What is High Solid Content PU Curing Agent and Why Do We Use Them?

Let us first understand what is a curing agent? A curing agent is a chemical substance used to promote or control the curing process of materials like coatings, adhesives, and resins. Curing is the process where a material changes from a liquid or viscous state to a solid state. The curing agent plays a crucial role in this process by initiating chemical reactions, such as cross-linking or polymerization, to solidify the material.

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Types of Curing Agents

• Epoxy Curing Agents: Used for curing epoxy resins. Common types include amine-based and anhydride-based curing agents. They react with epoxy resins to form a three-dimensional network structure, enhancing the mechanical strength and chemical resistance of the resin.
• Polyurethane Curing Agents: Used for curing polyurethane coatings or adhesives. They typically react with isocyanates to form polyurethane materials, which offer excellent abrasion resistance, flexibility, and weather resistance.
• Acrylic Curing Agents: Mainly used in acrylic coatings or resins. They cure through ultraviolet (UV) light or free radical initiation, commonly found in fast-drying coatings or adhesives.
• Silicone Resin Curing Agents: Used in silicone resin coatings or sealants. They react with silicone resins to form a cross-linked network, providing excellent heat resistance, weather resistance, and insulation properties. Refer to SIO-593 Waterborne Organosilicon Crosslinker.

The choice of curing agent depends on application requirements such as curing speed, durability, chemical resistance, and processing conditions. They are widely used in coatings, electronic encapsulation, composites, and construction materials.

SIOResin’s Curing Agent Products

Now let’s take a look at our company’s curing agent product introduction.

Aziridine Crosslinker Agent | SIOResin ®​​ H-

Aziridine is a special type of curing agent used for cross-linking reactive groups (like carboxyl and hydroxyl groups). It is a highly reactive compound commonly used in water-based coatings, adhesives, and inks to improve abrasion resistance, chemical resistance, and adhesion.

Our Aziridine curing agents H- are cross-linking agents that primarily react with acidic or active hydrogen groups (such as carboxyl, hydroxyl, or amide groups) to form durable three-dimensional network structures. This cross-linking significantly enhances the mechanical strength, solvent resistance, and water resistance of materials.

Common applications of aziridine curing agents:

• Water-based Coatings: Aziridine reacts with carboxyl groups in coatings to enhance chemical resistance, abrasion resistance, and adhesion.
• Inks: Used to improve drying speed, scratch resistance, and adhesion of inks.
• Adhesives: Improves strength and durability of adhesives.

Note that aziridine is toxic, so you must observe safety precautions and carefully follow handling procedures.

Carbodiimide Crosslinker | SIOResin ®​​ H-

Carbodiimide is a cross-linking curing agent used for reacting with carboxyl (-COOH) compounds. It is commonly used in water-based coatings, adhesives, and textile coatings to enhance water resistance, heat resistance, and chemical resistance.

Characteristics of carbodiimide curing agents:

• Carboxyl Cross-Linking Reaction. Carbodiimide reacts with carboxyl groups in polymers to form covalent bonds. This reaction does not require a catalyst and can occur at room temperature or lower, making it suitable for heat-sensitive materials.
• Water-Based Systems. Often used in water-based coatings and adhesives as a non-isocyanate cross-linking agent, providing good cross-linking effects without affecting storage stability.
• Water and Chemical Resistance. Improves water resistance, chemical resistance, and moisture resistance, widely used in textile coatings and paper coatings.
• Biomaterials Modification. Used in surface modification of biomaterials, such as proteins and polymers, promoting cross-linking between carboxyl and amino groups.

Isocyanate Curing Agents | SIOResin ®​​ H-/H-S

Isocyanate curing agents are widely used in polyurethane coatings, adhesives, and elastomers. The main active component is the isocyanate group (-NCO), which reacts with compounds containing active hydrogen (such as hydroxyl, amino, or carboxyl groups) to form strong cross-linked structures, thus curing the material.

Applications:

• Polyurethane Coatings: Forms polyurethane coatings with excellent abrasion resistance, chemical resistance, and elasticity, used in automotive coatings, floor coatings, wood finishes, and metal protection coatings.
• Adhesives: Provides excellent adhesion and durability in adhesives, suitable for structural bonding and elastic sealing.
• Elastomers: Used in manufacturing polyurethane elastomers with good mechanical strength and flexibility, applicable in seals and gaskets.
• Isocyanates are toxic and can irritate the skin, eyes, and respiratory system, so protective measures are necessary during handling to avoid direct contact or inhalation of vapors.

