What is urethane resin?
Polyurethane is a polymeric compound that is a resin material produced by the reaction of polyols that have isocyanate groups (-NCO) and hydroxyl groups (-OH).
Urethane resin has the following properties: With its excellent flexibility, strength, and processability, urethane resin has become an indispensable material in a wide range of fields.
- Elasticity and strength can be adjusted, allowing for a wide range of physical properties from soft to hard.
Soft urethane resin: Soft and with high cushioning properties (e.g. foam material).
Hard urethane resin: High rigidity and durability (e.g. structural components). - Highly abrasion and chemical resistant
Resistant to abrasion and many chemicals and solvents. - High workability
A variety of processing methods are possible, including heating, casting, and spray coating. - Good insulation and soundproofing
It has low thermal conductivity and is used as an insulating material, as well as having soundproofing properties. - Good adhesion
It adheres easily to other materials and is also used as an adhesive.
Applications of urethane resin
- Building materials: insulation, soundproofing
- Automotive parts: bumpers, seat foam
- Home appliances: heat-insulating materials, cushioning materials
- Textiles: elastic fibers (spandex, lycra)
- Coatings and paints: waterproof paints, abrasion-resistant paints
- Medical supplies: artificial skin, catheters
Urethane resin design
Select the type of urethane resin
- Soft urethane resin
Soft, elastic, and highly cushioned.
Mainly used for furniture, vehicle seating, and packaging materials. - Rigid urethane resin
High rigidity, high strength, and excellent insulating properties.
Mainly used for insulation and structural components. - Thermoplastic polyurethane (TPU)
Thermoplastic and can be molded.
Mainly used for films, shoe soles, tires, sealants, wear-resistant parts, etc.
Polyol
It is the main component that determines flexibility and elasticity.
Isocyanates
Isocyanates are components that determine crosslinking and hardness and play an important role in the design of urethane resins. The performance of the final product varies greatly depending on the type and characteristics of the isocyanate. Below, we introduce the major types of isocyanates and the major manufacturers that manufacture and supply them.
(1)Aromatic Isocyanates
・MDI (4,4′-diphenylmethane diisocyanate)
・TDI (toluene diisocyanate)
・Mixture of 2,4-TDI and 2,6-TDI (generally 80:20 ratio)
・XDI (xylylene diisocyanate)
(2)Aliphatic Isocyanate
・HDI (hexamethylene diisocyanate)
・IPDI (isophorone diisocyanate)
・Hydrogenated MDI (aliphatic MDI)
Catalysts/Additives
Urethane catalysts (polyurethane catalysts) are important chemicals used in the manufacturing process of polyurethane resin. Polyurethane is formed by the reaction of polyol (polyhydric alcohol) and isocyanate, and catalysts are used to make this reaction proceed efficiently, mainly promoting the following two reactions:
・Gelling reaction (polymer chains are formed by the reaction of polyol and isocyanate)
・Foaming reaction (isocyanate and water react to generate carbon dioxide and create foam)
This allows the hardness, density, and foaming speed of polyurethane foam to be adjusted.
The selection of a urethane catalyst depends on the product characteristics and manufacturing conditions. The following are some points to consider when selecting a catalyst:
・Reaction rate: If it is too fast, processing will be difficult, if it is too slow, it will be inefficient.
・Final physical properties: Select catalyst according to requirements such as softness, hardness, elasticity, etc.
・Environmental regulations: Sustainability and regulatory (e.g. low VOC catalysts) requirements must be met.
In recent years, the toxicity and environmental impact of conventional tin-based catalysts have come to be viewed as problematic. For this reason, bismuth-based catalysts, zirconium-based catalysts, and low-volatile organic compound (low-VOC) catalysts have been attracting attention as next-generation catalysts.
(1) Amine catalyst
It primarily promotes the foaming reaction, but may also affect the gelling reaction.
・Dimethylethanolamine（DMEA）
・Triethylenediamine（TEDA）
・Bis(dimethylaminoethyl) ether（BDMAEE）
・1,8-Diazabicyclo[5.4.0]-7-undecene（DBU）
(2) Metal catalyst
It mainly promotes the resinification reaction.
