Development and evaluation of terminally epoxidized triglycerides for coatings applications.

Epoxidized 10-undecenoic acid triglyceride, an experimental seed oil derivative that has a terminal epoxy group on each of the three acyl glyceride segments, has been found to have good reactivity with amine curatives and allows room temperature cures to be obtained. Coatings based on epoxidized 10-undecenoic acid triglyceride have also shown excellent UV stability. As an example, coatings samples placed in a QUVA chamber exhibit no loss in gloss after 3000 hr of a cycled exposure to high intensity UV lamps and moisture at temperatures of 50-60[degrees]C. In comparison, coatings based on standard liquid epoxy resins (LERs) and commercially available hydrogenated LERs lose gloss due to chalking/decomposition within 200-800 hr.

Keywords: Epoxy resins, reactive diluents, accelerated testing, weatherability, high-solids, thermoset, VOC control, epoxy

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As a part of the industrial biotechnology program at The Dow Chemical Company, we are working on the identification and development of epoxidized oils that will yield structurally robust, UV-stable thermoset polymers, a characteristic unavailable to conventional bisphenol A based epoxy resins. Work in this area is focused on experimental triglycerides, which contain from 5-11 carbon units and a terminal epoxy group in each of the three aliphatic segments. Terminally epoxidized triglycerides can be derived through a series of chemical transformations of low-cost seed oils obtained from plants. Industrial routes to these types of monomers have been investigated by others. (1)

It is hypothesized that the terminal epoxy functionality of these triglycerides would have good reactivity with amine curatives and would allow room temperature cures to be obtained for coatings applications. Due to their aliphatic character, they should also have excellent UV stability, another desirable property for coatings; conventional epoxy resins are quite labile to photolytic degradation. This paper presents coatings performance evaluations for epoxidized 10-undecenoic acid triglyceride, a terminally epoxidized triglyceride which could be obtained, for example, from castor oil.

PREPARATION OF EPOXIDIZED 10-UNDECENOIC ACID TRIGLYCERIDE

The epoxidized 10-undecenoic acid triglyceride (EUDA TAG) used in this study was synthesized by first esterifying glycerol with the [omega]-unsaturated fatty acid, 10-undecenoic acid. The 10-undecenoic acid triglyceride thus obtained was then epoxidized using aqueous peracetic acid. The structure for the EUDA TAG (Figure 1 with n = 6) was confirmed by (1)H NMR spectra and by an epoxy equivalent weight (EEW) determination. The EEW of the EUDA TAG was quite close to the calculated theoretical value of 213. Also, infrared analysis of the product using a Nicolet 5DX13 FTIR spectrometer showed the absence of any carbon-carbon double bond absorption at ~1650 [cm.sup.-1].

[FIGURE 1 OMITTED]

RESULTS AND DISCUSSION

Development of Coatings Formulations from Differential Scanning Calorimetry 

As a means to investigate the curing behavior of EUDA TAG, a large number of commercial curing packages based on accelerated, aliphatic amines were formulated with the resin and evaluated by differential scanning calorimetry (DSC). The DSC results obtained were compared in many cases to D.E.R.[TM] 331 epoxy resin, a standard bisphenol A diglycidyl ether (BADGE), and a commercially available epoxidized linseed oil (Flexol[TM] Plasticizer LOE). In all cases, the relative rates of reactivity were as follows: BADGE > EUDA TAG >> epoxidized linseed oil. To demonstrate this, DSC analyses for all three of these resins with the same isophoronediamine-based curing agent, Ancamine[R] 2423 by Air Products and Chemicals, Inc., are given in Figure 2.

[FIGURE 2 OMITTED]

In actual room temperature cures of films based on using Ancamine 2423, the dry-through times were 3 hr, 48 hr, and >1 week, respectively, for the three epoxy resins. For accelerated cures conducted at 60[degrees]C, the formulations based on epoxidized linseed oil were observed to cure in shorter periods of time, but they all yellowed badly. No yellowing was observed for the EUDA TAG formulations cured at 60[degrees]C. FTIR analysis was also conducted for EUDA TAG formulations cured at 60[degrees]C to determine if any amidolysis of the ester linkages had occurred. None was observed.

Coatings Properties

In testing of coatings prepared from EUDA TAG, excellent UV stability was observed. For example, clear-coatings placed in a QUVA chamber exhibited no loss of gloss after 4000 hr of a cycled exposure to high intensity UV lamps and moisture at temperatures of 50-60[degrees]C. In comparison, coatings based on BADGE and a commercially available, hydrogenated epoxy resin show loss of gloss due to chalking/decomposition within 200-800 hr.

