Mysterious color changing with natural fibres


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Marco Todeschini
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Hello everybody I'm experimenting with resin infusion for only a year, so maybe it's just a beginner question but I'm really stucked with it.
I'm making composites with jute+epoxy resin and I can't find the logic under the changing of color of the finished part: sometimes I get light colored part with clearly visible and contrasted jute texture (which I like), sometimes jute seems deeply soaked,and looks darker and uglier. In these bad cases it's also more likely to have problems like bubbles or voids.
I did some tests trying to understand why it happens, but without results.
Jute and resin (supersap biobased epoxy) are always the same, temperature and humidity don't change too much, mold is the same.

The only things changing is the vacuum bag: sometimes a standard bag, sometimes a custom self-made silicone bag (made with Smooth-on EX-Brush silicone), but I have color changes even using the same bag type.
Just to give an example: with a silicone vacuum bag I have a good result, with a identical silicone bag I have the "dark effect".
Again: with a standard vacuum bag I have a good result, the next time I change a bit the layout of flow mesh and I have the "dark effect".

I noted that if I do a hand-layup test it always comes light-colored (good), and this gave me the idea that the dark effect could be caused by the to high vacuum level: I imagined that in a perfectly sealed mold under a full vacuum the resin struggled to move, and was forced to pass INSIDE the jute fiber, wetting it more deeply and thus darkening it more (this would go along with my theory that the first silicone bag I made worked better, maybe because having less experience I did not make it perfectly sealing).
So I bought a SMC Vacuum Regulator, nothing seemed to change. Also, EasyComposite support informed me that it is not intended for resin infusion.

Has anyone ever come across such a problem?
I really can't find the cause, I'm sure I'm really missing something important!

Thank you very much,
Marco

I post a pic of the different results I get

The mold+silicone bags I'm using. Even if the seem identical, someone gives good results, someone gives the dark effect

Chris Rogers
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That's super strange.  The thing that puzzles me is why bagged wet layup always look light.  Is it always either light or dark or do you get a range of shades of dark?  You may be on to something with the vacuum infusion wetting out more thoroughly, because the wet layup will always have some trapped air, especially with natural fibers that are so hard to fully wet out and trap so much air. 

Are there any surface defects or porosity in the light ones?  

Is the thickness repeatable?  Are the light ones measurably thicker?

There's no way this is a material issue - like roll to roll or edge to edge this still happens randomly?

A vacuum regulator is fine for infusion, but you need to (very generally) reduce the flow speed if you are using lower vacuum level.  With natural fiber especially, water vapor is an issue and it boils off into gas at near full vacuum.  Some other people do not agree with me, but I like to reduce the vacuum on epoxy infusions from really high during the shoot to about half right before clamping the resin off.  I also ALWAYS leave the pump / vacuum on the part until it cures - another thing that is 100% normal in every factory I've ever been in but not with many people on this forum - who I must admit seem to get good results turning the vacuum off!  Clearly there are lots of gray areas in this composites business.

Good luck!  Very interested to hear what you learn!...




Hanaldo
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Can also confirm I have experienced this issue when playing around with these natural reinforcements. I never really gave it much thought as generally I put it down to being a different batch of reinforcement, and to be honest I like both colours, so I probably can't help much.

But I would say it has nothing to do with the fibre wet-out or void content of wet-laid vs infused - I always infused. And still had the same random colour variance. I do feel it is going to have something to do with the moisture content of the fibre, as that could change layup to layup depending on the weather conditions. But it isn't really a classic 'symptom' of moisture issues. 
Marco Todeschini
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Chris Rogers - 12/12/2020 2:33:09 AM
That's super strange.  The thing that puzzles me is why bagged wet layup always look light.  Is it always either light or dark or do you get a range of shades of dark?  You may be on to something with the vacuum infusion wetting out more thoroughly, because the wet layup will always have some trapped air, especially with natural fibers that are so hard to fully wet out and trap so much air. 

Are there any surface defects or porosity in the light ones?  

Is the thickness repeatable?  Are the light ones measurably thicker?

There's no way this is a material issue - like roll to roll or edge to edge this still happens randomly?

A vacuum regulator is fine for infusion, but you need to (very generally) reduce the flow speed if you are using lower vacuum level.  With natural fiber especially, water vapor is an issue and it boils off into gas at near full vacuum.  Some other people do not agree with me, but I like to reduce the vacuum on epoxy infusions from really high during the shoot to about half right before clamping the resin off.  I also ALWAYS leave the pump / vacuum on the part until it cures - another thing that is 100% normal in every factory I've ever been in but not with many people on this forum - who I must admit seem to get good results turning the vacuum off!  Clearly there are lots of gray areas in this composites business.

Good luck!  Very interested to hear what you learn!...

