I've been researching Innegra quite a bit as I prepare for a composite build. I have some innegra/carbon plain weave as well as 3.6oz innegra S being delivered tomorrow to run some test layup infusions with.
Innegra is a light, stretchy, fiber. It is one of the least dense composite fibers known. It will float in liquid resin, so is best used with some sort of vacuum compression or caul plate. Compared to Kevlar, it has a lower modulus (is more stretchy) and about 3 times the elongation-to-break. Ultimate strength is only about 60-80% that of kevlar). Innegra is a Polyolefin, and much like polypropylene or polyethylene it is very chemical resistant and therefore difficult to bond to just about anything- including most resins. Innegra S is made by drawing the fiber to a very fine state- around 24x its original length (compared to about 5x for most polypropylene fibers). This produces very fine fibers with surface ridges and striations- much like a bamboo shoot. The irregular surface is what allows a better mechanical bond in a resin matrix.
My brief understanding on Innegra is that its purpose in the layup is to add resilience to the overall part. Fiberglass can be moderately strong and stiff, but is heavy. Carbon adds strength and stiffness and is very light per unit strength, but is fairly brittle and tends to fail catastrophically. Kevlar adds resilience and will bend without breaking (duct tape your boat and paddle it home), and also some strength. Innegra will add more resilience than kevlar, but will not contribute much strength. It also fuzzes like kevlar when sanded. Each material on its own makes a sub-optimal hull, but combining them in the right ways can take advantage of the individual properties to make a boat that is strong, resilient, light, and overall tough. (strength plus resilience together = toughness). What we laypeople don't know is the layup schedule necessary to get the properties we want. How much of each property is enough? Mathematics might get close, but I think the better answer lies in destructive testing. A few manufacturers have taken those steps. I have a suspicion that the reason it is not widely adopted in general construction is that innegra might contribute little benefit until your canoe is wrapped taco-style around a rock.
The "flexible business card" video linked earlier is a good example of innegra's resilience. Another one is
https://vimeo.com/165791841 . (green is 4oz S glass and 4oz Innegra. white is two layers of 4oz S glass) - not my video but a convincing one!
The other property to compare is cost. Innegra costs about 25% that of Kevlar.
