CA2213547A1

CA2213547A1 - Acetophenone oxime-blocked pu powder coatings and their use

Info

Publication number: CA2213547A1
Application number: CA 2213547
Authority: CA
Inventors: Rainer Gras; Elmar Wolf
Original assignee: Individual
Current assignee: Huels AG
Priority date: 1996-08-23
Filing date: 1997-08-21
Publication date: 1998-02-23
Also published as: EP0825237A1; JPH1088035A; DE19634054A1

Abstract

Disclosed is a storage-stable, heat-curable polyurethane powder coating composition comprising:
a) at least one polyisocyanate component blocked totally or partially with acetophenone oxime, b) a hydroxyl-containing polymer, and c) if required appropriate auxiliaries or additives. The powder coating composition may be cured at a temperature, for example 160-180°C, lower than that employed in prior art.

Description

CA 02213~47 1997-08-21 Acetophenone oxime-blocked PU powder coatinqs and their use The present invention relates to a transparent or pigmented, heat-curable, weather-stable polyurethane (PU) powder coating of high reactivity.
The blocking of a polyisocyanate for the temporary protection of the NCO groups is a long-established procedure and is described, for example, in Houben-Weyl, Methoden der organischen Chemie XIV 2, page 61 ff.
For the preparation of PU powder, c-caprolactam has become established, from among the large number of blocking agents, for blocking the NCO component.
In DE-B 21 05 777 a range of advantages are cited as a reason for emphasizing ~-caprolactam-blocked adducts of isophorone diisocyanate (IPDI) as hardeners for powder coatings.
DE-A 25 42 191 relates to a blocked polyisocyanate of IPDI, ~-caprolactam and a polyol which is a mixture of a diol and a triol, and its use for preparing a heat-curable pulverulent coating composition.
DE-A documents 27 35 497 and 28 42 641 describe a pulverulent PU coating material based on a hydroxyl-containing polyester, a polyacrylate or an epoxy resin, whose hardener component is an ~-caprolactam-blocked mixture of IPDI isocyanurate and monomeric IPDI.
DE-A 30 04 876 claims an ~-caprolactam-blocked polyisocyanate which consists of an adduct of IPDI and a polyol in a fixed OH/NCO ratio.
The disadvantage of the known ~-caprolactam-blocked CA 02213~47 1997-08-21 polyisocyanates is the relatively high unblocking temperature of around 180-200~C. For a long time there has been great interest, on economic grounds, in reducing this temperature and/or the stoving time of such coating material. Attempts have been made to use, instead of ~-caprolactam, an oxime, whose adduct with an isocyanate is thermally cleaved at a much lower temperature than 170~C. In the cured coating to which an oxime has been added, however, there is the unwanted formation of blisters, as described in DE-A 33 12 028 (p.3, line 3) and Powder Coatings Bulletin, Dec. 1994, p.2.
Moreover, with such a reactive system, a slight yellowing is found, which increases greatly on overbaking, as our own experiments have shown.
A main object of the present invention is to overcome the disadvantages of the prior art and, in particular, to provide a PU powder coating which may be transparent or pigmented, which is notable in particular for high reactivity and which therefore permits curing at a low temperature.
The present invention accordingly provides a storage-stable, heat-curable polyurethane powder coating composition, essentially comprising:
a) at least one polyisocyanate component blocked totally or partially with acetophenone oxime, b) a polymer having at least two hydroxyl groups, and c) if required, appropriate auxiliaries or additives.

CA 02213~47 1997-08-21 The polyisocyanates in the context of this application are preferably a diisocyanate of aliphatic or (cyclo)aliphatic structure. Such a diisocyanate is very well known. Rather than listing individual representatives here, reference is made to the literature: Houben-Weyl, Methoden der organischen Chemie, Volume 14/2, p.61 ff. and J. Liebigs Annalen der Chemie, Volume 562, pp. 75-136. Preference is generally given to the readily industrially available aliphatic or (cyclo)aliphatic diisocyanates with 6-14 carbon atoms, especially 3-isocyanatomethyl-3,5-5-trimethylcyclohexyl isocyanate (isophorone diisocyanate) and dicyclohexylmethane 4,4'-diisocyanate. In addition to the monomeric diisocyanates, it is also possible to employ isocyanurates (i.e., a trimer) prepared therefrom, which can be prepared by known methods (cf. e.g DE-C documents 26 44 684, 28 21 109 and 29 16 201).
Finally, the polyisocyanates in the context of the present invention may also be understood as including those adducts which are obtained by reacting the abovementioned monomeric, predominantly bifunctional polyisocyanates with chain extenders which are common in isocyanate chemistry.
Compounds of this kind are listed, for example, in DE-A 27 07 660. Preference is given to polyols whose molecular weight is below 350, especially ethylene glycol and trimethylolpropane. The chain extenders should be reacted with the monomeric diisocyanates in such amounts that the resulting adducts have on average at least two isocyanate groups.

