Self-emulsifiable soybean oil phosphate ester polyols for low-VOC corrosion resistant coatings.

A series of soybean oil phosphate ester polyols (SOPEP) was prepared by reaction of fully epoxidized soybean oil with phosphoric acid and simultaneous hydrolysis in the presence of a polar solvent. The polyols were characterized by determination of acid value, oxirane number, hydroxyl value, molecular weight (GPC), and FTIR spectra. These polyols with varying amounts of acid phosphate groups could be self-emulsified to form aqueous dispersions after neutralization with organic base. These aqueous dispersions showed varying degrees of stability and their appearance ranged from opaque dispersions to translucent to clear solutions. Waterborne coating compositions were prepared using these aqueous dispersions as principal components and their thermally cured film properties were studied. It was found that by careful selection and formulation, SOPEPs can be successfully used for low-VOC waterborne coating formulations. SOPEPs with 3.5% phosphate ester content showed visibly superior corrosion resistance properties.

Keywords: Soybean oil phosphate ester polyols, self-emulsified aqueous dispersion, corrosion resistance, VOC, corrosion testing, hardness, scratch resistance, solvent resistance, alkyds, oils (castor, linseed, styrenated, sunflower), water-borne, adhesion, corrosion, weatherability, container, enamel, high-solids, water-based, VOC control, epoxy, esters, oxygenated, VOC, aluminum, iron, cast iron, steel

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Soybean oil is the most readily available and one of the least expensive vegetable oils in the world. There is currently significant interest in the use of soybean oil as a component in printing inks, (1) as plasticizers, (2,3) and as stabilizers in the manufacture of plastic parts due to its environmentally friendly, biodegradable, and noncorrosive properties. Moreover, it is increasingly attractive to incorporate soybean oil into the making of water-dispersible polymers and resins for packaging films and curing agents used in surface coatings, (4) which are more energy conserving and environmentally desirable than solvent-based paints. (5) Epoxidized soybean oil (ESO, Figure 1), produced by reaction of soybean oil with peracetic acid, has been investigated as an epoxy resin in UV-cured coatings. (6,7) Thames and Yu have reported cationic UV-cured coatings of epoxide-containing soybean oils that provide corrosion resistance. (6) Recently, ESO has been an attractive option for use in high-solids, low-/zero-VOC thermal cure coatings, owing to its low viscosity, commercial availability, low cost, and numerous reactive oxirane groups.

Our group has investigated direct incorporation of ESO into UV-cure systems as an additional epoxy component. (8) We also prepared soybean oil phosphate ester polyols (SOPEP, Figure 2) by careful hydrolysis of ESO with phosphoric acid as catalyst, and incorporated them into low VOC industrial bake coatings possessing improved adhesion properties due to the presence of the phosphate ester groups. (9,10) The authors investigated the effects of catalysts, solvents, and the amount of water on the hydrolysis of ESO. (11) Higher boiling hydroxylic solvents gave faster reactions and the water used in the hydrolysis of the phosphate triesters needs to be controlled to prevent hydrolysis of the monophosphate ester. Zhang et al. reported that polymeric structure has great effect on the adhesion and corrosion resistance of epoxy coatings. (12) Massingill reported that solventborne advanced epoxy phosphate esters of bisphenol A diglycidyl ethers containing 1% phosphoric acid gave coatings improved adhesion, flexibility, and corrosion resistance. (13) In that study, 1% phosphoric acid in the advanced epoxy coating increased adhesion (measured by Instron) by an order of magnitude. In the present work, the self-emulsification of neutralized soybean oil phosphate ester polyols, optimization of phosphate content for corrosion resistance, and application of the resins for corrosion resistant water-based coatings were investigated.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

EXPERIMENTAL

Materials

Epoxidized soybean oil (Vikoflex[TM] 7112 obtained from Atofina; iodine value -1.35, acid value 0.08 mg KOH, and % oxirane oxygen -7.0) was used as supplied. Commercial alkyd resin, Beckosol[TM] 12-054, and aminoplast crosslinker, Resimene[TM] 747, were supplied by Reichhold, Inc. A waterborne alkyd container coating was obtained from Precision Coatings as a commercial paint standard. Cycat[TM] 4040 acid catalyst was obtained from Cytec. The test panels used were B-1000 from Q-Panel, Inc. All other chemicals were supplied by Aldrich Chemical Co. and were used without further purification.

