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A review of recent development of sustainable waxes derived from vegetable oils
Accepted Manuscript Title: A Review of recent development of sustainable waxes derived from vegetable oils Author: Tao Fei Tong Wang PII: DOI: Reference:
S2214-7993(17)30049-8 http://dx.doi.org/doi:10.1016/j.cofs.2017.06.006 COFS 245
To appear in: Received date: Revised date: Accepted date:
24-3-2017 21-6-2017 22-6-2017
Please cite this article as: Fei, T., Wang, T.,A Review of recent development of sustainable waxes derived from vegetable oils, COFS (2017), http://dx.doi.org/10.1016/j.cofs.2017.06.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tao Fei and Tong Wang*
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Department of Food Science and Human Nutrition Iowa State University Ames, IA 50011 Submitted to Current Opinion in Food Science March 24, 2017 Revision submitted June 21, 2017
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ABSTRACT
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This review summarizes the recent development in the synthesis of biobased waxes using vegetable oils as feedstock. Driven by the increasing needs of sustainability in packaging and coating industry, research in developing renewable and eco-friendly waxes as replacers for paraffin or natural wax (e.g. beeswax and carnauba wax) has been conducted. We surveyed literature on synthesis, property, and applications of biobased waxes suitable for food contacts. The review also describes chemical structure-function relationship. Challenges in applications of such waxes are also discussed.
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* Corresponding author: Tong Wang, Professor 2312 Food Sciences Building Ames, IA 50011 Tel: 515-294-5448 Fax: 515-294-8181 Email: [email protected]
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A Review of recent development of sustainable waxes derived from vegetable oils
Highlights: •
Chemical reactions for the modification of vegetable oil are described
•
Properties similar to paraffin wax are targeted for the structural modification
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Structure-function relationships are discussed
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Opinions on future needs and challenges are given
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Introduction:
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The annual wax consumption in North America in 2016 is approximately 3 billion pounds (0.14 million metric tons), of which 60% is used for packaging and candle making [1, 2]. Although most waxes are not for direct food applications, they are widely used in contact with food. Fresh fruits and vegetables such as apples and green peppers are often coated with waxes for shelf life extension, and cardboard boxes used for food transportation are coated for improving mechanical strength and moisture resistance. The Code of Federal Regulations title 21 parts 176 and 178 state that substances that are in contact with food should be safe since they could be indirectly food additives. Waxes are also commonly used as an important ingredient in lubricant, adhesives, foods, cosmetics, pharmaceutics and inks. Based on the sources of waxes, they are generally categorized as petroleum-based, synthetic, and natural waxes. Petroleum-based wax constitutes 70% of the market [3]. However, the depleting oil reserves, dramatic fluctuation of crude oil price, and sustainability concerns from over-use of petroleum products have led to the search for alternatives. Many wax users are devoting significant efforts to implement ecodesign concepts that allow the coated materials to be repulpable or recyclable and to reduce the environmental impact from their disposal. Vegetable oils (VO) represent a promising source for producing renewable and eco-friendly wax alternatives due to their abundance and inherent biodegradability [4].
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Chemical modification of triacylglycerol (TAG) structure:
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Generally, the purpose of chemical modification is to increase the diversity of chemical structure and composition of a VO-based wax, so that new physical properties are obtained. Most of the reactions we summarized are mild and without the use of harmful chemicals, so the products derived are suitable to be in direct contact with foods.
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The most common method for obtaining VO-based waxes is partial or full hydrogenation. However, without additional structure modifications, the material cannot be directly used in the demanding applications such as corrugated cardboard coating. VO-based waxes need to possess equivalent physical and thermal properties to compete with the highly optimized petroleum-based waxes. To achieve the desirable functionalities, chemical modifications have been used to introduce functional groups [5, 6] and change physical properties. In this review, we describe the preparation methods for VO-based waxes, compare their physical properties with those of paraffin and natural waxes, discuss issues of the current methods, and summarize the observed trends of structure-function relationships.
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Epoxidation and ring opening reaction: Epoxidation and ring opening of the epoxide can functionalize the chain by incorporating functional groups such as pendent hydroxyl groups to impart high cohesiveness and wide melting range. Such wax can be used as a cohesiveness enhancer to blend with FHSO/PHVO (partially hydrogenated vegetable oil) or other base materials to generate waxes suitable for candle making [9]. By ring opening of epoxidized soybean oil TAG with polyhydroxy fatty acids to form polyols, and then polymerizing the polyols with isophorone diisocyanate using dibutyltin dilaurate as catalyst, polyurethanes with mechanical properties ranging from rigid and brittle to soft and ductile can be generated for surface coatings [10]. Other modifications that utilize the double bond is silyation of vegetable oils followed by hydrolysis and condensation to form a waxy material with excellent water repellency, thus a suitable material for coating packaging paper was formed [11]. However, with an extensive crosslinking and polymerization, the biodegradability of a wax material may become much less than that of the oil. This property should be evaluated to prove the environmental friendliness of the new coating materials.
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Transesterification reaction: A wide choice of raw materials can be used in this reaction and various types of catalysts including acid, base, metals, amines and enzymes have been investigated, as summarized in a review by Otera [7]. Products with various physical and thermal properties can be obtained by changing the fatty acid composition and chain length by the reaction. By transesterifying fully hydrogenated soybean oil (FHSO) with stearyl alcohol and triacetin at a molar ratio of 9:7:15 with sodium methoxide as catalyst, a wax comprising 31% diacetyl-monoacylglycerols, 12% monoacetyl-monoacylglycerols, 32% diacylglycerols, 11% acylglycerols and 14% wax esters was obtained. This product could be used as beeswax or paraffin substitute because of its high hardness and good cohesiveness [6]. Transesterification of oils from the kernels of apricot and sweet almond and corn germ with polyethylene glycol having a molecular weight of 200 to 800 at 205 to 225 °C produced a material capable of diffusing through tissues as well as assisting the formation of fine “oil in water” emulsions, which can be very useful for cosmetic, pharmaceutical and food industries [8].
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Transesterification using free fatty acid (FFA) or free fatty alcohol (FFAL): Pure fatty acid esters can be synthesized when pure FFA and FFAL derived from VO are used as starting materials. Many of these wax esters possess physical characteristics that are comparable to petroleum-based or natural waxes, so that they can be used in cosmetics, lubricants, and foods [12]. Monoesters with a carbon chain length of 36 to 44 were produced by esterification of C18:1 and C22:1 fatty acids with C18:1 and C22:1 alcohols at room temperature. These wax esters demonstrated very little polymorphic hysteresis during melting and cooling cycling, and can be used as a jojoba replacer in cosmetic and pharmaceutical applications [13]. Similar jojoba wax-like esters can also be obtained by esterification of 9-decenol with oleic acid or 9-decenoic acid with oleyl alcohol [14]. These esters have double bonds which can serve as starting materials of further reactions.
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Synthesis of very long chain wax esters with or without branches: When diacids or diols are used for esterification, the carbon chain length of wax esters can be further increased in the diesters or polyesters. Branched chains can also be introduced in these esters by epoxidation and ring opening esterification. Esters with functional groups on the linear end of the structure can be obtained by esterificaiton of acid anhydrides such as maleic anhydride with long chain alcohol such as docosanol in toluene at elevated temperatures (>110 °C). Such long chain esters with the dipolar groups spaced by a long carbon chain was claimed as carnauba wax substitutes and can be used in inks [18].
