I’m no expert on this subject but I update this article from time to time as I learn more (9/12)
Broadly speaking, quality inflatable kayakss are built in two ways and out of two types of material.
Synthetic Rubbers
These use a nylon or polyester woven fabric (see here for the difference) thickly coated with natural and/or synthetic rubber or neoprene. Examples are ’Hypalon’, invented and once produced by Du Pont but actually referring to the fabric coating, not the 
completed material it bonds to. A near identical product is now supplied, among others, by Pennel in France under the ‘Orca‘ brand.
Nitrilon (graphic top right) is similar, as I suspect is EPDM aka: Nordel.
These materials are respectively used by NRS, Gumotex (more Nitrilon details here), and Grabner (more here) as well as some folding kayak makers and raft manufacturers.
Gumotex in Czech Rep. actually manufacture Nitrilon fabric and many other rubber products in their factory where they also make boats, but as far as I can tell from the picture below left, their boats are purely glued together by hand. Austrian Grabners say their EPDM (right) boats are additionally ‘heat vulcanised’, a bonding process which which can be compared to RF or heat welding of plastics like PVC. 
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 rubber-to-rubber bonding sense vulcanisation is what happens when your punctured tubeless car tyre is professionally repaired from the inside, whereas glueing as you know is like sticking a patch on an inner tube.
Assuming I have the right end of the stick, then that means Grabners have a superior construction to Gumotexs, but then they are twice the price. It may also explain why Grabner boats do not feature pressure release valves (PRVs); their boats are so well bonded there is no risk of them bursting in the boat sun, while also claiming they can run at 0.3 bar or 4.5 psi – good for rigidity and therefor performance.
Don’t be put off by the picture of delaminated Grabner on the right – it’s from a 12-year-old Grabner Holiday 2 and occurred on that model at a point where a metal eyelet didn’t get on with the EPDM. (It’s since been renovated). It’s there to illustrate the densely woven – and crucially, stretch resistant - underlying fabric. On full inflation a good IK doesn’t want to be stretchy like a balloon (or a cheap Intex dinghy come to that), it wants to be taught like a basketball. The underlying woven yarn gives tensile strength, the natural rubber interior and exterior synthetic rubber or PVC/PU coating adds durability, a slick low friction surface, and of course airtightness.
PVC
Otherwise, IKs are made from a cheaper but stiffer PVC-coated fabric or more expensive polyurethane, the latter like Incept, Alpacka packrafts and a few more listed below.
Gumotex brought out a new material in 2012 called Hevealon based on their single-side coated Litepack of the last few years, but now with a teflon treatment on the uncoated side to repel water and so speed up drying. This was a problem with the former Litepack compared double coated classic Nitrilon which dried fast on both surfaces. The new Swing models use Hevealon.
As folding boatmakers, 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.’
Hypalon and PVC or polyurethane (PU)
Synthetic rubber ‘Hypalon’ and similar Nitrilon are both tough, durable and more expensive than PVC, metre for metre. On a same-sized boat they’re also more pliant and will therefor roll up or fold much more compactly (below left, a 4-metre Nitrilon Sunny). Synthetic rubbers also have very good resistance to UV, are heavier than comparable PVC/PU, but come is less colour options. They used to be the 
classic material for river running rafts (as pictured right at Lees Ferry on the Grand Canyon) that put in years and years of reliable service. But being a for m of rubber not a plastic, it cannot be easily heat welded in a machine like PVC/PU – it must be more labouriously glued (below right) which, when done well increases manufacturing costs. In Grabner’s case, it can then further be heat vulcanised which can 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 probably outlive your pet but might be considered over the top for a recreational IK. All cheapo IKs are made from PVC 
with the ‘pool toy’ connotation, but PU approaches the better characteristics of rubber and not all PVC is crap, just as not all metal 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 (left), 
thoughtful handling, drying and storage, a PVC or PU IK will still last for years provided the initial material and quality of manufacture are high. And while suitably chunky PVC or PU IKs like my Incept K40 are hard to fold, that stiffness translates to a more rigid boat once pumped up and on the water, which equates to a notably faster boat. This was apparent when I first tried a K40 after running a Nitrilon Sunny for years. 
Gumotex had difficulty in making a rigid boat as long as the K40 in Nitrilon. They tried with their decked, 5-metre Seaker sea kayak (discontinued) but for the hull had to use a PVC-coated fabric called Mirasol made in Korea. The problem was that made the weigh twice as much as the similar Incept.
