Waterproof Fabric Coatings 101: PU vs. PE vs. Silicone
One of the most important functions of a tent is (obviously) to keep you dry. This is almost always accomplished by using waterproof fabrics. In a traditional double-wall tent, there’s a waterproof outer layer called a fly or flysheet and a breathable inner tent body with waterproof floor and sidewalls. The fly is almost always made of a woven fabric (learn more about wovens vs. nonwovens in our article on fabrics). Usually it’s nylon in higher-end tents and polyester in less expensive tents.
However, woven fabrics alone are not waterproof. The most common way to produce a waterproof fabric is to apply a waterproof coating to an uncoated fabric. Coating involves the application of a liquid that adheres to the fabric and then is cured, leaving the fabric waterproof.
The fabric finishing process begins with an undyed, uncoated fabric, called greige (pronounced GRAYGE). After the fabric is dyed and finished, the fabric coater will apply one or more layers of coating (sometimes different coatings to each side of the fabric). Different coatings have different properties that make them ideal for specific applications. The weight and composition of the fabric as well as the intended application are both important factors in coating choice. In this article, we’ll be taking a journey of discovery through the most common waterproof coatings used in tents: polyester urethane (PU), polyether urethane (PE), and silicone, and combinations thereof.
Before we get into the really exciting stuff, a quick note about how the outdoor industry quantifies waterproofness: waterproofness is typically measured via a process called hydrostatic head (HH) testing. This technique determines how much water pressure must be exerted on a fabric before it begins to pass through. This is accomplished by measuring how tall a column of water is before the weight of the water exerts enough pressure to start pushing it through the fabric. A good way to visualize this is to picture a tall tube with a piece of fabric stretched across the bottom end. The tube gets filled with water until it soaks through the fabric. The height of the water (in millimeters) in the tube just as the water begins to penetrate the fabric is called the hydrostatic head rating. This is why the unit for waterproofness is millimeters (mm) and also why the waterproofness of a fabric is also referred to as a “water column”. A fabric is generally considered waterproof if its water column is around 1,000mm or more.
A quick aside so we don’t anger the technically inclined and easily incensed: it’s unwieldy to test waterproof fabrics in this method, as you would need a really long tube (some coated fabrics can exceed 10,000mm, which translates to a 32 feet 9 3/4 inch tall water column). These days, this test is usually done with a device called a Suter Tester, which uses a high-pressure pump to force water through the fabric instead of gravity, and then calculates the equivalent mm rating. We’ll go more in depth into Suter Testing in future articles.
Coatings have several attributes that must be taken into consideration when choosing a coating for a particular use case:
- Waterproofness: While all three of the main coatings are waterproof initially, some coatings cannot achieve very high hydrostatic head ratings, and some can absorb water, or “wet out” when exposed to moisture for extended periods of time.
- Durability: Coatings affect the tear strength of the fabric to which they’re applied by enhancing or reducing the fabric’s elasticity.
- Longevity: Coatings can have finite lifespans. Factors like mold growth and hydrolysis (breakdown of the coating) are more prone to affect some coatings more than others.
- Ability to accept fire retardants: Fire retardant chemicals must be mixed in with the coating before it is applied to the fabric. FR chemicals can only be combined with PU and PE coatings, not silicone. CPAI-84 is a standard adopted by a few states that specifies requirements for fire-retardancy in recreational tent fabric.
One last note before we get to the good stuff. Occasionally, you may come across tents made with fabrics that are waterproof without the use of a coating. These fabrics are usually laminates (multiple layers of material sandwiched together and adhered with heat or adhesive). Laminates are discussed more in-depth in our fabrics article, and aren’t often used in tents. The laminates you’ll usually see used in shelters are Dyneema Composite Fabric (aka Cuben Fiber) and waterproof breathable fabrics (WPBs) that are used in most single-wall tents. Single wall tents have a very limited use case as they manage condensation very poorly, even when made with waterproof breathable fabrics. While WPBs often have a Durable Water Repellent (DWR) finish, they don’t get their waterproofness from the DWR. Instead, WPBs utilize a microporous membrane that blocks water droplets but theoretically allows water vapor to pass through.
