Eco-Friendly Waterborne Polyurethanes

Synthesis, Properties, and Applications

Ram K. Gupta, Ajay Kumar Mishra, Ram K. Gupta, Ajay Kumar Mishra

• Publisher: CRC Press
• Year: 2022
• Print ISBN:
  9781032002866
• eBook ISBN:
  9781000532883
• Topic: Technik & Maschinenbau
• Subtopic: Chemie- & Biochemietechnik

The polyurethane industry is among the fastest growing, with polyurethanes used in consumer as well as industrial sectors. Waterborne polyurethanes (WPUs) exhibit many advantages over conventional volatile organic compounds (VOCs) based polyurethanes and have emerged as an environmentally friendly alternative. WPUs offer an opportunity to use sustainable raw materials to produce environmentally sustainable polymers, particularly, polyols derived from vegetable oils. Eco-Friendly Waterborne Polyurethanes: Synthesis, Properties, and Applications provides state-of-the-art knowledge of the synthesis, application, and property enhancement of WPUs.

• Covers various types of eco-friendly materials and technologies used to synthesize WPUs
• Presents an overview and applications of WPUs in several advanced research areas
• Provides fundamentals of synthetic processes and their chemistries for specific applications
• Elaborates on advanced approaches used to convert renewable resources into polymers
• Offers new direction to scientists, researchers, and students to better understand the chemistry, technologies, and applications

Written for polymer chemists, materials scientists, and other researchers and industry, this book serves as a comprehensive reference for readers interested in the development and application of sustainable polymers.

1 Introduction to Waterborne Polyurethanes

Felipe M. de Souza, Prashant Kote, and Ram K. Gupta

Department of Chemistry, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, USA

DOI: 10.1201/9781003173526-1

CONTENTS

1. 1.1 Introduction
2. 1.2 Synthesis of Waterborne Polyurethanes
3. 1.2.1 Prepolymer Method
4. 1.2.2 Acetone Process
5. 1.2.3 Melt-Dispersion Process
6. 1.2.4 Ketamine–Ketazine
7. 1.3 Classification of Waterborne Polyurethanes
8. 1.3.1 Cationic Waterborne Polyurethanes
9. 1.3.2 Anionic Waterborne Polyurethanes
10. 1.3.3 Zwitterionomers
11. 1.4 Applications of Waterborne Polyurethanes
12. 1.5 Conclusion
13. References

1.1 Introduction

Eco-friendly materials that can be used in commercial applications are very attractive for a sustainable future compared to fossil-based chemicals. Polyurethanes are widely used in commercial sectors; however, current efforts are to use eco-friendly materials and routes for their synthesis. Waterborne polyurethanes (WPUs) using renewable resources are emerging as a new class of polymers that are eco-friendly and sustainable. WPUs find wide applications in coatings, inks, adhesives, sealants, the biomedical field, etc. The market for WPU is increasing every year and is expected to reach $2.1 billion USD by 2025. The continuous increase in the market is due to the need for eco-friendly materials and many regulations that limit the amount of volatile organic compounds (VOCs) that can be released from commercial products, such as solvent-borne polyurethanes. That also includes other ingredients, such as physical blowing agents used in the synthesis of polyurethane foams like chlorofluorocarbons (CFCs), which were banned due to their harmful effects on the environment. The chemical reaction for the synthesis of polyurethane is very simple, as seen in Figure 1.1. The reaction proceeds through a polyaddition reaction between compounds containing several hydroxyl groups (polyols) and di- or multi-functional isocyanates that form the urethane linkage. The traditional polyols are mostly obtained through the polymerization of ethylene or propylene oxide, which leads to polyether polyols. These components are difunctional hydroxyl-terminated polymers with an aliphatic chain. Due to the flexibility, mobility, and hydrophobicity of the structure, polyols are referred to as the soft domains of polyurethane. On the other hand, isocyanates are composed of rigid groups with low mobility and polarized segments that properly interact with water through hydrogen bonding; hence, they are more dispersible in water or polar solvents, such as acetone.

Figure 1.1 A chemical reaction between polyol and diisocyanate.

