Motivation for entering the field of bio-based materials is primarily related to the desire to improve ecology and sustainability, but the approach depends on the position of a person or a company in the society. Manufacturers of bio-based materials primarily want to add value to their existing products, but the issue is the direction they should choose and how to achieve the goal. Usually, they are directed towards making raw materials for potential users, but they may develop a technology for making the final product. Companies making petrochemical products may look for bio-based substitutes, to replace some components in their products with renewable materials. In that case, they would utilize components made by the first group. Scientists have fewer restrictions and may consider a range of options by selecting different raw materials with a range of chemistries, creating new chemicals, and targeting different applications that may not be restricted by economic reasons. An important aspect of their research would be to establish principles and relationships between the structure and properties to provide suggestions for designing materials and processes. In this chapter, an attempt is made to analyze the selection parameters to facilitate the decision of why and how to use natural oils for the preparation of polymeric materials.

Biological oils are generated by plants, animals, fish, algae, planktons, and bacteria. Biological oils are triglycerides (triacylglycerols), i.e., esters of glycerin and different fatty acids (FA), but some non-triglyceride products such as cashew nutshell liquid are treated as oils. Tall oil is an extract from wood after digestion of wood chips with bases or acids and it is a mixture of chemicals rich in fatty acids and resin acids, which can be separated by distillation. The main value of biological oils is the possibility of their transformation into an array of chemicals, many of which are useful monomers for polymeric materials. Biological oils are usually liquid at room temperature due to the presence of double bonds. The presence of esters and double bonds is common in all oils, but some fatty acids may have a range of different functional groups. Most edible oils are triglycerides of five fatty acids: oleic, linoleic, linolenic, stearic, and palmitic acids, which are the major components but many other FA are minor components. Over 1000 fatty acids are identified but not more than 50 are technologically important.¹  Since almost every plant creates some kind of oil, the number of oils that were analyzed is enormous.²  The largest volume of oils on the markets are palm and soybean followed by rapeseed, sunflower, palm kernel, cottonseed, olive, peanut, and coconut oil. Table 1.1 shows the composition of the ten largest volume edible vegetable oils and two important industrial oils (linseed and castor). Rapeseed oil contains unhealthy erucic acid (22:1) and it is used mainly for industrial applications. When erucic acid is removed from rapeseed oil, it is called canola oil which is a popular cooking oil. Table 1.1 shows that these oils have no fatty acids with more than three double bonds and only palm kernel and coconut oil have saturated fatty acids with less than 16 carbons. In spite of the refining processes and removal of phospholipids from oils, there are still about two to five percent of non-triglyceride impurities including wax esters, hydrocarbons, and phenolic derivatives.³  These impurities may affect subsequent chemical transformations of oils.

Animal and fish fats have somewhat different compositions. Butter is the emulsion of milk fats (∼80%) and protein in water. Lard and beef tallow have about 27% palmitic (C16:0) acid, as shown in Table 1.2.

Generally, animal fats are low in unsaturation. On the other side, fish oils may have very high unsaturation and are rich in fatty acids with five or six double bonds. They also contain a significant amount of saturates. There are high hopes for algal oil utilization for industrial applications but their presence in the market today is symbolic. The composition of algal oils depends on the type of algae and can be tailored by genetic modifications. The values in Tables 1.1 and 1.2 are indicative and not exact, due to variability of compositions within the same species and market sources when production figures are cited. Currently, vegetable oils are the most promising raw materials for new oleochemicals due to their well-defined composition and favorable price. The dilemma of food versus industrial applications is only pronounced in the case of biodiesel due to large volumes, whereas the volume for oleochemicals is much smaller and tolerable. The utilization of oils for non-food applications such as in soaps, coatings, and lubricants is as old as the industry and is not in question. There are many oils that are not used for food, and intensive investigations are carried out to find their best applications.^4,5  Some specialty oils offer new properties. Calendula or marigold seed oil has one of the highest iodine values (IV) of 242. It has 59% calendic acid with three conjugated double bonds. Tung oil (IV = 160–175) is also rich in conjugated triene acid (69%). Oils with conjugated trienic fatty acids dry faster than non-conjugated (linseed) ones. Lesquerella oil has fatty acids with hydroxyl groups. Vernonia seed oil is rich in vernolic fatty acid (72–78%), which has one epoxy group. Dominating fatty acids in standard oils have 18 and 16 carbons and cis-double bonds, but some oils are rich in shorter or longer fatty acids. Figure 1.1 shows the structure of some important fatty acids. Comprehensive reviews of vegetable oils and their compositions are given in many books and articles.^1,4,6–12 

Generally, natural raw materials, such as natural oils, are heterogeneous in structure and vary from molecule to molecule. This may create issues when competing with synthetic products, which are generated from high purity starting materials. The composition depends on the origin of the oil, but in the same type of oil, it may differ due to the geographical region, climatic conditions, genetic makeup of the plant, and other variable factors.

