Water-Borne Coatings Prepared from High Molecular Weight Epoxy

5 Water-Borne Coatings Prepared from High Molecular

Downloaded by QUEENSLAND UNIV OF TECHNOLOGY on October 16, 2014 | http://pubs.acs.org Publication Date: December 3, 1979 | doi: 10.1021/bk-1979-0114.ch005

Weight Epoxy Resins EDWARD G. BOZZI and CAMILLA Y. ZENDIG Epoxy Resins Laboratory, CIBA-GEIGY Corporation, Ardsley, NY 10502

As air pollution controls tighten, as fuel reserves dwindle and as industrial hazards are explored, solvents become increasingly unattractive as diluents for coatings. Industry has formulated several alternative coating technologies, probably the most advanced and easily substituted being water-borne. Our water-borne investigations led us recently to the discovery of techniques for preparing oil-in-water emulsions of our highest molecular weight epoxy resins. Known commercially as ARALDITE ® 488, these resins have weight average molecular weights of 15,000-20,000 and are ideal for applications requiring excellent adhesion and formability. Emulsions of these resins with relatively high solids content can be prepared while the corresponding solvent cuts have viscosities too high for use in coatings. For example, at 40% solids in Cellosolve acetate, ARALDITE 488 has a viscosity of 6000 cP. while the water-borne emulsion at the same solids level is only 1000 cP. For most methods of application the solids content of the solvent cut would have to be dropped below 40% to give a workable viscosity and this would exclude the resin from some important end uses such as can end and 3-piece can body coatings. The resin i n i t s emulsified form, however, has the proper solids/viscosity relationship for this particular end use. In the emulsified state, the high molecular weight epoxy resin exists as small spherical droplets surrounded by a surfactant which forms a "skin" over the droplets and thus prevents coalescence. While the viscosity within these particles may be great due to the high viscosity of the solvent cut of resin, the overall emulsion viscosity w i l l only be dependent on the total solids content and any water-soluble additives. Therefore, coatings with a high non-volatiles content can be made from a very high molecular weight epoxy resin i n i t s emulsified form but cannot be made from the same resin at an equivalent solids level i n a solvent cut. 0-8412-0525-6/79/47-114-071$05.00/0 © 1979 American Chemical Society

In Epoxy Resin Chemistry; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

EPOXY RESIN CHEMISTRY

72


APPLICATIONS Can End Coatings. In most cases, a two-step process i s used when r o l l e r coating the interior ends of beer and beverage cans. After the f i r s t resinous coating i s applied, the pull-tab i s formed and a second coating follows to cover any cracks i n the o r i g i nal coating due to rivet and scribing operations. If the f i r s t coating were, however, flexible enough to withstand the rivet and scribing operations, the second repair coating could be eliminated. We have evaluated our high molecular weight epoxy resin emulsion for this end use and have found that the f l e x i b i l i t y i n herent i n the solvent-borne resin has not been lost during emulsification. At high resin/ hardener ratios we found excellent flexi b i l i t y and pasteurization resistance and good formulation s t a b i l i t y . Table I compares the properties of the high molecular weight epoxy emulsion to a "7" type epoxy resin emulsion formulated with melamine and urea-formaldehyde hardeners. TABLE I FORMULATIONS AND PHYSICAL PROPERTIES FORMULATION NO.

I

II

245

237.5

"7" Type Epoxy Emulsion, 50% Solids

-

-

196

Melamine Hardener (Cymel 303)

2

-

2

ARALDITE 488 Emulsion, 40% Solids

HI

10

Urea Formaldehyde Hardener (Beetle 65, 50% i n H 0) 2

Resin Crosslinker Wedge Bend (mm) Beer Pasteurization (160°F/45 m)

Stability

98/2

95/5

98/2

10

0-1

15-20

No Change/Excellent Adhesion 14

Slight Blush/ Slight Blush/ Excellent Ad- Excellent Adhesion hesion 58+ Weeks

(Separation at 15 Weeks)

Poor flow and leveling, particularly during r o l l e r coating, and the need for an external catalyst, are, however, problems with


5.