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Adding branched chain wax esters: With the double bonds in the esters, epoxidation and ring opening esterification can be done to incorporate functional groups and deliver new physical properties [15]. By introducing propionic acid, 9-decenoic acid, or 9-decenol to the epoxides of jojoba wax-like esters using ring-opening esterification, branched derivatives were obtained that had better low-temperature behavior [16, 17].
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Other chemical modification: Wax-like compounds such as amide and diamide, also can be synthesized by using VO-based materials as feedstocks, and these compounds often possess very high hardness and melting point. Esterified alkylolamide of hydroxysteric acid can be obtained by reacting hydrogenated castor oil with mono-ethanolamine at a temperature ranging from 150 °C to 175 °C. The melting point, hardness and gloss of the material were reported to be very similar to those of long chain ester waxes such as carnauba wax [19].
Advances and drawbacks of chemical modification: Overall, chemical modification using VO or its derivatives as feedstocks brings in numerous possibilities for synthesizing biobased waxes with desirable properties. However, some of the reactions are energy intensive and time consuming, and maybe difficult to control. Transesterification of TAG with alcohol usually generates a mixture of compounds which makes it difficult to subsequently identify structure-property relationships. The composition of the final product may not be fully controllable and predictable if the reaction inherently generates a mixture. Additional reaction optimization and fractionation may be required to achieve desirable end product. During a one-pot acetylation and stearylyzation of soywax, the reaction yield increased with the addition of stearyl alcohol, and decreased with the addition of triacetin, leading to significantly different compositions as well as hardness and cohesiveness of the end products [6]. In this case, additional study is essential to understand the reaction kinetics and equilibrium of such a three-component system.
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Some reactions may not be commercially feasible since they require expensive long chain fatty alcohols such as erucyl alcohol as a base material [13, 14]. A catalyst which often is a strong acid, base, or a metal is also required for transesterification, epoxidation, ringopening, or reduction reactions. However, these catalysts could cause molecular cleavage and waste disposal problems [20, 21]. Removing the catalyst and purifying the 5
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synthesized materials could also be problematic and it can increase the production cost considerably [13].
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Physical properties such as hardness, cohesiveness, melting point or range, viscosity, and optical transparency are used to characterize waxes. Hardness is usually measured by a penetration test either using a penetrometer or a texture analyzer. ASTM D1321 defines wax hardness as the distance that a standard needle can penetrate into the wax surface in 5 seconds with a 100 g standard load cell. Others have used the peak force generated by a texture analyzer’s probe penetrating into wax for a fixed distance as wax hardness [5, 6]. Cohesiveness of wax is usually quantified by using a compression or three-point bending test, and the area under the force-distance curve before the wax breaks is recorded as wax cohesiveness [5, 6, 22]. Melting point or range of a wax is commonly determined using differential scanning calorimetry (DSC).
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Physical properties and commercial applications of recently developed VO-based waxes:
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Among the VO-based waxes developed, majority is formed by physical blending for making candles. Waxes made by chemical modifications can have a board range of hardness, cohesiveness and melting temperature, so they can potentially be used in a variety of applications such as coating corrugated paper board and as a functional ingredient in cosmetics. Although many different types of VO-based waxes have been reported in the literature, very few have a complete set of physical parameters in comparison with petroleum-based waxes or natural waxes. Based on the available information, we provide a comprehensive table summarizing composition, physical properties, and potential applications of VO-based wax-like materials (Table 1). Due to the limited information available, some values are based on our best estimation.
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Although VO-based waxes are not designed as food additives, they do have a great potential as functional ingredients. Wax esters of n-3 polyunsaturated fatty acids that had low degree of susceptibility to oxidation and high degree of in vitro enzymatic hydrolysis were reported as a potential supplement of fish oil [23]. Natural waxes such as carnauba wax, candelilla wax, beeswax, sunflower wax, and rice bran wax have been extensively studied in oleogel formation [24-27]. Food grade VO-based waxes may also be used as oleogelators to replace these natural waxes. More studies on food applications of VObased waxes are needed. Correlations between chemical structure and physical property:
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With the development of VO-based waxes, some information about the relationship between chemical structure and property have been reported. A better understanding of the structure-function relationship is important and it will be useful for future structure modifications according to application needs. However, very few studies have comprehensively and systematically investigated structure-function relationship.
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Impact of pendent hydroxyl group and branched chain on hardness, cohesiveness and melting point: Pendent hydroxyl group and branched chain can significantly affect physical and thermal properties of waxes. Incorporating pendent hydroxyl group and branched chain (butyric ester) to a vegetable hardstock (KLX) discouraged ordered molecular packing and resulted in finer and more random crystallization which decreased both hardness and melting point of the wax mixtures [9]. However, these groups also greatly enhanced cohesiveness by improving intermolecular interactions (Figure 2). The pendent hydroxyl group led to finer and more needle-like crystals. It is believed that needle crystals allowed more contacts among the microstructural elements [30] and possibly contributed to the good cohesiveness. Similar trends were also observed in pure ester systems [8, 12, 13]. The branched monoesters tend to form glassy liquids rather than crystal phases, indicating that branches suppressed crystallization [31]. The incorporation of pendent hydroxyl groups in pure diesters increased their viscosity and resistance to shear stress due to the intermolecular hydrogen bonding (Figure 3) [32].
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Impact of chain length on hardness and melting point: Chain length is a key factor that determines the hardness of VO-based waxes. Analysis of six pure fatty acid esters of 1,3propanediol with carbon number range from 12 to 18 indicates that hardness and melting point increase with chain length of fatty acyl groups (Figure 1) [28]. The DSC of four jojoba wax-like monoesters with carbon number ranging from 36 to 44 also suggests a positive correlation between crystallization and melting temperature and carbon chain length [13]. As for esters, similar trend is also observed in linear hydrocarbons; melting point of straight-chain alkanes increased from about 45 to 80 °C when carbon number increased from 21 to 40 [29].
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Effect of bond type: Type of bond or linkage also affects physical properties of VO-based waxes. Amide bond leads to higher melting point than ester bond, and an acid group on the linear end of a fatty acyl chain leads to higher hardness due to their engagement in hydrogen bonding [18, 19].
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Future needs:
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The understanding of relationships between composition, microstructure, and properties of a wax, and of the interactions among various components in a mixed system are still very limited. The contribution of chemical structure and composition to the aesthetical properties such as shininess and smoothness of wax surface is still not fully understood. More systematic studies on structure-function relationships using pure lipids, as well as on mixing behaviors of multi-component systems are needed. The effect of molecular interactions and mixing ratio on mechanical strength, thermal properties, and appearance of waxes should be fully investigated.
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Conclusion:
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Substantial efforts have been devoted to developing eco-friendly biobased waxes over the past 20 years to replace petroleum-based products and natural waxes. However, to date, no ideal VO-based wax has been identified for a universal replacement of petroleumbased paraffin. Further research on structure modification is needed to create desirable properties. It is desirable to develop feasible methods to make long chain fatty diols and diacids. It is also essential to fully understand the roles of chemical structure, composition, microstructure, and interactions in a mixed system. The review of the current literature indicates that comprehensive and theoretical research is lacking in this field.
