CA1159992A - Melamine coatings - Google Patents

Abstract

Abstract of the Disclosure Ultraviolet resistant transparent coatings are pro-vided formed of (A) a melamine nucleus compound, (B) a benzo-phenone and preferably (C) a polyol. The compositions are effective in protecting polycarbonate resins and other UV
degradable materials.

Description

39'~

IMPROVED MELAMIN~ COATINGS
Background of the Invention Polymeric ultraviolet light absorbers are especially desirable in thin films of between about 0.05 mils and about 0.5 mils since monomeric absorbers are readily lost by diffu-sion, and solvent leaching because of the high surface area in relation to the volume of material employed. The preparation of polymeric UV-absorbers usually involves the vinyl polymeriza-tion of substituted UV-screeners such as (2-hydroxy-4-methacryl-oxybenzophenone) or the condensation polymerization of properly substituted UV-screeners such as 2-hydroxybenzophenone-4,4'-dicarboxylic acid with glycols or 2-hydroxy-4,4'-bishydroxy-methyl benzophenone with diacids.
It has now been discovered that improved polymeric films can be formed from a melamine nucleus compound, a benzo-phenone and preferably a polyol. Moreover, the benzophenone appears to be incorporated in the polymer as indicated by long term thermal aging tests, which renders the transparent coating ideally suited for protecting transparent polycarbonates, and other UV degradable materials.
Brief Description of the Invention Ultraviolet resistant surface coatings are provided of a transparent copolymer of (A) a melamine nucleus compound of the formula, R ~ N ~ / R
N-C C-N
R/ l ll R
N N
~ C/

N

~ 9 ~ 2 RD-~679 wherein the R groups are independently selected from H, -CH2O~I, and -CH2O(CH2)XH, wherein x is an integer of from 1 to 4; and a stabilizing amount (B) of a benzophenone of the formula, D-C~

R' wherein R' is selected from -OH and -NH2 groups in the 3,4 or 5 positions of the ring, and D is an aromatic radical of less than 4 six membered rings which can be substituted with -OH and -NH2 groups; and preferably (C) a polyfunctional compound con-taining at least two hydroxyl groups. The coatings are particu-larly well suited for protecting polycarbonate resins and otherUV light degradable materials.
Detailed Description of the Invention Melamine nucleus compounds which can be employed in the invention are those of the above formula wherein the R
groups can be methoxymethyl, ethoxymethyl, propoxymethyl, or butoxymethyl and hydrogen. Preferably the R groups are all the same and are alkoxymethyl.
The polyfunctional compound containing at least two hydroxyl groups can be aromatic or aliphatic. Representative aromatic compounds are phenols which include resorcinol, 2,2'-methylenediphenol, 2,4-methylenediphenol, 4,4'-isopropylidene-diphenol, 4,4'-(cyclohexylidene)diphenol, and 4,4'-dihydroxydi-phenol, and 4,4'-dihydroxydiphenylsulfone. Representative aliphatic compounds are alcohols which include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, ~ - 2 -ii5~992 RD-9679 propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentane-diol, 1,2,3-propanetriol, pentaerythritol and sorbitol. In acldition, the polyfunctional hydroxyl compound can be an alkyd resin, such as a hydroxyl containing epoxy resin, a soluble cellulose derivative, a vinyl polymer having free hydroxyl groups, such as poly(vinyl alcohol) or partial saponified poly (vinyl acetate). The polyfunctional hydroxyl compound (e.g.
polyol) can also contain carboxyl and amine groups but should contain at least two hydroxyl groups.
Among the dihydroxybenzophenones of the above formula which can be employed are the following:

2,3-dihydroxybenzophenone, 2,4-dihydroxybenzophenone, 2,5-dihydroxybenzophenone, 2,3'-dihydroxybenzophenone, 2,4'-dihydroxybenzophenone, 2-hydroxy-5-aminobenzophenone, 2-hydroxy-4'-aminobenzophenone, 2,2',4,4'-tetrahydroxybenzophenone, 2,2',3,3'-tetrahydroxybenzophenone, 2,2',5,5'-tetrahydroxybenzophenone, dihydroxynaphthophenones, dihydroxyanthrophenones, dihydroxydinaphthoketones, dihydroxyanthrones, etc.
The preferred compounds are where D is a substituted or unsubstituted benzene ring.
The benzophenone should be used in an amount suffic-ient to reduce the UV light degradation. Generally, from about one to about five percent by weight of the composition is sufficient.
X

