Epoxy/Polyester resins

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capt_courageous

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HI
Could someone explain the difference between Epoxy and Polyester resins when used in GRP boat construction. I know a minimal amount about chemistry but not a lot. I see that polyesters are often Lloyds approved and expoxies are not but I should like to know why.
Thanks in advance.
 
See here:
http://www.c-cyachts.com/images/com...91-785DC064755F/16cf4_Adv_of_Epoxy_v_Poly.pdf

The Gougeon Brothers, makers of West Epoxy, say that polyester resin is ok for initial boat construction where all the layers of resin cure together, primary bonding, but when making a repair on polyester, secondary or post bonding, you need the extra adhesive and other qualities of epoxy: doesn't shrink as much as polyester resin, better moisture barrier, more durable.

Google is your friend!
 
Epoxy is usually considered too expensive for boat construction I think. Maybe no Lloyds certification cos no-one builds boats out if it. I've worked with both, but am not an expert - what project do you have in mind? (or is it just curiousity?)
 
capt_courageous said:
HI
Could someone explain the difference between Epoxy and Polyester resins when used in GRP boat construction. I know a minimal amount about chemistry but not a lot. I see that polyesters are often Lloyds approved and expoxies are not but I should like to know why.
Thanks in advance.
Click to expand...

I suspect the reason for Lloyd's not approving all epoxy based hulls is the fact that it is so expensive that only light weight racing hulls are made from it, Epoxy is stronger than any of the poly resins so less material/weight is the end result.

Lloyd's may see this as a high risk so do not approve all hulls.

As a general rule any repair below the waterline, in shear or tension should be carried out with an Epoxy resin and cloth designed for use with epoxy resins, items in compression like lifting lugs and under deck pads padding can be be done with standard poly resins.

Also noteworthy is the fact that Epoxy will bond to timber buy poly resins will not at least not in the long term, this can be overcome by laying up more glass and effectively using the timber as a shaper only. Most modern boats use foam or even cardboard to provide a profile for glassing over, the shape and thickness being the actual strength.

Hope this helps.

Good luck. ;)
 
Epoxy resins

I can't imagine why Lloyds would not certify epoxy resin boats. Perhaps some methods of construction are considered doubtful.
In fact epoxy with carbon fibre is the material of choice for Boeing wing and tail components.
Here the carbon fibre is saturated with the (mixed) epoxy at the factory. The product known as prepreg is shipped to the builder refrigerated. The carbon cloth with epoxy is laid up in a mold then heated under vaccuum pressure in an autoclave. This gives the most consistent quality product with precise resin to carbon ratio with best weight for strength.
Now I am not aware of any manufacturers do prepreg fibreglass. There is no reason except that carbon gives so much more stiffness than glass. Top performance boats are made of prepreg carbon epoxy for min weight max strength and stiffness. A boat I sail on occasionally at out club was built this way in 1989 especially for Admirals cup.

However for mass boat building polyester is cheaper. As said it bonds well to itself in initial construction and is resistant to UV sun damage. (epoxy is susceptible so must be painted)
A very efficient form of fibreglass boat building using a chopper gun. Here bulk fibreglass thread is fed to a chopper in a gun form where polyester is added and the mix is blasted into the mold. It makes a hull which is cheap to build and while somewhat heavier than optimum it gains its stiffness from heavy layup. Hand laid glass cloth can be stronger for the weight provided the correct amount of resin is applied (resin to glass ratio) but is vastly more labour intensive. Hence most fibreglass mass produced hulls are made with a chopper gunb laying random orientated glass thread.
So as said Lloyds would have to ask questions of an epoxy hull simply because usually they use epoxy for best strength to weight which usually means min strength needed with min weight. While a polyester boat is made with less concern about weight but needing a lot of bulk of glass and resin to give stiffness resulting in a very thick strong hull.
Stiffness needed especially in a sail boat to combat the force of stay wires and mast load trying to bend the hull like a banana.

