Low VOC SB Epoxy for Corrosion Resistant Primers with Improved Flexibility

Daniel Haile and Tim MillerThe Dow Chemical Company, Dow Coating Materials

Spring House, PA USA

ABSTRACT

Primers used in coating systems designed for corrosion resistance applications in the Maintenance and Protective Coatings market are exposed to aggressive environments such as solvents, marine atmospheres, and abrasion, among others. In the US, as well as other areas of the world, coating systems designed for these applications have been primarily solvent borne systems with high levels of volatile organic compounds (VOC). There is a strong desire among stake holders in the Maintenance and Protective Coatings arena to reduce the emission of VOCs as well as the human health impact of these coatings. One hundred percent solids coatings are one route to that objective; however, at least in the case of epoxy coatings, these have the drawbacks of short pot lives, application difficulty due to too high viscosity, and reduced corrosion resistance compared to lower solids coatings. The other choice has been replacement of organic solvents with water but this option can limit the range of temperatures and humidity ranges in which they can be applied. Dow Coating Materials has designed a low VOC, flexible epoxy resin which when properly formulated provides application friendliness and robust performance rivaling industry standard high VOC 1-type epoxy solvent borne coatings. Performance of single coat and multicoat low VOC, industry standard high VOC and commercial solvent borne systems will be compared. The range of conditions under which low VOC systems can be applied and good performance achieved will also be summarized

INTRODUCTION AND BACKGROUND

Coatings for heavy duty corrosion resistance such as bridges in coastal areas, the exteriors of tanks in refineries or chemical plants, lining of storage tanks and the like are generally solvent borne. Epoxy is the main chemistry for these applications because of its superior adhesion, thermal, chemical resistance, and relatively low cost. Solvent borne epoxies can be formulated with acceptable pot lives and cure times having volume solids ranging from 60 to 100%. Increasingly stringent regulations on VOCs and a desire to minimize risks from the VOC emission and health hazards of organic solvents have pushed coating formulators toward higher solids products. Higher solids products frequently have short pot lives, long cure times, poorer corrosion resistance, and greater application difficulty than 60-65% volume solids products. Manufacturers of epoxy resins and curing agents have worked to minimize these draw backs by appropriate modification of their products. Another approach which has gained wide acceptance in North America is the replacement of traditional organic solvents such as toluene, xylene, methyl ethyl ketone, propylene glycol methyl ether with the VOC exempt solvents, p-chlorobenzotrifluoride, acetone, dimethyl carbonate and t-butyl acetate (tBAC). These solvents have problems of odor, cost, flammability, and in some cases poor solvency.

Developing a low VOC resin system requires the use of resins that are either oligomeric or nearly monomeric during application and from which polymeric systems are built up only after application1. One approach to obtaining high performance and low VOC’s is to replace high molecular weight solid epoxy with modified low molecular weight epoxy and epoxy diluents to reduce viscosity. Dow Coating Materials has developed a set of novel low molecular weight and flexible epoxies as well as supporting starting

point formulations and performance data. The objective of this paper is to present some of our formulation insights and coating performance data using these new raw materials especially in the area of metal primers.

EXPERIMENTAL METHODSTesting was performed following ASTM test method guidelines. Exceptions will benoted in the text. A brief description of the testing procedures is also provided below

.Salt Spray Corrosion Resistance: After curing the coated panels for seven days at 77˚F and 65% RH, panels were scribed and ASTM 1172 was followed for salt spray exposure. After exposure at various time intervals, the blistering of the coating was determined via ASTM D714-8743.

Pot Life: In order to determine the pot life of the coatings, part A and B were mixed and initiall viscosity measured. Viscosity was run every half hour and end of pot life estimated by the time required for the coating to increase viscosity by 20 KU.

Dry times were determined using a Byk linear dry time recorder using the 24-hour scale. Flexibility and hardness were evaluated by a conical mandrel bend and Koenig pendulum hardness on Bonderite panels respectively.

Small coating batches were prepared using a Cowles mixer. Commercial coatings were purchased from US manufacturers and mixed according to the manufacturer’s technical data sheet. Parts A and B of coatings were mixed on the 100-300 gram scale and drawn down on Bonderite B-1000, steel or other substrate generally using a 4-mil gap 3-inch draw down bar. The coatings were allowed to cure for 7 days at 25 ˚C and 50% relative humidity unless otherwise noted. The dry film thicknesses of the coatings were in the range of 2-3 mils

RESULTS AND DISCUSSION

Curing Agent Screening

We prepared a solvent borne 2K 30% PVC, 75% volume solids anticorrosive primer formulation using our modified epoxy, and several anticorrosive pigments shown in Table 3-6. In the formulations all of the pigments were on the epoxy side allowing for easy comparison of various solvent borne curing agents. We examined two curing agents in this formulation for viscosity, cure rate and film formation as a function of mixing time as well as Cleveland Humidity and Salt Spray performance. The results are summarized in Figure 1, Table 1 and Table 2.

