,4′-toluene diisocyanate (2,4′-TDI) and the polyol in a first step to form an intermediate and (ii) a reaction of the intermediate with methylene diphenyl diisocyanate (MDI) to form the polyurethane prepolymer.

16. The hot melt adhesive of claim 14 wherein the polyurethane prepolymer is the reaction product of a sequence of reaction steps comprising (i) a reaction of a molar excess of 2,4′-toluene diisocyanate (2,4′-TDI) and the polyol in a first step to form an intermediate and (ii) a reaction of the intermediate with a molar excess of methylene diphenyl diisocyanate (MDI) to form the polyurethane prepolymer.

17. The hot melt adhesive of claim 14 wherein the polyurethane prepolymer is the reaction product of a sequence of reaction steps comprising (i) a reaction of 2,4′-toluene diisocyanate (2,4′-TDI) and a molar excess of the polyol in a first step to form an intermediate and (ii) a reaction of the intermediate with a molar excess of methylene diphenyl diisocyanate (MDI) to form the polyurethane prepolymer.

18. The hot melt adhesive according to claim 14, wherein the chain extended polyurethane prepolymer comprises NCO moieties.

19. The hot melt adhesive according to claim 14, having an emission rate of toluene diisocyanate is less than 2 ppb.

Description

[0016] In a preferred embodiment the chain extender further reacted with the prepolymer is selected from the group consisting of 1,3-butane diol, 1,2-propane diol, 2-ethyl-1-hexanol, diethylene glycol, tripropylene glycol, isosorbide, resorcinol, 1,4-dimethylol cyclohexane, and mixture thereof. It was surprisingly found that by using low molecular weight diols and monols the content of monomeric isocyanates in the hot melt adhesive could be reduced while at the same time maintaining the melt stability of the adhesive, in particular at temperatures above 70° C.

[0017] The reactions of step (i) and (ii) of the inventive process may be conducted in the presence of a catalyst in order to enhance the turn-over rate of the reaction as well as reduce the reaction time and temperature. In a preferred embodiment, step (i) and/or step (ii) of the inventive process are therefore conducted in the presence of a catalyst. The catalyst is preferably chosen from the group consisting of metal organic compounds and amine-containing organic compounds. In an especially preferred embodiment, the catalyst is selected from the group consisting of metal organic compounds derived from tin, iron, titanium, bismuth and zirconium. In an alternatively preferred embodiment, the catalyst is c-caprolactam.

[0018] The polyol employed in step (i) of the inventive process is preferably a polyol having an average number molecular weight M.sub.n of 100 to 10000 g/mol, preferably 500 to 5000 g/mol, determined by GPC. The chain extender can in particular be chosen to control the viscosity properties of the polyurethane prepolymer. In an especially preferred embodiment, polyetherpolyols and/or polyesterpolyols are employed as chain extenders.

[0019] In a further preferred embodiment, the polyol has a hydroxyl value (OH value) of 50 to 1500 mg KOH/g, preferable 250 to 1500 mg KOH/g, most preferable 500 to 1500 mg KOH/g. The hydroxyl value is a measure of the content of free hydroxyl groups in a chemical substance, usually expressed in units of the mass of potassium hydroxide (KOH) in milligrams equivalent to the hydroxyl content of one gram of the chemical substance. The analytical method used to determine the hydroxyl value traditionally involves acetylation of the free hydroxyl groups of the substance with acetic anhydride in pyridine solvent. The hydroxyl value can be determined according to ASTM E222-10.

[0020] Although one obvious approach to reduce the content of monomeric isocyanates in the hot melt adhesive would be to use a low amount to begin with, it was found to be advantageous with regard to large-scale productions to employ a molar excess of the isocyanate component with respect to the hydroxyl-component in step (ii). While in step (i), the hydroxyl-component is molar excess to the isocyanate component. In a preferred embodiment, the molar ratio of isocyanate groups to hydroxyl groups (NCO:OH) in step (i) and/or step (ii) of the inventive process is therefore 1.1:1 to 4:1, preferably 1.5:1 to 3:1. Although the isocyanate groups are present in a molar excess in the step (ii), it was nevertheless possible to obtain a hot melt adhesive with a very low content of monomeric toluene diisocyanate and methylene diphenyldiisocyanate which also showed very low to not-measurable emission of either compound upon heating.

