Moisture- and heat-crosslinkable polyurethane-based adhesive composition comprising a low content of isocyanate monomer

Abstract

The present invention relates to a heat- and moisture-crosslinkable adhesive composition comprising: a) from 40 to 60 wt % of at least one polyurethane obtainable by a polyaddition reaction of at least one particular diisocyanate with at least one polyether diol, with or without the presence of at least one reaction catalyst, at a reaction temperature T1 below 95 C., in anhydrous conditions, and in amounts of diisocyanate(s) and of polyether diol(s) leading to an NCO/OH molar ratio in the range from 1.6 to 1.9; b) from 39 to 59 wt % of at least one compatible tackifying resin and c) from 0.01 to 1 wt % of at least one crosslinking catalyst. The present invention also relates to a method of preparing said adhesive composition, a self-adhesive support coated with the crosslinked adhesive composition and the use of said support for making labels and/or self-adhesive tapes.

Claims

1. A crosslinkable adhesive composition comprising: a) from 40 to 60 wt % of at least one polyurethane obtainable by a polyaddition reaction: of at least one aromatic or aliphatic diisocyanate that is: a1) isophorone diisocyanate, a2) 2,4-toluene diisocyanate, a3) diphenylmethane-2,4-diisocyanate, a4) an allophanate derivative of hexamethylene diisocyanate of formula (I): ##STR00011## in which: p is an integer in the range from 1 to 2; q is an integer in the range from 0 to 9; R represents a hydrocarbon chain, saturated or unsaturated, cyclic or acyclic, linear or branched, comprising from 1 to 20 carbon atoms; R.sup.3 represents a divalent alkylene group, linear or branched, having from 2 to 4 carbon atoms; or a mixture of said aromatic or aliphatic diisocyanates, with at least one polyether diol, with or without the presence of at least one reaction catalyst, at a reaction temperature T1 below 95 C., in anhydrous conditions, and in amounts of diisocyanate(s) and of polyether diol(s) leading to an NCO/OH molar ratio, designated r.sub.1, in the range from 1.6 to 1.9; said polyurethane having formula (II) ##STR00012## in which: R.sup.1 represents an aliphatic or aromatic divalent group: a1) a divalent group derived from isophorone diisocyanate: ##STR00013## a2) a divalent group derived from 2,4-toluene diisocyanate: ##STR00014## a3) a divalent group derived from diphenylmethane-2,4-diisocyanate: ##STR00015## or a4) a divalent group derived from an allophanate of hexamethylene diisocyanate (HDI) of formula (III): ##STR00016## in which: p is an integer in the range from 1 to 2; q is an integer in the range from 0 to 9; R represents a hydrocarbon chain, saturated or unsaturated, cyclic or acyclic, linear or branched, comprising from 1 to 20 carbon atoms; R.sup.3 represents a divalent alkylene group, linear or branched, having from 2 to 4 carbon atoms; R.sup.2, which may be identical to or different from R.sup.3, represents a divalent alkylene group, linear or branched, having from 2 to 4 carbon atoms; n is a non-zero integer such that the number-average molecular weight of the polyether block of formula [OR.sup.2].sub.n is in the range from 2000 to 12000 g/mol; m is an integer such that the number-average molecular weight of the polyurethane is in the range from 3000 to 21000 g/mol; b) from 39 to 59 wt % of at least one compatible tackifying resin, with a number-average molecular weight in the range from 200 Da to 5000 Da, and selected from the resins obtained by one of the following methods: b1) polymerization of terpene hydrocarbons in the presence of Friedel-Crafts catalysts, followed by reaction with phenols, b2) polymerization of alpha-methylstyrene, followed by reaction with phenols; and c) from 0.01 to 1 wt % of at least one crosslinking catalyst, said percentages being expressed by weight relative to the total weight of the adhesive composition.

2. The adhesive composition according to claim 1, wherein the allophanate derivative of hexamethylene diisocyanate of formula (I) is in the form of a composition in which it is comprised at a level of at least 99.5 wt %, said composition comprising less than 0.5 wt % of hexamethylene diisocyanate relative to the total weight of said composition.

3. The adhesive composition according to claim 1, wherein the at least one polyether diol has a number-average molecular weight in the range from 2000 to 12000 g/mol.

4. The adhesive composition according to claim 1, wherein the at least one polyether diol is a polyoxypropylene diol with a polydispersity index in the range from 1 to 1.4.

