Importance of Crosslinking or Curing of Formulation
Functionality of Polymer - Role in Curing
Functionality of Polymer - Role in Curing
The functionality of a polymer is the number of bonds a molecule can form with other molecules in a reaction. Only reactions between polyfunctional monomers can lead to thermosetting polymers. Reactions between monofunctional monomers can double in size but they do not grow to large polymers with repeating units. Because of their Cross-linking, thermoset polymers are infusible, insoluble, and dimensionally stable under load. These properties make thermoset polymers useful as load bearing structural adhesive.
Thermoset polymers consist of a continuous network of polymer chains that are crosslinked. The cross-links are formed by reaction of one polymer molecule with another or with a second polymer. These reactions can occur either at room or elevated temperatures, and there are catalysts that can accelerate the reaction mechanism.
Basically, cross-linking can be achieved either by:
- Applying radiation to the polymer, or
- Adding agents into the mix to promote chemical cross-linking.
The connecting chains of the thermoset polymer are held together with the same primary covalent bonds as are the atoms in the main chain. Thus, the molecules are interlocked and do not slip easily by one another.
Curing agents are, therefore, an important group of additives that influence both end-properties and curing. They can initiate the cure by catalyzing and promoting, or they can control the cure by accelerating or retarding it.
Selecting the Right Curing Agent
Selecting the Right Curing Agent
A number of common curing agents are known but each one has a unique chemical and physical properties along with their applications. Shown below are some curing agents and their applications:
Curing Agent |
Features & Applications |
Acrylates / Methacrylates |
- Primary use is as a cross-linking agent in acrylic pressure sensitive adhesives, or
- As a cross-linking agent for free radical polymerization in reactive adhesives including radiation (UV/EB) cured systems.
|
Amines / Amides |
- Primary use is as a cross-linking agent in polymers in reactive formulations such as curing agent for epoxy and polyurethane.
- Family can be subdivided into aliphatic amidoamine, aromatic, cycloaliphatic, dicyanomines and polyamine types.
|
Anhydrides
|
- Primary use is as a cross-linking agent in reactive epoxy structural adhesives.
- Requires long, elevated temperature cure. Has a critical mix ratio.
|
Azridines
|
- cross-links with various organic groups containing active hydrogen (e.g., carboxylic acids, alcohols, amines, mercaptans, epoxides, isocyanates, etc.).
- Used at levels of 2%-4% by weight of polymer system. Generally used in acrylic or polyurethane emulsions.
- Works at room temperature.
|
Carbodiimides
|
- Carbodiimides react with carboxylic acid and amine groups on adhesive resins.
- Carbodiimides can be used in 1-pack adhesives and provide up to a 90 day pot-life.
- They can also be used in 2 component adhesive systems, and can react at room temperature.
|
Diols / Triols
|
- These polyurethane crosslinkers are low molecular weight diols or triols, and
- Diamines which react with an isocyanate-terminated prepolymer to produce polyurethane (cross-linked).
|
Isocyanates
|
- There is a wide range of isocyanates that allow formulators to produce high quality polyurethane and polyurea adhesives.
- Isocyanates contain reactive NCO groups which react with hydroxyl groups on polyols to form polyurethanes and with amines on polyamines to form polyureas.
|
Peroxides / Persalts |
- Peroxides are used as catalysts for unsaturated polyester resins and other free radical curing resins.
- They generate free radicals to cause cross-linking and act at either elevated or ambient temperature.
|
Silanes |
- A silane is a molecule containing a central silicon atom bonded to two types of groups: Alkoxy groups and organo-functional groups.
- These two types of groups exhibit different reactivity and allow sequential reactions.
- In the cross-linking process, the first step is generally the grafting of the silane to the polymer backbone and, finally, the linking of the polymer chains via condensation of silanols.
- Most often used with polyurethane adhesives and to form polyurethane prepolymers.
|
Vinyl / Allyl Compounds |
- These are generally diallyl esters of linear dicarboxylic acids, and they are used as multi-functional cross-linking agents for vinyl polymers.
- These are often used in waterborne pressure sensitive adhesives.
|
Vegetable-based |
- Generally plant-based curing agents such as fatty oil acid or dicarboxylic acid.
- Also, diols and triols.
|
The appropriate curing agent selection process is dependent on:
- The base polymer
- The compounding processes used by the formulator
- The processing (curing) conditions employed by the end-user, and
- The physical and chemical service requirements of the application
Find out which curing agent chemistry is best for your formulation according to base polymer, end use application, reactivity, industry and available physical form below.
Curing Agents & Base Polymer Compatibility
Curing Agents & Base Polymer Compatibility
The curing agent must be compatible with the base polymer and other ingredients in the formulation. It must react with the functional groups provided on the base polymer. Tables shown below provide a guide that matches the appropriate curing agent family with the base polymer that is used in the formulation.
Note that this guide only facilitates the formulator to getting into the right ballpark. Within each family there are numerous derivations, and these various materials will affect the formulation even though they are within the same chemical family.