Polyester Urethane (PU)
Polyurethanes are a broad class of polymers encompassing two of the three common fabric coatings: polyester urethane (PU) and polyether urethane (PE/PeU). Because PU is much more common than PE, when the outdoor industry says “polyurethane”, they’re talking about polyester urethane, and that’s what this section is about. Yes, I know that from a strict organic chemistry perspective “PU” should technically encompass both polyester and polyether urethanes, but this isn’t an organic chemistry class (thank goodness) and I’m going to use the verbiage the outdoor industry has adopted as standard. If this bothers you then you should stop reading now and go camping instead. PU offers a solid balance of attributes, which is why it is by far the most popular waterproof coating for tent fabrics. PU can achieve high hydrostatic head ratings, it can accept fire retardants to meet CPAI-84 standard, it’s inexpensive, and it doesn’t reduce the fabric’s tear strength as much as PE coatings do. For tents that will be exposed to large amounts of moisture, PU coatings are often used because they can achieve hydrostatic head ratings in excess of 10,000mm (that’s a lot). This makes PU ideal for applications when there will be a lot of pressure exerted on a fabric, like if you’re using a cot in your tent.
Although PU is the standard and works pretty well under a variety of conditions, there are some drawbacks that make us think that PU is over-utilized in tents. The first issue we have with PU is that it decreases the tear strength of the fabric to which it is applied. This happens because of the way the coating affects the slippage of the fabric. Slippage is when the individual fibers that make up a fabric, or yarns, slide over one another. When you try to tear a fabric, slippage allows the yarns to bunch up, rather than remaining fixed in position. To propagate the tear, you’ll have to rip through several yarns at once, rather than one yarn at a time. This distributes the stress on the fabric over a larger area, thus reducing the force and making the fabric less likely to tear. In the case of fabrics with low slippage, adjacent yarns are not recruited to resist the stress, which then gets applied to only one yarn at a time. This means the stress is much more concentrated and the fabric is more likely to tear. As you would expect, ripping through five yarns at once is harder than ripping through one, and a fabric with more slippage will have a higher tear strength. PU coatings get in between the yarns, and lock them in place. This reduces slippage and consequently reduces the tear strength, making PU-coated fabrics slightly weaker than the fabric before coating.
Long-term waterproofness is another area in which PU coatings have some drawbacks. While it is quite counterintuitive, PU coatings are actually hydrophilic, which means they will slowly absorb water if they remain in prolonged contact with it. Water takes a long time to penetrate PU, which is why PU works as a waterproof coating. However, if there’s enough moisture in the environment, water will eventually saturate the PU coating and go into and through the fabric, causing the coated fabric to leak. This is rarely an issue in terms of water actually penetrating into a tent, as the process is quite slow. However, it can be problematic if you’re camping for a long time in especially wet areas or in a spot where water pools under your tent. This problem can be addressed by applying very thick layers of PU, meaning the water will take longer to penetrate the fabric. However, this has the disadvantage of adding considerable weight (coatings, especially PU coatings, are surprisingly heavy) and further reducing tear strength. PU’s water absorption causes a few additional problems. First, waterlogged PU coatings take longer to dry out. Second, as we discussed in our fabrics article, nylon will sag when it gets wet, so PU-coated fly fabric has the tendency to sag. This makes it difficult to maintain a taught pitch in humid and wet conditions. Some designers combat this by using polyester instead of nylon but that approach has its own problems. The final issue, and the biggest threat to your tent’s well being, is increased susceptibility to mold and mildew. Mold and mildew are some of the top tent killers out there. Like all living things, they need water to survive, and when they have a moisture-rich environment like a waterlogged PU-coated fabric the mold can destroy the coating, leading to a foul smelling tent with compromised waterproofness. For this reason, storing a PU-coated tent long-term when it’s still wet means almost certain death to your fabrics. Extra care must be taken to make sure it’s absolutely, positively bone-dry before you put it away for more than a day or two. Storing your tent in a dry environment will also prolong the life of your tent. Humidity alone can destroy coated fabrics and factory hot-taped seams. This isn’t to say that non-PU-coated tents should be stored wet. That’s also a huge no-no. Uncoated (i.e., breathable) tent canopy fabrics will absorb water and grow mold, but at least your silicone or PE-coated floor and fly might be spared.