The major components in Figure 1.2 have four, five, and six double bonds but the average content is f_n = 4.56 and f_w = 5.0. The gel point when crosslinking oils or functionalized oils is related to f_w rather than f_n.

The molar mass of oils can be calculated from their composition determined by GC/MS, assuming that oils are pure triacylglycerols (without free FA). Although the composition of oils is very heterogeneous, molar masses of standard oils vary in a narrow range, the strongest effect coming from the number of carbons in fatty acids. The molar mass of triacylglycerols consisting of FA with 18 carbon atoms lies between tristearin (M = 891) and trilinolenin (M = 873). The difference is two percent, which is within the experimental error of some experimental methods. The number average molar masses of selected oils calculated from FA composition have these approximate values: SBO glyc max (872), SBO low sat (878), sunflower (877), corn (870), canola (887), palm kernel (689), palm (864), and castor oil (925) with the experimental error below one percent. Weight average molar masses are close to M_n values, meaning that standard oils are essentially monodisperse. The GC/MS method for molar mass determination is not applicable to oils with a high free acid content, oligomerized oils, and modified oils or their mixtures. An alternative method is vapor pressure osmometry (VPO), which requires no knowledge of the composition and is particularly useful for studying mixtures. The error when applying to oils is below ten percent but could be less than two percent, and it is more precise at lower molar masses.¹⁴  NMR was used for the calculation of molar mass, but it could be applied to TAGs of known purity.^15,16  Size exclusion chromatography (SEC) also gives approximate M_n and M_w values, although the values relative to polystyrene (typical standards) give fairly large errors. The combination of SEC with light scattering (LS) is not precise at low molar masses (below 1000). Mass spectrometry techniques are good for identifying species but are not suitable for quantitative determination of average molar masses.

Heterogeneity of oil structures arises from the fact that triglycerides are usually composed of several fatty acids which differ in length, number of double bonds, the position of double bonds in fatty acids, and geometry of DB (cis–trans).

Heterogeneous structures have a lower crystallization tendency and produce materials with lower glass transitions than ordered systems, which could be beneficial for extending the elastic behavior to lower temperatures.

Consideration for making new polymeric materials from natural oils should include the cost, structure of available oils, and desired properties. Generally, there are two classes of polymers: thermoplastics and thermosets. Thermoplastic polymers are linear and are processed by rapid techniques such as extrusion and injection molding, but also by casting from solution. They are made from pure di-functional components generated by breaking triglyceride molecules. One of the largest oil-based commercial products by volume in this category is polyamide 11. It is obtained from castor oil in several steps involving methanolysis to obtain methyl ricinoleate, which is then thermally cracked to methyl undecylenic acid and heptaldehyde. Undecylenic acid is reacted with HBr to obtain 11-bromoundecanoic acid, which after treatment with ammonia gives the monomer-11-ω-amino undecanoic acid. Polycondensation produces polyamide 11.⁵  Various diacids, hydroxy acids, or diols can be made by methods including ozonolysis,⁶  metathesis,⁷  thiol–ene additions to double bonds,⁸  epoxidation, hydroformylation, and transesterification. Most frequently, such monomers are used for polyesters or polyester polyols for urethanes, polyamides, and epoxy resins with potential biodegradability as listed in several monographs.^9–12 

Thermosets are cross-linked systems and are natural products from modified triacylglycerols. Their applications vary from cast resins to foams, adhesives, elastomers, and coatings. Although the structure of coatings is in many ways the same as that of cast resins, elastomers, and foams, they are applied from solutions in thin layers and deserve special consideration.¹³  The selection of functional groups for thermosets is wider, covering the whole of organic chemistry. Points of attack for chemical transformation are usually double bonds (DB), ester bonds, and allylic positions, as indicated in Figure 1.3.

