IT202300022158A1 - AQUEOUS DISPERSIONS OF POLYURETHANES - Google Patents

Description

AQUEOUS DISPERSIONS OF POLYURETHANES

The present invention relates to aqueous polyurethane dispersions useful for the preparation of a base coat for transfer coating and to a method for coating a substrate by transfer coating.

These aqueous dispersions are particularly useful for coating paper, cardboard, plastic, wood and metals.

STATE OF THE ART

Transfer coating processes are commonly used to form a protective coating or decorate the surface of various substrates such as plastics, wood, inorganic substrates, and metals. In the transfer coating process, a removable base layer (removable primer) consisting of a resin composition with excellent surface properties such as surface energy and solvent resistance is applied in a releasable state to a flexible substrate, such as paper or a thermoplastic resin film, to produce a transfer film. One or more optional intermediate layers, such as a decorative layer, and an adhesive layer are applied to this primer, forming a removable multilayer film (transfer film). Subsequently, the surface of the resulting multilayer film, i.e., the adhesive surface, is adhered to the surface of a receiving substrate (laminating), bonding the multilayer film to the substrate. At the end of the process, the flexible substrate is removed (peeled) to obtain a transfer-coated substrate.

Unfortunately, known removable primer compositions for transfer coating are mainly based on resin compositions containing a large amount of organic solvents, and it is known that the evaporation of organic solvents can be harmful both to the environment and to the health and safety of workers.

To reduce the amount of organic solvents, the use of aqueous transfer-coating compositions has been proposed.

For example, WO 2016/150787 describes a primer for transfer-coating metallization comprising, based on the total weight of the primer: a) 60–80% by weight of a polyurethane dispersion; b) 20–40% by weight of a self-crosslinkable styrene-acrylic emulsion. Both ingredients are water-based.

However, there is still a need in the industry to develop environmentally friendly transfer coating ingredients that can form a removable film with good release properties, adequate surface energy, heat resistance, and high transparency.

Surprisingly, it has now been found that aqueous dispersions of anionic polyurethanes obtained from a nonionic polyether diol and a low molecular weight nonionic diol in a specific molar ratio give rise to films that are easily removable from the flexible substrate and have an excellent surface energy.

Furthermore, these polyurethane dispersions are stable and homogeneous (or easily redispersible) and do not gel or develop persistent sediments over time and in the presence of moderate heat.

To the best of the Applicant's knowledge, no one has described the aqueous polyurethane dispersions of the invention and their surprising and remarkable properties.

WO 2016/150787 does not provide much information on the polyurethanes that can be used. It only states that they can be obtained from a polyester polyol and common diisocyanates. Certainly, WO 2016/150787 does not describe anionic polyurethanes consisting of a nonionic polyether diol and a low molecular weight nonionic diol in the molar ratio specific to the present invention.

DESCRIPTION OF THE INVENTION

The fundamental object of the present invention is therefore an aqueous polyurethane dispersion useful as a primer for transfer coating, said aqueous polyurethane dispersion comprising:

a) from 15 to 50% by weight of an anionic polyurethane prepared by chain extension in water of a prepolymer obtained by reacting:

I) an aliphatic diisocyanate and/or a cycloaliphatic diisocyanate;

II) a non-ionic polyether diol with a molecular weight between 500 and 3,000 g/mol;

III) a non-ionic diol with a molecular weight less than 300 g/mol;

IV) a diol having at least one carboxyl or carboxylate group;

V) possibly, a compound other than diols II, III and IV having one or more? groups reactive towards isocyanate groups (NCO);

VI) if applicable, an aliphatic or cycloaliphatic polyisocyanate having an average isocyanate functionality greater than 2;

in proportions such that: A) the molar ratio of the sum of the NCO groups of compounds I and VI to the sum of the groups reactive towards the NCO groups of compounds II, III, IV and V is between 1.2 and 2.2; B) the molar ratio of the diols III and II is between 1.5 and 5.0; C) the amount of diols II and III represents 80 to 100 % by weight of the sum of compounds II, III and V; D) the amount of diol IV is such that the prepolymer contains 35 to 60 meq/100 g of prepolymer, as dry matter, of carboxylic or carboxylate groups; E) the amount of polyisocyanate VI, if present, does not exceed 3 % by weight of the sum of compounds I and VI;

b) possibly, up to 20% by weight of a suitable coalescing agent.

