ADHESIVE FOR BONDING A PLASTIC FILM TO A PAPER SUPPORT TO PROVIDE A PLASTIC COATING, AND PARTICULARLY SUITABLE FOR BONDING PLASTIC FILM TO SHEET PAPER OR CARDBOARD

Abstract: The invention provides an adhesive for bonding a plastic film to a paper support to provide it with plastic coating, and suitable in particular for bonding said plastic film to sheet paper or cardboard, consisting essentially of a 100 % solids one component polyurethane resin having a viscosity substantially of between 600 and 3500 Cp at 100 DEG C.

Description: ADHESIVE FOR BONDING A PLASTIC FILM TO A PAPER SUPPORT TO PROVIDE A PLASTIC COATING, AND PARTICULARLY SUITABLE FOR BONDING PLASTIC FILM TO SHEET PAPER OR CARDBOARD It is well known to coat paper or cardboard supports with plastic film, by a plastic-coating process, v/here paper and cardboard are plastic coated in order to give the sheet a considerable resistance to abrasion, repeated folding and wear, and to upgrade the appearance of print by giving it brilliance and preventing direct contact with the ink. This process is used in the most diverse fields, from cosmetics in which the highest quality perfume boxes are formed in this way, to jackets for books and records, covers of periodicals for collection and consultation, bags for shops, cardboard covers for childrens books, exercise books and diaries. It is therefore a very widespread process. In plastic coating, to obtain the aforesaid chracteristics each individual sheet is bonded by gluing to a film, usually of polypropylene and more rarely of cellulose acetate, over which the adhesive dissolved in a solvent has been spread. The process normally comprises, in succession, spreading the adhesive over the plastic film, evaporating the adhesive solvent, bonding the film to the sheet by the adhesive, and removing the sheet to recompose the pile. Specifically, the plastic film, wound on a roll, is unwound and passed through metering rollers where it is spread with the desired quantity of adhesive, after wnich it enters a heated tunnel in which the solvent SUBSTITUTE SHEET contained in the adhesive is completely evaporated. On leaving the tunnel the film is pressed by a calender to the paper or cardboard sheet, which is fed by a feeder. During this stage a continuous structure is obtained consisting of a number of side-by-side sheets bonded to a plastic film. The procedure can now continue in two different ways, either by winding the structure onto a reel and then cutting it into sheets, or by immediately separating the sheets to then recompose the bank. This latter procedure is most often used. The adhesive must have particular characteristics in order to adapt to the various types of paper and ink, to give the structure the aforestated characteristics and in particular to resist the mechanical stresses to which it is subjected during processing. Basically it must have a very high degree of initial adhesion to prevent the paper and film moving to different extents and causing wrinkles and channels. It must also have no influence on the tonality of the inks, and not result in colour change. In addition it must be ready to undergo in the shortest possible time further operations such as embossing, cartoning, punching and binding. All these requirements are totally satisfied by polyurethane adhesives in solution in organic solvents prepared specially for the specific sector. They are used by all plastic coaters. The main problem whicn arises from the use of this type of adhesive is related to the evaporation of the organic solvent during the plastic coating process, the evaporation involving serious consequences for the environment. SUBSTITUTE SHEET Most plastic coating firms are of craftsman type and operate on a jobbing basis: the various graphic firms send the printed sheets in the most varied shapes and quantities to the plastic coater, who then returns the sheets coated, piled and banked to the printer or bookbinder. Precisely because of their very small size and the modest turnover of each firm, there is no possibility of investing in plant for the recovery or combustion of the exit solvents. This is the reason for the effort expended in research by adhesives producers to find alternative non-pollutant solutions. In an attempt to obviate this problem adhesives of various chemical types in aqueous solution, including polyurethanes, have appeared. However this type of adhesive is still under development before finding practical application in industry. A further system involves the use of film comprising a bonding layer of low melting point. The film is bonded to the paper by using hot calenders. This is however a slow process with many limitations. The object of the present invention is therefore to provide an adhesive suitable for bonding paper to plastic coating film which overcomes the aforesaid problems of the known art. This adhesive must therefore be of the type which does not require the use of organic solvents, so avoiding all the problems connected with their removal