Just rattling on a bit. Some years back when Americas cup went to the huge J boat style Australians built a hull that weighed some 5 tonnes or so of lightest strongest prepreg carbon fibre. It had a keel of some 12 tonnes If I remember the numbers. They designed a strong ring from chain plates to keel mount to take the heeling loads. Extra strength was added under the jib sheet winches. The keel was 12ft long. The main sheet winches failed so spin sheet winches further aft were used for for the jib this combined with the harm done by pitching in the tow out to the race and the pendulum effect of the heavy keel led to the hull failing around the middle. She sunk in a few seconds. The moral is that min weight design can be dangerous.


All very interesting olewill
 
Funnily enough, I was just reading a book on GRP last night; agree with what you are saying.
The main issue with polyester resins is that older boats (eg 70s) were later discovered to be at risk from osmosis due the unforseen moisture permeability. Builders later started using epoxy resins to give a better resistance, with polyester on the inner lay-ups.
Its not incommon for example, to see older sailing boat hulls being blasted back and given an epoxy resin coating. I say sailing boats, as I guess there are more of them still about decades later than power.
 
What do they do in modern layup?

Is it just the gelcoat epoxy or the whole caboodle?

Actually, that'd create UV problems, so the gelcoat can't be epoxy can it?
 
xxyyzz said:
What do they do in modern layup?

Is it just the gelcoat epoxy or the whole caboodle?

Actually, that'd create UV problems, so the gelcoat can't be epoxy can it?
Click to expand...
I think the gel is polyester (given the hardener you use.)
Quite what is used in the laminate probably depends on the production method and costs. I guess vaccum might use epoxy all the way through.
Dont know.. only just started the book ;)
 
Some manufacturers may use an epoxy 'gel coat' barrier below the waterline to prevent possible osmosis, but I am pretty sure that all production boats will be polyester through out the laminate and as gelcoat.

Vacuum bagging etc. are all methods to improve the resin/glass infusion and ratio. The glass is the strength so resin just adds weight, if you can get all the glass bonded together using less resin you save weight (and money).

Additionaly sticking polyester to epoxy is a poor bond, epoxy to polyester is better.

Stuart
 
William_H said:
I can't imagine why Lloyds would not certify epoxy resin boats. Perhaps some methods of construction are considered doubtful.
In fact epoxy with carbon fibre is the material of choice for Boeing wing and tail components.
Here the carbon fibre is saturated with the (mixed) epoxy at the factory. The product known as prepreg is shipped to the builder refrigerated. The carbon cloth with epoxy is laid up in a mold then heated under vaccuum pressure in an autoclave. This gives the most consistent quality product with precise resin to carbon ratio with best weight for strength.
Now I am not aware of any manufacturers do prepreg fibreglass. There is no reason except that carbon gives so much more stiffness than glass. Top performance boats are made of prepreg carbon epoxy for min weight max strength and stiffness. A boat I sail on occasionally at out club was built this way in 1989 especially for Admirals cup.

However for mass boat building polyester is cheaper. As said it bonds well to itself in initial construction and is resistant to UV sun damage. (epoxy is susceptible so must be painted)
A very efficient form of fibreglass boat building using a chopper gun. Here bulk fibreglass thread is fed to a chopper in a gun form where polyester is added and the mix is blasted into the mold. It makes a hull which is cheap to build and while somewhat heavier than optimum it gains its stiffness from heavy layup. Hand laid glass cloth can be stronger for the weight provided the correct amount of resin is applied (resin to glass ratio) but is vastly more labour intensive. Hence most fibreglass mass produced hulls are made with a chopper gunb laying random orientated glass thread.
So as said Lloyds would have to ask questions of an epoxy hull simply because usually they use epoxy for best strength to weight which usually means min strength needed with min weight. While a polyester boat is made with less concern about weight but needing a lot of bulk of glass and resin to give stiffness resulting in a very thick strong hull.
Stiffness needed especially in a sail boat to combat the force of stay wires and mast load trying to bend the hull like a banana.

