Difference Between Epoxy and Polyurethane

The key difference between epoxy and polyurethane is that the epoxy contains epoxide groups whereas polyurethanes contain urethane linkages. In the application point of view, the main difference between epoxy and polyurethane is that the epoxy resins can resist moderate temperatures while the polyurethanes can withstand high temperatures.

Both epoxy and polyurethane are polymer materials. A polymer is a macromolecule that contains a large number of small repeating units (monomers). These monomers combine with each other to form a polymer. The term epoxy is used to name epoxy resins that consist of epoxy functional groups in a repeating pattern. Polyurethane is also a polymer material that contains urethane linkages.

CONTENTS

1. Overview and Key Difference
2. What is Epoxy
3. What is Polyurethane
4. Side by Side Comparison – Epoxy vs Polyurethane in Tabular Form
5. Summary

What is Epoxy?

Epoxy or epoxy resin is a class of polymer material consisting of epoxy functional groups. It is a form of polyurethane. These polymers can be either low molecular weight or high molecular weight polymers with at least two epoxide groups. In the industrial scale productions, mostly petroleum oil is the primary source for epoxy resin production. However, there are some plant-derived sources as well.

Epoxy resins can react with each other via catalytic homopolymerization and form cross-links between them. Or else, epoxy resins can react with some other compounds such as;

• Amines
• Acids
• Phenols
• Alcohols
• Thiols

These are co-reactants. Some other names for these co-reactants are hardeners or curatives. Therefore, the cross-linking reactions that epoxy resins undergo with these curatives refers to “curing”. The crosslinking process forms a thermosetting polymer that has favorable chemical and mechanical properties.

The method of calculation of the specific amount of epoxy groups present in the epoxy resin is as follows:

• The ratio between the number of epoxy groups and the mass of the polymer material gives the specific amount of epoxide groups in an epoxy resin.
• The unit for this measurement is “mol/kg”. We call this term sometimes as “epoxide number”.

Some Common and Important Forms of Epoxy Resins 

• Bisphenol A Epoxy Resin

This type of epoxy resins is a result of the combination of epichlorohydrin and bisphenol A. this combination gives bisphenol A diglycidyl ethers. If we increase the amount of bisphenol A (compared to epichlorohydrin), it provides high molecular weight polymer material. This polymer material is linear and is semi-solid crystalline materials.

• Bisphenol F Epoxy Resin

Here we use bisphenol F in the place of bisphenol A, but the way of formation of the polymer is same as the bisphenol A epoxy resin (as discussed above).

• Novolac Epoxy Resin

Novolac epoxy resins form when phenols react with formaldehyde and undergo glycosylation with epichlorohydrin. Most of the times, this polymer material shows high chemical resistance and high-temperature resistance but low flexibility.

• Aliphatic Epoxy Resin

These polymers form via glycosylation of aliphatic alcohols or polyols. This polymer material has a low viscosity at room temperature.

What is Polyurethane?

Polyurethane is a polymer material that has urethane linkages (carbamate linkages). Isocyanates and polyols undergo polymerization to form a polyurethane. Although the name polyurethane gives an idea that the polymer consists of urethane monomers, it in fact, consists of urethane linkages, not monomers.

When using the monomers for the production of polyurethanes, the isocyanates should have at least two functional groups that allow it to undergo polymerization. Moreover, polyol also should have at least two hydroxyl groups per molecule. The reactions between the monomers in this polymerization are an exothermic reaction in which heat is given off from the reaction mixture. The urethane linkage forms when the –N=C=O group of isocyanate react with the –OH groups of alcohol to form a urethane linkage (-NH-C(=O)-O).

What is the Difference Between Epoxy and Polyurethane?

Summary – Epoxy vs Polyurethane

Epoxies are a type of polyurethanes. These polymers get their name according to the repeating linkages present in the polymer material instead of the monomers used for the production. The difference between epoxy and polyurethane is that the epoxy resin contains epoxide groups whereas polyurethanes contain urethane linkages.

Reference:

1. “Epoxy.” Wikipedia, Wikimedia Foundation, 19 Apr. 2018. Available here
2. Johnson, Todd. “What Are Epoxy Resins?” ThoughtCo, ThoughtCo. Available here 
3. Lazonby, John. “Polyurethanes.” The Essential Chemical Industry Online. Available here 

Image Courtesy:

1.’Epoxy’ By Dzhang2680 – Own work, (CC BY-SA 3.0) via Commons Wikimedia 
2.’Polyurethane chair, Walker Art Gallery’ By Reptonix free Creative Commons licensed photos (CC BY 3.0) via Commons Wikimedia