Select Curing Agents for Coating Formulations

Curing, or crosslinking, is the mechanism in which a functional entity (single molecule or functionality part of an oligomer, pre-polymer or polymer) reacts during the polymerization process with the other elements to be incorporated in the new polymer chains system.

There are various crosslinking methods, and curing agents can improve the polymerization mechanism. Sometimes, curing agents are absolutely required to reach the final coating’s characteristics. Between all the varieties of crosslinking chemistries, learn how to select the best curing agent for your coating formulations.

Curing Agent, Its Types and Selection Process

Nowadays, in many coatings, the final film appears through a crosslinking reaction. This final film results from a reaction between the main polymer and a crosslinking or curing agent.

Select the Right Curing Agent for Coating Formulation

What is Curing Agent?

A curing agent can be defined as a substance able to participate in the chemical reaction between the oligomer, pre-polymer and polymer, to achieve the polymerization process and provide the final film.

It means that the curing agent is consumed by this reaction. Consequently, it will stay in the final film and participate to its chemical and mechanical properties. With the binders, the curing agent will help to build the 3D network. This mechanism can be further modified using a Catalyst Agent.

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As the curing agent is participating in the polymerization, mechanical and chemical properties of coating systems are different with and without the use of a curing agent or crosslinker. The table and figure below shows the comparison between the polymerization processes with and without the curing agent:

A + A → A-A-A 
No Curing Agent
With Curing Agent
Remarks Sometimes the polymerization cannot starts without a curing agent Curing agent reacts and participates in the final film build and give some specific properties, allow the 3D network

Polymerization Processes with and without Curing Agent

Continue reading or click to go on specific section of the page:

Need for Curing Agent

A curing agent, or crosslinker, allows the achievement of the polymerization process by reacting with the functionalities of the polymer material:

  • The curing agent stays in the polymer build after the reaction
  • Specific curing agents may bring some extra-properties to the system through the proper build of 3D network with the binder

Now, let's understand, in detail, what are the main types or curing agents/ crosslinkers used in coating formulations...

Curing Agents: General Types in Coatings

The choice for a curing agent is large. Their conclusions are not limited and can be enlarged to most of coatings systems. This is because, a same functionality may react with various curing agents. Different polymers may have similar functional sites (such as –OH or –COOH sites). Thus, we will focus on specificities of curing agents and not on the main polymer type.

The selection of the proper one is the key to achieve the required high-end quality coatings. Through the achievement of the proper 3D network, it will enhance the film properties.

And, as long as the delivery forms are fitting the paint system characteristics, the curing agents are used in either:

Applications of Curing Agents

Various types of curing agents used in coatings formulations are:

i. Amine Type Curing Agents

Amine types curing agents are derivative from ammonia (NH3). They are named according to the number of amine in a molecule, like: mono-, di-, tri- and polyamines. According to the type of hydrocarbon involved, they are classified as:

  1. Aliphatic - Aliphatic amine types curing agents can be selected to enhance the chemical resistance. In general they are low viscous and have a low color impact on the film. This allows a fast curing even at room temperature. Such products are:

    • Aminoethylpiperazine
    • Diethylenetriamine
    • Triethylenetetramine

    Through modifications they can have adjusted reactivity and targeted enhanced properties, such as:

    • Chemical resistance
    • Blush resistance
    • Water spot resistance, or
    • Adhesion

  2. Cycloaliphatic - Cycloaliphatic amines from IPDA are one of the most commonly used curing agents for their excellent influence on the chemical and mechanical properties. They can be used for low VOC coatings with:

    • Fast cure rate
    • Short pot life, and
    • Low temperature curing suitability

  3. Aromatic - Mannich type curing agents are usually amine types modified to get further improvement such as :

    • Enhanced chemical resistance
    • Influence the curing time, even under low temperatures
    • Good adhesion, even on wet surfaces
    • Longer pot-life

    Aromatic amines have better chemical resistance than aliphatic amines

    Melamine curing agent will react with the OH functionalities at high temperature (above 140°C). Mainly used in stoving curing systems, this reaction can be accelerated using a catalyst.

Reaction of Melamine with OH Functionality
Reaction of Melamine with OH Functionality 

Amine curing agents are recommended to cure the epoxy resins. The curing speed will depend on:

  • Type and dosage of the amine, and
  • Type of epoxy resin

» Select Optimum Amine Curing Agent for Epoxy Resin as per Your Need

In general, the dosage of the amine curing agent is optimal when the number of moles in epoxy groups is equal to that of active hydrogen. Shown below is the reaction of amine type curing agent with epoxy resin.

