Acetoacetylated castor oil in coatings applications

Progress in Organic Coatings 44 (2002) 49–54

Acetoacetylated castor oil in coatings applications A.S. Trevino, D.L. Trumbo∗ S.C. Johnson Polymer, 8310 16th Street, P.O. Box 902, Sturtevant, WI 53177, USA Received 1 September 2000; received in revised form 17 July 2001; accepted 6 August 2001

Abstract The hydroxyl functionalities of castor oil were converted to ␤-ketoesters by reaction with t-butyl acetoacetate. The reaction is relatively rapid and proceeded to high yield under mild conditions. The resulting acetoacetate esters were used to formulate thermosetting coating compositions. Films were cured at ambient and elevated temperatures using multifunctional amines as crosslinkers. The films obtained had only modest solvent resistance even when cured at elevated temperatures. The pencil hardness of the films was reasonably given the cure level obtained. The 60 ◦ glosses of the films and film flexibilities were good. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Castor oil; Acetoacetylation; Ambient cure; Diamine

1. Introduction Castor oil has been used in paints, coatings and inks for many years [1–4] and has quite a number of advantages as a raw material. Castor oil is obtained from a naturally occurring renewable resources. As such it is readily available, relatively inexpensive and environmentally benign. Castor oil’s disadvantages include having only one double bond in each fatty acid moiety as shown below:

Having only one double bond in each fatty acid chain means that castor oil is appropriately classified as a semi-drying oil [1]. As a result, coatings that incorporate castor oil alone will never achieve complete cure through oxidative crosslinking as do coatings that contain oils with multiple double bonds in each fatty acid moiety. This shortcoming is often addressed by dehydrating the castor oil as shown in Scheme 1. ∗ Corresponding author. Fax: +1-262-631-4039. E-mail address: [email protected] (D.L. Trumbo).

Dehydration transforms castor oil into a drying oil, which improves the properties of films made with it. Alternatively, one can choose to have castor oil participate in crosslinking reactions through the hydroxyl functionalities in the fatty acid moieties. Typically, isocyanates or melamines are used, but anhydrides have also reported utility in cure reactions. Several research groups have demonstrated these types of cures while employing castor oil in the formation of molded articles or films [5–8]. The properties of such molded articles and films proved to be excellent. But, melamines usually require relatively high temperatures to effect cure and isocyanate systems that cure at ambient temperature must be two pack systems to avoid premature gellation. In addition, isocyanates have worker safety issues associated with them. It seems to be that the modification of the hydroxyl groups of castor oil with a moiety capable of reacting with a variety of species safely under a broad range of conditions would be useful. Recently, we have been exploring the synthesis and applications of ␤-ketoester functional species [9–11]. The ␤-ketoesters were chosen because they are readily synthesized in high yields and are known to react with a large variety of functionalities over a broad range of conditions [12–14]. As shown in Scheme 2, in order to convert the hydroxyl groups of castor oil to ␤-ketoesters we reacted the oil with t-butyl acetoacetate. The product we obtained was then used to formulate coatings. The films from these coatings were cured at elevated and ambient temperature and the properties were assessed. This paper summarizes the results we obtained.

0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 0 - 9 4 4 0 ( 0 1 ) 0 0 2 2 3 - 5

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Scheme 1. Dehydration of castor oil.

Scheme 2. Synthesis of castor oil ␤-ketoester.

