postcure problem

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#1

Hi, I have a problem with post cure. I made some flat test panel with gel coat, glass fiber and epoxy resin. I post cure it untill 80°C with a 10°C/h ramp. After 6 hours, if I take off a panel from the oven, it sags. If I switch off the oven and the themperature decreases slowly, the panels sags again. Panels are made with gel coat and 2 layers of 300 gr twill glass, both 0/90°

I used 2 differnt kinds of gel coat and epoxy resin, but there arent’t different, panels sags on the side of gel coat.

I’m trying now to cool down slowly. Panels are small, about 15x20 cm

#2

They are unsupported? Does the resin has a Tg high enough? Epoxy will become somewhat softer during postcure, especially with a short period between hardening and postcure. Cool down and it will become stable again, or postcure further until fully cured.

#3

They are lay flat, with gel coat upwards. When they cool down, they sags with the sides upwards. I used 2 different resin, one with (fully cure) a Tg of +90°C, the second has a Tg of 190/200°C. If I cool down, panels sags, if I heat the then to 80°C, they return flat. I just tried, if I cool down the oven from 80°C to 60°C, they sags

gel coats are

  1. MGS T30
  2. ebalta OH82-1

resin

  1. hexion EPR 320 + EPH 161
    2 hexion EPR 4434 + EPH 4434
#4

Have you tried flipping over one of the layers of 300gsm glass? You might have an issue with symmetry about the panel’s neutral axis. I have heard that some woven fabrics are not truly symmetric about the thickness (one side is more 0 dominant and the other more 90 dominant), I’m not sure if twill weaves are this way, but if you flip over the second layer (or rotate it 90 degrees) and run the panel again and it doesn’t warp, then that is the issue.

#5

I have not tried to do this… From the catalog, weight rate is warp 49%, weft 51%. I don’t know if this can give problems. It is also true that a panel il made with a different 300 gsm fabric, and it has the same problem. I have cool down to 70 °C and panels sags again, then heat uo to 80, and they are almost flats. Mystery. The panels are bent always on the side of the gel coat

#6

T30 gelcoat is polyester based, and has a quite high shrinkage. Tg might be not that much.

To get the most out of 4434 you need to postcure to 200 degrees or more, supported.

To get the most out of 161 (do not know the 320 resin) you need to postcure up to 120 degrees or so.

#7

herman, you are right for T30, but ebalta is a epoxy hi temp gel coat…
Yes, I need to post cure to 200°C the 4434, but I suppose that if I cure it to a lower temperature, I have a lower Tg ok, but not a distorsion like this after several hours of post cure.
I will do a panel without gel coat, to see if it si the cause

#8

How long is the resin curing at room temperature before post curing? What was the layup method? Are the fabrics on each face mirroring each other? Is the plate cupping, twisting, or potato chipping?

#9

Hi wyowindworks, I wait 2 days before post cure.
Layup is
day 1, gel coat
day2 - a panel 2 layers of 300 gsm twill glass fabric, both 0/90
- a panel 2 layers of 300 gsm glass fabric (but a different type, more coarse, I don’t know the technical name in english, I think is matting)
An other panel is made with 225 gsm mat and poliester resin, and it sag.
days 4, 5 and 6 post cure…day 4 for a few hours untill 50°C, the day after up to 80°C…I haven’t time to do it in once
Plate cupping, monday I take some photos

#10

Even if your fabric is symetric, adding a gelcoat is adding more material to one side. If different resins, then shrink rate will be different. If only a flat panel, try adding a metal sheet on top for the whole post-cure.

By sag, i think you mean warp, if you are curing flat panels on a flat oven surface. More or less the same idea, but one does it on its own, and one uses gravity to help.