Glue and vapor permeability

[Eelco]

Goretex is a little different still; its a thin hydrophophic membrane with tiny holes. The hydrophobic nature makes that liquid water will bead up on the outside, while gaseous water can diffuse through the holes by virtue of it being such a thin membrane. Waxed leather is similar in that regard really

[Eelco]

9 hours ago, fredk said:

To me; its the ability of a material to allow some air in and out. To allow damp air out of an item made from a material. Leather has no 'breathability'. Canvas deck shoes have loads

Yeah indeed the holes in a coarse canvas to allow for a direct pumping effect; like if youd hold it to your face you could quite easily blow through a significant amount of air (unlike leather). But I mean it here in the sense of all water transport; not just bulk convection. Clearly, a single layer of leather, while more closed than canvas, is far superior to a gum boot or a latex glove.

[Eelco]

8 hours ago, dikman said:

Probably why they developed Goretex and similar materials? Seems to me that any glue applied between two layers of leather is likely to clog the "pores" of said leather. Otherwise there would be no bonding between the layers.

True; any glue will clog the pores; which probably does matter, since there is also of course such a thing as wicking, which plays a role in leather, whereby the large surface tension in microscopic hydrophilic pores will rapidly suck small channels of bulk liquid water around in a way that the much smaller atmospheric air pressure differences never could. But the glue line would interrupt those continuous channels.

So im having a hard time imagining a glued piece of leather to be quite as vapor permeable as a single piece of equal thickness; but the open question to me is how close you can approximate that ideal in practice. 

[Eelco]

Tested out some PVB today; made a thin sheet which is really nice and thin and uniform since its fully dissolved in ethanol. Looks and feels like snake skin.

Didnt add any plasticizer since in pure form it should have a shore hardness similar to leather. Was kinda worried about it being brittle; though it isnt even in the freezer. In pure form it does not have a ton of stretch before breaking though... but enough to match leather I suppose.

You can notice that it is water permeable; the thin sheet pretty much instantly becomes more rubbery when put into water, showing that water dissolves into it fast. I kinda like that it seems like itd work without a plastizicer; since PEG is also used as a leather conditioner and its a fairly small molecule; so if the PEG would diffuse out of the PVAc into the leather over time the glue line would become progressively more brittle. Also I the PVAc/PEG sheets leave a bit of a greasy residue on my fingers, indicating that there is PEG weeping out onto the surface I suppose.

Its interesting that PVB glue manufacturers frequently cite their product as being suitable for leather; but ive never heard of a leatherworker actually using it.

[TomE]

3 hours ago, Eelco said:

Tested out some PVB today; made a thin sheet which is really nice and thin and uniform since its fully dissolved in ethanol. Looks and feels like snake skin.

Didnt add any plasticizer since in pure form it should have a shore hardness similar to leather. Was kinda worried about it being brittle; though it isnt even in the freezer. In pure form it does not have a ton of stretch before breaking though... but enough to match leather I suppose.

You can notice that it is water permeable; the thin sheet pretty much instantly becomes more rubbery when put into water, showing that water dissolves into it fast. I kinda like that it seems like itd work without a plastizicer; since PEG is also used as a leather conditioner and its a fairly small molecule; so if the PEG would diffuse out of the PVAc into the leather over time the glue line would become progressively more brittle. Also I the PVAc/PEG sheets leave a bit of a greasy residue on my fingers, indicating that there is PEG weeping out onto the surface I suppose.

Its interesting that PVB glue manufacturers frequently cite their product as being suitable for leather; but ive never heard of a leatherworker actually using it.

Great information, @Eelco.  Prior to retirement, I was growing protein crystals from PEG solutions and shooting x-rays at the crystals.  Fun to learn about more practical chemistry.  Did not know about PVB, and I'm interested in your use of PEG for flexibility.  Two criteria I have for gluing are short dry time before sewing and not interfering with burnishing edges.  Using Barge cement for most projects.

[SUP]

Oh Goody!  I like that. Scientific information and explanations.

6 hours ago, Eelco said:

The way water gets through then, is by dissolving into the leather, diffusing along the leather fibers, and evaporating out at the other end.

Does it dissolve into the leather fibers? I should think it will only diffuse along the fibers. That is why we use waxes and other material which will not allow that, to water proof footwear, is it not? Wax being hydrophobic, technically allows water vapor to diffuse through, but in such small amounts that it is even used to seal food. So it should be minimal.

In PVAc too, the water molecules move through between the polymer chains, in a similar manner, I believe.