2. Experimental All the chemicals, including castor oil, used in this study were obtained from the Aldrich Chemical and were used without any further purification. Nuclear magnetic resonance spectra were obtained at ambient temperature on CDCl3 (10% (w/v)) solutions of material using a Varian Gemini 300 FT NMR. Infrared spectra were recorded with a Nicolet 5DXB FTIR on films cast on sodium chloride plates or on potassium bromide pellets of material. Molecular weights were measured using a GPC equipped with a waters 510 pump, 410RI detector and two 30 cm polymer lab columns. Numerical values for the molecular weights were obtained by comparison to a polystyrene calibration curve. The degree of cure of each film was estimated by counting the number of methyl ethyl ketone double rules required to break through the film to the substance below. The greater the number of rubs required, the higher the degree of cure was judged to be. This test was performed with an ATLAS AATCC

Crockmeter. Making solvent extraction measurements also assessed the degree of cure. Samples for these measurements were prepared by pouring 8 g of formulated coating solution (␤-ketoester of castrol oil, crosslinker and solvent) into an aluminum weighing pan and either placing the pan in an oven at 130 ◦ C for the desired length of time or leaving the pan at ambient temperature for an arbitrary length of time. The resulting cured masses were removed from the pans and after cooling (for films cured at elevated temperature) and drying to a constant weight, were placed in 5 ml of tetrahydrofuran in screw cap vials. The vials were tightly sealed with Teflon lined screw caps and were kept at ambient temperature for 5 days with periodic agitation. Any remaining solid was isolated by filtration and dried to a constant weight. The percentage of insoluble material was calculated as Wr /Wo × 100, where Wr is the weight of remaining solid, and Wo the original sample weight. Impact resistances of the films were measured with a Gardner impact tester employing a 4 lb weight. Pencil

A.S. Trevino, D.L. Trumbo / Progress in Organic Coatings 44 (2002) 49–54

hardnesses were measured according to ASTM D3363. Gloss measurements were made with a BYK-Gardner microtrigloss meter. 2.1. β-Ketoester synthesis Castor oil (1000 g, 1.07 mol) was charged to a 2 l flask equipped with a mechanical stirrer, reflux condenser, Dean-Stark trap and thermometer. t-Butyl acetoacetate (507.2 g, 3.2 mol) was added followed by 200 ml of xylene. The reaction mixture was heated with stirring to 110 ◦ C. At ∼95–105 ◦ C a vigorous evolution of t-butanol commenced.

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In ∼40–45 min 96 wt.% of the theoretical quantity (theor = 236.8 g) of n-butanol had been collected. The reaction mixture was cooled to ambient temperature and the flask was adapted for distillation. The remaining n-butanol and the xylene were removed under reduced pressure. Gas chromatography revealed that the material was ∼97–98% pure so that no further purification was done. 2.2. Coating formulations Fifty grams of the acetoacetylated castor oil was dissolved in 25 ml of methyl ethyl ketone. A stoichiometric (based on

Fig. 1. 300 MHz 1 H NMR spectra of: (A) castor oil; (B) acetoacetylated castor oil.

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functional groups) quantity of crosslinker, a multifunctional amine, was dissolved in methyl ethyl ketone and added to the acetoacetylated castor oil solution. It was necessary to dissolve the amine in sufficient quantity of methyl ethyl ketone so that all the amine groups would form imines [14]. If this was not done, the reaction between the amine and ␤-ketoester was too fast for any drawdowns to be made, i.e. the material gelled in the container. Films were made by drawing the coating solution over Bonderite 1000 steel panels with a No. 3 bird bar (dry film thickness 1.0–2.0 mil). Some of the coated panels were placed in a forced air oven at 130 ◦ C for varying lengths of time and some panels were allowed to remain at ambient temperature. The panels that were heated in the oven were removed and allowed to cool to ambient temperature. These panels were then tested immediately for property development. The panels kept at ambient temperature were evaluated for property development

after an arbitrary length of time. Controls for the film studies were made from acetoacetylated castor oil without added crosslinker. The infrared spectra of unreacted castor oil (A) and acetoacetylated castor oil (B) are shown in Fig. 1. While the spectra are non-quantitative they do show that acetoacetylation has taken place. The spectrum of acetoacetylated oil shows a significant decrease in the absorbance at 3400 cm−1 , the –OH stretching frequency. New absorbances at 1700 cm−1 (carbonyl stretch of both ketone and ester) and 1350 cm−1 (C–O stretch from the ester bond of ␤-ketoester) [15] shows that the –OH group has been esterified. Nuclear magnetic resonance spectroscopy, specifically 1 H NMR, was used to quantify the extent of acetoacetylation. The spectra of the reacted (B) and unreacted oil (A) are shown in Fig. 2. The peaks are assigned as shown in the figure [16,17]. The area of the double bond resonances

Fig. 2. Infrared spectra of: (A) castor oil; (B) acetoacetylated castor oil.