Reaction of Amine Type Curing Agent With Epoxy Resins
Reaction of Amine Type Curing Agent With Epoxy Resins 

The table below shows the amine type recommendation regarding the curing temperature for a DGEBA-type resin:

Temperature°C 40 100 110 125 145 160 200
Type Polyether amines Polyamides Ethylene amines
Cycloaliphatic Aromatic

Amine types curing agents are often accused to create some blushing. Indeed, as hygroscopic (absorbing moisture), they can react with moisture in the air to form ammonium carbamate products. These products are creating the undesirable blushing. Amines with a longer pot-life are more sensitive to blush. This is because, they are much slower to form oligomers with the epoxy resin. This allows more time to negatively interact with the humidity, instead of the epoxide groups.

ii. Amide Type Curing Agents

Polyamide curing agents are formed by the reaction between a fatty acid and a polyamine. They contain reactive primary and secondary amines in its molecules. Classical polyamides are providing more flexible films with a very good adhesion and water resistance. This family can be used in nearly all coatings type, included the powder coatings (dicyandiamide based agents).

» See Various Amide-type Curing Agent Available for Powder Coatings

Polyamide amine reacts with bisphenol-A-type epoxy resin to cure at or below normal temperature with moderate heat generation. The slow curing rate offers a long pot life.

Shown below is the reaction of polyamide type curing agents with epoxy resin:

Reaction of Polyamide Type Amine with Epoxy Resin
Reaction of Polyamide Type Amine with Epoxy Resin

They are less volatile and can be used in low VOC and High Pigment load formulations. They offer a good adhesion to concrete, and can enhance the corrosion resistance. Through proper modification, the adhesion to metal and impact resistance may be enhanced.

iii. Silane Type Curing Agents

Silane curing agents are based on silane compounds. Based on Silicon atom, they usually present two functionality classes:

  1. Hydrolysable group: Here R is typically alkoxy, acyloxy, halogen or amine. Following hydrolysis, a reactive silanol group is formed, which can form chemical bonds with inorganic materials such as glass or metal.
  2. Non-hydrolysable reactive group: Here X is typically amino, epoxy or vinyl group. It forms bonds with organic materials such as resins, to obtain specific characteristics.

Shown below is the general silane structure:
Silane Structure

Structure of Silane

  • Amino-functional silanes are used as classical amine curing agents for epoxy or urethane functional resins. They offer a good curing at low temperature and can improve the chemical and corrosion resistance, improving the substrate adhesion.

  • Vinyl-functional silane as curing agent can improve the substrate adhesion.

  • Hydrolyzed silanes are often used as waterborne epoxy curing agent.

Due to their versatility, they can react with various kinds of resin:
Curing of Acrylic Resin
Curing of Acrylic Resin
Curing of Styrene Unsaturated Polyester Resin
Curing of Styrene Unsaturated Polyester Resin
Curing of Polyurethane Resin
Curing of Polyurethane Resin
Curing of Epoxy Resins
Curing of Epoxy Resins

Silanes are generally used as curing or coupling agents. Corresponding functionality regarding the resin system can be summarized as follow:

Silane Functionality Acrylic Epoxy Melamine Phenolic
Unsaturated Polyester Polyurethane Rubber

iv. Isocyanate Type Curing Agents

Polyisocyanates are very effective curing agents. They are versatile and can by react with hydroxyle or amine functionalities to form polyurethane or polyurea chains. They are unfortunately sensitive to moisture. When unblocked, they react fast reducing the pot-life significantly.

With the various types of polyisocyanates available on the market, using polyisocyanate can enhance many parameters such as the mechanical and chemical film resistance.

Polyisocyanates are labeled, but may be a good choice in term of efficiency and reactivity.

Shown below are reactions of isocyanate type curing agent with different functionalities:

Reaction of ISocyanate with OH Functionality 
Reaction of Isocyanate with OH Functionality 

Reaction of ISocyanate with NH2 functionality
Reaction of Isocyanate with NH2 Functionality 

Reaction of ISocyanate with COOH Functionality
Reaction of Isocyanate with COOH Functionality 

v. Other Types of Curing Agents

Anhydride Type - Anhydrides used as curing agents are mainly used for epoxy electrical insulating materials. They severer curing conditions than amine-based curing agents, but are suitable for making large moldings. This is because they have a long pot life and form cured resins having electrical, chemical, and mechanical properties. Usually they are used with an amine to accelerate the curing.

Reaction of Anhydride curing agent with epoxy resin is shown below:

Reaction of Anhydride Curing Agents with Epoxy Resins

Reaction of Anhydride Curing Agents with Epoxy Resins 

Arizidine Type - Arizidine types are very effective and often used for their low impact on the pot-life. They are reacting with the carboxylic sites:

Reaction of Arizidine Curing Agent with Epoxy Resins

Reaction of Arizidine Curing Agent with Epoxy Resins 

Nevertheless, they are hazardous material and labeled as:

Hazardous Material Label

Polymercaptan Type - Polymercaptans are used to cure epoxy resin rapidly at ambient temperature. They are recommended when a fast curing is required (pot-life from 2 to 30 minutes!). They provide excellent results in thin film build. They present:

  • Low toxicity compared to other hardeners
  • Lighter color
  • Excellent adhesion, and
  • Ability to accelerate other amines and polyamides (that is why they are commercially available with and without amine accelerator)

Unfortunately, as fast curing agents, they often provoke some skinning and generally have a strong odor.

vi. Crosslinkers for Water-Based Systems

The most commonly used crosslinkers for water-based systems are:

  1. Polyaziridine - Polyaziridine reacts under ambient temperature with the carboxylic acid groups, and then can be used in various systems. This reaction is quite slow. Even if the reaction allows a longer pot-life, some temperature curing helps in curing the system. Polyaziridine are sensitive to water, and the formulation needs a quite high pH (over 8.0) to stay stable. Unfortunately, they are labelled materials and their toxicity often causes some allergic reactions.