during the application of the adhesive and film to the sheet. As the adhesive of the present invention is SUBSTITUTE SHEET particularly designed for bonding plastic coating film to sheet paper, a particular object of the invention is to provide an adhesive which develops its adhesiveness within a short time and with a high degree of initial adhesiveness, to prevent the paper sheet and film moving relative to each other and so create wrinkling. A further object of the invention is to provide the final plastic coated product with ideal functional and aesthetic properties. These and further objects which will be apparent from the description are attained according to the present invention by an adhesive for bonding a plastic film to a paper support to provide it with plastic coating, and suitable in particular for bonding said plastic film to sheet paper or cardboard, characterized by consisting essentially of a 100% solids one component polyurethane resin having a viscosity substantially of between 600 and 3500 Cp at 100°C. The preferred embodiment of the invention relates to bonding a plastic film to paper or cardboard in form of sheets. Said film can be made from any suitable plastic such as polypropylene, polyester, cellulose acetate, PVC, or others. The polyurethane resin according to the invention can be prepared by reacting suitable polyols, such as polypropylene glycols, with a trialcohol or a trifunctional aliphatic amine, and isocyanate in excess, in order to achieve a one component polyurethane. Said trialcohols can be preferably trimethylol propane, SUB S TITUTE S and said a ine can be triisopropanolamine. Said isocyanate can be preferably 4,4'-diphenylmethane diisocyanate or 2,4'-diphenylmethane diisocyanate; 2,4 toluenediisocyanate, or 2,6 toluenediisocyanate, or a mixture of the two isomers in question. As regards the polyols, these are quite, preferably polypropylenglycols, chosen among those having an average molecular weight approximately inside the range 400-J-4000. In order to select the desired average molecular weight, said polyols can be monomodal or bimodal. Suitable viscosity modifiers can be added to the reaction mixture in order to control the targeted viscosity, which is essential according to the adhesive of the invention. In this regard, OH-bearing compounds can be used, such as for example glycols such as 1,4-butandiols, or monoethylene glycol. A suitable acid modifier can also be added to the reaction mixture, such as benzoyl chloride, or phosphoric acid. As stated above, in order to obtain a one component polyurethane, a residual amount of isocyanate is necessary at the end of the reaction. In this regard, a preferred ratio NC0:0H is approximately 2:1. Some non- limiting examples are given hereinafter to better describe the characteristics and advantages of the invention. EXAMPLE 1 A polyurethane resin suitable for forming an adhesive SUBSTITUTE SHEET according to the invention is prepared from the following initial composition: a) polypropyleneglycol M.W. 2000" *b,) . polypropylieneglycol-. Mi t. . u W. 4„,0 . ,0■ . Jf 68.874 (OH No.=86.7) c) trimethylolpropane 1.275 d) 4,4"-diphenylmethane diisocyanate 29.815 e) benzoyl chloride 0.036 100.000 (M.W. = molecular weight). Addition and reaction sequence 1) Add the propyleneglycols a) + b) while stirring. Raise the temperature to 50°C. 2) Add and dissolve the trimethylolpropane c). 3) When c) has completely dissolved, add the diisocyanate d) and the benzoyl chloride e). React under stirring at 90-95°C for 2 hours. Check the viscosity at 100°C and the residual isocyanate percentage against the values specified below. 4) Discharge to filter. The polyurethane resin obtained has the following characteristics: Appearance: clear yellow % solid: 100 Viscosity in Cp at 100°C: 1000 (+ 200) Brookfield 3 x 50 % NCO: 4.3 (+ 0.15) The viscosity of this resin was measured as a function of temperature within the temperature range of 40-65°C, which can be considered the useful operating range for application of the adhesive. SUBSTITUTE SHEET The results are given in Table A below. TABLE A Viscosity as a function of temperature Temperature Viscosity (Brookfield RVF100) °C Cp 40 48000 (6 x 20) 45 33000 (6 x 20) 50 22500 (6 x 20) 55 16000 (6 x 20) 60 10250 (6 x 20) 65 7250 (6 x 20) 100 1200 N.B. The numbers in parentheses to the side of the viscosity values indicate the rotor/rotational speed combination used for the measurement. EXAMPLE 2 A polyurethane resin suitable for forming an adhesive according to the invention is prepared from the following initial composition: a) polypropyleneglycol M.W. 2000 57.88 b) polypropyleneglycol M.W. 400 10.13 c) trimethylolpropane 1.25 d) 4,4'-diphenylmethane diisocyanate 29.40 e) benzoyl chloride 0.03 f) dipropylene glycol 1.31 100.00 (M.W. = molecular weight) Addition and reaction sequence 1) Add the propyleneglycols a) + b) while stirring. Raise the temperature to 50°C. 2) Add and dissolve the trimethylolpropane c). SUBSTITUTE SHEET 3) When c) has completely dissolved, add the diisocyanate d) and the benzoyl chloride e). React under stirring at 90°C for 2 hours. Check the residual isocyanate percentage. 