[FIGURE 3 OMITTED]

SYNTHESIS OF SOYBEAN OIL PHOSPHATE ESTER POLYOLS: The reaction product of one phosphoric acid molecule with three epoxidized soybean molecules is the phosphate triester. The triester can be selectively hydrolyzed to the more stable monoester with water. Soybean oil phosphate ester polyols (SOPEPs) were prepared by reaction of ESO with phosphoric acid and water, in the presence of t-butyl alcohol. The process involved the addition of phosphoric acid diluted in solvent, at a controlled rate, to the mixture containing ESO, solvent, and water, with vigorous mechanical stirring. A series of SOPEPs was synthesized using phosphoric acid varying from 2-30% by weight of ESO. The reaction temperature was slowly raised to 85[degrees]C (reflux temperature) and maintained until completion of reaction. The reaction was monitored by periodic determination of percent oxirane oxygen content and acid value of the sample. Upon completion of the reaction, solvent and residual water were removed using a rotary vacuum evaporator. Unreacted phosphoric acid was removed by successive washing an ethereal solution of the product with water. SOPEPs were then characterized for various physical, chemical, and spectroscopic parameters.

PREPARATION OF AQUEOUS DISPERSIONS OF SOPEP: SOPEP was neutralized with calculated amount of amine (based on 100% neutralization) while stirring at room temperature. Dimethylethanolamine was used as amine neutralizer. Deionized water was added slowly to the neutralized SOPEP and stirred at 2000 rpm. The mixture was stirred for another 30 min after addition of water was complete. The resultant dispersions were milky to translucent to clear solution, depending upon phosphate ester content of the product. All the dispersions were prepared to have 50% SOPEP, with no co-solvent.

CHARACTERIZATION OF POLYOLS: Acid values (A.V.), oxirane numbers (O.N.), and hydroxyl values (H.V.) of the SOPEP samples were determined by ASTM methods (ASTM D 1639-90, ASTM D 1652-97, and ASTM D 1957-86, respectively). The viscosities of the samples were measured using a Brookfield viscometer at 25[degrees]C. The molecular weight was determined by gel permeation chromatography (GPC) using a polystyrene standard. The SOPEP samples were also characterized with FTIR spectra. Salt fog corrosion resistance tests were carried out as per ASTM B-117, for 96 hr. Results are reported in Table 1.

RESULTS AND DISCUSSION

SOPEPs with Different Phosphate Ester Content

In this article, the term "phosphate ester content" is used to indicate the amount of phosphoric acid used in the synthesis of SOPEP. A series of soybean oil phosphate ester polyols with varying phosphate ester content (2, 3, 3.5, 4, 5, 6, 8, 10, 20, and 30 wt%) were synthesized from fully epoxidized soybean oil using phosphoric acid and water, in the presence of a polar water-miscible solvent. The characteristics of product SOPEP are summarized in Table 1. For all the samples synthesized, the oxirane number (% by weight of oxirane oxygen) was used to monitor the progress of reaction; and all the reactions were continued until the oxirane number was less than 0.1%.

As expected, increasing the amount of phosphoric acid increased the acid value of the products. This indicates the formation of acid phosphate, both mono- and diphosphate esters. In Table 1, the values of molecular weight and polydispersity index for all the products clearly indicate the formation of polymeric or oligomeric species. Increase in molecular weight can result from the intermolecular phosphate di- and triester formation. However, the significantly lower hydroxyl values, as compared to the theoretical values, indicate oligomerization through epoxy-epoxy and/or hydoxy-epoxy reaction. Epoxy compounds are known to undergo such reactions (Figure 3) through an acid-catalyzed cationic mechanism. (6,7) Under the conditions used, it appears that reactions involving formation and subsequent hydrolysis of triphosphate ester to the di- and monoesters is taking place simultaneously with oligomerization.

[FIGURE 4 OMITTED]

There is no specific trend observed for the hydroxyl value as a function of the amount of phosphoric acid added, and resultant products have almost constant hydroxyl values. This might be due to increased rate of oligomerization at higher acid contents, resulting in lower hydroxyl values.