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References:
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[1] American fuel and petrochemical manufactures wax facts (accessed Dec 2016). https://www.afpm.org/Wax-Facts/
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[2] National candle association facts and figures about candles (accessed Dec 2016). http://candles.org/facts-figures-2/
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Sustainability is another desirable aspect of using VO-based waxes. However, recyclability and biodegradability of waxes or repulpability of the wax-coated corrugated boards or papers are largely unreported in the literatures. Although the recyclability or repulpability of corrugated fiberboard is a voluntary standard, these tests should be conducted as a key performance indicator for sustainability. The introduction of novel properties into the wax, such as antimicrobial, antioxidant, and anticorrosion is very desirable. These novel properties will further expand wax applications in food, packaging, cosmetic and pharmaceutical industries.
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[3] Global wax industry 2016: market analysis and opportunities 306
[4] Alam M, Akram D, Sharmin E, Zafar F, Ahmad S: Vegetable oil-based eco-friendly coating materials. Arabian Journal of Chemistry 2014, 7:469-479.
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[5] Yao L, Wang T: Textural and physical properties of biorenewable “waxes” containing partial acylglycerides. J Am Oil Chem Soc 2012, 89:155-166.
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[6] Yao L, Lio J, Wang T: Synthesis and characterization of acetylated and stearylyzed soy wax. J Am Oil Chem Soc 2013, 90:1063-1071.
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This interesting article monitored the changes of physical properties including hardness and cohesiveness of soy wax along with the incorporation of acetyl and stearyl groups, and demonstrated that length of the fatty acyl chain as well as diversity of the structural composition can significantly impact physical properties of vegetable oil-based waxes.
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[7] Otera J: Transesterification. Chem. Rev. 1993, 1449-1470. •
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[8] Mahler E, Gattefosse M: Esterification of triglyceride with polyethylene glycols and product. US Patent 1966, 3,288,824.
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[9] Wang L, Wang T: Chemical modification of partially hydrogenated vegetable oil to improve its functional properties for candles. J Am Oil Chem Soc 2007, 84:1149-1159.
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[10] Chen R, Zhang C, Kessler M: Polyols and polyurethanes prepared from epoxidized soybean oil ring-opened by polyhydroxy fatty acids with varying OH numbers. J Appl Polym Sci 2015, 132:41213.
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This article presented a chemical modification route for improving cohesiveness of vegetable oil-based waxes, and demonstrated that pendent hydroxyl group and branched chains can significantly affect hardness, cohesiveness, melting point as well as morphology of vegetable oil-based waxes.
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This article comprehensively reviewed transesterification. Different types of catalysts used for the reaction were thoroughly discussed.
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[11] Tambe C, Graiver D, Narayan R: Mositure resistance coating of packaging paper from biobased silyated soybean oil. Progress in Organic Coatings 2016, 101:270-278.
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[12] Patel S, Nelson D, Gibbs A: Chemical and physical analyses of wax ester properties. J Insect Sci 2001, 1:4.
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[13] Bouzidi L, Li S, Biase S, Rizvi S, Narine S: Lubricating and waxy esters, I. Synthesis, crystallization, and melt behavior of linear monoesters. Chemistry and Physics of Lipids 2012, 165: 38-50.
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This article is a good example of using pure ester to study structure-function relationship. Jojoba wax-like esters were successfully synthesized and the correlation between carbon chain length and crystallization and melt behavior of monoesters was also elucidated.
[14] Bouzidi L, Li S, Biase S, Rizvi S, Narine S: Lubricating and waxy esters. 4. Synthesis, crystallization behavior, melt behavior, and flow behavior of linear monoesters incorporating 9-decenol and 9-decenoic acid. Ind Eng Chem Res 2013, 52:2740-2749.
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[15] Moser BR, Sharma BK, Doll KM, Erhan SZ: Diesters from oleic acid: synthesis, low temperature properties, and oxidation stability. J AM Oil Chem Soc 2007, 7:675-680.
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[16] Bouzidi, L, Li S, Biase S, Rizvi S, Dawson P, Narine S: Lubricating and waxy esters II: Synthesis, crystallization, and melt behavior of branched monoesters. Ind Eng Chem Res 2012, 51:14892-14902.
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This study demonstrated the synthesis of branched derivatives of jojoba wax-like esters and elucidated their crystallization and melt behavior as a function of chain length, branching, symmetry, and functional groups.
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[17] Li S, Bouzidi L, Narine S: Lubricating and waxy esters, V: Synthesis, crystallization, and melt and flow behaviors of branched monoesters incorporation 9-decenol and 9decenoic acid. Ind End Chem Res 2014, 53:12339-12354. [18] Callinan T, Crimi J, Mcgee H, Parks A, Schwartz P: Synthetic wax substitute for carnauba wax and transfer ink compositions containing such substitutes. US Patent 1964, 3,129,140.
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[19] Kelly M: Hard wax substitute. US Patent 1941, 2,356,408.
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[20] Swern D: Organic peroxy acids as oxidizing agnets. In: Organic peroxides, vol 2. Wiley-Interscience 1971, New York, pp 355-533.
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[21] Park S, Jin F, Lee J: Synthesis and thermal properties of epoxidized vegetable oil. Macromol Rapid Commun. 2004, 25:724-727.
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[23] Gorreta F, Bernasconi R, Galliani G, Salmona M, Tacconi MT, Bianchi R: Wax esters of n-3 polyunsaturated fatty acids: a new stable formulation as a potential food supplement. 1 – digestion and absorption in rats. Lebensm-Wiss U-Technol 2002, 35:458465. [24] Zulim Botega DC, Marangoni AG, Smith AK, Golf HD: Development of formulations and process to incorporate wax oleogels in ice cream. J Food Sci 2013, 78:C1845-1851.
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[22] Liu L, Wang N, Xu L, Yu X, Zhang R, Wang T: A novel method of determining wax cohesiveness by using a texture analyzer. Journal of Texture Studies 2015, DOI:10.1111/jtxs.12171.
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[25] Doan CD, Van de Walle D, Dewettinck K, Patel AR: Evaluating the oil-gelling properties of natural waxes in rice bran oil: rheological, thermal, and microstructural study. J Am Oil Chem Soc 2015, 92:801-811.
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[26] Blake AI, Co ED, Marangoni AG: Structure and physical properties of plant wax crystal networks and their relationship to oil binding capacity. J Am Oil Chem Soc 2014, 91:885-903.
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[27] Hwang HS, Kim S, Singh M, Winkler-Moser JK, Liu SX: Organogle formation of soybean oil with waxes. J AM Oil Chem Soc 2012, 89:639-647.
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[28] Abes M, Narine S: Crystallization and phase behavior of fatty acid esters of 1,3propanediol I: pure system. Chem Phys Lipids 2007, 149:14-27.
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[29] Gibbs A, Pomonis G: Physical properties of insect cuticular hydrocarbons: the effect of chain length, methyl-branching and unsaturation. Comp Bichem Physiol 1995, 112:243-249. [30] Blake AI, Co ED, Marangoni AG: Structure and physical properties of plant wax crystal networks and their relationship to oil binding capacity. J Am Oil Chem Soc 2014, 91:885-903.
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[31] Rodrigues J, Cardoso F, Lachter E, Estevao L, Lima E, Nascimento R: Correlating chemical structure and physical properties of vegetable oil esters. J Am Oil Chem Soc 2006, 83:353-357.
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This article evaluated the influence of fatty acid ester structures on their rheology and crystallization behavior. Shorter fatty acid esters have weaker random interactions among molecules, and branched chains only significant affects crystallization temperature.