CA 02213~47 1997-08-21 The polyisocyanates employed as hardeners in the novel coating composition preferably have a content of blocked NCO groups of about 9-20%, more preferably 11-15% by weight. Their melting point is generally within a wide range and is preferably from about 60 to about 150~C, preferably from about 70 to about 150~C. They are suitable for preparing solvent-containing one-component PU stoving enamels. The blocking of the polyisocyanates can be undertaken either in inert solvents or else in bulk in the polyisocyanate initial charge.
The inert solvents are those which do not react with the polyisocyanates. Examples of them include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone and cyclohexanone; aromatic compounds, such as toluene, xylene, chlorobenzene and nitrobenzene; cyclic ethers, such as tetrahydrofuran; esters, such as methyl acetate and n-butyl acetate; aliphatic chlorinated hydrocarbons, such as chloroform and carbon tetrachloride, and polar aprotic solvents, such as dimethyl-formamide, dimethylacetamide and dimethyl sulfoxide.
The blocking agent, i.e., acetophenone oxime, is employed relative to the polyisocyanate preferably in a ratio such that there is 0.5 - 1 mol of acetophenone oxime per NCO
group.
The reaction of the polyisocyanate with acetophenone oxime is usually carried out in the temperature range between 0 and 150~C. To carry out blocking rapidly and completely a relatively high reaction temperature is CA 02213~47 1997-08-21 preferred. On the other hand, the reaction temperature must be at least 10~C below the unblocking temperature of the blocked polyisocyanate. Preference is given to the temperature range which is about 15 to 35~C below the unblocking temperature, i.e., at about 120~C.
In the blocking reaction, a catalyst may be used to promote the isocyanate polyaddition reaction, for example tin(II) octoate, dibutyltin dilaurate (DBTL), tertiary amines, etc.
The polymers having at least two hydroxyl groups are preferably polyesters, epoxy resins, and hydroxyl-containing acrylates, each having a molecular weight of from 800 to 40,000.
For the powder coatings, the polyesters i.e., polyester polyols, are preferably those prepared from terephthalic acid with a polyol such as 1,6-hexanediol, neopentylglycol, 1,4-dimethanolcyclohexane and 2,2,2-trimethylolpropane.
The epoxy resins are preferably those listed in DE-A 29 45 113, page 12, line 1 to page 13, line 26.
The hydroxyl-containing polyacrylates are preferably those described in DE-A- 30 30 359, page 14, line 21 to page 15, line 26.
The polymers having at least two hydroxyl groups are employed generally in such an amount that the OH/NCO
molar ratio is l:n where n is a number from about 1.2 to about 0.8, preferably from about 1.05 to about 0.95.
The auxiliaries and additives are very well known CA 02213~47 1997-08-21 - 5a -in the name and examples of them include leveling agents, pigments, dyes, fillers, catalysts, thixotropic agents, W
stabilizers and antioxidants. The amount of these substances, relative to the amount of the solid binder, may vary within a wide range.
The PU powder coatings are prepared, for example, by mixing the polymer having at least two hydroxyl groups and the blocked polyisocyanate, possibly following addition of the desired coating additives, in the stated ratios and extruding the mixture at an elevated temperature. This temperature must be above the melting points of the polymer having at least two hydroxyl groups and blocked polyisocyanate but below the unblocking temperature of the blocked polyisocyanate.
Extrusion is followed by cooling and by milling to a particle size of less than 250 ~m, preferably less than 100 ~m. Subsequently, when required coarser fractions may be removed by sieving and are returned to the mill.
The application of the powder coating to bodies that are to be coated may take place in accordance with known methods, for example by electrostatic powder spraying, by fluidized-bed sintering or by electrostatic fluidized-bed sintering. The coated articles are then cured in the temperature range between 140 and 250~C for a sufficient time, e.g., from 60 minutes to 1 minute, preferably at a temperature between 150 and 200~C for from 20 to 5 minutes, more preferably at a temperature between 150 and 180~C.
Substrates suitable for coating with the novel PU

