A critical choice? Definitely. For many applications, making the right selection between these resins can affect strength, durability, product life and, of course, cost. They have different chemical compositions and these differences express themselves in their physical properties. Before choosing between them for a particular application, it is important to have a clear idea of what performance is required from the build. Understanding the differences between these resins will help a user to compile the list of critical material performance factors required from the finished article and inform the selection.
Let's get the painful chemistry out of the way first:
Polyester resins are formed by the reaction between polyols such as glycol or ethylene glycol with dibasic acids such as pthalic acid or maleic acid. These unsaturated resins are combined with other chemicals sometimes called hardeners or, catalysts. This changes the molecular structure and the resulting compound cures, generating heat in the process. Methyl ethyl ketone peroxide ('MEKP') is one such 'hardening' agent.
Vinyl ester resins are produced by the reaction ('esterification') between an epoxy resin and an unsaturated monocarboxylic acid. Essentially they comprise a base of polyester resin strengthened with epoxy molecules in the backbone of the molecular chain. Vinyl esters also use peroxides (e.g. MEKP) for hardening.
Both resins can be 'thinned' by reaction with chemicals such as styrene.
These chemical differences between the resins result in several difference in physical properties.
Advantages and Disadvantages
- On a broad scale of viscosity, vinyl esters are midway between polyesters and epoxy resins, before adding styrene. Thinning affects workability and strength - 'thinning' reduced strength but makes it easier to brush or spray.
- Vinyl esters are more tolerant of stretching than polyesters. This makes them more able to absorb impact without damage. They are also less likely to show stress cracking.
- Vinyl ester has fewer open sites in its molecular chain. This makes it much more resistant to water penetration ('hydrolysis') which can cause osmotic blistering.
- Vinyl esters shrink less on curing, which means that 'pre-release' of a laminate from a mold is less significant.
- The cross bonding of vinyl esters is superior to that of polyesters. This means that vinyl esters bond to core materials much more effectively than polyesters and delamination is less of an issue.
- Vinyl esters are less sensitive to ambient conditions (temperature and humidity) than are polyesters.
- Vinyl esters are more expensive than polyesters though careful calculations are required to assess the cost impact for a significant build project such as a luxury yacht. This is because the relative strengths need to be factored in - you can use less vinyl ester to achieve a given strength.
- Both resins are susceptible to 'chalking' - UV breakdown at the surface - unless an additive is incorporated in the mix.
Which to Use?
Despite the superiority of vinyl ester (apart from cost), polyester still has a large part to play in composite fabrications.
Where prolonged exposure to water is likely (such as a boat hull or water tank), then by using polyester for the bulk construction with a surface barrier of vinyl ester, water penetration can be reduced considerably without a significant increase in cost.
If improved durability and impact resistance is important, then vinyl esters win over polyesters - and again the build can be tailored to use the vinyl esters in those areas with higher impact probability. However, these are relative and other resins or composites may be superior (and more expensive).
Vinylesters and polyesters are widely used, and for many similar applications. However where the physical properties of vinylester are more important than cost, then vinylester takes the lead:
- Transportation: parts for automobiles and other surface transportation vehicles
- Building and Infrastructure: fascias for buildings, reinforcements for bridges
- Military / aerospace applications
Before making a decision, consider the requirements for durability very carefully, and weigh up the cost. It may be that the extra cost of vinylester will be offset by its superior strength and durability. Then again, maybe both will work well in combination for the application.