The modified epoxy-1and 2 in the anticorrosive primer are low viscosity fatty acid and dimer acid modified epoxies respectively. The curing agents used are low viscosity polyamide and fast cure Mannich base hardener which is used to improve cure time. The stoichiometric ratio used in all formulation is 1:1 epoxy to amine.

Formulation Comparisons

The formulation used for comparison purpose is given in Table 3-6. The VOCs of the commercial products were taken from the manufacturer’s product literature. We cured all of the coatings at 25 ˚C. The dry to touch and dry through times are generally in the same range except the commercial and solid epoxy based primers which have faster dry through time. It also shows that pot lives are similar except for one of the commercial product which only has only 1.5 hours. The VOC of the solid epoxy based formula is very high, 380g/l compared to 248g/l for the modified epoxy formulation. One of the commercial

formulations has a VOC below 200g/l and as result, the initial mixed viscosity is very high which requires that the paint has to be reduced with solvent to facilitate application. This will make the actual VOC higher.

Relative corrosion resistance evaluations as determined by Salt Spray and Cleveland Humidity are summarized in Table 2 and further illustrated by photographs in Figures 2. The modified epoxy anticorrosive primer shows very similar corrosion resistance as the industry standard solid epoxy based system. After 1500 - 2000 hrs exposure, the coating was removed. This is shown in the pictures below, figure 2. A very good performance was obtained relative to the commercial and 1-type epoxy based. There is also information available comparing Waterbased to Solventbased system that shows comparable result.4

CONCLUSIONS

The modified epoxy, from Dow Coating Materials allows the formulation of anticorrosive primers with application friendliness and performance comparable to conventional much higher VOC SB products. The modified epoxy in combination with epoxy diluents also gave very good corrosion resistance. When the modified epoxy is paired with the Dow adducted polyamine and polyamide curing agents, primers having good corrosion and humidity resistance, adhesion, cure time, flexibility and hardness can be formulated.

Reference

1 See for example, “Protective Coatings, Fundamentals of Chemistry and Composition” Clive Hare, Chapter 28

2 ASTM B117 – Standard Practice for Operating Salt Spray ( Fog ) apparatue, 2009

3 ASTM D714 – Standard Test Method for Evaluating Degree of Blistering of Paints, 1987

4 Francisco, J., Claudino, T., “ Waterborne Coatings For Heavy Duty Applications in M&PC Market, 2011, p. 3-7.

FIGURES AND TABLES

Figure 1. Viscosity as a function of time from initial mixing for modified epoxy based anticorrosive primer formulation.

0 30 60 90 120

150

180

210

240

270

300

60

70

80

90

100

110

120

130

140

Coating-4Coating-2Coating-1Commercial-2Coatring-3Commercial-1

Time, minutes

Visc

osity

, KU

Table 1. Coating performance for curing agents in a modified epoxy based anticorrosive primer formulation.

Coating Description Resin Type 4-hr Delta KU CommentCoating-1 Fatty Acid Modified

Epoxy7 Polyamide curing agent

Coating-2 Fatty Acid Modified Epoxy

13 Polyamide curing agent exempt solvent

Coating-3 Type 1 Solid epoxy 3 Polyamide curing agentCoating-4 Dimer Acid Modified

Epoxy10 Polyamide curing agent

Commercial-1   20 20 KU change in 1.5 hrsCommercial-2   7  

Table2. Coating performance for six anti-corrosive primer starting point formulation

Coating Description VOC

Pot Life

Dry to Touch

Dry Through

Direct Impact

Koenig Pendulum Hardness

Cleveland Humidity 2 Weeks

Salt Spray

  g/L hrs hrs hrs in-lb sec B10001000 Hours

Coating-1 245 >4 3.4 12.6 20 115 10 10Coating-2 141 4 3.3 8.5 20 108 10 10Coating-3 418 >4 5.5 7.8 20 100 10 8Coating-4 246 >4 3.3 13.5 20 107 10 9Coating-5 237 >4 1.8 5 20 10 10 Coating-6 255 1.5 10 10 Commercial-1 300 >4 3 5 20 71 10 10Commercial-2 180 1.5 3.6 5.8 20 81  10  6

