The revisionist mythology of Wharram

In the technical data sheet they say the nominal ratio by weight for 105 resin / 205 or 206 hardener is 5.19/1, but you can use 4.83 to 6.2/1 which is acceptable.
So the accuracy of mixing ratio is not critical. But the package contains 5 volume of resin and 1 volume of hardener, wich corresponds in weight to 5.19 / 1 .
If you use 5/1 in place of the exact weight ratio 5.19/1 , you will be short of hardener and you are wasting money ! this can be a lot for a big project !

 

Great explanation.
THAT also explains why I was always running out of hardener in past projects.

 

The best mixing regime is the one that works for you, many home building projects are stretched out over such time and un workman like work hygiene that the pumps are barely recognisable let alone working.
I like working by volume as for me it bares the closest resemblance to the resin required estimate. (from my polyester days)
To facilitate that I make up a chart of volume/ratios to refer to when mixing for a job, the other handy think is having a "spare resin" job on hand to use up any left overs. As for running out of hardener, its small bananas on a boat building project, most largish projects will have several speeds of hardener on hand so its really not an issue.

 

the "running out of hardener" is not the thing that worries me! It is the obviously (slightly) incorrect mixing ratio...-therefore: electronic scale!

 

That's the point of the discussion. Its not "incorrect". Its close enough if the manual says so.

 

...the Gurit manual does not give a "from - to" ratio...

 

There must be an acceptable +- ratio otherwise you are chasing .1 or .01 or .001 in order to get the ratio perfect. There is more resin stuck to the resin container than the catalyst container has that upset the ratio ? No. Get real.

 

Vinylester is not only a less expensive option but is also superior in many ways to the epoxies most of us have available. I don't know where you are located but most of the common epoxies used in the US for example have a tensile strength of anywhere from around 7000 to 8000 psi (depending on which hardener is used) while the infusion VE that i use is 11800 psi. all the other properties are excellent also so you are by no means settling for second best when choosing VE. This may come as a surprise to a lot of people who have been brainwashed over the last 50 years by marketing. But don't take my word for it or anyone else who may dispute this. All of the legit resin manufacturers can supply you with Technical data sheets on their products so you can compare for yourself. VE is particularly suitable for foam core construction and depending on the method you use may yield a lighter structure. It has a much better viscosity for hand layup and as redreuben pointed out, faster gel and cure times so a lot less waiting for resin to kick so you can move forward. I learned resin infusion from Derek Kelsal who was an early adopter/pioneer of foam core construction of multihulls back in the 1960's and right until he passed away a few years ago he was still building with polyester and could see no reason to change. Polyester is even cheaper than VE. I have not looked at the TDS for poly though. The biggest reason to use epoxy is that it is a lot less likely to have the neighbors complaining about the smell. It's also easier to dispense in small quantities and easier to use for woodworking.

 

Nothing amazing

The tensile strength of polyester resin can range from 1,450 to 17,900 psi (

10.0123MPa10.0123 cap M cap P a
10.0123

Examples of polyester resin tensile strengths

• Fibre Glast 90L Isophthalic Polyester Resin: 17,900 psi (
  
  123MPa123 cap M cap P a
  123)
• Polynt DISTITRON® 411 V4 Unsaturated Polyester Resin: 11,600 psi (
  
  80.0MPa80.0 cap M cap P a
  80.0)
• GPO-3 Glass Polyester: 8,000–11,000 psi

Plus

After a cure period of seven days it can be seen that the tensile strength of the epoxy resin is 20 to 30% higher than those of polyester and vinyl ester. More importantly, after post cure the difference becomes ever greater


Epoxy stars in several other areas

"Fatigue Resistance and Micro-Cracking"
"The superior ability to withstand cyclic loading is an essential advantage of epoxies vs. polyester resins. This is one of the main reason epoxies are chosen almost exclusively for aircraft structures"

Degradation from Water Penetration An important property of any resin, particularly in a marine environment, is its ability to withstand degradation from water penetration.
Both polyester and vinyl ester resins are prone to water degradation due to the presence of hydrolysable ester groups in their molecular structures. As a result, a thin polyester laminate can be expected to retain only 65% of its inter-laminar shear strength after immersion over period of one year, whereas an epoxy laminate immersed for the same period will retain around 90%.

https://www.mjmyachts.com/images/stories/pdf/sp advantages of epoxy resin.pdf

 

Never said anything was amazing, just pointing out that polyester and vinylester are not some second grade choice. Most production boats these days are still built with a skin coat of vinylester and the remaining structural layers with polyester and they are just fine. In real life vinylester does not degrade from water intrusion and is used to laminate the skin coat for that reason. I work at a good sized marina and we havn't seen blisters on any boat built in the last 20 years. we have peeled a couple that were built in the 1970's without the VE skin coat. VE is used in the US for most, if not all of the underground fuel tanks at gas stations that have fuel on the inside and groundwater on the outside. It is really not possible to blanket compare Polyester to vinylester to epoxy as the graphs shown above as there are hundreds of formulations of each for different purposes, there really is no general example of each. We can really only look at what we have available to us. A high elongation VE for example may well have better resistance to micro cracking than a low elongation epoxy formulation. In boatbuilding we typically design for stiffness so fatigue is less of an issue than say, the wing structure of an aero plane, consequently polyester boats from 50 years ago are still holding together just fine, we cut up several each summer that are still quite sound.

 

Since we are on the subject and we have smart and experienced posters here, perhaps you could educate us on the normal elongation to failure of each resin type: a marine quality iso polyester, vinylester, and epoxy. What little I have heard, it's important that whatever reinforcement used it must reach its maximum elongation before the resin system's, otherwise cracking of the resin will occur. I know there are elongation variances within each resin system but looking for the most commonly used percentage of each when applied to stitched glass and carbon. It seems like this might be relevant to resin choice.