5. The adhesive composition according to claim 1, comprising: a) from 40 to 60 wt % of the polyurethane of formula (II), in which: R.sup.1 represents the divalent group derived from 2,4-toluene diisocyanate, R.sup.2 represents the divalent propylene group, n is a non-zero integer such that the number-average molecular weight of the polyether block of formula [OR.sup.2].sub.n is in the range from 3500 to 8500 g/mol, m is an integer such that the number-average molecular weight of the polyurethane is in the range from 4600 to 16800 g/mol; b) from 39 to 59 wt % of the compatible tackifying resin, with a number-average molecular weight in the range from 200 Da to 5000 Da, that is a resin obtained by method b2); and c) from 0.01 to 1 wt % of the least one crosslinking catalyst, relative to the total weight of the adhesive composition.

6. The adhesive composition according to claim 1, comprising less than 0.5 wt % of unreacted aliphatic diisocyanate monomer, and less than 0.1 wt % of unreacted aromatic diisocyanate monomer, relative to the total weight of the adhesive composition.

7. The adhesive composition according to claim 1, having a total content of NCO group in the range from 0.15 to 1.7 wt % relative to the total weight of the adhesive composition.

8. A method of preparing a composition as defined in claim 1, comprising mixing the at least one polyurethane the at least one tackifying resin and the at least one crosslinking catalyst.

9. A self-adhesive support obtainable by a method comprising: (i) preheating the adhesive composition as defined in claim 1 to a temperature T4, in order to make it liquid, and then (ii) coating said adhesive composition on a support layer, and (iii) crosslinking said adhesive composition by heating the coated support to a temperature T5 in the range from 70 C. to 150 C., in the presence of humidity.

10. A label and/or self-adhesive tape comprising the self-adhesive support as defined in claim 9.

Description

EXAMPLES 1 TO 7

Preparation of the Polyurethanes

(1) Polyurethanes 1 to 7 of examples 1 to 7 were prepared identically using the various ingredients shown in Table 1. The amounts of diisocyanate and of diol used correspond to an NCO/OH molar ratio in the range from about 1.7 to 1.8.

(2) The amounts shown in Table 1 are expressed in grams.

(3) Experimental Protocol:

(4) The diisocyanate and the diol, and if applicable the reaction catalyst, are mixed in a reactor, stirring continuously, under nitrogen, at a temperature T1 in the range from 72 to 80 C. The temperature is controlled so that it does not exceed 80 C.

(5) The progress of the reaction is monitored by measuring the NCO level by back-titration of dibutylamine, using hydrochloric acid according to standard NF T52-132. The reaction is stopped when the measured NCO level is roughly equal to the desired NCO level.

(6) TABLE-US-00001 TABLE 1 Ingredients of the polyurethane 1 2 3 4 5 6 7 Acclaim 8200 750 Acclaim 4200 929 929 912 912 929 412 Scuranate T100 71 28 71 71 Tolonate X FLO 118 100 IPDI 88 88 Borchikat 315 0.1 0.1 0.1 0.05 TIB Kat 217 0.1
Characterization:

(7) For each of the polyurethanes obtained in examples 1 to 7: the NCO level in the polyurethane synthesis medium is determined according to standard NF T52-132. The measured values are expressed in percentage by weight relative to a 100-gram sample, are then referred to the total weight of the adhesive composition and are presented in Table 4. the content by weight of unreacted diisocyanate monomer present in the polyurethane synthesis medium is measured by HPLC equipped with a UV detector as described above (reverse phase on C18, mobile phase: aqueous solution of acetonitrile, buffered with an aqueous solution at 0.2 wt % of tetrabutylammonium bisulphate to pH equal to 2.5, detection wavelength: 254 nm). The values measured are expressed in percentage by weight and are then referred to the total weight of the adhesive composition and are presented in Table 4. the viscosity of the polyurethane is estimated by measuring the viscosity of the polyurethane synthesis medium at 23 C. at the end of the reaction. This measurement is carried out 24 hours after the end of reaction (J+1) at 23 C., using a Brookfield RVT viscosimeter, with a number 6 needle at a rotary speed of 20 revolutions per minute (rev/min). The value measured is expressed in millipascal second (mPa.Math.s) and is presented in Table 2.

(8) TABLE-US-00002 TABLE 2 Characterization of the polyurethane 1 2 3 4 5 6 7 Viscosity at 23 C. 8710 10800 19500 25920 25920 18400 83400 (mPa .Math. s)

EXAMPLES 1A TO 10A

Preparation of the Adhesive Compositions

(9) Compositions 1A to 10A were prepared identically using the various ingredients shown in Table 3. In particular, these compositions were prepared from the polyurethanes obtained in examples 1 to 7 without undergoing a purification step or separation of the residual diisocyanate, not consumed by the polyurethane synthesis reaction. In other words, these compositions were prepared from the polyurethanes of the examples (designated 1 to 7) comprising the corresponding unreacted diisocyanate monomer, which represents less than 0.1% of the weight of the polyurethane (for examples 1-3 and 6) and less than 0.5 wt % of the polyurethane (for examples 4-5 and 7). The concentrations of unreacted diisocyanate monomer were measured according to the method described above in the description.

