What’s Behind Unsaturated Polyester Resins?

Today we are proud to present you our first guest blogger article, written by Luca Boso from Poliver SpA about Unsaturated Polyester Resins. He points out several types of Unsaturated Polyester Resins and that Resins are represented in a lot of application sectors in everyday life products. Furthermore, he explains the different types of technologies used in the production process and shares insights about the pricing.

Thank you, Luca, and enjoy reading, everyone!

“Dear readers, to start with this interesting topic, I would like to take you on a short trip through time and introduce you to the historical beginnings of Unsaturated Polyester Resins as this is important to understand before going more into market and quality management details.

A Short Look at History.

The Unsaturated Polyester Resin (short: UPR) was discovered in the early ’30s about one century ago. It was made by William Carothers and his group in the DuPont laboratories during the rush to discover synthetic alternatives for natural Fibers.


There are many types of UPRs. Nevertheless, few product families can be identified based on the chemistry of the substances used in the synthesis. Glycols, Anhydrides/Acids (Ortho-Phthalic Anhydride, Iso-Phthalic Acid), Maleic Anhydride (unsaturated and responsible for the crosslinking) and Styrene are the most used elements in producing UPRs.

Apart from the Glycols, Resins have been identified mostly in two categories: ORTHO and ISO – depending on whether Ortho-Phthalic Anhydride or Isophthalic-Acid is used in the reaction. This choice will affect both: reaction time and final product property.

Without going into detail, ISO UPRs offer better properties considering above all water resistance and are mostly used in outdoor applications.

ORTHO UPRs are cheaper in terms of sourcing and mainly used in indoor applications. Glycols (MEG, DEG, MPG, NPG, etc.) account for different subcategories of resins based on different properties. Generally speaking, the majority of UPRs are made by a mixture of different Glycols for ORTHO and the use of the only MPG for ISO.


The manufacturing process of UPRs consists of two steps. The first step is the reaction to form Alkyd where all the raw materials (Glycols, Acids, and Anhydrides) react to form the Polyester at high temperature (called “esterification“) and in the final stage with the vacuum. The vacuum allows to extract the water and complete the reaction (minimum yield 99%) as stated by Le Chatelier’s principle.

The second step is the dilution of Alkyd in Styrene (never the opposite!) and Additives until the formulation is obtained. Final filtration is necessary since no distillation is possible. This is a crucial point in the quality of UPRs: everything you use in the preparation will remain in the product, so any Catalyst or Promoter of esterification, any recycled material, and side stream production will affect the quality and safety profile of the UPRs.

The standard ingredients can be found in the positive list of food contact substances according to EU legislation. That means they are sufficient to produce a high-quality UPR.


Nowadays, with regards to how products are marketed, we can identify three types of UPRs. Since the purity of the resins is not possible to determine, there is a mix up of all three types.

  • Specialty products whose marketing is like in the old days
  • Standard products whose marketing is like Commodities (raw material + a fee)
  • Technical grade made by recycled, out of standard or by side products (below raw materials costs)

UPRs have a lot of application sectors in everyday life products such as:

  • Agro (Silos)
  • Adhesives and Sealants (Body Panels, Fenders, Boat hulls/Decks and other large Glass Fiber)
  • Automotive (Smc, Bmc, Putties, Mastics, Flat Sheet)
  • Building and Construction (Abrasive, Engineered Stone, Sanitary Ware, Profiles, Tanks, Pipes, Poles, Corrugated Sheets, Polymer Concrete)
  • Distribution (Gelcoat, Various)
  • Leisure (Marine, Buttons)
  • Paints and Coatings

Generally, two different types of technology are used: UPRs combined with mineral products (Casting) and UPRs combined with fiberglass (Fiber Reinforced Polymer, short: FRP). These two technologies account for the majority of applications. Total consumption of about 10 billion tons/year of UPRs and a total turnover of 20 billion USD/year is considered. [1]

UPRs were discovered during the ’30s and ’40s.  It was easy to find the technology for obtaining the products (FRP or casting). These technologies derived mostly from the work already done with the Phenolic Resin discovered at the beginning of the 1900s.