Base Polymer ↓ |
Curing Agents |
Acrylates / Meth-acrylates |
Amines / Amides |
Anhydrides |
Aliphatics |
Amido-amines |
Cyclo-aliphatics |
Aromatics |
Dicyano-amines |
Poly-amines |
Acrylics and acrylic copolymers |
✔ |
|
|
|
|
|
|
|
Epoxies(EP) |
|
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
Polyamide |
|
✔ |
|
✔ |
|
|
|
|
Polychloro-vinyls (PVC, PVDC) |
✔ |
|
|
|
|
|
|
|
Polyesters |
✔ |
|
|
|
|
|
|
✔ |
Polyimides |
|
✔ |
|
|
|
|
|
|
PU - prepolymers |
|
✔ |
|
|
✔ |
|
✔ |
✔ |
PU - isocyanates |
|
✔ |
|
|
✔ |
|
✔ |
✔ |
PU - thermoplastics |
|
✔ |
|
|
✔ |
|
✔ |
|
PU - dispersions |
|
✔ |
|
|
✔ |
|
✔ |
|
PU - polyols |
|
✔ |
|
|
✔ |
|
✔ |
|
Sulfone polymers |
|
|
|
|
✔ |
|
✔ |
|
Base Polymer ↓ |
Curing Agents |
Azridines |
Carbodi-imides |
Diols / Triols |
Isocyanates |
Peroxides / Persalts |
Silanes |
Vinyl / Allyl Compounds |
Vegetable Based |
Acrylics and acrylic copolymers |
✔ |
|
|
✔ |
|
✔ |
|
|
Amines |
|
|
|
|
|
✔ |
|
|
Aminoplasts / Phenoplasts (UF, MUF) |
|
|
|
|
|
✔ |
|
|
Epoxies (EP) |
|
|
✔ |
|
✔ |
✔ |
|
|
Ethylene copolymers - emulsions (VAE, EVC) |
|
|
|
|
|
✔ |
|
|
Natural polymers |
|
|
|
|
|
|
|
✔ |
Natural rubbers (NR) |
|
|
|
✔ |
✔ |
|
|
|
Polyamide |
|
|
|
|
|
✔ |
|
|
Poly-caprolactones |
|
|
|
✔ |
✔ |
|
|
|
Poly-chlorovinyls (PVC, PVDC) |
|
✔ |
|
✔ |
✔ |
✔ |
|
|
Polyesters |
|
|
|
✔ |
✔ |
✔ |
✔ |
|
Polyimides (PI) |
✔ |
|
|
|
|
✔ |
|
|
Polyolefins (PO) |
|
|
✔ |
|
|
✔ |
✔ |
|
Polysulfides (PS) |
|
|
|
|
|
✔ |
|
|
Polyurethane - prepolymers |
✔ |
|
✔ |
✔ |
|
✔ |
|
|
PU - isocyanates |
|
|
✔ |
✔ |
|
✔ |
|
|
PU - thermo-plastics |
|
|
|
✔ |
|
|
|
|
PU - dispersions |
✔ |
|
|
✔ |
|
|
|
|
PU - polyols |
|
|
|
✔ |
|
✔ |
|
|
Polyvinyl acetate emulsions (PVAc) |
|
|
|
|
|
✔ |
|
|
Polyvinyl alcohols (PVOH) |
|
|
|
✔ |
|
|
|
|
Silicones (Si) |
|
|
|
|
|
✔ |
✔ |
|
Silyl modified polymers (SMP) |
|
|
|
|
|
✔ |
|
|
Styrene copolymers (SBR, SBC) |
|
|
|
|
|
✔ |
|
|
Synthetic-rubbers |
|
|
✔ |
✔ |
✔ |
✔ |
|
|
Curing Agents Selection Based on Type of End-Use Product
Curing Agents Selection Based on Type of End-Use Product
The selection of curing agent will depend on the type of adhesive / sealant that is being formulated. These end-uses include hot melt, solvent based, waterborne, reactive (100% solids), radiation curing (UV/EB), anaerobic and film / web.
Table below provides the various types of adhesives or sealants that are generally formulated with curing Agents:
Base Polymer ↓ |
Curing Agents |
Acrylates / Meth-acrylates |
Amines / Amides |
Anhydrides |
Aliphatics |
Amido-amines |
Cyclo-aliphatics |
Aromatics |
Dicyano-amines |
Poly-amines |
Solvent-based |
✔ |
|
|
|
|
|
|
✔ |
Water-borne |
✔ |
|
|
|
|
|
|
|
Reactive |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
Radiation curing (UV/EB) |
✔ |
|
|
|
|
|
|
|
Anaerobic |
✔ |
|
|
|
|
|
|
|
Film / web |
|
✔ |
✔ |
|
✔ |
✔ |
|
|
Adhesive Type ↓ |
Curing Agents |
Azridines |
Carbodi-imides |
Diols / Triols |
Isocyanates |
Peroxides / Persalts |
Silanes |
Vinyl / Allyl Compounds |
Vegetable Based |
Solvent-based |
|
|
✔ |
✔ |
✔ |
|
|
✔ |
Water-borne |
✔ |
✔ |
|
✔ |
|
✔ |
✔ |
|
Reactive |
|
|
|
✔ |
✔ |
✔ |
|
✔ |
Radiation curing (UV/EB) |
|
|
|
|
|
✔ |
✔ |
|
Hotmelt |
|
|
|
✔ |
|
✔ |
|
|
Film / web |
|
|
|
✔ |
|
✔ |
|
|
Curing Agent Selection According to the Reactivity
Curing Agent Selection According to the Reactivity
Curing agents have various reactivities. Some can cure at room temperature, some require elevated temperature cures, and others are suited for radiation cure. Once added to the formulation, pot life can be an issue depending on the type of application.
