Heat-curable powder coating composition

Abstract

The invention relates to a one component heat curable powder coating composition comprising a resin containing reactive unsaturations and wherein all said reactive unsaturations are carbon carbon double bonds connected directly to an electron withdrawing group, a thermal initiation system comprising a peroxide chosen from the group of compounds represented by formula (1) ##STR00001##
wherein R.sup.1 and R.sup.2 each independently stand for an optionally substituted alkyl of 1 to 30 C-atoms, wherein the 1 to 30 C-atoms do not include the C-atoms of the substituents or for an optionally substituted aryl of 6 to 18 C-atoms, wherein the 6 to 18 C-atoms do not include the C-atoms of the substituents, and a co-crosslinker chosen from the group of vinylethers, vinyletherurethanes, vinylesters, vinylamides, itaconates, enamines, vinylureas and mixtures thereof.

Claims

1. A one component heat curable powder coating composition comprising: (i) a polyester resin containing reactive unsaturations, the polyester resin having a weight per the reactive unsaturations (WPU) as determined using .sup.1H-NMR which is higher than 250 and lower than 1500 g/mole, (ii) a thermal initiation system comprising a peroxide chosen from the group consisting of compounds represented by formula (1) ##STR00010## wherein R.sup.1 and R.sup.2 each independently stand for an optionally substituted alkyl of 1 to 30 C-atoms, wherein the 1 to 30 C-atoms do not include the C-atoms of the substituents or for an optionally substituted aryl of 6 to 18 C-atoms, and wherein the 6 to 18 C-atoms do not include the C-atoms of the substituents, and (iii) a co-crosslinker having a weight per unsaturation (WPU) as determined using .sup.1H-NMR higher than 150 and lower than 870 g/mole, said crosslinker being chosen from the group consisting of vinylethers, vinyletherurethanes, vinylesters, vinylamides, itaconates, enamines, vinylureas and mixtures thereof, wherein all of the reactive unsaturations are carbon-carbon double bonds connected directly to an electron withdrawing group and are reactive towards radicals generated by the peroxide, and wherein the thermal initiation system is present in an amount such that when the powder coating composition is applied to a substrate and cured at a temperature of 130 C. for 20 minutes, the resulting coating resists at least 50 acetone double rubs (ADR), wherein each ADR is a back and forward movement over a surface of the coating having a thickness of approximately 60 m using a cotton cloth drenched in acetone covering a hammer head having a weight of 980 grams and a contact surface area with the coating of 2 cm.sup.2.

2. The composition according to claim 1, wherein the resin has a WPU higher than 250 and less than 1150 g/mole.

3. The composition according to claim 1, wherein the resin has a WPU higher than 500 and less than 1500 g/mole.

4. The composition according to claim 1, wherein the resin has a WPU higher than 500 and less than 1150 g/mole.

5. The composition according to claim 1, wherein the co-crosslinker has a WPU higher than 150 and lower than 650 g/mole.

6. The composition according to claim 1, wherein the co-crosslinker has a WPU higher than 150 and lower than 630 g/mole.

7. The composition according to claim 1, wherein the resin has a hydroxyl value in the range of from 0 to 70 mg KOH per g resin.

8. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on maleic acid, fumaric acid, itaconic acid, acrylic acid and/or methacrylic acid.

9. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on maleic acid, fumaric acid, citraconic acid, itaconic acid, and/or mesaconic acid.

10. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on fumaric acid and/or maleic acid.

11. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on fumaric acid.

12. The composition according to claim 1, wherein the peroxide is benzoyl peroxide or lauroyl peroxide.

13. The composition according to claim 1, wherein the co-crosslinker is chosen from the group of vinylethers, vinylesters and mixtures thereof.