Hi Chris!
First of all thank you for your earlier big help in beginning 2020, I emailed you and you gave me a lot of useful information. Then the world begun to go wrong and I realized only months later that I didn't respond you, sorry I didn't want to be rude. I hope I can make up for it sharing some info gained in these months' experiments.

Once again you had a good idea: I focused on weight (that doesn't seem to vary with a logic depending on dark/light) and only today I try to measure the thickness of laminates. I cut 10 bad experiments with my bandsaw in order to be able to measure thicknesses also in the center part.
Every dark shell is 2.2mm thick (sometimes 2.0 in some areas).
Every light shell is about 2.5mm thick. Sometimes 2.4, sometimes 2.6 and it seems to goes with a proportionally slightly change of shade.

So now we know that the color change (at least also) with the thickness of the stack. Since the layers are always the same I think we should talk about the compression of these layers.

About surface defects: even if I guess theory should say the opposite the light/less compressed shells are not only better in look for the clear visibility of the texture but they are also much freer of defects.

Another reasoning, but first of all I have to show you my current procedure:
This is the procedure I follow to make the silicone vacuum bag (for brevity "slimer" given the green color)
The gray part is the mold, first of all I make a test shell using hand layup and vacuum bagging (that how I saw that it always comes light if hand laid). I use this test shell as offset to simulate the real shell and then I start to brush the silicone in different coats in order to create the slimer.

This is how I make the infusion: I simply lay 2 jute cloth layers, I add a layer of jute net that I use as flow mesh (but that I'll leave IN the laminate, adding stiffness and reducing waste), I add few pieces of peelply+breather material on the edge to connect the center of the slimer with the vacuum channel.


Now the reasoning: could be the compression difference be caused by the probably slightly different thickness of different slimers? They are created just hand brushing coat after coat and it's likely to have different final thickness even with the same number of coats.
And here the doubt: does a thicker slimer give more or less compressed(>>Dark) laminates?
Unfortunately I find ideas that can support both one thesis and the other...I guess that I only need to try to make an exaggeratedly thick slimer and an exaggeratedly thin slimer and see...the bad part is that those brushable silicone cans are not so cheap Sad

A last doubt after the measures taken today: the test-offset-shell is only 1.6mm thick. So the room left for the laminate (1.6mm)is naturally less than the necessary (2.2mm or 2.5mm) because it does not take into account the jute net.
However, I don't know what to do with this news, since both slimers (both the functional one and the "darkener") were made with the same (wrong) offset.

About temperature and moisture, I did another test that I'll explain the the reply below.

Marco Todeschini
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Hanaldo - 12/12/2020 4:24:48 AM
Can also confirm I have experienced this issue when playing around with these natural reinforcements. I never really gave it much thought as generally I put it down to being a different batch of reinforcement, and to be honest I like both colours, so I probably can't help much.

But I would say it has nothing to do with the fibre wet-out or void content of wet-laid vs infused - I always infused. And still had the same random colour variance. I do feel it is going to have something to do with the moisture content of the fibre, as that could change layup to layup depending on the weather conditions. But it isn't really a classic 'symptom' of moisture issues. 


Hello Hanaldo, thank you very much for sharing your experience.
I hate to look very self assured (I never am, especially with things I don't understand), but beyond a reasonable doubt ALL the samples I have done with hand-layups are light.

Yesterday I did an experiment to test the incidence of temperature and moisture during the infusion.
I tried to keep the mold warm, about 27 ° C, during the infusion. I was afraid that the resin, in contact with the cold mold, would take too long to cure and therefore would have a long time to penetrate deeper into the fabric. Unfortunately, even though I noticed the curing time speed up, the result remained dark.

I also tried to leave the jute in a box at 35 ° C for a couple of hours, and then let the pump run for a long time before starting the infusion ... but nothing to do.
I also store the pre-cut pieces of jute I use in a box with many of those tiny silica bags that come with shippings, hoping to help reduce moisture. All the dark and light parts are made with the pieces from the same box, coming form the same roll.

In the answer above to Chris I saw a close correlation with the thickness (and therefore the compression) of the laminate.
I will continue to investigate and I'll let you know!
Thank you
Massimiliano
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Marco Todeschini - 12/12/2020 3:17:42 PM
Hanaldo - 12/12/2020 4:24:48 AM
Can also confirm I have experienced this issue when playing around with these natural reinforcements. I never really gave it much thought as generally I put it down to being a different batch of reinforcement, and to be honest I like both colours, so I probably can't help much.

But I would say it has nothing to do with the fibre wet-out or void content of wet-laid vs infused - I always infused. And still had the same random colour variance. I do feel it is going to have something to do with the moisture content of the fibre, as that could change layup to layup depending on the weather conditions. But it isn't really a classic 'symptom' of moisture issues. 


