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Adhesives Ingredients

Select Curing / Vulcanizing Agents for Adhesives

The transformation of a liquid resin to a solid adhesive or sealant is achieved by the addition of chemically active compounds known as curing, cross-linking, or vulcanizing agents. Cross-linking increases the thermal stability and mechanical properties of the polymer. Cross-linking, curing, or vulcanizing agents are substances that are added to a polymeric resin to assist the curing reaction.

Does Cross-Linking affects the mechanical strength of a sealant? Are there some important factors that need to be considered for selecting vulcanizing agents?

Explore the Chemistry behind cross-linking in detail and learn the criterion of selecting right curing agent/vulcanizing agents for your adhesive formulation.

Vulcanizing Agents - Chemistry & Selection


TAGS:   Crosslinking / Curing / Vulcanizing Agents    

Crosslinking Agents for AdhesivesCrosslinking or curing is the formation of chemical links between molecular chains to form a three dimensional network of connected molecules.

The crosslinking or Curing Agent is the formulation component that causes this reaction to take place. These terms are generally used with all polymeric materials that are capable of forming a thermosetting structure.

The crosslinking of rubber with sulfur is called vulcanizing agents for rubber. Crosslinking bonds the chains together to form a network. The term “Vulcanizing Agent is generally used with rubber or elastomers.

Crosslinking agents in polymers are multifunctional chemical compounds,that react with functionality of the molecular chains in the base polymer and thereby form a thermoset of three-dimensional polymeric materials.

The curing agents are of di- or higher functionality, and they become an integral part of the final thermoset Curing Vulcanizationmaterial except for materials lost in a condensation crosslinking process. These crosslinking agents can range from low molecular weight to polymeric materials. Crosslinking causes changes in physical and chemical properties. It causes changes in hardness, tensile strength, modulus, elongation, solution, swelling and other properties.

The resulting product is called a thermoset, because it does not flow on heating. Crosslinking increases the thermal stability and mechanical properties of the polymer. It provides important effects on both the curing features and on the fundamental properties of the adhesive or sealant system.

The figure below illustrates a crosslinked resin and shows how the number of crosslink sites, the length of the crosslinking agent molecules, and the distance between crosslinks on the base polymer affect the properties of the final adhesive.


Functionality of Polymer

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 crosslinking, thermoset polymers are infusible, insoluble, and dimensionally stable under load. These properties make thermoset polymers useful as load bearing structural adhesive.

Thermoset polymers


Thermoset polymers consist of a continuous network of polymer chains that are crosslinked. The crosslinks 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, crosslinking can be achieved either by applying radiation to the polymer or by adding agents into the mix to promote chemical crosslinking. 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.

crosslinking

crosslink


Effect of crosslinking on the final properties of the adhesive / sealant


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


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
Applications
Acrylates / Methacrylates
  • Primary use is as a crosslinking agent in acrylic pressure sensitive adhesives, or
  • As a crosslinking agent for free radical polymerization in reactive adhesives including radiation (UV/EB) cured systems.
Amines / Amides
  • Primary use is as a crosslinking 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 crosslinking agent in reactive epoxy structural adhesives.
  • Requires long, elevated temperature cure. Has a critical mix ratio.
Azridines
  • Crosslinks 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 crosslinking 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 crosslinking 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 multifunctional crosslinking 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.


Now lets considers the Criteria for Selecting the Right Curing Agent

The appropriate curing agent used in selection 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

Base Polymer


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: Aliphatics
Amines/ Amides: Amido-amines
Amines/ Amides: Cyclo-aliphatics
Amines / Amides: Aromatics
Amines/ Amides: Dicyano-amines
Amines/ Amides: Poly-amines
Anhydrides
Acrylics and acrylic copolymers

Epoxies(EP)

Polyamide
Polychloro-vinyls (PVC, PVDC)
Polyesters
Polyimides (PI)
Polyurethane -prepolymers
Polyurethane -isocyanates
Polyurethane-thermoplastics
Polyurethane -dispersions
Polyurethane -polyols
Sulfone-
based polymers


Base Polymer
Curing Agent
Azridines
Carbodiimides
Diols /
Triols
Isocyanates
Peroxides /Persalts

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
Polyurethane- isocyanates
Polyurethane- thermoplastics
Polyurethane-dispersions
Polyurethane-polyols
Polyvinyl acetate emulsions (PVAc)
Polyvinyl alcohols (PVOH)
Silicones (Si)
Silyl modified polymers (SMP)
Styrene copolymers (SBR, SBC)
Synthetic-rubbers


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:

Type product

Curing Agents
Acrylates /
Methacrylates
Amines / Amides: Aliphatics
Amines / Amides: Amido-
amines
Amines / Amides: Cyclo-aliphatics
Amines / Amides: Aromatics
Amines / Amides: Di-cyano-
amines
Amines / Amides: Poly-
amines
Anhydrides
Solvent based
Waterborne
Reactive
Radiation curing (UV/EB)
Anaerobic
Film / web


Type Product

Curing Agents

Azridines
Carbodiimides
Diols /   Triols
Isocyanates
Peroxides/ Persalts
Vinyl /Allyl Compounds
Vegetable Based
Hotmelt
Solvent based
Waterborne
Reactive
Radiation curing (UV/EB)
Film / web


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: Aliphatics
Amines /Amides: Amido-amines
Amines / Amides: Cyclo-aliphatics
Amines / Amides: Aromatics
Amines / Amides:Di-cyano-amines
Amines /Amides: Poly-amines
Anhydrides
Room temperature
Elevated temperature
Radiation cure (UV/EB)
Pot life (< 1 hr)
Pot life (> 1 hr)
Latent (temperature activated)


Reactivity

Curing Agents

Azridines
Carbodiimides
Diols / Triols
Isocyanates
Peroxides / Persalts
Vinyl /Allyl Compounds
Vegetable Based
Room temperature
Elevated temperature
Radiation cure (UV/EB)
Pot life (< 1 hr)
Pot life (> 1 hr)
Latent (temperature activated)


Form


Curing agents come in a variety of forms, from powder and liquids to filler supported products. The selection of form is generally dependent on the mixing conditions and processes employed by the compounder.

Table below shows the various forms that are commercially available for the curing agents discussed in this guide:

Form

Curing Agents
Acrylates/
Meth-acrylates
Amines / Amides: Aliphatics
Amines / Amides: Amido-amines
Amines / Amides: Cyclo-aliphatics
Amines / Amides: Aromatics
Amines / Amides:Di-cyano-amines
Amines / Amides: Polyamines
Anhydrides
Beads
Crystals
Dispersions /Emulsions
Flakes
Granules
Liquid
Paste
Powder
Solid
Various


Form

Curing Agents
Azridines
Carbodiimides
 Diols/Triols
Isocyanates
Peroxides / Persalts
Vinyl / Allyl Compounds
Vegetable  Based
Crystals
Dispersions / emulsions
Water soluble
Solvent based
Flakes
Liquid
Pellets
Powder
Solid
Spheres
Various


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: Aliphatics
Amines /Amides: Amido-amines
Amines / Amides: Cyclo-aliphatics
Amines / Amides: Aromatics
Amines /Amides: Dicyano-amines
Amines / Amides: Poly-amines
Anhydrides
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 Agent
Azridines
Carbodiimides
Diols /Triols
Isocyanates
Peroxides /Persalts
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


Commercial Curing Agents Available in Market






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