Converting oils directly to products is a desirable goal. Drying oils such as linseed and tung oil can be used directly as coatings. Coating films are formed by oxidative polymerization through double bonds or allylic sites. Direct polymerization of oils is carried out on an industrial scale to give higher molar mass and higher viscosity oils for the coating industry or as rubber processing aids (factice). “Blown oils” are produced by bubbling air through soybean or linseed oil at temperatures of 100–110 °C for a relatively long time (30–50 hours), while “bodied oil” is partially polymerized (linseed) oil by heating at 330 °C. Polymerization of oils can be carried out at temperatures below 100 °C in the presence of superacids such as HBF₄.¹⁴  Although oxidative polymerization may lead to solid products, their usefulness was not confirmed. For controlled polymerization to useful products, oils must be modified by introducing reactive functional groups.

The largest volume polymers from vegetable oils are alkyds and polyurethanes which are produced from polyols and isocyanates. Although there are some commercial aliphatic isocyanates based on fatty acids, the main use of oils is in polyols. Thermoplastic polyurethane elastomers are made from fatty acid-based diols (macrodiols), usually diphenylmethane diisocyanate (MDI), and a short diol (chain extender). Macrodiols are usually polyesters of molar mass from 1000 to 2000. Ricinoleic acid is a natural hydroxy containing fatty acid that would self-polymerize just by heating, but a more elegant way is to run transesterification of methyl ricinoleate. Control of molecular weight can be achieved by adding a short diol, preferably with more reactive primary hydroxyls.¹⁵  Since the OH group in ricinoleic acid is located on the 12th carbon, the polymerized polyol will have a linear structure with a hanging six carbon chain as a side group (dangling chain). When a hydroxy fatty acid is prepared by epoxidation and ring opening of oleic acid, or by hydroformylation, dangling chains are longer: eight or nine carbons as shown in the polyester in Figure 1.4. Dangling chains in polyurethanes act as plasticizers in elastomers and, when compared with petrochemical analogues, they lower and slow down the elastic recovery after deformation. If a higher elastic response is desired, then the elimination of dangling chains is preferable. However, when a memory effect in foams is desired, then polyols with dangling chains are useful.

Ozonolysis and reductive hydrogenation of triolein produce a triol with terminal hydroxyl groups, while hydroxylated triolein by epoxidation and reduction with hydrogen gives a triol with dangling chains. The effect of dangling chains on polyols and MDI cross-linked polyurethanes from polyols with and without dangling chains is illustrated in Figure 1.5.

Polyurethane with MDI without dangling chains is glassy solid, stronger, with a high modulus and a higher glass transition by about 25 °C, but with low elongation at break. Polyurethanes plasticized with dangling chains behave as elastomers at room temperature and are more suitable for low temperature applications where high elongations are required.

The influence of double bonds on the properties of polyurethane networks is best illustrated with polyurethanes from castor oil and hydrogenated castor oil, as shown in Figure 1.6. Double bonds disturb the regularity of chains and lower the crystallization temperature and glass transition of polymers.

While the presence of double bonds in oils has a profound effect on the crystallization temperature, their influence on polymer properties is less pronounced. The glass transition in PU with double bonds is around 5 °C lower, and the tensile strength and modulus at 50% elongation are comparable. Elongation at break is higher in double bond containing samples, and the effect of double bonds would be higher at higher concentrations. Generally, double bonds in the chain increase the elasticity, lower the T_g, decrease the oxidative stability, and give a brownish tint to the material.

Table 1.3 displays the properties of the polyol from SBO with heterogeneity displayed in Figure 1.2 and is compared with those from high oleic oil, having a structure close to triolein. The functionality of two polyols was 3.5 for the SBO polyol and around three for triolein.

Despite the lower OH number, PU from the triolein polyol gave almost identical T_g and tensile strength to those of PU from the SBO polyol (heterogeneous), but a lower modulus and higher elongation. Whereas triolein-based polyurethane contained dangling chains of eight or nine carbon atoms, SBO also had around 15% of saturated fatty acids bearing no functional groups, acting as a stronger plasticizer. SBO polyurethane made from the polyol of higher OH number also had a higher content of MDI, resulting in a higher modulus.