Casting is a technology that is applied in Abrasive, Sanitary Ware, Engineered Stone, Buttons, Polymer Concrete, Gelcoat, Putties, and Mastics. FRP is a technology that is applied in Profiles, Corrugated Sheets, Flat Sheet, Tanks, Pipes, Poles, Smc, Bmc, Marine, Silos, Various.

The resins content in each product is variable. Generally, Casting resins share is between 5% and 100% and FRP between 15% and 60%. [2] The major difference between casting and FRP is that casting is mainly used for adhesive properties of the UPR and FRP used to create plastic behavior.

Quality and Safety.

The quality of UPRs is difficult to estimate because UPRs do not have an identified molecular. This is due to the fact that it is a mixture of different Polymer chains of different molecular structures, which are typically between 1.000 and 10.000 daltons. [3]

In general, two documents are usually supplied by the UPR producer and describe the quality.

The first document is the certificate of analysis (COA) reporting the result of the analysis of some typical physicochemical properties (viscosity, acid number, appearance, styrene content) and curing properties (gel time, curing time, exothermic peak temperature) usually determined on each batch of production.

The second document is the technical data sheet (TDS) reporting typical values of mechanical and thermal properties (strength, modulus, elongation, heat deflection temperature etc.) based on the seldom random analysis. These documents are useful in the choice of resin type. Nevertheless, the UPRs have to be tested in the customer process and in the final product in order to verify the desired technical features (this process is called homologation).

Most of the safety issues of using UPRs are related to the solvent used to dissolve the Alkyd, which in most cases is Styrene. Styrene, first of all, represents a risk of flammability and furthermore is harmful to humans and the environment (aquatic life). Most of the risks are reported in the material safety data sheet (short: MSDS) together with much other useful information on handling UPRs.

As said before, UPR is made by Glycols Acid and Anhydrides dissolved in Styrene: any other substances added must be carefully evaluated for their safety profile and must be reported in the MSDS if they represent a risk – independently from the quantity level in the resin. Nevertheless, several alternatives that do not use Styrene as Solvent of UPRs are available.

These alternatives are Alkyd or UPR Styrene-free to avoid the risk of flammability and reduce the risk for humans and the environment to the minimum. The use of these alternatives is recommended as much as possible in any process that involves the use of UPRs.


Primarily, the pricing of UPRs is related to raw material costs. This means the raw materials costs of Glycols, Acid/Anhydrides and Styrene. These raw materials account for 60 to 90 percent of the costs of UPRs. [3]

Then there are production costs (manpower and utilities). Furthermore, distribution costs like packaging and transport need to be taken into account when considering the full cost of UPRs. In most cases, what remains is a small profit – usually of a single-digit percent margin. This fact allows only companies with integrated production of raw materials/final products to be profitable.

Even though the market seems to favor integrated companies, there is still space for small, flexible, and customer-oriented companies. These smaller companies, focused on only UPRs, are mostly for customers for whom the price is not the only concern, but who are looking for more flexible solutions.

If we consider UPRs as a Specialty, the price should be based on the value-added to the final product and be affordable for the customer. The commodity approach fails in several positions in UPR market.

Now It Is up to You.

If you are interested in buying Unsaturated Polyester Resins from Poliver SpA via CheMondis, click here.

Thank you for taking the time to read this article. I hope you enjoyed it. Let’s start a discussion by leaving me a comment below or reach out to me via e-mail LBOSO@POLIVER.IT”

Thanks for taking the time to read the CheMondis Blog. 


[1] Composite Germany. (2020). 14th Composites Market Survey.
[2] Thomas, S., Hosur, M., Chirayil, C.J. (2019). Unsaturated Polyester Resins: Fundamentals, Design, Fabrication, and Applications.
[3] Bruins, P.F. (1976). Unsaturated Polyester Technology.

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