All About Tow-component 2K Polyurethane Adhesives

29th April 2024

Two-component (2K) polyurethane adhesives are one of the most important categories within polyurethane adhesives, widely used and in large quantities. Typically composed of two components, referred to as Part A and Part B, which are separately packaged and mixed in certain proportions before use. Part A (resin) consists of hydroxyl components, while Part B (hardener) consists of components containing free isocyanate groups. Some resins are based on polyurethane prepolymers with terminal NCO groups, while the hardener consists of low molecular weight polyols or polyamines. Part A and Part B are mixed in specific proportions to produce polyurethane resin.

Since their introduction, two-component polyurethane adhesives have become the most diverse and highest-yielding products in the polyurethane adhesive industry due to their adjustable performance, high bonding strength, and wide bonding range.

Features of Two-component Polyurethane Adhesives

Two-component polyurethane adhesives have the following characteristics:

  1. They are reactive adhesives that undergo crosslinking reactions and produce cured products after mixing the two components.
  2. During preparation, the raw material composition and molecular weight of the two components can be adjusted to achieve suitable viscosity at room temperature, resulting in high solid content or solvent-free two-component adhesives.
  3. They typically cure at room temperature, and curing speed can be adjusted by selecting adhesive raw materials or adding catalysts. Generally, two-component polyurethane adhesives exhibit high initial bonding strength. Upon heating, their final bonding strength exceeds that of single-component adhesives, meeting the requirements of structural adhesives.
  4. The amounts of the two components can be adjusted within a certain range, with a certain tolerance. The NCO/OH molar ratio of the two components is generally greater than or equal to 1. During curing, some NCO groups participate in the adhesive curing reaction, forming chemical bonds, while excess NCO groups can react to form urethane methacrylate, urea, etc., increasing crosslinking density and improving the cohesive strength and heat resistance of the adhesive layer.

Storage of Adhesives

Two-component polyurethane adhesive formulations contain organic solvents such as acetone and ethyl acetate and should be stored and transported according to flammable substance regulations. Part B contains active isocyanate groups and should be kept away from contact with water or other reactive groups. Part A may crystallize or form flocculent precipitates in cold weather, which is a normal physical change. It can be melted and used by warming in warm water or using suitable heating equipment (avoid open flames). The storage life of Part A and Part B is one year and six months, respectively. The product is available in tin cans and glass bottles.

Applications of Adhesives

1. Automotive

The introduction of lightweight metals, composite materials, and plastics in new car structures has led to a continuous increase in the demand for adhesives and sealants in the automotive industry. The most widely used polyurethane adhesives in automobiles include single-component moisture-curing polyurethane sealants for windshield assembly, structural adhesive for bonding glass fiber-reinforced plastics and sheet molding compound composites, two-component polyurethane adhesives for interior parts, and water-based polyurethane adhesives. Interior parts of automobiles are also a rapidly growing area for adhesive usage.

Water-based polyurethane adhesives widely used in automobiles refer to adhesives where polyurethane is dissolved or dispersed in water. Most water-based polyurethanes are linear thermoplastic polyurethanes. Due to their non-crosslinked film structure and low molecular weight, their water resistance, solvent resistance, and film strength are relatively poor and need to be modified to improve their performance. Polyester and acrylic hybrids, along with uretdione and isocyanurate, are used to prepare automotive repair clear coats, which do not require high-speed mixing equipment, are easy to blend together, and exhibit good adhesion properties.

2. Wood

The single-component moisture-curing polyurethane adhesive used in the woodworking industry is liquid and used at room temperature. Typically, it exhibits high bonding strength, flexibility, water resistance, and can bond with many non-wood substrates such as textiles, metals, plastics, rubber, etc. The dry and wet strength of single-component polyurethane adhesives in testing is generally superior to phenol-formaldehyde adhesives. Prior to bonding, applying a hydroxymethyl resorcinol (HMR) coupling agent on the bonding substrate surface can enhance bonding strength. HMR can strengthen the bonding strength of all thermosetting wood adhesives. When the wood surface is pre-coated with HMR coupling agent, the strength and durability of single-component polyurethane adhesives can meet most rigorous testing requirements.

3. Flexible Packaging

Flexible packaging, also known as soft packaging or pouch packaging, has surpassed rigid packaging such as plastic, glass bottles, and cans due to its lightweight, long shelf life, hygiene, ease of storage and transportation, easy opening, minimal waste, and good shelf appeal. Polyurethane adhesives, due to their excellent performance, are used to bond different types of film materials together to produce composite films for flexible packaging with properties such as cold resistance, oil resistance, resistance to chemicals, transparency, and abrasion resistance. Polyurethane adhesives have become the primary adhesives for processing composite films for flexible packaging. In the adhesive market in China, polyurethane adhesives rank second in usage for packaging composite film manufacturing, only after the footwear industry.