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How do you know how much hardener to mix with how much resin?

When thermoset resins and hardeners are mixed it is important that the correct amount of hardener is used for a certain weight of resin.  Most thermoset systems have one fixed mixing ratio that must not be varied if the optimum cured properties are to be obtained.  For some special resin/hardener systems the weight ratio can be varied to produce cured material with differences in properties, usually in flexibility or hardness. You must consult with the supplier to determine if the mixing ratio is a fixed ratio or if some variation is permissible.

The basis for determining the weight ratio is dependent on the “equivalent weight” of the reacting materials.  This term is defined as “the weight of material that is considered to have one reactive site”

Example:  An epoxy resin contains epoxy groups that can react with the curing agent to produce a cross-linked polymer.  If an epoxy material has an equivalent weight of 150 g/eq, it means that 150 g of resin contains one reactive epoxy group.  It may also mean that 300 g of epoxy has two reactive epoxy groups, i.e. there is one epoxy for every 150g.  Some epoxy resins may have 3 or 4 reactive epoxy groups, but the equivalent weight still represents the weight of resin that has one reactive group.  

Example:  Just as an epoxy resin has an equivalent weight, a curing agent or hardener also has an equivalent weight.  An amine hardener has reactive N-H groups than can react with an epoxy group. The equivalent weight of an amine may be expressed as having an amine equivalent weight 30g/eq.  This means that 30 g of material has one N-H group or that 60 g has 2 N-H groups or that 90 g has 3 N-H groups.  NOTE:  a NH2 group on an amine molecule is considered to have 2 N-H groups.

The resin supplier or the hardener supplier will measure the equivalent weight by analytical methods or they can often calculate the weight by the structure of the material.  When these weights are provided to the supplier who makes the chemical system consisting of epoxy resin and hardener, they will use the equivalent weight of each material to calculate the mix ratio for their system.

Example 1:

  • A resin with an equivalent weight of 150 g/eq epoxy is being mixed with an amine hardener with an equivalent weight of 30 g/eq amine.  For complete reaction with no unreacted components left over it is important that one equivalent of epoxy must react with one equivalent of N-H amine. 
  • One equivalent epoxy weighs 150 g
  • One equivalent amine weighs 30 g.
  • The mix ratio by weight is 150 g epoxy per 30 g amine.  This is 5 g epoxy per 1 g of amine.
  • For this system the mix ratio is a fixed weight ratio.
  • It can be expressed in several different ways:  50 g epoxy for 10 g amine; 20 g amine for 100 g of epoxy
  • It is common in the resin industry for a mix ratio to be specified as 20 phr. The term ‘phr’ means per hundred resin or 20 parts by weight amine per 100 parts by weight of resin.

Example 2:

  • A resin system contains fillers which are materials with no reactive groups. 
  • An epoxy base contains 30 g resin (with an equivalent weight of 150 g/eq) and 70 g of filler. 
  • Because the resin is only 30% of the base, the base has an epoxy equivalent weight of 500 g/eq epoxy. 
  • To properly mix this base with an amine hardener having an equivalent weight of 30 g/equivalent N-H it would be specified that the weight ratio is 30 g amine for every 500 g of epoxy base.  This could also be expressed as amine hardener at 6 phr.

The supplier of a chemical system containing a reactive base and a reactive hardener will use the equivalent weight of the reactive components in the actual base and hardener being supplied and will calculate the proper mix ratio for the customer to use.  In certain instances the supplier may indicate that the weight ratio can be varied due to the chemistry of the components.  The supplier may say you could use hardener at 20 phr or at 40 phr with some difference in properties for each mixed system.

The important point to remember is that the supplier will provide you with a specified mix ratio which must be used to obtain optimum properties for the mixed and cured material.  Variation in the mix ratio means the cured material will have less-than-optimum properties.