Table 3. Coating-1 Anticorrosive Primer Formulation

Material Name Pounds GalGrind Modified Epoxy-1 199.66 21.36 Monofunctional epoxy diluent

29.62 4.00

Xylene 94.06 13.06 Dispersant 3.25 0.41 Defoamer 1.08 0.13 Calcium Silicate 303.23 12.52 Zinc Phosphate 90.81 3.61 Barium Sulfate 162.26 4.52 Red Ironoxide 95.87 2.20 Thickener 13.93 1.03 Grind Sub-total 993.76 62.85Grind above in order on Cowles, then reduce speed and add following Modified Epoxy-1 96.10 10.28 PM Acetate 14.64 1.81 MIBK 17.90 2.68 Xylene 17.06 2.37Grind total 1139.46 79.99Part B Premix Polyamide 113.26 14.16 Tertiary Amine 14.66 1.81 Xylene 29.02 4.03 Premix Sub-total 156.94 20.00Totals 1296.40 100.00Property ValueTotal PVC 32.2 %Volume Solids 70.9 %Weight Solids 83.0 %Density 12.96 lb/galVOC 245 g/l

Table 4. Coating-2 Anticorrosive Primer Formulation

Material Name Pounds GallonsPart AGrind Modified Epoxy-1 200.09 21.40 Monofunctional epoxy diluent

29.68 4.01

Xylene 40.02 5.56 t-butylacetate 60.00 8.32 Dispersant 4.31 0.55 Defoamer 4.31 0.52 Calcium Silicate 292.68 12.09 Zinc Phosphate 86.00 3.42 Barium Sulfate 162.61 4.53 Red Ironoxide 91.08 2.09 Thickener 15.21 1.13 Grind Sub-total 116.46 0.00Grind above in order on Cowles, then reduce speed and add following Modified Epoxy-1 96.30 10.30 PM Acetate 0.00 0.00 MIBK 0.00 0.00 Xylene 0.00 0.00 t-butylacetate w 44.21 6.13 Grind total 218.23 0.00Part B Premix Polyamide 113.51 14.19 Tertiary Amine 14.69 1.82 Xylene 28.81 4.00 Premix Sub-total 20.01 0.00Totals 1283.53 100.05Property ValueTotal PVC 31.5 %Volume Solids 70.3 %Weight Solids 82.4 %Density 12.83 lb/galVOC 141 g/l

Table 5. Coating-3 Anticorrosive Primer Formulation

Material Name Pounds GallonsPart AGrind Solid Epoxy Resin Solution 277.46 30.56 Urea Resin 10.01 1.16 Xylene 128.06 17.79 Dispersant 2.68 0.34 Defoamer 1.15 0.14 Calcium Silicate 232.20 9.59 Zinc Phosphate 68.22 2.72 Barium Sulfate 100.65 2.80 Red Ironoxide 100.65 2.30 Thickener 12.07 0.89 Grind Sub-total 933.14 68.29 Grind above in order on Cowles, then reduce speed and add following PM Acetate 24.77 3.07 MIBK 57.81 8.65 Grind total 1015.73 80.00Part B Premix Polyamide 129.44 16.60 Xylene 24.48 3.40 Premix Sub-total 153.92 20.00Totals 1169.65 100.00  Property ValueTotal PVC 33.3 %Volume Solids 52.3 %Weight Solids 69.0 %Density 11.70 lb/galVOC 418 g/l

Table 6. Coating-4 Anticorrosive Primer Formulation

Material Name Pounds GallonsPart AGrind Modified Epoxy-2 200.16 22.04 Monofunctional epoxy diluent 29.69 4.01 Xylene 108.19 15.03 Dispersant 3.06 0.39 Defoamer 1.31 0.16 Calcium Silicate 292.79 12.09 Zinc Phosphate 86.03 3.42 Barium Sulfate 162.67 4.53 Red Ironoxide 91.11 2.09 Thickener 15.21 1.13 Grind Sub-total 990.22 64.89Grind above in order on Cowles, then reduce speed and add following Modified Epoxy-2 96.34 10.61 PM Acetate 14.67 1.82 MIBK 17.94 2.68 Xylene 0.00 0.00 Grind total 1119.18 80.00Part B Premix Polyamide 109.57 13.70 Tertiary Amine 14.70 1.82 Xylene 32.26 4.48 Premix Sub-total 156.52 20.00Totals 1275.70 100.00Property ValueTotal PVC 31.3 %Volume Solids 70.8 %Weight Solids 82.6 %Density 12.76 lb/galVOC 246 g/l

Figure 2. Photographs of anticorrosive (AC) primers on bonderite B-1000 panels after salt spray exposure.

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