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[33] Kiado A: Applications of paraffin waxes and liquid paraffin. Development in Petroleum Science 1982, 14:240-329.
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[32] Li S, Bouzidi L, Narine S: Lubricating and waxy esters. 6. Synthesis and physical properties of (E)-didec-9-enyl octadec-9-enedioate and branched derivatives. Ind End Chem Res 2014, 53:20044-20055.
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[34] Tulloch A: Beeswax – composition and analysis. Bee World 1980, 61:47-62. [35] Basson I, Reynhardt EC: An investigation of the structures and molecular dynamics of natural waxes. I. Beeswax. J Phys D: Appl Phys 1988, 21:1421-1428.
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[36] Basson I, Reynhardt EC: An investigation of the structures and molecular dynamics of natural waxes. II. Carnauba wax. J Phys D: Appl Phys 1988, 21:1429-1433.
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[37] Endlein E, Peleikis K: Natural waxes – properties, compositions and applications. SOFW Journal 2011, 137.
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[38] Rezaei K, Wang T: Hydrogenated vegetable oils as candle wax. J Am Oil Chem Soc 2002, 79: 1241-1247.
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[39] Murphy T: Vegetable oil based wax compositions. US Patent 2004, 6,824,572.
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[40] Calzada JF, Upadhyaya J: Non-paraffin candle composition. US Patent 2000, 6,063,144.
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[41] Murphy T, Doucette MK, Richards M: Triacylglycerol-based alternative to paraffin wax. US Patent 2013, 8,529,924.
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[42] Depablo J, Biddy M: Nanoparticle modified lubricants and waxes with enhanced properties. US Patent 2009, 0,053,268 A1.
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[43] Lee Y: Partial acyl glyceride based biowaxes, biocancles prepared therefrom and their preparation. US Patent 2008, 0,145,808.
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[44] Phadoemchit T, Boonvichitr S: Paraffin wax replacer. US Patent 1989, 4,842,648. 422
[45] Clock A, Mckeown S, Wisnefsky E, Frenkel P: Bio-based wax compositions and applications. US Patent 2014, 8,876,918.
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Mainly linear hydrocarbons (C20-40)
ed
Commercial paraffin
pt
14% hydrocarbons 35 % monoesters 14% diesters 3% triesters 4% hydroxyl monoesters 8% hydroxyl polyesters 12 % free fatty acid 1% free fatty alcohol and others 2% hydrocarbon 84% aliphatic and aromatic esters 3% free fatty acids 3% free fatty alcohol 3% lactides 4% resins
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Beeswax
Carnauba wax
AS soywax
Physical properties
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Table 1. Compositions, physical properties and potential applications of current VO-based waxes.
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75% acetylated glycerol esters 14% wax esters 11% acylglycerols
High hardness: ~ 6,000 g Medium cohesiveness: ~ 8,000 g.mm Medium melting point: ~ 65 °C
Medium hardness: ~ 3,500 g Very high cohesiveness: ~ 40,000 g.mm Medium melting point: ~ 65 °C
Applications
References
Candles, coatings, packaging
Yao et al., 2013 [6] Kiadó, 1982 [33]
Candles, encaustic painting
Yao et al., 2013 [6] Tulloch et al., 1972, 1980 [34] Basson and Reynhardt, 1988 [35]
Very high hardness: ~ 60,000 g Low cohesiveness: ~ 2,000 g.mm High melting point: ~ 84 °C
Paper, surfboard coating, car waxes, foods
Basson and Reynhardt, 1988 [36] Endlein and Peleikis, 2011 [37]
Medium hardness: ~ 4,000 g High cohesiveness: ~ 18,000 g.mm Low melting point: ~ 50 °C
Candles
Yao et al., 2013 [6]
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cr us Medium hardness: ~ 3,500 g Very high cohesiveness: ~ 40,000 g.mm Low melting point: ~ 55 °C
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AM-MD wax
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35% monoacylglycerol 15% diacylglcerol 50% acetylated monoacylglycerol
Hydroxylated partially hydrogenated vegetable oil
FFA-TAG
30-40% free fatty acids 50-70% commercial KLX or other VO-based waxes
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OH-KLX
Low hardness: ~ 2,000 g High cohesiveness: ~ 10,000 g.mm Low melting point: ~ 47 °C Medium hardness: ~ 4,000 g Low cohesiveness: ~ 2,000 g.mm Low melting point: ~ 50 °C
Beeswax and paraffin substitute, encaustic painting medium
Yao et al., 2012 [5]
Cohesiveness enhancer for candles
Wang and Wang, 2007 [9]
Candles
Rezaei and Wang, 2002 [38] Murphy, 2004 [39] Calzada and Upadhyaya, 2000 [40] Murphy et al., 2013 [41]
Hardness: N/A Cohesiveness: N/A Low melting point: ~ 50 °C
Candles
Depablo and Biddy, 2007 [42]
RBD/PAG-wax
0%-95% palm Stearin 0%-5% glyceryl monostearate 0%-100% partial acyl glycerols
Hardness: N/A Cohesiveness: N/A Low melting point: ~ 55 °C
Paraffin substitute
Lee, 2008 [43] Phadoemchit et al, 1989 [44]
Epoxy-VO-wax
5%-95% epoxidized VO 5%-95% fully hydrogenated VO
Hardness: N/A Cohesiveness: N/A Medium melting point ~ 60 °C
Candles
Clock et al., 2014 [45]
Ac
Nano-soywax
90% Soywax 10% 10nm SiOx
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100% straight and branched chain monoesters
Amide-wax
Mainly esterified alkylolamide of hydroxystearic acid
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pt
ce
425
N/A
Moisture resistant packaging paper coating
Tambe et al., 2015 [11]
Hardness: N/A Cohesiveness: N/A Low melting point: ~ 35 °C
Jojoba wax substitutes
Boudidi et al. 2012; 2013 [13, 14] Li et al., 2014 [17, 32]
Very high hardness: ~ 15,000 g Cohesiveness: N/A High melting point: ~ 92 °C
Carnauba wax substitute
Kelly, 1944 [19]
Long chain esters formed by acid anhydride and alcohol
Hardness: N/A Cohesiveness: N/A High melting point: 80 – 82 °C
Carnauba wax substitute in ink
Callinan et al., 1964 [18]
Ac
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Figure 1. Variation of (a) hardness and (b) peak and onset melting temperature with carbon chain length of pure fatty acid esters (adapted from Abes and Narine, 2007 [28])
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Figure 2. Effect of adding OHKLX or BuoKLX (OH or butyl pendent group on KLX) on (a) Hardness and (b) Cohesiveness (lower slope value means higher cohesiveness) of FHSO and KLX base materials (adapted from Wang and Wang, 2007 [9]).
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Figure 3. a. Crystallization; b-c. viscosity characteristics of esters with different numbers of pendent hydroxyl groups and branched chains. No OH groups in H6, and 1, 2, 3 OH 16
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groups in H3, H4, H5, respectively (adapted from Bouzidi et al., 2012; Li et al., 2014 [16, 32]).
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A Review of recent development of sustainable waxes derived from vegetable oils
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Tao Fei and Tong Wang*
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There is no conflict of interest!
