Chapter 11
Advances in the Use of BiOH® Polyols in Polyurethanes Downloaded by COLUMBIA UNIV on March 21, 2013 | http://pubs.acs.org Publication Date (Web): August 16, 2012 | doi: 10.1021/bk-2012-1105.ch011
Timothy W. Abraham* Cargill Inc. Minnesota, MN 55343, U.S.A. *E-mail:
[email protected] An exponential increase in the utilization of petroleum for transportation fuels, energy and industrial chemicals that has taken place over the past several decades is widely accepted as being unsustainable. Additionally, increased concerns about the environment and global warming have given rise to worldwide efforts devoted to utilizing biobased resources for fuels and industrial chemicals. Natural oils are seen as one such inexpensive, abundant renewable resource. The versatility of natural oils in industrial applications is demonstrated by the ability to introduce various functional groups, such as epoxy, hydroxyl and carboxylic acid groups, onto the fatty acid chains, which enables their use in numerous applications. The utilization of natural oils in polyurethanes is accomplished by introducing hydroxyl groups. Cargill has introduced a family of natural oil-based polyols for polyurethane foams under the BiOH® tradename. The basic technology involves epoxidation and reaction of the epoxides with nucleophiles yielding polyols that have the desired molecular weight and functionality range. BiOH® polyols have been used to partially replace petrochemical-based polyols in flexible molded foams and flexible slabstock foams, including viscoelastic foams, and to partially substitute copolymer polyols in high resilience foams.
© 2012 American Chemical Society In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
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Introduction The beginnings of the modern petroleum industry can be traced back to the 1850s when fractionation of petroleum by distillation was first demonstrated. Subsequently, a rapid expansion in the utilization of petroleum took place in the 1900s resulting from the introduction of the internal combustion engine, followed by a steady growth in the exploration and discovery of petroleum resources. A rapid increase in the utilization of petroleum for transportation fuels, energy and industrial chemicals has taken place over the past several years, mainly driven by increasing energy demands due to population growth and improved living standards across the globe. However, it has also become quite clear in recent years, even to the most ardent supporters of the petroleum industry, that petroleum as a portable, dense energy source powering the vast majority of vehicles, and as the base for many industrial chemicals, is not sustainable. A consequence of this acute dependence on a finite resource is an increase in carbon dioxide emissions, which is known to be a major contributor to global warming and climate change. Increasing concerns about the environment and sustainability has fueled growing worldwide efforts devoted to utilizing renewable resources for energy and industrial chemicals, with the aim of reducing the world’s dependence on fossil fuels. Bio-based resources are renewable and CO2 neutral, in contrast to fossil fuels. In some instances, bio-based products may even possess unique properties, such as biodegradability and biocompatibility, thus providing additional benefits. There has been a positive change in the attitude of the general public over the past several years towards environmentally friendly products. However, the demand from many customers and consumers is for products that are equivalent in price and performance to the petroleum-based products that are being replaced. Although a few bio-based products are able to garner a higher price than their petrochemical-based counterparts, processes for the conversion of renewable raw materials into industrial chemicals need to be cost competitive for a vast majority of biobased products, if they are to compete against the incumbent petrochemical-based products. This competitiveness is inevitable, with the continuous increase in the price of petroleum as the world approaches peak oil production, as well as advances being made in new technologies for biobased products. Ironically, it’s the same natural materials that had originally been displaced as raw materials, when fossil fuels became cheaper to extract and convert to useful products at the turn of the last century, that are once again attracting attention as renewable, environmentally friendly materials for fuels and industrial chemicals. The development of technologies that utilize agricultural, animal, forestry and municipal waste as renewable feedstocks presents a significant opportunity for the manufacture of biobased fuels and industrial chemicals. Significant progress is being made in developing such value-added chemicals (1–3). Cargill Inc. has been at the forefront of investing in new technologies to manufacture industrial chemicals from renewable resources. It has been supplying starch-based products to the papermaking industry for over 75 years, for use in coatings and sizing, and has also supplied starch-based products to other industries, 166 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
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such as corrugated boards for packaging, construction materials, and feedstocks for various biochemicals. Recent additions to this portfolio of starches are xanthan gum, scleroglucan, carageenan, alginates and pectin. In 2002, Natureworks LLC, a subsidiary of Cargill, opened the world’s first global-scale manufacturing facility for polylactic acid, a biobased plastic resin from carbohydrates, demonstrating its vision and commitment to such efforts. Cargill has also, for many decades, used natural oils and their derivatives in a variety of applications, such as paints & coatings, inks, dielectric fluids, lubricants, adhesives, biodiesel, water treatment, dust control and oilfields. Its access to various oilseeds across the global, from soy in the US and South America, to canola and sunflower in Europe, palm in Asia, and cottonseed in Australia, coupled with a strong global supply chain, places Cargill in a unique position to supply these various industries. In 2005, Cargill introduced natural oil-based BiOH® polyols for the replacement of petroleum-based polyols in polyurethane foams.
