Enter the total mass of the epoxy resin you are using.
Enter the theoretical HEW of your hardener.
The molar ratio of reactive groups in the hardener to the epoxy groups. Typically 1 for full cure.
Calculation Results
Required Hardener (grams)–
Moles of Epoxy Resin–mol
Moles of Hardener Needed–mol
Formula UsedRequired Hardener (g) = (EEW / HEW) * R * Grams of Epoxy
Comparison of Epoxy Resin Mass vs. Required Hardener Mass at varying EEW values.
Calculation Breakdown Summary
Metric
Value
Epoxy Equivalent Weight (EEW)
–
Grams of Epoxy Resin
–
Hardener Equivalent Weight (HEW)
–
Stoichiometric Ratio (R)
–
Required Hardener (grams)
–
Moles of Epoxy
–
Moles of Hardener Needed
–
What is Epoxy Equivalent Weight (EEW)?
Epoxy Equivalent Weight (EEW) is a fundamental characteristic of epoxy resins. It quantifies the mass of the resin in grams that contains one mole of epoxide groups. Essentially, it's a measure of the resin's reactivity and molecular weight related to its functional groups. Understanding EEW is crucial for formulators to accurately determine the amount of hardener needed for a complete and effective cure in epoxy systems. It directly influences the final properties of the cured epoxy, such as its strength, flexibility, and chemical resistance.
Who should use it: Anyone involved in formulating or using epoxy resins, including chemical engineers, material scientists, formulators, R&D chemists, manufacturers of adhesives, coatings, composites, and even advanced DIY enthusiasts working with specific epoxy systems.
Common misconceptions: A common misunderstanding is that EEW is simply the molecular weight of the resin. This is incorrect; EEW specifically relates to the mass per reactive epoxide group, not the total molecule. Another misconception is that a lower EEW always means a "better" or "stronger" resin. While lower EEW resins are generally more reactive and can lead to higher crosslink density, the suitability depends entirely on the application's performance requirements.
EEW Formula and Mathematical Explanation
The core concept behind calculating the amount of hardener needed revolves around achieving a specific stoichiometric ratio, which dictates the molar equivalence between epoxy groups and the reactive groups on the hardener. The most common target is a 1:1 molar ratio (R=1) for a complete cure. The formula to determine the required amount of hardener is derived from this principle.
Step-by-step derivation:
Calculate Moles of Epoxy Resin: The number of moles of epoxy resin is found by dividing the mass of the epoxy resin by its Epoxy Equivalent Weight (EEW).
Moles of Epoxy = Grams of Epoxy / EEW
Determine Moles of Hardener Needed: To achieve the desired stoichiometric ratio (R), the moles of hardener needed are the moles of epoxy multiplied by R.
Moles of Hardener Needed = Moles of Epoxy * R
Calculate Required Hardener Mass: Finally, to convert the moles of hardener needed into a usable mass, we multiply it by the Hardener Equivalent Weight (HEW).
Required Hardener (grams) = Moles of Hardener Needed * HEW
Substituting the formulas:
Required Hardener (grams) = (Grams of Epoxy / EEW) * R * HEW
Rearranging for clarity and to match the calculator's output:
Required Hardener (grams) = (EEW / HEW) * R * Grams of Epoxy
Variable explanations:
Variable
Meaning
Unit
Typical Range
EEW
Epoxy Equivalent Weight
grams/equivalent
150 – 2000+ (depends heavily on resin type)
Grams of Epoxy Resin
Mass of the epoxy resin being used
grams
Any positive value
HEW
Hardener Equivalent Weight
grams/equivalent
20 – 300+ (depends heavily on hardener type)
R
Stoichiometric Ratio (Hardener Moles / Epoxy Moles)
Unitless
0.8 – 1.2 (commonly 1.0 for full cure)
Required Hardener (grams)
The calculated mass of hardener needed
grams
Derived value
Moles of Epoxy Resin
The number of moles of epoxy groups
mol
Derived value
Moles of Hardener Needed
The number of moles of hardener functional groups required
mol
Derived value
Practical Examples (Real-World Use Cases)
Example 1: Standard Epoxy Adhesive
A formulator is creating a two-part epoxy adhesive. They are using a common Bisphenol-A based epoxy resin with an EEW of 190 g/eq. They want to achieve a full cure using a standard amine hardener with an HEW of 85 g/eq. They plan to mix 100 grams of the epoxy resin.
Result: The formulator needs approximately 44.7 grams of the hardener for 100 grams of this epoxy resin to achieve a stoichiometric cure.
Example 2: High-Performance Composite Resin
A company is developing a resin system for a high-performance composite application. They are using a specialty epoxy resin with a lower EEW of 150 g/eq for faster cure and higher crosslink density. The hardener selected has an HEW of 60 g/eq. They need to mix 500 grams of the epoxy resin.
Result: For 500 grams of this specialty epoxy resin, 200 grams of the selected hardener are required to ensure proper curing and optimal mechanical properties.
How to Use This Epoxy Equivalent Weight Calculator
Our Epoxy Equivalent Weight (EEW) Calculator is designed for simplicity and accuracy. Follow these steps to get your precise hardener requirements:
Enter Epoxy Equivalent Weight (EEW): Input the EEW value of your specific epoxy resin. This is usually found on the manufacturer's Technical Data Sheet (TDS).
Enter Grams of Epoxy Resin: Specify the total weight of the epoxy resin you intend to use in your formulation.
Enter Hardener Equivalent Weight (HEW): Input the HEW of the hardener you are using. This is also typically available on the hardener's TDS.
Enter Stoichiometric Ratio (R): For most applications aiming for a full cure, set this to 1.0. If you are intentionally seeking an under-cured or over-cured state (which is rare and application-specific), you can adjust this value. Consult your resin system's specifications.
