AU713911B2 - Coating with improved dirt pick-up resistance - Google Patents

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION NAME OF APPLICANT(S): Rohm and Haas Company ADDRESS FOR SERVICE: DAVIES COLLISON CAVE Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

INVENTION TITLE: Coating with improved dirt pick-up resistance The following statement is a full description of this invention, including the best method of performing it known to me/us:a a a The present invention relates to coatings having improved dirt pick-up resistance, particularly at high ambient temperatures and in high humidity.

After exposure to the atmosphere, exterior coatings tend to pick up particles of atmospheric dirt, which causes a slight discolouring of the coating with time. This problem is greater in hot and humid conditions when coatings are more tacky. It can be reduced by employing a polymer having a relatively high glass transition temperature however, coatings containing high Tg polymers have other disadvantages such as lack of flexibility and a need for volatile organic coalescents to effect film formation, and thus a coating containing purely high Tg polymers is generally not desirable.

Another solution to the problem is to employ a two-stage polymer, having a low Tg portion and a highTg one. US 5063091 discloses two-stage polymers for use as a thick elastomeric coating comprising methyl methacrylate, butyl acrylate and N-methylol methacrylamide.

Such polymers are said to have relatively low tack at high temperatures, and thus good dirt pick-up resistance, despite at least one stage having a low glass transition temperature (Tg); this is due to the presence of the hard, high Tg phase. However, two stage polymers are more difficult to manufacture and are undesirable in certain applications because of problems caused by the incompatibility of the hard and soft phases. Furthermore, the coatings disclosed in US 5063091 are relatively thick, and are not suitable for all applications. Accordingly there is still a need for a single stage polymer which has a suitable dirt pick-up resistance without the attendant disadvantages of a very high Tg component.

SUS 3749690 discloses latex paints comprising an aqueous single-stage polymer emulsion of, inter alia, methacrylic acid, N-methylolacrylamide, ethyl acrylate and acrylonitrile. Such compositions are said to have a good spread of properties such as burnish resistance, scrub resistance, stain removal and low-temperature coalescence. There is no indication as to .whether or not these compositions have good dirt pick-up resistance, particularly at high temperatures and at high humidity.

We have discovered that certain known single stage polymer compositions, despite having relatively low Tgs, provide coatings which exhibit surprisingly good dirt pick-up resistance, particularly in hot and humid conditions, compared with coatings based on compositions having significantly higher Tgs.

Accordingly the present invention comprises in one aspect the use of a single-stage polymer composition comprising as monomer components: N-methylol (meth)acrylamide, and one or more of alkyl acrylates, alkyl methacrylates, styrene, vinyl acetates and/or vinyl esters; 4 and having a Tg of from -20°C to Sto improve the dirt pickup resistance of an aqueous coating composition.

The invention is especially valuable for improving the dirt pickup resistance of coating compositions in conditions of high ambient temperature (eg over 25 0 C, particularly over and high relative humidity (eg over 80%, particularly over It is believed that the presence of the N-methylol (meth)acrylamide is responsible for the surprisingly good dirt pickup performance. Thus in a further aspect the invention provides a method of improving the dirt pickup resistance of an aqueous coating composition which contains a single-stage polymer having a Tg of f-om -20°C to 40°C and comprising monomer units of one or more of alkyl acrylates, alkyl methacrylates, styrene, vinyl acetates and/or vinyl esters, comprising incorporating with said monomer units, prior to polymerization, units of N-methylol (meth)acrylamide.

The N-methylol (meth)acrylamide is generally present in the polymer composition at a level of from 1 to 15 by weight, although more may be used if its presence is desired for other purposes.

Preferred polymer compositions for use in the invention comprise monomer units of Nmethylol (meth)acrylamide,

(C

1 -Cs)alkyl acrylate and (C 1

-C

4 )alkyl methacrylate. More preferred are compositions comprising 1-6 by weight N-methylol (meth)acrylamide, 20-70 by weight (Ci-C 4 )alkyl acrylate, and 20-70 by weight (Ci-C 4 )alkyl methacrylate.

It is also preferred that polymer compositions for use in the invention have a Tg of between 0 and 40 0 C, more preferably between 10 and 30 0

C.

EXAMPLES

Coatings were prepared containing as binders three different compositions as specified below.