Bozzi AND ZENDIG

73

Water-Bome Coatings


the high molecular weight epoxy emulsion. To solve these two problems, we formulated the epoxy emulsion with 15-20% of various water-base alkyds, polyesters and acrylics. After much experimentation we found only a few systems with good film properties and s t a b i l i t y and found in particular one alkyd, Kelsol 3900 from Spencer-Kellogg, to perform very well with our epoxy resin emulsion. Table II shows the formulation evaluated and the properties of the coating. TABLE II FORMULATION FOR CAN END COATING Epoxy Resin Emulsion (40% Solids) Alkyd (Kelsol 3900) (48.6% Solids) Melamine Hardener (Cymel 303) Water

-

73.4 15.1 4.1 7.4 100.0

Resins/Crosslinking Agent Epoxy/Alkyd Solids, % Viscosity, (#4 Ford Cup), sec.

-

90/10 4/1 40.8 45

PERFORMANCE PROPERTIES Substrate Application Method Cure Schedule (min./°F) Appearance 209

Can

End

Immersed in C a S 0 4 ,

min.

Beer Pasteurization, 45 m/160°F (Closed Container) Appearance: Adhesion: Water Pasteurization, 45 m/160°F (Closed Container) Appearance: Adhesion: Wedge Bend, mm

Treated Aluminum Roller Coated 3/400 Good Flow and Leveling Pass, 6

No Change Excellent

No Change Excellent 22

Automotive Primer Coatings. To those working in the f i e l d i t i s generally known that water-borne versions of "7" and "9" type epoxy resins do not meet the high corrosion resistance requirements for an automotive primer. However, the high crosslinking density and excellent adhesion properties afforded by the high molecular weight of our epoxy resin should make i t ideal for such end uses. Table III shows the formulation and performance properties


74

EPOXY RESIN CHEMISTRY


of our epoxy resin emulsion and appropriate primer coating f i l l e r s cured with a proprietary water-borne phenolic hardener. The best property of this coating i s i t s outstanding salt spray resistance. We obtained over 1500 hours salt spray resistance on Bonderite P-60 steel, with less than 1 mm creep. Even on cold rolled steel, the coatings displayed 300 hours resistance (< 3 mm creepage) while those of a commercial water-borne coating failed after only 24 hours exposure. TABLE III FORMULATION FOR AUTOMOTIVE PRIMER COATINGS Epoxy 1Resin Emulsion (40% Solids) Alkyd (Kelsol 3900) (48.6% Solids) •Borne Phenolic Hardener (32% Solids) Water-! Water Clay BaS04

Lampblack Ti02

38.3 13.0 18.0 8.6 6.5 13.0 1.0 1.6 100.0 88/12 70/30 48.5 50

Resins/Crosslinking Agent Epoxy/Alkyd Solids % Viscosity, (//4 Ford Cup), sec.

PERFORMANCE PROPERTIES Substrate Application Method Cure Schedule (min./°F) Appearance Salt Spray Resistance

Bonderite P60 Steel 0.003 mil Doctor Blade 30/325 Good Flow and Leveling 1500 Hours, < 1 mm Creep

Other Applications. We have evaluated our epoxy resin emul* sion with various thermoplastics. Results are shown graphically in Table IV.


5.

BOZZI AND ZENDIG

Water-Borne Coatings

TABLE IV THERMOPLASTIC MODIFIED EPOXY RESIN EMULSION


THERMOPLASTIC Polyvinyl Acetate (Gelva S55H, Monsanto)

Improved Impact and Acid Resistance

Vinyl Acetate/Acrylic Co« Polymer (Polyco 2151, (Borden)

Improved Adhesion and Acid Resistance

Alkyd (Kelsol 3906, Spencer-Kellogg)

Improved F l e x i b i l i t y and Dry Time

Acrylic (WS32, Rohm and Haas)

Improved Hardness and Dry Time

Adhesion i s usually improved while f l e x i b i l i t y i s maintained when using the high molecular weight epoxy emulsion as an upgrader i n water-borne thermoplastic systems. CONCLUSION Our investigations of very high molecular weight epoxy resin emulsions as evaluated i n can end, automotive primer and thermoplastic-type maintenance coating systems have yielded some very exciting and useful data. The largest technological challenge that remains i s i n obtaining proper rheology. This problem i s helped by the addition of very specific water-soluble resins. Commercial success of the high molecular weight epoxy resin emulsion w i l l depend, therefore, on how well we can solve these Theological problems. RECEIVED July 16, 1979.


Water-Borne Coatings Prepared from High Molecular Weight Epoxy

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