Click "Calculate": The calculator will instantly process your inputs.
How to read results:
Required Hardener (grams): This is the primary output, showing the exact mass of hardener needed to mix with your specified amount of epoxy resin for the desired stoichiometric ratio.
Moles of Epoxy Resin: This intermediate value shows the molar quantity of reactive epoxy groups.
Moles of Hardener Needed: This intermediate value indicates the molar quantity of reactive hardener groups required.
Formula Used: A clear display of the formula used helps in understanding the calculation.
Decision-making guidance: Use the "Required Hardener (grams)" value as the precise measurement for mixing your epoxy system. Deviating significantly from the stoichiometric ratio can compromise the cured properties of the epoxy, potentially leading to reduced strength, flexibility issues, or incomplete cure. Always double-check your inputs against the manufacturer's data sheets.
Key Factors That Affect Epoxy Equivalent Weight Results
While the calculation itself is straightforward, several external factors and nuances related to EEW and hardener calculations can influence the outcome and the final properties of the cured epoxy:
Manufacturer's Data Accuracy: The EEW and HEW values provided by manufacturers are typically based on theoretical calculations or standardized testing. Actual batch variations, though usually small, can exist. Always rely on the most up-to-date TDS.
Resin Type and Structure: Different epoxy resin chemistries (e.g., Bisphenol-A, Bisphenol-F, Novolacs, Cycloaliphatics) have inherently different molecular weights and numbers of epoxide groups, leading to varied EEW values. This impacts the required amount of hardener.
Hardener Chemistry: Amine, anhydride, Lewis acid, and other hardener types react differently and have distinct equivalent weights. The choice of hardener profoundly affects the required quantity and the resulting crosslink density and properties.
Desired Stoichiometry (R): While R=1.0 is standard for full cure, intentionally using a different ratio (e.g., R=0.9 for slight under-cure, or R=1.1 for slight over-cure) will directly change the required hardener mass. This might be done to modify cure speed, Tg, or flexibility, but it sacrifices optimal performance.
Additives and Fillers: Introducing fillers (like silica, talc, or glass fibers) or other additives into the epoxy formulation can affect the apparent EEW or the volume available for the hardener reaction, potentially requiring adjustments beyond the basic calculation. High filler loadings can significantly change the system's rheology and cure characteristics.
Temperature During Mixing and Cure: While not directly affecting the EEW calculation, ambient and substrate temperatures are critical for the curing reaction's kinetics. Incorrect temperatures can lead to incomplete cure even with the correct ratio, affecting final properties and seemingly invalidating the calculation's outcome.
Pot Life and Working Time: The calculated amounts are for initial mixing. The pot life (working time) is influenced by the reactivity of the resin-hardener combination, influenced by EEW, HEW, and temperature. A faster reaction (often associated with lower EEW/HEW) means a shorter pot life.
Moisture Sensitivity: Some hardeners, particularly amines, can react with atmospheric moisture, reducing their effective reactivity. This can lead to an apparent need for more hardener or an incomplete cure if not accounted for or if proper handling procedures aren't followed.
Frequently Asked Questions (FAQ)
General Questions
What is the difference between EEW and molecular weight?
Molecular weight is the total mass of one mole of a molecule. EEW is the mass of the resin containing one mole of *epoxide groups*. A resin molecule might have one, two, or more epoxide groups, so EEW is typically less than the resin's molecular weight.
Why is the stoichiometric ratio (R) usually 1.0?
A ratio of 1.0 means that for every mole of epoxy groups, there is one mole of reactive sites on the hardener. This allows for the maximum possible crosslinking, leading to the optimal mechanical, thermal, and chemical properties of the cured epoxy.
What happens if I don't use the correct hardener amount?
Using too little hardener (R < 1.0) results in an under-cured epoxy with reduced strength, heat resistance, and chemical resistance. Using too much hardener (R > 1.0) can also lead to issues like brittleness, reduced flexibility, potential exotherm problems, and can sometimes leave unreacted hardener groups which might affect surface properties or longevity.
Can I use the EEW calculator for any epoxy system?
This calculator is based on the fundamental stoichiometry of epoxy-amine or epoxy-other reactive group reactions. It works for most common thermosetting epoxy systems where EEW and HEW are defined and a target stoichiometric ratio is known. It's not applicable to systems that don't rely on this type of molar equivalence for curing.
How do I find the EEW and HEW for my materials?
These values are almost always found on the manufacturer's Technical Data Sheet (TDS) or Safety Data Sheet (SDS) for the specific epoxy resin and hardener. Always refer to the official documentation.
Is EEW the same for all epoxy resins?
No. EEW varies significantly based on the chemical structure of the epoxy resin. For example, standard liquid Bisphenol-A diglycidyl ether (BADGE) resins typically have an EEW around 170-190 g/eq, while higher molecular weight solid resins or specialty resins can have much higher EEWs.
Does EEW affect cure time?
EEW itself doesn't directly dictate cure time, but resins with lower EEW are often more reactive, and when paired with appropriate hardeners, can lead to faster cure cycles. The hardener type, concentration (stoichiometry), and temperature are primary drivers of cure speed.
What is an "equivalent" in EEW and HEW?
An equivalent refers to a mole of reactive functional groups. For EEW, it's one mole of epoxide groups. For HEW, it's one mole of reactive sites on the hardener (e.g., for an amine hardener, it's one mole of active hydrogen atoms).
Related Tools and Internal Resources
Viscosity CalculatorCalculate and adjust the viscosity of your resin formulations for optimal processing.
Gel Time CalculatorEstimate the pot life and gel time of your epoxy mixture based on formulation and temperature.