According to another aspect, the invention relates to a method for transfer-coating a receiving substrate comprising:

i) prepare the polyurethane aqueous dispersion described above;

ii) applying said aqueous polyurethane dispersion to the surface of a flexible substrate to form a removable primer;

iii) if necessary, add an intermediate layer over the removable primer; iv) apply an adhesive over the removable primer or over the intermediate layer, if present, to prepare a removable multilayer film (transfer-film);

v) adhering the removable multilayer to a receiving substrate; and

vi) remove (peel off) the flexible substrate.

A further object of the invention is a substrate coated following the method described above.

DETAILED DESCRIPTION OF THE INVENTION

Preferably, the aqueous polyurethane dispersion of the invention comprises 20 to 45% by weight of anionic polyurethane, based on the weight of the polyurethane dispersion. Typically, the aqueous polyurethane dispersion contains 45 to 80% by weight of water.

Examples of aliphatic and cycloaliphatic diisocyanates useful for the preparation of the polyurethane of the present invention are 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (or isophorone diisocyanate, IPDI), hydrogenated diphenylmethane-4,4'-diisocyanate (H12MDI), 1,6-hexamethylene diisocyanate (HDI), 2,4,4-trimethyl-1,6-hexamethylene diisocyanate (TMDI), 1,5-pentane diisocyanate (PDI), and mixtures thereof. IPDI and H12MDI are the preferred aliphatic and cycloaliphatic diisocyanates.

The terms ?non-ionic polyether diols? and ?non-ionic diols? refer to diols and polyether diols that lack anionic or cationic or potentially anionic or cationic functionality.

The non-ionic polyether diols usable as diols II in the preparation of the prepolymer are those commonly used in the field and known to those skilled in the art and have a molecular weight between 500 and 3,000 g/mol, preferably between 700 and 2,200 g/mol, more preferably between 800 and 1,500 g/mol, even more preferably about 1,000 g/mol.

The term molecular weight used in this text, when referring to diols, is the molecular weight (MW) in g/mol calculated from their hydroxyl number (NOH), determined according to the standard test method ASTM D4274-11 (i.e., MW=56,100 *FOH/NOH, where FOH is the nominal hydroxyl functionality per polymer chain).

Examples of suitable polyether diols include products obtained from the polymerization of cyclic oxides, such as ethylene oxide, propylene oxide, butylene oxide, tetrahydrofuran, and mixtures thereof. Particularly useful polyether diols include polypropylene glycol, poly(ethylene-propylene) glycol, and polytetramethylene glycol (also known as polytetrahydrofuran). The preferred polyether diol is polytetramethylene glycol.

Polyether diols are the preferred diols for the preparation of the anionic polyurethane of the invention, since they are stable to hydrolysis and provide better release properties, especially when applied to a plastic substrate.

Diol III is a non-ionic diol having a molecular weight less than 300 g/mol, preferably less than 250 g/mol, and more preferably less than 200 g/mol. Examples of suitable III diols are ethylene glycol, di-, tri-, tetra-ethylene glycol, 1,2-propane diol, di-, tri-, tetra-propylene glycol, 1,3-propane diol, 1,4-butane diol, 1,3-butane diol, 2,3-butane diol, 1,6-hexane diol, 1,5-pentane diol, 2,2-dimethyl-1,3-propane diol (neopentyl glycol), 1,4-dihydroxycyclohexane, 1,4-dimethylol cyclohexane, 1,8-octane diol, 1,10-decane diol, 1,12-dodecane diol, 2,2,4- and/or 2,4,4-trimethyl-1,3-pentane diol, and mixtures thereof.

Ethylene glycol, 1,4-butanediol, 1,4-dimethylol cyclohexane, and 1,6-hexane diol. 1,4-butanediol and 1,4-dimethylol cyclohexane are the most preferred diols with a molecular weight of less than 300 g/mol. Diols derived from renewable sources, such as 1,4-butanediol from renewable sources, are most preferred.