14. The composition according to claim 13, wherein the co-crosslinker is a vinylether.

15. The composition according to claim 13, wherein the resin has an acid value of less than 10 mg KOH per g resin.

16. The composition according to claim 14, wherein the resin has an acid value of less than 10 mg KOH per g resin.

17. The composition according to claim 13, wherein the resin has an acid value of less than 5 mg KOH per g resin.

18. The composition according to claim 14, wherein the resin has an acid value of less than 5 mg KOH per g resin.

19. The composition according to claim 1, wherein the composition further comprises an inhibitor.

20. The composition according to claim 19, wherein the inhibitor is a hydroquinone or a catechol.

21. The composition according to claim 1, wherein the resin has a glass transition temperature of at least 40 C. as measured via DSC at a heating rate of 5 C./min.

22. The composition according to claim 1, wherein the resin has a glass transition temperature of at least 45 C. as measured via DSC at a heating rate of 5 C./min.

23. The composition according to claim 1, wherein the resin has a glass transition temperature of at least 40 and of at most 65 C. as measured via DSC at a heating rate of 5 C./min.

24. The composition according to claim 1, wherein the resin has a number average molecular weight in the range of from 1500 to 8000 Da.

25. The composition according to claim 1, wherein the resin has a number average molecular weight in the range of from 2100 to 4000 Da.

26. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on maleic acid, fumaric acid, itaconic acid, acrylic acid and/or methacrylic acid, said resin has a hydroxyl value in the range of from 0 to 70 mg KOH per g resin and a number average molecular weight in the range of from 1500 to 8000 Da and a glass transition temperature of at least 40 C. as measured via DSC at a heating rate of 5 C./min; and the co-crosslinker has a WPU higher than 150 and lower than 630 g/mole and said co-crosslinker is chosen from the group of vinylethers, vinylesters and mixtures thereof.

27. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on maleic acid, fumaric acid, itaconic acid, acrylic acid and/or methacrylic acid, said resin has a hydroxyl value in the range of from 0 to 70 mg KOH per g resin and an acid value of less than 10 mg KOH per g resin, and a number average molecular weight in the range of from 1500 to 8000 Da and a glass transition temperature of at least 40 C. as measured via DSC at a heating rate of 5 C./min; and the co-crosslinker has a WPU higher than 150 and lower than 630 g/mole and said co-crosslinker is a vinylether; and the composition comprises an inhibitor.

28. The composition according to claim 27, wherein the reactive unsaturations of the resin are based on fumaric acid and/or maleic acid.

29. The composition according to claim 28, wherein the reactive unsaturations of the resin are based on fumaric acid and/or maleic acid.

30. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on maleic acid, fumaric acid, itaconic acid, acrylic acid and/or methacrylic acid, said resin has a hydroxyl value in the range of from 0 to 70 mg KOH per g resin and a number average molecular weight in the range of from 1500 to 8000 Da and a glass transition temperature of at least 40 C. as measured via DSC at a heating rate of 5 C./min; and the co-crosslinker has a WPU higher than 150 and lower than 630 g/mole and said co-crosslinker is chosen from the group of vinylethers, vinylesters and mixtures thereof; and the composition comprises an inhibitor.

31. The composition according to claim 30, wherein the reactive unsaturations of the resin are based on fumaric acid and/or maleic acid.

32. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on maleic acid, fumaric acid, itaconic acid, acrylic acid and/or methacrylic acid, said resin has a hydroxyl value in the range of from 0 to 70 mg KOH per g resin and a number average molecular weight in the range of from 1500 to 8000 Da and a glass transition temperature of at least 40 C. as measured via DSC at a heating rate of 5 C./min; and the co-crosslinker has a WPU higher than 150 and lower than 630 g/mole and said co-crosslinker is a vinylether; and the composition comprises an inhibitor.

33. The composition according to claim 32, wherein the reactive unsaturations of the resin are based on fumaric acid and/or maleic acid.

34. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on maleic acid, fumaric acid, itaconic acid, acrylic acid and/or methacrylic acid, said resin has a hydroxyl value in the range of from 0 to 70 mg KOH per g resin and an acid value of less than 10 mg KOH per g resin, and a number average molecular weight in the range of from 1500 to 8000 Da and a glass transition temperature of at least 40 C. as measured via DSC at a heating rate of 5 C./min; and the co-crosslinker has a WPU higher than 150 and lower than 630 g/mole and said co-crosslinker is a vinylether.