Other important properties for coatings such as methyl ethyl ketone double rubs, pendulum hardness, cross hatch adhesion, and 1/8 in. conical bend were also obtained for coatings based on EUDA TAG. These data are given in Table 1, along with data obtained for standard thermosets used in coatings applications. In summary, very good properties were observed for the coatings based on EUDA TAG which were comparable or better than the standard thermoset resins.

Coatings Properties for Blends of EUDA TAG and BADGE

Blends of EUDA TAG with BADGE have also been evaluated. An important finding from this work is that EUDA TAG co-cures with BADGE producing films and coatings that are homogenous. The homogeneous nature of these new network polymers is indicated from analyses, which have been conducted using attenuated total reflectance infrared and x-ray photon spectroscopy. (2)

From the coatings properties obtained for blends it has been observed that the BADGE imparts improved cure rates, higher [T.sub.g]s, and better corrosion resistance to the EUDA TAG, as is shown in Table 2. In turn, EUDA TAG imparts significantly improved UV resistance and toughness to the BADGE.

CONCLUSIONS

EUDA TAG, a terminally epoxidized triglyceride containing 11 carbons in each of the aliphatic, ester segments, has greatly improved reactivity over internally epoxidized triglycerides derived, for example, from soybean and linseed oils. This improved reactivity allows room temperature cures to be obtained for coatings applications. For coatings applications, outstanding UV resistance and good properties in other areas such as adhesion, hardness, and solvent resistance, have been observed for formulations based on EUDA TAG. EUDA TAG has also been observed to improve the UV stability of conventional LERs, e.g., D.E.R. 331 epoxy resin, in blends with these resins. For blends, the diglycidyl ether of bisphenol A in turn imparts better reactivity, increased hardness, higher [T.sub.g]s, and improved corrosion resistance to the terminally epoxidized triglyceride.

Table 1 -- Coatings Properties for EUDA TAG vs Other Thermoset Resins

                                                             QUVA
                                                             Stability,
            Glass       1/8 in.  Cross-                      Retention
            Transition  Conical  Hatch     MEK     Pendulum  of
            Temp.,      Bend     Adhesion  Double  Hardness  60[degrees]
Formation   [degrees]C  Test     Rating    Rubs    (OSC)     Gloss, hr

EUDA TAG +  16          Pass     5b        200+     96       > 4000
Ancamine                         (No
  2423                           Failure)
BADGE +
Ancamine
  2423      65          Failed   0         200+    130          100-200
HLER (a) +
Ancamine
  2423      39          Pass     5b        200+    119          200-300
Aliphatic
Isocyanate
  (b)       17          Pass     5b        200+     91       > 4000

BADGE and aliphatic isocyanate formulations were cured at room
temperature; EUDA TAG and HLER formulations were cured at 60[degrees]C.
(a) HLER = Eponex[R] 1511 (hydrogenated diglycidyl ether of bisphenol
A).
(b) Aliphatic Isocyanate = Desmodur N75 + 867PX60.
Coating Thickness = 2-3 mils.

Table 2 -- Coatings Properties for Blends of EUDA TAG with BADGE

                                                                100%
Weight Ratio of EUDA TAG to BADGE  100% EUDA TAG  2:1    1:1    BADGE

Dry-through time (hr)                 48            19     12     3
QUVA stability, retention of
  60[degrees] gloss, hr            >4000          3000   1400   100-200
Ultimate glass transition
  temperature, [degrees]C             15            40     50    65
Pendulum hardness (OSC)               10            41     64   130
Cross hatch adhesion
(5 rating = no failure)                5b            5b     5b    0b
Corrosion rating @ scribe (a)          0             7     10    10
Corrosion rating (unscribed area)
  (a)                                  0            10     10    10
(10 rating = no failure)

Curing agent = Ancamine 2423; Cure schedule = seven days at room
temperature + 1.5 hr at 60[degrees]C.
(a) After 1030 hr in salt fog chamber.
Coating Thickness = 2-3 mils.

Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology, November 12-14, 2003, in Philadelphia, PA.

References

(1) Bjornberg, H., "Novel Primary Epoxides," WIPO Publication 00/18751 (WO 00/18751), Perstorp AB, April 6, 2000.

(2) Davis, B.L., unpublished.

Jim D. Earls, Jerry E. White, Marvin L. Dettloff, and Marty J. Null--The Dow Chemical Company*

* 2301 N. Brazosport Blvd., Bldg. B-1214, Freeport, TX 77541.