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* Corresponding author: Tong Wang, Professor 2312 Food Sciences Building Ames, IA 50011 Tel: 515-294-5448 Fax: 515-294-8181 Email: [email protected]
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Department of Food Science and Human Nutrition Iowa State University Ames, IA 50011 Submitted to Current Opinion in Food Science March 24, 2017
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S2214-7993(17)30049-8 http://dx.doi.org/doi:10.1016/j.cofs.2017.06.006 COFS 245
To appear in: Received date: Revised date: Accepted date:
24-3-2017 21-6-2017 22-6-2017
Please cite this article as: Fei, T., Wang, T.,A Review of recent development of sustainable waxes derived from vegetable oils, COFS (2017), http://dx.doi.org/10.1016/j.cofs.2017.06.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Tao Fei and Tong Wang*
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Department of Food Science and Human Nutrition Iowa State University Ames, IA 50011 Submitted to Current Opinion in Food Science March 24, 2017 Revision submitted June 21, 2017
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ABSTRACT
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This review summarizes the recent development in the synthesis of biobased waxes using vegetable oils as feedstock. Driven by the increasing needs of sustainability in packaging and coating industry, research in developing renewable and eco-friendly waxes as replacers for paraffin or natural wax (e.g. beeswax and carnauba wax) has been conducted. We surveyed literature on synthesis, property, and applications of biobased waxes suitable for food contacts. The review also describes chemical structure-function relationship. Challenges in applications of such waxes are also discussed.
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* Corresponding author: Tong Wang, Professor 2312 Food Sciences Building Ames, IA 50011 Tel: 515-294-5448 Fax: 515-294-8181 Email: [email protected]
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A Review of recent development of sustainable waxes derived from vegetable oils
Highlights: •
Chemical reactions for the modification of vegetable oil are described
•
Properties similar to paraffin wax are targeted for the structural modification
•
Structure-function relationships are discussed
•
Opinions on future needs and challenges are given
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Introduction:
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The annual wax consumption in North America in 2016 is approximately 3 billion pounds (0.14 million metric tons), of which 60% is used for packaging and candle making [1, 2]. Although most waxes are not for direct food applications, they are widely used in contact with food. Fresh fruits and vegetables such as apples and green peppers are often coated with waxes for shelf life extension, and cardboard boxes used for food transportation are coated for improving mechanical strength and moisture resistance. The Code of Federal Regulations title 21 parts 176 and 178 state that substances that are in contact with food should be safe since they could be indirectly food additives. Waxes are also commonly used as an important ingredient in lubricant, adhesives, foods, cosmetics, pharmaceutics and inks. Based on the sources of waxes, they are generally categorized as petroleum-based, synthetic, and natural waxes. Petroleum-based wax constitutes 70% of the market [3]. However, the depleting oil reserves, dramatic fluctuation of crude oil price, and sustainability concerns from over-use of petroleum products have led to the search for alternatives. Many wax users are devoting significant efforts to implement ecodesign concepts that allow the coated materials to be repulpable or recyclable and to reduce the environmental impact from their disposal. Vegetable oils (VO) represent a promising source for producing renewable and eco-friendly wax alternatives due to their abundance and inherent biodegradability [4].
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Chemical modification of triacylglycerol (TAG) structure:
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Generally, the purpose of chemical modification is to increase the diversity of chemical structure and composition of a VO-based wax, so that new physical properties are obtained. Most of the reactions we summarized are mild and without the use of harmful chemicals, so the products derived are suitable to be in direct contact with foods.
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The most common method for obtaining VO-based waxes is partial or full hydrogenation. However, without additional structure modifications, the material cannot be directly used in the demanding applications such as corrugated cardboard coating. VO-based waxes need to possess equivalent physical and thermal properties to compete with the highly optimized petroleum-based waxes. To achieve the desirable functionalities, chemical modifications have been used to introduce functional groups [5, 6] and change physical properties. In this review, we describe the preparation methods for VO-based waxes, compare their physical properties with those of paraffin and natural waxes, discuss issues of the current methods, and summarize the observed trends of structure-function relationships.
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Epoxidation and ring opening reaction: Epoxidation and ring opening of the epoxide can functionalize the chain by incorporating functional groups such as pendent hydroxyl groups to impart high cohesiveness and wide melting range. Such wax can be used as a cohesiveness enhancer to blend with FHSO/PHVO (partially hydrogenated vegetable oil) or other base materials to generate waxes suitable for candle making [9]. By ring opening of epoxidized soybean oil TAG with polyhydroxy fatty acids to form polyols, and then polymerizing the polyols with isophorone diisocyanate using dibutyltin dilaurate as catalyst, polyurethanes with mechanical properties ranging from rigid and brittle to soft and ductile can be generated for surface coatings [10]. Other modifications that utilize the double bond is silyation of vegetable oils followed by hydrolysis and condensation to form a waxy material with excellent water repellency, thus a suitable material for coating packaging paper was formed [11]. However, with an extensive crosslinking and polymerization, the biodegradability of a wax material may become much less than that of the oil. This property should be evaluated to prove the environmental friendliness of the new coating materials.
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Transesterification reaction: A wide choice of raw materials can be used in this reaction and various types of catalysts including acid, base, metals, amines and enzymes have been investigated, as summarized in a review by Otera [7]. Products with various physical and thermal properties can be obtained by changing the fatty acid composition and chain length by the reaction. By transesterifying fully hydrogenated soybean oil (FHSO) with stearyl alcohol and triacetin at a molar ratio of 9:7:15 with sodium methoxide as catalyst, a wax comprising 31% diacetyl-monoacylglycerols, 12% monoacetyl-monoacylglycerols, 32% diacylglycerols, 11% acylglycerols and 14% wax esters was obtained. This product could be used as beeswax or paraffin substitute because of its high hardness and good cohesiveness [6]. Transesterification of oils from the kernels of apricot and sweet almond and corn germ with polyethylene glycol having a molecular weight of 200 to 800 at 205 to 225 °C produced a material capable of diffusing through tissues as well as assisting the formation of fine “oil in water” emulsions, which can be very useful for cosmetic, pharmaceutical and food industries [8].
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Transesterification using free fatty acid (FFA) or free fatty alcohol (FFAL): Pure fatty acid esters can be synthesized when pure FFA and FFAL derived from VO are used as starting materials. Many of these wax esters possess physical characteristics that are comparable to petroleum-based or natural waxes, so that they can be used in cosmetics, lubricants, and foods [12]. Monoesters with a carbon chain length of 36 to 44 were produced by esterification of C18:1 and C22:1 fatty acids with C18:1 and C22:1 alcohols at room temperature. These wax esters demonstrated very little polymorphic hysteresis during melting and cooling cycling, and can be used as a jojoba replacer in cosmetic and pharmaceutical applications [13]. Similar jojoba wax-like esters can also be obtained by esterification of 9-decenol with oleic acid or 9-decenoic acid with oleyl alcohol [14]. These esters have double bonds which can serve as starting materials of further reactions.
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Synthesis of very long chain wax esters with or without branches: When diacids or diols are used for esterification, the carbon chain length of wax esters can be further increased in the diesters or polyesters. Branched chains can also be introduced in these esters by epoxidation and ring opening esterification. Esters with functional groups on the linear end of the structure can be obtained by esterificaiton of acid anhydrides such as maleic anhydride with long chain alcohol such as docosanol in toluene at elevated temperatures (>110 °C). Such long chain esters with the dipolar groups spaced by a long carbon chain was claimed as carnauba wax substitutes and can be used in inks [18].