Just rattling on a bit. Some years back when Americas cup went to the huge J boat style Australians built a hull that weighed some 5 tonnes or so of lightest strongest prepreg carbon fibre. It had a keel of some 12 tonnes If I remember the numbers. They designed a strong ring from chain plates to keel mount to take the heeling loads. Extra strength was added under the jib sheet winches. The keel was 12ft long. The main sheet winches failed so spin sheet winches further aft were used for for the jib this combined with the harm done by pitching in the tow out to the race and the pendulum effect of the heavy keel led to the hull failing around the middle. She sunk in a few seconds. The moral is that min weight design can be dangerous.


All very interesting olewill
Click to expand...
Yeah - poor old Jeremy Rogers tried the chopped strand blower system. That's what many consider the cause of his folding.

Carbon in resin is old-hat now - they're using metallic composites - lots of development work going on for EADS.

How late was the 787? Mostly due to production problems in the carbon fibre components?
 
Thanks guys for all your replies.
Amulet - it is really just out of interest. Years ago I went on a boat which did not seem to have gel coat below the waterline. It was really weird looking into underberth lockers. Talk about a glass bottomed boat. By the way, carbon fibre was invented in Farnborough but I have not heard Boeing give us Brits any credit for their new wonder construction.
 
capt_courageous said:
Thanks guys for all your replies.
Amulet - it is really just out of interest. Years ago I went on a boat which did not seem to have gel coat below the waterline. It was really weird looking into underberth lockers. Talk about a glass bottomed boat. By the way, carbon fibre was invented in Farnborough but I have not heard Boeing give us Brits any credit for their new wonder construction.
Click to expand...

But it would have had a gel coat - just not pigmented. Quite common with some builders as a way of making visible any defects before it was coated.

Carbon fibre is widely used and developed in this country. Ask John Barnard and McLaren Racing, for example.
 
charles_reed said:
Yeah - poor old Jeremy Rogers tried the chopped strand blower system. That's what many consider the cause of his folding.
Click to expand...

Not sure that is correct. Chopper guns have never been very popular in Europe, but more common in USA. I thought JR's problems were exactly the opposite - getting involved with Colin Chapman in advanced composite construction using vacuum moulding, a forerunner of the now popular resin infusion method.
 
To answer a few of the points raised here - An epoxy gel coat on a polyester hull isn't feasible as it won't bond. If epoxy is used it is applied after moulding. Production builders use different types of polyester. The basic vanilla resi is 'orthohthalic' whereas it is normal to use 'isophthalic at least for all below waterline applications as it is more resistant to osmosis. There are other types used for increased resistance, Vinylester for example.

And just to bust another myth - it is perfectly feasible to bond fresh polyester to existing, it is not necessary to use epoxy. The old mantra 'epoxy is a glue, polyester isn't' is a myth. Polyester to wood is a no-no though.

The most important thing for preventing osmosis is laminate quality - it is essential to avoid air holes or patches of uncured styrene.
 
It always surprises me that even expensive production Yachts such as Halberg Rassy do not use epoxy resin.
Forget the teak deck and give me the best quality hull material.
 
noelex said:
It always surprises me that even expensive production Yachts such as Halberg Rassy do not use epoxy resin.
Forget the teak deck and give me the best quality hull material.
Click to expand...

But other (lesser?) brands such as Hanse and Elan do offer epoxy laminated hulls as an option on some models.

As already noted one of the great advantages is a lighter stronger hull and maybe HR have worked out that buyers are not that interested, but prefer to spend their money on teak decks, finely finished joinery (even if it is now mostly machine made) rather than a light strong hull.
 
Well, I might as well parade my ignorance further...

How much of the weight of a typical 32 footer is in the hull anyway? Ballast, engine, rigging and fit out must dwarf the fibreglass surely?
 
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