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polyester vs vinylester vs epxoy

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  • polyester vs vinylester vs epxoy

    I was doing research into what resin I should use for my upcoming repairs and found this quote online from I also searched the classicmako forum and there seems to be differing opinions. Any comments?

    Epoxy Resin vs. Vinylesters and Polyesters

    Here is some technical data that you may find interesting regarding the differences between Epoxy Resin and Vinylester or Poyester Resins.

    In the marine industry, liquid plastics, namely epoxies, polyesters, and vinylesters are used to saturate (wet out) the fibers of wood, glass, kevlar amarid, or carbon to form a fiber reinforced plastic (FRP). To create a quality part, adhesion to the fibers is the most important factor. Not all resins keep their grip on fibers equally.

    Epoxy resin is known in the marine industry for its incredible toughness and bonding strength. Quality epoxy resins stick to other materials with 2,000-p.s.i. vs. only 500-p.s.i. for vinylester resins and less for polyesters. In areas that must be able to flex and strain WITH the fibers without micro-fracturing, epoxy resins offer much greater capability. Cured epoxy tends to be very resistant to moisture absorption. Epoxy resin will bond dissimilar or already-cured materials which makes repair work that is very reliable and strong. Epoxy actually bonds to all sorts of fibers very well and also offers excellent results in repair-ability when it is used to bond two different materials together.

    Vinylester resins are stronger than polyester resins and cheaper than epoxy resins. Vinylester resins utilize a polyester resin type of cross-linking molecules in the bonding process. Vinylester is a hybrid form of polyester resin which has been toughened with epoxy molecules within the main moleculer structure. Vinyester resins offer better resistance to moisture absorption than polyester resins but it's downside is in the use of liquid styrene to thin it out (not god to breath that stuff) and its sensitivity to atmospheric moisture and temperature. Sometimes it won't cure if the atmospheric conditions are not right. It also has difficulty in bonding dissimilar and already-cured materials. It is not unusual for repair patches on vinylester resin canoes to delaminate or peel off. As vinylester resin ages, it becomes a different resin so new vinylester resin sometimes resists bonding to your older canoe. It is also known that vinylester resins bond very well to fiberglass, but offer a poor bond to kevlar and carbon fibers. Do to the touchy nature of vinylester resin, careful surface preparation is necessary if reasonable adhesion is desired for any repair work.

    Polyester resin is the cheapest resin available in the marine industry and offers the poorest adhesion, has the highest water absorption, highest shrinkage, and high VOC's. Polyester resin is only compatible with fiberglass fibers and is best suited to building things that are not weight sensitive. It is also not tough and fractures easily. Polyesters tend to end up with micro-cracks and are tough to re-bond and suffer from osmotic blistering when untreated by an epoxy resin barrier to water. This is really cheap stuff.

    Summary - Epoxy resin has far more to offer in its ability to flex, prevent delamination, and ease of use for repairwork. Using epoxy resin leads to better quality products.
    3N2, 76 mako 23, marblehead, ma[br]

  • #2
    When looking at "stick together" numbers you have to keep in mind that the prep on the piece being tested and the test method have quite a bit to do with the published numbers. You can take the information for what it is however, epoxy resins do present the strongest secondary bond characteristics of the three. Your resin choice will depend largely on the repair to be made and your method of repair.
    Greenwich, NJ[br]1976 22B


    • #3

      thanks for the replies. i still confused at what to use as warthog mentions in a post that he only uses epoxy. it also seems that most people who replace their transom are using epoxy? i believe that all makos are built in polyester? If this is the case why wouldn't you make all repairs in polyester? Or is there certain areas that should have epoxy ie the transom and floor? thanks in advance
      3N2, 76 mako 23, marblehead, ma[br]


      • #4
        Yes mako boats and about 95% of the boats on the water are built from polyester resin. But you have to understand something about bonding. There are 2 different types of bonds... Primary and secondary.

        Primary bonds are chemical bonds between molecules to form one soild piece. When a boat is constructed, the polyester resin is laid into the boat layer by layer and part by part. All in a wet-to-wet fashion. So it is essentially one piece all chemically bonded together.

        Secondary bonds are much different. They have nothing to do with molecular binding and everything to do with surface adhesion. They are mechanical bonds.. actually micromechanical bonds. The polyester resin of the boat is fully cured... So it doesn't matter what resin you use, the repaired area will never become "one" with the rest of the boat. It is essentially glued to the boat. That is why prep work is so important in repairs. The surface that you are bonding, or better said... glueing to, has to be clean and roughed preferably with some scratches too. That way there is something for the repair resin to mechanically lock itself into and hold tight... Much like prepping something to be painted.

        Now that we have talked about the different bonds... We can talk about resin properties.

        Polyester resin has the weakest properties of the 3 resin types. It is more brittle and not very water resistant. And in a repair situation, it forms the weakest secondary bond of the 3 by a landslide. It also has very poor penetration/coating properties with wood surfaces. It does not sink into wood like epoxy. Vinylester isn't great with wood, but its better than polyester. Polyester and vinylester resins can be thinned to sink into wood a little better (hot coating), but they will never beat the penetrating and coating properties of epoxy. Polyester resin uses MEKP catalyst and the amount of catalyst can be varied from 3/4% up to 3%+ to alter the cure rate.