PU (and rarely PE) coatings are often used in tents in conjunction with silicone coating. This combination is usually referred to as sil/PU. This means one side of the fabric (usually the inside) is coated with PU and the other side is coated with silicone. This combination provides some of the benefits of silicone (more hydrophobic, better long-term waterproofness, etc.) and still allows the requisite fire retardants to be mixed into the PU side to achieve CPAI-84 standard. However, there are some drawbacks to this. First, the tear strength of sil/PU fabric is still lower than the tear strength of the fabric before coating because of the PU. Also, because the two coatings are not applied simultaneously, achieving the ideal thickness on each side of the fabric is a tricky balancing act. The silicone cannot be allowed to penetrate all the way through the fabric. If it soaks in too far while it’s being applied, it can lead to coating adhesion failures when the PU is applied. Almost nothing adheres to silicone (which is why it’s the material of choice for non-stick spatulas and many industrial lubricants), including PU coatings. If the PU is applied to silicone-saturated fabric it can peel off prematurely, leading to compromised waterproofness. Additionally, the PU component of this coating technique is still not immune to hydrolysis (discussed below), and will eventually break down.
Hydrolysis: PU’s Achilles’ Heel
Many of you may have noticed a peculiar smell emanating from your old tent. It falls somewhere on the spectrum around vomit, smelly gym clothes and old raisins and permeates my early memories of camping with my parents. Despite its nostalgia, “Old Tent Smell” means that your beloved tent has entered its sunset years. The odor is a byproduct of a reaction called hydrolysis. The propensity of a coating to undergo hydrolysis, or hydrolyze, is called hydrolytic stability. Of the three coatings discussed here (PU, PE, and silicone), PU has the worst hydrolytic stability. Over time, the PU coating reacts with moisture in the air (this process is accelerated by heat) and splits the polyester urethane molecule. The products of this reaction are smelly, sticky, and are not waterproof. The irony that the cause of the stench is “PU” is not lost on us. The reaction can be delayed as long as possible by storing your tent in a cool, dry, dark place, but sooner or later it will succumb to the stinky inevitability of organic chemistry. When I first encountered this on my dad’s 1970s North Face VE-24, I tried as many homespun cures as I could, to no avail. Unfortunately, once hydrolysis has begun, there’s no stopping it. While a good scrubbing may temporarily get rid of some of the most offensive volatiles, it’s a purely symptomatic treatment. Even if you can get rid of the smell, your coating is still on its way out and won’t stay waterproof for much longer. Because of hydrolysis, fabrics with PU coatings essentially have a shelf life. It may take a decade or more, but even if you treat your tent with the utmost care, it will eventually hydrolyze. This doesn’t factor in much to most companies’ decisions to coat their fabrics with PU, as it’s assumed that something else on the tent will break or you’ll upgrade before hydrolysis becomes a problem. However, if you’re hoping to be using your tent a decade from now, then it would be a good idea to seek out tents without PU coatings.
Polyether Urethane (PE)
Polyether urethane is the other member of the polyurethane family that is used in outdoor applications. It is far less common than PU, probably because it’s more expensive and simply because it’s not as well known. PE has similar characteristics to PU in that it can be combined with fire retardants and is applied in a similar manner, but with a few major differences that make it better for some applications and worse for others. Generally speaking, PE is used in burlier tents in harsher environments. It’s a fairly popular choice for floor fabric in expedition tents, where weight is less of a concern and long-term durability is paramount.
Waterproofness, durability, and longevity are all intertwined with PE coatings, so it’s hard to address each characteristic separately. It’s easiest to start with PE’s greatest disadvantage, and work from there. PE coatings stretch even less than PU coatings, and they all but eliminate slippage in the fabrics to which they’re applied. The result is that PE coatings often decrease the fabric’s tear strength. We use PE-coated fabrics on the 70D and 210D floors of our expedition series tents, because those fabrics start out with an extremely high tear strength, so the burliness of the fabric itself is more than enough to compensate for the reduction in tear strength by the PE coating. It is uncommon to find PE coatings applied to lightweight fabrics. After years of searching, we finally found a 20D lightweight PE-coated fabric that was strong enough to use on the floor of our Portal tent. It’s the only PE-coated fabric we’ve seen under 70D with a tear strength high enough for us to use in a tent.
