NO143666B - COATING AGENT, SPECIFICALLY IN POWDER FORM, CONTAINING A POLYMER WITH REACTIVE GROUPS AND A CROSS-BONDING AGENT WITH BLOCKED ISOCYANATE GROUPS - Google Patents

Description

This invention relates to a coating agent, in particular a powder coating agent, which consists of a polymer, preferably an acrylic resin, and a cross-linking agent which has isocyanate reactivity.

The known coating agents, especially those in powder form,

, have little ability to flow together and level out during curing. The film formation requires high temperatures, and the cross-linking even higher, around 190-260°C. Furthermore, the known powders have a tendency to clump together during storage even at temperatures of 21-27°C, and the resulting coatings are brittle and have poor gloss and adhesiveness.

The solution to these problems would be to use a cross-linking agent which is as pure as possible and which is compatible with the reactive resin. The cross-linking agent should be able to provide flowability at a relatively low temperature, and act as a plasticizer for the resin, and still be stable during storage. It should also be able to cross-link the resin at as low a temperature as practically possible and, after cross-linking, enter the coating film as an internal plasticizer.'

US patent 3,317,463 deals with the use of blocked isocyanate mixtures (blocked with hindered phenols) in the production of urethanes. According to example III B, a coating is produced which is cured for 60 minutes at 160°C.

US patent 3,583,943 deals with the blocking of an isocyanate group with a ketoxime.

US Patent 3,621,000 relates to an isocyanate which is blocked with a bifunctional polyether.

It is a difficulty with all known powder mixtures that the coating cannot be quickly hardened without being heated to a high temperature.

However, the coating agent, especially the powder coating agent, according to the present invention can be cured at a relatively low temperature. There has been a need to achieve such

"low-temperature curing", and yet avoid agglomeration of the powder. The powder should have the ability to flow together into a film before curing. For reasons of energy saving, among other things, it is preferred that the powder mixture has as low a curing or cross-linking temperature as possible. Therefore, the area for softening

temperature be relatively narrow. Furthermore, when using a blocked isocyanate as a cross-linking agent, it is preferred that when the blocking agent is split off, there should be little or no gas formation.

The object of the present invention is thus a coating agent for the production of solvent-resistant coatings, which firstly comprises a polymer with isocyanate-reactive groups, and secondly a cross-linking agent that contains at least two blocked isocyanate groups per molecule, and which is made from isophorone diisocyanate, a blocking agent and a compound with at least two isocyanate-reactive groups, the last-mentioned compound having a softening effect on the coating agent both during and after curing. The coating agent is characterized in that the cross-linking agent is prepared by first reacting the isophorone diisocyanate with the blocking agent so that the most reactive of the two isocyanate groups is blocked, and then reacting with the softening compound.

From US patent 3,867,347 it is true that a coating powder based on a hydroxyl-containing acrylic resin and a blocked diisocyanate as cross-linking agent is known. Several examples show a cross-linking agent which is the reaction product ("adduct") of isophorone diisocyanate (IPDI) and hexanediol, blocked with caprolactam. However, there is no indication that the blocking should be carried out before the adduct formation, and the finished, blocked cross-linking agent is already added to the monomer batch which is to give the acrylic resin. However, from NO patent 137 553, which deals with a similar technique, it is known to melt the blocked ("masked") isophorone diisocyanate together with the hydroxyl-containing resin, after which the solidified mass is pulverized in an impact mill. Here too, IPDI can be "extended" with polyols. A hydroxyl-containing polyester is used as resin. The advantage of the present invention is that the blocking agent is split off from the most reactive of the isocyanate groups, whereby curing occurs more quickly. According to the two above-mentioned patents, the isocyanate groups are first reacted with the polyol (the internal plasticizer), and thereby the most reactive isocyanate groups are bound to this. Hardening will then not be as fast.

As stated, the coating agent is preferably a powder mixture, usually with a range of particle sizes from about 0.1 to about 250 µm, preferably 1-150 µm and even more preferably from about 10 to 100 µm (average less than 35 µm).

The substrate on which the coating is applied can be non-primed metal, plastic, glass and the like. It should be understood that on non-conductive surfaces a conductive deposit, such as carbon or metal, can be placed before application. This is especially the case if the powders are to be deposited electrostatically. If one does not wish to apply such a conductive layer, it will often be possible to heat the substrate so that the particles can stick to it. The metallic substrate can be cleaned in the usual way, and the powder can be applied directly to the substrate, but in some cases it has been found desirable, in order to achieve long-term corrosion resistance, to apply a corrosion-protective primer.