Diol IV of the prepolymer is an anionic or potentially anionic diol having at least one carboxyl or carboxylate group, which is substituted in the 2-position by two hydroxymethyl groups. Preferably, diol IV is dimethylol propionic acid (DMPA), dimethylol butanoic acid (DMBA), a salt thereof, or a mixture thereof; more preferably, diol IV is dimethylol propionic acid or a salt thereof.

Components V are preferably nonionic and contain isocyanate-reactive groups, i.e., functional groups that can react with isocyanate groups. They can be selected from diols other than II and III, polyols, primary or secondary amines, or polyamines, thiols, and monools. Monools are preferred as component V.

Examples of V-polyols are glycerin, pentaerythritol, trimethylol propane and its derivatives, such as the trifunctional polypropylene glycol initiated on trimethylol propane.

Preferably, polyols and other components with more than two groups reactive with isocyanates represent, at most, 3% by weight of the sum of compounds II, III and V. More preferably, polyols are not present in the prepolymer of the invention.

Examples of suitable monools (monofunctional alcohols) V are cycloaliphatic or aliphatic C1-C8 alcohols, linear or branched, glycol monoethers, alkoxylated alcohols and mixtures thereof.

Isopropanol, butyl alcohol, 1-hexanol, and 2-ethyl-1-hexanol are examples of suitable alcohols. Examples of suitable glycol monoethers include 2-methoxyethanol, 2-propoxyethanol, 2-ethoxyethanol, ethylene glycol monobutyl ether, propylene glycol monobutyl ether, diethylene glycol monomethyl ether, dipropylene glycol monomethyl ether, diethylene glycol monoethyl ether, dipropylene glycol monoethyl ether, diethylene glycol monobutyl ether, dipropylene glycol monobutyl ether, triethylene glycol monoethyl ether, triethylene glycol monobutyl ether, tripropylene glycol monobutyl ether, and the like.

Examples of suitable alkoxylated alcohols are methoxy and ethoxy polyethylene glycol, having 5 to 30 oxyethylene units.

Examples of usable primary or secondary V-amines are butyl amine, isobutyl amine, isopropyl amine, and diethanol amine. Propoxylated and/or ethoxylated monoamines (e.g., Jeffamine? M series from Huntsman) are also useful as V-amines.

Examples of V-thiols are methanethiol, ethanethiol, isopropanethiol, n-propanethiol, n-butanethiol, isobutanethiol, sec-butanethiol, tert-butanethiol.

Component VI is an organic polyisocyanate having an average functionality greater than 2, preferably between 2.3 and 4.

Examples of suitable polyisocyanate VI are hexamethylene diisocyanate isocyanurate, hexamethylene diisocyanate biuret, isophorone diisocyanate isocyanurate, and isophorone diisocyanate biuret.

Preferably, components V and VI are not used in the preparation of the prepolymer.

In the prepolymer, the molar ratio of the sum of the NCO groups of compounds I and VI to the sum of the groups reactive with respect to the NCO groups of compounds II, III, IV and V is between 1.2 and 2.2, preferably between 1.5 and 2.0.

The molar ratio of diols III to II is between 1.5 and 5.0, preferably between 2.0 and 4.0, more preferably between 2.2 and 3.6.

The amount of diols II and III represents 80 to 100% by weight, preferably 85 to 100% by weight, more preferably 95 to 100% by weight, more preferably 100% by weight of the sum of compounds II, III, and V.

The total amount of carboxyl and carboxylate groups in the polyurethane is measured in milliequivalents (meq) of -COOH and -COO<- > groups per 100 g of prepolymer, as a dry substance. In the anionic polyurethanes of the present invention, this value is between 35 and 60 meq/100 g, preferably between 40 and 55 meq/100 g.

The anionic polyurethane of the invention can be prepared according to a process comprising the following steps:

i. prepare a prepolymer by reacting ingredients I?VI in the proportions described above;

ii. extend the prepolymer chain in water.

In the process, step i is preferably carried out at a temperature between 40 and 110 °C. A catalyst can be used to accelerate the reaction.