Blocked Isocyanate Crosslinker | SIOResin ®​​ H-

Blocked isocyanates are chemically modified isocyanate curing agents where the isocyanate groups (-NCO) are “blocked” by specific blocking agents. This makes them stable at room temperature, preventing reaction with moisture or other active hydrogen compounds. The isocyanate groups become reactive again when exposed to higher temperatures, typically between 100°C and 180°C. The blocking agent decomposes and releases the free isocyanate groups to cross-link with resins or other compounds.

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The characteristics of blocked isocyanates:

• Stable at Room Temperature. Blocked isocyanates do not react at room temperature, allowing them to be stored with hydroxyl or amino group-containing compounds without premature curing.
  
• Thermal Unblocking. At elevated temperatures, the blocking agent decomposes, releasing active isocyanate groups. This restores the chemical activity of the isocyanate to start the cross-linking reaction with active hydrogen compounds, forming a cross-linked network.

Applications:

• Powder Coatings: Used in thermosetting powder coatings. During the baking process, the isocyanate unblocks and reacts with resins to form a durable polyurethane coating with excellent weather resistance and mechanical properties.
• Water-Based Coatings: Mixed with water-based resins containing hydroxyl or amino groups, and cross-links during heating to improve water resistance, chemical resistance, and adhesion.
• Adhesives: Applied in hot-melt adhesives and structural adhesives, where heating activates the curing process for quick solidification under specific conditions.

Amino Crosslinker | SIOResin ®​​ H-

Amine curing agents (crosslinkers) widely serve in coatings, adhesives, and composites, especially in epoxy resin systems. They contain active amine groups (-NH₂) or other amine groups that react with epoxy groups (-CH-CH₂-O-) in resins to form a cross-linked structure, achieving curing.

Reaction Principle

Amine groups in curing agents react with epoxy groups in resins to form new hydroxyl and amine compounds. Because amine groups typically have multiple reactive sites. They can react with multiple epoxy groups, forming a three-dimensional network structure that enhances the strength, toughness, and chemical resistance of the material.

Applications of amine curing agents:

• Anti-Corrosion Coatings: Widely used in anti-corrosion coatings, especially for ships, chemical pipelines, storage tanks, and bridges requiring corrosion resistance and high strength.
• Structural Adhesives: Provides high strength and good bonding performance in epoxy resin adhesives, suitable for construction, automotive, and aerospace applications.
• Composites: Aromatic amine curing agents are used in high-performance composites, such as those for aerospace components, wind turbine blades, and sports equipment.

About SIOResin (SIO New Material)

Have you been confused about the term "Solid Content"? Or ever felt frustrated of even overwhelmed in conversation with a sales rep, and they keep throwing out terms and numbers like 70, 85, acrylic, polyaspartic, 90, polyurethane, epoxy, 100%, and so on until you get are at the point of pulling your hair out and shouting "What does it all Mean"!? While all these terms and numbers are important, the concept does not have to be that difficult. In fact, it is a fairly easy theory to gasp if we start from the beginning.

The first thing we need to understand is the difference between the solids and the carrying agent. This is what all the numbers were referring to. 

The Solids-The word "Solid" is defined as; a substance or object that is solid rather than liquid or fluid. So, obviously that bucket of cure & seal at your shop or the kit of epoxy you just picked for your next job is a liquid form and is definitely not solid. This is why we use the term "solid content" to describe the part of liquid that will become solid after it has been applied and had the proper time to harden. For example, if you are applying a coating that has a solid content of 85%, only 85% of that liquid will stay behind as solid film and 15% will evaporate off. If you are spraying an acrylic sealer that is 25% solids, 75% of whatever you sprayed on the concrete will evaporate and only 25% will actually form a solid film. 

The Carrying Agent- So what about the part that evaporates? What does that part do, And why do we need it? This part of liquid is referred to as the carrying agent or the carrier. The Solid content of concrete sealers and coatings are made up of a variety of different resins that all need to be spread out at a certain rate per square foot. These different resins also have differences in their workability, and some of them wouldn't be usable in their pure form. This is where the carrier comes in to play. It is what allows you to work with the resin and spread it out at the proper rate. The main two carrying agents found in concrete sealers and coatings are water and solvents. Most of the odor associated with sealers and coatings come from the carrier, this is why you generally see most of the low odor options to be water based. Although in the concrete coatings world things are changing fast and we now have a great low odor solvent based option as well. 

Understanding the relationship between solid content and carrying agents helps to paint a clearer picture of why a concrete sealer or coating will behave in a certain manner. Things like dry time, breathability, sheen, bond, and workability are all affected by the same amount of solids that a coating or sealer contains. Over the next few weeks, we will take a deeper look at the different kinds of carrying agents as well as sealers and coatings. 

-Jeff Hershberger

Please feel free to send us an with any questions you might have about concrete sealers and coatings.

Are you interested in learning more about High Solid Content PU Curing Agent(ru,bn,th)? Contact us today to secure an expert consultation!