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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-62016-7
Cover image: Watercolor "Water Lilies in September" by Deny Kyriacos.
Copyright reserved by the artist
Cover design by Russell Richardson
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
The use of naturally occurring molecules in the production of industrial polymers and polymeric intermediates attracts more and more the attention of manufacturing companies. From a scientific point of view, the laboratory preparation of commodity polymers based on natural products was already examined many decades ago. However, the recent advent of issues related to terms such as biobased, biodegradable, sustainability and cyclic economy, all of which concern the protection of the environment from the deleterious effects of some petrochemicals as well as from the irreversible accumulation of thermoplastics and thermosets in nature, has prompted governments and industries alike to examine the marketing of polymers that consist at least partly of naturally sourced components in their macro-molecular structure.
This book is addressed to readers interested in learning the basics of the chemistry of biobased polyols in the manufacture of commercial polymers. The latter include, among others, polyurethanes, epoxides and polyesters, both saturated and unsaturated.
The introductory chapter of this book gives an account of the various biobased polyols and their initiators, as well as the prices of vegetable oils compared to crude oil. The ubiquitous word, sustainability, is also subject to the author’s comments. The second chapter briefly describes most applications in which the polyols may be commercially valuable. This is followed by a thorough investigation of the chemical structures as well as the extraction processes of fatty acids, which are the major constituents of naturally occurring fats and oils. The fourth chapter is dedicated to an understanding of the basic chemistry of the groups present in triglyceride molecules. Several examples of routes to the synthesis of biobased polyols from fatty acids, as well as from vegetable oils, are given in the fifth chapter. Carbohydrate initiated polyols are not new in the industrial world. They cannot be considered as fully biobased unless they are ethoxylated or propoxylated with epoxides originating from natural products. The synthesis of those epoxides from natural sources is described in the sixth chapter and several practical examples are included. The last chapter addresses the technology of products made from biobased polyols.
Accompanying the text of each chapter of this book are many graphs and photographs.
The author wishes to thank all the scientists, engineers, technicians and marketers whose work is mentioned in this book, often in great detail. Thanks are also extended to the originators of the photographs included herein. Finally, the initiatives of all manufacturing companies, the management of which operate their businesses with a commitment to solving environmental problems, are also acknowledged.
This chapter describes polyols in detail, including diols, the chemical components of which are obtained from sources other than crude oil.
The polyols are used in the manufacture of commercial polymers and polyurethanes, for example.
Among the major natural chemicals from which polyols can be derived are:
For industrial purposes vegetable oils and carbohydrates are the most approachable sources of chemicals. Work on polyols derived from animal fats can be found in the patent literature [1].
Those products are, quite rightly, described as green products because they originate from natural sources, the production of which humans can control almost at will. They are renewable because, in contrast to crude oil, they originate from non-depletable sources. Their availability is not the monopoly of some countries which possess vast amounts of oil reserves. Agricultural products require the right weather conditions to grow as well as an area large enough for them to be cultivated on.
The first detailed studies on the use of vegetable oils and animal fats in polyurethane technology date back to the late fifties and early sixties.
Among the reasons given for utilizing polyols from natural sources were:
In general, the price of oil (Figure 1.1) used to produce the components of polyether and polyester polyols is determined by speculation largely founded on the production policies of the OPEC cartel.
Unfortunately, the pricing of basic carbohydrates or bean oils generally is not much different from that of crude oil.
Soybean oil futures are traded at the Chicago Futures Market, where the price of soybean oil is still lower than that of petroleum (Figure 1.2).
This means that, triglycerides, even if considered renewable sources of chemicals, are subject to speculative pricing the same way crude oil is. However, the difference is that their production is not restricted to only a few countries. Bean oil, or carbohydrate cartels, will be difficult to establish and organize on a global scale. This trading approach does not exclude speculative price hikes similar to those of crude oil.
Figure 1.1 Crude oil (petroleum) price chart (1barrel of crude ~ 140 kg) [7].
Figure 1.2 Soybean oil price variation compared to that of crude oil in USD/ton [7].