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Adding branched chain wax esters: With the double bonds in the esters, epoxidation and ring opening esterification can be done to incorporate functional groups and deliver new physical properties [15]. By introducing propionic acid, 9-decenoic acid, or 9-decenol to the epoxides of jojoba wax-like esters using ring-opening esterification, branched derivatives were obtained that had better low-temperature behavior [16, 17].
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Other chemical modification: Wax-like compounds such as amide and diamide, also can be synthesized by using VO-based materials as feedstocks, and these compounds often possess very high hardness and melting point. Esterified alkylolamide of hydroxysteric acid can be obtained by reacting hydrogenated castor oil with mono-ethanolamine at a temperature ranging from 150 °C to 175 °C. The melting point, hardness and gloss of the material were reported to be very similar to those of long chain ester waxes such as carnauba wax [19].
Advances and drawbacks of chemical modification: Overall, chemical modification using VO or its derivatives as feedstocks brings in numerous possibilities for synthesizing biobased waxes with desirable properties. However, some of the reactions are energy intensive and time consuming, and maybe difficult to control. Transesterification of TAG with alcohol usually generates a mixture of compounds which makes it difficult to subsequently identify structure-property relationships. The composition of the final product may not be fully controllable and predictable if the reaction inherently generates a mixture. Additional reaction optimization and fractionation may be required to achieve desirable end product. During a one-pot acetylation and stearylyzation of soywax, the reaction yield increased with the addition of stearyl alcohol, and decreased with the addition of triacetin, leading to significantly different compositions as well as hardness and cohesiveness of the end products [6]. In this case, additional study is essential to understand the reaction kinetics and equilibrium of such a three-component system.
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Some reactions may not be commercially feasible since they require expensive long chain fatty alcohols such as erucyl alcohol as a base material [13, 14]. A catalyst which often is a strong acid, base, or a metal is also required for transesterification, epoxidation, ringopening, or reduction reactions. However, these catalysts could cause molecular cleavage and waste disposal problems [20, 21]. Removing the catalyst and purifying the 5
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synthesized materials could also be problematic and it can increase the production cost considerably [13].
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Physical properties such as hardness, cohesiveness, melting point or range, viscosity, and optical transparency are used to characterize waxes. Hardness is usually measured by a penetration test either using a penetrometer or a texture analyzer. ASTM D1321 defines wax hardness as the distance that a standard needle can penetrate into the wax surface in 5 seconds with a 100 g standard load cell. Others have used the peak force generated by a texture analyzer’s probe penetrating into wax for a fixed distance as wax hardness [5, 6]. Cohesiveness of wax is usually quantified by using a compression or three-point bending test, and the area under the force-distance curve before the wax breaks is recorded as wax cohesiveness [5, 6, 22]. Melting point or range of a wax is commonly determined using differential scanning calorimetry (DSC).
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Physical properties and commercial applications of recently developed VO-based waxes:
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Among the VO-based waxes developed, majority is formed by physical blending for making candles. Waxes made by chemical modifications can have a board range of hardness, cohesiveness and melting temperature, so they can potentially be used in a variety of applications such as coating corrugated paper board and as a functional ingredient in cosmetics. Although many different types of VO-based waxes have been reported in the literature, very few have a complete set of physical parameters in comparison with petroleum-based waxes or natural waxes. Based on the available information, we provide a comprehensive table summarizing composition, physical properties, and potential applications of VO-based wax-like materials (Table 1). Due to the limited information available, some values are based on our best estimation.
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Although VO-based waxes are not designed as food additives, they do have a great potential as functional ingredients. Wax esters of n-3 polyunsaturated fatty acids that had low degree of susceptibility to oxidation and high degree of in vitro enzymatic hydrolysis were reported as a potential supplement of fish oil [23]. Natural waxes such as carnauba wax, candelilla wax, beeswax, sunflower wax, and rice bran wax have been extensively studied in oleogel formation [24-27]. Food grade VO-based waxes may also be used as oleogelators to replace these natural waxes. More studies on food applications of VObased waxes are needed. Correlations between chemical structure and physical property:
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With the development of VO-based waxes, some information about the relationship between chemical structure and property have been reported. A better understanding of the structure-function relationship is important and it will be useful for future structure modifications according to application needs. However, very few studies have comprehensively and systematically investigated structure-function relationship.
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Impact of pendent hydroxyl group and branched chain on hardness, cohesiveness and melting point: Pendent hydroxyl group and branched chain can significantly affect physical and thermal properties of waxes. Incorporating pendent hydroxyl group and branched chain (butyric ester) to a vegetable hardstock (KLX) discouraged ordered molecular packing and resulted in finer and more random crystallization which decreased both hardness and melting point of the wax mixtures [9]. However, these groups also greatly enhanced cohesiveness by improving intermolecular interactions (Figure 2). The pendent hydroxyl group led to finer and more needle-like crystals. It is believed that needle crystals allowed more contacts among the microstructural elements [30] and possibly contributed to the good cohesiveness. Similar trends were also observed in pure ester systems [8, 12, 13]. The branched monoesters tend to form glassy liquids rather than crystal phases, indicating that branches suppressed crystallization [31]. The incorporation of pendent hydroxyl groups in pure diesters increased their viscosity and resistance to shear stress due to the intermolecular hydrogen bonding (Figure 3) [32].
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Impact of chain length on hardness and melting point: Chain length is a key factor that determines the hardness of VO-based waxes. Analysis of six pure fatty acid esters of 1,3propanediol with carbon number range from 12 to 18 indicates that hardness and melting point increase with chain length of fatty acyl groups (Figure 1) [28]. The DSC of four jojoba wax-like monoesters with carbon number ranging from 36 to 44 also suggests a positive correlation between crystallization and melting temperature and carbon chain length [13]. As for esters, similar trend is also observed in linear hydrocarbons; melting point of straight-chain alkanes increased from about 45 to 80 °C when carbon number increased from 21 to 40 [29].
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Effect of bond type: Type of bond or linkage also affects physical properties of VO-based waxes. Amide bond leads to higher melting point than ester bond, and an acid group on the linear end of a fatty acyl chain leads to higher hardness due to their engagement in hydrogen bonding [18, 19].
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Future needs:
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The understanding of relationships between composition, microstructure, and properties of a wax, and of the interactions among various components in a mixed system are still very limited. The contribution of chemical structure and composition to the aesthetical properties such as shininess and smoothness of wax surface is still not fully understood. More systematic studies on structure-function relationships using pure lipids, as well as on mixing behaviors of multi-component systems are needed. The effect of molecular interactions and mixing ratio on mechanical strength, thermal properties, and appearance of waxes should be fully investigated.
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Conclusion:
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Substantial efforts have been devoted to developing eco-friendly biobased waxes over the past 20 years to replace petroleum-based products and natural waxes. However, to date, no ideal VO-based wax has been identified for a universal replacement of petroleumbased paraffin. Further research on structure modification is needed to create desirable properties. It is desirable to develop feasible methods to make long chain fatty diols and diacids. It is also essential to fully understand the roles of chemical structure, composition, microstructure, and interactions in a mixed system. The review of the current literature indicates that comprehensive and theoretical research is lacking in this field.
