Vinyl Ester vs Polyester Resin: Which One to Choose
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Polyester and vinyl ester are the two resin families behind most fiberglass parts, from boats and tanks to FRP pipe. They look and handle in a similar way, both cure with MEKP catalyst, but they do not perform the same. Here is how to choose, with the trade-offs that matter in practice.
The short answer
- Choose isophthalic polyester for general laminating, structural layers and parts exposed to water and weather, when cost matters.
- Choose vinyl ester for anything in contact with chemicals, for corrosion barriers, for parts that must resist water absorption over time (osmosis) and where you need a tougher, more impact resistant laminate.
- Choose high temperature vinyl ester when the part works hot.
- Often the best answer is both: a vinyl ester liner or skin coat in contact with the fluid, and isophthalic polyester for the bulk of the laminate. This is how our FRP flanges and tank manways are built.
Polyester resins: orthophthalic and isophthalic
Unsaturated polyester resins are the most widely used laminating resins. They come in two main grades:
- Orthophthalic: the cheapest, fine for non-critical parts, moulds of short life and decorative items.
- Isophthalic: better mechanical properties, better resistance to water and to mild chemicals, the usual choice for structural FRP, marine laminates and swimming pools.
Our isophthalic polyester resin is supplied in 2, 3, 5 and 20 kg cans, with or without MEKP catalyst.
Vinyl ester resins
Vinyl ester is chemically a hybrid between polyester and epoxy: the backbone is epoxy based, and the reactive groups sit only at the ends of the molecule. This structure has fewer points that water and chemicals can attack, which gives vinyl ester its key advantages:
- chemical and corrosion resistance, which is why it is the standard resin for chemical tank liners, scrubbers and FRP pipe corrosion barriers;
- resistance to hydrolysis and water absorption, useful against osmotic blistering in boat hulls;
- toughness and elongation: laminates crack less under impact and fatigue;
- good adhesion, which makes it a common choice for repairs and skin coats.
Our vinyl ester resin and high temperature (HT) vinyl ester resin come in the same 2 to 20 kg formats.
Side by side
| Isophthalic polyester | Vinyl ester | |
|---|---|---|
| Chemical resistance | good for water and mild chemicals | very good, the reference for corrosion barriers |
| Water absorption / osmosis | moderate | low |
| Toughness, impact | standard | higher |
| Curing | MEKP + cobalt accelerator | MEKP + cobalt accelerator (check the data sheet) |
| Cost | lower | higher |
| Typical use | structural laminates, marine, pools | tank and pipe liners, chemical service, skin coats |
To give an idea of the cost difference, a 5 kg can on our store costs EUR 31.50 in isophthalic polyester and EUR 46.50 in vinyl ester (excluding VAT, without catalyst).
What about epoxy?
Epoxy offers the highest adhesion and mechanical properties and is the reference for carbon fiber parts, but it is more expensive, needs exact mixing ratios and does not cure with MEKP. In industrial FRP and in most fiberglass work, vinyl ester gives most of the chemical performance of epoxy at a lower cost and with the same workshop process as polyester.
Curing: catalyst and accelerator
Both polyester and vinyl ester cure with MEKP catalyst, usually together with a cobalt accelerator that may already be in the resin. The right percentage depends on resin, temperature and working time: as a reference, when you order resin with catalyst we supply MEKP at 2% (100 ml every 5 kg). Always follow the resin data sheet, and measure the catalyst with a dispenser rather than by eye.
Shipping resins abroad
Resins and catalysts are dangerous goods (ADR) and travel only with authorised carriers. For deliveries outside Italy, ask us for a quote including transport.