Aliphatic Polyurethanes Containing Acryloyl Groups and a Process for Their Preparation

1. Field of the Invention

The present invention relates to certain polyurethanes which contain acryloyl groups and can be converted into crosslinked products by radiation curing and to a process for their preparation.

2. Description of the Prior Art

Polyurethanes which contain acryloyl groups and are known and may be obtained, as described, for example, in DE-OS (German Published Specification) 1,644,798, DE-OS 2,115,373 or DE-OS 2,737,406, as the reaction product of, inter alia, polyisocyanates and hydroxyalkyl acrylates. They are cured with the aid of high-energy rays, such as UV rays or electron beams, and are used for coating, for example, wood or for the production of coatings for graphical purposes.

A wide variety of polyisocyanates can be employed for the preparation of the urethane acrylates. If the production of surface coating films which are resistant to yellowing is desired, (cyclo)aliphatic polyisocyanates are employed for the preparation of the urethane acrylates. Although hexamethylene diisocyanate, inter alia, is mentioned in DE-OS (German Published Specification) 2,115,373 as a suitable starting diisocyanate, it can be said that this is a purely hypothetical disclosure, since prior to the present invention it has not been possible to employ this inexpensive diisocyanate in practice because of the crystallinity of the urethane acrylates formed, which prohibits their use as surface coating binders.

Therefore, an object of the present invention is to provide aliphatic polyurethanes containing acryloyl groups which are prepared from hexamethylene diisocyanate (HDI) as the essential starting diisocyanate and which nevertheless are liquid and crosslink rapidly under UV irradiation.

This object has been achieved by the aliphatic polyurethanes containing acryloyl groups according to the invention which are described in more detail below.

The present invention relates to aliphatic polyurethanes which contain acryloyl groups and are based on the reaction product (at an NCO/OH equivalent ratio of 0.5:1 to 1:1) of

a) 1.0 mole of a diisocyanate component containing at least 50 equivalent percent, based on NCO equivalents, of hexamethylene diisocyanate with

b) 0.5 to 0.85 moles of at least one monohydric alcohol having a molecular weight of 116 to 144 and containing acryloyl groups,

c) 0.15 to 0 5 moles of a monohydric alcohol having a molecular weight of 154 to 2000 and containing ester groups or a mixture of alcohols having an average molecular weight of 154 to 2000 and an average hydroxyl functionality of 0.8 to 1.5 and containing ester groups and

d) 0.3 to 0.5 moles of at least one polyhydric alcohol having a molecular weight of 62 to 2000.

The present invention also relates to a process for the preparation of aliphatic polyurethanes containing acryloyl groups by reacting at an NCO/OH equivalent ratio of 0.5:1 to 1:1

a) 1.0 mole of a diisocyanate component containing at least 50 equivalent percent, based on NCO equivalents, of hexamethylene diisocyanate with

b) 0.5 to 0.85 moles of at least one monohydric alcohol having a molecular weight of 116 to 144 and containing acryloyl groups,

c) 0.15 to 0.5 moles of a monohydric alcohol having a molecular weight of 176 to 2000 and containing ester groups or a mixture of alcohols having an average molecular weight of 154 to 2000 and an average hydroxyl functionality of 0.8 to 1.5 and containing ester groups and

d) 0.3 to 0.5 moles of at least one polyhydric alcohol having a molecular weight of 62 to 2000.

Polyisocyanate component a) contains at least 50 equivalent percent, based on NCO equivalents, of HDI. Polyisocyanate component a) can also contain up to 50% NCO equivalent percent of other polyisocyanates containing aliphatically and/or cycloaliphatically bonded isocyanate groups. These other polyisocyanates include 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (IPDI), polyisocyanates based on IPDI or HDI which contain isocyanurate groups and polyisocyanates based on IPDI or HDI and polyhydric alcohols (such as trimethylolpropane and/or propylene glycol) and contain urethane groups. Polyisocyanate component a) preferably contains at least 80 NCO equivalent percent, more preferably 100 NCO equivalent percent, of hexamethylene diisocyanate (HDI).

Component b) is based on a monohydric alcohol containing acryloyl groups and mixtures of these alcohols. These are understood to be esters containing on average one free hydroxyl group per molecule and prepared by reacting acrylic acid with dihydric alcohols. The esters have a molecular weight of 116 to 144. Examples of such ethylenically unsaturated esters include hydroxyethyl acrylate, 1-methyl-2-hydroxyethylacrylate, 2- and 3-hydroxypropyl acrylate and 2-, 3- and 4-hydroxybutyl acrylate.