~ 2 RD-9679 The benzophenone can be reacted solely with the mel-amine nucleus compound but preferably a polyol is employed such as described in the specification. Generally, the melamine nucleus compound will constitute from about 20 to about 80 per-cent of the mixture and the polyol the remainder, exclusive ofthe benzophenone.
In order to form the UV light resistant composition and apply it to a suitable substrate, the reactants can be dissolved or suspended in a suitable solvent such as n-butanol, ethanol and the like, preferably with a suitable acid catalyst which is activated at elevated temperature such as benzene sul-fonic acid and sulfamic acid and preferably with a surface active agent to aid in forming a film of the composition. A
variety of catalysts and surface active agents can be employed and are commercially available.
The coating composition can be applied to a suitable substrate by conventional means such as spraying, dipping and the like. The thickness of the coating is not critical but will generally be between about 0.05 mil and about 0.5 mil for a sub-strate of between about 1 mil and about 0.5 inches. After appli-cation, the reaction can be accomplished at a temperature between about 100 and about 150C in a period of from 15 minutes to about four hours in an air oven. The resultant article is then resistant to UV degradation and it is found that the benzophenone does not leach out as it is reacted within the composition.
Among the materials which can be protected by the compositions of the invention are those which are readily degraded by UV light such as, for example, polycarbonates, poly-carbonate-polysiloxane copolymers, polystyrene, polyvinyl chloride, ABS polymers~ poly(2,6-dimethylphenylene oxide) alone l i ~g ~ ~ z RD-9679 or copolymerized with high impact polystyrene, or even wood.
The following examples will serve to illustrate the invention, but are not meant to be limiting. All parts and percentages in said examples and elsewhere in the specification and claims are by weight unless otherwise indicated.
A coating blend of 750 parts of hexamethoxymethyl-melamine and a like amount of caprolactone polyol (Nyax Polyol PCP-0300) was mixed with 7.5 parts surface active agent (Mall-inckrodt BYK-300) and catalyzed with 1.5% of p-toluene sulfonic acid. The reaction mixture was then diluted with 1500 parts N-butanol to 50% solids and a 10 mil Lexan polycarbonate sheet cleaned with isopropanol dipped into this coating blend, with-drawn slowly and allowed to drain for 5 minutes at room temper-ature to permit the solvent to evaporate. The coated sheet was cured for an hour at 125C in a circulating air oven.
The above procedure was repeated several times but for the exception that to 600 parts of the above mixture was added one of the following W-stabilizers in the amount indicated.
Examples 1-7 20Example 1: 15 parts (5% on solids) 2,4-dihydroxy-benzophenone (DHBP) Example 2: 15 parts (5% on solids) Resorcinol mono benzoate Example 3: 15 parts (5~ on solids) 2(2'-hydroxy-255'-octylphenyl)benzotriazole (Cyasorb 5411) Example 4: 9 parts (3% on solids) 2,4-dihydroxybenzo-phenone (DHBP) Example 5: 9 parts (3% on solids) 2-hydroxy-4-n-octoxybenzophenone (Cyasorb UV 531) 30Example 6: 9 parts (3% on solids) ethyl-2-cyano-

-3,3-diphenyl acrylate (Uvinul N-35) Example 7: 9 parts (3% on solids) 2-cyano-3,3-diphenyl acrylic acid X

~ 9~2 RD-9679 Lexan polycarbonate panels coated with the various blends of samples 1, 2 and 3 were exposed to UV light from RS sunlamps. The protection afforded by the various screeners is most easily seen by the change in yellowness index (~ YI) of the various samples. The sample number 1 had the lowest A YI
of 1.66 while sample number 2 had a ~YI of 13.63 compared to 13.68 for an unprotected control and sample number 3 had a ~YI
of 3.74. The change in yellowness was determined by the method of ASTM D-1925-70 for samples tested for 1000 hours.
Ten mil Lexan polycarbonate film samples were then coated on one side with one of the coating blends of Examples 4, 5, 6 and 7 and allowed to air dry for 30 minutes at room temperature (25-30C). The UV absorbances of these uncured coatings were measured with a UV-spectrometer before and after curing for 1 hours at 125C. The coated films were then baked for 50 hours at 125C while UV-absorbance measurements were taken at intervals.