Binder A: Polymer of methyl methacrylate/ethyl acrylate/N-methylol acrylamide (45/51/4 wt%) S Binder B: Polymer of methyl methacrylate/butyl acrylate/N-methylol acrylamide (59.8/36.2/4 wt%) Binder C (comparative): Polymer of methyl methacrylate/butyl acrylate/ureido-functional methacrylate/methacrylic acid (52.4/45.6/1/1 wt%) The coatings had the following compositions (all weights in kg): BINDER A BINDER B BINDER C Grind preparation: Propylene glycol 40 40 Water 130 130 130 TAMOL 731 18 18 18 NOPCO NXZ 5 5 PRIMAL QR-708 16.23 1.0 HMT SKANE M8 KATHON LX FI-PURE R960 Calcium carbonate Ground for 15-20 minutes, then let down Binder 485 (A) Water 42.24 OPAQUE OP-62 135 NOPCO NXZ 2 PRIMAL RM-1020 28 TEXANOL 34 with the following: 485 (B) 42.24 135 2 28 34 442 (C) 100 135 2 28 22 99 9 99 9*9* 99 9 9 .9 9*9* 99

*C

9 999999 99 9 99 9 The three coatings were coated on asbestos panels, which had been sealed with PRIMAL

WL-

91. Two coats were applied at intervals of 24 hours, and panels exposed to North (vertical) and South (vertical, at 50 and at 450) in Singapore.

After one month's exposure, the panels containing Binders A and B were much cleaner than that containing C, with Binder A slightly better than B.

The various trade names referred to in the above Examples designate the following: 1) TAMOL 731 Sodium salt of carboxylated polyelectrolyte pigment dispersant Rohm and Haas Co.

2) NOPCO NXZ defoamer Henkel Corp.

3) PRIMAL QR-708 hydrophobically modified polyurethane- rheology modifier Rohm and Haas Co.

4) SKANE M-8 isothiazolone biocide Rohm and Haas Co.

5) KATHON LX 1.5 isothiazolone biocide Rohm and Haas Co.

6) TI-PURE R960 titanium dioxide pigment DuPont Co.

7) ROPAQUE OP-62 emulsion polymer, particles hollow when dried pigment Rohm and Haas Co.

8) PRIMAL RM-1020 hydrophobically modified polyurethane rheology modifier Rohm and Haas Co.

9) TEXANOL 2,2,4-trimethyl-1,3-pentanedioimonoisobutyrate coalescent -Eastman Chemical Co.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer group of integers or steps.

Claims (9)

1. Use of a single-stage polymer composition comprising as monomer components: N-methylol (meth)acrylamide, and one or more alkyl acrylates, alkyl methacrylate styrene, vinyl acetate and/or vinyl esters; and having a Tg of from -20°C to 40 0 C, to improve the dirt pickup resistance of an aqueous coating composition.

2. Method of improving the dirt pickup resistance of an aqueous coating composition which contains a single-stage polymer having a Tg of from -20"C to 40°C and comprising monomer units of one or more of alkyl acrylates, alkyl methacrylates, styrene, vinyl acetate and/or vinyl esters, comprising incorporating with said monomer units, prior to polymerization, units of N-methylol (meth)acrylamide.

3. Use or method according to claim 1 or 2 wherein the dirt pickup resistance is improved at a temperature of 25 C or higher, and a relative humidity of at least 15 4. Use or method according to claim 3 wherein the dirt pickup resistance is improved at a temperature of 30 0 C or higher, and a relative humidity of at least Use or method according to any one of the preceding claims wherein the Tg of the polymer is between 0 and 40 0 C.

6. Use or method according to claim 5 wherein the Tg of the polymer is between 10 and 30 0 C.

7. Use or method according to any one of the preceding claims wherein the polymer comprises units of N-methylol (meth)acrylamide, (C 1 -C)alkyl acrylate and (C, -C 4 )alkyl methacrylate.

8. Use or method according to any one of the preceding claims wherein the N- methylol (meth)acrylamide is present in the polymer at a level of from 1 to 15% by weight.

9. Use or method according to any one of the preceding claims wherein the polymer comprises 1-6% by weight N-methylol (meth)acrylamide, 20-70% by weight (Cl- Cs)alkyl acrylate, and 20-70% by weight (CI-C 4 )alkyl methacrylate a a a. a a. a as a. a a. P:\OPER\AXD\1743028.RS 19/10/99 Use or method according to any one of the preceding claims wherein the polymer comprises units of N-methylol (meth)acrylamide, butyl or ethyl acrylate and methyl methacrylate.

11. Use of a single-stage polymer composition substantially as hereinbefore described with reference to the Examples.

12. Method of improving dirt pickup or resistance substantially as hereinbefore described with reference to the Examples. DATED this 19th day of OCTOBER, 1999 ROHM AND HAAS COMPANY by DAVIES COLLISON CAVE Patent Attorneys for the Applicant a o