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Table 1 Film properties Film

Diamine crosslinkera

Cure temperature ( ◦ C)

Cure time (min)

Methyl ethyl ketone double rubs’

Pencil hardness

60 ◦ Gloss

Forward impact

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18b 19b

1,5 PDA 1,5 PDA 1,5 PDA 1,5 PDA 1,9 NDA 1,9 NDA 1,9 DNA 1,9 NDA 1,9 NDA 1,12 DDA 1,12 DDA 1,12 DDA 1,12 DDA TAEA TAEA TAEA TAEA – –

130 130 130 22 130 130 130 130 22 130 130 130 130 130 130 130 22 130 22

30 60 90 2800 30 60 90 120 2880 30 60 120 180 30 60 120 2200 120 2800

65 90 120 30 10 16 20 32 5 8 14 26 39 15 45 67 24 3 1

F F H 2B 2B 2B B HB – 2B 2B B B 2B HB F HB – –

83 80 80 85 88 86 86 84 – 90 88 88 83 89 84 83 86 – –

100 80 80 140 140 120 120 120 – 140 130 120 120 140 110 100 110 – –

a b

1,5 PDA: 2-methyl-1,5-pentane diamine; 1,9 NDA: 1,9-nonane diamine; 1,12 DDA: 1,12-dodecanediamine; TAEA: tris(2-aminoethyl) amine. Control films.

at δ = 5.35 was compared with the area of the CH2 group of the acetoacetate moiety at δ = 3.46 to determine the extent of reaction. This comparison shows that 98–99% of the available hydroxyl groups have been esterified. The molecular weight values for the acetoacetylated castor oil were Mn = 1150 and Mw = 1300. The theoretical formula weight of completely acetoacetylated castor oil is ∼1180, so the molecular weight results confirm nearly quantitative acetoacetylation. The film properties obtained are summarized in Table 1. The results of the extraction study are listed in Table 2. We used tetrahydrofuran for the extraction solvent instead of methyl ethyl ketone because tetrahydrofuran is a much more aggressive solvent than methyl ethyl ketone, i.e. if a material is not soluble in tetrahydrofuran it is highly doubtful whether it will be soluble in methyl ethyl ketone. Also, the Table 2 Solvent extraction results Sample

Diamine crosslinkera

Cure temperature ( ◦ C)

Cure time (min)

Insoluble material (%)

1 2 3 4 5 6 7 8 9b

1,5 PDA 1,5 PDA 1,9 NDA 1,9 NDA 1,12 DDA 1,12 DDA TAEA TAEA –

130 22 130 22 130 22 130 22 130

120 1440 120 1440 120 1440 120 2880 120

95.0 68.9 84.9 61.8 80.3 57.5 87.8 86.2 0

a b

Control film. Amine designations are the same as in Table 1.

