For many years, I didn't care much for polyurethane vacuum bags for one simple reason...the cost. I've argued in the past that polyurethane wasn't worth 2 to 3 times more money than vinyl. Though there are several characteristics of polyurethane material that make it better than vinyl, it was still not worth the added expense. To me (an average woodworker with an average income) it seemed that only the pro's could afford the expense and appreciate the difference associated with this premium material. But...
Things Have Changed
Polyurethane bagging material has become much more affordable and it is now within the reach of hobbyist woodworkers and small cabinet shops. Additionally, polyurethane vacuum bags are now more flexible and can withstand greater pressure and heat. With so many advantages, I'd bet most people who try a polyurethane bag will never switch back to vinyl.
How Good Is It?
Before a durability comparison can be made between vinyl and polyurethane, a standard has to be set. Both materials come in several grades which can affect their performance and durability. In fact, a high grade of vinyl can be more durable than a low grade of polyurethane. For the purpose of this article, let's assume that this comparison is between a high grade of vinyl and a high grade of polyurethane.
With this is mind, polyurethane vacuum bags are significantly more durable than vinyl bags. With everyday use, vinyl has a tendency to develop tears and pinholes (which can be repaired). A polyurethane bag can withstand much more use and abuse because it is chemically engineered to be elastic. This elasticity is what makes it more puncture resistant than vinyl. Additionally, polyurethane has a much greater "memory". In other words, it goes back to its original shape after being stretched. And unlike many other polyurethane materials, VS Elite™ and Extreme™ polyurethane can be repaired and patched.
- Vinyl material tends to get very stiff in a cold shop environment and can make it difficult to use.
- Vinyl bags tend to be rigid and difficult to use when new. The bag will soften up a bit after several uses, but vinyl will never be as soft and flexible as polyurethane.
- Vinyl bags tend to be somewhat tacky (especially when new) and this can make it difficult to slide the project panel inside.
- Thickness affects the life of the vacuum bag. It is generally accepted that 20 mil vinyl is the minimum acceptable thickness for vacuum bagging most materials. For most projects, 27 to 30 mil vinyl vacuum bags are fine. Anything thicker than 30 mil can be too rigid and very expensive.
Polyurethane stretches more than vinyl and is therefore more resilient.
Vacuum bags made from polyurethane tend to be far less frustrating to use in a cold shop since this material is minimally affected by lower temperatures.
The 20 mil thick polyurethane material is very affordable and more durable than 30 mil vinyl. The step up from 20 mil to 30 mil polyurethane is really only needed for the most demanding vacuum pressing users where ultimate durability is the highest concern.
Polyurethane can be formulated with non-transferring lubricants which prevents most adhesives from sticking to the material. This opens up a lot of possibilities for vacuum bagging fiberglass and other tricky projects that require powerful adhesives. Of course, testing with sample material is recommended.
A New Breed of Polyurethane
I researched many of the components used to make polyurethane that affect its use in vacuum press veneering, and I have worked with several manufacturers to find a polyurethane film that meets the requirements that are crucial to professional and hobbyist woodworkers. First and foremost, it had to be ultra-resilient and durable. Second, it had to be affordable. Other important factors include clarity, bond-ability, and resistance to veneer glues and some epoxies. The truth is that finding the right combination of these characteristics is like solving a Rubik's Cube while wearing a blindfold. It took a lot of time and testing to find the right polyurethane.
Here's What You Need To Know
Strength and Resilience - American made vinyl and polyurethane can both be very durable materials. You'll find that polyurethane stretches more than vinyl and for some applications, this can be an important factor. If you are pressing a wickedly curved project, you may find the polyurethane conforms to the shape of the project much more readily than vinyl. In fact, VS Elite™ and Extreme™ polyurethane can stretch up to 6 times its width and length and still return to its original shape and size after years of use. If you opt to build a frame press or vacuum press table, polyurethane is critical because it allows you to load projects of various sizes with the same setup. VS Elite™ and Extreme™ are better for vacuum pressing than every other polyurethane formulation I've tested.
Cost - Until recently, polyurethane bags cost 3 to 4 times more than vinyl and the expense was hardly worth the increased life of the bag and the additional ease of use. However, the cost of vinyl has increased considerably since the pandemic started. Polyurethane has also increased in cost, but not nearly as much as vinyl so the gap between costs on these materials is now much smaller. I think this makes polyurethane an easy decision and my guess is that vinyl bags will soon be antiquated and obsolete.
Weight - There is little difference in weight between vinyl and polyurethane of the same thickness. But since 20 mil polyurethane is much more durable than 30 mil vinyl, it is better to compare them in that configuration. With 20 mil polyurethane you get a more durable product that weighs 33% less than 30 mil vinyl.
Adhesive Resistance - There is a wide array of chemicals that can be used to alter the performance of polyurethane films. One additive that makes the VS Elite™ and Extreme™ polyurethane easier to use is the non-transferring lubricant. It prevents glues and some epoxy types from sticking to the inside of the bag. It also make it easier (compared to vinyl) to slide the project into the bag. As an added benefit, this lubricant adds significant life to the vacuum bag because it also blocks UV light.
Storage - One of my favorite aspects of the polyurethane material is that it is so flexible that you can roll or fold it up into a surprisingly small package. My 4x4 polyurethane bag folds up and easily fits in my work bench drawer when not in use. On the other hand, a comparably sized vinyl bag must be rolled somewhat loosely and stored in a closet or behind the work bench.
Bond-Ability - The ability to chemically bond polyurethane to itself is a characteristic that allows the user to make a custom size vacuum bag. This important property eluded nearly every manufacturer who helped me with this material. After several months of tests with dozens of potent adhesives and countless polyurethane concoctions, we found something that made it possible to chemically weld polyurethane to itself with vinyl cement. This felt like a small miracle and it's a feature that is not found in most polyurethane films. Polyurethane it's not as easy to bond as vinyl, but smaller bags can be built with the right technique.
A Tough Decision
You should now have a better understanding what makes polyurethane different from vinyl. Despite the somewhat higher price, I think most would agree that polyurethane is the best value due to it's durability and overall ease of use.