Glue and vapor permeability

[Eelco]

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?

[RockyAussie]

If it is for indoor use a couple of eyelet holes put into the instep area just above toe sole will allow air to be move in and out as you walk which might be a lot easier option. I do this even on outdoor wear shoes which is only a problem when I step in water. Keeps the shoes from getting smelly as well.

 

[Eelco]

True; or I could make a sandal instead . My interest here is somewhat theoretical though, to figure out how much glue matters to vapor permeability in practice.

[Eelco]

I did some experiments with PVAc; made a small sheet out of pure PVac, 10wt% and 20wt% PEG400. The pure material is not very flexible and can be cracked even at room temperature, as expected. The 10% can be cracked right out of the freezer, the 20% remains flexible in all conditions tested. Both are very tough and ductile and seem like they would make for perfectly nice leather glue (as is the conventional wisdom).

Ill guess Ill make a bigger thin sheet on a piece of silicone sheet so I can fold it into a little bag, then put in a damp sponge, plus a paper and PP bag as controls, then weight them in over time to see if it makes any real world difference in water permeability.

I got one shoemaker to talk to me about what they use for their boot linings, and its latex. Again not sure what it matters in practice but it might explain why the lined leather boots I have are about as breathable as a latex glove.

I also have PVB resin that I think in theory might also work great for leather with some PEG400 added, and its also a highly hygroscopic polymer, capable of absorbing almost 10% water; even before mixing in any PEG. Probably should also add that to my experiments. Havnt been able to find anyone directly commenting on this topic; except for this guy; though he does not offer much in the way of real world tested solutions; more a general observations of 'yeah adhesives can ruin your shoes breathability'.

[SUP]

What is meant by breathability?

[fredk]

29 minutes ago, SUP said:

What is meant by breathability?

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

[SUP]

If air and moisture go in and out, it means that water can as well. So leather is either water resistant or breathable. Or, the leather does not absorb water but it lets the water in, through the 'pores' that people seem to be talking about online everywhere. So your shoes are safe but your feet are wet  but will dry because of those very same 'pores'.

Or am I getting everything wrong @Eelco?

Of course, for indoor use, this should not be a problem.

 

Edited Friday at 09:37 PM by SUP

[dikman]

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.

[fredk]

No. eg A rubber balloon is 'breathable'; it lets air out along with moisture in the air, but it doesn't allow water in

I once had a top-level pair of leather motorcycling driving boots; they were 'breathable'. In hot weather they kept my feet cool, by letting out the hot air, but in wet weather they kept my feet dry by not allowing water in

Edited Friday at 10:09 PM by fredk

[Eelco]

9 hours ago, SUP said:

If air and moisture go in and out, it means that water can as well. So leather is either water resistant or breathable. Or, the leather does not absorb water but it lets the water in, through the 'pores' that people seem to be talking about online everywhere. So your shoes are safe but your feet are wet  but will dry because of those very same 'pores'.

Or am I getting everything wrong @Eelco?

Of course, for indoor use, this should not be a problem.

 

Im not sure breathable is a very well defined term; but if you want to get technical about it; yeah there is a difference between convection, as you would get from bulk a bulk gas being pumped around, versus molecular diffusion through the material. Subjectively the net result is the same though; how fast does a given amount of moisture go from one side to the other, and how much moisture will build up inside the shoe.

While a solid piece of leather technically is porous, the holes are so tiny that I think the amount of bulk convection through an unbroken piece of leather can be ignored. The way water gets through then, is by dissolving into the leather, diffusing along the leather fibers, and evaporating out at the other end. Same with PVAc; its a solid sheet of rubber without holes, but its a very hydrophilic rubber so water can diffuse through it. Hopefully fast enough to make a difference. Something like neophrene is super hydrophopic though; close to zero water molecules will dissolve into a rubber like that, and close to zero will make it to the other side.

[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 Saturday at 03:38 PM 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 Saturday at 03:09 PM 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. 

[Eelco]

30 minutes ago, chuck123wapati said:

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

Google seems to have a hard time deciding on what a Crepida is exactly; but the term has led me to some other interesting historical designs. The term calceus seems to come pretty close to what im thinking of.

Great that its already been done; not trying to win any originality points; I just want a shoe that will hold up to hours of indoor walking, while looking good in the process. In a perfect world id both find the time to give a decent shot at doing it myself; and find someone capable of executing on it professionally. The more theyd have done it before the better.

[Eelco]

58 minutes ago, chuck123wapati said:

soaking your shoes in acetone wont be a good thing

Im hearing mixed things about it; depends on the timing and type of leather I guess.

But the melting point of the PVB + plasticizer can be tuned to be super low, like 50C or so; plus water also acts like a further plasticizer on PVB. So if you just soak the leather outsole in warm water, I think you could just tear it right off with a bit of force, without meaningful heat exposure to your upper.

[chuck123wapati]

15 minutes ago, Eelco said:

Im hearing mixed things about it; depends on the timing and type of leather I guess.

But the melting point of the PVB + plasticizer can be tuned to be super low, like 50C or so; plus water also acts like a further plasticizer on PVB. So if you just soak the leather outsole in warm water, I think you could just tear it right off with a bit of force, without meaningful heat exposure to your upper.

it's highly flammable and very dangerous to inhale who cares about the leather think safety. not to mention the quantity you would need, simply not worth the potential problems. A crepida is a light shoe.  Good luck.

 

[Eelco]

15 minutes ago, chuck123wapati said:

it's highly flammable and very dangerous to inhale who cares about the leather think safety. not to mention the quantity you would need, simply not worth the potential problems. A crepida is a light shoe.  Good luck.

Acetone isnt all that different from ethanol in its toxicology. And I frequently handle both by the gallon. Not taking any baths in it though; and proper ventilation never hurts. Seems like these would be the least of my VOC-worries as a leatherworker though. I personally dont let any chloro-anything into my house; or any glue with an unspecific solvent composition; so its not like im not paranoid about these things. But yeah I acetone ethanol propanol and ethyl-acetate... those are the only ones I allow into my house really. But to each his own.

[Eelco]

Found some numerical values for water permeability of PVAoh here in Figure 76 and some values for leather here.

To put that in perspective, even 20mu of the most water permeable glue I can think of, has a 10x greater barrier to moisture than the most permeable leathers; and is kinda on par with a less permeable leather.

So I think that pretty much proves that if you care about permeability, you better stay away from linings at all and stick with the single layer leather; and also if you want your sole to contribute, it better be a single layer.

Also a single male foot thats active can produce some 20g of sweat per hour. Even with a leather shoe on the more permeable side of things, like a single layer thin mocasin, itd only barely keep up. Thats of course not taking into account the ability of your leather to store/buffer moisture; not like im going to be working out 8 hours continuously. 

But yeah... stay away from glue if you care about moisture, seems indeed like it should be the moral of the story here, at least if im reading these numbers correctly. Not super happy with that conclusion because I kinda like the idea of replaceable outsoles; but perhaps only glueing the outer rim of an outsole with a strong cement would suffice?

Also it is quite possible I am not reading these numbers correctly. If you take a thin layer of permeable glue and squeeze it out as much as possible, you dont get a homogenous 20mu line, but rather a variable thickness with both sides of the leather in near contact in many places. So what the net resistance of such a glue line is can really only be established empirically I think; not hard to imagine that in practice things could be an order of magnitude better; in which case properly applied glue would be back on the menu as a viable option...