Alternative Methods for Producing Biomass-Based Rigid Polyurethane Foams and Ecopolyols

Polyurethanes (PU), with their superior physical and chemical properties compared to many other polymer types, have become one of the most widely consumed polymer types today. One of the most important classes commonly used is rigid polyurethane foams (RPF), because their low density and thermal conductivity, combined with strong mechanical properties, make them ideal heat and sound insulation materials. As with many polymer types, petro-based raw materials are used in PU production. Today, many researchers and industry organizations have focused on replacing oil-based sources with renewable sources to increase the sustainability of PUs, due to growing environmental concerns, the rapid depletion of oil, and increasingly stringent regulations related to these issues. At this point, biomass sources emerge as the most important alternative materials. Although the synthesis of polyols, one of the most important components of PUs, from various biomass sources using different chemical approaches is easy and fast, it remains a very important area that still needs development and the creation of new alternatives. Today, the vast majority of polymers are produced from oil-based raw materials. Since oil-based polymers have a chemically durable structure, they can persist in nature for long periods without degradation/decomposition, creating a major waste problem for the environment. The rapid depletion of petroleum resources and the future shortage also pose a separate problem. Because of these problems, researchers have begun searching for cheap and sustainable sources that could replace raw materials derived from depleting petroleum. This search has made natural biomass increasingly popular, and it is thought that they could replace petroleum-derived raw materials in the future [1]. Biomass is among the most promising renewable sources for the chemical industry. They stand out due to their biodegradable nature and easy decomposition without releasing polluting/toxic compounds, as well as their abundance and low cost [2]. Biomass, particularly that produced by agricultural, food, and forestry industries and with low economic value, is seen as an important source for producing polymeric materials. The use of biomass has additional benefits beyond reducing the use of non-renewable petro-chemical products and increasing biodegradability, such as reducing greenhouse gas emissions, lowering production costs, and improving the physico-mechanical properties of the resulting product [3]. Polyurethane foams, consisting of solid and gas phases, are engineering materials preferred in various industrial fields, particularly packaging, insulation, coatings, biomedical and automotive sectors, due to their superior properties such as low thermal conductivity, high mechanical strength and low density [4, 5]. The polymeric foam market, which exceeded USD 100 billion in 2015, is estimated to reach approximately USD 122.4 billion by 2021 [6]. Among polymeric foams, PU foams are the most widely used for industrial and domestic purposes. World consumption of PU foams is estimated to reach approximately USD 92 billion by 2024 [7]. Urethane was first synthesized by Wurtz in 1849, but was considered useless at that time. Later, in 1937, Dr. Otto Bayer synthesized PU from a reaction between a polyester diol and an isocyanate (Figure 1) [8]. The chemical structures of PUs are specific for each application, but fundamentally, all PUs are high molecular weight polymers formed as a result of a polymerization reaction called polyadition between polyols containing -OH groups and isocyanates containing -N=C=O groups [9]. PUs can be divided into many different groups, such as rigid, flexible, thermoplastic, coatings, and adhesives, depending on their applications [10]. [caption id="attachment_104240" align="aligncenter"] Figure 1. Polyurethane formation reaction[/caption]   Rigid polyurethane foams (RPF), which comprise a large portion of world polyurethane consumption, are formed when a polymeric diisocyanate compound (for example, polymeric methylene diphenyl diisocyanate, pMDI) reacts with a polyol (formulated polyol, FP) containing blowing gas, catalyst and surfactant, expanding to 30-40 times its original volume [11]. Thanks to their closed-cell structures trapping low thermal conductivity blowing gas molecules, RPFs are among the best heat insulation materials in the world. The types and amounts of raw materials in the foam formulation determine both the application field and properties of the foam. Although flexible and rigid polyurethane foams have basically similar chemistry, the differences in their properties are directly related to the properties of their constituent components, particularly polyols and isocyanates [12]. Polyols play a major role in determining the physico-mechanical properties of the final product. The rigidity or flexibility of the foam, its chemical resistance, and its gas and moisture permeability are largely determined by the polyol used. Three fundamental properties of polyols—hydroxyl numbers, functionality, and average molecular weight—are generally the most important parameters affecting the final properties of foams. For example, if a polyol with high hydroxyl number is used without changing the molecular weight, the rigidity, dimensional stability, and heat stability of the foam increase, while small decreases are observed in tensile and elongation strength properties [10]. Another component of foams as important as polyols is isocyanates. The most commonly used isocyanate types are methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI). Hexamethylene diisocyanate (HDI), which is resistant to yellowing under sunlight, is used in the coatings and adhesives industries, but is not preferred in the foam industry due to its lower reactivity compared to TDI and MDI. In foam formulation, oil-based TDI and polymeric MDI (pMDI) with average functionalities of 2 and 2.7 respectively are used. In RPF applications with closed-cell structures, pMDIs are generally preferred because they provide more controlled reaction kinetics than TDI. TDI, on the other hand, is more commonly preferred in flexible foam production with open-cell structures [13]. Although pMDI is less harmful to health than TDI, toxicological studies have determined that diisocyanates can cause asthma in susceptible individuals even at very low concentrations [14]. This situation shows that there may be at least some restrictions related to isocyanate use, therefore the development of alternative isocyanate synthesis methods is becoming increasingly important [15]. Blowing agents that enable the formation of cellular structures in foams can be divided into two main groups based on their blowing mechanisms: (i) Chemical and (ii) physical blowing agents. Water, used as a chemical blowing agent, reacts with the isocyanate compound to form an unstable carbamic acid compound. With the help of the heat released due to the reaction between polyol and isocyanate, this carbamic acid compound is converted to an amine compound while releasing carbon dioxide gas (Figure 2). The carbon dioxide gas cannot escape from the medium due to the increasing viscosity during the reaction and forms the foam structure. Physical blowing agents are compounds with low boiling points and low molecular weights, such as chlorofluorocarbons, hydrochlorofluorocarbons, some alkanes or cycloalkanes. The rapid evaporation of the physical blowing agent together with the heat released during the reaction is the driving force in foam formation. However, due to the high potential of chlorine and fluorine-containing compounds to thin the ozone layer, their use was banned by the Montreal Protocol in 1987. After this, the RPF industry shifted to blowing agents with zero ozone depletion potential, such as cyclopentane, isopentane, and water. Water's low cost, easy availability, and harmlessness make it advantageous as a blowing agent. However, RPFs inflated using physical blowing gases have lower thermal conductivity coefficients than those inflated with water, thus their heat insulation performance is higher. However, these materials can ignite much more easily in the event of a fire due to the low flash point of the blowing gas inside them. Surfactants, although used in small amounts, are other important components that play a role in both controlling and stabilizing the distribution and size of gas bubbles formed during the foaming reaction. A typical surfactant is a copolymer with silicon in its backbone. Surfactants have important functions such as enabling the mixing of immiscible liquids (emulsification), generating air bubbles during mixing, nucleating until the foam gels, and stabilizing the foam [16, 17]. Through the use of catalysts, the reaction occurs rapidly and the blowing gas remains inside the cells without escaping from the structure. The amine compounds responsible for the reaction between water and isocyanate (1,4-diazabicyclo[2.2.2]-octane) [18] and metal complexes responsible for gelation (polymerization) (dibutyl tin dilaurate) [19] are the most commonly preferred catalysts. It is recommended to use these two compounds together in certain proportions to obtain good foam structure. Trimerization reactions occurring between diisocyanates take place at high temperatures. The main catalyst families for these reactions are carboxylate derivatives and quaternary ammonium salts [20].