Natural Oils in Industrial Applications Worldwide economic and scientific interest in natural oils (lipids) as inexpensive, abundant renewable resource has grown, and has already been the subject of significant research efforts globally. It represents a major potential alternative source of chemicals suitable for developing environmentally safe and consumer friendly products. Natural oils have been used since the beginning of civilization in non-food applications, and its use in coatings for wood and metal, decorative arts, printing inks, lacquers, and as lubricants goes back many thousands of years. These applications take advantage of the natural oil’s lubricating and solvent like nature, and its ability to cross-link in the presence of air, resulting in polymeric films that are tough, elastic, waterproof, and adhere tightly to the substrate (4). Natural oils are primarily fatty acid triesters of glycerol (also known as triacylglycerols or triglycerides). The fatty acids usually have an even number of carbons and can be 6-carbons to 22-carbons in length, but the most common fatty acids are 12-carbons to 18-carbons long. Many of the fatty acids possess one or more double bonds, and a few of them have other functional groups, such as hydroxyl groups or epoxide groups. The general structure of soybean oil (Figure 1) shows the glycerol backbone onto which are attached fatty acid chains, primarily oleic, linoleic, linolenic, stearic and palmitic acids. Modern industrial methods of extracting and refining yield vegetable oils with well defined specifications and in high purity. In some cases, natural fats and oils can be used directly as building blocks for polymers, but often they are modified or functionalized to intermediates which are more suitable for polymer formation. Although currently not economical for industrial applications, genetic engineering offers a way to further optimize the properties of natural oil-based polymers by controlling the fatty acid distribution, and changing the composition of plant oils to control the structures and produce polymers with improved properties. Natural oils can be used directly for energy, or easily processed into biofuels, such as their conversion to biodiesel or jet fuel. However, the value of lipids as an abundant 167 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
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source for industrially useful chemicals is demonstrated by the versatile methods that have been developed to introduce functional groups, such as hydroxyl, epoxy, carbonyl and carboxylic acid groups, on the fatty acid chains (5).
Figure 1. General structure of soybean oil.
Synthetic methods involving radical, electrophilic, nucleophilic, and pericyclic reactions have been applied extensively to the carbon-carbon double bonds of natural oils and fatty acids for selective functionalization (6–12). Natural oils that already possess hydroxyl or epoxide groups can be used directly in some applications (13, 14). Polymeric materials from natural oils have also been investigated extensively (15, 16), with the above-mentioned reactions being utilized to synthesize useful monomers for polymers which are used as coatings, and toughening agents for epoxy resins (17). Certain vegetable oils and their derivatives, such as polyol products, are utilized as alternative feedstocks to produce additives or components for composites or polymers (18). More recently, natural oils have been used in creating engineering composites (19, 20) and pressure-sensitive adhesives (21).
168 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
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Natural Oils in Polyurethanes Polyurethanes can be broadly classified into Flexible foams (~50%), Rigid foams (~30%) and CASE (Coatings, Adhesives, Sealants and Elastomers, ~20%) applications. Flexible polyurethane foams are used in such items as home and office furniture, mattresses, carpet backing and automotive seats. Rigid polyurethane foams are used in insulation materials (eg. coolers, vending machines, refrigerators, and panels for walls and roofs), automotive parts, and various structural materials. CASE applications span a variety of industries from coatings for cars, floors, and basketball courts, to various adhesives, sealants (eg. caulking materials), and elastomeric materials (eg. tracks, shoe soles, and artificial leather). The polyurethane industry is about a 20 billion dollar global industry which used about 11 billion pounds of polyols in 2006 (22). About a third of polyurethanes are sold in North America, another third in Europe, and the rest in Asia Pacific and South America. The seminal work of Frankel and others, beginning in the 1960s, at the USDA’s Northern Regional Research Laboratory, demonstrated the potential for utilizing hydroformylation reactions to convert natural oils into polyols for use in polyurethanes (23–32). There has been a renewed interest (33–35) in recent years to utilize such hydroxylated natural oils in polyurethanes due to the volatility of the petroleum-based polyols market. Various natural oils, such as soybean oil (36–39), sunflower oil (38, 40), rapeseed oil (40, 41), and palm kernel oil (42), have been tested in the synthesis of polyols. Polyurethanes produced from natural oil polyols present some excellent properties, such as enhanced hydrolytic and thermal stability (43, 44). In recent years, the availability of inexpensive blown soybean oil, which has hydroxyl groups that are