3b) Add the dipropylene glycol f) and react under stirring at 90°C for 1 hour. Check the viscosity at 100°C and the residual isocyanate percentage against the values specified below. 4) Discharge to filter. The polyurethane resin obtained has the following characteristics: Appearance: clear yellow % solid: 100 Viscosity in Cp at 100°C: 2400 (+ 200) Brookfield 3 x 50 % NCO: 3.2 (+_ 0.15) The viscosity of this resin was measured as a function of temperature within the temperature range of 40-65°C, which can be considered the useful operating range for application of the adhesive. The results are given in Table A' below. TABLE A 1 Viscosity as a function of temperature Temperature Viscosity (Brookfield RVF100) °C Cp 40 190000 (6 x 20) 45 125000 (6 x 20) 50 82000 (6 x 20) 55 52000 (6 x 20) 60 36000 (6 x 20) 65 23000 (6 x 20) 100 2400 N.B. The numbers in parentheses to the side of the viscosity values indicate the rotor/rotational speed combination used for the measurement. EXAMPLE 3 A polyurethane resin according to the invention is prepared from the following initial composition: a) polypropylenglycol M.W. 2000 43.100 * ) (OH No.=120.3) b) polypropylenglycol M.W. 400 19.81δ c) Triisopropanolamine 2.295 d) 4,4'-diphenylmethane diisocyanate 34.757 e) benzoyl chloride 0.030 100.000 Addition and reaction sequence 1) React while stirring a) + b), at 45°C. 2) Add c), previously dissolved and mix. 3) When c) is dissolved, add d) . Temperature should be not higher than 80°C, check the NCO content, which should be % NC0=4.51, and viscosity at 100°C. When viscosity and NCO content are as planned, add and mix e). 4) Discharge to filter. The adhesive of Example 1 of the invention was used for plastic-coating paper sheets with a polypropylene film (thickness 12 microns). The object of the test is to determine an important property of the coated sheet, ie its brightness, following bonding with said adhesive, as a function of the quantity of adhesive spread, ie the coating weight 2 (g/m ) expressed as the grams of adhesive spread per SUBSTITUTE SHEET square metre of sheet surface. During the bonding, a silicone-coated paper sheet of adequate size is interposed between the two substrates to be joined, taking care to place the silicone coated side of the paper on the substrate with the adhesive. 2 After calendering, a square of 100 cm is cut from the bonded zone containing the silicone-coated paper. The adhesive -coated substrate and the silicone- coated paper are separated from the rest and weighed on an analytical balance. The adhesive is washed with the chosen solvent and the substrate (clean) and silicone-coated paper are re-weighed. 2 Expression of result: g/m = (A - B) x 100 where: A = weight of substrate + adhesive + silicone-coated paper (before washing) B = weight of substrate + silicone-coated paper (after washing). In this test the adhesive of the example of the invention is compared with a polyurethane adhesive of known type dissolved in solvent. The results obtained are summarized in Table B below. Two different tests (A and B) at different temperatures were conducted on the adhesive of the invention. TABLE B Description of adhesives ADC0TΞ B16685: Solvent-free adhesive of the above example. % NV 100 Viscosity 1100 Cp at 100°C (Brookfield 3x50) % NCO 4.2 ADC0TΞ C2163: Adhesive dissolved in solvent, used for SUBSTI comparison. % NV 26 Viscosity 50 Cp at 25°C (Brookfield 1x100) % NCO 0.6 Comparative machine tests ADCOTE VISCOSITY ADHESIVE CALENDER g/m 2 BRIGHTNESS B16685 cp TEMPERATURE(°C)TEMPERATURE(°C) 20°C 60°C TEST(A) 11750 54/55 60 4.3 65 90 TEST(B) 4550 68 69/70 2.0 60 90 ADCOTE C2163(known)50 25 60 5.0 60 90 From the results of Table B, it can be generally noted that analogous brightness effects are obtained with the two compared adhesives but using a lower coating weight of the adhesive of the invention. In particular, it can be seen that for an adhesive temperature of around 54/55°C (Test A), a brightness at 20°C is obtained for the adhesive of the invention which is in fact better than for the comparison adhesive (again using less adhesive of the invention). At a higher adhesive application temperature (Test B, 68°C), the same brightness effect as the comparison adhesive is 2 obtained using only 2 g/m of the adhesive of the 2 invention against 5 mg/m of the comparison adhesive. The test therefore demonstrates that in addition to the basic advantages of the invention as initially stated, and which are discussed hereinafter, the important property of brightness of the plastic-coated sheet is obtainable in the case of the invention with a decidedly lower amount of adhesive. The tests also showed that all the initially stated basic advantages of the invention are attained, the SUBSTITUTE SHEET advantages obtainable with the adhesive of the invention are in particular the elimination of atmospheric and environmental