Industrial Crops and Products Surface Modification of Zein Films

Industrial Crops and Products 30 (2009) 168–171

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Industrial Crops and Products

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Short communication Surface modiﬁcation of zein ﬁlms

Atanu Biswas a,∗, Gordon W. Selling a, Kristen Kruger Woods a, Kervin Evans b a Plant Polymers Research Unit, USDA/Agricultural Research Services, 1815 N. University Street, Peoria, IL 61604, United States b New Crops and Processing Technology Research, USDA/Agricultural Research Services, 1815 N. University Street, Peoria, IL 61604, United States article info abstract

Article history: A novel method to derivatize the surface of zein is devised that can modify the water absorption and Received 20 October 2008 surface wetting behavior. The reagents used to impart the desired properties in a reasonable amount Received in revised form 23 January 2009 of time include octenyl succinic anhydride and alkyl and alkenyl ketene dimers. The method is easy to Accepted 4 February 2009 apply and involves baking with an appropriate concentration of a derivatizing agent. Decreased water absorption and increased contact angle with water relative to control demonstrate the advantages this Keywords: methodology provides. Atomic force microscopy was used to demonstrate that after derivatization the Zein surface of the ﬁlm became much different, having large globular domains that extended as much as Surface energy Hydrophilic reagents 122 nm above the lowest surface. Given the hydrophobic character of the reagents and their relative incompatibility with zein, it was anticipated that derivatization would be somewhat inhomogeneous. Published by Elsevier B.V.

1. Introduction zein films (James, 1944) and fibers (Zhang et al., 1997). Likewise, zein has been crosslinked (Veatch, 1941) with cyanuric chloride, Zein is a natural polymer obtained as a product of industrial corn formaldehyde, carbodiimide, glyoxal (Woods and Selling, 2007), processing (Lawton, 2002; Momany et al., 2006). With the growth and glutaraldehyde (Sessa et al., 2007). Biswas et al. reported the of the bio-ethanol industry, the amount of zein potentially available acylation of zein with anhydrides and acyl chlorides in dimethyl- has grown considerably, to the extent that techniques are needed formamide (Biswas et al., 2005a,b) and ionic liquids (Biswas et to develop new uses for this material. It has been widely studied al., 2006). Wu et al. (2003) synthesized zein/nylon copolymer in and has historically had many industrial and food uses (Wang and dimethylformamide. In all of the techniques described, the deriva- Padua, 2003; Padua et al., 2005; Ghanbarzadeh et al., 2007). Pre- tization has taken place in solution. However, at times only the viously the main application has been in the textile fibers market, surface may need to be derivatized in order to impart value to the but now it is used as a coating for candy, nuts, fruit, pills, and other article. Additive, non-bound coating techniques have been utilized encapsulated foods and drugs. Zein can also be incorporated into to vary the surface properties of zein (Wang and Padua, 2006). How- resins and other bioplastic polymers (Lai et al., 1997; Padua and ever, given that additives and coatings can be deposited or removed Santosa, 1999; Lawton, 2004; Selling et al., 2004; Selling and Sessa, at various points during processing of the article a technique that 2007). bonds the reagent to the zein film has merit. A major drawback in the use of zein as a renewable raw mate- In this work, we sought to find a new method to derivatize zein. rial is the lack of amine moieties (lysine amino acid) which limits Since a major application of zein is in the form of films and fibers, we the number of typical protein derivatizing techniques available. believe a surface reaction of zein would be useful. There are several Another major problem in modifying and derivatizing zein is its advantages of this approach. First, there is no need to alter the cur- lack of solubility. It dissolves in an ethanol/water (90:10) mixture, rent processes for making films, coatings, or fiber. The reaction can but this mixture cannot be readily used for chemical modifica- be carried out after the film, coating, or fiber is made. Secondly, we tion because alcohol and water may react faster with electrophilic can keep the desirable properties of the film, coating, and fiber and reagents than zein does. In spite of these shortcomings, a num- only impart additional desired properties onto the surface. Thirdly, ber of articles have been reported over the years on zein reactions. as the reaction only takes place on the surface, less derivatizing For example, a method has been devised to prepare zein acetate in agent is consumed, thereby decreasing the cost of the reaction. In order to increase the water resistance, strength, and flexibility of addition by chemically bonding the desired agent onto the film, the agent cannot be easily removed physically from the article. Finally, the reaction is easy to do and effective. To our knowledge, there is no ∗ Corresponding author. Tel.: +1 309 681 6406; fax: +1 309 681 6691. prior report of similar surface modification of zein films, coatings, E-mail address: [email protected] (A. Biswas). or fibers.