Sorry, I stopped reading after that because I am not really interested in synthetic materials, but thank you. If I ever need it, I will know to read this before using anything synthetic.

And leather and pores. Saying leather is porous does not mean that there are tiny pores, like open channels on the surface, is what I meant. Unfortunately, many people  do think that, like they think about our skin with pores that open and shut like windowpanes! Nothing like that.  It just means that there are spaces in between the fibers through which water molecules can move. What we see as pores on the surface of leather is actually the points where hair follicles were.  They are, fortunately or unfortunately, not little corridors from one side to the other. If only they were. That would make stitching so much easier. No need to use stitching chisels; just widen the 'pores' with an awl, like doing so when using pricking chisels!

But I digress.

This is about moisture permeability in leather and breathability. It can be used in a sensible way to achieve what we want.

I use deerskin moccasins that I made for indoor use. The soles have a synthetic midsole and the outer and inner soles are both deerskin. I like to use a generic Gorilla glue for all my leatherwork. It is easy to use, dries to the color of wood, which is never visible anyway and is pretty strong. Are the soles permeable to moisture? Maybe, maybe not. But the rest of the shoe certainly is. The upper and the sides are all a single layer of deerskin and my feet are always dry. All through the hot summers here the moccasins get wet often but my feet remain dry. The moccasins dry as rapidly as well. I have never had an issue with moist or damp feet.  So this means that the soles do not need to be permeable to moisture, which is a good thing. Like anyone who has a pet knows, accidents happen and I would not like to be wearing moisture permeable footwear when I discover one by stepping into it!

So, my moccasins are certainly breathable, in a practical way. As long as there is a path for the water vapor to evaporate, the sides and top work as well because I am always moving and the water vapor molecules move as well... and evaporate. After all, water vapor rises anyway, so it will gradually come to the top of my feet and then evaporate through the deerskin upper. They keep my feet dry, the soles are soft and I do not step into pet accidents. And they are certainly far, far more comfortable than any synthetic material, or indeed, any store bought leather footwear too.. 

And if I want the 'earthing' experience, I do what I did as I was growing up - walk around at home barefoot; during the day, in bright light.

You have provided some really interesting information. While I, being a scientist, albeit in Biology, understand, perhaps you could express the same information in layman's terms so that everyone can understand. e.g. instead of hydrophilic, saying 'water repelling' will make things more clear; the difference between diffuse and dissolve, what permeable means and so on.

Thank you again for all the information. Coming to this site is always a learning experience and quite delightful.

[Eelco]

1 hour ago, SUP said:

I use deerskin moccasins that I made for indoor use. The soles have a synthetic midsole and the outer and inner soles are both deerskin. I like to use a generic Gorilla glue for all my leatherwork. It is easy to use, dries to the color of wood, which is never visible anyway and is pretty strong. Are the soles permeable to moisture? Maybe, maybe not. But the rest of the shoe certainly is. The upper and the sides are all a single layer of deerskin and my feet are always dry. All through the hot summers here the moccasins get wet often but my feet remain dry. The moccasins dry as rapidly as well. I have never had an issue with moist or damp feet.  So this means that the soles do not need to be permeable to moisture, which is a good thing. Like anyone who has a pet knows, accidents happen and I would not like to be wearing moisture permeable footwear when I discover one by stepping into it!

So, my moccasins are certainly breathable, in a practical way. As long as there is a path for the water vapor to evaporate, the sides and top work as well because I am always moving and the water vapor molecules move as well... and evaporate. After all, water vapor rises anyway, so it will gradually come to the top of my feet and then evaporate through the deerskin upper. They keep my feet dry, the soles are soft and I do not step into pet accidents. And they are certainly far, far more comfortable than any synthetic material, or indeed, any store bought leather footwear too.. 

Yeah; single layer upper leather shoes have a good reputation for breathability; which is also my experience for single layer tough boots I own. Though its definitely not as breathable as a fabric upper... so that made me curious how much breatability we could be getting from our soles. 

But im having a hell of a time trying to find my ideal indoor all single layer leather boot. I should buy some type of single layer mocasin type shoe though, just to have something to compare against, even if its not stylistically what im looking for.

Note what im looking for is the ideal all natural material desktop threadmill shoe. Its going to see some serious marching so it needs to stand up to that in terms of construction and comfort, as well as breathability. I kinda want ankle height because I hate heel slip or tight laces on top of my foot. And im not sure I want to larp as a roman soldier although to be fair the right kind of roman sandal would be kinda ideal for my purposes... but id rather want something more boot-looking, less sandal looking. So yeah... 

