Updated Summer 2020

Part 2 covers hull forms and rigidity • More about glues and repairs here

A ‘PVC vs rubber’ article from 2011 by The Boat People
PVC vs PU vs synthetic rubber: a good if slightly biased page. Skip to: 2. Characteristics of the Material to read about PVC (vinyl’), TPU (polyurethane’) and synthetic rubber (‘hypalon, etc).

Broadly speaking, quality inflatable kayaks are made from two types of material: a woven fabric core coated in either ‘rubber‘ or ‘plastic‘. ‘Plastic’ can further be split into PVC (cheaper) and TPU, more modern, more expensive and less toxic.
IKs are also fabricated or assembled in two ways: either with ‘inner tubes’ which inflate a fabric shell to support a rigid form, like a bicycle tyre inner tube. Or sections of hull fabric carefully cut and glued together into fully airtight chambers making a load-carrying monocoque. I call this ‘tubeless‘ and the pros and cons of both are outlined below.

Synthetic rubber

These use a nylon or polyester (PES) woven fabric (see right). Nylon is more stretchy which is good for impacts and punctures but less good for rigidity. It is coated with tough natural and/or synthetic rubber and sometimes a softer neoprene on unseen inner surfaces. Examples include the original hypalon, invented decades ago by DuPont, no longer made or trademarked but actually referring to the coating, not the finished material.

Hypalon fabric is composed of three plies all bonded together:
• an inner coating of neoprene makes it airtight and easy to glue
• a fabric core of polyester or nylon weave gives tensile strength and resists tearing
• the hypalon outer resists UV and chemicals very well.

A near-identical product is now supplied, among others, by Pennel in France under the ‘Orca‘ brand (graphic, above left) as used on US-made NRS IKs.
Gumotex’s various Nitrilons (left) are similar, as is EPDM aka: ‘Nordel’ used by Grabner as well as some folding kayak makers and raft manufacturers. In 2012 Gumotex brought out a material called Hevealon based on their single-side coated Lite-Pack (renamed Nitrilon Light and since dropped). Now all boats are Nitrilon.

Gumotex in Czech Rep. produces Nitrilon fabric in their factory as well as various other rubber-based sheeting and products besides inflatables. The tougher boats in their inexpensive recreational IKs are made from regular Nitrilon (see graphic above) and are glued together by hand.

The gluing of some of these boats limits the operating pressure to 0.2 bar (2.9); still-impressive by the standards of many well known American-branded IKs which run as little as 0.1 bar. But notably, a couple of Gumotex IKs including the long-established and expensive K whitewater series and the Seawave sea kayak run 0.25 bar (3.6 psi). It is possible that these boats are not hand-glued, but heat vulcanised, a join that can contain higher pressures.

Austrian Grabners say their EPDM boats are heat vulcanised which can be compared to heat- or radio-frequency (RF) welding of plastics like PVC. Heat welding PVC is so easy and effective you can do it yourself with roller and a heat gun fitted with a flat nozzle (left), though trying to make an IK like this would be quite a challenge (as would be gluing up an IK from Nitrilon sheet).

Apparently, heat welding PVC was invented in 1948 in Vitry, France by SEVY, the company that went on to become Sevylor (‘Sevy of gold’). They produced an inflatable PVC bath that became a hit in post-war austerity France. A velour coated air mattress followed and since that day Sevylor has never looked back.

Vulcanising is a chemical reaction which either molecularly bonds rubber to itself or makes a rubber product, such as a tyre more durable. In a bonding sense, vulcanisation is what happens when your punctured tubeless car tyre is professionally repaired on the inside surface, whereas gluing is like sticking a patch on an inner tube (stop me if I’m going too fast, here). Assuming I have the right end of the sticky stick, that means Grabners have a superior construction process to most Gumotexs, and may also explain how, despite running 50% higher pressures, Grabner boats don’t feature pressure release valves (PRVs); their boats are so well bonded there’s little risk of them bursting if left in the hot sun. But that’s still not recommended.