A component of the coating agent is thus a polymer (resin) which has isocyanate-reactive functionality. In other words, the resin is already polymerised, but has protruding groups which can react with the isocyanate groups from the cross-linking agent. The cross-linking will take place during the curing (normally by heating) of the powder.

Both thermosetting and thermoplastic resins can be used as resins for isocyanate-reactive groups. Preferably, the protruding groups should contain replaceable hydrogen atoms, e.g. the hydrogen atoms present in a hydroxyl group, an amino group (primary or secondary), a mercapto group or an amido group. The polymerized resin is preferably a linear polymer. The resin may be a polymer of an ethylenically unsaturated monomer, e.g. of the acrylic, methacrylic or vinyl type.

A preferred resin is one where the monomer has the formula 4_[R1-0-(0)C]-A[C(0)0-R] where n and t are 0 or 1 and n + t is t n ■ 3

1-2, where A is alkenyl or alkenylene with from 2 to 8 carbon atoms or a substituent thereof where the substituent can be chlorine, carboxyl or cyano, and it is to be understood that A can be branched or straight-chain, and R and R<1 > can be the same or different and can be hydrogen, alkyl with from 1 to 20 carbon atoms, preferably 2-8 carbon atoms, hydroxyl-substituted alkyl with from 1 to 20 carbon atoms, preferably 2-8 carbon atoms, and even more preferably 2-5 carbon atoms , a primary or secondary amino-substituted alkyl with from 1 to 20 carbon atoms, preferably 2-8 carbon atoms, where the substituent on the amino group is an alkyl group or a hydroxyalkyl group with from 1 to 6 carbon atoms,

mercapto-substituted alkyl group with from 1 to 6 carbon atoms, mercapto-substituted alkyl with from 1 to 20 carbon atoms, preferably 2-8 carbon atoms, alkylthio-alkylene with up to 20 carbon atoms, preferably up to 8 carbon atoms and a group of

up to 8 carbon atoms containing an oxirane ring, and most preferably 3 carbon atoms, e.g. glycidyl.

Of the above-mentioned monomers, those where n + t = 1 are most preferred, especially acrylic, methacrylic and the like.

Of the above-mentioned monomers, suitable examples of acids are acrylic acid, methacrylic acid, itaconic acid, ethacrylic acid, maleic acid, fumaric acid and the like, suitable examples of alkyl esters are those where the alkyl group is methyl, ethyl, propyl, isopropyl, butyl, tert-butyl, pentyl, hexyl, ethylhexyl, octyl, decyl, dodecyl and the like, suitable examples of hydroxy-substituted alkyl are hydroxyethyl, hydroxypropyl, hydroxybutyl, hydroxypentyl, hydroxyhexyl, hydroxyoctyl, hydroxydecyl, hydroxydodecyl and the like, suitable examples of aminoalkyl are aminomethyl, aminoethyl, amino -propyl, aminobutyl, aminopentyl, aminohexyl, aminooctyl, amino-decyl, aminododecyl where the amino group is a primary amino group or a secondary amino group and the substituent on the amino group is an alkyl group with from 1 to 6 carbon atoms, preferably 1-4 carbon atoms, suitable examples of mercaptoalkyl are methyl mercapto, ethyl mercapto, propyl mercapto, butyl mercapto, pentyl mercapto, hexyl mercapto, octyl mercapto, decyl mercapto and dod ecylmercapto, and in a similar way the alkoxyalkyl and alkylthioalkyl groups can be methoxymethyl, ethoxymethyl, ethoxyethyl, methylthiomethyl, ethylthio-

ethyl and the like.

It should be understood that although the polymerized resin

can be a homopolymer, then one should preferably use a mixture of monomers to obtain coating films with desirable properties such as e.g. improved flaking resistance, chalking resistance, adhesion, gloss, flexibility, durability, hardness, smoothness and solvent resistance.

A preferred monomer mixture is one having a major amount of ester (R or R in the above formula is alkyl or substituted alkyl where the substituent is non-functional or non-reactive) (about 50-90%), a small amount (about 0 .1-5%) of an acid (R or R<1> is hydrogen) and a significant amount (about 5-25%) of a functionally substituted alkyl (R or R<1>

is an alkyl group having a substituent having isocyanate-

reactive functionality).