Edited February 1 by Eelco

[Eelco]

2 hours ago, TomE said:

Great information, @Eelco.  Prior to retirement, I was growing protein crystals from PEG solutions and shooting x-rays at the crystals.  Fun to learn about more practical chemistry.  Did not know about PVB, and I'm interested in your use of PEG for flexibility.  Two criteria I have for gluing are short dry time before sewing and not interfering with burnishing edges.  Using Barge cement for most projects.

Yeah good question about the working properties; I imagine that makes all the difference in practical shoe making. I think PVB is kinda nice since you can use it either as a low melting point hot glue, or as a solvent goop, or a mix of the two. You can clamp a sheet in a frame, put it over an IR heater, squeeze something in between and it will stick right away (which is how carglas is made); or alternatively use a slow vaporizing solvent to get a long working time. But thats just theory... I dont have much shoemaking practice.

I made a 20%PEG PVB sheet as well which is very flexible; and does not have the same oily feeling as the PEG-PVAc, suggesting that the plasticizer is bound better. It barely needs it anyway to be sufficiently flexible so I dont think it matters much.

I put a piece of wet paper towel folded inside, and put a wet paper towel next to it as control. The PVB sheet is about 0.05mm thick I guess.

In the same time that the control sample lost 20% of its weight, the PVB/PEG wrapped sample lost 10% of its weight.

Thats pretty good id say; in terms of diffusion-resistance, we are comparing R_air_boundary_layer to R_air_boundary_layer + R_glue, suggesting the diffusion resistance of the water through the adhesive is of a similar order as the transfer to the surrounding air. In a real shoe we have of course a whole chain, of R_sock + R_lining + R_glue + R_outer + R_air. And im guessing normally in that chain, the R_leather dominates, since it sure is noticeably different from the pure R_air youd experience barefeet. So I think in total a boot glued like this would not be terribly different from the same amount of leather without any glue.

I should buy some latex adhesive and pour a sheet of that. I think my paper would lose some moisture eventually but probably not enough to be measurable after an hour.

Edited February 1 by Eelco

[chuck123wapati]

On 1/26/2025 at 12:23 PM, Eelco said:

Ive been wanting to make my own indoor leather minimal shoe for a while, really optimizing for breathability.

Ive noticed that the leather boots I have vary a lot in breathability. Some of that makes sense; thicknesses and leather types; but in part its a bit of a mystery to me. Especially lined leather can be a real disappointment in breathability; even when the total thickness isnt that much. Im inclined to explain this in terms of shoe cement used. If you look at the chemistry of the most popular shoe cements, they are not vapor permeable at all. Infact they often prize themselves on their ability to block water. But that goes both ways.

The construction im currently considering is an upper sticheddown to the insole, and then a glue only leather outsole. This being a barefoot indoors design, I want to keep the total sole thickness minimal. And ideally, the outsole should be easily replacable.

What im hoping is that the right use of PVAc glue will serve me for these indoor soling purposes. What im hoping is:

* the glue I find wont make my sole fall off all the time

* if I want to replace the outsole, it should be easy to get off by soaking in warm water / maybe acetone?

* the thin leather sole plus water permable glue will noticeably contribute to overall breathability

* I can find a PVAc sufficient flexible for this purpose in the first place (adding 15%wt PEG400 should help with that?)

 

Does this sound at all like a reasonable experiment, or should I save myself the time? Im having a hard time finding shoe cements explicitly advertizing themselves as breathable.

If it sounds like this is unlikely to work, perhaps a good option would be thin rim of a stronger cement, with the bulk interior of the sole being PVAc?

it sounds like you want to make a double-sole moccasin. or maybe a Roman Crepida. already beeen done lol.

glue doesn't make your sole fall off, using the wrong glue just doesn't work to keep the sole on. Like using water-soluble glue in a possibly wet environment, ie sweaty leather shoes.

soaking your shoes in acetone wont be a good thing

think about this if the glue has tiny holes in it making it permeable then it won't glue anything with any strength or reliability especially if it's water soluble glue and water can get in the tiny holes. 

 

Reasonable experiment IMO no.  Make a nice pair of mocs and be done overthinking this.

[Beehive]

I once glued a liner on a belt using some kind of industrial two part epoxy. This freaking stuff was being used to glue panels on the outside of the building...overhead. The workers had thrown away a brand new double tube of it. The tubes, side by side, took a dedicated gluing gun. 

Afterwards, trying to push a needle through it was like trying to stitch a plastic coke bottle.