  2. Search Commercially Available Crosslinkers for Your Water-based Coatings System

    Crosslinkers for Water-based Coating Systems

  3. Carbodiimide - Carbodiimide also reacts with the carboxylic acid groups at ambient temperature. This alternative has been developed to replace the toxic polyaziridine. Unfortunately they still contain some toxic material (rest of monomeric isocyanate due to the manufacturing process) and are expensive alternatives.

  4. Polyisocyanates - Regarding the polyisocyanates curing agents, the isocyanate groups will react with the hydroxyl and carboxyl groups. Side reaction with water will generate an amine. This will react further with isocyanate to form a partially interpenetrating network. This reaction with water will also generate some CO2 which may cause some gassing issue such as entrapped bubbles or disturbed surface.

    To avoid the unwanted water interaction with the isocyanate, some water dispersible products exist. There, the isocyanate functionality is “blocked” and becomes less sensitive to water.

  5. Silane - Silane curing technology requires the use of stabilized products. Indeed, silane agent is sensitive to water and will form some unstable silanol groups. Controlled pH and co-solvents may help in improving the silanol stability, and then make the product usable in a coating formulation. Usually, alcoxysilanes are used, but often requires the use of acid or alkali as catalyst.

To avoid the negative impact of hydrolysis, prior to the application they have to be:

  • Separated as a second component, and
  • Mixed with the resin containing part

The crosslinking reaction takes place upon drying of the coating, when pH drops because of evaporation of the neutralizing agent.

Select the Right Curing Agent for Coating Formulation

Curing agents are an essential component in many formulations. The large choice of curing agents available on the market offers numerous possibilities to the formulator. It makes possible the selection of the right product for any specific system, at the right cost.

Testing a curing agent with care may not only save time and energy on the long-term production. But also improve the paint characteristics to achieve the high-end performance system.

Selecting the right curing agent technology and modification empowers the manufacturer to:

  • Improve operational efficiency
  • Reduce production costs, and
  • Manipulate the finished coating's characteristics

The table below summarizes the characteristics of each curing agent type used in coating formulations:

Characteristic Reactive with Moisture sensitivity Impact on Pot-life Recommended for Water-based Systems
Amine Type -OH Epoxy Medium Medium No
Amide Type Epoxy Very High Medium Yes
Isocyanate Type -OH / -NH2 /-COOH Very High High Yes
Silane Type Depends on the functionality Yes
Anhydride Type Epoxy Epoxy Very High No
Aziridine Type -COOH Medium Low Yes
Mercapto Type Epoxy Medium Very High No

Various curing agents exists, and each family has its own properties, and can be further adapted to fit your requirements.

The selection of a curing agent will be determined by:

  • The chemical nature and functionality of the resin system
  • The end-use of the paint
  • The application conditions

Important Characteristics to Test Curing Agents

When testing and evaluating a new curing agent may be fastidious, it is possible to focus on critical points to reduce the time and material spent on the study keeping a high pertinence of results.

Before leading some complete tests, first the correct curing agent dosage has to be determinate. When a stoichiometric ratio of 1:1 between the curing agent functionality and the binder reactive group seems a good compromise, some specific wanted characteristics may appear only when this ratio is not respected.

For instance, an off-ration (under cured system) may give a more flexible film, when an over cured system may improve the chemical resistance.

Additionally, it is highly recommended to prevent moisture. This is due to the high risk of curing agent side reaction that may lead to abnormal results and unwanted negative side effects.

Other improtant characteristics include:

Important Characteristics to Test Curing Agents
  • Viscosity: In 1K system stability or in 2K systems to determine the pot life, viscosity evaluation is a key point. That too in time and under specific temperature. Even in 1K systems, due to the different interactions in the paint, a curing agent may have a latent action

  • Gloss: As the curing agent participates in the reaction, the film gloss may be influenced

  • Hardness: As the curing agent participates in the reaction, the film hardness will also be influenced

  • Impact test: As the curing agent participates in the reaction, flexibility of the film may be influenced

  • Chemical resistance: Solvent, acid, alkali, water…

  • Influence on the color, blushing: As the curing agent participates in the reaction, yellowing may faster occur especially for stoving systems. Moreover using an amine curing agent may show some blushing

  • And of course: The general aspect of the film

Besides all these considerations, the right reaction process using the right curing agent will also depends on important factors, such as:

  • The functionality of curing agent
  • The functionality of functional binder
  • The dosage and type of curing agent
  • Temperature and curing time
  • Moisture

» View All Commercially Available Curing Agent/Crosslinkers Grades in Coatings Database

This coatings database is available to all, free of charge. You can filter down your options by suitable resin, system or application (coatings, inks...), supplier and regional availability.

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