Post Curing and Tg Question

Discussion in 'Fiberglass and Composite Boat Building' started by CatBuilder, Jul 10, 2010.

  1. CatBuilder

    CatBuilder Previous Member

    Final selection of epoxy for my project is taking place right now.

    First set of questions is regarding post curing:

    1) Does all epoxy improve the same way if post cured? As in... if Epoxy A has a Tg of 118 and Epoxy B has a Tg of 121, will the same post curing technique improve both epoxies equally?

    2) It's been 99 degrees here lately. If I just close up the translucent tent I'm building in on a 100% sunny day at 99 degrees, wouldn't that post cure the entire boat at once to a reasonable effect? What temperature is needed to be reached (and held) for a good post cure?

    Questions regarding Tg:

    1) What epoxy has the highest Tg both with and without post curing?

    2) What is the slowest setting high-quality epoxy anyone knows of in the States? I have incredible heat here and a very long project to do (vacuum bag 1/4 of the boat on a mold in individual panels... should take quite some time).
     
  2. Herman
    Joined: Oct 2004
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    Location: The Netherlands

    Herman Senior Member

    About Tg:

    This is the temperature at which the product (plastic, epoxy) has a transition from a glass-like behaviour to a rubbery behaviour.

    With epoxy resins, the Tg can range from about 45 degrees C to as much as 250 degrees and possibly even more.

    This Tg needs to be developed by post curing. In general, and this is a very strong generalisation, we keep in mind that the Tg can possibly be about 15 degrees C above the temperature at which the resin is cured. (which is some 40-45 degrees when cured at room temperature). There are some cold curing epoxies which develop a Tg more agressively, but these have short curing times.

    OK, onto practical curing: You are performing a free-standing postcure.

    Heating up the boat is OK, but I do not recommend more then 10 degrees C ramp rate. The epoxy needs time to cross link further, and raise the Tg. You do not want to "run over" the Tg, which, for a short time, will give you a rubbery boat, with minor or major consequences. (This could be done, however, when the product still is in the mold, and vacuum still applied.)

    Most epoxy suppliers will be able to produce a chart at which you can see the development of the Tg at certain temperatures. With this chart you can determine the amount of time needed for the postcure. (usually 18-24 hours.)

    Please also note that the ultimate Tg is something determined by the epoxy, and can be found on the datasheet. It is of no use to cure at a higher T then the ultimate Tg. (although most resin suppliers will do that, to speed up the cure of test specimens, which do not have any cosmetical demand.)

    I have no idea of what is available over there. I sell Hexion and Sicomin resins in NL and Europe, which both are well documented. At least find someting with decent documentation.
     
  3. apex1

    apex1 Guest

    In general I concur with Herman (no surprise I know), but like to get more specific on some points.

    First we have to leave this focussing on the resin.
    For any given resin you can have hundreds of hardeners behaving different and leading to a much different end product, that includes post curing and Tg issues.

    Next we have to warn you that a matrix which is explicite marketed for postcuring processing cannot be just heated up under a tarp as the sun rises!

    (Herman mentioned that to some extend)

    But you can improve almost all resins properties when you ramp up the temperature of your completed part to some 70°C in slow steps. (and decrease accordingly) Thats about 5°C hr.


    No, there is no such relation, each will behave a bit different.

    There is no product in the higher Tg rates without the need of postcuring!

    As Herman I don´t know about the US market, but Raka seems to be a helpful supplier.

    Regards
    Richard
     
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