[0021] It was surprisingly found that the content of monomeric toluene diisocyanate in the hot melt adhesive could be reduced to less than 0.02 wt.-% when the hot melt adhesive was produced according to the inventive process. In a preferred embodiment, the hot melt adhesive therefore has a content of monomeric toluene diisocyanate of less than 0.02 wt.-%, preferably less than 0.01 wt.-%, based on the total weight of the hot melt adhesive.

[0022] Since toluene diisocyanate is not the only critical isocyanate contained in the hot melt adhesive, it is desirable to reduce the content of all monomeric isocyanates in the hot melt adhesive since they were found to be a potential health risk which also limits the application of such adhesives, especially with regard to their employment in the production of food packaging. Therefore, an embodiment of the present invention is preferred wherein the monomeric content of methylene diisocyanate in the hot melt adhesive is less than 0.1 wt.-%, preferably less than 0.05 wt.-%, based on the total weight of the hot melt adhesive.

[0023] It was surprisingly found that the inventive process is in particular suitable for the production of hot melts having a wide range of softening points. In a preferred embodiment of the inventive process, the hot melt adhesive has therefore a softening point of more than 40° C., preferably more than 50° C., determined according to the method of ring & ball.

[0024] The inventive process is especially suitable for the production of hot melt adhesives having low contents of monomeric isocyanates. A further object of the present invention is therefore a hot melt adhesive obtained by the inventive process, the obtained hot melt adhesive having a content of monomeric toluene diisocyanate of less than 0.02 wt.-%, preferably less than 0.01 wt.-%, based on the total weight of the hot melt adhesive.

[0025] In a preferred embodiment, the monomeric content of methylene diisocyanate in the inventive hot melt adhesive is less than 0.1 wt.-%, preferably less than 0.05 wt.-%, based on the total weight of the hot melt adhesive.

[0026] It was surprisingly found that the inventive hot melt adhesive showed a good melt stability in combination with low amounts of monomeric isocyanates. A poor melt stability usually results in an increase in viscosity if the hot melt is kept at application over a longer period of time. The inventive hot melt adhesive therefore shows a viscosity increase while being kept at application temperature for a period of 2 hours of no more than 20%, determined according to ASTM D3835.

[0027] Although common polyurethane hot melt adhesive may show a low content of monomeric isocyanates, there is always the risk of emission of volatile contents such as toluene diisocyanate due to the high application temperatures of hot melt adhesives. It was surprisingly found that in most cases, the emission of toluene diisocyanate upon melting of the inventive hot melt adhesive was below the level of detection. In a preferred embodiment, the inventive hot melt adhesive therefore has an emission rate of toluene diisocyanate of less than 2 ppb, preferably less than 1 ppb, in particular less than 0.5 ppb, the ppb referring to parts by weight of the hot melt adhesive. The emission rate was measured at 130° C. over a period of at least 30 minutes.

[0028] The present invention will be explained in more detail by the following examples which by no means are to be understood as limiting the scope or spirit of the invention.

EXAMPLES

Prepolymer

[0029] The prepolymer was obtained by mixing polypropylene glycol (487.5 g, hydroxyl number 237 mg KOH/g) and a polyester of adipic acid, isophthalic acid/PPG/diethylene glycol (56.2 g, hydroxyl number 137 mg KOH/g) with 2,4′-TDI (269 g). An NCO value of 6.8% was obtained after heating to ca. 80° C. 4,4′-MDI (57.5 g) was subsequently added and stirring was continued at 80° C. An NCO end value of ca. 6.41% was obtained.

