PVC vs. KEE: What are the Pros and Cons?
Earlier this year, I discussed the differences between PVC and TPO. That blog post continues to generate a lot of interest, so I thought I’d follow up with a closer look at PVC and the KEE sheets.
Just as a refresher, remember that PVC is a very rigid solid – think about PVC pipes. To make a good membrane, it has to be made flexible, which involves mixing the PVC with what are called plasticizers. When PVC roofing first came out, those plasticizers were fairly volatile and on hot roofs they migrated out and evaporated. The result: the PVC got brittle and often cracked.
Over the years, the plasticizers have gotten better; they are less volatile and stay in the sheet longer. But they do migrate out – just a lot more slowly than was the case for those early formulations.
So, where does KEE come in? Well, conventional plasticizers are liquids, whereas KEE is a very soft and flexible polymer. So, if you used KEE instead of a liquid plasticizer, there’d be no liquid to evaporate out. Sounds great, but a pure PVC-KEE sheet would be very difficult to weld. Therefore, some liquid plasticizer is always needed.
To put it simply, KEE is like an insurance policy – once the roof is down, even if the liquid plasticizers migrate out and are lost, the KEE will ensure that there’s at least some flexibility.
Now that I’ve (hopefully) explained the use of KEE, let’s review the various membranes. PVC is specified by ASTM D4434 which requires the PVC content to be above 50% of the total polymer amount. KEE sheet is specified by ASTM D6754 that requires KEE to be greater than 50% of the polymer content. So, more KEE than PVC – which is why it’s called KEE membrane.
So far this is straightforward – we have PVC and KEE membranes. But, things are never so simple. Many PVC manufacturers add some KEE, but not enough to classify their membranes as KEE. They keep the amount below 50% and are doing it to provide some baseline of flexibility in case the liquid plasticizer is lost. I would argue that we have PVC, PVC-KEE, and KEE membranes on the market.
Let’s take a look at a comparison of the specifications for the PVC and KEE membranes:
|Property||PVC – ASTM D4434||KEE – ASTM D6754|
|Heat Aging – key attribute for use in high-heat conditions||176° F for 8 weeks||176° F for 8 weeks|
|Accelerated Weathering Test – fundamental requirement for UV resistance||6,300 KJ/sq meter at 145° F||6,300 KJ/sq meter at 176° F|
|Retention of Properties after Heat Aging, %||90||90|
|Elongation at Break, %||15||18|
|Tearing Strength, lbf||45||108|
|Breaking Strength, lbf||200||348|
|Change in Weight after Immersion in Water, max||+/-3.0||+6.0|
In terms of heat and UV resistance–in other words, those attributes that are key to weathering performance–these sheets appear to be the same. The marked improvement in tearing and breaking strengths in the KEE specification is worth discussing. There is only one KEE sheet on the market and it was used to develop the ASTM specification. It uses a heavier fabric than the PVC and PVC-KEE sheets and is responsible for most of the performance increase.
Superior chemical resistance of PVC and the KEE variations is often a claimed advantage over membranes such as TPO. However, this is not as simple as it seems. Firstly, there aren’t any standard tests of chemical resistance. Also, if roofs were being exposed to any number of industrial chemicals, there would be other concerns beyond how the roof was performing. Having said that, it’s certainly the case that all of the PVC-based sheets have better resistance to grease, oils, and fats than TPO.
So, there are the PVC and KEE specifications but nothing specific for the PVC-KEE sheets. My advice when looking for a way to differentiate between products is to consider the following:
- Manufacturer’s reputation – have you had consistently good results using product from the manufacturer, and does that manufacturer have a solid reputation within the industry?
- Product performance claims – does the manufacturer have solid test data, preferably obtained by an independent third party, to validate their performance? Do the tests follow industry standards, such as ASTM?
- Do not try to guess about performance based on the formulation, such as the amount of KEE or PVC in the sheet. It is performance that matters, not formulation.
Remember that good installation is critical for the long-term performance of a roof. Thinner sheets can be more easily punctured by falling sharp objects regardless of the reinforcement fabric. Also, sheets containing KEE can be harder to weld, so make sure test-cuts are taken and welder settings validated at the beginning of any installation.