It is interesting to note that the products containing above 6% phosphate ester exhibit lower viscosities and molecular weights. We attribute these characteristics to an enhanced rate of di/triester hydrolysis in the more acidic medium. Further, increased phosphate ester content will reduce the epoxide group concentration on the oil molecule, resulting in a reduced rate of molecular weight building oligomerization reaction.

The FTIR spectra of these polyols (Figure 4) show the disappearance of the peak at 824 [cm.sup.-1], indicating the loss of the epoxy groups, while two characteristic peaks appeared at 3400 [cm.sup.-1] (-OH groups) and 1020 [cm.sup.-1] (phosphate groups). Figure 1 shows the peak change at 1020 [cm.sup.-1] of the SOPEP with phosphate ester content of 2%, 4%, and 10%. The most noticeable feature of the spectra is the increasing intensity of the peak at 1020 [cm.sup.-1] as a function of phosphate ester content, indicating the presence of increasing degrees of phosphate incorporation.

Aqueous Dispersions of SOPEP

SOPEPs containing ionizable acid phosphate groups are capable of self-emulsification in water, after neutralization with suitable amines. N,N-dimethylethanolamine (DMEA), which is widely used as a neutralizer for carboxylic acid-containing polymers, was used in this study at 100% neutralization. The aqueous dispersions made from different samples showed different behavior. SOPEPs having less than 3% phosphate ester content showed poor emulsion stability and phase separation within 24 hr of their preparation, while those with 3% and 4% yielded opaque and stable dispersions for 30 days or longer. Samples between 4% and 10% were stable translucent systems and those above 10% phosphate ester content were clear solutions. These results are not surprising since with increasing phosphate ester content, the acid values of the samples are increasing and, hence, upon neutralization with base, they become increasingly hydrophilic. High temperature stability and freeze/thaw stability were not examined.

Formulation of Coatings and Film Properties

Waterborne coating formulations were prepared using the aqueous dispersions of SOPEP as principal binder component. Hexamethoxymethylmelamine (HMMM) was used as crosslinker. Aromatic sulfonic acid was used as curing catalyst. All the coatings were formulated with SOPEP/HMMM weight ratios of 70/30 or 80/20. The coating compositions were prepared and applied as follows:

[FIGURE 5 OMITTED]

To the aqueous dispersions of SOPEPs, 30% formaldehyde resin was added and well mixed. Coating formulation was applied on test panels using an eight path wet film applicator (Gardner) to give a dry film thickness (DFT) of 1.0 mil. The coated panels were cured in a forced air oven at 140[degrees]C for 30 min. The baked film properties of these compositions are shown in Table 2. The films of the compositions containing SOPEP with >8% phosphate ester content showed significant wrinkling and poor adhesion and, hence, other properties could not be tested and reported.

The results in Table 2 indicate that while all of the films had good hardness, they lacked in adhesion and impact resistance properties. This indicates that the films were brittle, probably due to very high crosslink density. It is also possible that under the conditions of curing, HMMM might have undergone excessive self-crosslinking. We attributed these results mainly to higher HMMM content in the compositions. To study the effect of reduced HMMM amount, another series of compositions were prepared with a SOPEP/HMMM weight ratio of 80/20. A typical formulation is shown in Table 3. A ladder study of the effect of crosslinker type and content would help optimize the amount of crosslinker for a particular application.

The results of the film properties of two compositions are presented in Table 4. These results indicate that it is possible to formulate coating compositions by varying SOPEP/HMMM ratio to yield useful film properties. However, it appears that there exists an optimum level for phosphate ester content above which film properties become inferior. A comparison of SOPEP 3.5 in a black paint formulation was made with a commercial paint (Table 5).

