Polyurethane Synthesis from Carbon Dioxide

1. Introduction

Today, the effectiveness of crude oil as the most important chemical feedstock is increasingly growing. In addition to its use as fuel, crude oil provides carbon-containing molecules for chemical synthesis and monomers for plastic materials. Current consumption rates are rapidly depleting global crude oil reserves, increasing the importance of alternative renewable carbon-based sources for feedstocks. One possible source of CO₂ arises in many combustion processes and contributes to an event called the greenhouse effect (along with methane). Current global carbon dioxide (CO₂) emissions are estimated to exceed 31 billion tonnes annually. Carbon capture and storage (CCS), a method to reduce the amount of CO₂ in the atmosphere, is expected to significantly lower CO₂ concentration. Intensive research activities in this field promise encouraging results in the near future, however, storing large amounts of CO₂ and keeping it out of the atmosphere is not particularly attractive. Global warming is assumed to result from CO₂ produced as a consequence of excessive fossil fuel consumption being released to the atmosphere rather than being processed. Independent of such environmental concerns, excess CO₂ chemical utilization is an important issue. Non-toxic and non-flammable CO₂ is considered as a carbon source (C1) for chemical reactions; however, the thermodynamic stability of CO₂ generally makes its use in chemical reactions challenging.

Methods to overcome high energy barriers through reduction include: (i) low-valence metal compounds through oxidative coupling, (ii) increasing the electrophilicity of unsaturated compounds and carbonyl carbon, and (iii) reactions of epoxides with high reactivity with CO₂, such as these approaches.[1]

When examining the effect of greenhouse gases on global warming, CO₂ gas accounts for a significant share. For this reason, an effective method is urgently needed to reduce the amount of CO₂ in the atmosphere. From a chemical standpoint, CO₂ (approximately 28% by weight) is considered a stable and abundant carbon source. The conversion of carbon dioxide into valuable chemicals is a promising method not only for reducing its amount, but also for obtaining useful chemical substances from renewable sources (Figure 1). In the literature, there are many studies on the conversion of CO₂ into various functional chemicals such as cyclic diols, aminoalcohols, diamines, cyclic carbonates, cyclic carbamates and cyclic ureas.  

Many research groups continue to study the use of various heterogeneous catalysts and the mechanisms of these reactions.[2, 3]

Global warming is a problem created by increasing emissions of greenhouse gases such as CO₂, methane, nitrogen oxide, chlorofluorocarbons and the like with each passing year. Large amounts of CO₂ are released into the atmosphere as a result of burning important energy sources such as oil, coal and natural gas. This situation is increasing greenhouse gas formation with each passing day. Reducing the amount of CO₂ in the atmosphere to extend the life of the world and prevent climate irregularities and droughts resulting from global warming is an urgent problem that scientists need to solve. Four current approaches are proposed to overcome this problem (Figure 2); i) Increasing efficiency and reducing energy consumption, ii) Renewable energy sources instead of fossil fuels, iii) CO₂ capture/storage (CCS), iv) CO₂ capture/utilization (CCU). Currently, the CCS method appears to be the most effective way to reduce the amount of CO₂ in the air. On the other hand, the CCU approach is promising not only in terms of CO₂ consumption but also in the use of abundant, inexpensive, non-flammable and non-toxic natural resources for the production of high value-added chemicals.[6]

2. Synthesis of Valuable Chemicals from CO₂

Carbon dioxide has a very stable structure. Its content in high oxidation state and use as a C1 feedstock requires a high amount of energy input. There are two known different strategies for converting CO₂ into useful chemicals. One is reductive CO₂ conversion, the other is non-reductive CO₂ conversion. Reductive conversion of carbon dioxide to target compounds requires very high energy and strong reducing agents (hydrogen), such as formic acid and methanol. On the other hand, non-reductive conversion of CO₂ is partially exothermic or endothermic to maintain the +4 oxidation of CO₂. In non-reductive conversions, compounds containing carbonates, carbamates, ureas, carboxylates, polycarbonates, polyurethanes and the like are obtained from CO₂. [7] Chemicals derived from biomass derivatives showing chemically neutral properties are replacing chemicals obtained from fossil fuels such as coal or oil. Cyclic carbonates, cyclic carbamates and cyclic ureas attract considerable attention due to their applications in many organic syntheses and industries.