When diol IV is used in acid form, the prepolymer obtained at the end of step i is normally neutralized at temperatures below 100 °C, preferably with tertiary amines, such as N-alkyl morpholines, trialkyl amines, alkyl alkanolamines, trialkanol amines, and mixtures thereof. Particularly suitable for this purpose are triethyl amine, dimethyl ethanolamine, and N-ethyl morpholine. Advantageously, diol IV is neutralized before the reaction of step i takes place. Neutralization can also be performed during the prepolymer's dispersion in water (step ii).

Step II can be carried out in water (or in water and a neutralizing agent) under mechanical stirring. In step II, chain elongation is preferably carried out in water in the presence of diamines as chain extenders. Diamines that can be used as chain extenders are diamines having at least two hydrogens active towards the NCO group, such as hydrazine, ethylene diamine, piperazine, 1,5-pentane diamine, 1,6-hexane diamine, isophorone diamine, diethylene triamine, and mixtures thereof. Typically, in step II, the equivalent amount of chain extender ranges from 30 to 100%, preferably 70 to 90%, of the free NCO groups contained in the prepolymer.

Step ii of the invention's process is preferably carried out by percolating the prepolymer in water. After dispersing the prepolymer, the chain extender is added to obtain the anionic polyurethane of the invention. The temperature of this step is preferably kept below 40°C. In one embodiment, the process is conducted in the presence of a solvent that can be distilled at the end of the preparation. Examples of suitable solvents are methyl ethyl ketone (MEK), methyl isobutyl ketone, acetone, and ethyl acetate.

In another embodiment, the process is conducted in the presence of the coalescing agent b of the invention.

The aqueous polyurethane dispersion of the invention may contain up to 20% by weight, preferably between 3 and 17% by weight, of a suitable coalescing agent b. Coalescing agents, also known as cosolvents, are high-boiling solvents that function as temporary plasticizers for polymer particles by reducing the minimum film formation temperature (MFFT). Coalescing agents enable the formation of polymer films at room temperature, particularly for polymers that exhibit a MFFT higher than room temperature.

Coalescing agents suitable for carrying out the present invention may be selected from water-soluble or partially water-soluble coalescing agents.

Examples of water-soluble coalescing agents are glycols and glycol ethers, such as butyl glycol, butyl diglycol, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol monobutyl ether, dipropylene glycol dimethyl ether, and mixtures thereof. Other suitable water-soluble coalescing agents include N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-butyl-2-pyrrolidone, and mixtures thereof.

Partially water-soluble coalescing agents have low solubility in water, usually less than 10%, preferably less than 5%. Examples of suitable partially water-soluble coalescing agents are glycol esters, such as 2,2,4-trimethyl-1,3-pentanediol diisobutyrate and monoesters of propylene glycol with C6-C10 aliphatic acids, and esters of polycarboxylic acids with C1-C4 alcohols.

N-ethyl-2-pyrrolidone and dipropylene glycol monomethyl ether are the preferred coalescing agents. Dipropylene glycol monomethyl ether is the most preferred coalescing agent.

If the coalescing agent b is not already present at the end of the anionic polyurethane preparation process, it is simply added at the end of the process described above.

The aqueous polyurethane dispersion of the invention may further comprise between 1 and 15% by weight, preferably between 3 and 12% by weight, as the active substance, of an aqueous dispersion of natural or synthetic polymers, compatible with the anionic polyurethane of the invention, having a surface energy greater than 44 mN/m. Examples of suitable aqueous dispersions of synthetic polymers are dispersions of polyurethane, acrylic-polyurethane hybrid, and acrylic resins, such as Alberdingk® AC 4600 (from Alberdingk Boley) or Joncryl® FLX 5201 (from BASF). These polymers with a surface energy greater than 44 mN/m help to increase the surface energy of the base coat, improving its printability.

According to one embodiment of the invention, the aqueous polyurethane dispersion can be thickened by adding 0.1 to 5 wt. %, preferably 0.3 to 2 wt. %, of a thickener, preferably a polyurethane thickener.