Thus, to improve the water dispersibility, more polarized, ionic groups or surfactants should be introduced to form a stable polyurethane–water suspension, such as polyester polyols that contain a –C(O)–O– group that interacts with water through hydrogen bonding. High hydrophilicity might be an issue for many applications as these polymers can be more susceptible to react with moisture or degrade under the presence of oxidizing agents, such as acid or alkaline media [1]. Therefore, to counter such issues, other approaches such as the use of emulsifiers (external or internal) can be applied. External emulsifiers are generally surfactants, and their applications are cost-effective; however, sometimes they lead to the formation of unstable big particles. On the other hand, chemical implementation of an ionic group into a polyurethane's backbone as a pendant group that can contain a negative (anionic), positive (cationic), or species that contains both charges (zwitterion) can provide a balance of hydrophilic and hydrophobic nature to obtain stable dispersion in aqueous media.

Di-methylol propionic acid (DMPA) as an effective internal emulsifier is regularly used to introduce a side carboxylic acid group into the polyurethane's chain to synthesize anionic WPU. It can then be neutralized by a quaternary ammonium salt, such as triethylamine (TEA), to further improve its water dispersibility [2]. Cationic WPU is another type that consists mostly of introducing a pending tertiary amine group, such as methylene diethanolamine (MDEA), into the main chain that can be converted into a quaternary amine salt by the addition of an acid. There are many applications of both anionic and cationic WPUs. However, a recent approach used by Zhang et al. demonstrated an interesting example of simultaneous use of both anionic and cationic WPUs to serve as a pesticide delivery system using castor oil-based WPUs [3]. The WPUs were synthesized by reacting castor oil as the bio-based polyol and isophorone diisocyanate (IPDI) with 2,2-bis(hydroxymethyl) butyric acid (DMBA) or N-methyl diethanolamine (MDEA). The latter two were responsible for yielding an anionic or cationic WPU, respectively. The use of WPUs enhanced the pesticide's retention time and, thus, its efficiency.

Zwitterionic-based WPUs present lower adhesion of bacteria or biomaterials, such as proteins, blood, fibrinogen, and others on its surface. Additionally, these polymers have effective biocompatibility and chemical stability compared with other polymers, such as poly(ethylene glycol) (PEG). A group of researchers developed an anti-biofouling polyurethane with viable applications for the transport of proteins [4]. The synthesis consisted of a prepolymer method that introduced positive and negative charges into the backbone through a quaternary ammonium salt and sulfate group, respectively. [2-(dimethylamino)ethyl methacrylate dihydroxy terminated (DMA(OH)₂)] was synthesized and used as a chain extender. Then 1,3-propane sulton (1,3-PS) was introduced through a ring-opening reaction. The result showed an improvement in the resistance for adhesion of protein to the polyurethane due to the zwitterionic segment, suggesting the importance of such polyurethanes for biomedical equipment to prevent microbial infections and clot formation during the transportation of fluids or prolonged periods exposed to blood or other body fluids.

1.2 Synthesis of Waterborne Polyurethanes

Most of the synthetic approaches for WPUs consist of two steps. In the first step, a prepolymer is prepared using the desired di or polyols along with the di or polyisocyanate. In the second step, the terminal isocyanate groups from the prepolymer are used as reactive sites to introduce chain extenders that contain hydrophilic groups to disperse the polymer into aqueous media. Several other synthetic routes, such as acetone process, melt-dispersion, and ketamine–ketazine, have been developed. This session briefly describes the main aspects of these procedures.

1.2.1 Prepolymer Method

The main motivation behind developing WPUs was to find alternative synthetic routes that use eco-friendly solvents or reduce the consumption of solvents while providing properties comparable to petrochemical-based polyurethanes. The prepolymer or emulsification method consists of dispersing the prepolymer in water by initially introducing an excess of isocyanate to obtain isocyanate function as an end group to increase the hydrophilicity. Then, a chain extender is introduced in the heterogeneous phase. There are some requirements to successfully perform this method, such as executing the dispersion step in a short period and maintaining a suitable temperature so that isocyanate does not react with water. Cycloaliphatic isocyanates are desired for this approach due to their lower reactivity toward water. For the success of synthesis, careful control of viscosity and proper functionality are also required. Generally, 15% of organic solvents, such as N-methyl-2-pyrrolidone, are used to reduce viscosity. The main purpose of adopting this method is to avoid VOCs and decrease overall production costs. However, some components, such as DMPA, that introduce the ionic segment are derived from non-renewable sources.