CA 02213~47 1997-08-21 - 5b -powder coatings are all those which withstand the curing temperatures indicated without suffering a deterioration in their mechanical properties. Examples are metals, glass, ceramic and plastic.
The stoving enamels and powder coatings that are obtained are notable relative to their conventional counterparts for a more favorable relationship between stoving temperature, curing time and defined quality requirements.
Experimental section I. Preparation of the hardener component Example 1 270 parts by weight of acetophenone oxime were added at 100~C to 222 parts by weight of IPDI at a rate such that the temperature did not rise - 6 - O.Z. 5~94 above 120~C. After the end of the addition of acetophenone oxime, the reaction mixture was held at 120~C for 0 5 h more in order to complete the reaction.

free NC0 content: 0.2 %
5 total NC0 content: 16 7 %
melting point: 55 - 58~C

Example 2 106 parts by weight of diethylene glycol were added dropwise over the course of about 1 h at 80~C to 444 parts by weight of IPC?I. The reaction 10 mixture was then heated further at 80~C until the NC0 content had reached 15 3 %. Then 270 parts by weight of acetophenone oxime were added in portions at 110CC and after the end of the addition of acetopnenone oxime!
the reaction mixture was heated further at 120~C until the NC0 content had reached 0 2 %

15 free NC0 content: 0 2 %
total NC0 content: 10.1 %
melting point: 77 - 83~C

The Examples listed in Table 1 below were prepared in analogy to the process described in Examples 1 and 2.

- 7 - o. z . 5094 Table 1:

Example Composition of the blocked isocyanate FreeTotal m.p.
No. NCO NCO
Mol of Diisocyanate Mol of chain extender Mol of acetophe- [~C
none oxlme 3 41PDI 1 ethyleneglycol 6 0.1 14.170-75 4 41P[)I 1 1,4-di(hydroxymethy!)cyciohexane 6 0.2 13.576-81 3 IPDI 1 trimethylolpropane (TMP) 3 0.1 13.7133 -138 D
6 6 IPDI 1 TMP 9 0.1 13.984 - 90 7 5 IPDI 1 triethanolamine 7 0.3 13.286 - 91 r 8 3 IPDI 1 tris(hydroxyethyl) isocyanurate 3 0.2 9.3148 -154 9 4 HMDI 1 TMP 5 0.1 11.1106 -111 ~1 1 HXDI - 2 0.1 17.766-71 HMDI: 4,4'-Methylenebiscyclohexyl isocyanate HXDI: 1,4-bis(isocyanatomethyl)cyclohexane - 8 - O.Z. 5094 Example 1 1 a) 3 parts by weight of DABC0 TMR were added with intense stirring at 80~C to 1000 parts by weight of iPI~I. After an induction period of about 1 minute the terrlper2ture of the reaction mixture rose to 136~C
s over the course of about 3 minutes. When the temperature maximum has been reached the reaction is at an end. Further heating at 1 30~C
brings no change in the NC0 content. After it has cooled to room temperature the reaction mixture had an NC0 content of 26 %.

b) 836 parts by weight of acetophenone oxime were added in portions at a rate such that the temperature of the reaction mixture did not rise above 120~C to 1000 parts by weight of the partially trimerized IPDI
with an NC0 c~ntent of 26 % described under a). After the end of the addition of acetophenone oxime the reaction mixture was heated further at 120~C for about 0 5 h to complete the reaction.

free NCO content: 0.2 %
total NC0 content: 13.8 %
melting point: 88 - 91 ~C

Example 12 (Comparison example) 1000 parts by weight of the partially trimerized IPDI described under 11 a were reaG.ed with 538 parts by weight of MEK oxime in analogy to 11 b.