control films showed that the non-crosslinked acetoacetylated castor oil is very soluble in methyl ethyl ketone, so the results from tetrahydrofuran can be extrapolated to methyl ethyl ketone. The results obtained show that films cured at elevated temperature developed more solvent resistance than films cured at ambient temperature. This result is not surprising as higher temperatures result in increased film mobility bringing more reactive groups into contact with each other during the allotted cure time. Overall, the solvent resistance of these films, both those cured at ambient temperature and those cured at elevated temperature, is only modest. However, solvent extraction studies show that a very high percentage of gel is being obtained. Granted, the samples prepared for the gel experiments were not thin films and often bulk samples cure more thoroughly than thin films because of increased mobility in the bulk samples [1], but the quantity of gel obtained still suggests that films with greater solvent resistance should have been obtained. On close examination of the failure of these films, it seems that they were not dissolving in the methyl ethyl ketone but were being abraded off the metal panels. It seemed a possibility then that what we thought might be lack of solvent resistance could be a lack or loss of adhesion. At the suggestion of a referee, we tested the adhesion of our coatings to the panels using ASTM D3359. The ratings for the film were between 2B and 3B. These ratings reflect only moderate adhesion and so loss of adhesion could be responsible, in part, for the modest perceived solvent resistance observed for the films on the panels. The acetoacetylated castor oil is a relatively low molecular weight material so when crosslinked the molecular weight

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between crosslinks would be low. This could cause some shrinkage in the film, which would result in the film pulling away from the substrate somewhat. Pencil hardnesses are relatively low overall, which could also be due to lack of good adhesion. The glosses are reasonable and the forward impacts are high. Again, if the films are not tightly adhered to the substrate then the impact resistances might appear higher than they really are. However, even if the impact values were somewhat lower than measured, they would still be fairly good.

3. Conclusions We were able to acetoacetylate castor oil’s hydroxyl groups in very high yield by reaction with t-butyl acetoacetate. The resulting ␤-ketoester yielded cured films when reacted with multifunctional amines. The films cured at elevated temperature had better properties than those films cured at ambient temperature. However, the ambient temperature films did develop some properties, which might make acetoacetylated castor oil useful in some applications where high performance is not required and heating substrates is impractical. Also, some of the lack of a high degree of solvent resistance is most likely due to less than optimal adhesion between the coating and substrate. However, the coating formulations were very simple. It is possible that a more fully formulated coating with adhesion promoter(s) added would perform

much better. This possibility is currently being investigated in these laboratories. References [1] P. Oldring, G. Hayward, Resins for Surface Coatings, SITA Technol. (London) 1 (1993). [2] C.D. Smith, K. Wise, J. Paint. Technol. 41 (1969) 338. [3] D.H. Solomon, The Chemistry of Film Formers, Krieger, New York, 1977. [4] E.A. Apps, Printing Ink Technology, Hill Books, London, 1961. [5] L.W. Barrett, H. Sperling, Polym. Mater. Sci. Eng. Prep. 65 (1991) 180. [6] P. Patel, T. Shah, B. Suthai, J. Appl. Polym. Sci. 40 (1990) 1037. [7] L.H. Sperling, J.A. Manson, J. Am. Oil Chem. Soc. 60 (1983) 1887. [8] L.H. Sperling, C.E. Carraher, S.P. Qureshi, J.A. Manson, L.W. Barrett, in: J.C.G. Gebelen (Ed.), Polymers from Biotechnology, Plenum Press, New York, 1991. [9] R.J. Esser, US Patent 5 498 659 (1996). [10] R.J. Esser, US Patent 5 605 952 (1997). [11] R.J. Esser, J.E. Devona, D.E. Setzke, L. Wagemans, Prog. Org. Coat. 36 (1999) 45. [12] F.D. Rector, W.W. Blount, D.R. Leonard, J. Coat. Technol. 61 (1989) 31. [13] J.S. Witzeman, W.D. Nottingham, F.D. Rector, J. Coat. Technol. 62 (1990) 101. [14] K. Hoy, C.H. Gardner, J. Paint. Technol. 46 (1974) 70. [15] In-house Computer Program for Calculating Chemical Shifts, S.C. Johnson Wax, Inc., Racine, WI, 1992. [16] R.J. Clemens, J.S. Witzeman, F.D. Rector, in: Proceedings of the 16th International Conference Org. Coat. Technol., Athens, 1990, pp. 127–142. [17] D.H. Williams, I. Fleming, Spectroscopic Methods in Organic Chemistry, McGraw-Hill, London, 1980.