What Is Urethane Resin?
Urethane resin is a generic term for polymers produced by the polyaddition reaction of compounds with isocyanate groups and hydroxyl groups. The bonding site is called a urethane bond (Figure 1) and is generally formed by the polyaddition of diisocyanate and polyol.
As abbreviation symbols, PU is used for plastics and U for rubber. Urethane resins have excellent tensile strength, abrasion resistance, oil resistance, and chemical resistance and are therefore also applied in paints and coatings. However, they also have the disadvantage of gradual degradation due to hydrolysis and ultraviolet rays.
Applications of Urethane Resin
Urethane resins are used in a wide range of applications, from familiar household goods to industrial products, due to their high versatility. Examples of familiar products include clothing and car bumpers, while industrial applications include soundproofing materials and adhesives.
It is also used in the soles of sports shoes due to its high flexibility and is one of the resins that are indispensable in our daily lives. This material has a history of being used as a substitute for natural rubber. Therefore, the development of applications utilizing its high elasticity and tensile strength, which are characteristics of rubber, is underway.
Environmental Residuality and Recycling of Urethane Resin
Urethane resin is widely used as a heat insulator for refrigerators, freezers, and coolers because of its excellent heat insulation properties. However, their chemical properties make them difficult to recycle and post-process, and this is an area that is the subject of ongoing research.
The decomposition and recycling technologies have already been established, including thermal decomposition, hydrolysis, glycol decomposition, ammonia decomposition, and amine decomposition, but there are still problems to be solved in terms of decomposition efficiency and cost.
Characteristics of Urethane Resins
Urethane resins have excellent flexibility, impact resistance, tensile strength, and chemical resistance, but they deteriorate after about 3 years. While it has many of these characteristics, its properties vary greatly depending on the manufacturing method.
Degradation can be caused by a wide variety of factors, including oxygen, ozone, light, and heat, hydrolysis through reaction with water, and decomposition by microorganisms. In addition, polyurethanes are characterized by the ability to change their physical properties depending on the manufacturing method. As a result, there are various types of polyurethane resins depending on their intended use. Typical examples are as follows.
1. Foam Type
Foam type is a urethane resin in which air is incorporated into the resin by adding a foaming agent and polymerizing. This material is divided into soft urethane foam and hard urethane foam. The former is used for automobile seats, etc., because of its superior cushioning and durability and lightweight. The latter has excellent heat insulation properties and is used as a heat insulator for refrigerators, freezers, and air conditioners.
Among urethane foam manufacturing methods, slab molding, molding, and laminate molding require large-scale manufacturing equipment. On the other hand, the spray foaming method can be easily handled because the foam can be mixed and foamed on the spot at the point where insulation is needed.
2. Non-foam Type
A typical example of a non-foam type is an elastomer, or urethane resin with rubber elasticity, which is used in sports shoes. Because of their high elasticity, urethane resins with such properties are used for sportswear and slacks.
Differences Between Urethane, Epoxy and Acrylic Resins
Although these three terms are often used interchangeably, perhaps because they all contain the word “resin,” they refer to completely different materials.
Urethane resin is a generic term for a polymeric material with urethane bonds, as explained at the beginning of this article. On the other hand, epoxy resins refer to thermosetting resins with reactive epoxy rings at the ends of their molecules.
Acrylic resin is also a polymer or copolymer of acrylic acid and its derivatives. In other words, their meanings are strictly defined by their different structures and chemical properties, so care must be taken in their usage.
Adhesives
There are two types of adhesives: solid adhesives, such as double-sided tape and seaming tape, and liquid adhesives, in which the applied adhesive is pressed and cured to produce an adhesive effect.
Liquid adhesives can be classified into two major types based on the adhesion method: dry-setting type and reaction-curing type. In the dry-setting type, adhesion occurs when the water or solvent contained in the adhesive volatilizes. On the other hand, the reaction curing type is an adhesive that uses curing caused by a chemical reaction between the components contained in the adhesive.
Reaction curing type can be further classified into two types. One is epoxy resin-based adhesives, in which the main agent and hardener cause curing, and the other is urethane resin-based adhesives, in which curing is caused by moisture in the air. Among these, urethane resin-based adhesives are used in a wide range of general applications because of their high adhesion and quick bonding, making them versatile.
Floor Coating Agents Using Urethane Resins
Floor coating agents can be classified into two types: water-based and oil-based. Originally, coating agents using urethane resins were also divided into water-based and oil-based systems. However, since there were questions about the safety of oil-based systems, water-based systems are generally used today.
Water-based urethane coating agents are characterized by lower durability than oil-based floor coating agents such as glass and silicone coatings. However, they also have the feature that they can be finished at a lower cost. Therefore, compared to other floor coating agents, water-based urethane coating agents are often chosen.
The advantages of choosing water-based urethane coatings are that they are quick-drying due to their fast curing speed and that they are virtually odorless, without the volatile odor of thinner. In addition, it is compatible with most flooring materials, so it can be used for any type of flooring.
On the other hand, one demerit is deterioration over time, which can be considered a weakness of urethane resins. Most water-based urethane coatings have a durability of 10 years or less. However, in addition to the aforementioned merits, water-based urethane coating agents are still chosen as popular coating agents because of their superior safety and ease of use.