In addition, other copolymerizable monomers having an ethylenically unsaturated group may be included, in smaller amounts (from about 0.1 to 10%), e.g. aliphatic or aromatic vinyl compounds (such as vinyl chloride, vinyl toluene and styrene).

All that is required of the polymerized resin is that it has protruding isocyanate-reactive groups.

In the polymerization reaction of the aforementioned monomers, ordinary polymerization techniques are used, preferably solution polymerization in an organic solvent.

Since the mixture is preferably to be used as a pulverizing agent, the temperature during curing should be as low as possible. It is therefore preferred that the molecular weight of the polymerized resin should be relatively low, e.g. from 10,000 to 75,000, with as little deviation from the average figure as possible. In other words, the molecular weight range should be as narrow as possible, within the above range.

By using a low molecular weight resin, the desired fluidity, less tendency to an "orange peel" appearance and the like is achieved. A narrow molecular weight distribution gives a high agglomeration temperature (for powder), better flowability, better gloss and better compatibility with the other components in the mixture.

The cross-linking agent that is used as a component of the powder coating mixture is, as mentioned, isophorone diisocyanate where the most reactive isocyanate group is blocked, and the other has been reacted with a compound which after curing becomes an internal plasticizer for the resin, but which before curing is a external plasticizer for the resin. By "internal softener" is meant a material which gives a softening effect to the resin and is chemically bound to it. By "external plasticizer" is meant a corresponding material that is not chemically bound. The most reactive isocyanate group is the one attached to the six ring over a methylene group. During curing, this group will be unblocked and react with the functional groups in the polymerized resin.

By "blocked isocyanate" is meant an isocyanate group that has been reacted with a blocking agent so that the reaction product is stable at up to 50°C for long periods of time. During curing, the blocked isocyanate is de-blocked during heating.

Saturated aliphatic glycols or polyhydroxyalkanes or cycloalkanes can be used as an isocyanate-reactive, softening compound, e.g. fcutylene glycol, 1, 6-dihydroxyoctane, 1,7-dihydroxyoctane, 1,10-dihydroxydecane, 1,12-dihydroxydodecane, 2,2,2-trimethylolpentane, pentaerythritol, 1,3,5-trihydroxypentane, 2,4, 6-trihydroxyhexane, neopentyl glycol (2,2-dimethyl-1,3-dihydroxypropane), 2,2,4-trimethyl-1,3-pentanediol and 1,4-cyclohexanedimethanol, polyoxyalkylene glycol where the alkyl has from 2 to 4 carbon atoms e.g. polyoxyethylene glycol, polyoxypropylene glycol and polyoxybutylene glycol, with molecular weight up to 500, hydroxyl-containing materials having the repeating unit -[CH2~CH(CH20H)]n where n is from 1 to 10, polyamino compounds such as 1,4-diaminobutane, 1,6 -diamino-hexane and 1,8-diamino-dodecane, it should be understood that the amino compound should be a primary or secondary amine with at least one reactive hydrogen atom, a substituent may be attached to the nitrogen atom, the substituent may be an alkyl group or hydroxyalkyl group with from 1 to 6 carbon atoms.

Suitable examples of mercapto compounds are: 1,4-dimercaptopentane, 1,6-dimercaptohexane, 1,8-dimercaptooctane, 1,10-dimercaptodecane and 1,12-dimercaptododecane.

The following are suitable examples of amido compounds which can be used: 1,4-diamidobutane, 1,6-diamidohexane, 1,8-diamidooctane, 1,10-diamidodecane and 1,12-diamidododecane. It should be understood that a hydrogen atom attached to the amido group can be replaced by an alkyl or hydroxyalkyl group and thereby have at least one replaceable hydrogen atom that can be converted from the amido group.

As said, the most reactive isocyanate group must be blocked. The most common blocking agents are phenols, alcohols, caprolactam, aldoximes or ketoximes. Preferably, a ketoxime is used. Suitable ketoximes are described in US patent 3,583,943. Those that are preferred are dialkyl ketoximes, e.g. dimethylketoxime, methylethylketoxime, methylisobutylketoxime and diisobutylketoxime. Preparation of cross-linking agent a) Isophorone diisocyanate (IPDI) + blocking agent in X

b) X + polyfunctional plasticizer y Y

where X contains one blocked and one free isocyanate group, and Y contains the blocked isocyanate group and a group originating from the reaction with the plasticizer.