        Vinylester resin is in the middle... but closer to the properties of epoxy. Vinylesters are produced by the esterification of an epoxy resin with an unsaturated monocarboxylic acid. The reaction product is then dissolved in a reactive diluent at 35-45 weight %... Vinylester resin has much improved water resistance and physical properties. Its secondary bond strength is very close to that of some epoxy resins. Vinylester resin also uses MEKP catalyst but the amount of catalyst cannot be varied as much (according to FGCI tech guys). It needs to be as close to 2% as possible for a correct cure.

        Epoxy resin is the cadillac of resins. It has fantastic physical properties and water resistance. It is also very light weight compared to the ester resins. Thats why its used in airplanes and stuff that has to be very strong and very light. There are many different epoxy resins. All the way from water thin penetrating epoxy to 5:1 resins.. Generally speaking... As an epoxy resin:hardner ratio increases so does its physical properties and water resistance. But its pot life and working time diminish very quickly. For instance a 2:1 resin will have physical properties very similiar to those of vinylester resin and a very good working time, but a 4:1 or 5:1 epoxy resin will have outrageously strong physical properties and secondary bond strengths with a super short potlife/working time.. It is not really good for wood coating/penetration because it doesn't have very long to sink in. But a 1:1 or 2:1 resin is great for wood coating/bonding. You can coat the wood with a 1:1 or 2:1 epoxy and let it sit for a while to soak in and then do the lamination with a 3:1 or 4:1 resin if its in a high strength area. With epoxy resins, the resin:hardner ratio for a particular resin cannot be deterred from... If the resin calls 3:1, then you have to mix it at 3:1. Otherwise the resin will not cure completely or at all, or it will cure with very weak properties. Epoxy resin cannot be thinned with acetone or styrene either. If you want a thin epoxy resin, then you can buy epoxy in different viscosities. Penetrating epoxy resin is just like water and will sink into anything, but isn't good for laminating fiberglass.

        Problem is that epoxy resin is not compatable with ester resinis in a wet-to-wet situation. In other words, you cannot apply polyester resin, gelcoat, or vinylester resin. To uncured epoxy... Nor can you apply epoxy to uncured ester resin products. The sytrene in the ester resins will inhibit the cure in both and you'll end up with a big sticky uncured mess. Now... You can put epoxy resin over fully cured polyester or vinylester resins, but mechanical abrasion of the bonding surface has to be done to achieve a secondary bond.

        Some polyester resin products... gelcoat.. have difficulty with being placed over fully cured epoxy products. There is something in the epoxy that inhibits the cure of the gelcoat. I have put gallons of gelcoat over West System epoxy repairs and had great success. The epoxy surface must be fully cured,the amine blush must be washed off, and the surface must be mechanically abraded so that the gelcoat can form a secondary bond with the epoxy surface. If you ever run into a situation where the epoxy surface is cured, but the gelcoat still won't set, a tie coat of vinylester resin can be placed over the epoxy and then the gelcoat can be placed over the vinylester... But the easiest thing do do is use West System epoxy when you are planning to gelcoat the repair. Or use vinylester or polyester resin for the repair in the first place.

        Some guys prefer the working characteristics of different resins. I like the working characteristics of epoxy for filleting and laminating with fiberglass cloths and fabrics... But if I am using alot of fiberglass mat, I prefer vinylester resin becasue it breaks down the binder in the mat and allows the stuff to be shaped around corners and angles. I also use vinylester resin when building molded parts that will have gelcoated surfaces. When doing a repair in a high strength area or coating/bonding to wood... You can bet that I'll pick epoxy.

        Like I said before... You just can't beat the binding strength of epoxy for repairs and secondary bonding. Some guys have excellent success with repairs using polyester resin, but its not as forgiving as epoxy. If you don't have all your ducks in a row the repair has a good chance of breaking down over time. Vinylester resin is a little more forgiving (stronger too). And epoxy is very forgiving and very strong.
        Slidell, LA 1993 Mako 261B - Temperance


        • #5
          Wow! thanks for the explanation. I am going to be undertaking my rail repair this week and feel after all of the input that I should use epoxy and gelcoat over it. Ringleader you had posted the pics under a previous thread 76 mako. I am inclined to use epoxy on the rail and was hoping that you could give me a yeah or neah on that as it is in an area that will require meticulous (spelling) work as it is in a very visible space. I also sounds like I should use the west system vs a fgci epoxy?

          For all repairs it sounds like you would recommend using epoxy if possible.

          thank again for the help

          sorry for the long time between post as i am away and have a very slow dial up access

          3N2, 76 mako 23, marblehead, ma[br]


          • #6
            If you are going to gelcoat the reapir, then you should use West Epoxy. Just remember that you have to let the epoxy cure completely (I allow a week) and then wash off the amine blush and sand it down really good. For an area like that you'll have lots of sanding/shaping to do anyways. What I would recommend is to build your repair area in expoy and fiberglass and sand that down to as close to the desired shape as possible. Then come back and use 3M premium filler (its a vinylester resin based product)(green label can) and wipe it and do your fine shaping with that before gelcoating the repaired area. Areas like that take alot of work sometimes and very often you end up doing it over 2-3 times before you get it just right.
            Slidell, LA 1993 Mako 261B - Temperance