Don’t be put off by the picture of delaminated Grabner on the left – it’s from a 12-year-old Grabner Holiday 2 and occurred at a point where a metal eyelet on the demi-deck didn’t get on with the EPDM fabric (that boat has since been renovated). It’s there to illustrate the densely woven and stretch resistant underlying fabric. On full inflation a good IK doesn’t want to stretch like a balloon (or a PVC Slackraft come to that), it wants to be taught like a basketball. As folding boat makers, Pakboat put it on their website: ‘The abrasion resistance [and so, waterproofing] is in the coating, and the tear strength and tensile strength are in the woven fabric.’

PVC and TPU

These days most IKs are made from a cheaper but stiffer PVC-coated fabrics. Remember, with PVC this is not the plastic film of the pool toy and Slackraft, but PVC coated or bonded onto a nylon or polyester fabric core and, just like Hypalon rubber, makes a flexible, durable and airtight coating that’s less pliant or stiffer.
The difference is a tent made out of a plastic bag; light and waterproof yes, durable no. Or a tent made of normal woven fabric with a PU waterproofing coating: much more durable. TPU is superior to PVC in all ways bar price, but is less familiar as a brand name. It’s also superior in some ways to synthetic rubber. This page explains.
In environmental terms, PVC has become a bit of a dirty word (Greenpeace report) which is why Gumotex US make such a song and dance about their IKs not being made from PVC. Apparently toxic chemicals like DEHP (below) leach out of the material and it degrades internally over time. This outgassing may be what what gives PVC it’s distinctive plasticy smell.

It’s staggering to note the lengths that boat makers like Ally Canoe, Pakboat, Sea Eagle and even Aire will go to not to mention ‘PVC‘ by name. They will just describe hull material as ‘1100 Decitex Reinforced’ or ‘Base Fabric Denier 650/1000’, but meantime the bladder cells (see below) are proudly identified as urethane. But PVC it is and so all US vendors must post a warning above if they dare sell PVC products in the Sunshine State. Will using a PVC IK normally make you ill? It’s extremely unlikely but Californian environmental law – the strictest in the US – insists on the warning.

Rubber vs plastic

Synthetic rubbers like Nitrilon and EPDM are both tough, durable and more expensive than PVC, metre for metre. On a same-sized boat, they’re also more pliant and will roll up much more compactly than PVC (right, a 4-metre Nitrilon Sunny). Importantly, this means creases and folds cannot develop into cracks and leaks, as I’ve found on PVC boats.

Synthetic rubbers also have very good resistance to UV and solvents, are heavier than comparable PVC/PU and come is fewer colour options. Rubber used to be the classic material for river-running rafts (as pictured above at Lees Ferry, Grand Canyon) that put in years and years of reliable service. But being a form of rubber, not plastic, it cannot be heat welded in a machine like PVC/PU; it must be labouriously glued which, when done well, increases costs. It can also be heat vulcanised which might be regarded as a form of heat welding.

This is why Grabners, NRS MaverIKs and proper river running rafts all cost so much. They’ll easily outlive your pet and might be considered over-the-top for a recreational IK’ing. Inexpensive recreational IKs are made from PVC-coated fabrics. Polyurethane (PU) approaches the better characteristics of rubber and it’s likely that not all PVC is nasty, toxic crap, just as not all alloy bicycle frames are the same quality. In the hands of a careful owner – as opposed to the hard use from a rental- or river-running outfit –with proper care and maintenance involving anti-UV 303 protectorant (right), thoughtful handling, drying and storage, a PVC or PU IK should still last for years provided the initial material and quality of manufacture is high. And while suitably chunky PVC or PU IKs like my old Incept K40 are hard to fold, that stiffness translates to a more rigid and therefore faster boat on the water without needing to resort to high pressures. This was apparent when I first tried a K40 after running a Nitrilon rubber Sunny for years.