The Brookfield viscosity at 25°C of the aqueous polyurethane dispersion of the present invention is generally between 10 and 1,000 mPa*s, preferably between 10 and 500 mPa*s, more preferably between 15 and 150 mPa*s.

The aqueous polyurethane dispersions of the present invention are stable, i.e. homogeneous or redispersible, and do not develop excessive viscosity during storage.

Aqueous polyurethane dispersions are defined as homogeneous when they remain visibly homogeneous when stored in a closed container at 50°C for at least 30 days.

Aqueous polyurethane dispersions are defined as redispersible when, when stored in a closed container at 50°C for at least 30 days, they develop a sediment that can be dispersible by simply manually shaking the container.

The aqueous polyurethane dispersions described above are film-forming and can be advantageously used as a base coating for the transfer-coating of substrates, such as paper, plastics such as polycarbonates and PVC, wood and metals according to the method of the invention.

In step ii of the method of the invention, the aqueous polyurethane dispersions are applied to the flexible support substrate using known methods. Examples of such application methods include coating (e.g., brushing, hand rollers, sponges, or squeegees), spraying (e.g., air spray, airless spray, hot spray, and electrostatic induction spray), showering, dipping, curtain coating, roller coating, and reverse roller coating, rotogravure and flexographic printing, and digital printing, such as inkjet printing.

The flexible substrate of the inventive method preferably comprises a plastic film, such as polyethylene terephthalate (PET), polypropylene, polyethylene, polyvinyl chloride, polyamide, and polycarbonate. Silicone-coated paper or polytetrafluoroethylene is also suitable as a support substrate. Polyethylene terephthalate, especially untreated polyethylene, is preferred.

Optionally, intermediate layers can be applied between the base coat and the adhesive to achieve decorative and/or technical effects. For example, the base coat can be printed, or a thin metallic layer can be applied over the base coat to provide a barrier or holographic effect.

Any adhesive commonly used in the industry can be used in step iii) of the method of the invention. Particularly advantageous adhesives include those based on acrylates, polyurethanes, and combinations thereof. Examples of suitable adhesives include Herberts HP 1020-F Purbinder C1 from Bostik, Epotal CF 605X Basonat LR 9056 from BASF, and similar products.

The flexible substrate preferably has a thickness of between about 10 and 100 μm, particularly between 12 and 40 μm. The film obtained by applying and drying the aqueous polyurethane dispersions of the invention to the flexible substrate preferably has a dry thickness of between about 0.2 and 15 μm, preferably between about 0.4 and 5.0 μm, and the adhesive film normally has a dry thickness of between about 1 and 15 μm, preferably between about 2 and 5 μm.

Drying of the flexible substrate after application of the coating layer(s) can take place between 0 and 160°C, preferably between 40 and 140°C. Drying at temperatures other than room temperature can be done in an oven; alternatively, drying can be supported by infrared and/or near-infrared radiation or by ventilation.

In step iv) of the process, the removable multilayer film (transfer film) is adhered to a receiving substrate, leaving the flexible substrate on the outside. If necessary, pressure can be applied to achieve better adhesion between the multilayer film and the surface of the receiving substrate.

Finally, in step v), the flexible substrate is removed from the multilayer film by peeling.

The transfer-coating method of the invention is particularly suitable for the preparation of coated paper and plastic films for use as packaging materials, preferably as food packaging materials.

The following examples show the preparation and transfer-coating application of aqueous polyurethane dispersions according to the present invention and of comparative dispersions, together with the main characterizations of the products obtained.

EXAMPLES

Characterization Methods

The residual isocyanate group content in the prepolymer was determined by reacting the NCO groups of the prepolymer with excess di-n-butyl amine in ethyl acetate and by titrating the excess unreacted di-n-butyl amine in isopropanol with HCl. The residual content value is calculated as % NCO groups by weight.

The dry matter was determined with an IR dryer, Mettler Toledo HB 43, set at a temperature of 160 °C.

The stability of the aqueous polyurethane dispersions was determined by an oven aging test for 30 days at 50 °C.

