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 3 to 4 times more money than vinyl. Though there were a few characteristics of polyurethane material that made 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 serious hobbyists and small cabinet shops. Additionally, polyurethane vacuum bags are now more flexible and can withstand greater pressure and heat. With so many advantages, I think 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 last significantly longer than vinyl bags. With everyday use, vinyl has a tendency to develop tears and pinholes (which can be repaired). Polyurethane bags 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.
- Vinyl film that is made in the USA is almost always more durable than imported vinyl.
- Vinyl with a high percentage is plasticizers is better for vacuum pressing than ordinary vinyl.
- 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, 30 mil vinyl vacuum bags are ideal. Anything thicker than 30 mil can be difficult to work with and unnecessarily expensive.
Polyurethane stretches more than vinyl and is therefore more resilient.
Just like vinyl, the thickness does affect the life of the bag. The good news is that the 20 mil polyurethane material is very well suited for vacuum bagging and is very affordable.
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, we recommend a quick "corner" test to make sure any non-standard veneer adhesive that you might use is compatible with the polyurethane material.
A New Breed of Polyurethane
I've researched the components in polyurethane that affect its use in veneering with a vacuum press. I have worked with several manufacturers and a few chemists to make 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 must be affordable. The 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.
Here's What You Need To Know
Strength and Resilience - American made vinyl and polyurethanes are both 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, 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.
Cost - This is a tough one. Until now, polyurethane bags cost 3 to 4 times more than vinyl and the expense was hardly worth the increased life span of the bag. The VS Elite™ and Extreme™ polyurethane bags have changed everything. This material is notably less expensive than ordinary polyurethane sheeting. Despite its low cost, it's better than every other polyurethane formulation I've tested. Let's look at the nitty gritty... the real price difference between polyurethane and vinyl. A vinyl vacuum bag in the 4' x 8' size is priced at $180 and a VS Elite polyurethane bag in the same size is $295.00 (Update: they are now on sale for less).
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, its best to compare them in that configuration. Ultimately, this means that you get a more durable product that weighs 33% less than 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 polyurethane easier to use is a non-transferring lubricant. It prevents glues and some epoxy types from sticking to the inside of the bag. It also make it easier to slide the project into the bag. As an added benefit, this lubricant adds significant life to the vacuum bag by blocking 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 a user to make his/her own custom size vacuum bag. This important property eluded every chemist who helped me with this material. After several months of tests with dozens of highly potent adhesives and countless polyurethane concoctions we found something that made "bond-ability" possible. Ultimately, we ended up with a true polyurethane material that can be chemically welded with HH-66. This could best be described as a small miracle and it's a feature that is not found in any other commercially available polyurethane. Polyurethane it's not easy to bond like vinyl, but smaller bags can be built with the right technique. For more information on building your own custom sized vacuum bag, click here.
A Tough Decision
You should now have a better understanding what makes polyurethane different from vinyl. The decision to spend more money on a vacuum bag might not be an easy one. Here are the most common reasons for opting for polyurethane:
- The user wants the most durable vacuum bag regardless of price;
- The user plans to vacuum press panels at a commercial or industrial pace;
- The user has a highly curved or complex project and needs a bag to conform to its shape;
- The user demands the very best of everything he/she purchases, regardless of whether they need it or not;
- The user has a ton of excess income and wants to spend it.
Whether it's vinyl or polyurethane, stick with a good quality material and treat it well. You won't be disappointed!