35. The composition according to claim 34, wherein the reactive unsaturations of the resin are based on fumaric acid and/or maleic acid.

36. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on maleic acid, fumaric acid, itaconic acid, acrylic acid and/or methacrylic acid, said resin has a hydroxyl value in the range of from 0 to 70 mg KOH per g resin and an acid value of less than 5 mg KOH per g resin, and a number average molecular weight in the range of from 1500 to 8000 Da and a glass transition temperature of at least 40 C. as measured via DSC at a heating rate of 5 C./min; and the co-crosslinker has a WPU higher than 150 and lower than 630 g/mole and said co-crosslinker is a vinylether.

37. The composition according to claim 36, wherein the reactive unsaturations of the resin are based on fumaric acid and/or maleic acid.

38. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on maleic acid, fumaric acid, itaconic acid, acrylic acid and/or methacrylic acid, said resin has a hydroxyl value in the range of from 0 to 70 mg KOH per g resin and a number average molecular weight in the range of from 1500 to 8000 Da and a glass transition temperature of at least 40 C. as measured via DSC at a heating rate of 5 C./min; and the co-crosslinker has a WPU higher than 150 and lower than 630 g/mole and said co-crosslinker is a vinylether.

39. The composition according to claim 38, wherein the reactive unsaturations of the resin are based on fumaric acid and/or maleic acid.

40. The composition according to claim 1, wherein the reactive unsaturations of the resin are based on maleic acid, fumaric acid, itaconic acid, acrylic acid and/or methacrylic acid, said resin has a hydroxyl value in the range of from 0 to 70 mg KOH per g resin and an acid value of less than 5 mg KOH per g resin, and a number average molecular weight in the range of from 1500 to 8000 Da and a glass transition temperature of at least 40 C. as measured via DSC at a heating rate of 5 C./min; and the co-crosslinker has a WPU higher than 150 and lower than 630 g/mole and said co-crosslinker is a vinylether; and the composition comprises an inhibitor.

41. The composition according to claim 40, wherein the reactive unsaturations of the resin are based on fumaric acid and/or maleic acid.

42. A process for the preparation of a powder coating composition according to claim 1 comprising the steps of: (a) mixing the components of the powder coating composition to obtain a premix; (b) heating the premix to obtain an extrudate; (c) cooling down the extrudate to obtain a solidified extrudate; and (d) breaking the solidified extrudate into smaller particles to obtain the powder coating composition.

43. A process for coating a substrate comprising the following steps: (1) applying a powder coating composition according to claim 1 to a substrate; and (2) heating the substrate.

44. A substrate that is fully or partially coated with a powder coating composition according to claim 1.

45. The substrate according to claim 44, wherein the substrate is a heat-sensitive substrate selected from the group consisting of wood and plastic.

Description

EXAMPLES

(1) The invention is explained in more detail with reference to the following non-limiting examples.

Experimental Section

Synthesis and Application of the Powder Coating

(2) TABLE-US-00002 TABLE 2 Chemicals Description Chemical name Commercial name or use Neopentyl glycol Monomer Trimethylol propane Monomer Hydrogenated bis-phenol A Monomer Terephthalic acid Monomer Isophthalic acid Monomer Fumaric acid Monomer Hydroxylbutyl vinylether Monomer Isophoronediisocyanate Monomer Ethylene carbonate Monomer Dilauroyl peroxide Laurox S Initiator from Akzo Nobel Dibenzoyl peroxide (BPO) Luperox Initiator A75 from Arkema Tert-butyl peroxybenzoate Trigonox C from Initiator Akzo Nobel Tert-butyl hydroquinone Inhibitor Cobalt bis(2-ethylhexanoate), COMMET Cobalt Accelerator also known as Cobalt Octanoate from De octanoate Monchy International B.V. Byk-361 Flow agent N from Byk
Synthesis of Resins: General Procedure

(3) The chemicals used in the following examples are described in table 2.