Ingredients

The following ingredients were used in the examples described below:

IPDI = isophorone diisocyanate;

H12MDI = hydrogenated diphenylmethane 4,4'-diisocyanate;

PTMEG = polytetramethylene glycol, MW = 1000 g/mol;

BDO = 1,4-butanediol;

CHDM = 1,4-cyclohexane dimethanol;

DMPA = 2,2-dimethylol propionic acid;

EDA = ethylene diamine;

Hydr = hydrazine hydrate;

TEA = triethyl amine;

NEP = n-ethyl-2-pyrrolidone;

DPM = dipropylene glycol monomethyl ether;

MEK = methyl ethyl ketone;

MPEG = methoxy polyethylene glycol, MW = 750 g/mol;

Pol1 = aqueous dispersion of a polyurethane from adipic acid and 1-4-butanediol polyester diol, methoxy polyethylene glycol, IPDI, DMPA and hydrazine as a chain extender; 30 wt % dry substance, surface energy 46 mN/m;

Pol2 = aqueous dispersion of a polyurethane from adipic acid/phthalic acid and 1,6-hexanediol polyester diol, H12MDI, DMPA, methoxy polyethylene glycol and ethylene diamine as chain extender; 35 wt% dry substance, surface energy >48 mN/m.

Example 1 (comparative)

In a reactor equipped with a thermometer, stirrer and condenser, under nitrogen atmosphere, 19.02 g (0.13 mol) of 1,4-CHDM, 131.88 g (0.13 mol) of PTMEG and 16.32 g of DMPA (0.12 mol) and 99.72 g of NEP were charged and heated to 40 μC. Subsequently, 137.07 g of IPDI (0.617 mol) were added and the mixture was homogenized for 10 min at 40 μC. The reaction mixture was then heated to 90 μC and held at this temperature until a residual NCO group content in the prepolymer was approximately 4.8 wt %.

Once the required NCO group value was reached, the reaction mass was diluted with 50.24 g of NEP (coalescing agent b). The prepolymer solution was cooled to 70 °C and 11.32 g of TEA were added while stirring.

A 365 g aliquot of the prepolymer solution was then dispersed in 389.85 g of demineralized water under vigorous stirring and was subsequently extended with 29.12 g (0.909 mol) of Hydr.

The aqueous polyurethane dispersion thus obtained had a dry matter content of 31% by weight.

Examples 2 (comparative) and 3-7

The aqueous polyurethane dispersions of Examples 2-7 were prepared following the same procedure as in Example 1 using the reagents and molar ratios reported in Table 1.

Example 8

In a reactor equipped with a thermometer, stirrer and condenser, 18.13 g (0.201 mol) of BDO, 86.203 g (0.086 mol) of PTMEG and 20.20 g of DMPA (0.15 mol) and 106.74 g of MEK were charged under nitrogen atmosphere and brought to 40 ?C.

170.35 g of IPDI (0.76 mol) was added with stirring and homogenized for 10 min at 40 ?C. The reaction mass was then heated and held at 80 ?C until a residual NCO group content in the prepolymer of approximately 6.9 wt.%.

After cooling to 70 °C, 14.46 g of TEA was added while stirring. A 316 g aliquot of the neutralized prepolymer was dispersed in 361.71 g of demineralized water while stirring vigorously and was subsequently extended with 13.62 g (0.227 mol) of EDA diluted in 68.11 g of water.

At the end of the process, the MEK was distilled under reduced pressure.

The aqueous polyurethane dispersion thus obtained had a dry matter content of 35% by weight.

Examples 9-18

The aqueous polyurethane dispersions of Examples 9-18 were prepared following the same procedure as in Example 8 using the reagents and molar ratios given in Tables 2 and 3.

After distillation of the MEK, different amounts of DPM were added to the dispersions of Examples 9-11, 13-15 and 17-18 (see Tables 2 and 3).

Example 19

In a reactor equipped with a thermometer, stirrer and condenser, 16.71 g (0.185 mol) of BDO, 79.50 g (0.079 mol) of PTMEG and 18.63 g of DMPA (0.14 mol) and 103.31 g of MEK were charged under nitrogen atmosphere and heated to 40 °C. 170.55 g of IPDI (0.77 mol) were added while stirring and the mixture was homogenized for 10 min at 40 °C. The reaction mass was then heated and held at 80 °C until a residual NCO group content of approximately 8.0 wt % was achieved.