So why would you want to use a PE coating if it compromises the fabric’s tear strength? PE has several advantages that make it a great choice for floor fabrics. First, it does not absorb water like PU coatings. It’s hydrophobic and even when left in prolonged contact with water, it doesn’t wet out, even when compared to a PU fabric with a better water column. The main benefit of this in terms of durability is that PE-coated fabrics are extremely mold and mildew-resistant. Because the coating keeps the fabric dry on the inside, there’s no moisture to support mold growth.
The other big advantage to PE coatings is their great hydrolytic stability. They are far less prone to hydrolyzing than PU coatings. We almost never come across hydrolyzed PE coatings, and the ones that we have encountered have usually been used for a long time in particularly hot and humid conditions (if you want your tent to last a long time, don’t move to the Philippines or Singapore).
If long-term durability is a concern for you, or you’re mostly going to be using your tent in hot, humid places, look into a tent with PE coated fabrics. PE coatings are not nearly as easy to come by, as they haven’t been fully embraced by the industry yet. I know of only three companies that use them, and two of them (SlingFin and Mountain Hardwear) were founded by the same person. However, because they can be mixed with fire retardants, PE makes it possible to avoid many of the shortfalls of PU coatings while avoiding state-by-state shipping restrictions. In our opinion, PE coatings are under-appreciated and underutilized. As tent designers start thinking harder about why they’re using the materials they’re using (and read this article), expect to see PE coatings becoming more and more popular.
Spatulas, lubricants, sealant, greases, implants, wire insulation, and tents? The chemical properties that make silicone such a diversely utilized class of compounds (nonstick, chemically inert, hydrophobic) also make it an excellent material for waterproofing tent fabrics. To differentiate pure-silicone coatings from sil/PU coatings, they are most commonly referred to as sil/sil fabrics, which indicates that silicone was applied to both sides of the fabric. Some names for specific fabrics using silicone coatings are silnylon (referring to nylon that has been silicone coated) and silpoly (silicone coated polyester, which is less common due to polyester’s poor tear strength but is sometimes used in tarps). Sometimes you’ll see misleading things like “fly and floor are silicone treated”, “coated in a durable silicone repellent”, or “silicone-treated ripstop nylon”, when companies are actually using sil/PU or sil/PE fabrics, so make sure to get into the nitty-gritty tech specs to make sure you actually know what you’re getting. Sometimes you even have to call brands to get a straight answer.
Unlike polyurethanes (both PU and PE), silicone is an inorganic polymer. The molecules that comprise organic polymers have carbon backbones, whereas silicones are built around a highly stable Si-O bond. This is the basis for silicone’s chemical inertness. Silicone is much more inert than polyurethanes, which means it’s less prone to react with other compounds (e.g. hydrolysis). This is why it’s possible to make silicone that’s safe for use in cooking tools or internal applications like implants. But you’re not eating (much) food off your tent, so what makes silicone so great for tents?
The amount of silicone that can be applied to a fabric is limited by the thickness of the fabric. With PU coatings, you can keep slapping coat after coat on the outside of the last coat, which is how you can get such high HH results. That’s not so easy with silicone, so the main drawback of silicone coatings is that it’s hard to get water columns much higher than 1500mm when you’re using lightweight fabrics. This hasn’t been an issue in our experience. Even with the 1500mm 30D sil/sil nylon used in the floor of our WindSaber, we have not had any issues with leakage, except when one user set theirs up on top of a small stream. Under normal conditions, silicone is so hydrophobic (when’s the last time you had to wring out a spatula?) that wet-out is not a concern. Sil/sil fabrics absorb little if any water. Because silicone keeps your nylon from soaking up water, fabric stretch due to moisture is greatly reduced and your fly (or tarp) will stay much more taught when wet. You can get your fly almost totally dried out just by giving it a good shake. This also means that silicone imparts an unmatched level of mold/mildew resistance. We have never seen a case of mold growth on a silicone-coated fabric (again, this doesn’t mean you should put your tent away wet! Don’t do it! Don’t even think about it!). The cherry on top (or maybe the sundae itself, depending on your priorities) is that silicone coatings don’t hydrolyze. We have never seen a sil/sil fabric go bad. This is a big deal. As long as you store your tent properly, your sil/sil fabrics will outlast all other coatings and coating combinations.