(4) Resin Synthesis (Resin A)

(5) A reaction vessel fitted with a thermometer, a stirrer and a distillation device, was filled with a tin catalyst and the monomers for the first step (all the (poly)alcohols and terephthalic acid) as listed in table 3. Stirring was then applied and a light nitrogen flow was passed over the reaction mixture while the temperature was raised to 220 C. Subsequently, for the second step fumaric acid together with a small amount of radical inhibitor was added at a temperature of 180 C. followed by esterification at 220 C. When an acid value of less than approximately 15 mg KOH/g resin was reached, the reaction mixture was cooled to 205 C. The third stage of the polyester preparation was carried out under reduced pressure at 205 C. till an acid value of approximately 5 mg KOH/g resin was reached. The acid value of the resin was lowered further via reaction of the remaining acid-groups of the resin with ethylene carbonate. The used amount was dependent on the acid value before addition.

(6) Resin and Co-Crosslinker Analysis:

(7) Glass transition temperature (Tg) measurements (inflection point) and melting temperature measurements were carried out via differential scanning calorimetry (DSC) on a Mettler Toledo, TA DSC821, in N.sub.2 atmosphere and at a heating rate of 5 C./min. Viscosity measurements were carried out at 160 C., on a Rheometric Scientific CT 5 (Rm 265) apparatus (Mettler Toledo). A 30 mm spindle was used. The applied shear-rate was 70 s.sup.1. The acid and hydroxyl values of the resins were determined titrimetrically according to ISO 2114-2000 and ISO 4629-1978, respectively.

(8) The weight per unsaturation (WPU) was determined via .sup.1H-NMR on a 300 MHz Varian NMR-spectrometer using pyrazine as internal standard. Recorded spectra were analyzed in full with ACD software and peak areas of all peaks were calculated.

(9) The weight resin per mole unsaturation was calculated with the following formula:

(10) $\mathrm{WPU}={\left[\frac{W_{\mathrm{pyr}}}{W_{\mathrm{resin}}}\frac{1}{M{W}_{\mathrm{pyr}}}\frac{A_{c=c}/N_{c=c}}{A_{\mathrm{pyr}}/N_{\mathrm{pyr}}}\right]}^{-1}$
W.sub.pyr and W.sub.resin are weights pyrazine (is internal standard) and resin, respectively, expressed in the same units. MW.sub.pyr is molecular weight pyrazine (=80 gr/mole). A.sub.CC is the peak area for hydrogens attached to the carbon carbon double bonds of the reactive unsaturations (CC component) in the resin; N.sub.CC is the number of hydrogens of that particular CC component. A.sub.pyr is the peak area for pyrazine and N.sub.pyr is the number of hydrogens (=4).

(11) Powder Coating Composition Analysis

(12) Thermal analysis measurement of the initial and stored (2 months at 25 C. or 72 hours at 40 C.) powder coating composition were carried out via differential scanning calorimetry (DSC) on a DSC Q2000 apparatus from TA Instruments in N.sub.2 atmosphere. A powder coating composition sample of approximately 5-10 mg was used. The sample was first stabilized at 0 C. (2 minutes), then heated with 5 C./min to 200 C. With the accompanied analysis software from TA Instruments, the onset temperature and the peak temperature of the DSC trace were calculated.

(13) TABLE-US-00003 TABLE 3 Synthesis and properties of the resins used Resin no. A Amount Monomers (mole %) Neopentylglycol 47.9 Trimethylol propane 3.7 Terephthalic acid 37.5 Fumaric acid 10.9 Ethylene carbonate X Resin characterization Weight per unsaturation (WPU) (theoretical) 1028 Weight per unsaturation (WPU) (measured with NMR) 1130 Mn (theoretical) 2723 Hydroxyl value (mg KOH/g) 42.7 Acid value (mg KOH/g) 3.1 Tg ( C.) 46.5 Viscosity at 160 C. (Pa .Math. s) 21.2
Synthesis of Vinyl Ether Based Co Crosslinkers: General Procedure
Method to Determine Presence of Free-NCO.

(14) An FT-IR spectra was recorded on a Varian Excalibur apparatus equipped with an ATR (Golden Gate) accessories. A characteristic peak for free NCO can be found at 2250 cm.sup.1. Presence of a peak at this position refers to free NCO groups.