Note by ‘PVC-coated fabric’, I mean a multi-layer coated fabric similar to the Nitrilon shown above, not ‘vinyl’ which is a cheap, single layer plastic good for beach toys like some Intex dinghies, but not durable IKs.
Construction ‘air tube’ or ‘tubeless’
Sometimes an IK’s hull tubes are just a casing or ‘envelope’ into which slip light, removable, airtight ‘sponsons’ or air bladders, made either from stiffer, more ‘brittle’ vinyl, more durable and flexible urethane, or PU-coated nylon. Examples include Aire, Advanced Elements and the BP Trinity II (see Other IKs). Pictured on the right, vinyl and urethane might be compared to Platypus water bladders (vinyl, stiff) and the blue Camelbak (more rubbery).
Comparing one with the other, you could say that the Gumotex, Grabner, NRS and Incept style is like a tubeless car tyre; the good seal of the tough hull tubes keeps the air in. And Aire-style is like a tyre with an inner tube which pumps up to give rigidity and form to the outer cover or hull casing tubes. Both are repairable with patches in broadly speaking the same way (see below). I notice my ‘tubeless’ analogy has now been adopted by Innova, the US Gumotex importers, except they try to make out that ‘tubeless’ is superior. It certainly is for automotive use, but with IKs I believe it’s more down to manufacturing ease and costs.
PVC (welded is best, like Incept or 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 together, and is slipperier in the water, so giving better response when combined with the superior stiffness. My current IK is a PVC Incept K40 which is indeed stiff to the point of being less easy to roll up than a Gumboat, but much faster on the water.
The difference between the ‘tubeless’ or ‘tubed’ construction style is merely down to the cost of manufacture and materials. Bladder boats can use cheaper PVC shells and so save costs (or spend it on design) because air-proof welding of the hull tubes is not critical; they can just be easily heat welded, sewn, glued, or zipped together and the inexpensive sponson ‘inner tubes’ (or ‘cells’ as Aire call them, similar to Camelbak bladders) can be slipped in. But durability of the outer hull is a factor in how the panels fitted together to make the tubes, and here quality heat welding is best, certainly compared to vulnerable stitching. (Although you can run a seam of Aquaseal/Aquasure along a stitch to give it long term protection to abrasion). Length for length, the weight and rigidity of PVC covered bladders may end up a little heavier but become more rigid and so you paddle faster, depending on design and all the rest.
I-beam floors
You may read about I-beam floors and think: ‘Nice, what is it and does it matter?’ As it’s rarely illustrated I’ve only lately realised what it means, and yes it does matter with regards to over-pressurisation due to heating and so expansion when the boat is left out of the water in the hot sun.
A basic IK like a Gumo Sunny or the profile of a Bic or Sevy pictured above has three main chambers: two round sides and a flatter, wider floor or many tubes all linked. Obviously the round sides are easy to construct and take on the desired tubular form on inflation; it’s just a roll of material glued into a pointy-ended tube. Over inflating this is no great drama as the round shape distributes the pressure equally.
The flatter, lilo-like floor section is another matter. To make it of a series of burst-proof parallel tubes like the sides would be heavy and require a valve for each chamber. So instead, a flatter rectangular ‘airbed-like’ section is made for the floor and I-beam sections – like the steel beam pictured right – are glued to the insides of the top and bottom of the floor before it’s glued up. It’s said this is the most labour-intensive and so expensive part of IK construction and again could explain why easy, slip-in bladders are preferred; it saves time and effort and so costs which equals more customers.
I-beams help constrain a flat, floor shape once inflated. Without the I-beams the floor would balloon into a useless rounded form, but with the I-beams, it wouldn’t be hard for too much pressure to tear away the I-beams inside and separate the floor upper and lower with the same ballooning effect. This is why my Sunny Mk1 and other IKs including better Aires and the Incepts have a pressure relief valve (PRV) in the I-beam floor, even though this part of the boat is in the cooling water more than the exposed rounded side tubes which can handle higher pressures, just like a raft or RIB. In fact the Incept K40 has twin tube, I-beam sides which also feature PRVs to avoid separation. I-beam is a good if awkward design, but an over-inflated I-beam floor without a PRV could mean a ruined boat.