The following conditions were observed:
NCO:OH (step (i))=1.38:1
NCO:OH (step (ii))=2.2:1

[0030] The content of monomeric TDI was determined to be 0.04 wt.-% and the monomeric content of MDI was found to be 0.8 wt.-%. The total NCO-content of the prepolymer was determined to be 6.41%. The prepolymer showed a viscosity of 77 500 mPa*s, determined at 50° C.

Example 1

[0031] 450 g of the prepolymer were heated up to 80° C. and 7.12 g of 1,3-butanediol as were added.

[0032] The amount of 1,3-butanediol was calculated so that 20% of the total NCO groups of the prepolymer were converted.

[0033] The content of monomeric TDI in the final product was found to be less than 0.01 wt.-%. The content of monomeric MDI in the final product was to be 0.09 wt.-%. The total NCO-content was determined to be 4.47%.

[0034] The resulting product showed a viscosity of 33 000 mPa*s at 100° C. After 2 hours at 100° C. the viscosity reached 38 500 mPa*s, corresponding to an increase in viscosity of 16.6%.

Example 2

[0035] 450 g of the prepolymer were heated up to 80° C. and 8.67 g of 1,2-propane diol were added.

[0036] The amount of 1,2-propane diol was calculated so that 20% of the total NCO groups in the prepolymer were converted.

[0037] The content of monomeric TDI in the final product was found to be less than 0.01 wt.-%. The content of monomeric MDI in the final product was to be 0.08 wt.-%. The total NCO-content was determined to be 4.03%.

[0038] The resulting product showed a viscosity of 50 500 mPa*s at 100° C. After 2 hours at 100° C. the viscosity reached 58 100 mPa*s, corresponding to an increase in viscosity of 15.1%.

Example 3 (One Pot Approach)

[0039] Calculation of necessary amounts of TDI and MDI were done as described above for the prepolymer preparation.

[0040] Polypropylene glycol (487.5 g, hydroxyl number 237 mg KOH/g) and a polyester of adipic acid, isophthalic acid/PPG/diethylene glycol (56.2 g, hydroxyl number 137 mg KOH/g) were mixed and heated up to ca. 80° C. The calculated amount of 2,4′-TDI (269 g) and 4,4′-MDI (57.5 g) were added at 80° C. and homogenate for 10 min at 80° C.

[0041] 13.77 g of 1,3-butanediol were added and the mixture was reacted at 80° C. for 120 min.

[0042] The content of monomeric TDI in the final product was found to be less than 0.02 wt.-%. The content of monomeric MDI in the final product was to be 0.08 wt.-%. The total NCO-content was determined to be 4.39%.

[0043] The resulting product showed a viscosity of 41 000 mPa*s at 100° C. After 2 hours at 100° C. the viscosity reached 48 350 mPa*s, corresponding to an increase in viscosity of 18%.

[0044] Films having a thickness of 100 μm were prepared using a hot melt adhesive of examples 1 to 3, produced according to the claimed process. The film was outgassed at 130° C. for 2 hours by placing the film on a hot plate and the emission concentration of toluene diisocyanate (TDI) and methylene diphenyldiisocyanate (MDI) were measured at head height (30 cm) above the hot plate and 15 cm above the hot plate, respectively. The results are summarized in Table 1:

TABLE-US-00001 TABLE 1 Distance 30 cm Distance 15 cm TDI  0.0035 mg/m.sup.3 0.0169 mg/m.sup.3 MDI <0.0035 mg/m.sup.3 0.0042 mg/m.sup.3

[0045] In all cases, the official threshold of 0.035 mg/m.sup.3 for TDI and 0.05 mg/m.sup.3 for MDI, respectively, was not reached. The inventive hot melt adhesive showed a low emission rate below the threshold for TDI and MDI while at the same time having a low content of the respective monomeric isocyanates and maintaining a good melt stability.