(15) Co-Crosslinker Synthesis (I)

(16) A reaction vessel fitted with a thermometer, a stirrer and a distillation device, was filled with a tin catalyst and the monomers for the first step (all the (poly)alcohols, isophthalic acid) as listed in table 4. Stirring was then applied and a light nitrogen flow was passed over the reaction mixture while the temperature was raised to 220 C. Subsequently, for the second step a vinylether as listed in table 3 and a tin catalyst were added at a temperature of 120 C. Subsequently, an isocyanate as listed in table 3 was dosed such that the reaction mixture was kept below 120 C. during addition. After all isocyanate was dosed, the temperature was kept or set at 120 C. and maintained at this temperature for approximately half an hour. Next,

(17) n-butanol was added until all free NCO had reacted (measured using FT-IR as described above). The temperature was kept at 120 C. and vacuum (0.1 bar) was applied to remove all volatiles. After vacuum the content of the vessel was discharged.

(18) TABLE-US-00004 TABLE 4 Synthesis and properties of the co-crosslinker Co-crosslinker I Type Urethane vinylether Amount (mole %) Hydroxyl butyl vinyl ether 28.5 Isophorone diisocyanate 28.5 Hydrogenated bisphenol A 14.3 Neopentylglycol 14.3 Isophthalic acid 14.3 Co-crosslinker characterization Mn (theoretical) 1152 Weight per unsaturation in g/mole (WPU) 576 (theoretical) Weight per unsaturation in g/mole (WPU) 623 (determined using .sup.1H NMR) Tg ( C.) 41 Hydroxyl value (mg KOH/g) 1.0 Acid value (mg KOH/g) 0.5 Viscosity at 160 C. (Pa .Math. s) 3.9
Preparation of the Powder Coating Composition, Application and Analysis:

(19) The compositions of the tested powder coating composition are given in the tables below. The components were extruded at 60 C. using a Prism Twin Screw extruder (200 rpm, torque >90%). The extrudate was grinded and sieved; the sieving fractions smaller than 90 microns were used as a powder coating composition. The powder coating compositions were applied with a corona powder application spray gun on an aluminum ALQ panel and cured at various temperatures for 15 minutes in a convection oven (Heraeus Utah 6120). The applied coating layer thickness was approximately 60 m.

(20) Preparation of the Powder Coating Composition

(21) The ratio resin:co-crosslinker is chosen 3:2 on mole unsaturation. The amount of initiator in the thermal initiation system is based on the total weight of the resin system (e.g. x mole initiator per kg resin system). The amount of inhibitor in the initiation system is based on the total weight of the resin system. The amount of accelerator in the initiation system is based on the total weight of the resin system (e.g. x mole accelerator per kg resin system). The amount of flow agent is calculated in wt % of the total powder coating composition. In all powder coating composition 0.8 wt % flow agent was used, unless described differently.

(22) Flow of the Powder Coating Composition

(23) Flow characteristics (flow) of the powder coating compositions on the substrate can be determined by comparing the flow of the coating with PCI Powder Coating Flow panels (ACT Test Panels Inc.) at a coating thickness of approximately 60 m. The rating of flow is from 1 to 10, with 1 representing the roughest coating and 10 representing the coating with the best flow.

Example 1 and Comparative Experiment A

(24) TABLE-US-00005 TABLE 5 Flow and stability of powder coating composition after storing for 2 months at 25 C. Exp-# comparative example A.1 1.1 1.2 Resin A A A Co-crosslinker I I I Initiation system Initiator Trigonox C Luperox A75 Laurox S 92.0 mmol/kg 92.0 mmol/kg 92.0 mmol/kg Chemical name Tert-butyl Dibenzoyl Dilauroyl peroxybenzoate peroxide peroxide Inhibitor Tert-butyl Tert-butyl Tert-butyl hydroquinone hydroquinone hydroquinone 500 ppm 500 ppm 500 ppm Accelerator Co Co Co 3.0 mmol/kg 3.0 mmol/kg 3.0 mmol/kg Extrusion Ok Ok Ok Flow (PCI scale) 2 2 1 Flow 0 2 1 After 2 months Initial onset 107.7 111.6 100.2 temperature ( C.) Initial peak 111.8 122.1 106.5 temperature ( C.) Onset temperature 92.9 116.8 101.7 after 2 months at 25 C. ( C.) Peak temperature 98.3 125.8 108.4 after 2 months at 25 C. ( C.)