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References:
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[1] American fuel and petrochemical manufactures wax facts (accessed Dec 2016). https://www.afpm.org/Wax-Facts/
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[2] National candle association facts and figures about candles (accessed Dec 2016). http://candles.org/facts-figures-2/
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Sustainability is another desirable aspect of using VO-based waxes. However, recyclability and biodegradability of waxes or repulpability of the wax-coated corrugated boards or papers are largely unreported in the literatures. Although the recyclability or repulpability of corrugated fiberboard is a voluntary standard, these tests should be conducted as a key performance indicator for sustainability. The introduction of novel properties into the wax, such as antimicrobial, antioxidant, and anticorrosion is very desirable. These novel properties will further expand wax applications in food, packaging, cosmetic and pharmaceutical industries.
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[3] Global wax industry 2016: market analysis and opportunities 306
[4] Alam M, Akram D, Sharmin E, Zafar F, Ahmad S: Vegetable oil-based eco-friendly coating materials. Arabian Journal of Chemistry 2014, 7:469-479.
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[5] Yao L, Wang T: Textural and physical properties of biorenewable “waxes” containing partial acylglycerides. J Am Oil Chem Soc 2012, 89:155-166.
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[6] Yao L, Lio J, Wang T: Synthesis and characterization of acetylated and stearylyzed soy wax. J Am Oil Chem Soc 2013, 90:1063-1071.
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This interesting article monitored the changes of physical properties including hardness and cohesiveness of soy wax along with the incorporation of acetyl and stearyl groups, and demonstrated that length of the fatty acyl chain as well as diversity of the structural composition can significantly impact physical properties of vegetable oil-based waxes.
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[7] Otera J: Transesterification. Chem. Rev. 1993, 1449-1470. •
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[8] Mahler E, Gattefosse M: Esterification of triglyceride with polyethylene glycols and product. US Patent 1966, 3,288,824.
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[9] Wang L, Wang T: Chemical modification of partially hydrogenated vegetable oil to improve its functional properties for candles. J Am Oil Chem Soc 2007, 84:1149-1159.
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[10] Chen R, Zhang C, Kessler M: Polyols and polyurethanes prepared from epoxidized soybean oil ring-opened by polyhydroxy fatty acids with varying OH numbers. J Appl Polym Sci 2015, 132:41213.
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This article presented a chemical modification route for improving cohesiveness of vegetable oil-based waxes, and demonstrated that pendent hydroxyl group and branched chains can significantly affect hardness, cohesiveness, melting point as well as morphology of vegetable oil-based waxes.
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This article comprehensively reviewed transesterification. Different types of catalysts used for the reaction were thoroughly discussed.
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[11] Tambe C, Graiver D, Narayan R: Mositure resistance coating of packaging paper from biobased silyated soybean oil. Progress in Organic Coatings 2016, 101:270-278.
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[12] Patel S, Nelson D, Gibbs A: Chemical and physical analyses of wax ester properties. J Insect Sci 2001, 1:4.
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[13] Bouzidi L, Li S, Biase S, Rizvi S, Narine S: Lubricating and waxy esters, I. Synthesis, crystallization, and melt behavior of linear monoesters. Chemistry and Physics of Lipids 2012, 165: 38-50.
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This article is a good example of using pure ester to study structure-function relationship. Jojoba wax-like esters were successfully synthesized and the correlation between carbon chain length and crystallization and melt behavior of monoesters was also elucidated.
[14] Bouzidi L, Li S, Biase S, Rizvi S, Narine S: Lubricating and waxy esters. 4. Synthesis, crystallization behavior, melt behavior, and flow behavior of linear monoesters incorporating 9-decenol and 9-decenoic acid. Ind Eng Chem Res 2013, 52:2740-2749.
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[15] Moser BR, Sharma BK, Doll KM, Erhan SZ: Diesters from oleic acid: synthesis, low temperature properties, and oxidation stability. J AM Oil Chem Soc 2007, 7:675-680.
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[16] Bouzidi, L, Li S, Biase S, Rizvi S, Dawson P, Narine S: Lubricating and waxy esters II: Synthesis, crystallization, and melt behavior of branched monoesters. Ind Eng Chem Res 2012, 51:14892-14902.
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This study demonstrated the synthesis of branched derivatives of jojoba wax-like esters and elucidated their crystallization and melt behavior as a function of chain length, branching, symmetry, and functional groups.
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[17] Li S, Bouzidi L, Narine S: Lubricating and waxy esters, V: Synthesis, crystallization, and melt and flow behaviors of branched monoesters incorporation 9-decenol and 9decenoic acid. Ind End Chem Res 2014, 53:12339-12354. [18] Callinan T, Crimi J, Mcgee H, Parks A, Schwartz P: Synthetic wax substitute for carnauba wax and transfer ink compositions containing such substitutes. US Patent 1964, 3,129,140.
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[19] Kelly M: Hard wax substitute. US Patent 1941, 2,356,408.
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[20] Swern D: Organic peroxy acids as oxidizing agnets. In: Organic peroxides, vol 2. Wiley-Interscience 1971, New York, pp 355-533.
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[21] Park S, Jin F, Lee J: Synthesis and thermal properties of epoxidized vegetable oil. Macromol Rapid Commun. 2004, 25:724-727.
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[23] Gorreta F, Bernasconi R, Galliani G, Salmona M, Tacconi MT, Bianchi R: Wax esters of n-3 polyunsaturated fatty acids: a new stable formulation as a potential food supplement. 1 – digestion and absorption in rats. Lebensm-Wiss U-Technol 2002, 35:458465. [24] Zulim Botega DC, Marangoni AG, Smith AK, Golf HD: Development of formulations and process to incorporate wax oleogels in ice cream. J Food Sci 2013, 78:C1845-1851.
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[22] Liu L, Wang N, Xu L, Yu X, Zhang R, Wang T: A novel method of determining wax cohesiveness by using a texture analyzer. Journal of Texture Studies 2015, DOI:10.1111/jtxs.12171.
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[25] Doan CD, Van de Walle D, Dewettinck K, Patel AR: Evaluating the oil-gelling properties of natural waxes in rice bran oil: rheological, thermal, and microstructural study. J Am Oil Chem Soc 2015, 92:801-811.
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[26] Blake AI, Co ED, Marangoni AG: Structure and physical properties of plant wax crystal networks and their relationship to oil binding capacity. J Am Oil Chem Soc 2014, 91:885-903.
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[27] Hwang HS, Kim S, Singh M, Winkler-Moser JK, Liu SX: Organogle formation of soybean oil with waxes. J AM Oil Chem Soc 2012, 89:639-647.
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[28] Abes M, Narine S: Crystallization and phase behavior of fatty acid esters of 1,3propanediol I: pure system. Chem Phys Lipids 2007, 149:14-27.
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[29] Gibbs A, Pomonis G: Physical properties of insect cuticular hydrocarbons: the effect of chain length, methyl-branching and unsaturation. Comp Bichem Physiol 1995, 112:243-249. [30] Blake AI, Co ED, Marangoni AG: Structure and physical properties of plant wax crystal networks and their relationship to oil binding capacity. J Am Oil Chem Soc 2014, 91:885-903.
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[31] Rodrigues J, Cardoso F, Lachter E, Estevao L, Lima E, Nascimento R: Correlating chemical structure and physical properties of vegetable oil esters. J Am Oil Chem Soc 2006, 83:353-357.
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This article evaluated the influence of fatty acid ester structures on their rheology and crystallization behavior. Shorter fatty acid esters have weaker random interactions among molecules, and branched chains only significant affects crystallization temperature.
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[33] Kiado A: Applications of paraffin waxes and liquid paraffin. Development in Petroleum Science 1982, 14:240-329.