As with many polymer industries, the basic components of the RPF industry—polyol and isocyanate—are produced using oil-based raw materials primarily. Raw material acquisition for polymer production from renewable biomass has attracted great interest in both academia and industry. Growing interest in using these materials has led to increased use of renewable and long-term sustainable products such as green and bio-based polyols in RPF production, which has a large market. Polyols produced from bio sources:

(i) Have low carbon emissions, (ii) Are recyclable, (iii) Originate from non-polluting sources, (iv) Are quickly biodegradable/decomposable in nature, (v) Are inexpensive, (vi) Are abundantly available, stand out with these superior advantages. As a result of this interest, the green and bio-based polyol market is expected to reach USD 4.7 billion by 2021 [21]. On the other hand, world giant companies such as Dow Chemical, Bayer Material Science, BASF, Huntsman, DuPont, and Shell Chemicals Ltd. have been commercializing such polyols for a long time [22, 23]. Additionally, over the past 10 years, academic work has largely focused on ecopolyol production from biomass residues, vegetable oils, and industrial by-products [24-28]. Figure 3 shows the number of scientific publications over the years regarding ecopolyol acquisition from biological sources and RPF production with these ecopolyols. It can be clearly seen from the graph that the academic interest in this field has increased rapidly, especially in the last 5 years. [caption id="attachment_104242" align="aligncenter"] Figure 3. Change in the number of publications on ecopolyols from biological sources and PU foam production over the years[/caption]   Extensive studies have been ongoing for years to obtain ecopolyols from renewable sources such as edible oils and lignocellulosic biomass resulting from agricultural and forestry industries. Although ecopolyols can be efficiently produced from these sources, new, easy and fast methods need to be developed due to excessive energy and time consumption in the production stages. There are two basic methods used in the PU industry to obtain ecopolyols using lignocellulosic biomass: acid liquefaction [29] and oxypropylation [30] methods. The oxypropylation method is based fundamentally on heating the biomass in propylene oxide in the presence of a basic catalyst and under pressure [31]. Although oxypropylation is a suitable method for the synthesis of alternative ecopolyols in polyurethane foam production, the use of propylene oxide, which is easily flammable, seriously toxic, and carcinogenic, reduces interest in this method. Additionally, this method requires the use of steel reactors that increase investment costs as a precaution against explosions caused by very rapid and uncontrolled exothermic polymerizations [32]. For these reasons, using a different process to convert a biomass into a polyol would be safer. The acid liquefaction method is a much more attractive alternative because harmful reactants such as propylene oxide are not used in this method, nor are dangerous reaction conditions such as high pressure required. The solvents used in the acid liquefaction method are biologically compatible substances used even in the cosmetics industry. Additionally, the oxypropylation method does not increase hydroxyl numbers but increases functionality, and the resulting ecopolyols generally have higher viscosity values. In the acid liquefaction method, liquefied products with high hydroxyl numbers are obtained, and high hydroxyl numbers are very important for RPF production [33].

