What is Curing Agent?
What is Curing Agent?
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.
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 in 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.
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 show the comparison between the polymerization processes with and without the curing agent:
| Reaction |
A + A → A-A-A
No Curing Agent
|
A + B → ABA-ABA
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 |
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 a 3D network with the binder
Now, let's understand, in detail, what are the main types of curing agents/crosslinkers used in coating formulations.
Types of Curing Agents in Coatings
Types of Curing Agents 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, the 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 the 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: waterborne coatings, solvent-borne coatings, powder coatings and radiation curing coatings.
Various types of curing agents used in coatings formulations are:
Amine Type Curing Agents
Amine Type Curing Agents
Amine types curing agents are derivatives from ammonia (NH3). They are named according to the number of amines in a molecule, like: mono-, di-, tri- and polyamines. According to the type of hydrocarbon involved, they are classified as:
- 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 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:
- Cycloaliphatic - Cycloaliphatic amines from IPDA are one of the most commonly used curing agents for their excellent influence on chemical and mechanical properties. They can be used for low VOC coatings with:
- Fast cure rate
- Short pot life, and
- Low temperature curing suitability
- 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
Amine curing agents are recommended to cure 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
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 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.
Amide Type Curing Agents
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, including the powder coatings (dicyandiamide based agents).
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
They are less volatile and can be used in low VOC and High Pigment load formulations. They offer good adhesion to concrete, and can enhance corrosion resistance. Through proper modification, the adhesion to metal and impact resistance may be enhanced.
Silane Type Curing Agents
Isocyanate Type Curing Agents
Isocyanate Type Curing Agents
Polyisocyanates are very effective curing agents. They are versatile and can react with hydroxyl 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 mechanical and chemical film resistance.
Polyisocyanates are labeled, but may be a good choice in terms of efficiency and reactivity.
Shown below are reactions of isocyanate type curing agents with different functionalities:
Reaction of Isocyanate with OH Functionality
Reaction of Isocyanate with NH2 Functionality
Reaction of Isocyanate with COOH Functionality
Other Types of Curing Agents
Other Types of Curing Agents
Anhydride Type - Anhydrides used as curing agents are mainly used for epoxy electrical insulating materials. They have severe 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
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
Nevertheless, they are hazardous material and labeled as:
Polymercaptan Type - Polymercaptans are used to cure epoxy resin rapidly at ambient temperature. They are recommended when 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.
Crosslinkers for Water Based Systems
Crosslinkers for Water Based Systems
The most commonly used crosslinkers for water-based systems are:
- 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.
- 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.
- 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 issues such as entrapped bubbles or disturbed surfaces. 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.
- 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, alkoxysilanes are used, but often require the use of acid or alkali as a 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
Select the Right Curing Agent
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 exist, 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
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 important characteristics include:
- 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, the 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 depend 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
Crosslinking/Coupling/Curing Agents for Paints & Coatings
View a wide range of crosslinking/coupling/curing agents available in the market today, analyze technical data of each product, get technical assistance or request samples.