0926-6690/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.indcrop.2009.02.002 A. Biswas et al. / Industrial Crops and Products 30 (2009) 168–171 169

2. Experimental

2.1. Materials

The zein samples were obtained from Freeman Industries. Octenyl succinic anhydride (OSA) was purchased from Aldrich. Two ketene dimer samples were obtained from Hercules Incorporated (Wilmington, DE): alkyl ketene dimer (AKD) is derived from satu- rated fatty acids and alkenyl ketene dimer (ALKD) is derived from unsaturated fatty acids. Deuterated sodium hydroxide and water were purchased from Cambridge Isotope Laboratories (Andover, MA). The other chemicals were procured from Aldrich Chemical Co. Fig. 1. Structures of reagents used to modify zein surface.

2.2. Procedures the zein surface. Additional work would be needed to select a Zein was dissolved in a 90:10 ethanol/water mixture and cast more environmentally safe solvent; a solvent such as acetone may into films of an approximate thickness of 0.21 mm. Solutions of OSA, also be suitable. These materials have been shown to modify the AKD, and ALKD were dissolved in methylene chloride at 10% con- hydroxyl groups of starch (Qiao et al., 2006). Zein has been shown centration. The solutions were then applied onto the zein surface to have numerous hydroxyl groups due to the presence of many ser- as a thin film of roughly 0.25 mm thickness and 10 mm by 40 mm ine and threonine amino acids (Cheryan and Shukla, 2001). Under area. The film and the derivatizing agent were first air-dried, and the reaction conditions it is expected that the ketene dimers (AKD then placed in an oven at 100 ◦C overnight. The reaction product and ALKD) or the anhydride (OSA) will react with the hydroxyl was then washed with toluene and methylene chloride to remove groups located on the surface of the film. The OSA would react any free, unbound chemicals. The final products were characterized in the typical fashion giving an ester moiety bound to the zein with ATR, 1H NMR, water absorption, and contact angle measure- and a free carboxylic acid. The ketene dimers would provide ␤- ment. keto esters on reaction with the hydroxyl groups present in the Water absorption was carried out by immersing 1 cm × 2cm zein protein (Scheme 1). The terminal amine of zein would react films of zein, with or without surface derivatization, in deionized with OSA or the ketene dimer to give an amide or a ␤-keto amide water, samples were run five times. The dry mass of each film was respectively. Given that this chemistry is occurring at the surface first obtained, and the mass of each immersed film (with excess only, it was not surprising to find that differences between test water removed) was determined at 5, 15, 60, 120, and 1440 min. and control could not be found by attenuated total reflectance From this the mass% (water) gained was determined. (ATR) IR or NMR. In ATR the top one or two micrometers is eval- The contact angle measurements (measurements were made uated. In NMR, the bulk material is evaluated. In either case a between two and four times) were made on a First Ten Angstroms true surface derivatization would not be detected by these tech- FTA4000 Microdrop Instrument. Zein films were held in place with niques. two-sided tape to a glass microscope slide. A 22-gauge needle was After the chemical treatments differences in surface properties used to release a five microliter drop. The initial contact angle were observed. Changes in the hydrophobicity of zein were seen (time=0s)was calculated using the first stable image of the full in bulk water absorption experiments. In this case, thin films of drop on the film surface. The second contact angle was calcu- zein (0.21 mm thick) were separately immersed in water at different lated using the image captured 2 s after the initial image. Images times, and the mass of water absorbed was measured. Care was were recorded every 0.0083 s and a total of 500 images were taken to remove free water droplets on the film surface. The results collected. for different samples of zein are detailed in Fig. 2. It is clear that the The atomic force microscope (AFM) experiments were car- surface reaction with OSA, AKD, and ALKD has decreased the water ried out with a Nanoscope IV microscope and controller (Veeco absorbency of zein films. At 120 min, zein films modified with ALKD Instruments, Santa Barbara, CA). Silicon probes (purchased from only absorbed 40% of the water of the unmodified zein. The trend NanoAndMore Inc., Lady’s Island, SC) having a force constant of towards decreased water absorbency is ALKD > AKD > OSA except 1.9 N/m were used. Scans were made in tapping mode in air at at very long soaking times. Although the reactions of OSA, AKD, rates of 0.1–0.3 Hz. Samples were prepared by slicing pieces of zein and ALKD are only on the zein surface, their effect on bulk water treated and untreated thin films with a razor blade and attach- absorption is significant. ing to a steel puck using double-sided tape. Care was taken to As an indication of the change in surface properties, we mea- scan the samples in the middle in order to avoid scanning over sured the contact angle of the modified and unmodified zein any induced stress fractures caused by the razor blade. Zein films surface. The results are shown in Fig. 2. The contact angle was larger were allowed to adhere to the tape for a minimum of 10 h prior to immediately after the water droplet is deposited on the zein surface scanning. Scans were repeated on three separate samples of each (t = 0 s) and decreased in time as the droplet equilibrates on the sur- film. face. For comparison, the contact angle data for waxed paper and

