PROCESS OF COATING WOOD

Abstract

The present invention relates to a process of coating wood with at least two waterbased coating compositions the process comprising: a) applying a coating composition A comprising at least one anionic polymer A with a Hansch parameter ≤1.6, and drying or allowing to dry said aqueous coating composition A, b) applying a coating composition B comprising at least one anionic polymer B with a Hansch parameter ≥1.7 and pigment, and drying or allowing to dry said aqueous coating composition B, with the proviso that the difference of the Hansch Parameters of anionic polymer B and anionic polymer A is at least 0.5. The invention also relates to wood coated accordingly, and the use of a coating composition A, comprising an aqueous anionic polymer A with a Hansch parameter ≤1.6, as an undercoat for coating wood.

Claims

1.-13. (canceled)

14. A process of coating wood with at least two waterbased coating compositions the process comprising: a) applying a coating composition A comprising at least one anionic polymer A with a Hansch parameter ≤1.6, and drying or allowing to dry said aqueous coating composition A, b) applying a coating composition B comprising at least one anionic polymer B with a Hansch parameter ≥1.7 and pigment, and drying or allowing to dry said aqueous coating composition B, with the proviso that the difference of the Hansch Parameters of anionic polymer B and anionic polymer A is at least 0.5.

15. The process according to claim 14, wherein the difference of the Hansch Parameters of anionic polymer B and anionic polymer A is at least 0.7.

16. The process according to claim 14, wherein the anionic polymer A is obtained by free radical aqueous emulsion polymerization of a mixture comprising ethylenically unsaturated monomers A, wherein the monomer mixture comprises: 0.1 to ≤5% by weight, based on the total amount of monomers A, of at least one acid-functional monomer, as a monomer A1, and 95 to 99.9% by weight, based on the total amount of monomers A, of at least one nonionic monomer, as a monomer A2.

17. The process according to claim 14, wherein the anionic polymer A is formed from 0.1 to ≤5% by weight, based on the total amount of monomers A, of at least one acid-functional monomer, as a monomer A1, and 95 to 99.9% by weight, based on the total amount of monomers A, of at least one nonionic monomer, as a monomer A2, whereas at least 60% by weight of the monomer A2 is selected from the group consisting of vinyl acetate, ethyl acrylate, methyl acrylate, methyl methacrylate, styrene, n-butyl acrylate, n-butyl methacrylate and 2-ethylhexyl acrylate, and up to 40% of monomer A2 are other monomers A2.

18. The process according to claim 14, wherein anionic polymer B is obtained by free radical aqueous emulsion polymerization of a mixture comprising ethylenically unsaturated monomers B, wherein the monomer mixture comprises: 0.5 to ≤5% by weight, based on the total amount of monomers B, of at least one acid-functional monomer, as a monomer B1 and 95 to 99.5% by weight, based on the total amount of monomers B, of at least one nonionic monomer, as a monomer B2.

19. The process according to claim 14, wherein the anionic polymer B is formed from: 0.5 to ≤5% by weight, based on the total amount of monomers B, of at least one acid-functional monomer, as a monomer B1, and 95 to 99.5% by weight, based on the total amount of monomers B, of at least one nonionic monomer, as a monomer B2, whereas at least 60% of the monomer B2 is selected from the group consisting of styrene, α-methyl styrene, methyl methacrylate, n-butyl acrylate and 2-ethylhexyl acrylate, and up to 40% of monomer B2 are other monomers B2.

20. The process according to claim 16, wherein the acid-functional monomer, as a monomer A1 and/or B1 are acrylic acid and/or methacrylic acid.

21. The process according to claim 14, wherein the pigment is selected from the group consisting of titanium dioxide, calcium carbonate, clay and talcum.

22. The process according to claim 14, wherein the anionic polymer A is obtained by free radical aqueous emulsion polymerization of a monomer mixture comprising 0.1 to ≤5% by weight, based on the total amount of monomers A of acrylic acid and 95 to 99.1% by weight, based on the total amount of monomers A, of at least one nonionic monomer A2, whereas at least 60% by weight of the monomer A2 is selected from the group consisting of vinyl acetate and ethyl acrylate; and the anionic polymer B is obtained by free radical aqueous emulsion polymerization of a monomer mixture comprising 0.5 to ≤5% by weight, based on the total amount of monomers B of acrylic acid and/or methacrylic acid and 95 to 99.5% by weight, based on the total amount of monomers B, of at least one nonionic monomer B2, whereas at least 60% by weight of the monomer B2 is styrene.

23. A process according to claim 14 wherein the anionic polymers A and B independently have a glass transition temperature in the range form −20° C. to 60° C.

24. Wood coated according the process of claim 14.

25. Wood coated according to claim 23, wherein the wood is pine.

26. A method for blocking stains comprising applying the process according claim 14.

Description

EXAMPLES

[0101] Unless the context suggests otherwise, percentages are always by weight. A reported content is based on the content in aqueous solution or dispersion if not stated otherwise.

