See also
Prediction: TPU will be the next big thing with IKs
… and here it could be: Aquaglide Cirrus
Hull forms and rigidity
Tubeless Vs Bladders
Glues and repairs
A ‘PVC vs rubber’ article from 2011 by The Boat People
PVC vs PU vs synthetic rubber: slightly biased. Skip to: 2. Characteristics of the Material

Cheap IKs are made from a film of heat-welded vinyl, like a slackraft. They’ll float, but run 1psi to avoid bursting and so sag horribly and track poorly. Some might also say they look a bit crap and are responsible for the ‘pool toy’ image from which IKs suffer. These boats are OK for messing about near the water’s edge, but often end up in the trash or back on eBay after being used just a couple of times.

Broadly speaking (I say that a lot for legal reasons), quality inflatable kayaks use material comprised of a densely woven fabric core (left) coated in either synthetic rubber or much more commonly now, PVC.
The almost ballistic core adds strength and all but eliminates elasticity which means pressures can be pushed higher, greatly improving boat performance, unlike a pure vinyl inflatable pooltoy which just keeps expanding but never gets firm.

Synthetic rubber (CSM, ‘Hypalon’)

This uses a nylon or polyester woven core coated with natural rubber or neoprene on unseen inner surfaces. The exterior surface is then gets a durable synthetic rubber coating, the best known was Hypalon®, invented after WWII by DuPont and which helped kick-start the inflatable boat and RIB boom. DuPont stopped making Hypalon® in 2009, but today various off-brand outlets continue to make it, or similar rubber-based fabrics. As folding boat makers, Pakboat put it on their website: ‘The abrasion resistance [and waterproofing] is in the coating, and the tear strength and tensile strength are in the woven fabric.’
Hypalon and similar fabrics are excellent for what they do, but have had their day. Hypalon needs to glued by hand which adds days to assembly times, while plastics like age-old PVC and up-and-coming TPU (as used in packrafts) can be heat welded in hours. This ease of assembly is the main reason PVC in various grades is so commonly used to make IKs.

Hypalon fabric is composed of three plies all bonded together:
• an inner coating of neoprene or natural rubber makes it airtight and easy to glue
• a fabric core of polyester or nylon weave resists stretching and tearing
• the durable hypalon outer coating resists UV and chemicals better than just about anything.

A near-identical product is supplied, among others, by Pennel in France under the ‘Orca‘ brand (above left) as used on US-made NRS IKs. Gumotex’s various in-house Nitrilons (below) are similar, as is Dow’s ‘Nordel‘ EPDM, as bought in by Grabner (above right) and other folding kayak and raft manufacturers.

Some not very illuminating Gumotex photos here with no gluing to be seen. Gumotex limit the operating pressure of most of their boats to a modest 0.2 bar (2.9psi), still more than many well known IKs.
A couple of round tube Gumotex IKs like the K whitewater series and the Seawave sea kayak run 0.25 bar (3.6 psi). Somewhere along the line the fabric is vulcanised, probably in the chemically bonding the coating sense, rather than assembly.

Austrian Grabner make a big deal of their EPDM boats being vulcanised, but again this is related to preparing or curing the fabric, not assembling the sections of EPDM. Vulcanising is a chemical reaction which either molecularly bonds rubber to itself (as in the autoclave drum, above left) or makes a rubber product such as a tyre more durable. As said, synthetic rubbers is they cannot be ‘machine welded’ – they need to be laboriously glued by hand.
However they do it, Grabners manage to run high pressures for a traditional round-tube IK and amazingly, don’t feature pressure release valves (PRVs), even in the I-beam floor. That shows confidence in manufacturer.

PVC

These days most IKs are made from less expensive PVC-coated fabrics. Remember, with PVC this is not necessarily the plastic film of a novelty pool toy, but PVC bonded onto a nylon or polyester fabric core which, just like Hypalon, makes a flexible, durable and airtight coating. PVC is less pliant (hard to roll up) but also stiffer (good on the water). PVC is the quality varies. There is such a thing as recycled PVC which you might expect will last less long and bond less well than the brands like Duratex or Mehler or Valmex. If a PVC kayak maker mentions a brand of PVC fabric, that’s probably a good thing.

The great thing with PVC is that it can be easily and quickly heat welded. No gluing required. Heat welding PVC is so easy and effective you can even do it yourself with roller and a heat gun. The process was invented in 1948 in France by SEVY, the company that went on to become Sevylor. They produced an inflatable PVC bath that became a hit in post-war austerity France and since that day Sevylor has never looked back. Though some paddlers have.

In environmental terms, PVC has become a bit of a dirty word which is why Gumotex and Grabner 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 is what gives PVC its 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 the warning below if they dare sell PVC products in the Sunshine State. Will using a PVC IK make you ill? It’s extremely unlikely but Californian environmental law – the strictest in the US – insists on the warning.

TPU? It’s coming: see here

So, Rubber or Plastic?
Synthetic rubbers like Nitrilon and EPDM are 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 (left, a 4-metre Nitrilon Sunny). Importantly, this means creases and folds cannot develop into cracks and leaks, as I’ve found on PVC boats, while hard folded corners (below) can be vulnerable to abrasion.

Synthetic rubbers also have very good resistance to UV and solvents, are said to be heavier than comparable PVC and come is fewer colour options. Rubber used to be the classic material for river-running rafts (above, 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; 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 film or PVC-coated fabrics – usually shells containing separate air bladders. 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 river-running outfit – with proper care and maintenance involving anti-UV 303 protectorant (right), thoughtful handling, drying and storage, a PVC IK should still last for years provided the initial material and quality of manufacture is high. And while suitably chunky PVC 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.