3. Results and discussion

The structures of the reagents used in this study are detailed (Fig. 1). Zein was first cast into a film by the conventional method. Solutions of OSA, AKD, and ALKD in methylene chloride (at 10% concentration) were applied to the film surface and heated at 100 ◦C overnight. Methylene chloride was selected as it can readily dis- Scheme 1. Formation of zein alkyl ␤-ketone ester from reaction of zein with ketene solve the selected reagents and it is polar enough that it can wet dimer. 170 A. Biswas et al. / Industrial Crops and Products 30 (2009) 168–171

Table 1 Contact angle measurementa for water on modiﬁed zein surface measured at 0 and 2 s after application of the water drop.

Sample 0 s 2 s

Control 84 ± 367± 3 10% OSA 88 ± 172± 1 10% AKD 101 ± 196± 1 10% ALKD 87 ± 187± 1 Waxed paper 98 ± 398± 2 Mylar 86 ± 474± 2

a Error represents ±one standard deviation.

Given that it is expected that the reagents used in this study may modify the surfaces of the films only, AFM was used to deter- mine if changes in surface morphology occurred with treatment. AFM height (a) and phase (b) images were analyzed (Figs. 3 and 4). The surface of the control film was relatively uniform having evenly distributed pores ranging in size from 1.5 to 19.8 nm (Fig. 3a and b). In contrast, the sample treated with 10% AKD had a very inhomogeneous surface (Fig. 4a and b). The globular domains extended as Fig. 2. Water absorption of modified and control zein films, error bars represent high as 122 nm above the lowest surface of the scanned region. The ± one standard deviation. globules consisted of an inhomogeneous mix of material (seen as light and dark area of the globules—Fig. 4b). Given that the AKD, a material with a long hydrocarbon tail, would not be compatible Mylar (obtained on the same instrument) are included in Table 1.It with zein, it is not surprising that the reagent did not uniformly is interesting that the contact angle for Mylar is somewhat similar to cover the zein surface. Once one reagent reacted with the appropri- that of zein. In general, surface treatment with OSA, AKD, and ALKD ate moiety on zein and bonded to the surface, then the next reagent caused the surfaces to increase in contact angle, corresponding to molecule would probably react near this other molecule in order decreasing wetting by water and increasing surface hydrophobicity. to take advantage of any hydrophobic–hydrophobic interactions. This is expected because OSA, AKD, and ALKD contain alkyl chains This would result in an inhomogeneous surface if the chemistry which are hydrophobic. In the case of AKD, the contact angle almost were stopped before complete reaction. Due to crowding, complete approached that of waxed paper. coverage may not be achievable.

Fig. 3. AFM pictures of control zein ﬁlm—1.7 ␮m2.

Fig. 4. AFM pictures of zein ﬁlm after reaction with AKD—1.0 ␮m2. A. Biswas et al. / Industrial Crops and Products 30 (2009) 168–171 171