・Tin compounds (dibutyltin dilaurate (DBTDL) and tin octylate)
・Bismuth compound
Chain extender
Chain extenders are used in the manufacturing process of urethanes. They are important components in adjusting the molecular weight of polyurethanes and controlling the final physical properties (hardness, strength, elasticity, etc.) during the reaction between polyol and isocyanate.
Chain extenders are low molecular weight compounds that usually have two or more reactive functional groups (mainly hydroxyl and/or amino groups). They form intermolecular crosslinks during the curing stage of polyurethane to strengthen the hard segments.
They are mainly used in the production of polyurethane elastomers, rigid foams, and thermoplastic polyurethanes (TPUs). Chain extenders are broadly classified into two types depending on their structure:
(1) Diol Chain Extender
A molecule with two hydroxyl groups (-OH), which gives it flexibility and elasticity.
・1,4-Butanediol (BDO): The most common chain extender, often used in polyurethane elastomers and TPUs.
・Ethylene glycol (EG): Increases hardness in rigid polyurethanes.
・1,6-Hexanediol (HDO): Enhances durability and flexibility.
(2) Diamine chain extender
Features: A molecule with two amino groups (-NH₂). It is characterized by fast reactivity and improves hardness and mechanical strength.
・Ethylenediamine (EDA)
・4,4′-Methylenebis(2-chloroaniline) (MOCA): Once widely used, but toxicity concerns have led to the development of alternatives.
・Diaminodiphenylmethane (MDA)
(3) Other compounds
Amino alcohol type
・Diethanolamine (DEA): Contains both hydroxyl and amino groups, allowing for adjustment of hardness and flexibility.
Aromatic compounds: Contains an aromatic structure (benzene ring), improving hardness and heat resistance.
Method for manufacturing urethane resin
Soft urethane resin
Manufacturing process for soft urethane resin
- Preparation of raw materials
・Polyol: The main component that determines the flexibility of the resin.
・Isocyanate: Reacts with polyol to form urethane bonds.
・Blowing agent: Uses a CO2 generator or water to create foaming.
・Catalyst: Controls the reaction rate (amine-based, tin-based, etc.).
・Other additives (foam stabilizers, flame retardants, colorants, etc.) - Mixing/reaction
The above raw materials are mixed in the appropriate ratio.
A chemical reaction results in the formation of a cross-linked polyurethane structure.
The foaming agent generates gas, causing the resin to expand. - Foaming/hardening
As the reaction progresses, the urethane foam expands and hardens.
The hardening time can be adjusted by adjusting the composition and temperature conditions. - Molding/finishing
It is poured into a mold and formed into the required shape.
After hardening, it is cut and finished.
Type of manufacturing method
- Continuous foaming method (slabstock method)
It is continuously foamed on a conveyor belt to create large blocks of foam.
It is used to manufacture furniture, mattresses, insulation materials, etc. - Batch foaming method
It is poured into a mold and foams and hardens in fixed amounts.
Applied to products with specific shapes (cushioning materials, automotive parts, etc.). - Injection Molding
The urethane is reacted inside a mold using an injection molding machine.
This method is suitable for creating high-precision shapes.
Hard urethane resin
Rigid urethane resin manufacturing process
Rigid urethane resins (such as polyurethane foam) are a type of polyurethane that is primarily used as insulation and structural materials, and have greater rigidity and heat resistance than soft urethane resins.
The basic chemical reaction is the same as for soft urethanes, but the degree of cross-linking is higher and the formulation of foaming agents and additives is different.
- Preparation of raw materials
・Polyol: High-functional polyol (to increase crosslink density) is used
・Isocyanate: Polymethylene polyphenylisocyanate (PMDI) is commonly used
・Blowing Agent: Physical blowing agent (HFC, HC, CO₂, etc.)
Chemical blowing agent (CO₂ is generated by the reaction
of water and isocyanate)
・Catalyst: Reaction promoter (amine-based, tin-based)
・Other additives: Flame retardants, surfactants (foam stabilizers), stabilizers, etc. - Mixing/reaction
・Polyol, isocyanate, blowing agent, catalyst, etc. are mixed in the appropriate ratio.