[0102] The solids content was measured according to ISO 3251.

[0103] The pH-value was measured according to ISO 976.

[0104] The minimum film forming temperature was measured according to ISO 16808.

[0105] The glass transition temperature (Tg) was measured according to DIN EN ISO 11357-2 (2013-09) (differential thermo analysis, midpoint temperature, heating rate 20 K/min).

[0106] The weight average particle size of the polymer dispersions was measured according ISO 13321 using a High Performance Particle Sizer (Malvern) at 22° C. and a wavelength of 633 nm.

Preparation of the Polymer Dispersions D1:

[0107] A 4L-reactor was inertized by passing nitrogen through for 10 minutes, then charged with 700 g of demineralized water, 25 g of a 33% seed latex of polystyrene with a particle size of 33 nm. The reactor containing the above charge was heated to 85° C. and stirred over the complete time of synthesis. 5 g of 7 wt % sodium peroxosulfate aqueous solution was added at 85° C. After the addition, an emulsion feed was started and fed within 210 minutes. In parallel to the emulsion feed, 95 g of 7 wt % sodium peroxosulfate aqueous solution was started and fed to the reactor with 240 min. After the end of the initiator feed, the reaction mixture was cooled to 75° C. To the reaction mixture was then added 43 g of a 8 wt % aqueous solution of sodium hydroxide within 5 minutes. After that 26 g of a 10 wt % aqueous solution of tert-butyl hydroperoxide solution and 20 g of a 13 wt % solution of sodium sulfite were added within 60 minutes. After the end of the feed, the reaction mixture was cooled to room temperature.

[0108] The emulsion feed was prepared by mixing 520 g of demineralized water, 26 g of a sodium salt of a fatty alcohol polyglycol ether sulfate and 6.5 g of sodium dodecyl sulfate, and 1278 g of a mixture of monomers with was composed from 16 weight % of n-butyl acrylate, 14 weight % of 2-ethylhexyl acrylate, 57 weight% of styrene, 10 weight % of methyl methacrylate and 3 weight % of acrylic acid. The resulting latex D1 had a solids content of 48.1 wt %, the particle size was 146 nm measured by dynamic light scattering. The pH was 6.9. The glass transition temperature was measured to 40° C.

Preparation of Dispersion Dx (x=1 to x=7)

[0109] The dispersion Dx was prepared identical to the procedure of dispersion D1, only using the respective monomer mixtures of table 1. The results are summarized in table 2.

TABLE-US-00003 TABLE 1 Monomer compositions of the polymers A and B nBA EHA EA S BMA MMA AA MAA dispersion [wt %] [wt %] [wt %] [wt %] [wt %] [wt %] [wt %] [wt %] D1 16 14 57 10 3 D2 18 21 56 5 D3 37 60 3 D4 49 48 3 D5 75 20 5 D6 19 19 57 5 D7 97 3 nBA = n-butyl acrylate EHA = 2-ethylhexyl acrylate EA = ethyl acrylate S = styrene BMA = n-butyl methacrylate MMA = methyl methacrylate AA = acrylic acid MAA = methacrylic acid wt % = % by weight

TABLE-US-00004 TABLE 2 Characteristics of Dispersion D1 to D7 and comparative Dispersions D8 to D10 Hansch- solids content particle size parameter of T.sub.g Dispersion [wt %] [nm] pH polymer [° C.] D1 48.1 146 6.9 2.63 41 D2 48.3 150 7.0 1.60 53 D3 48.4 152 6.9 1.51 33 D4 48.2 149 6.8 1.62 15 D5 47.2 155 6.7 1.08 15 D6 48.4 145 6.9 2.72 27 D7 48.3 143 7.0 2.61 35 D8 0.79* 21 D9 0.83* 12 D10 0.73* 42 T.sub.g = glass transition temperature *estimated from T.sub.g

[0110] Dispersion D8, D9 and D10 are commercially available product based on the monomer vinyl acetate. The monomer composition was calculated using the glass transition temperature and the Fox equation under the assumption that the polymers are composed on only vinyl acetate and ethylene. With this theoretical composition, the respective Hansch parameter was calculated.

TABLE-US-00005 D8: Mowilith ® LDM 1852 (Celanese), theoretical monomer composition: 88 weight % vinyl acetate; 12 weight % ethylene; D9: Mowilith LDM 1871 (Celanese), theoretical monomer composition: 82 weight % vinyl acetate; 18 weight % ethylene; D10: Vinnapas ® DP 55 (Wacker); theoretical monomer composition: 100% vinyl acetate;

Preparation of the TiO.SUB.2.-Paste:

[0111] 14.6 g of tap water was put in a 100 ml vessel. 6.2 g of Dispex Ultra PX 4575 (BASF SE) was added to the water as dispersing agent. Subsequent, 0.3 g of Hydropalat WE 3650 (BASF SE) was added as wetting agent and 0.3 g of Foamstar SI 2210 (BASF SE) as defoamer. Subsequent 78 g of Kronos 2190, a titanium dioxide (TiO.sub.2) from Kronos Worldwide was added. The mixture was stirred in an Ultraturrax for 10 minutes at 5000 rounds per minute to disperse the TiO.sub.2.