Another renewable source used in ecopolyol production to develop environmentally friendly PU foam materials is edible oils. Edible oils are basically composed of triglycerides and long-chain fatty acids, which are important raw materials for PU foam production due to the abundance and diversity of their sources (Figure 4) [34]. These oils have many advantages:

(i) They have versatile compositions and structures, (ii) They are easily biodegradable, (iii) They are environmentally friendly, (iv) Since they can be dissolved in many industrial solvents, they allow mixing with petro-based polyols, (v) Moreover, foams obtained from these oils show good properties in terms of flexibility, mechanical strength, wear resistance, toughness, adhesion, chemical and corrosion resistance [35]. [caption id="attachment_104243" align="aligncenter"] Figure 4. Triglyceride structure of oil[/caption]   With the exception of castor oil and lesquerella oils, edible oils do not contain hydroxyl groups that can react with isocyanates. For this reason, it is impossible to directly synthesize PU with edible oils [36]. Additionally, the primary use of edible oils is not to obtain ecopolyols but for cooking. Therefore, ecopolyol production from edible oils cannot compete with food production. When edible oils are desired to be used as a raw material for polyol production, they must be chemically modified and hydroxyl groups must be added to their structures. Carbon-carbon double bonds and ester bonds are two important functional groups found in the structures of vegetable oils. There are some synthetic methods starting from one of these two functional groups to obtain vegetable oil-based polyols: (i) Epoxidation followed by oxirane ring opening, (ii) hydroformylation followed by hydrogenation, (iii) ozonolysis, (iv) thiol-ene coupling, (v) transesterification, and (vi) amidation [37]. Epoxidation is one of the most common methods used for functionalizing carbon-carbon bonds [38, 39]. Using epoxidation reactions, many types of oils such as soybean, rapeseed, linseed, olive, corn, safflower, karanja, melon seed, and cottonseed oils have been used to produce ecopolyols on an industrial scale [35]. Epoxidation is generally carried out at temperatures between 30 and 80°C and reaction times between 5-20 hours, depending on the feedstock type and reactant ratios [40]. Under optimized conditions, conversion yields of more than 90% can be achieved. After the epoxidation reaction, polyols are obtained from epoxidized vegetable oils through oxirane ring-opening reactions using compounds containing active hydrogen such as alcohols, inorganic and organic acids, amines, and water [39-41].

The greatest problem encountered when obtaining RPFs from biological materials is the reduction in physico-mechanical properties compared to those obtained from oil-based polyols. To prevent reductions in the properties of the resulting RPFs, many different strategies have been applied in the literature. These can simply be listed as follows:

(i) Increasing the rigidity of RPFs by adding a cross-linker with high hydroxyl numbers, such as glycerin, to ecopolyols obtained at low hydroxyl numbers [42], (ii) Applying certain chemical modifications to increase the hydroxyl numbers of the obtained ecopolyols [43], (iii) Blending the obtained ecopolyols with commercial oil-based polyols [44, 45], (iv) Conducting formulation studies by varying the isocyanate ratio, catalyst, surfactant and blowing agent types and amounts for RPF that will provide the best physico-mechanical properties with the obtained ecopolyol [46, 47], (v) Creating RPF composites with various types of additives/fillers [48-50]. Although edible oils, lignocellulosic biomass and similar biological materials have become industrially significant in recent years and have found many different application areas, in recent years, obtaining polyurethane materials from biological sources has become extremely important both ecologically and economically. Both academia and industry organizations have focused on replacing oil-based polyols with polyols derived from biological sources. Although biological materials are abundant and inexpensive, obtaining polyurethane materials from them involves some difficulties. The greatest of these difficulties is that ecopolyols seeking commercialization do not have economically viable chemical processes. While green materials are very attractive, most consumers are not willing to pay extra for green-labeled products. Therefore, it is necessary to develop new technologies that will reduce their production costs.

References

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  Prof. Dr. Murat Erdem Eskişehir Technical University Faculty of Science, Department of Chemistry Research Assistant Emre Akdoğan Eskişehir Technical University Faculty of Science, Department of Chemistry