It should be understood that any plasticizer having at least two replaceable hydrogen atoms from functional groups can be used. It may possibly have three functional groups, which may react with the diisocyanate. The preferred plasticizer contains at least 4 carbon atoms and has functional groups separated by at least 1 carbon atom. The softener preferably has from 6

to approx. 20 carbon atoms.

The plasticizer can itself be a reaction product, e.g. an ester resulting from the reaction of a polyhydroxyl material and an acid, its anhydride or a dibasic acid. Suitable polyhydroxy compounds are described above. Suitable acids, dibasic acids and their anhydrides may have up to 12 carbon atoms, e.g. oxalic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid and sebacic acid. A preferred ester is one derived from the reaction of neopentyl glycol and 2,2-dimethyl, 3-hydroxy-propionic acid, having the structure

HO-CH2-C(CH3)2-CH2-O-C(O)-C(CH3)2-CH2"OH.

Other suitable polyhydroxy compounds which can be reacted with a polyisocyanate are polyhydroxy hydantoin compounds such as apTiprpl 1- har sl-rnkt.nrpn

where R and R can be the same or different and can be hydrogen, alkyl with 1-6 carbon atoms, hydroxyalkyl with 1-6 carbon atoms, phenyl and the like.

R 3 and R 4 can be the same or different and can be hydrogen, alkyl with from 1 to 6 carbon atoms, hydroxyalkyl with from 1 to 6 carbon atoms, the polyoxyalkylene with from 2 to 4 carbon atoms per alkylene group with a hydroxy end group, repeating units of

-[OCH2-CH(CH2OH)]-n

where n is from 1 to 10, and the like. It should also be understood that the hydantoin materials must contain at least two hydroxyl groups. Typically these compounds are prepared by reacting dimethyl hydantoin (R5 and R<6> are methyl and R3 and R<4> are hydrogen) with a suitable hydroxyl introducing compound, e.g. epichlorohydrin,

ethylene oxide, propylene oxide or butylene oxide. For the reaction product of epichlorohydrin, hydrolysis may be required to remove the chlorine groups. Generally, "polyhydroxy-hydantoin" can be said to contain the basic 5-membered ring structure listed above.

It has also been demonstrated that a bis-hydantoin derivative can also be used, i.e. a suitable hydroxy derivative of methylene-bis-(substituted hydantoin).

The preferred hydantoin is one in which R~* and R are both methyl and R 3 and R 4 are both - 02^ 2^ 2 ' Another preferred hydantoin is one in which R^ and R^ are both methyl, and R 3 and R 4 are both

-CH 2 OH.

Because the final powder mixture should cure rapidly, the blocking agent will be released at a suitable temperature which is preferably above 121°C. Since the first reacted -NCO group is the most reactive and is now unblocked (121°C), it can be quickly cross-linked with the reactive groups protruding from the polymerized resin.

The cross-linking agent and the resin can be used as a basis for a paint after suitable dilution with an organic solvent. In addition, they can be used in the production of powder paint as described in US patent 3 737 401.

The preferred method for producing powder particles according to the present invention is to produce the particles by the precipitation process in US patent 3 737 401. In this method, finely divided drops of film former plus organic solvent for the film former are formed. When the liquid paint is brought into contact with another solvent which is a non-solvent for the film former, but is a mutual solvent (partially miscible) with the first solvent, the droplets are formed. The powder particles are precipitated when the first solvent is removed from the droplets by dilution therefrom in contact with a solvent in which the first solvent is miscible. A preferred pair of solvents is a ketone (for the film former) and water. With regard to further discussion of the production of powder particles, reference is made to US patent 4,112,214.

Other components can be added before the production of the powder, e.g. pigments, fillers, organic dyes and varnishes, plasticizers and flow control agents, catalysts to accelerate curing and antistatic agents. A further, preferred plasticizer that can be mixed with the cross-linking agents is an ester derived from the reaction of a saturated aliphatic triol of up to 12 carbon atoms (about 2 parts by weight) with a long-chain monobasic fatty acid (with at least 5 carbon atoms - about 2 parts by weight), which reaction product is then reacted with an aromatic or saturated aliphatic dibasic acid or anhydride (about 1 part), and this results in a mixture having an excess, preferably a 50% excess, of hydroxyl groups. The hydroxyl groups of this plasticizer can react with the unblocked isocyanate groups in the cross-linking agent during the curing of the powder.