Ring-forming reactions shown in Figure 3 are highly valuable from an environmental and organic synthesis perspective. To date, various synthesis methods have been developed without catalysts or with homogeneous or heterogeneous catalysts.[8]

Environmentally and practically, a reusable heterogeneous catalytic synthesis system is important both for catalyst reuse and for easy separation of the resulting product. Traditionally, these cyclic compounds are synthesized using phosgene, a toxic and hazardous reagent. For this reason, an environmentally and practically alternative method is needed for the synthesis of cyclic compounds obtained from CO₂. Direct synthesis methods for cyclic compounds from carbon dioxide are proposed as follows. These intramolecular ring reactions occur through two addition reactions: 1. Formation of carbamate or carbonate intermediate compounds by adding a functional group to CO₂. 2. Adding the other functional group to these intermediates (the second reaction is the rate-determining step).

3. Polyurethane Synthesis from CO₂

Another major industrial application of carbon dioxide is its use in the production of biodegradable thermoplastics poly(propylene carbonate) (PPC) and poly(cyclohexene carbonate) (PCHC) (Figure 4). These polymers, in addition to their characteristics of durability, light weight, toughness, heat resistance, easy processing, high transparency and good electrical insulation, also have high industrial importance in the medical and health sectors as well as in automotive, electronics, optical media, glass and coating industries due to their biodegradability. These polymers have also been extensively researched for use as polyols in polyurethane and polyester production.   In recent years, many companies have brought polyurethane-based materials produced using these polymers to the commercialization stage. Due to both environmental and economic attractiveness, major chemical companies are targeting completion of production facility installations in the United States and Europe in the coming years for the production of CO₂-based polyols.

3.1. Sustainable Polyurethane Shoe Sole Production from CO₂-Polyols

Most walking and sports shoe soles use polyurethane elastomers, which show lighter weight and better impact resistance properties compared to rubber. However, the biggest problem is that these materials have low hydrolysis resistance and undergo degradation when exposed to hot/humid environments for long periods, with the appearance of cracks. Today's polyurethanes are produced entirely from fossil fuels, causing extensive environmental damage. The "PUFOOTCO₂" project developed by SYNTHELAST aims to produce polyurethane elastomers sustainably and improve their properties. Accordingly, the benefits to be obtained through the use of CO₂ as a feedstock:

Technical Benefit

It is targeted to provide better properties to CO₂-based polyurethanes to be produced for use in shoe soles, thereby reducing their service life (Figure 5).

Environmental Benefit

CO₂ is abundant in nature and exists as a renewable resource. It is also known that it can be used in polymer synthesis. Dependence on fossil fuels in polyurethane production and the associated carbon footprint can be reduced. Furthermore, by reducing carbon dioxide emissions released into the atmosphere, a sustainable benefit is provided to global warming (Figure 6).

Economic Benefit

Carbon dioxide is considerably cheaper compared to petroleum-based raw materials. For this reason, the production cost of CO₂-based polyurethane appears quite reasonable when compared to conventional polyurethane production (Figure 7).

3.2. Use of CO₂-Polyols in High-Performance Polyurethane Hot Melt Adhesives

Polyurethane reactive hot melt adhesives are polyurethane systems with isocyanate end groups. These adhesives, which are solid at room temperature, require heating to 110-140°C before application. Initial strength develops rapidly with cooling and curing of the adhesive. Continued curing for a few more days with moisture removal ensures complete strength development. In a typical polyurethane formulation, there is diisocyanate (usually MDI), crystalline polyol (melting point ~50-70°C) and alongside amorphous polyols, curing catalysts and additives. Polyester polyols are the preferred crystalline compounds in hot melt adhesive systems. Polyether polyols, on the other hand, are preferred as amorphous. This combination gives the material the property of being usable as a hot melt adhesive. These two materials have disadvantages such as resistance stability to open air conditions. Polyesters have low hydrolysis stability, while polyethers have poor oxidation resistance under ultraviolet (UV) light. Commercial polycarbonate polyols offer many performance advantages. They have high hydrolytic and oxidative stability, but their high cost makes their use unattractive except for special applications.[10]

Novomer is conducting research on the production of polycarbonate polyols containing carbon dioxide within the polymer chain. In addition to environmental impact, its production from waste carbon dioxide provides economic advantage.

