Tacky T-88: Time to panic??

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Aerowerx

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Last night, after about 48 hours of cure time, I went out and checked my glue-up.

Some areas were still a little bit tacky.

The temperature here has been in the 40s at night, though, and in the 70s during the day (Farenheit).

Time to rip everything out, or should I give it a few more days?

What I did different this time was make two smaller batches so as to not waste as much. It took just a small dab of hardener. But I have been thinking that may not be a good idea. Over on the resurrected Canard Forum there was a thread about saving money by not wasting epoxy. The problem I see with smaller batches, and what may be my problem with this glue-up, is that any measurement/dispensing errors would be a higher percentage of the total batch. I know it is wasteful, but I think I would rather throw away some epoxy than have a joint failure at 5000 feet!

My first glue-up has turned out fine, by the way.
 

davidb

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Last night, after about 48 hours of cure time, I went out and checked my glue-up.

Some areas were still a little bit tacky.

The temperature here has been in the 40s at night, though, and in the 70s during the day (Farenheit).

Time to rip everything out, or should I give it a few more days?

I'd give it a couple more days before panicking. Per their literature, it can take 7 days to cure at temps in the 30s so 4 days is reasonable for temps in the 40s. Temperature does have a vast effect on cure. FWIW, I always measure by volume rather than weight. You might try that for smaller batches to get a more accurate ratio. For small batches I just eyeball equal sized blobs on a plastic plate. The clear part is more viscous than the amber so I squirt the clear first and then match the amber blob size to that.
 

Brian Clayton

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sometimes the surface layer stays tacky for a few days. Seems like it happens when the humidity is high in the shop. I would be willing to bet, if you knock off that top layer with a bit of sandpaper, you will find that it has cured fine.
 

Aerowerx

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sometimes the surface layer stays tacky for a few days. Seems like it happens when the humidity is high in the shop. I would be willing to bet, if you knock off that top layer with a bit of sandpaper, you will find that it has cured fine.

It has been quite humid here recently. As we get into fall the humidity will drop, but of course it will get colder, too.
 

Aerowerx

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I checked it when I got home from work tonight---that's over 72 hours.

It definitely feels less tacky now, so I guess it will be OK.
 

Synergy

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Long cure times with varying temperatures can allow powerful thermal stresses to weaken the bonds as they develop. Always try to cure within the specified time at a single, fixed temperature until B-stage, when the bonds can handle mechanical stress. Incomplete cure due to cold is usually a recoverable condition: the temperature merely stops the reaction from continuing, and it resumes upon warming, however this allows plenty of time for things to weaken and/or move.

Next time, tape some cheap foam panels together to make a curing environment. A variety of low-heat sources will warm the box nicely, even in large sizes. It will do wonders for quality control to aim for the curing temps that provide the best results. Just be careful about overheating and possible ignition sources!

100_2055.jpg 101_1809.jpg
 

Brian Clayton

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Long cure times with varying temperatures can allow powerful thermal stresses to weaken the bonds as they develop. Always try to cure within the specified time at a single, fixed temperature until B-stage, when the bonds can handle mechanical stress. Incomplete cure due to cold is usually a recoverable condition: the temperature merely stops the reaction from continuing, and it resumes upon warming, however this allows plenty of time for things to weaken and/or move.

Next time, tape some cheap foam panels together to make a curing environment. A variety of low-heat sources will warm the box nicely, even in large sizes. It will do wonders for quality control to aim for the curing temps that provide the best results. Just be careful about overheating and possible ignition sources!

View attachment 34949 View attachment 34950

I need to make one big enough to put my shop in!
 

Aerowerx

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Next time, tape some cheap foam panels together to make a curing environment. A variety of low-heat sources will warm the box nicely, even in large sizes. It will do wonders for quality control to aim for the curing temps that provide the best results. Just be careful about overheating and possible I ignition sources!
I have been thinking about something like this so I can keep working through the winter. But I have never heard of the stress problem.
 