4. Conclusions Cheryan, M., Shukla, R., 2001. Zein: the industrial protein from corn. Ind. Crops Prod. 13, 171–192. Ghanbarzadeh, B., Musavi, M., Oromiehie, A.R., Rezayi, K., Razmi Rad, E., Milani, We have found a new method to derivatize the zein surface in J., 2007. Effect of plasticizing sugars on water vapor permeability, surface order to modify the surface wetting behavior. The method is easy energy and microstructure properties of zein films. Food Sci. Technol. 40, 1191– to apply and entails only the application of a suitable concentra- 1197. James, A.L., 1944. Increasing the water-resistance of zein coatings. U.S. Patent No. tion of the derivatizing agent with subsequent heating at about ◦ 2,364,792. 100 C. AFM has been used to demonstrate that the morphology of Lai, H.M., Padua, G.W., Wei, L.S., 1997. Properties and microstructure of zein sheets the film surface changes after treatment. While the modified zein plasticized with palmitic and stearic acids. Cereal Chem. 74, 83–90. articles produced are not food grade at present, due to lack of data, Lawton, J.W., 2002. Zein: a history of processing and use. Cereal Chem. 79, 1–18. Lawton, J.W., 2004. Plasticizers for zein: their effect on tensile properties and water the improved hydrophobicity will have inherent value. The modi- absorption of zein films. Cereal Chem. 81, 1–5. fied surface is inhomogeneous having wide, tall, irregular globules. Momany, F.A., Sessa, D.J., Lawton, J.W., Selling, G.W., Hamaker, S.A.H., Willett, J.L., ␣ Since zein is used often as films, coatings, and fibers, changes in 2006. Structural characterization of -zein. J. Agric. Food Chem. 54, 543– 547. the hydrophobicity of the surface can impart desirable properties, Padua, G.W., Santosa, F.X.B., 1999. Tensile properties and water absorption of e.g., decreased water absorption, increased water repellency, and zein sheets plasticized with oleic and linoleic acids. J. Agric. Food Chem. 47, improved compatibility with organic additives. 2070–2074. Padua, G.W., Rakotonirainy A.M, Ha, T.T., 2005. Method of manufacturing improved corn zein resin films, sheets, and articles. U.S. Patent No. 6,849,113. Acknowledgements Qiao, L., Qu-Ming, G., Cheng, H.N., 2006. Enzyme-catalyzed synthesis of hydropho- bically modified starch. Carbohyd. Polym. 66, 135–140. Selling, G.W., Sessa, D.J., Palmquist, D.E., 2004. Effect of water and tri(ethylene) glycol We gratefully acknowledge Dr. H.N. Cheng of Hercules Incor- on the rheological properties of zein. Polymer 45, 4249–4255. porated, Wilmington, DE, for helpful discussions. Janet Berfield is Selling, G.W., Sessa, D.J., 2007. Multivalent carboxylic acids to modify the properties thanked for producing the control and modified zein films and car- of zein. Ind. Crops Prod. 25, 63–69. Sessa, D.J., Mohamed, A., Byars, J.A., Hamaker, S.A.H., Selling, G.W., 2007. Properties rying out the bulk water absorption tests. of films from corn zein reacted with glutaraldehyde. J. Appl. Polym. Sci. 105, 2877–2883. References Veatch, C., 1941. Zein acetate. U.S. Patent No. 2,236,768. Wang, Y., Padua, G.W., 2003. Tensile properties of extruded zein sheets and extrusion blown films. Macromol. Mat. Eng. 288, 886–893. Biswas, A., Sessa, D.J., Lawton, J.W., Gordon, S.H., Willett, J.L., 2005a. Microwave Wang, Y., Padua, G.W., 2006. Water barrier properties of zein–oleic acid films. Cereal assisted rapid modification of zein by octenyl succinic anhydride. Cereal Chem. Chem. 83, 331–334. 82, 1–3. Woods, K.K., Selling, G.W., 2007. Improved tensile strength of zein films using glyoxal Biswas, A., Sessa, D.J., Gordon, S.H., Lawton, J.W., Willett, J.L., 2005b. Synthesis of zein as a crosslinking reagent. J. Biobased Mater. Bioenerg. 1, 281–287. derivatives and their mechanical properties. In: Cheng, H.N., Gross, R.A. (Eds.), Wu, Q.X., Sakabe, H., Isobe, S., 2003. Studies on the toughness and water resistance Polymer Biocatalysis and Biomaterials. ACS Symposium Series, vol. 900. Oxford of zein-based polymers by modification. Polymer 44, 3901–3908. University Press, New York, pp. 141–148. Zhang, M., Reitmeier, C.A., Hammond, E.G., Myers, D.J., 1997. Production of textile Biswas, A., Shogren, R.L., Stevenson, D.G., Willett, J.L., Blowmik, P.K., 2006. Ionic liq- fibers from zein and a soy protein–zein blend. Cereal Chem. 74, 594–598. uids as solvents for biopolymers: acylation of starch and zein protein. Carbohyd. Polym. 66, 546–550.