Other softeners can also be added which can react with excess, unblocked isocyanate groups.

Any of the softeners listed above can be used. Preferred materials are polyhydroxy compounds, e.g. cellulose derivatives (cellulose acetate, cellulose acetate butyrate), alkane polyols, hydroxyl-containing lactones and lactams with rings of 5 to 7 members and hydroxyl-containing hydantoin materials.

Based on film-forming components, a total of 100% by weight, powder mixtures according to the present invention can be described as follows:

Additional (External)

plasticizer 0-10%, preferably at least 1%.

The powder mixture can also be described by specifying the ratio between the different reactive groups.

The total amount of B + C should have an excess of NCO groups to be able to react with the resin A.

The invention is now described in broad terms, and examples of the implementation of the invention are given below. All percentages are percentages by weight unless otherwise stated.

An acrylic resin was prepared by adding 1 part acrylic acid, 1 part ethylhexyl acrylate, 6 parts ethylhexyl methacrylate, 20 parts ethyl methacrylate and 5 parts hydroxypropyl methacrylate to an organic ketone solvent, and the solution was polymerized with an azo catalyst at approx. 80°C.

The cross-linking agent was prepared by reacting 1 mole part of isophorone diisocyanate with 1 mole part of methyl ethyl oxime in an organic ketone solvent until the reaction was complete. Then half a mole part of 1,6-hexanediol was added. Dibutyltin oxide was added to complete the reaction.

A solution of acrylic resin and cross-linking agent was then prepared in a weight ratio of 2:1. Carbon black was added at a ratio of 1 part per 25 parts acrylic resin, and a powder was prepared in accordance with US Patent 3,737,401 by adding it with stirring to a mixture of water and the ketone, at which point droplets began to form. Then the solution was added to additional amounts of water to cause precipitation of powder by diluting the ketone from the droplets. The powder was then separated from the liquid by filtration, washing and drying.

The particles had diameters in the range between 5 and 50 µm.

A cleaned steel plate was used as a substrate onto which the particles were electrostatically sprayed. The film was cured to a hard, adherent, high-gloss coating by heating the plate to approx. 163°C. The curing time was 30 minutes.

Example 2

The procedure from example 1 was followed, and an excess of isocyanate (50% excess) was used in the preparation of the cross-linking agent. Before the precipitation from the agitated medium, further plasticizer was added (0.1 part per part acrylic resin). The other plasticizer is the reaction product of trimethylolpropane (2 mole parts), pelargonic acid (2 mole parts) and phthalic anhydride (1 mole part). The powder was formed in the same way as in example 1. The powder particles were sprayed electrostatically onto a steel plate and then hardened by exposing the plate to approx. 163°C

for 30 minutes. The film was hard and sticky, and it had a high gloss.

Example 3

The procedure from Example 2 was followed except that caprylactonediol (1 part per 10 parts acrylic resin) was added before the precipitation. Equally good films were obtained.

Example 4

The procedure from Example 1 was used in the preparation of the resin and powder, except that the following composition was used to prepare the cross-linking agent.

The curing temperature was 163°Cf and the curing time was 30 minutes.

The films prepared according to this example are substantially equivalent to those of example 1.

Example 5

The method from example 1 was used in the preparation of the resin and the powder, except that the following composition was used to prepare the cross-linking agent.

The curing temperature was 163°C, and the curing time was 30 minutes.

The films produced according to this example are approximately the same as those produced according to example 1.

Claims (1)

Coating agent for the production of solvent-resistant coatings, comprising I) a polymer (resin) with isocyanate-reactive groups and II) a cross-linking agent containing at least two blocked isocyanate groups per molecule, and which is produced from isophorone diisocyanate, a blocking agent and a compound with at least two isocyanate-reactive groups, which compound has a softening effect on the coating agent both during and after curing, characterized in that the cross-linking agent is prepared by first reacting the isophorone diisocyanate with the blocking agent so that the most reactive of the two isocyanate groups is blocked, and then with the softening compound.