pollution because of the absence of any solvent; energy saving in that no drying tunnel is required; no risk of flammability due to solvents; no problem of disposal of special toxic or harmful refuse; paper-film bonding rate much higher than that of known adhesives (3-4 times higher) and in any event dependent solely on feed capacity of the feeder; to obtain the same aesthetic and functional characteristics it is sufficient to apply an adhesive quantity 2-3 times less than the quantity currently necessary using known adhesives; because of the small adhesive quantity the appearance of the plastic-coated product is much more planar; when cross-linking is complete the degree of adhesion obtained is higher. In addition, according to the invention, the sheets can be separated and piled after only one hour following bonding. Moreover, they can be immediately fed to subsequent processing, such as embossing, cartoning or binding, this representing a considerable advantage. SUBSTITUTE SHEET

Claims: CLAIMS 1. A method for plastic coating a paper support by bonding a plastic film on it by an adhesive, particularly suitable for bonding said plastic film to sheet paper or cardboard, characterized by the fact of interposing between said paper support and plastic film an adhesive essentially consisting of a 100% solids one component polyurethane resin having a viscosity substantially between 600 and 3500 Cp at 100°C. 2. A method according to claim 1, characterized by the fact that said polyurethane resin is the product of a reaction of a polyol, such as polypropyleneglycol having an average molecular weight inside the range 400 r 4000, with a trialcohol or a trifunctional aliphatic amine, and isocyanate in excess. 3. A method according to claim 2, characterized by the fact that said trialcohol is trimethylol propane. 4. A method according to claim 2, characterized by the fact that said amine is triisopropanol amine. 5. A method according to claim 2, characterized by the fact that said isocyanate can be selected among 4,4'-diphenylmethane diisocyanate; 2,4'-diphenylmethane diisocyanate; 2,4 toluenediisocyanate; 2,6 toluene¬ diisocyanate; a mixture of the isomers in question. 6. A method according to claim 2, characterized by the fact that said reaction is carried out in the presence of a viscosity modifier. 7. An adhesive for bonding a plastic film to a paper support to provide it with plastic coating, and suitable in particular for bonding said plastic film to SUBSTITUTE SHEET sheet paper or cardboard, characterized by consisting essentially of a 100% solids one component polyurethane resin having a viscosity substantially of between 600 and 3500 Cp at 100°C. 8. An adhesive as claimed in claim 7 characterized by being prepared from the following reaction mixture a) polypropyleneglycol M.W. b) polypropyleneglycol M.W. c) trimethylolpropane d) 4,4'-diphenylmethane diisocyanate e) benzoyl chloride (M.W. = average molecular weight). 9. An adhesive as claimed in claim 7, characterized by being prepared from the following reaction mixture: a) polypropyleneglycol M.W. 2000 57.88 b) polypropyleneglycol M.W. 400 c) trimethylolpropane d) 4,4*-diphenylmethane diisocyanate e) benzoyl chloride f) dipropylene glycol (K.W. = average molecular weight). 10. An adhesive as claimed in claim 7 characterized by containing an isocyanate percentage of between 4.15 and 4.35. 11. An adhesive as claimed in claim 7, characterized by being prepared from the following reaction mixture: a) polypropylenglycol M.W. 2000 43.100 b) polypropylenglycol M.W. 400 19.818 (OH No.=120.3) c) Triisopropanolamine 2.295 d) 4,4'-diphenylmethane diisocyanate 34.757 e) benzoyl chloride 0.030 100.000 and having a % NCO content of around 4.51. 12. An adhesive as claimed in claim 7 characterized by possessing the following viscosity values as a function of temperature: Temperature Viscosity (Brookfield RVF100) °C in centipoise 40 48000 45 33000 50 22500 55 16000 60 10250 65 7250 100 1200 13. An adhesive as claimed in claim 7 characterized by possessing the following viscosity values as a function of temperature: Temperature Viscosity (Brookfield RVF100) °C in centipoise 40 190000 45 125000 50 82000 55 52000 60 36000 65 23000 100 2400 SUBSTITUTE SHEET 14. Use of a 100% solids one component polyurethane resin having a viscosity substantially inside the range of 600 and 3500 Cp at 100 °C as an adhesive for bonding a plastic film on a paper support. 15. Process for preparing a 100% solids one component polyurethane resin having a selected viscosity inside the range between 600 and 3500 Cp at 100°C, characterized by reacting polypropylene glycols with a trifunctional alcohol or amine, adding a diisocyanate and an acid modifier, such as benzoyl chloride, and reacting the mixture under stirring at a selected temperature. SUBSTITUTE

Inventors: ROSSI MASSIMO, ROSSI, MASSIMO
Applicants: MORTON INT SPA, MORTON INTERNATIONAL S.P.A.
Application year: 1992
Main classification: C08G18
IPC codes: B32B7, C08G18, C09J175, D21H27