(25) The examples 1.1 and 1.2 and the comparative example A.1 clearly demonstrate the beneficial use of the peroxides according to the invention with respect to their chemical storage stability as can be seen from the decrease in flow after storage of the powder coating composition.

(26) Furthermore these experiments indicate that the DSC can be used to easily assess the storage stability and the flow. In case the onset/peak temperature of curing remains the same or increases the flow remains the same, whereas in case the onset/peak temperature decreases also the flow decreases.

(27) In order to assess the effect on the flow characteristics using minimal amounts of powder coating formulations, this DSC methodology was employed on the samples stored at 40 C. as depicted in table 6 and 7.

Example 2

(28) TABLE-US-00006 TABLE 6 Stability of powder coating composition after 72 hours at 40 C. Exp-# comparative example A.2 2.1 2.2 Resin A A A Co-crosslinker I I I Initiation system Initiator Trigonox C Luperox A75 Laurox S 92.0 mmol/kg 92.0 mmol/kg 92.0 mmol/kg Chemical name Tert-butyl Dibenzoyl Dilauroyl peroxybenzoate peroxide peroxide Inhibitor Tert-butyl Tert-butyl Tert-butyl hydroquinone hydroquinone hydroquinone 500 ppm 500 ppm 500 ppm Accelerator Co Co Co 3.0 mmol/kg 3.0 mmol/kg 3.0 mmol/kg Extrusion Ok Ok Ok Initial onset 107.7 111.6 100.2 temperature ( C.) Initial peak 111.8 122.1 106.5 temperature ( C.) Onset temperature 91.2 114.2 101.5 after 72 hours at 40 C. ( C.) Peak temperature 97.3 123.8 108.9 after 72 hours at 40 C. ( C.)

(29) As can clearly be observed from the data in table 6 upon storage at 40 C. the flow employing the peroxides according to the invention remains good as indicated by the DSC data whereas at 40 C. an even further deterioration of the flow characteristics is observed. Compare to this end the examples 2.1 and 2.2 (DSC onset/peak temperature remains the same or increases) with comparative experiment A2 (DSC onset/peak temperature decreases).

Example 3

(30) TABLE-US-00007 TABLE 7 Stability of powder coating compositions comprising Luperox A 75 after 72 hours at 40 C. Exp-# 3.1 3.2 3.3 3.4 Resin A A A A Co-crosslinker I I I I Initiation system Initiator Luperox A75 Luperox A75 Luperox A75 Luperox A75 92.0 mmol/kg 92.0 mmol/kg 92.0 mmol/kg 92.0 mmol/kg Chemical name Dibenzoyl Dibenzoyl Dibenzoyl Dibenzoyl peroxide peroxide peroxide peroxide Inhibitor Tert-butyl Tert-butyl hydroquinone hydroquinone 500 ppm 500 ppm Accelerator Co Co 3.0 mmol/kg 3.0 mmol/kg Extrusion Ok Ok Ok Ok Initial onset 111.6 906.0 111.0 96.3 temperature ( C.) Initial peak 122.1 122.5 121.4 121.3 temperature ( C.) Onset temperature 114.2 98.3 113.8 98.5 after 72 hours at 40 C. ( C.) Peak temperature 123.8 124.0 123.1 122.0 after 72 hours at 40 C. ( C.)

(31) These examples clearly show that the presence of a transition metal catalyst (the accelerator) does not influence the flow characteristics upon curing (example 3.4 vs 3.2 where the peak/onset temperatures remain the same). Furthermore, these examples demonstrate the positive effect on the flow characteristics when employing a hydroquinone type inhibitor (3.1 vs 3.2 and 3.3 vs 3.4 as the higher peak/onset temperatures indicate a better flow).