Types, application fields, advantages and disadvantages of sulfonic acid water-borne polyurethanes

文: 蘇智禾 / 聚和國際 分析技術組

攝影師：Ivan Samkov: https://www.pexels.com/zh-tw/photo/9628854/

What is polyurethane (PU)

Polyurethanes are polymerized by isocyanates and polyols, with the elasticity and toughness of rubber, and the strength of plastic and excellent processability, often used in paints, sponges, adhesives, coatings for leather and other materials, dressings, biomedicine and other fields.

 

Conventional solvent-borne polyurethanes V.S. water-borne polyurethanes

In the conventional production process of solvent-borne polyurethanes, a large amount of organic solvents would be used. With raising environmental awareness globally in recent years, water-borne polyurethanes produced with significant reduction in the use of solvents, or even by solvent-free processes (CN1869091A, 2006) have attracted more attention.

Furthermore, the amounts of volatile organic compounds (VOCs) generated from the processes are greatly reduced, that is not only beneficial to environments, but also to good to the production operators’ health by minimizing the exposure to those hazardous chemicals.

Types of water-borne polyurethanes

Water-borne polyurethanes (WPU) can be divided into 3 categories - cationic, anionic and non-ionic.

1. Cationic WPUs：mostly, quaternary ammonium salts (CN105601876A, 2016)

The quaternary ammonium salt group of the cationic WPUs would get easy to decompose or produce reverse reaction when heated, and its heat resistance is poor.

In recent years, cationic WPUs can be found in the applications of textile, leather finishing, papermaking, and wood lacquer, for the better permeability contributed by favorable binding between positive charges and fibers (CN111533875A, 2020). Another study indicated that the WPUs combined with chitosan would be given with excellent anti-bacterial property, biodegradability and biocompatibility, and can be used in food packaging and drug release carriers. (CN106700021A, 2017)

2. Non-ionic WPUs：with groups of ethoxy (CN101638472A, 2009), polyethylene glycol ether (CN108164650B, 2017)

With poor emulsion stability, poor film performance and low solid content, non-ionic WPUs are less popular and mostly used with functional additives, such as wetting agents, leveling agents, defoaming agents, antioxidants, according to the applications. (WO2021103376A1, 2019)

3. Anionic WPUs：with groups of carboxylic acid (CN104356330B, 2014), phosphoric acid (KR101858774B1, 2010) and sulfonic acid (CN107446105B, 2017)

Anionic WPU’s are the most widely studied category, and carboxylic acid-based WPU’s are the most common ones. Due to the low dissociation degrees, carboxylic acid-based WPU’s have poor heat resistance and low solid content, furthermore, the fact of high water content also means high evaporation heat and long drying time (CN111454420B, 2020). Carboxylic acid-based WPU’s need to be stored under alkaline conditions. Besides, since volatile amines are often used in the processes (to neutralize and form salts), during storage the amines would be easy to decompose and volatilize, furthermore, to cause a peculiar smell.

Oppositely, sulfonic acid-based WPUs that have high dissociation degrees, stabilized by the strong electrostatic repulsion of the negatively charged colloids, high solid content, that further contribute to the advantages of excellent thermal stability, water resistance, and good adhesion. And, they are not produced through amine neutralization, thus the finished products have no peculiar odors (CN108976371B, 2018 ). The following is a brief introduction to the applications and development of sulfonic acid-based WPUs.

 

Applications and Development of Sulfonic Acid Waterborne Polyurethanes

Surface Modification

With the development of economy, the requirements for the product surface quality of consumers nowadays have been increased. There are two major development aspects for surface modification, one is high transparency (high gloss), and the other is matting.

In the conventional PU process, matting is proceeded by increasing the surface roughness via addition of matting agents, such as wax powder or silica, into the PUDs, in order to reduce reflection of light, but the dispersibility of matting agents is the main technical bottleneck.

Later on a new matting technique without using any matting agents was developed—which can make the roughness of the dry surface by controlling the PU particle sizes during synthesis, furthermore, to get rid of various coating problems like whitening, frosting, brittleness, being easy to break and fall off caused by poor dispersion of matting agents. (CN106519133B,2016)
 

Adhesive tapes

Reducing the dependence on petroleum raw materials is also a research direction that cannot be ignored in future. After certain processing, the biologically derived soybean oil can replace petroleum polyols as the raw material for PUs. Besides, as a natural renewable resource, soybean oil that is large in output, cheap in price, and degradable can relieve environmental pressure. (CN105713175B, 2016)
 

Further Reading:  About Poly-EPS

Further Reading:  The Application and market trend of Lignin.