free NC0 content: 0.1 %
total NC0 content: 16.7 %
melting point: 53 - 61 ~C

Example 13 (ComParison example) 2 5 700 parts by weight of c~aprolactam were added in portions at a rate such that the temperature of the reaction mixture did not rise above 130~C and at _ 9 _ o. z . 5094 120~C to 1000 parts by weight of the partially trimerized IPDI with an NCO
content of 26 % described under 11 a. After the end of the addition of c-caprolactam, the reaction mixture w, s heated further at 120~C for about 2 h to complete the reaction.

free NCO content: 0.5 %
total NCO content: 15 0 %
rnelting point: 86 - 89~C

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- 11 - O.Z. 5094 C Pol~furethane powd~r cQatEnqs General preparation procedure The comminuted products - blocked polyisocyanates ~crosslinkers), polyesters, leveling-agent masterbatch and, if appropriate, catalyst 5 rnasterbatch - are intimately mixed, together if appropriate with the white pigment, in an edge runner mill and the mixture is then homogenized in an extruder at up to 13û~C. After it has cooled, the extrudate is fractionated and milled using a pin mill to a particle size < 100 ,um. The powder thus prepared is applied using an electrostatic powder spraying unit at 60 kV to degreased, 10 optionally pretreated iron panels, which are stoved in a convection oven at temperatures between 160 and 180CC.

Le~elin~-a~ent masterbatch 10 % of ~he leveling agent - a commercial copolymer of butyl acrylate and 2-ethylhexyl acrylate - is homogenized in the melt in the corresponding 15 polyester, and the solidified melt is cornminuted.

C~talyst masterbatch 5 % by weight of the catalyst - DBTL - is homogenized in the melt in the corresponding polyester, and the solidified melt is comminuted.

The abbreviations in the tables below have the foilowing meanings:

LT = Layer thiekness in ~Im El = Erichsen indentation in mm ~DIN 53156) CH = Crosshatch test (DIN 53151) GG 60~ ~ = Gardner gloss (ASTM-D 5233) Irnp. rev. = Impact reverse in g m HK = Konig hardness in sec (DIN 53157) ~ _ 2_,, ~ ~ ~ ~ OA ~

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- 13 - o. z . 5094 C 2 Transparent powder coatinqs The transparent powder coatings as well were prepared, applied and stoved at between 150 and 1 70~C in accordance with the process described.

Table 4:

Example C 1 2 3 4 5 6 r~ 8~') g 10~ D
Formulation ~
Crosslinker~romAExample() 23.5523.8014.3117.29 16.362394 23.94 20.11 13.59 13.59 ~"

(3) (6) (6) (9) (9) (1~ 11 ) r Polyester as per B 1 76.4576.20 - - - 76.0676.0679.89 - -Polyesteras per B 2 - - 85.6982.71 - - - - - -Polyester as per B 3 - - - - 83.64 - - - 86.41 86.41 Notes All ~ormulations containec 0.5 % by weight leveling agent and 0.3 "~ by weight benzoin;
xj 0.1 % by weight DBTL; OH/NCO ratio is 1 : l, o) 1:0.8 Coatings data El > 10 > 10 > 10~ 10 ~ 10 9/10> 10 > 10 > 10 > 10 Notes Curing conditions: 170''C/10 - 12', 160~C/15 - 17', 150~CI25 - 30' - CA 022l3547 l997-08-2l - 14 - O.Z. 5094 C 3 PU powder coatin~s - Comparison examples Example 1 Owing to the low melting range (53 - 61 ~C) the crosslirlker of Example A 12 is in principle unsuitable for preparing transparent and pigmented powder s coatings in accordance with conventional powder-coating technology. The cross!inker itself bakes during storage, and does therefore not have adequate storage stability at room temperature.

Ready-to-spray powder coatings prepared therefrom do not comply with DIN
Standard 5599~1 part 7 - "Determination of blocking resistance". Moreover, 10 the coatings have a tendency varying from foaming to pinholing; in other words, the surface of the coating is not flawless. The yellowing of the transp~rent and pigmented coating films was at a relatively high level.

