What is Curing in Adhesives?

When talking about Adhesives, Curing is a chemical process of the toughening or hardening of a polymer material by crosslinking polymer chains. Curing is achieved by chemical reactions which might or might not require mixing with a chemical curing agent. After curing, the final properties or the strength of the adhesive builds up.

Curing by Chemical Additives

Curing by chemical additives is the transformation of a liquid adhesive into a solid adhesive by a chemical active ingredient called a curing agent or crosslinker.  By adding the crosslinker, the physical and chemical properties of such as tensile, elongation, hardness, modulus, swelling etc. of the adhesive changes. The crosslinker also increases the thermal stability of the adhesive. Crosslinker react with the functionality of the molecular chains in the base polymer and thereby form a thermoset of three-dimensional polymeric materials. 

Polymers consist of a continuous network of polymer chains. The cross-links are formed by reaction of one polymer molecule with another. These reactions can occur either at room or elevated temperatures, and there are catalysts that can accelerate the reaction mechanism.

Curing agents are 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.

What are Some of the Curing Agents and their Applications?

  1. Acrylates: used as a crosslinking agent in acrylic pressure sensitive adhesives
  2. Amines: curing agent for epoxy and polyurethane
  3. Isocynates: Isocyanates contain reactive NCO groups which react with hydroxyl groups on polyols to form polyurethanes and with amines on polyamines to form polyureas
  4. Peroxides: used as catalysts for unsaturated polyester resins and other free radical curing resins
  5. Anhydride: cross-linking agent in reactive epoxy structural adhesives.

What are the Different types of curing in Adhesives?

  1. Heat curing:

Adhesives get cured with the help of heat. Epoxides are class of the most common heat curing adhesives, especially single component adhesive, some of the two component adhesives are cured in moisture, but heat can speed up the process.  Adding heat in curing improve crosslink density, hardness, rigidity, toughness and impact resistance. But flexibility, elongation and crack resistance may be reduced as a result of heat exposure. 

  1. UV Curing: 

Ultraviolet curing (UV curing) is a processing technology that quickly cures the liquid materials by ultraviolet light. Under the irradiation of ultraviolet light, the polymerization of liquid adhesive material can react quickly. UV curing adhesives have the advantages of convenient operation and use: fast curing speed, high production efficiency, no pollution, and energy effective. 

The photo-initiator in the glue system is first excited by ultraviolet light to generate free radical active species. Then the polymerization process proceeds with the unsaturated pre-polymers and monomers through crosslinking processes to obtain a network structured polymer.  UV-cured adhesives can cure in a very short time.

  1. Microwave curing:

Microwave curing is a new type of curing method that has been used in adhesives. The microwave frequency is about 0.3~300 GHz, and it will produce heating, melting, chemical reaction, and other physical phenomena with the assistance of microwave. 

Therefore, microwave curing is more efficient than the conventional methods. It can speed up the chemical reactions. Due to such high efficiency, microwave curing can easily convert the liquid resin into a solid state, which could benefit the curing of adhesive. Microwave curing can reduce the curing time and manufacturing cost and the curing speed is also extraordinarily fast. 

However, the high speed of the curing process also has a weakness. When the adhesive is cured, the volume shrinkage due to solvent volatilization or chemical reaction will generate internal stress, which will affect durability and bonding strength. 

What is the Importance of Accelerators in Adhesives?

The term accelerators are often confused with curing agents. 

Accelerators are used along with a catalyst, curing agent or hardener to increase the rate of reaction, to lower the polymerization temperature, or to improve the efficiency of the reaction. 

Curing agents will have a significant effect on the curing characteristics and on the ultimate properties of the adhesive system. 

Accelerators affect the storage time, gel time, working life of the adhesive. 

They act when one needs to control the cure rate. 

What are some types of Accelerators?

  • Zinc diethyl dithiocarbamate (ZDEC)
  • Zinc N-dibutyl dithiocarbamate (ZDBC)
  • Zinc mercaptobenzothiazole (ZMBT)
  • Zinc butyl xanthate (ZBX)
  • Tetramethyl thiuram disulfide (TMTD)

Why do PSA tapes require drying time?

Curing and drying are the two stages of coating. Drying is the evaporation or the solvent removal from the adhesive during the time of coating. In drying there is no chemical process involved. 

Solvent retention occurs in PSA tapes due to incomplete drying. Solvent retention causes problems such as low shear strength, cohesive failure as well as reducing other inherent properties of adhesive tapes. 

Drying time is important in PSA tapes and can be quickened by heat or the air flow. Temperature, humidity, substrate, thickness of the adhesive layer influences the drying and curing time of adhesive.

PSA tapes, after coating, require 14 days of drying for the complete removal of solvent.  They are stored in the curing /drying room.

Adhesive Specialities, a leader amongst PSA Tapes manufacturers, have made such a curing room for providing the best quality products for their customers.

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