These new polycarbonate polyols can be used in the synthesis of reactive polyurethane hot melt adhesives. Polypropylene carbonate diols with molecular weights of 1000 and 2000, crystalline polyhexamethylene adipate polyester polyols and diisocyanate are reacted to form isocyanate prepolymers with 2-5% isocyanate content. Prepolymers are subsequently used in polyurethane-based hot melt adhesive production. Polyurethane adhesives contain more than 40% carbon dioxide by weight. Polypropylene carbonate polyols developed for use in hot adhesive systems are aimed at replacing petroleum-based polyether, polyester and polycarbonate polyols. This product obtained from copolymerization of carbon dioxide and epoxide contains more than 40% CO₂ by weight.

Through the use of waste CO₂ as a feedstock, the carbon footprint of the product is considerably reduced. Waste CO₂ has a considerably lower price compared to petroleum-based feedstocks (Figure 8).

Polycarbonate backbone in polyurethane products increases strength and stability. The polyols developed by Novomer have a polycarbonate backbone. The strength and stability of polyurethanes produced from these polyols are increased due to the polycarbonate backbone. Foams containing such polyols in their formulations have been observed to have not only high tensile and tear strength but also increased resistance under load. In adhesives and coatings, it increases adhesion, adhesion strength and weather resistance. In elastomers, it provides high tensile and bending strength. Compared to other commercial polyols, they have 40-50% lower calorific content. In addition to high CO₂ content, The polyols developed by Novomer have 40-50% lower calorific content compared to other commercial polyether, polyester and polycarbonate polyols. This property is particularly important for polyurethane systems requiring flame retardancy. Early samples of first products are being produced in molecular weights between 1000 and 2000.[12]

3.3 Hydrolysis/Oxidation Resistance of Water-Based Polyurethane Synthesized from CO₂-Polyols

For the synthesis of water-based polyurethane containing carbon dioxide, CO₂ and propylene oxide were copolymerized with a zinc-cobalt dual metal catalyst. The obtained CO₂-polyol was first reacted under dibutyltin dilaurate catalyst with 4,4'-diphenylmethane diisocyanate and 2,2-dimethylol propionic acid to form a prepolymer, which was then reacted with additional 4,4'-diphenylmethane diisocyanate and 1,4-butanediol to obtain acid-functional polyurethane. Water-based polyurethane synthesis was completed by reacting this polymer with triethylamine in water (Figure 9). When examining the mechanical properties of the obtained polyurethane material, an improvement was observed due to carbonate units from CO₂-polyol, and oxidation resistance also increased. On the other hand, ether units on CO₂-polyol increase hydrolysis resistance. The tensile strength of water-based polyurethane in sodium hydroxide showed no decrease when compared to oligoester-based polyurethane. Furthermore, the thermal-mechanical performance of these carbon dioxide-based polyurethanes can be easily modified by adjusting the amount of carbonate units. For example, when carbonate content increased from 30% to 66%, the glass transition temperature rose from -7.8°C to 18.8°C. This also increased the material's tensile strength from 35.6 MPa to 52.2 MPa, while reducing elongation at break from 630% to 410%. The use of carbon dioxide in the production of water-based polyurethanes is an alternative to commercial equivalents derived from fossil fuels, and the hydrolysis/oxidation resistance of the obtained polyurethane materials can provide superiority over commonly used oligoester-based polyurethanes.[13]

4. Conclusions and Recommendations

With the transfer of successful steps by countries with limited oil resources in the energy sector to the materials sector, polymer-based materials produced using alternative sources have begun to enter our daily lives. Particularly, significant investments by chemical companies in Germany and the USA in CO₂-based polyols will come into operation within a few years. Following the depletion or reduction of oil resources, the plastics industry's orientation towards alternative sources will be inevitable, and the importance of this method will grow with each passing day. From the perspective of our country, this method would be a highly effective solution to prevent import dependence in plastic feedstocks, one of the major problems facing the Turkish plastics industry. Assoc. Prof. M. Atilla Taşdelen / Polymer Engineering Department Faculty of Engineering - Yalova University Serhat Durak / Master's Student - Yalova University Süleyman Güngör / Master's Student - Bursa Technical University Serhat Oran / Doctoral Student - Yalova University

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