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[32] Li S, Bouzidi L, Narine S: Lubricating and waxy esters. 6. Synthesis and physical properties of (E)-didec-9-enyl octadec-9-enedioate and branched derivatives. Ind End Chem Res 2014, 53:20044-20055.
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[34] Tulloch A: Beeswax – composition and analysis. Bee World 1980, 61:47-62. [35] Basson I, Reynhardt EC: An investigation of the structures and molecular dynamics of natural waxes. I. Beeswax. J Phys D: Appl Phys 1988, 21:1421-1428.
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[36] Basson I, Reynhardt EC: An investigation of the structures and molecular dynamics of natural waxes. II. Carnauba wax. J Phys D: Appl Phys 1988, 21:1429-1433.
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[37] Endlein E, Peleikis K: Natural waxes – properties, compositions and applications. SOFW Journal 2011, 137.
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[38] Rezaei K, Wang T: Hydrogenated vegetable oils as candle wax. J Am Oil Chem Soc 2002, 79: 1241-1247.
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[39] Murphy T: Vegetable oil based wax compositions. US Patent 2004, 6,824,572.
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[40] Calzada JF, Upadhyaya J: Non-paraffin candle composition. US Patent 2000, 6,063,144.
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[41] Murphy T, Doucette MK, Richards M: Triacylglycerol-based alternative to paraffin wax. US Patent 2013, 8,529,924.
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[42] Depablo J, Biddy M: Nanoparticle modified lubricants and waxes with enhanced properties. US Patent 2009, 0,053,268 A1.
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[43] Lee Y: Partial acyl glyceride based biowaxes, biocancles prepared therefrom and their preparation. US Patent 2008, 0,145,808.
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[44] Phadoemchit T, Boonvichitr S: Paraffin wax replacer. US Patent 1989, 4,842,648. 422
[45] Clock A, Mckeown S, Wisnefsky E, Frenkel P: Bio-based wax compositions and applications. US Patent 2014, 8,876,918.
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Mainly linear hydrocarbons (C20-40)
ed
Commercial paraffin
pt
14% hydrocarbons 35 % monoesters 14% diesters 3% triesters 4% hydroxyl monoesters 8% hydroxyl polyesters 12 % free fatty acid 1% free fatty alcohol and others 2% hydrocarbon 84% aliphatic and aromatic esters 3% free fatty acids 3% free fatty alcohol 3% lactides 4% resins
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Beeswax
Carnauba wax
AS soywax
Physical properties
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Table 1. Compositions, physical properties and potential applications of current VO-based waxes.
Ac
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75% acetylated glycerol esters 14% wax esters 11% acylglycerols
High hardness: ~ 6,000 g Medium cohesiveness: ~ 8,000 g.mm Medium melting point: ~ 65 °C
Medium hardness: ~ 3,500 g Very high cohesiveness: ~ 40,000 g.mm Medium melting point: ~ 65 °C
Applications
References
Candles, coatings, packaging
Yao et al., 2013 [6] Kiadó, 1982 [33]
Candles, encaustic painting
Yao et al., 2013 [6] Tulloch et al., 1972, 1980 [34] Basson and Reynhardt, 1988 [35]
Very high hardness: ~ 60,000 g Low cohesiveness: ~ 2,000 g.mm High melting point: ~ 84 °C
Paper, surfboard coating, car waxes, foods
Basson and Reynhardt, 1988 [36] Endlein and Peleikis, 2011 [37]
Medium hardness: ~ 4,000 g High cohesiveness: ~ 18,000 g.mm Low melting point: ~ 50 °C
Candles
Yao et al., 2013 [6]
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cr us Medium hardness: ~ 3,500 g Very high cohesiveness: ~ 40,000 g.mm Low melting point: ~ 55 °C
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AM-MD wax
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35% monoacylglycerol 15% diacylglcerol 50% acetylated monoacylglycerol
Hydroxylated partially hydrogenated vegetable oil
FFA-TAG
30-40% free fatty acids 50-70% commercial KLX or other VO-based waxes
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OH-KLX
Low hardness: ~ 2,000 g High cohesiveness: ~ 10,000 g.mm Low melting point: ~ 47 °C Medium hardness: ~ 4,000 g Low cohesiveness: ~ 2,000 g.mm Low melting point: ~ 50 °C
Beeswax and paraffin substitute, encaustic painting medium
Yao et al., 2012 [5]
Cohesiveness enhancer for candles
Wang and Wang, 2007 [9]
Candles
Rezaei and Wang, 2002 [38] Murphy, 2004 [39] Calzada and Upadhyaya, 2000 [40] Murphy et al., 2013 [41]
Hardness: N/A Cohesiveness: N/A Low melting point: ~ 50 °C
Candles
Depablo and Biddy, 2007 [42]
RBD/PAG-wax
0%-95% palm Stearin 0%-5% glyceryl monostearate 0%-100% partial acyl glycerols
Hardness: N/A Cohesiveness: N/A Low melting point: ~ 55 °C
Paraffin substitute
Lee, 2008 [43] Phadoemchit et al, 1989 [44]
Epoxy-VO-wax
5%-95% epoxidized VO 5%-95% fully hydrogenated VO
Hardness: N/A Cohesiveness: N/A Medium melting point ~ 60 °C
Candles
Clock et al., 2014 [45]
Ac
Nano-soywax
90% Soywax 10% 10nm SiOx
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100% straight and branched chain monoesters
Amide-wax
Mainly esterified alkylolamide of hydroxystearic acid
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N/A
Moisture resistant packaging paper coating
Tambe et al., 2015 [11]
Hardness: N/A Cohesiveness: N/A Low melting point: ~ 35 °C
Jojoba wax substitutes
Boudidi et al. 2012; 2013 [13, 14] Li et al., 2014 [17, 32]
Very high hardness: ~ 15,000 g Cohesiveness: N/A High melting point: ~ 92 °C
Carnauba wax substitute
Kelly, 1944 [19]
Long chain esters formed by acid anhydride and alcohol
Hardness: N/A Cohesiveness: N/A High melting point: 80 – 82 °C
Carnauba wax substitute in ink
Callinan et al., 1964 [18]
Ac
AE-wax
an
Silylated then polymerized soybean oil
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Silylated-TAG
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Figure 1. Variation of (a) hardness and (b) peak and onset melting temperature with carbon chain length of pure fatty acid esters (adapted from Abes and Narine, 2007 [28])
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Figure 2. Effect of adding OHKLX or BuoKLX (OH or butyl pendent group on KLX) on (a) Hardness and (b) Cohesiveness (lower slope value means higher cohesiveness) of FHSO and KLX base materials (adapted from Wang and Wang, 2007 [9]).
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Figure 3. a. Crystallization; b-c. viscosity characteristics of esters with different numbers of pendent hydroxyl groups and branched chains. No OH groups in H6, and 1, 2, 3 OH 16
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groups in H3, H4, H5, respectively (adapted from Bouzidi et al., 2012; Li et al., 2014 [16, 32]).
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A Review of recent development of sustainable waxes derived from vegetable oils
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Tao Fei and Tong Wang*
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There is no conflict of interest!
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* Corresponding author: Tong Wang, Professor 2312 Food Sciences Building Ames, IA 50011 Tel: 515-294-5448 Fax: 515-294-8181 Email: [email protected]
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Department of Food Science and Human Nutrition Iowa State University Ames, IA 50011 Submitted to Current Opinion in Food Science March 24, 2017
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