The History of the Development of Polyurethane Adhesives

Oct 21, 2021

Polyurethane adhesive refers to an adhesive containing urethane groups (-NHCOO-) or isocyanate groups (-NCO) in the molecular chain. Polyurethane adhesives are divided into two categories: polyisocyanate and polyurethane. Polyisocyanate molecular chain contains isocyano group (-NCO) and carbamate group (-NH-COO-), so polyurethane adhesive shows a high degree of activity and polarity. It has excellent chemical adhesion with substrates containing active hydrogen, such as foam, plastic, wood, leather, fabric, paper, ceramics and other porous materials, as well as metals, glass, rubber, plastics and other materials with smooth surfaces.

Introduction
Polyurethane adhesive is an important part of the polyurethane resin that is currently under rapid development. It has excellent properties and has been widely used in many aspects. It is one of the eight important types of synthetic adhesives.
Polyurethane adhesives have excellent shear strength and impact resistance, are suitable for various structural bonding fields, and have excellent flexibility properties.
Polyurethane adhesive has excellent rubber properties and can adapt to the adhesion of substrates with different thermal expansion coefficients. It forms a soft-hard transition layer between the substrates, which not only has strong adhesion, but also has excellent cushioning and reducing Shock function. The low temperature and ultra-low temperature properties of polyurethane adhesives exceed all other types of adhesives.
Waterborne polyurethane adhesives have the characteristics of low VOC content, low or no environmental pollution, non-combustibility, etc., which is the key development direction of polyurethane adhesives.

Development history
In 1940, German researchers discovered that triphenylmethane-4,4'-triisocyanate can bond metal and sodium butadiene rubber, which was used on tank tracks in World War II. After the 1950s, Bayer developed the esmorhurs series (diisocyanates and polyisocyanates) and esmophens series (low molecular weight hydroxyl-terminated polyester polyols). A certain amount of esmbd and esmoplen can be formulated into: Polystal series of products (two-component solvent-based polyurethane adhesive).
Japan introduced German and American technologies in 1954, produced polyurethane materials in 1960, and started producing polyurethane adhesives in 1966. It successfully developed vinyl polyurethane water-based adhesives and put them into industrial production in 1981. Japan is very active in the research and production of polyurethane adhesives, and together with the United States and Western Europe, it has become a major country in the production and export of polyurethane.
China developed and produced triphenylmethane triisocyanate (Lekner Gum) in 1956, and soon produced toluene diisocyanate (TDI) and two-component solvent-based polyurethane adhesives. In 1966, polyurethane adhesives were put into production. , It is still the most productive variety among China's polyurethane adhesives. Since the 1980s, many advanced production lines and products have been introduced from abroad. Among them, a large amount of imported polyurethane adhesives are needed to support them. Therefore, domestic research units have accelerated the development of polyurethane adhesives, especially after 1986. China's polyurethane industry has entered a period of rapid development. In 1994, the state formally approved the establishment of the "China Polyurethane Industry Association, with a "Polyurethane Adhesive Committee" under which it has become the national polyurethane adhesive technology and information exchange center.

Classification
(1) Classification by reaction composition According to reaction composition, it can be divided into polyisocyanate adhesives, isocyanate group-containing polyurethane adhesives, hydroxyl-containing polyurethane adhesives and polyurethane resin adhesives.
(2) Classification by use and characteristics According to the classification of uses and characteristics, it can be divided into general-purpose adhesives, food packaging adhesives, shoe adhesives, paper-plastic composite adhesives, construction adhesives, and structures. Adhesives, ultra-low temperature adhesives, foaming adhesives, anaerobic adhesives, conductive adhesives, hot melt adhesives, pressure-sensitive adhesives, closed adhesives , Water-based adhesives and sealing adhesives, etc. [1] 
principle
Regardless of the polyurethane adhesive, the isocyanate groups in the system react with the active hydrogen-containing substances in or outside the system to generate polyurethane groups or polyurea, which greatly improves the strength of the system and achieves the purpose of bonding.

Features
High-performance waterborne polyurethane adhesives have the following characteristics:
(1) Good water resistance and medium resistance.
(2) High bonding strength and high initial adhesion.
(3) Good storage stability.
(4) Resistance to freezing and thawing, and high temperature resistance.
(5) Fast drying speed and good film-forming properties at low ambient temperature.
(6) Good construction technology.

In order to achieve the above points, in addition to the synthesis of high-performance waterborne polyurethane, the choice of film formulation additives is also particularly important.
The synthetic raw materials of polyurethane prepolymer are mainly oligomer polyol and diisocyanate. Oligomer polyols are usually divided into two types: polyether polyols and polyester polyols. The prepolymers made from polyether polyols have good hydrolytic stability, good flexibility and extensibility, and low temperature resistance. Good; and the polyester polyol type prepolymer has large internal cohesion and high bonding strength.

How to use polyurethane adhesive
Ke Nuo chemical polyurethane adhesive has the advantages of high solid content, low viscosity, good process applicability, strong adhesion, non-toxicity, etc. It is suitable for aluminum foil (OPA), pretreated polyethylene (HDPE, LDPE), polypropylene ( OPP, CPP), polyester (PET), polyamide (PA, ONY) film, metal sprayed film, two-component solvent-based polyurethane adhesive compounded between cellophane.