Absorbance Cured Baked Baked Baked Baked Sample Peak~ Uncured 1 hr 3 hrs 13 hrs 20 hrs 50 hrs

4 328 nm 2.391 2.184 2.090 2.184 2.223 2.226 293 nm 2.283 1.468 1.086 0.650 0.557 0.539 6 305 nm 2.592 0.253 0.041 0.053 0.059 0.070 7 304 nm 2.568 2.700 2.625 1.905 1.276 0.275 From an examination of the data it can be seen that the composition of the invention, sample 4, is considerably more resistant to thermal degradation after an extended period of 50 hours.
In repeating the examples, similar results are achieved with other benzophenones of the invention, such as ~ - 6 -2,3-dihydroxybenophenone, 2,5-dihydroxybenzophenone, 2-hydroxy-5-aminobenzophenone, 2-hydroxy-4'-aminobenzo-phenone, 2,2',4,4'-tetrahydroxybenzophenone and 2,4-dihydroxynaphthophenone; and with polyols such as polyvinyl alochol, ethylene glycol, and propylene glycol;
melamine compounds such as hexa(ethoxymethyl)-melamine and polymers such as a copolymer of Lexan polycarbonate and polydimethylsiloxane.

Claims (14)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A composition resistant to ultraviolet radiation when cured consisting essentially of by weight (A) from about 20% to 80% of a melamine compound having the formula wherein the R groups are independently selected from H, -CH2OH, and -CH2O(CH2)xH groups, wherein x is an integer from 1 to 4, (B) from 80% to 20% of a polyol and (C) a stabilizing amount of a benzophenone of the formula, wherein R' is selected from -OH and -NH2 groups in the 3,4 or 5 positions of the ring, and D is an aromatic radical of less than 4 six membered rings which can be substituted with -OH
and -NH2 groups.

2. The composition of claim 1 where the polyol is an aliphatic straight chain polyol.

3. The composition of claim 2, wherein the melamine nucleus compound is hexa(methoxymethyl)melamine.

4. The composition of claim 2, wherein the melamine nucleus compound is hexa(methylol)melamine.

5. The composition of claim 2, wherein the benzo-phenone is 2,4-dihydroxybenzophenone.

6. The composition of claim 1, wherein D is a benzene radical.

7. An article comprising a substrate and a surface coating of a transparent ultraviolet light resistant polymer consisting essentially of the reaction product of (A) from about 20% to 80% of a melamine compound having the formula, wherein the R groups are independently selected from H, -CH2OH, and -CH2O(CH2)xH groups, wherein x is an integer from 1 to 4, (B) from 80% to 20% of a polyol and (C) a stabilizing amount of a benzophenone of the formula wherein R' is selected from -OH and -NH2 groups in the 3,4 or 5 positions of the ring, and D is an aromatic radical of less than 4 six membered rings which can be substituted with -OH and -NH2 groups.

8. The coating of claim 7 wherein the additional compound is an aliphatic straight chain polyol.

9. A coating of claim 8 wherein the melamine nucleus compound is hex(methoxymethyl)melamine.

10. A coating of claim 8 wherein the melamine nucleus compound is hexatmethylol)melamine.

11. A coating of claim 8 wherein the benzophenone compound is 2,4-dihydroxybenzophenone.

12. The article of claim 7 wherein the substrate is a transparent polycarbonate resin.

13. A process for inhibiting the UV light degradation of a UV light degradable substrate comprising (1) coating said substrate with a mixture consisting essentially of by weight (A) from about 20% to 80% of a melamine compound having the formula, wherein the R groups are independently selected from H, -CH2OH, and -CH2O(CH2)xH groups, wherein x is an integer from 1 to 4, (B) from 80% to 20% of a polyol and (C) a stabilizing amount of a benzophenone of the formula, wherein R' is selected from -OH and -NH2 groups in the 3,4 or 5 positions of the ring, and D is an aromatic radical of less than 4 six membered rings which can be substituted with -OH
and -NH2 groups, and (2) heating said mixture to effect reaction.

14. The process of claim 13 wherein the film is between about 0.05 mils and about 0.5 mils.