To find an alternative, researchers developed an approach that used soybean oil for the synthesis of WPU [5]. The synthesis consisted of performing a ring-opening reaction of epoxidized soybean oil with adipic and pimelic acid, separately, to introduce a carboxylic acid as an ionic segment. The synthesized WPU was used as a pressure-sensitive adhesive. Shear strength of about 1 h to more than 100 h, a tack of 1.78–5.66 N, and peel strength of 1.23–2.77 N/mm were observed for this adhesive. Castor oil was used as a natural polyol source and provided a total bio content of about 77% in the WPU. Also, castor oil demonstrated good compatibility with the bio-derived ionic segments. The results demonstrated that the WPU obtained through the emulsification process had effective thermal stability that presented the max thermal decomposition temperature (~383°C) along with good adhesive characteristics. These properties, accompanied by renewable sources of materials, show potential for large-scale applications.

1.2.2 Acetone Process

The acetone process consists of synthesizing a prepolymer and implementing a chain extender in acetone to obtain a homogenous phase. After completion of the synthesis, a solvent exchange process is performed by adding water and removing acetone through distillation or rotary evaporation. Acetone is used as it is a convenient solvent due to its chemical inertness, proper dispersion of polyurethanes in it, water solubility, and low boiling point, which facilitates its easy removal. Some advantages of this method include the formation of a homogenous system that yields small particles and reduces the reaction rate between chain extenders toward isocyanate, leading to higher WPU reproducibility. Acetone also decreases the viscosity of the system, which eases processing. However, this process requires a high volume of acetone, which increases the cost and makes it less eco-friendly. It also adds an extra distillation step and a requirement for the polyurethane structure to be soluble in acetone to avoid precipitation [2]. Despite these factors, the acetone process is a viable industrial method for the production of adhesives due to its high reproducibility and better control of the process. As an alternative option, methylethylketone was used in place of acetone. Nanda et al. compared the properties of polyurethanes prepared using acetone and prepolymer methods [6]. The study showed that a low concentration of ionic segments may be required to achieve a stable dispersion. A stable dispersion of polyurethanes was achieved by using 2% and 4% of DMPA in acetone and prepolymer methods, respectively.

Sardon et al. synthesized a WPU using the acetone process to understand the influence of DMPA concentration, amount of acetone in dispersion, temperature for solvent exchange, and conditions of solvent evaporation [7]. Use of 60 wt.% of acetone, lower than 30°C solvent exchange temperature, and 0.1 mmol of DMPA per gram of polyol (at PU of 75 wt.%) were some of the optimal experimental conditions to form stable dispersions. The procedure consisted of using poly(propylene glycol) as the diol, DMPA as an internal ionic emulsifier, TEA to neutralize the acidic gr...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Preface
• Editors’ Biographies
• Chapter 1 Introduction to Waterborne Polyurethanes
• Chapter 2 Chemistry and Technology of Waterborne Polyurethanes
• Chapter 3 Green Materials for Waterborne Polyurethanes
• Chapter 4 Eco-Friendly Synthesis for Waterborne Polyurethanes
• Chapter 5 Synthesis, Properties, and Applications of Waterborne Polyurethanes
• Chapter 6 Nanocomposites of Waterborne Polyurethanes
• Chapter 7 Waterborne Polyurethanes for Flexible and Rigid Foams
• Chapter 8 Flame-Retardant Waterborne Polyurethanes
• Chapter 9 Synthesis, Characterization, and Applications of Smart Waterborne Polyurethanes
• Chapter 10 Shape Memory Waterborne Polyurethanes
• Chapter 11 Waterborne Polyurethanes for Self-Healing Applications
• Chapter 12 Waterborne Polyurethanes for Biomedical Applications
• Chapter 13 Waterborne Polyurethanes for Tissue Engineering
• Chapter 14 Waterborne Polyurethanes for Biodegradable Coatings
• Chapter 15 Recent Developments in Waterborne Polyurethanes for Coating Applications
• Chapter 16 Waterborne Polyurethanes for Weather Protective Coatings
• Chapter 17 Recent Developments in Waterborne Polyurethanes for Corrosion Protection
• Chapter 18 Waterborne Polyurethane for Electrically Conductive Coating
• Chapter 19 Waterborne Polyurethanes for Electrical Applications
• Chapter 20 Waterborne Polyurethanes for Sensors
• Chapter 21 Waterborne Polyurethanes for Sealants
• Chapter 22 Waterborne Polyurethanes for Packing Industries
• Chapter 23 Waterborne Polyurethanes for Automobile Industries
• Chapter 24 Non-Isocyanate-Based Waterborne Polyurethanes
• Chapter 25 Waterborne Polyurethanes: Challenges and Future Outlook
• Index