Subsequently, 13.628 g of MPEG (0.018 mol) was added. The reaction mass was then heated and held at 80 °C until the residual NCO group content in the prepolymer was approximately 7.4 wt %.

After cooling to 70°C, 13.34 g of TEA was added while stirring. An aliquot of 355.67 g of the neutralized prepolymer was dispersed in 387.66 g of demineralized water while stirring vigorously and was subsequently extended with 15.87 g (0.26 mol) of EDA diluted in 79.34 g of water.

At the end of the process, the MEK was distilled under reduced pressure.

The aqueous polyurethane dispersion thus obtained had a dry matter content of 35% by weight.

To evaluate the properties of the polyurethanes of the invention, the aqueous polyurethane dispersions of the examples were applied to the flexible support substrate (untreated A4 BOPET sheet with a surface energy of 40 mN/m) using a K Control Coater. The resulting films were then dried at 80°C for 10 seconds in a convection oven. The dry thickness of the base coat was approximately 0.5 μm, unless otherwise specified.

To evaluate the release properties of the base coat obtained from the aqueous polyurethane dispersions in the examples, a strip of adhesive tape (Intertape TM 51596 from Synflex Insulation Systems) was carefully applied to the top of each removable primer. After 30 seconds, the primer was peeled off by pulling firmly on the tape. The release rating was expressed on a scale from 1 to 10. 10 indicates perfect release with clean, crisp edges and a long tail of the released coating, 1 indicates no release (complete adhesion to the substrate). A value of 7 or higher indicates no primer residue on the surface of the flexible substrate. These values are considered industrially acceptable.

The surface energy of polyurethanes (SE in mN/m) was determined on films obtained with the same procedure described above. Dyne Test pens (from ) were used for the determinations in a range of values from 32 to 48 mN/m.

Tables 1-3 report the molar ratio of diol III to diol II (Diol Ratio) and the molar ratio of NCO groups to reactive NCO groups (NCO/Diols) used to prepare the polyurethane dispersions of Examples 1-18. Tables 1-3 also show the DMPA content in meq/100 g prepolymer (as dry substance), the residual NCO group content, in % by weight, before chain extension (% NCO), the % dry substance content (% Dry) before dilution with coalescing agent, and the amount of coalescing agent added to the polyurethane dispersions in grams per 100 grams of dispersion (Cosolvent).

Table 1

*comparative

Table 2

*comparative

** dried primer thickness equal to 2 ?m Table 3

** Dried primer thickness of 2 μm. The aqueous dispersions in Examples 1-19 are stable, i.e., homogeneous or redispersible, for 30 days at 50°C.

The results of the release test and the surface energy test (in mN/m) on the aqueous polyurethane dispersions of Examples 1-18, reported in Tables 1-3 demonstrate that the aqueous polyurethane dispersions of the invention give films with good release properties, superior to those of the comparative Examples, and adequate surface energy.

Examples 20-23

To evaluate the possibility of increasing the surface energy of the removable film, 100 grams of the aqueous polyurethane dispersion from Example 12 were mixed under stirring at 300 rpm in varying amounts with Pol1 or Pol2. After a few minutes, the polymer mixtures were diluted with DPM. The resulting aqueous polyurethane dispersions were homogenized for 30 minutes and tested.

Table 4 reports the quantities in grams of Pol1, Pol2, as active material, and DPM added to the aqueous polyurethane dispersion of Example 12.

Table 4

Comparison of the release test and surface energy test results (in mN/m) on the aqueous polyurethane dispersions of Examples 19-25 with those of Examples 12 and 13 demonstrates that the addition of a polymer with higher surface energy can increase both the ease of release and the surface energy of the removable primer.