Hello Hanaldo, thank you very much for sharing your experience.
I hate to look very self assured (I never am, especially with things I don't understand), but beyond a reasonable doubt ALL the samples I have done with hand-layups are light.

Yesterday I did an experiment to test the incidence of temperature and moisture during the infusion.
I tried to keep the mold warm, about 27 ° C, during the infusion. I was afraid that the resin, in contact with the cold mold, would take too long to cure and therefore would have a long time to penetrate deeper into the fabric. Unfortunately, even though I noticed the curing time speed up, the result remained dark.

I also tried to leave the jute in a box at 35 ° C for a couple of hours, and then let the pump run for a long time before starting the infusion ... but nothing to do.
I also store the pre-cut pieces of jute I use in a box with many of those tiny silica bags that come with shippings, hoping to help reduce moisture. All the dark and light parts are made with the pieces from the same box, coming form the same roll.

In the answer above to Chris I saw a close correlation with the thickness (and therefore the compression) of the laminate.
I will continue to investigate and I'll let you know!
Thank you

Hi Marco, why don't you try a different brand of infusion epoxy? Just ti be sure there is not anything in the super sap that, under full vacuum, interacts with the jute.

Hanaldo
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Massimiliano - 12/13/2020 6:12:23 AM
Marco Todeschini - 12/12/2020 3:17:42 PM
Hanaldo - 12/12/2020 4:24:48 AM
Can also confirm I have experienced this issue when playing around with these natural reinforcements. I never really gave it much thought as generally I put it down to being a different batch of reinforcement, and to be honest I like both colours, so I probably can't help much.

But I would say it has nothing to do with the fibre wet-out or void content of wet-laid vs infused - I always infused. And still had the same random colour variance. I do feel it is going to have something to do with the moisture content of the fibre, as that could change layup to layup depending on the weather conditions. But it isn't really a classic 'symptom' of moisture issues. 


Hello Hanaldo, thank you very much for sharing your experience.
I hate to look very self assured (I never am, especially with things I don't understand), but beyond a reasonable doubt ALL the samples I have done with hand-layups are light.

Yesterday I did an experiment to test the incidence of temperature and moisture during the infusion.
I tried to keep the mold warm, about 27 ° C, during the infusion. I was afraid that the resin, in contact with the cold mold, would take too long to cure and therefore would have a long time to penetrate deeper into the fabric. Unfortunately, even though I noticed the curing time speed up, the result remained dark.

I also tried to leave the jute in a box at 35 ° C for a couple of hours, and then let the pump run for a long time before starting the infusion ... but nothing to do.
I also store the pre-cut pieces of jute I use in a box with many of those tiny silica bags that come with shippings, hoping to help reduce moisture. All the dark and light parts are made with the pieces from the same box, coming form the same roll.

In the answer above to Chris I saw a close correlation with the thickness (and therefore the compression) of the laminate.
I will continue to investigate and I'll let you know!
Thank you

Hi Marco, why don't you try a different brand of infusion epoxy? Just ti be sure there is not anything in the super sap that, under full vacuum, interacts with the jute.

I dont think this will be it. I was using a completely different resin on the other side of the world and I still experienced these variances. That said, at this point we sort of have no idea where to start - so this is as good a place as any.

I do hope we can get to the bottom of it, I'm very intrigued now!

Massimiliano
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Hanaldo - 12/13/2020 8:34:32 AM
Massimiliano - 12/13/2020 6:12:23 AM
Marco Todeschini - 12/12/2020 3:17:42 PM
Hanaldo - 12/12/2020 4:24:48 AM
Can also confirm I have experienced this issue when playing around with these natural reinforcements. I never really gave it much thought as generally I put it down to being a different batch of reinforcement, and to be honest I like both colours, so I probably can't help much.

But I would say it has nothing to do with the fibre wet-out or void content of wet-laid vs infused - I always infused. And still had the same random colour variance. I do feel it is going to have something to do with the moisture content of the fibre, as that could change layup to layup depending on the weather conditions. But it isn't really a classic 'symptom' of moisture issues. 


Hello Hanaldo, thank you very much for sharing your experience.
I hate to look very self assured (I never am, especially with things I don't understand), but beyond a reasonable doubt ALL the samples I have done with hand-layups are light.

Yesterday I did an experiment to test the incidence of temperature and moisture during the infusion.
I tried to keep the mold warm, about 27 ° C, during the infusion. I was afraid that the resin, in contact with the cold mold, would take too long to cure and therefore would have a long time to penetrate deeper into the fabric. Unfortunately, even though I noticed the curing time speed up, the result remained dark.

I also tried to leave the jute in a box at 35 ° C for a couple of hours, and then let the pump run for a long time before starting the infusion ... but nothing to do.
I also store the pre-cut pieces of jute I use in a box with many of those tiny silica bags that come with shippings, hoping to help reduce moisture. All the dark and light parts are made with the pieces from the same box, coming form the same roll.

