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Polyurethane curing agents typically refer to the components containing isocyanate groups (NCO) in two-component polyurethane products, mainly used in combination with active hydrogen. Common components with active hydrogen include polyether polyols, polyester polyols, epoxy resins, hydroxypropyl acrylic resin, etc. In these products, the addition of curing agents not only increases the crosslinking and cohesion of the products but also enhances the strength, weather resistance, and other properties of the final products. Currently, polyurethane curing agents are widely used in adhesives, coatings, inks, and other fields.
Generally, polyurethane curing agents can be categorized into three main types: solvent-based, water-dispersible, and blocked polyisocyanates.
Solvent-based curing agents are the most common in the applications, offering enhanced strength, weather resistance, and mechanical properties to the cured materials. It is consisted of polyisocyanates and solvents. The isocyanates within the solvent-based curing agents react with components containing active hydrogen, such as polyols and resins, to form a durable and crosslinked polyurethane network.
The inclusion of solvents in this type of curing agent facilitates the application and processing of the mixture, ensuring uniform coverage and penetration into substrates. Moreover, solvent-based systems contribute to the enhancement of various material properties, including improved adhesion, flexibility, and resistance to external environmental factors.
Solvents used in polyurethane curing agents include Acetic ethyl ketone, butyl acetate, propylene glycol monomethyl ether acetate, acetone, toluene, mixed xylene, dichloromethane, and others. It depends on various factors to decide which solvent is suitable for the application including chemical compatibility with other components, volatility for drying time, viscosity.
Solvents play a crucial role in the curing agents, especially in solvent-based formulations. Generally, the main goal of using solvents is to reduce the viscosity for ease of application during construction. Rather than that, solvents help facilitate the dissolution and dispersion of isocyanates, promoting their effective reaction with hydroxy, such as polyols and resins. The addition of solvents also contributes to the enhanced properties of cured polyurethane products including improved adhesion, flexibility.
Components of solvent-based polyisocyanate curing agents
With the growing emphasis on environmental awareness, waterborne polyurethane materials have gained increasing attention. In two-component waterborne polyurethane products, the isocyanate component plays a crucial role. A common practice involves hydrophilic modification of isocyanates to achieve dispersion in water. The primary methods of modification include non-ionic modification, anionic modification.
Non-ionic modification involves the use of hydrophilic substances such as polyethylene glycol monomethyl ether (MPEG) and polyethylene glycol (PEG) to chemically modify polyisocyanates. The resulting modified polyurethane curing agent not only exhibits a certain degree of hydrophilicity but also encapsulates the residual isocyanate groups (NCO groups), enabling them to remain stable in water for a period of time. This modification contributes to the preparation of waterborne polyurethane products by ensuring the uniform dispersion of polyisocyanates in water and maintaining stability for a specified duration.
Components of Non-ionic Hydrophilic Curing Agents
Hydrophilic polyisocyanates can also be obtained through ionic modification. Common ionic modifications include carboxylate modification and sulfonate modification. Carboxylate modification typically involves using dimethylolpropionic acid (DMPA), while sulfonate modification generally utilizes ethylenediamine-based sulfonic acid salts and amino sulfonic acid salts. These compounds introduce carboxyl or sulfonic acid groups onto the molecular chain, allowing the polyurethane curing agent to emulsify in water.
Components of Aionic Hydrophilic Curing Agents
The NCO groups in the curing agent component exhibit high reactivity and can react with atmospheric moisture at room temperature, leading to degradation. To facilitate storage and minimize raw material loss, a closed-cell curing agent can be obtained by reacting a blocking agent with NCO, resulting in a stable curing agent at room temperature.
Upon heating, the closed-cell curing agent can revert to its original isocyanate structure and react with components containing hydroxyl groups. The formulation strategy typically involves introducing a blocking agent into common solvent-based and water-dispersible polyurethane curing agents, effectively capping excess NCO groups. Different blocking agents exhibit varying unblocking temperatures, allowing for tailored selection based on specific usage scenarios. Common blocking agents and their unblocking temperatures are detailed in the table below:
|Methyl ethyl ketoxime
Blocked polyisocyanate curing agents are commonly used in can&coil coatings. Curing occurs after heating up to the activation temperature, which forms a durable and stable structure with improved toughness, wear and scratch resistance.