Polyols based on renewable raw materials, such as fatty acid triglycerides, sugar, sorbitol, glycerol and dimer fatty alcohols, are already used in diverse ways as raw materials in the preparation of polymer chemicals.
It is claimed that soybean-oil-based polyols cost less than the petroleum polyols they replace, because they require considerably less energy to produce; can be used in a broad range of polyurethane applications; and produce polyurethane products with equivalent or better physical characteristics [8].
In any event, polyols manufactured from petrochemical sources constitute the majority of the polyols, polyesters as well as polyethers used in industry.
Another source of polyols has emerged from the co-polymerization of CO2 and epoxides [9].
During the last few years, the term sustainability has been mentioned repeatedly in published articles, speeches, presentations as well as in company reports, to say the least. This was not the case when fluorocarbons, for example, were widely used as blowing agents in the polyurethanes industry. Many decades have elapsed since their deleterious effect on the ozone layer was discovered.
The negative effect of CO2 on the atmosphere and the migration of bisphenol A from polycarbonate utilized in feeding bottles are additional examples which indicate that the consequences of chemicals are spotted only after a type of specific damage has already been inflicated on the environment, the economy, human health, etc.
According to the Cambridge Dictionary, the verb “to sustain” has the following meanings:
A succinct but detailed definition of the name derived from the verb “to sustain,” i.e., sustainability is given in Wikipedia. Accordingly, “Sustainability is the process of maintaining change in a balanced fashion, in which the exploitation of resources, the direction of investments, the orientation of technological development and institutional change are all in harmony and enhance both current and future potential to meet human needs and aspirations.”
Not long ago, the course of the polyurethanes industry was sluggish. Sustainablility studies carried out by some multinationals pointed to the closure of old isocyanate plants. A few months later, the state of the economy changed. The polyurethanes market picked up and the same plants, instead of being shut, were upgraded. Ironically, a short time later, the same companies showed poor earnings because the market did not follow the predicted growth trend. But this is not the exception.
The biobased chemicals market has recently seen the collapse of bio-succinic acid producer BioAmber, despite the numerous reorganizations aimed at reviving the sales of the company. A year before the company was shut, BioAmber was planning a seven-fold increase of its production capacity. According to their management, the business plan the company put forward to its creditors was sustainable.
Succinic acid is a dicarboxylic acid widely used in the manufacture of polyester polyols. The manufacturing process from natural sources proves to be expensive, even if the science involved is brilliant. Therefore, the profit margins generated to sustain its production must be high. BioAmber’s capacity was 30 Ktpa, but the returns did not justify the operation of the company.
Whereas the applications of succinic acid in the polymers industry are well known, the production of polyols from natural sources has only recently gained momentum. Their successful inclusion in current technologies will show whether their use is sustainable. There is no need to use a polyol produced from palm or rapeseed oil in a polyurethane formulation if its contribution to the properties of the end product does not offer any economic or qualitative appeal to the consumer.
There is no doubt that the final outlet of all industrial and agricultural products are aimed at the direct or indirect consumption by humans. The higher the production rates, the more energy will be required by the production processes. A simple mathematical model will certainly prove that sustainabilty as well as cyclic economy will be convincigly achieved and implemented, at least, when the growth of the world population will be controllable. But this is a very difficult target to attain.
Vegetable oils have been known to mankind since prehistoric times. Humans have used fats and oils for food, healing and other ends. Over the years, the extraction of oils from agricultural products has been elaborated.
Nowadays, for some polymerization purposes, many vegetable oil molecules must be chemically transformed in order to include hydroxyl groups in their structure.
For instance, soybean oil does not contain any hydroxyl groups but has an average of 4.6 double bonds per triglyceride molecule. The unsaturation of the vegetable oil molecule can accommodate hydroxyl groups. However, many reactions for preparing polyols from vegetable oils are not very selective.
By-products are created during the transformation. Furthermore, many conventional methods of preparing polyols from vegetable oils do not produce polyols having a significant content of hydroxyl groups, and the available methods do not produce products having a desirable viscosity. Greases or waxes often result as a consequence of such chemical transformations.
