"Hardness" can be confusing as some people use the term to mean "stiffness" or lack of flexibility or elasticity.  Strictly speaking, "hardness " refers to the resistance of one substance to be indented or scratched by another (see Moh's scale of hardness). 

Crosslinking refers to the process whereby linear polymer chains are joined together at points other than the ends of the chain.  Crosslinking reduces elasticity and increases stiffness and possibly some increase in hardness.  Hardeners may be substances, e.g., carbon black, silica, glass fiber, carbon fiber, and other solids, added to and blended into a polymer.  Often, a surface-active coupling agent, e.g., a functional organosilicon alkoxide, is used to chemically bond the polymer to the added solid.  Such composites are generally harder (and stiffer) than the unblended polymer.

Hardener term has come as it hardened the mass after reaction with other component. The term is specifically used for epoxy resin curative. Crosslinker is used for crosslinking. Not necessarily it will harden the mass. Any level of reaction of multifunctional (>2) reactant with other reactant is called crosslinking. Best example is rubber. Crosslinking occurs during vulcanization. Crosslinker is general term and hardener, vulcanization etc are specific terms

As mentioned from Dr. Samui above, both are doing the same. For example, the cosslnker is added to (meth)aclylates before the polymerization takes  place : e.g. a dimetharylate can be used to obtain hard or soft polyacrlate-Materials. The hardness is depending on the Tg. The hardener for epoxy resins (eg polyamine) reacts with the oligomeric prepolymers under ring opening of the oxirane rings. You may obtain also soft materials with hardeners if you use aliphatic died oxides and Jeff amines. T he cross linked material is harder the the starting resins in all cases! it depends on the degree of cross linking and generally on the glass transition temperature!

"Hardness" can be confusing as some people use the term to mean "stiffness" or lack of flexibility or elasticity.  Strictly speaking, "hardness " refers to the resistance of one substance to be indented or scratched by another (see Moh's scale of hardness). 

Crosslinking refers to the process whereby linear polymer chains are joined together at points other than the ends of the chain.  Crosslinking reduces elasticity and increases stiffness and possibly some increase in hardness.  Hardeners may be substances, e.g., carbon black, silica, glass fiber, carbon fiber, and other solids, added to and blended into a polymer.  Often, a surface-active coupling agent, e.g., a functional organosilicon alkoxide, is used to chemically bond the polymer to the added solid.  Such composites are generally harder (and stiffer) than the unblended polymer.

Dear all,

just a small comment on the contribution of Dr. M. David Curtis. In the case of rubbers and elastomers, the elasticity is enhanced to a great deal with crosslinking density. So I think it is not so sharp to say that the elasticity is reduced with crosslinking. regards

Dear Casey Kzin:

coming back to your original question. "Crosslinking" and "hardening" gives the same result, namely networks. However, If you start from short chain polymers, the crosslinking may be a result of terminal group reaction (e.g. epoxy Resins). In case of long chain polymer, the crosslinking may be carried out between the chains (e.g. physically with elektron beam or photo-chemically). As mentioned above, networks can be obtained during polymerization in presence of crosslinkers. A typical example are superabsorbers: acrylic acid is copolymerized with ca 1% bisacrylate.

I hope that your queries is answered satisfactorily.

Kind regards H. Ritter

They are the same: hardening a polymer is done by using a cross linking agent

Good question answered well.  For a reference with respect to Epoxies

Chemical and Process Technology Douglas M. Considine Ed. p415 McGraw-Hill 1974.

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