CORROSION RESISTANCE: Since phosphate esters are known to improve corrosion resistance properties of the coatings on metallic substrates, (14) it was thought to be interesting to study the effect of phosphate ester content on corrosion resistance properties. The SOPEPs having 3, 3.5, 5, and 8% phosphate ester content were selected for this study. The coating compositions (SOPEP/HMMM weight ratio of 80/20) were applied on B1000 steel panels and cured at 140[degrees]C for 30 min. The panels were evaluated for salt fog corrosion resistance per ASTM B-117 test method for 96 hr. Two commercial alkyd coatings were also evaluated under identical conditions, for comparison. Figure 5 shows photographs of the coating films on the panels after the 96-hour salt fog corrosion test. The results show that the composition containing 3.5% phosphate ester content (photograph D) had visibly superior corrosion resistance among all the samples studied. Further, the performance was significantly better than both commercial alkyd coatings tested. Acid phosphate esters are believed to react with the substrate metal and bind the coating tightly to the substrate. (15) Theoretically, more phosphate gives better adhesion and corrosion resistance of the coating film until the phosphate ester saturates the metal surface. Once the metal surface is saturated, then phosphate esters are left in the binder. The poor corrosion resistance of high phosphate (5% and 8%) compositions is thought to be due to higher hydrophilicity of unreacted acid phosphate esters in the polymer.

CONCLUSIONS

It is possible to synthesize polymeric/oligomeric polyols varying in phosphate ester content from epoxidized soybean oil. It is possible to derive self-emulsifying polyols from these SOPEPs by neutralization of the acid phosphate groups with suitable amines. By selection of proper acid value of SOPEP, it is possible to produce stable aqueous dispersions of SOPEP having a wide range of properties in coatings. These stable aqueous dispersions can be used as the principle component of thermally cured coating compositions with melamine type crosslinkers. By proper selection of type and amount of SOPEP and crosslinker, it is possible to formulate waterborne coatings with useful film properties. Besides low cost and low-VOC of such coatings, SOPEPs containing 3.5% phosphate ester content exhibited visibly improved corrosion resistance properties.

ACKNOWLEDGMENT

The authors would like to acknowledge the United Soybean Board and the California Air Resources Board (ICAT Grant No. 02-02) for their financial support.

References

(1) Erhan, S.Z. and M.O. Bagby, "Polymerization of Vegetable-Oil and Their Uses in Printing Inks," J. Am. Oil Chem. Soc., 71, 1223-1226 (1994).

(2) Gibbons, W.S. and Kusy, R.P., "Effects of Plasticization on the Dielectric Properties of Poly(vinyl chloride) Membranes," Thermochim. Acta., 284, 21-45 (1996).

(3) Saad, A.L.G., Sayed, W.M., and Ahmed, G.M., "Preparation and Properties of Some Filled Poly(vinyl chloride) Compositions," J. Appl. Polym. Sci., 73, 2657-2670 (1999).

(4) Sato, S.H., Peloggia, T., Hunt, J., Schwarzer, R., Bueno, C., and Lovald, R.A., "Process for the Preparation of Polyamides," U.S. Patent 6,011,131, 2000.

(5) Jiratumnukul, N. and Van de Mark, M.R., "Preparation of Glycol Esters of Soybean Oil Fatty Acids and Their Potential as Coalescent Aids in Paint Formulations," J. Am. Oil Chem. Soc., 77, 691-697 (2000).

(6) Thames, S.F. and Yu, H., "Cationic UV-Cured Coatings of Epoxide-Containing Vegetable Oils," Surf. Coat. Technol., 115, 208-214 (1999).

(7) Gu, H.Y., Ren, K., and Martin, D., "Cationic UV-Cured Coatings Containing Epoxidized Soybean Oil Initiated by New Onium Salts Containing Tetrakis(pentafluorophenyl)gallate Anion," J. COAT. TECHNOL., 74, No. 927, 49 (2002).

(8) Raghavachar, R., Sarnecki, G., Baghdachi, J., and Massingill, J. Jr., "Cationic UV Cured Coatings Using Epoxidized Soybean Oil." RadTech Report, 12, 36-40 (1998).

(9) Zhong, B., Shaw, C., Rahim, M., and Massingill, J. Jr., "Novel Coatings From Soybean Oil Phosphate Ester Polyols," J. COAT. TECHNOL., 73, No. 915, 53 (2001).

(10) Lin, G., Baghdachi, J., and Massingill, J. Jr., "Alkyd Coatings from Hyperbranched Polyols," Proc. 79th Annual Meeting of the Federation of Societies for Coatings Technology, Atlanta, GA, November 4-7, 2001.

(11) Guo, Y., Hardesty, J., Mannari, V., and Massingill, J. Jr., "Hydrolysis of Epoxidized Soybean Oil," under review at J. Am. Oil Chem. Soc.