When the mixture is poured into a mold or continuous production equipment,
the following reaction takes place: urethane bond formation (crosslinking reaction)
Polyol and isocyanate react to create a high-density urethane structure. - Foaming/hardening
・Foaming reaction
Water and isocyanate react to generate CO₂ (chemical foaming).
Foaming is also promoted by the evaporation of the physical foaming agent.
・Curing
The material is cured at high temperature or room temperature to achieve
its final strength. - Molding/finishing
・Block molding: Creates large chunks that are later cut and processed.
・Cast molding: Pours material into a mold and hardens to create a specific shape.
・Continuous molding (spray, panelization): Used to make building materials and
insulation.
Type of manufacturing method
- Block foaming method
A large urethane block is made and then cut for use. Used for insulation and building panels, etc. - Injection molding (RIM: reaction injection molding)
Two liquids (polyol + isocyanate) are mixed and molded by allowing a chemical reaction to proceed inside a mold.
Used for automobile parts, electrical equipment cases, etc. - Spray foaming
As an insulation application method, it is sprayed on the construction site and foamed and hardened.
Used for insulation in homes and cold storage warehouses. - Sandwich panel molding
Rigid urethane foam is used as a core material and is integrated with metal or plastic plates.
Used in insulation panels and refrigerator wall materials.
Thermoplastic polyurethane (TPU)
Urethane elastomer manufacturing process
Urethane elastomers (polyurethane elastomers) are materials that combine the elasticity of rubber with the durability of plastic.
There are two general types: thermoplastic polyurethane (TPU) and thermoset polyurethane (CPU), each of which is manufactured using a different method.
- Preparation of raw materials
・Polyol: A soft component that determines the elasticity of the elastomer.
Polyester and polyether types are common.
・Isocyanate: A hard component that determines rigidity and abrasion resistance.
MDI (diphenylmethane diisocyanate)
TDI (toluene diisocyanate)
NDI (naphthalene diisocyanate), etc.
・Chain Extender: Short-chain diols and diamines are used to adjust the molecular weight
and create a crosslinked structure.
1,4-butanediol (BDO), ethylenediamine (EDA), etc.
・Catalyst: Used to promote reactions (amine-based, tin-based, etc.). - Urethane elastomer reactions
2-1 Thermoplastic urethane (TPU) manufacturing method
TPU is a urethane elastomer that can be softened and molded by heat,
and can be used for injection molding or extrusion molding.
① Batch polymer method
Polyol, isocyanate, and chain extender are mixed and reactively polymerized.
The reaction temperature is approximately 180-250°C, and the reaction is
controlled to adjust the appropriate molecular weight.
The resulting polymer is pelletized in an extruder and provided as a raw material
for the final product.
② Continuous Polymer Method (Extrusion)
Polyol and isocyanate are mixed and reacted in an extruder.
The material is molded in a molten state and then cooled and solidified.
The material is pelletized and processed by injection molding or
extrusion depending on the final use.
➡ Applications: Shoe soles, belts, tubes, films, mobile phone cases, machine parts
2-2 Manufacturing method of thermosetting urethane (CPU)
CPU is a strong, abrasion-resistant urethane elastomer with a cross-linked structure
that cannot be remolded by heat.
① Casting method (cast molding)
The polyol and the isocyanate are mixed and reacted.
The reaction is allowed to proceed at an appropriate temperature (80-120°C)
and the viscosity is allowed to rise to an appropriate level.
The resin is then poured (cast) into a mold and allowed to harden.
Once hardened, it is removed and finishing processes (trimming and
polishing) are carried out.
➡ Applications: Industrial rollers, seals, gears, springs, wheels
② RIM (Reaction Injection Molding)
It is mixed at high speed and poured into a mold. The reaction time is short,
making it suitable for mass production. Used for automotive and industrial parts.
➡ Applications: Automobile bumpers, dashboards, sealing materials,
industrial machinery parts