Example 2 c-caprolactam-blocked polyisocyanates, i.e. including the crosslinker of 15 Example A 13, are less reactive; in other words, the curing conditions required for such coatin~s involve either higher temperatures or longer baking times, eg. 200CC/12 - 15', 180~CJ15 - 20', 170~C/25 - 30' for pigmented coating and 200~C/8 - 10', 180~C/12 - 15', 170~C/18 - 22', 160CC/25 - 30' for transparent coating.

Claims

1. A storage-stable, heat-curable polyurethane (PU) powder coating composition comprising:
(a) at least one polyisocyanate component blocked totally or partially with acetophenone oxime, and (b) a polymer having at least two hydroxyl groups, at an OH/NCO molar ratio of 1:n where n is a number of from about 1.2 to about 0.8.

2. A powder coating composition as claimed in claim 1, wherein the polyisocyanate component (a) is an adduct of a monomeric diisocyanate and polyol having a molecular weight below 350.

3. A powder coating composition as claimed in claim 2, wherein the polyol is ethylene glycol or trimethylolpropane.

4. A powder coating composition as claimed in claim 1, wherein the polyisocyanate component (a) is an oligomeric diisocyanate, alone or in a mixture with a monomeric diisocyanate.

5. A powder coating composition as claimed in claim 4, wherein the oligomeric diisocyanate is a trimerized monomeric diisocyanate.

6. A powder coating composition as claimed in any one of claims 2 to 5, wherein the monomeric diisocyanate is isophorone diisocyanate, 4,4'-methylenebiscyclohexyl isocyanate or 1,4-bis(isocyanatomethyl)cyclohexane.

7. A powder coating composition as claimed in claim 1, wherein the polyisocyanate component is a monomeric diisocyanate blocked with acetophenone oxime and the monomeric diisocyanate is isophorone diisocyanate, 4,4'-methylene-biscyclohexyl isocyanate or 1,4-bis(isocyanatomethyl)-cyclohexane.

8. A powder coating composition as claimed in any one of claims 1 to 7, wherein the polyisocyanate component has a content of blocked NCO groups of 9 - 20%.

9. A powder coating composition as claimed in any one of claims 1 to 8, wherein the polyisocyanate component is blocked such that there is 0.5 - 1 mol of acetophenone oxime per equivalent of NCO.

10. A powder coating composition as claimed in any one of claims 1 to 8, wherein a polyester, an epoxy resin or a hydroxyl-containing acrylate, each having a molecular weight of from 800 to 40,000, is employed as the polymer having at least two hydroxyl groups.

11. A powder coating composition as claimed in claim 10, wherein the polymer having at least two hydroxyl groups is a polyester polyol produced from terephthalic acid and an alcohol selected from the group consisting of 1,6-hexanediol, neopentylglycol, 1,4-dimethanolcyclohexane and 2,2,2-trimethylolpropane.

12. A powder coating composition as claimed in any one of claims 1 to 11, wherein the OH/NCO ratio is 1:n where n is a number of from 1.05 to 0.95.

13. A storage-stable heat-curable polyurethane powder coating composition comprising:
(a) at least one polyisocyanate component blocked totally or partially with acetophenone oxime such that there is 0.5 - 1 mol of acetophenone oxime per equivalent NCO, wherein the polyisocyanate component has a content of blocked NCO groups of from about 9 to about 20% by weight and a melting point of from about 60 to about 150°C and is selected from the group consisting of:
(i) a monomeric aliphatic or (cyclo)aliphatic diisocyanate having 6 to 14 carbon atoms, (ii) an isocyanurate prepared from the monomeric diisocyanate (i), and (iii) an adduct obtained by reacting the monomeric diisocyanate (i) with a polyol as a chain extender such that the resulting adduct has on average at least two isocyanate groups; and (b) a polymer having at least two hydroxyl groups and a molecular weight of from 800 to 40,000 and being selected from the group consisting of a polyester polyol, an epoxy resin and a hydroxyl-containing polyacrylate, wherein the polyisocyanate component (a) and the polymer (b) are contained in such amounts that an OH/NCO
molar ratio is 1:n where n is a number of from about 1.2 to about 0.8.

14. A method of coating a substrate, which comprises:
applying the powder coating composition as defined in any one of claims 1 to 13 to the substrate, and then curing the coating composition at a temperature of 140 to 250°C, wherein the substrate is capable of withstanding the curing temperature without suffering a deterioration in mechanical properties.