Chemically, vegetable oils are defined as triglycerides (also called glyceryl trialkanoates) because they are esters of glycerol and fatty acids (Figure 1.3).
The structures in Figure 1.4 put the glyceride definition in a broader context.
Figure 1.3 Products resulting from the hydrolysis of triglycerides.
Figure 1.4 The definition of glycerides reflects the number of esterified hydroxyl groups of glycerol.
In practice, the carboxylic acid moieties are not all the same, but mixtures of several ones, as shown in Figures 1.3 and 1.4. They are also present in different triglyceride molecules in variable ratios. The acids are called fatty because their structure is similar to the acidic constituents of triglycerides found in fats. Fats are solid triglycerides whereas oils are liquids. The carboxylic acids are monobasic with a long hydrocarbon tail chain. Fatty acids, as shown in Figure 1.5, can be fully saturated but they can also contain unsaturated sites as well as hydroxyl groups (ricinoleic acid for example).
Further down in the text, it will be shown how unsaturated triglycerides are hydroxylated.
The hydroxylated compounds can be made useful, for example, in the formation of urethanes, by reacting the hydroxyl groups with isocyanates. Coatings, adhesives, elastomers, foams and composites can be made from elastomers using such hydroxy functional compounds.
For example, in a first step an excess of a diisocyanate, such as MDI or TDI, the structures of which are shown in Figure 1.6, is reacted with a hydroxyl-containing triglyceride, such as castor oil, so as to form a pre-polymer containing an excess of isocyanate groups [10, 11].
Figure 1.5 Structures of some saturated and unsaturated fatty acids.
Figure 1.6 Chemical structures of some commercial aromatic diisocyanates.
Those free NCO groups originate either from free isocyanates or from the reaction products of castor oil with TDI (or MDI), as shown in the reaction scheme in Figure 1.7.
By reacting the isocyanate mixture with water in the presence of an amine catalyst a foamed product is obtained, because of the evolution of CO2. The reaction is shown in Figure 1.8.
Figure 1.7 Reaction products of castor oil with an excess of 2,4-toluene diisocyanate (TDI).
Figure 1.8 Reaction of isocyanate with water.
Polyols obtained from triglycerides are very often propoxylated and/or ethoxylated with propylene oxide or ethylene oxide respectively in order to increase their molecular weight and subsequently their chain flexibility. The structure of each alkylene oxide is shown in Figure 1.9.
An example of such a polyol synthesis is described below [12]:
First, 267.2 g castor oil and 5.73 g KOH are flushed with nitrogen in an autoclave at 110°C with stirring. Then, 747.3 g of propylene oxide are added. After a reaction time of 4 h, 186.8 g ethylene oxide is metered under pressure. After 1 h, the contents of the reactor are cooled to 40°C and neutralized by the addition of 132 g distilled water and 32.4 g, 11.85% sulfuric acid. After addition of 0.65 g Irganox 1076 (antioxidant), dehydration is carried out in vacuo and the mixture is heated thoroughly for 3 h at 110°C and then filtered.
The OH number of the product is 51.7 mg KOH/g, and the viscosity at 25°C is 500 mPas.
The reaction sequences of the above-described experiment are shown in Figure 1.10. It should be noted that propoxylation leads to alcohols with secondary hydroxyl end groups for steric reasons. Ethoxylation, in turn, introduces primary OH groups, which are more reactive towards carboxylic acids and isocyanates.
Therefore, in order to synthesize a completely biobased polyol, chemicals such as propylene oxide and ethylene oxide must also emerge from natural sources. This text will examine if such a process is feasible.
Nevertheless, the final polyurethane cannot be defined as fully biobased as long as the isocyanate component is aromatic. The source of aromatics being, until now, petrochemical.
The same argument is valid for other technologies where polyols originating from natural sources are constituents of thermosets, such as unsaturated polyesters, for example.
Figure 1.9 Chemical structures of ethylene and propylene oxide.
Figure 1.10 Propoxylation followed by ethoxylation of castor oil.