(10) The amounts shown in Table 3 are expressed in grams.

(11) Experimental Protocol:

(12) Composition 1A is prepared firstly by putting the tackifying resin and the antioxidants in a glass reactor under vacuum, heated to a temperature T2 above or equal to the softening point of the tackifying resin and less than or equal to 130 C. Then, once the resin is properly molten, the vacuum is switched off and half of the polyurethane composition obtained in the preceding example 1 is introduced, under nitrogen. The mixture is maintained at a temperature T2 as defined above, stirring continuously. After addition, the mixture is stirred under vacuum for at least 2 hours, and then the vacuum is switched off again and the other half of the polyurethane composition 1 is introduced under nitrogen, stirring the mixture at a temperature T2 as defined above. Then the mixture is cooled to temperature T3, at about 80 C., the vacuum is switched off and then the catalyst is introduced under a nitrogen atmosphere, stirring vigorously. After addition, the vacuum is restored and the mixture is stirred for a further 10 minutes.

(13) The composition obtained is stored in a moisture-proof aluminium cartridge previously stove-dried at 100 C.

(14) The experimental protocol applied for example 1A is reproduced identically for examples 2A to 10A taking Table 3 into account.

(15) TABLE-US-00003 TABLE 3 Ingredients of the adhesive 8A 9A 10A composition 1A 2A 3A 4A 5A 6A 7A (comp) (comp) (comp) Polyurethane 1 52.0 Polyurethane 2 52.1 Polyurethane 3 52.1 52.1 52.1 52.1 Polyurethane 4 52.1 Polyurethane 5 52.0 Polyurethane 6 52.0 Polyurethane 7 52.1 Dertophene H150 46.8 47.1 46.8 Sylvares 525 47.1 47.1 46.8 47.1 Kristalex F100 47.1 Sylvalite RE105XL 47.1 Acrynax 4326 47.1 DBU 0.5 0.1 0.1 0.1 0.5 0.5 0.1 0.1 0.1 0.1 Antioxidants 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7
Characterizations:

(16) The following measurements are carried out identically on the various adhesive compositions 1A to 10A obtained: The viscosity of the composition is measured at 100 C. using a Brookfield RVT viscosimeter coupled to a heating module of the Thermosel type made by Brookfield, with a number 27 spindle at a rotary speed of 20 revolutions per minute. This viscosity is expressed in millipascal.Math.second. In addition, the general appearance of each composition was evaluated visually (visible light). Compositions 1A to 7A are all transparent. For the comparative examples 9A and 10A, gelling of the composition was observed, probably resulting from incompatibility between the tackifying resin and the polyurethane used.

(17) The measured values are presented in Table 4.

(18) TABLE-US-00004 TABLE 4 Characterizations of the adhesive 8A 9A 10A compositions 1A 2A 3A 4A 5A 6A 7A (comp) (comp) (comp) Viscosity at 8350 6000 7000 3000 3700 9025 7000 6100 gel gel 100 C. (mPa .Math. s) % NCO (in wt % 0.77 0.38 0.77 0.74 0.74 0.77 0.66 0.77 0.77 0.77 of the adhesive composition) Level of free 0.04 0.05 0.04 0.49 0.49 0.04 0.3 0.04 0.04 0.04 monomer (wt % of the adhesive composition)

Preparation of a PET Support Layer Coated With the Crosslinked Adhesive Composition at a Rate of 50 g/m2

(19) The support layer used is a rectangular sheet of PolyEthylene Terephthalate (PET) with a thickness of 50 m and dimensions of 20 cm by 40 cm.

(20) The composition 1A obtained is preheated to a temperature T4 in the range from 70 C. to 150 C. and preferably from 100 C. to 120 C. and it is introduced into a cartridge, from where a cord is extruded, which is deposited near the edge of the sheet, parallel to its width.

(21) The composition contained in this cord is then distributed on the entire surface of the sheet, so as to obtain a uniform layer of roughly constant thickness. This is done using a film-pull (also called filmograph) which is moved from the edge of the sheet to the opposite edge. In this way a layer is deposited with a composition corresponding to a basis weight of 50 g/m.sup.2, which roughly represents a thickness of the order of 50 m.