The double bond in oleic acid is on the ninth or tenth carbon. Linoleic acid also has a double bond in that location, but also has an additional double bond between the 12th and 13th carbon. Linolenic has yet a third double bond between the 15th and 16th carbon. When functional groups are introduced at these positions, they will be inside the fatty acid chains. This has strong implications on the crosslinking density and properties. When epoxidized fatty acids are polymerized by cationic initiators, close epoxy groups (in linoleic and linolenic) may lead to cyclization and reduce the functionality of crosslinked networks or reduce the OH number in polyols made by ring opening with alcohols. Terminal OH or acid groups may be introduced by oxidizing methyl groups using metabolic engineering, although the process is tedious.¹⁷  In standard polyurethanes from high oleic oils, the polyol has an OH group either on the ninth or on the tenth carbon, as shown in Figure 1.7. The part from the first and ninth carbon will be a part of the network chain (see Figure 1.5) that is subjected to stress in mechanical testing, whereas the pendant (dangling) chain is not, but has a plasticizing function. Moving the OH group to the left will result in shorter elastic chains and longer dangling chains. A shorter elastic chain increases glass transition, strength, and modulus but longer dangling chains act in the opposite way. The net effect should be checked experimentally.

Saturated dangling chains may crystallize when they reach a certain length. When polyitaconic acid was esterified with alcohols of different lengths, crystallization occurred when the hydrocarbon side-chain reached ten carbons.¹⁹  Crystallization may occur even if the main chain is long enough. Saturated hydrocarbon chains from fatty acids with a terminal hydroxy group on the 18th carbon are expected to be crystalline. Polyesters and polyurethanes from nonanoic hydroxy fatty acids are highly crystalline materials and display excellent memory effects.²⁰ 

Terminal hydroxy groups are highly desirable but there are only a few natural fatty acids of this type. Some fatty acids with functional groups at different positions in the fatty acid chain with potential for application in polyurethanes are shown in Figure 1.8.

Gunstone lists hydroxy fatty acids with different positions of the OH group in the fatty acid chain: ricinoleic with OH on the 12th carbon, lesquerolic and auricolic on the 14th, coriolic on the 13th, and helenynolic on the 9th.²¹ 

Genetic modification of plants to produce desirable fatty acids with specific functional groups is possible.²²  Algae are especially conducive to this because of their rapid growth rate and accumulation of lipids. This could be the future in designing new molecules. Designing oils with preferable structures typically requires both the manipulation of endogenous genes as well as the introduction of heterologous constructs from new plant or algal species. The process can be complex and, in the case of soybean producing ricinoleic acid, the success was limited. The possibility of creating new functionalized oils offers exciting opportunities, however. Factors to consider are as follows: (a) terminal OH groups are desirable for utilization of the total FA length as an elastic chain for higher elasticity, higher elongation, and lower modulus for elastomers and flexible foams; (b) fatty acids with shorter chains are useful for rigid applications, while long chains are for elastic materials; (c) the position of a functional group in the FA chain; and (d) primary versus secondary OH groups determine reactivity and other factors. Long chains between functional groups may allow crystallization of hydrocarbon chains, decreasing low temperature elasticity.

When making thermosetting polyurethane cast resins, coatings, adhesives, and foams, oil-based polyols are reacted with diisocyanates or polyisocyanates. The ratio of polyols and isocyanates depends on the applications. For hard and rigid materials, a higher aromatic isocyanate content is required. Also, higher polyol functionality and lower molar mass are desired for higher rigidity. Typically, rigid foams have less than 50% oil-based polyols and flexible polymers have above 80%. However, bio-polyols in foams on a commercial scale are mixed with petrochemical polyols and their content generally does not exceed 30% except in applications such as carpet backing, where deterioration of properties is tolerated. Figure 1.9 shows epoxy resins and foams from epoxidized oils with 100% oil content.^23–26 

Transformation of oils and fats with low unsaturation to polyols can be carried out by transesterification with polyhydric alcohols. Theoretically, transesterification of triglycerides with glycerol at a molar ratio of 1 : 2 should give monoglycerides, i.e., diols. This does not occur since transesterification by chemical means is a reversible process and one obtains a mixture of glycerol, monoglycerides, diglycerides, and triglycerides (starting oil).^13,27  The mixture of non-functional oil, mono-functional diglycerides, di-functional monoglycerides, and tri-functional free glycerol would give polyols of functionality varying between two and three, but the OH value will be controlled by the excess of glycerol. Typically, at two moles of glycerol per mole of triglyceride, the yield is 40–60% with a base catalyst and could reach 70% with ionic liquids.²⁸  Using an excess of glycerol shifts the equilibrium to a higher monoglyceride content and free glycerol. Since glycerol is not compatible with oil, it creates a cloudy polyol above some critical excess. It is interesting that non-functionalized triglycerides may not be detrimental if kept within certain limits. A high monoglyceride content has been obtained by enzymatic transesterification.²⁹  Free glycerol has a higher density than oil-based polyols and may precipitate with time. Transesterification with higher alcohols such as sorbitol leads to higher functionality and higher viscosity polyols.