In the answer above to Chris I saw a close correlation with the thickness (and therefore the compression) of the laminate.
I will continue to investigate and I'll let you know!
Thank you

Hi Marco, why don't you try a different brand of infusion epoxy? Just ti be sure there is not anything in the super sap that, under full vacuum, interacts with the jute.

I dont think this will be it. I was using a completely different resin on the other side of the world and I still experienced these variances. That said, at this point we sort of have no idea where to start - so this is as good a place as any.

I do hope we can get to the bottom of it, I'm very intrigued now!

Doing some research I found this https://www.slideshare.net/mobile/Farhanullahbaig/jute-fiber-71021585

According to the article, the core of a single jute fiber is air.
Please see attached pics from the article.
It could therefore be that, under very high vacuum, the core of every single fiber will be filled with resin which is not happening in wet layup at lower vacuum.
If my theory is right, an infusion at lower vacuum is worth trying.
The article also says (par. 19) that the fiber is also sensitive to chemical agents, so trying with another resin is something I would do too.




Hanaldo
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Massimiliano - 12/14/2020 10:19:07 PM
Hanaldo - 12/13/2020 8:34:32 AM
Massimiliano - 12/13/2020 6:12:23 AM
Marco Todeschini - 12/12/2020 3:17:42 PM
Hanaldo - 12/12/2020 4:24:48 AM
Can also confirm I have experienced this issue when playing around with these natural reinforcements. I never really gave it much thought as generally I put it down to being a different batch of reinforcement, and to be honest I like both colours, so I probably can't help much.

But I would say it has nothing to do with the fibre wet-out or void content of wet-laid vs infused - I always infused. And still had the same random colour variance. I do feel it is going to have something to do with the moisture content of the fibre, as that could change layup to layup depending on the weather conditions. But it isn't really a classic 'symptom' of moisture issues. 


Hello Hanaldo, thank you very much for sharing your experience.
I hate to look very self assured (I never am, especially with things I don't understand), but beyond a reasonable doubt ALL the samples I have done with hand-layups are light.

Yesterday I did an experiment to test the incidence of temperature and moisture during the infusion.
I tried to keep the mold warm, about 27 ° C, during the infusion. I was afraid that the resin, in contact with the cold mold, would take too long to cure and therefore would have a long time to penetrate deeper into the fabric. Unfortunately, even though I noticed the curing time speed up, the result remained dark.

I also tried to leave the jute in a box at 35 ° C for a couple of hours, and then let the pump run for a long time before starting the infusion ... but nothing to do.
I also store the pre-cut pieces of jute I use in a box with many of those tiny silica bags that come with shippings, hoping to help reduce moisture. All the dark and light parts are made with the pieces from the same box, coming form the same roll.

In the answer above to Chris I saw a close correlation with the thickness (and therefore the compression) of the laminate.
I will continue to investigate and I'll let you know!
Thank you

Hi Marco, why don't you try a different brand of infusion epoxy? Just ti be sure there is not anything in the super sap that, under full vacuum, interacts with the jute.

I dont think this will be it. I was using a completely different resin on the other side of the world and I still experienced these variances. That said, at this point we sort of have no idea where to start - so this is as good a place as any.

I do hope we can get to the bottom of it, I'm very intrigued now!

Doing some research I found this https://www.slideshare.net/mobile/Farhanullahbaig/jute-fiber-71021585

According to the article, the core of a single jute fiber is air.
Please see attached pics from the article.
It could therefore be that, under very high vacuum, the core of every single fiber will be filled with resin which is not happening in wet layup at lower vacuum.
If my theory is right, an infusion at lower vacuum is worth trying.
The article also says (par. 19) that the fiber is also sensitive to chemical agents, so trying with another resin is something I would do too.






All very interesting, however both myself and Marco have experienced the variations from an infusion result. It's not like hand layup = light coloured and infusion = dark coloured, it changes. Which would indicate that at least sometimes, the high vacuum level is not an issue. I wonder if perhaps there is another influence that means that hollow fibre in some situation is 'sealed' and doesn't get wet out, and in other situations it is open and does allow full wet out.

Thing that gets me is, the colour difference is drastic. Which makes me feel like it has to be more than just the fibre wet out. If the fibres fully wet out appear as the dark colour, then surely for the fibres to appear SO much lighter when not fully wet out then it would be very noticeable in other ways as well, eg. noticeably dry fibres in a cross-sectional cut, or differences in strength between the two laminates. I don't see how that microscopic channel of air through the middle of the single fibre could possibly make such a massive difference to the colour.
Lester Populaire
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I have quite a bit of experience with all kinds of entropy resins and natural fibres and I don't think it is related to the resin (which work really well). Never seen anything like that but don't usually work with jute.
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