The pot life must provide sufficient time for application and mating of the substrates before the bond begins to cure. There are also latent curing agents that become activated only on application of temperature. With these systems, the pot life can be very long until the curing agent becomes activated by temperature.
Table below shows the various reactivities that are associated with common curing agents:
Reactivity ↓ |
Curing Agents |
Acrylates / Meth-acrylates |
Amines / Amides |
Anhydrides |
Aliphatics |
Amido-amines |
Cyclo-aliphatics |
Aromatics |
Dicyano-amines |
Poly-amines |
Room temperature |
✔ |
✔ |
✔ |
|
✔ |
|
✔ |
|
Elevated temperature |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
Radiation cure (UV/EB) |
✔ |
|
|
|
|
|
|
|
Pot life (< 1 hr) |
|
✔ |
✔ |
|
✔ |
|
✔ |
✔ |
Pot life (> 1 hr) |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
|
✔ |
Latent (temperature activated) |
✔ |
|
|
|
|
✔ |
|
|
Reactivity ↓ |
Curing Agents |
Azridines |
Carbodi-imides |
Diols / Triols |
Isocyanates |
Peroxides / Persalts |
Silanes |
Vinyl / Allyl Compounds |
Vegetable Based |
Room temperature |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
Elevated temperature |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
Radiation cure (UV/EB) |
|
|
|
|
✔ |
|
|
|
Pot life (< 1 hr) |
|
|
|
✔ |
✔ |
|
|
✔ |
Pot life (> 1 hr) |
✔ |
|
✔ |
✔ |
|
✔ |
|
✔ |
Latent (temperature activated) |
|
|
|
✔ |
|
✔ |
|
|
Curing Agents Selection According to Industry
Curing Agents Selection According to Industry
Certain curing agents have found significant use in specific industries. This is primarily related to the base polymer that is used, but it also depends on the reactivity, form, etc. of the curing agent.
Table below shows the curing agents that are generally approved in various adhesives / sealant industries:
Industry ↓ |
Curing Agents |
Acrylates / Meth-acrylates |
Amines / Amides |
Anhydrides |
Aliphatics |
Amido-amines |
Cyclo-aliphatics |
Aromatics |
Dicyano-amines |
Poly-amines |
Adhesives |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
Sealants |
✔ |
✔ |
✔ |
|
|
|
✔ |
✔ |
Automotive |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
|
Building & construction |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
Consumer / household / office |
✔ |
|
✔ |
|
|
|
✔ |
|
Electrical & electronic |
✔ |
|
|
|
|
|
|
✔ |
Footwear & leather |
✔ |
|
✔ |
|
|
|
✔ |
|
Industrial assembly |
|
|
|
|
|
|
|
✔ |
Labels / signs / decals |
✔ |
|
|
|
|
|
|
|
Medical |
✔ |
|
✔ |
|
|
|
|
|
Non-woven |
✔ |
|
✔ |
|
|
|
|
|
Paper and packaging |
|
|
|
|
|
|
|
|
Tapes |
✔ |
|
|
|
|
|
|
|
Transportation (excluding automotive) |
|
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
|
Industry ↓ |
Curing Agents |
Azridines |
Carbodi-imides |
Diols / Triols |
Isocyanates |
Peroxides / Persalts |
Silanes |
Vinyl / Allyl Compounds |
Vegetable Based |
Adhesives |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
✔ |
Sealants |
✔ |
|
✔ |
✔ |
|
✔ |
✔ |
✔ |
Automotive |
|
|
|
✔ |
|
✔ |
|
✔ |
Building & construction |
✔ |
|
✔ |
✔ |
|
✔ |
|
✔ |
Consumer / household / office |
✔ |
✔ |
|
|
|
✔ |
|
|
Electrical & electronic |
|
|
✔ |
|
|
✔ |
|
|
Footwear & leather |
|
|
|
✔ |
|
✔ |
|
|
Industrial assembly |
|
✔ |
|
|
|
✔ |
|
|
Paper and packaging |
|
✔ |
|
✔ |
|
✔ |
|
|
Tapes |
✔ |
✔ |
|
|
|
|
|
|
Transportation (excluding automotive) |
|
|
✔ |
|
|
✔ |
|
|
Wood and related industries |
|
✔ |
|
✔ |
|
✔ |
|
✔ |
Available Physical Forms of Curing Agents