introduced by blowing air or oxygen through the heated oil (45, 46), provided an impetus for the development of various natural oil-based polyols. Subsequent generations of blown oil-based polyols include products with higher functionality, obtained by transesterifying blown oils with polyols like glycerol and sorbitol (45, 46). The initial success demonstrated with blown oils led to the accelerated development of other proven technologies for the introduction of hydroxyl groups onto fatty acids and natural oils. Although the use of polyols obtained by the hydroformylation of oils in making rigid polyurethanes had been demonstrated by Frankel and others (23–32), the versatility of these polyols in a number of different polyurethane applications was demonstrated by Rogier (47), who synthesized liquid polyols for elastomers and microcellular foams. The hydoroformylation reaction provides a valuable method to introduce primary hydroxyl groups onto fatty acid chains, while maintaining the molecular weight of the oil. More recently, Guo and others (48–51) studied the physical and mechanical properties of polyurethane foams made with polyols derived from the hydroformylation of natural oils. In order to build high-MW polyols from hydroformylated fatty acids, Peerman and Rogier (52) had developed a novel method of polymerizing hydroxymethyl-containing alkyl esters of fatty acids onto polyol, polyamine, and aminoalcohol initiators, which enabled the synthesis of previously unseen high molecular weight, non-gelling polyols. These polyols were used in the preparation of elastomeric polyurethanes that exhibited low water absorption, good retention 169 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
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of strength after exposure to hot water, and good flexibility. Building on this work, the Dow Chemical Company has synthesized polyols from natural oils in a similar manner, and have tested them in various polyurethane applications (53–58). In order to understand structure-property relationships, Petrovic et. al. (59) synthesized a series of triols from methyl oleate, which have a more uniform structure than polyols synthesized from a heterogeneous mixture of fatty acids, and studied the effect of the molecular weight of these polyols on the properties of the resulting polyurethanes. Epoxidation of natural oils, followed by reaction of the epoxides with nucleophiles is another method to introduce hydroxyl groups (60). Industrially, the epoxidation of natural oils is performed utilizing a peracid, such as performic acid or peracetic acid (61–64). Various nucleophiles have been used in the reaction with epoxidized oils, such as water, alcohols, carboxylic acids, amines, amino alcohols and thiols (39, 65–69). The reaction with carboxylic acids results in polyols with vicinal hydroxyl and ester groups. These reactions usually result in the introduction of secondary hydroxyl groups. Ozonolysis (70) is another method of introducing primary hydroxyl groups onto fatty acids. However, unlike in the case of hydroformylation, ozonolysis results in oxidative cleavage of the carbon-carbon double bonds. Azelaic acid (nonanedioic acid) and Pelargonic acid (nonanoic acid) are produced as bulk industrial chemicals by ozonlysis. The reaction of double bonds in natural oils with ozone results in the formation of ozonides, which are converted to polyols using different reducing agents (71). Hydroformylation introduces primary hydroxyl groups without decreasing the molecular weight of the oil, while ozonolysis of fatty acid chains results in low molecular weight compounds, which limits their use to mostly rigid polyurethanes. A mixture of low molecular weight polyester polyols and highly functionalized glyceride alcohols can be obtained when vegetable oils are directly subjected to ozonolysis in the presence of polyols like glycerol (72–74).
BiOH® Polyols BiOH® polyols are synthesized utilizing the epoxidation route, where the double bonds in natural oils are first converted to epoxides, and the epoxides are then reacted with nucleophiles, such as monoalcohols, in the presence of an acid catalyst, to introduce hydroxyl groups ((75, 76) Figure 2). A variety of polyols have been synthesized using this basic technology, by choosing different types of natural oils, partially or fully epoxidizing the oils, and varying the type and functionality of the nucleophiles. The reaction of epoxidized oils with nucleophiles can also be performed in a manner as to cause intermolecular reactions resulting in the formation of polyether bonds (77), yielding natural oil-based oligomeric polyols that are suitable for use in manufacturing flexible foams (78, 79). Polymerization of epoxidized oils, via ether bonds, can also be accomplished by reacting the epoxidized oils with multifunctional nucleophiles, such as ethylene glycol, trimethylolpropane, or an amino alcohol. A portfolio of BiOH® polyols have been developed utilizing the epoxidation technology, and is shown in Table I. 170 In Biobased Monomers, Polymers, and Materials; Smith, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
Table I. Typical properties of BiOH® polyols BiOH® polyol
5000
5300
5400
2300
1000
2100
56
119
129
160
200
225
Viscosity (cp @ 25C)
4,000
5,500
4,400
4,900
3,900
8,900
Molecular weight (Mn)
1721
1,447
1,300
1,335
1,164
1,215
Functionality (Fn)
1.7
3.1
3.0
3.8
4.2
4.9
Acid value (mg KOH/g)
0.5
0.72
0.6
0.84
0.55
1.7
Water (ppm)
500
1,600
500
1,300
700
3,000
Color (Gardner)