Claims (9)

1) Aqueous polyurethane dispersion, useful as a primer for transfer coating, comprising: a) from 15 to 50% by weight of an anionic polyurethane prepared by chain extension in water of a prepolymer obtained by reacting: I) an aliphatic diisocyanate and/or a cycloaliphatic diisocyanate; II) a non-ionic polyether diol with a molecular weight between 500 and 3,000 g/mol; III) a non-ionic diol with a molecular weight less than 300 g/mol; IV) a diol having at least one carboxyl or carboxylate group; V) possibly, a compound other than diols II, III and IV having one or more? groups reactive towards isocyanate groups (NCO); VI) if applicable, an aliphatic or cycloaliphatic polyisocyanate having an average NCO functionality greater than 2; in proportions such that: A) the molar ratio of the sum of the NCO groups of compounds I and VI to the sum of the groups reactive with respect to the NCO groups of compounds II, III, IV and V is between 1.2 and 2.2; B) the molar ratio of the diols III and II is between 1.5 and 5.0; C) the amount of diols II and III represents 80 to 100% by weight of the sum of compounds II, III and V; D) the amount of diol IV is such that the prepolymer contains 35 to 60 meq/100 g of prepolymer, as dry matter, of carboxylic or carboxylate groups; E) the amount of polyisocyanate VI, if present, does not exceed 3% by weight of the sum of compounds I and VI; b) possibly, up to 20% by weight of a suitable coalescing agent. 2) The aqueous polyurethane dispersion of claim 1, wherein the molar ratio between diols III) and II) is between 2.0 and 4.0. 3) The aqueous polyurethane dispersion of claim 2, wherein the molar ratio of the sum of the NCO groups of I and VI to the sum of the groups reactive towards the NCO groups of II, III, IV and V is between 1.5 and 2.0. 4) The aqueous polyurethane dispersion of claim 1, wherein the non-ionic polyether diol II) has a molecular weight between 700 and 2,200 g/mol. 5) The aqueous polyurethane dispersion of claim 1, wherein the diol III has a molecular weight less than 250 g/mol. 6) The aqueous polyurethane dispersion of claim 1, wherein the diol IV is dimethylol propionic acid, dimethylol butanoic acid, a salt thereof, or a mixture thereof. 7) The aqueous polyurethane dispersion of claim 1, wherein the isocyanate is isophorone diisocyanate, hydrogenated diphenylmethane-4,4'-diisocyanate or mixtures thereof. 8) The aqueous polyurethane dispersion of claim 7, wherein the coalescing agent is N-ethyl-2-pyrrolidone, dipropylene glycol monomethyl ether or mixtures thereof. 9) A method of transfer-coating a receiving substrate comprising: i) prepare an aqueous polyurethane dispersion comprising: a) from 15 to 50% by weight of an anionic polyurethane prepared by chain extension in water of a prepolymer obtained by reacting: I) an aliphatic diisocyanate and/or a cycloaliphatic diisocyanate; II) a non-ionic polyether diol with a molecular weight between 500 and 3,000 g/mol; III) a non-ionic diol with a molecular weight less than 300 g/mol; IV) a diol having at least one carboxyl or carboxylate group; V) possibly, a compound other than diols II, III and IV having one or more? groups reactive towards NCO groups; VI) if applicable, an aliphatic or cycloaliphatic polyisocyanate having an average isocyanate functionality greater than 2; in proportions such that: A) the molar ratio of the sum of the NCO groups of compounds I and VI to the sum of the groups reactive with respect to the NCO groups of compounds II, III, IV and V is between 1.2 and 2.2; B) the molar ratio of the diols III and II is between 1.5 and 5.0; C) the amount of diols II and III represents 80 to 100% by weight of the sum of compounds II, III and V; D) the amount of diol IV is such that the prepolymer contains 35 to 60 meq/100 g of prepolymer, as dry matter, of carboxylic or carboxylate groups; E) the amount of polyisocyanate VI, if present, does not exceed 3% by weight of the sum of compounds I and VI; b) optionally, up to 20% by weight of a suitable coalescing agent. ii) applying said aqueous polyurethane dispersion to the surface of a flexible substrate to form a removable primer; iii) if necessary, add an intermediate layer over the removable primer; iv) apply an adhesive over the removable primer or over the intermediate layer, if present, to prepare a removable multilayer film (transfer-film); v) adhering the removable multilayer to a receiving substrate; and vi) remove the flexible substrate.