As with inflatable anything that doesn’t have PRVs all round like an Incept, never leave it out of the water in the hot sun for long. I found this out the hard way with my Feathercraft Java. One thing I like about my Incept K40 is that I no longer need to be paranoid about bursting my £1500 boat by letting it overheat accidentally out of the water. I can leave it on the car roof or on a beach all day and it will just purge via the PRVs and then be a bit limp once it all cools down. On a cool morning after a hot day you may find you IK a bit flaccid for the same reason – it doesn’t necessarily have a leak, the air inside has cooled and contracted a little.
There are other ways of making an IK hull stiff, and therefor responsive and fast. The neatest solution are twin side beams; two thinner tubes of say 5 inches on top of each other as clearly visible on the Grabner, left. This also has the advantage of making more room inside than a fat, single side tube design: higher sides to keep out waves, but more windage – not so good when conditions get severe at sea. Other methods include metal frames like Advanced Elements, or extra rigid inflatable floors, but this might be seen as complicating matters. I got halfway to trying it myself with the Sunny. A folder needs a frame of course, but as Incept and possibly Grabner have proved, a long, touring IK can be made usefully stiff just using twin-tube design, materials and air pressure.
Repairs
On a Hypalon boat repairs are like an inner tube: rough up, clean off, apply the right rubber glue (not Aquaseal called ‘Aquasure’ in UK or Europe, left) and same-matertial patch – all which needs to be done very well as the patch is vulnerable on the outside. However, I have succeeded in 
glueing on 
non-critical D-rings onto Nitrilon Gumotex boats using Aquasure; allow it to become touch dry on both surfaces (as when repairing a bicycle inner tube) then press together with a roller (right).
It’s worth noting that McNetts Aquaseal/Aquasure (same thing sold in different territories) calls itself a waterproofing sealant not a glue. Their Seam Grip is made of the same stuff – a runnier version of Aquaseal. There are stronger glues out there suited to making boats but in the US they can’t mail them. Alpacka has a good page on gluing and in Europe the Packrafting.de have one too and can sell you super sticky Helapast PU glue. I think the Polymarine glue I’ve bought in the UK is a similar thing but even before Alpacka suggested it, I used Aquasure as a glue without really thinking about it, applying thin measures to both surfaces and allowing it to get tacky dry (like inner tube repairs) before pressing them together hard with a roller. You need to do this as Aqusseal needs air to set – like any sealant. Allowing it to half cure in air to a surface and then letting it ‘seal’ to itself is a way of bonding anything – even rubber-based Nitrilon to PVC, as I did here.
In the UK you can buy Aqausure in 28g tubes from £6, or 250g for around £24. Unless you have a lot to glue/seal, be wary of saving money of the 250-g tube; give it a chance and it will split the tube or harden before you get to use it all, even if it’s effectively over half price. The other glue I used on my PU/PVC Incept IK (and Slackrafts) is Bostik 1782, being unable to buy thr Bostik that NZ-based Incept recommended in the UK, and not least because it was going cheap on ebay.
Aire-style repairs are actually easier on the rubbery urethane cells. According to their youtube vid, you unzip the shell, slap on a bit of ‘get you home’ Tyvec tape on the split, tape up the inner side of the envelope-shell gash too to keep out grit, reflate and off you go. You can glue up in the usual way later, if necessary. Having said that, I had the feeling the thin urethane sponsons laminated to thin ripstop nylon fabric on the Feathercraft Java I had once couldn’t have been so easily or securely repaired with tape; it would be difficult to bond it to the fabric nylon compared to smooth urethane, but perhaps once inflated the seal would have been fine.
Conclusion
‘Hypalon’/Nitrilon ‘tubeless’ construction is the traditional or European method, and if well made will last for many, many years, as rafters know well. Our Nitrilon Gumo Solar looks as good as new and I wish my Incept could made from the same material wiothout having to pay for a Grabner. Sponsons/bladders seem to be a more convenient North American may of doing it.
You’ll see me go on about this because the problem with bladder boats is that although they may perform better, it’s normal for some water to get inside the hull sleeves which contain the pumped-up bladders. Result: the boat takes ages to dry. 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 sea water. Mildew may develop and who knows, something may rot and shorten the life of a boat (although Aire say that a little water in the chambers will not be bad, even long term). Until I know better, quick-drying non-bladder construction is my preference for where I do my paddling.