(12) Zhang, S., Ding, Y., Li, S., Luo, X., and Zhou, W., "Effect of Polymeric Structure on the Corrosion Protection of Epoxy Coatings," Corros. Sci., 44, 861-869 (2002).

(13) Massingill, J. Jr., "Organic Solvent Solutions of Phosphate Esters of Epoxy Resins," U.S. Patent 5,086,094, 1992.

(14) Brooman, E.W., "Modifying Organic Coatings to Provide Corrosion Resistance--Part I: Background and General Principle," Metal Finishing, 100 (1), 48-53 (2002).

(15) Massingill, J. Jr., "Adhesion and Flexibility Contributions to the Performance of Epoxy Phosphate Ester Coatings," J. COAT. TECHNOL., 63, No. 797, 47 (1991).

Yinzhong Guo, ([dagger]) Vijay M. Mannari,** Pulin Patel, and John L. Massingill Jr. ([double dagger]) -- Texas State University*

* Institute for Environmental and Industrial Science, San Marcos, TX 78666.

([dagger]) Current address: Polytechnic University at Brooklyn, Brooklyn, NY 11201.

** Current address: Coatings Research Institute, Eastern Michigan University, Ypsilanti, MI 48917.

([double dagger]) Author to whom correspondence should be addressed. Institute for Environmental and Industrial Science, Texas State University, 601 University, San Marcos, TX 78666; email: [e-mail].

Table 1 -- Percent Phosphate Ester vs Properties of SOPEP

%PO4               2        3        3.5      4         5        6

O.N. (%)              0.07     0.06     0.04     0.04      0.03     0.04
Acid No. (mg KOH)    11.8     26.4     31.8     32        59.7     65.9
Hydroxyl #          258      253      211      244       260      255
(mg KOH)
Viscosity          5250     8700     8400     1687 (a)  4790     4410
cPs@50[degrees]C
Mw                 3770     6813     7893     7125      7917     7285
Mw/Mn                 1.8      2.7      1.08     2.8       2.7      2.8

%PO4               8        10       20       30

O.N. (%)              0.03     0.02     0.02     0.01
Acid No. (mg KOH)    89.2    104      132      176
Hydroxyl #          263      259      256      279
(mg KOH)
Viscosity          3160     4350     2690     2310
cPs@50[degrees]C
Mw                 3918     2717       --       --
Mw/Mn                 2        1.7     --       --

(a) 14% solvent.

Table 2 -- Film Properties of SOPEP-Based Waterborne Compositions (a)

SOPEP                     SOPEP 3  SOPEP 3.5  SOPEP 4  SOPEP 6  SOPEP 8

Dry-film thickness (mil)  0.8      0.9        0.7      0.7      0.8
Pencil hardness           6H       6H         6H       6H       6H
Impact resistance (D/R)   80/30    90/30      60/20    60/20    60/20
  (in/lb)
MEK double-rub            >200     >200       >200     >200     >200
Adhesion (cross-hatch)    1B       1B         1B       1B       1B

(a) Cured at 140[degrees]C for 30 min.

Table 3 -- Typical Coating Composition

(SOPEP/HMMM ratio 80/20)

Ingredients                 Parts by wt

SOPEP dispersion (50% NVM)   72.00
Resimene 747                  9.2
Cycat-4040                    0.60
Surface wetting additive      0.20
Water                        18.00
Total                       100.00

Table 4 -- Film Properties of SOPEP-Based Waterborne Compositions (a)

(SOPEP/HMMM = 80/20)

SOPEP                     SOPEP 3.5  SOPEP 5

Dry-film thickness (mil)  0.9        0.9
Pencil hardness           3H         3H
Impact resistance (D/R)   160/90     100/60
MEK double-rub            150        100
Adhesion (cross-hatch)    4B         2B

(a) Cured at 140[degrees]C for 30 min.

Table 5 -- Comparison with Commercial Paint

                   POLYOL 3.5  Container Enamel (a)

Film appearance    Both smooth and glossy
Pencil hardness    H           H
Impact resistance  160/100     160/140
MEK DR (pass)      50          75
Adhesion           4B          4B
VOC, lb/gal        1.45        2.7

(a) Commercial product.