Preparation of the Lacquers

Preparation of Clear Lacquers (CLx):

[0112] 100 g of the dispersion Dx was diluted by tap water to a viscosity of 200 mPas (Brookfield, 25° C., spindle 2, 20 rounds per minute) is necessary. Then, a 10% aqueous solution of ammonia was added to reach a pH value of 8.5. Subsequent, 0.5% of Hydropalat® WE 3221 (BASF SE) was added as wetting agent as well as 0.5% of Byk 24 (BYK) as defoamer. Subsequent, 6 g of ethylene glycol butyl ether was added as coalescing agent. 10 minutes stirring resulted in CLx.

Preparation of a Pigmented (“white”) Lacquer (PLx):

[0113] The pigmented lacquer PLx was prepared using the respective clear lacquer CLx. The pigmented lacquer was prepared by the addition of a TiO.sub.2-paste. To 80 g of clear lacquer CLx 20 g of TiO.sub.2-paste was added and stirred for 10 minutes.

Process of Coating Wood:

[0114] The barrier properties of the lacquers against penetration of colored substances from the wood was tested using fresh cut wood species containing knots. The wood pieces were cut in such a way that the knots in the wood were halved. The wood used was pinus cembra, in German “Zirbelkiefer”.

[0115] The freshly cut wood was coated afterwards with the respective lacquers using a slit coater with a slit of 200 μm. The whole surface including the knot cut into halves was coated.

[0116] The first coating (Composition A), the “primer” was dried at 25° C. for 24 hours in the lab. Subsequent, a second coating (Composition B), the “top coat”, was applied. The piece of wood, coated with a primer and top coat, was dried for 14 days at 25° C. in the lab.

Testing

[0117] Subsequent, the coated wood was put into a chamber for 72 hours for artificial weathering. Within the chamber, the coated, dry wood was irradiated using a Xenon arc lamp for 102 minutes and subsequent 18 minutes water was sprayed over the wood. 102 minutes of irradiation and 18 minutes of spraying water resembles 1 cycle. Within the 72 hours, 36 cycles were applied to the coated wood. The procedure is according to EN ISO 4892-2:2013 procedure A, cycle 1.

[0118] The coated wood was ranked visually according to the discoloration of the white coating over a knot.

[0119] Ranking 0: no discoloration visible

[0120] Ranking 1: a slight, yellowish-brown discoloration is visible.

[0121] Ranking 2: a distinct discoloration is visible.

TABLE-US-00006 TABLE 3 Application test results of the coating resulting from coating process with composition A and composition B Composition HP.sub.A Composition B E A (primer) polymer A (top coat) HP.sub.B polymer B Δ HP.sub.B − HP.sub.A Result E1 CL5 1.08 PL1 2.63 1.55 0 E2 CL3 1.51 PL1 2.63 1.12 1 E3 CL8 0.79 PL1 2.63 1.84 0 E4 CL9 0.83 PL1 2.63 1.80 0 E5  CL10 0.73 PL1 2.63 1.90 0 E6 PL5 1.08 PL1 2.63 1.55 0 E7 PL3 1.51 PL1 2.63 1.12 1 E9 PL5 1.08 PL1 2.63 1.55 0 E10 CL5 1.08 PL2 1.6 0.52 1 E11 CL3 1.51 PL6 2.72 1.21 0 E12 CL8 0.79 PL7 2.61 1.82 0 E13 CL3 1.51 PL2 1.6 1.09 1 CE CE14 CL5 1.08 PL5 1.08 0 2 CE15 CL1 2.63 PL1 2.63 0 2 CE16 PL1 2.63 PL1 2.63 0 2 CE17 PL1 2.63 CL1 2.63 0 2 CE18 CL8 0.79 PL8 0.79 0 2 CE19 CL9 0.83 PL5 1.08 0.25 2 CE20 CL3 1.51 PL3 1.51 0 2 CE22 CL6 2.72 PL6 2.72 0 2 CE23 CL6 2.72 PL2 1.74 −0.98 2 CE24 CL8 0.79  PL10 0.73 −0.06 2 CE25  PL10 0.73 CL8 0.79 0.06 2 E: Example CE: Comparative Example HP.sub.A: Hansch Parameter of Polymer A; HP.sub.B: Hansch Parameter of Polymer B Result: Ranking of discoloration after weathering

[0122] From the experimental data it can be seen that only the combination of a polar polymer with a Hansch parameter below 1.6 and unpolar polymer with a Hansch parameter above 1.6 are able to stop discoloration. Two polymers with Hansch parameters each below 1.6 or a single polymer applied twice as primer and top coat with Hansch parameters below 1.6 stop discoloration. Also two polymers or a single polymer applied twice each having a Hansch parameter above 1.6 do not stop discoloration.