(22) The sheet of PET thus coated is then put in a stove at a temperature T5 in the range from 70 C. to 150 C. and preferably from 100 to 120 C. for about 15 minutes, and then at 23 C. for 7 days at a relative humidity of 50%, for crosslinking of the composition.

(23) The sheet is then laminated on a protective non-stick layer consisting of a sheet of silicone film, rectangular and with the same dimensions.

(24) The PET support layer thus obtained is submitted to the tests described below.

(25) The experimental protocol applied and the tests carried out for the multilayer system obtained using composition 1A are reproduced identically for examples 1A to 7A.

Peel Test at 180 on Stainless Steel Plate

(26) The adhesive power is evaluated by the peel (or peeling) test at 180 on a stainless steel plate as described in FINAT method No. 1, published in the FINAT Technical Manual 6th edition, 2001. FINAT is the International Federation of manufacturers and processors of self-adhesive labels. The principle of this test is as follows.

(27) A test specimen in the form of a rectangular strip (25 mm150 mm) is cut from the PET support layer coated with the crosslinked composition obtained previously. This test specimen is fixed on of its length (after removing the corresponding portion of protective non-stick layer), on a substrate consisting of a degreased stainless steel plate, applying two passes of a 1-kilogram roller. The assembly obtained is left for 15 minutes at room temperature. It is then placed in a pulling apparatus that is able, starting from the free end of the rectangular strip, to effect peeling or detachment of the strip at an angle of 180 and with a separation speed of 300 mm per minute. The apparatus measures the force required for detaching the strip in these conditions. The results are expressed in newton per centimeter (N/cm) and are shown in Table 5 below.

(28) In addition, the fracture surface is assessed visually, according to the state of the unglued surfaces. The designation AR is used for adhesive rupture, when it is observed that all of the adhesive seal remains glued to the PET support layer. The designation CR is used for cohesive rupture, when it is observed that the adhesive seal was broken and part remained glued to the PET support layer and the other part was glued to the substrate. The results are shown in Table 5.

Instantaneous Adhesion Test (Also Called Loop Test)

(29) The immediate tackiness or tack is evaluated by the instantaneous adhesion test, called loop test, described in FINAT method No. 9, the principle of which is as follows.

(30) A test specimen in the form of a rectangular strip (25 mm150 mm) is cut from the PET support layer coated with the crosslinked composition obtained previously. After removing all of the protective non-stick layer, the 2 ends of this strip are joined so as to form a loop whose adhesive layer is facing outwards. The 2 joined ends are put in the moving jaws of a pulling apparatus that is able to impose a speed of movement of 300 mm/minute along a vertical axis with possibility of going forwards and backwards. The lower part of the loop placed in the vertical position is first brought into contact with a horizontal glass plate of 25 mm by 30 mm on a square zone with side of about 25 mm. Starting from this contacting, the direction of movement of the jaws is reversed. The immediate tackiness is the maximum value of the force required for the loop to be unglued completely from the plate. The results are expressed in newton per square centimeter (N/cm.sup.2) and are shown in Table 5.

(31) In addition, the fracture surface is assessed visually, according to the state of the unglued surfaces. The designation AR is used for adhesive rupture: in particular, it is observed that all of the adhesive seal remains glued to the PET support layer. The designation CR is used for cohesive rupture: it is observed that the adhesive seal was broken and part remained glued to the PET support layer and the other part remained glued to the substrate. The results are shown in Table 5.

(32) TABLE-US-00005 TABLE 5 Tests of the crosslinked adhesive composition on 8A 9A 10A support 1A 2A 3A 4A 5A 6A 7A (comp) (comp) (comp) 180 peel on stainless 11.81 3.26 9.45 18.50 10.24 10.24 6.30 0.55 n.a. n.a. steel (N/cm) CR CR AR CR CR CR CR CR Instantaneous 12.40 4.82 6.05 2.12 11.94 12.87 7.44 1.24 n.a. n.a. adhesion of the loop CR CR AR CR CR CR CR CR on glass (N/cm.sup.2) n.a.: not applicable

(33) Thus, all the adhesives obtained from examples 1A to 7A according to the invention lead to adhesive performance that is satisfactory in terms of adhesive strength and tack. Moreover, the values obtained at the end of the peel test and after the instantaneous loop adhesion test are strictly above 2N/cm and 2N/cm.sup.2 respectively, which are the minimum desired values for obtaining self-adhesive power.

(34) For comparative example 8A, these cumulative conditions are not fulfilled, and in fact the adhesive obtained has poor performance and is not suitable as a self-adhesive.

(35) For examples 9A and 10A, the adhesive strength and the instantaneous tackiness could not be measured as the composition had gelled.

(36) The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

(37) From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.