Component c) is based on a monohydric alcohol containing ester groups or a mixture of alcohols containing ester groups and having an average hydroxyl functionality of 0.8 to 1.5 and an average molecular weight of 154 to 2000, preferably 176 to 1000. Component c) can contain olefinically unsaturated double bonds (for example because of the use of acrylic acid as a component for the preparation of the ester alcohols), but can also be exclusively prepared from from saturated compounds. Examples of suitable compounds c) include ester mixtures having an average hydroxyl functionality of 0.8 to 1.5, which can be obtained, for example, by the reaction of acetic acid, acetic anhydride or acrylic acid with polyhydric alcohols at an equivalent ratio of carboxyl groups to hydroxyl groups of 1.5:3 to 2.2:3, preferably 2:3. An equivalent amount of an acid anhydride may be used. Examples of polyhydric alcohols include trimethylolpropane, glycerol and the alkoxylation products of polyhydric alcohols (e.g., the addition product of 4 moles of ethylene oxide to 1 mole of trimethylolpropane).

The polyols d) include polyhydric alcohols having a molecular weight of 62 to 2000, preferably alcohols having a molecular weight of 92 to 200 which are at least trihydric such as glycerol, trimethylolpropane, pentaerythritol and sorbitol. Also suitable are polyether and polyester polyols having molecular weights of up to 2000, preferably up to 1000. The polyether polyols may be obtained by the alkoxylation of monomeric alcohols of the type previously mentioned. The polyester polyols may be obtained by the reaction of polyhydric alcohols and polybasic acids or anhydrides of polybasic acids in known manner. It is also possible to use relatively small amounts of diols, such as ethylene glycol, thiodiglycol or hexanediol. However, the diols should be used in amounts of less than 50 hydroxyl equivalent percent, based on the hydroxyl equivalents of component d). Otherwise, it is not possible to obtain the desired properties of the polyurethanes.

The polyurethanes can be prepared by reaction of the starting components in the presence or absence of solvents which are inert towards isocyanates such as acetone, ethyl acetate, butyl acetate, low molecular weight esters of (meth)acrylic acids or mixtures of such solvents. The reaction temperature is preferably 20.degree. to 100.degree. C., more preferably 40.degree. to 80.degree. C. The reaction is preferably carried out as a batch process with the optional addition of suitable catalysts. The reaction temperature is maintained until no further free isocyanate groups are present. It accordance with one embodiment polyisocyanate component a) is initially introduced into the reaction vessel, components b) and c) are added together or in succession (the sequence is not critical), and component d) is then added. In accordance with another embodiment components b), c) and d) are initially introduced into the reaction vessel and the polyisocyanate component a) is subsequently added.

In all cases, the reaction is carried out under mild conditions at an NCO/OH equivalent ratio of 0.5:1 to 1:1 until the NCO content has fallen to below 0.1% by weight.

The addition reaction resulting in the polyurethane can be accelerated in known manner by the addition of a suitable catalyst, e.g., tin octoate, dibutyltin dilaurate or tertiary amines such as dimethylbenzylamine. Alternatively, the urethane acrylate obtained as the reaction product can be protected from premature and undesirable polymerization by the addition of suitable inhibitors and antioxidants such as phenols and/or hydroquinones, in amounts of in each case 0.001 to 0.3% by weight, based on the urethane acrylate. These auxiliaries can be added before, during and/or after the reaction.

The surface coatings prepared from the products according to the invention can contain the known auxiliaries and additives of surface coating technology.

These include fillers, pigments, dyestuffs, thixotropic agents, glossing agents, matting agents and flow control agents. These additives and auxiliaries can be used in the customary amounts. The polyurethanes containing acryloyl groups can also be dissolved in known surface coating solvents, mixed with copolymerizable monomers or employed as an emulsion in water.

Examples of suitable solvents include butyl acetate, cyclohexane, acetone, toluene or mixtures of such solvents.

Examples of suitable copolymerizable monomers are organic compounds which contain at least one copolymerizable olefinic double bond per molecule and have a viscosity of not more than 500 mPa.s at 23.degree. C. Suitable monomers include styrene, 1,6-hexanediol diacrylate, trimethylolpropane triacrylate or N-vinylpyrrolidone.