Another huge benefit of silicone is that unlike PU and PE, silicone coatings actually increase the tear strength of the fabric to which they are applied. As I mentioned above, silicone is often used as a lubricant. When silicone coats the yarns in a fabric and is cured, as it does when it is applied as a coating, it makes it easier for the yarns to slide over one another. This increases slippage and therefore the tear strength. Sil/sil fabrics often have significantly higher tear strengths than the same fabric with a PU or PE. For this reason, silicone is often applied in small quantities to enhance the tear strength of non-waterproof fabrics.
You may have gathered from this that at SlingFin, we prefer to use sil/sil fabrics wherever possible. So why aren't they ubiquitous? I can’t vouch for what other designers are thinking, but there are a few hurdles between silicone’s current level of use and the total market domination it deserves. The first is price, which is unfortunately always a consideration when selecting materials. Sil/sil fabrics cost significantly more on average than their sil/PU counterparts. In my experience, the same base fabric with a sil/sil coating costs 10-30% more than it would with a sil/PU coating. That’s not a huge issue for SlingFin, since the price point market has never been (nor will be) our focus. Another issue is that PU is simply the way everything has been done for many years, and it can be surprising how reluctant many companies are to deviate from the standard.
There are a few minor issues with silicone from a functional standpoint. One is that it can’t be seam-taped effectively. Seam tape is what keeps sil/PU fly fabrics from leaking through the needle holes created when the tent is sewn. There is one company that offers stuff sacks with seam-taped sil/sil fabric, but I've seen enough of that seam tape fail to make me think they haven’t perfected it yet. Fortunately, there’s a solution for tents. A seam style known as “double needle lap-felled” reduces the need to seal or tape the seam when you're using sil/sil fabrics. With the DNLF seam, each stitch goes through four layers of fabric. It’s a fairly specialized technique but most premium factories have this capability. During extended hard rain, some water can wick in through the thread, but we've only seen this issue with consistent heavy rain that lasts at least several hours. Should that occur, it's an easy fix with any silicone-based seam sealer. DNLF seams aren't as effective with PU or Sil/PU fabrics as the PU wets out and facilitates moisture wicking in through the thread. Single needle seams will need to be sealed after construction with a liquid seam sealer. All our sil/sil flies are sewn with double needle lap-felled seams. Another issue is that because sil/sil fabrics can be quite slippery, some people don’t like to use them in floors. We currently use sil/sil fabric for the floor of our WindSaber tent, and the slipperiness of the fabric hasn’t proved to be an issue for us or our customers.
Perhaps the biggest obstacle preventing sil/sil fabric from gaining market share is that silicone coatings can’t be combined with fire retardants to meet CPAI-84 standard. A few years ago, there were seven states that still used CPAI-84 as the standard for flame resistance in recreational camping tents. Amidst growing concerns around the health risks of fire retardants, the number of states that restrict tent sales through CPAI-84 has dropped to three. Unfortunately, one of those states is California, which I am told has at least several people and one or two parks. This is why our Portal and SplitWing tents (both of which use a drool-worthy nylon 66 sil/sil fly fabric) can’t be shipped to our home state. Many online retailers don’t have the ability to restrict sales on a state-by-state basis (most notably Backcountry.com), so as long as the 800-pound gorilla that is California restricts sales of tents made with sil/sil fabrics, the industry will be slow to switch.
In our experience, silicone has proven to be the best tool for making a fabric durably, reliably waterproof. Silicone-coated fabrics last longer and have numerous advantages over PU and PE coatings. At SlingFin, we’ve been using sil/sil nylon 66 ripstop in our expedition series tents since their inception. As we expand our use of sil/sil fabrics into the Portal and SplitWing tents this year, we’re confident that SlingFin is on the forefront of an industry-wide shift in the way fabrics are used in the design and construction of tents.
***UPDATE - The industry has helped push to update the codes and sil/sil, non-FR coatings are now commonly sold and shipped in California!