Polyester resins are typically used in the manufacture of small dinghies and other production boats. The primary reason for this is the inherent low cost of the resin, combined with its ease of use for hand lamination of parts. The mechanical properties of Polyester resins has improved over the years, but they are still the lowest quality of the different resin families We manufacture all of our boats with a high grade Vinyl-Ester resin. Vinyl-Esters have mechanical properties rivaling that of Epoxy resins, but in a more user friendly package. This resin is more expensive than Polyesters, but in exchange we gain considerable mechanical properties that result in a more reliable and longer lasting boat. Below are some comparisons of these properties.
Tensile and Flexural Strength
Tensile strength is the measure of a materials resistance to breaking under tension. That is, the amount of force that can be used to pull the resin apart without failure. Think in terms of how hard you can pull on a length of rope before it fails. Flexural strength is the measure of a materials ability to resist deformation under load. That is, how hard you can push against a material before it fails. Think in terms of laying a piece of plywood across two sawhorses and piling weight up in the middle of the board, stopping just before the board breaks. Below is a chart outlining the differences in these strengths between typical vinyl-ester and polyester resins.
Let’s start with the above example for rope with tensile strength. We’ve already learned that Vinyl-Ester resin has a tensile strength of 8,600 psi, while Polyester has a tensile strength of only 6,000 psi. Now, where does tensile elongation come into play? Essentially, tensile elongation is the measure of how much a material will stretch as it approaches its tensile strength. A good example of this might be in the comparison of household trash bags. When lifting a heavy bag of trash, a bag with high tensile strength and elongation will stretch slightly, but ultimately stay together, while a bag with lower tensile strength and elongation with stretch very little, ultimately tearing apart. Below is a chart showing the different tensile elongation between typical vinyl-ester and polyester resins.
A materials heat distortion is the temperature at which it will deform under a specified load. The higher a materials heat distortion temperature, the more dimensionally stable a material is in high temperature environments. As it turns out, boats spend the vast majority of their lives living under the sun, receiving constant glare of water. A boat with a low heat distortion may experience deformation when just sitting on the docks waiting for use. This might be seen as dents in the hull from the bunks, or bowing between the bunks. A boat with higher heat distortion will resist these deformations until higher temperatures are experienced. Below is a chart showing the different heat distortion temperatures between typical vinyl-ester and polyester resins.