The inert solvents and also the copolymerizable monomers can be employed in amounts in each case of up to 100% by weight, preferably up to 50% by weight, based on the weight of the polyurethanes containing acryloyl groups.

It is also possible to simultaneously use both inert solvents and copolymerizable monomer.

If the polyurethanes containing acryloyl groups are to be processed from an aqueous emulsion, the emulsion can be prepared, for example, with the aid of external emulsifiers and if appropriate the customary auxiliaries used in emulsion technology.

Emulsifiers which are suitable for this purpose are known and described, for example, in Ullmanns Encyclopadie der tech. Chemie (Ullmanns Encyclopaedia of Industrial Chemistry), Volume 10, 4th Edition, chapter on Emulsions, page 449 et seq.

Preferred emulsifiers are copolymerizable polyurethane emulsifiers containing acryloyl groups, such as those described in DE-OS (German Published Specification) 3,900,257.

The aqueous emulsions contain 10 to 70% by weight, preferably 30 to 70% by weight, of the polyurethanes containing acryloyl groups according to the invention. The emulsions can be prepared by stirring water into the mixture of polyurethane acrylate according to the invention and emulsifier, for example by simple stirring or by means of a dissolver.

To form a finely divided emulsion, it is advantageous to add the water in portions at temperatures below 30.degree. C. Oil-in-water emulsions are formed under optimum shear.

In all the variants of the process according to the invention, the curing (if appropriate after evaporation of volatile auxiliaries, such as inert solvents or water) is carried out by means of high-energy radiation, such as UV light, electron beams or gamma rays.

In the case of crosslinking by means of UV irradiation, it is necessary to add photoinitiators to the coating compositions. Suitable photoinitiators are known and include the compounds described, for example, in the monograph by J. Korsar, Light-Sensitive Systems, J. Wiley & Sons, New-York - London - Sydney 1965.

Preferred photoinitiators are benzoin ethers such as benzoin isopropyl ether, benzil ketals such as benzil dimethyl ketal, and hydroxyalkylphenones such as 2-hydroxy-2-methylphenylpropan-1-one.

The photoinitiators are employed in amounts of 0.1 to 10% by weight, preferably 0.1 to 5% by weight, based on the weight of the reaction products, depending on the intended purpose of the composition according to the invention. The photoinitiators can be used individually or in combination with one another to obtain advantageous synergistic effects.

The coating compositions or surface coatings prepared using the polyurethanes according to the invention as binders are suitable for coating diverse substrates such as paper, cardboard, leather, wood, plastics, non-wovens, textiles, ceramic materials, mineral materials, glass, metal, imitation leather and photographic materials (for example, paper provided with a photographic layer).

In the following examples, all the percentages and amounts are by weight unless otherwise indicated.

The amount of component a) set forth in Table 1 was dissolved, together with 0.2% of tin dioctoate and 0.1% of 2,6-di-t-butyl-p-cresol, in butyl acetate (in an amount to provide a solids content of 80%) and the solution was heated to 50.degree. C. while air was passed through. The amount of component b) set forth in Table 1 was then added dropwise, followed by the amount of component c) and finally the amount of component d). During this procedure, the reaction temperature was kept at a maximum of 60.degree. C., if necessary by external cooling. The mixture was then kept at 60.degree. C. until the NCO content had fallen to less than 0.1%

The product from Comparison Example 4 was not suitable for use as a coating because of its crystallinity. For clarity, the numbering of the use examples corresponds to the previous numbering of the preparation examples.

The products of the examples according to the invention and Comparison Example 5 were adjusted with butyl acetate to a processing viscosity of about 50 seconds (DIN cup 4 mm, 23.degree. C.), and in each case 5% of benzophenone, based on the weight of the urethane acrylate, was added. After the surface coating films had been applied to cardboard (application thickness of 10 .mu.m), the coated substrate was passed through the beam of a Hanovia lamp (80 W/cm, 10 cm distance). The value given for the reactivity in Table 2 indicates the belt speed at which a coating which was resistant to solvents and scratching was achieved.

"Resistant to solvents" in this case means that the surface coating film still looked perfect after at least 20 double rubs with a cloth soaked in butyl acetate.

Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.