Fabrication ‘air tube’ or ‘tubeless’

An IK’s hull can be a casing or ‘envelope’ made of PVC, Cordura or any similar hard-wearing fabric into which slip light, removable sponsons or air bladders, made either from stiffer, ‘brittle’ vinyl, more durable and flexible urethane, or even PU-coated nylon. Examples include Aire (an AireCell’ bladder pictured left), Advanced Elements and the BP Trinity II (see Other IKs). I call this the ‘American’ method.

Pictured right: vinyl and urethane may all be just ‘plastics’, but might be compared to Platypus water bladders (vinyl, stiff, slippery) and the blue Camelbak (softer and rubbery). That is why in bladder boats like Aires, PVC is good for the shell and urethane makes an ideal, slightly stretchy bladder. Cheaper Aire Tributary IKs us vinyl.

Compare that to the Gumotex, Grabner, NRS and Incept style of fabrication which is like a tubeless car tyre: the perfect gluing of the tough hull sections keeps the air in. Both are repairable in broadly speaking the same way.
I notice my ‘tubeless’ analogy has been adopted by Innova, the US Gumotex importers, except that they try to make out that ‘tubeless’ is superior. It certainly is for automotive tyre use, but with IKs it’s more down to manufacturing ease and therefore, costs. PVC (welded is best, like Incept or some Aires, not glued like Advanced Elements or sewn like Tributary) is stiffer once pumped up, less durable, doesn’t abrade so well on grit (out of the water), but is less expensive than synthetic rubbers, quicker to weld or sew, and is slipperier in the water, so giving better response when combined with its superior stiffness. The difference between the ‘tubeless’ or ‘tubed’ construction style is merely down to the cost of manufacture and materials.

Bladder boats can use PVC shells because fully sealed welding of the hull envelope isn’t critical; they can just as easily be heat welded, sewn, glued, or even zipped together and the ‘inner tube’ bladders can be slipped in and pumped up.
But durability of the outer hull is a factor in how the panels fit together to make the sleeved hull, and here quality heat welding is best, certainly compared to vulnerable stitching. I also read that on cheaper IKs sponsons can get twisted in the sleeves during unrolling and inflation which can get to be a faff. That, and much quicker and easier drying/cleaning is why I prefer tubeless IKs. If your boat is in and out of your car boot or motorhome hatch, then tubeless is the way to go. If you’re more into multi-day trips, a bladder boat has no disadvantages once pumped up correctly.

The problem with bladder boats is that although the best-made ones may perform better, it’s normal for some water to seep inside the hull sleeves which contain the pumped-up bladders. Result: the boat takes ages to dry properly. This may not matter in sunny Californi-yay, but it sure does in Scotland or Scandinavia. Packing a wet boat is as undesirable as packing a wet anything, not least if it’s seawater. Mildew may develop, grit may get in and who knows, something may rot and shorten the life of a boat (although Aire says that a little water in the chambers does no harm, even long-term). When I come back from a sea paddle I always hose my IK down.

Drop-stitch fabric (right) – a type of construction from flat tubeless panels – is discussed in the next article.

Conclusion
Tubeless construction seems to be the traditional or ‘European’ method, and if well made will last for decades as rafters know well (left, a hypalon Semperit from the 80s just before it got sawn in half). Our Nitrilon Gumotex Solar looked as good as new when I sold it some nine years on, and had my Incept K40 been made from the same material I’d have probably kept it. Bladdered IKs are a cost-saving way of doing it. All cheap IKs use bladders, but the most expensive IKs are tubeless.

I like the simplicity of tubeless IKs: a tough outer shell that is well sealed. There is no cheap way of doing it and so any tubeless IK ought to be a well-made IK. In my experience in the US, IK rental outfitters tend to use tubeless Hypalon IKs like NRS, even if most recreational IKs sold are bladdered. Tubeless will cost much more but they’ll last much longer, especially if made from synthetic rubber.

Part 2: Hull forms, Rigidity and drop stitch