Abraham Leket

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Aerowerks-
In order to get a C of A you are requested to keep track and maintain every glue batch left-overs for inspection. that is being done in order to judge your work
and also to build up confidence in yourself. If you are not sure about a batch- get rid of the items- they will hunt you on each and every bump at 5000 feet regardless the final quality. We have a saying in aviation: "when in doubt-there is no doubt"
 

fly2kads

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In order to get a C of A you are requested to keep track and maintain every glue batch left-overs for inspection.

No such requirement here in the USA. Thankfully!

How does one go about that? Label each container with an ID, and record that ID in your build log? It seems like such a policy would encourage waste, as it turns a simple task into a paperwork exercise, encouraging builders to mix up larger batches in order to minimize the paperwork.
 

4trade

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No such requirement here in the USA. Thankfully!

How does one go about that? Label each container with an ID, and record that ID in your build log? It seems like such a policy would encourage waste, as it turns a simple task into a paperwork exercise, encouraging builders to mix up larger batches in order to minimize the paperwork.

Here, in Finland we must glue one test piece with every glue mix, and keep record for parts we glue with that mix. I always make 2 test pieces, one for record and one for me to break after cure. Those test pieces what i use is short (2") 10 mm wide sticks. No need to worry when i see that test piece crack in a wood, not in a glue joint.
 

Pops

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Here, in Finland we must glue one test piece with every glue mix, and keep record for parts we glue with that mix. I always make 2 test pieces, one for record and one for me to break after cure. Those test pieces what i use is short (2") 10 mm wide sticks. No need to worry when i see that test piece crack in a wood, not in a glue joint.

Back when I built my first homebuilt in the mid 70's, I did the same thing. It was a KR-2. I broke one and saved the other for the FAA inspector. He never ask to see them. Dan
 

TFF

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If you are going to leave it at a temp below 50 and I would really be closer to 60 I would make that hot box or get a rack of the quartz lamps and place it close to the joints overnight. I used some T-88 when it was cold on a model and it did not set until I brought it inside overnight. If you get it warm in the shop for the initial cure it will go better, but cold all the time is to iffy.
 

Aerowerx

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According to the T-88 data sheet it can be used down to 35 degrees F with no loss of strength, but could take 2 weeks to cure.

When I checked this morning (6.5 days) there were no tacky spots.
 

TFF

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6 days is a long wait, and testing the fringes does not leave warm and fuzzies. Temp cycling down to extremes is not stacking the deck in favor either. Depends on how you have the part clamped. A spring clamp for 6 days would worry me.
 

Aerowerx

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From the System 3 web site FAQ:
My epoxy resin is taking too long to cure. How can I speed it up?
The only way to speed the cure of our epoxy resin products, once they've been applied, is to heat the room or the area that your project is in. Every 18°F increase in temperature cuts the time it takes for the resin to cure in half.

Will the ultimate strength of an epoxy coating be affected if it gets cold while it's curing?
No. As long as the epoxy is completely cured, the physical properties will not be affected.​
Note the statement about the cure time cutting in half for every 18 degrees.

And from the T-88 data sheet:
At 70°F, T-88 will harden in 6-8 hours and will reach functional strength in 24 hours. T-88 has been specifically formulated to cure as low as 35°F without reduction in strength; this cure will require approximately one week. At 150°F, T-88 will set within 30 minutes and develop maximum bond strength and impact resistance after 2 hours. If excessive flow-out occurs, allow 2-4 hours at room temperature before heat cure.


My previous statement about 2 weeks was in error.

If System 3 doesn't know how to use their product, then who does?
 
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bmcj

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If System 3 doesn't know how to use their product, then who does?

Not sure what you mean by this. According to their 18 degree doubling the speed, that fits the stated times reasonably well (35 degrees will be at least 4 times slower than 70 degrees, and 150 degrees should cure about 16 times faster). The numbers aren't exact, but they follow the 18 degree rule somewhat closely.
 

Aerowerx

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Not sure what you mean by this.....
I was referring to all those statements about cure time, temperature, "warm fuzzies", etc.

Seems very few know how the stuff really works.

According to System Three themselves, who make T-88, all that extra stuff doesn't matter. So long as the temperature remains above 35 degrees F, and the mix ratio is correct, T-88 will eventually cure with out any loss in strength.

That is what I meant by my statement. I would rather go with what they say first, until proven otherwise.
 
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