It can be shown that the OH number in polyols is directly proportional to the crosslinking density. Glass transition, T_g, and mechanical properties increase with the crosslinking density, Figure 1.11.

The majority of applications of oil-based polymers in coatings, foams, and adhesives require a crosslinked structure. Economics favors processing in the fewest number of steps. Preparation of polyols for polyurethanes directly from oils (Schemes 1.1–1.3) is possible by thiol–ene addition of mercapto alcohols,⁸  hydroformylation, and metathesis or with an extra step involving epoxidation, ring opening with alcohols, acids, hydrogen, or inorganic acids, and ozonolysis/hydrogenation.¹⁶  Epoxidized oils can be directly polymerized to polymers using cationic catalysts.^40–44 

Radical polymerization of oils is carried out by introducing acrylic vinyl groups.^45–49  These raw materials may be co-reacted with a range of vinyl monomers for coatings or for unsaturated polyester binders for composites.^50–52  A great advantage of radical polymerization is that no by-products are generated as in condensation reactions, and it can be carried out at lower temperatures. However, the chain polymerization reaction may be affected by a large number of allylic hydrogens which are good radical blockers. Olefinic terminal double bonds as in oils with undecylenic fatty acid or ones obtained by co-metathesis of oils with ethylene or by ring opening of epoxy groups with allyl alcohol react with radicals to a low degree of conversion, but maleic anhydride may be a useful co-monomer for linking two double bonds. It also allows additional crosslinking through esterification with hydroxyls when they are present (dual curing mechanism) and gives excellent thermosetting resins.⁵³ 

A new trend in using environmentally friendly technologies is the application of “click” chemistry processes such as a thermally activated azide–alkyne reaction.^54–56  Since the presence of catalysts and solvents is undesirable, useful resins were prepared just by heating azidated oil with alkynated oils at 100 °C (see Figure 1.12). The conversion of the thermally activated reaction reached 90% conversion after 24 hours.⁵⁷ 

Copolymerization of functionalized oils with different reactants not only affects the polymer properties but also may reduce the bio-content. Using aromatic co-reactants introduces rigidity in the final polymer and may improve strength, as in polyurethanes cured with different isocyanates. Aliphatic diisocyanates give lower strength and more flexible materials, whereas aromatic triisocyanates were found to impart the highest strengths. Cycloaliphatic diisocyanates give similar properties to aromatic diisocyanates but have better light stability. Components with hydrogen bonds, such as polyurethanes and polyamides, usually offer greater strength and toughness. Polymers entirely made from triglycerides, such as epoxy resins from epoxidized oils, lack hydrogen bonding and have somewhat reduced strength, which is primarily controlled by the crosslinking density. This is well-illustrated by comparing epoxy resins from linseed and soybean oils, shown in Figure 1.13 and Table 1.4.

Thus, copolymerization with different, primarily petrochemical co-reactants can be used for controlling the mechanical and other properties at the expense of bio-content.

Ester bonds hydrolyze under conditions of high humidity and in the presence of bases. This is a serious issue in the case of thin products such as coatings, exposed to environmental elements like heat and humidity. In the urethane industry, oil-based polyol systems may be delivered with base (amine) catalysts causing a change in properties (viscosity, acid and hydroxyl numbers) on storage with time. However, ester bonds are well-shielded by long, hydrophobic aliphatic chains and do not hydrolyze readily at room temperature in solid samples at moderate relative humidity. Rapid deterioration of cast polyurethane samples based on soybean oil polyols was found to occur in a water-saturated atmosphere above 80 °C.⁵⁸ 

Natural oils are relatively low polarity materials and have good electrical properties. Purified soybean oil is commercially used as an insulating liquid in transformers. The room temperature dielectric constant of oils (ε′) is ∼2.8. Epoxidized soybean oil has somewhat higher ε′ of ∼4.8.

Epoxy resins from epoxidized soybean oil displayed excellent insulation properties: a dielectric strength of 232 kV cm⁻¹, a volume resistivity of 1.1 × 10¹⁵ Ω cm, and a surface resistivity of 5.2 × 10¹⁵ Ω square⁻¹, comparable with those of standard polymeric insulating materials.⁴⁴  The dielectric properties of polyurethanes and other polymers are related to their composition and the presence of polar components. Generally, the permittivity (dielectric constant) of oil-based polyurethanes should be lower than those of standard polyurethanes with polyester and polyether polyols. It is not a constant and increases significantly above the glass transition temperature.