Matt is right about cross-linked polyethylene. Their rotomolding process uses the cross-linked PE, and the entire hull becomes one single molecule because the chemical agent added to the raw material PE causes each of the individual molecules to cross-link to adjascent molecules. Many of the kayaks are blow molded using Ultra High Molecular Weight Polyethylene (UHMWPE). This process does not become cross-linked to form a single molecule. A blow molded kayak could be ground up and fed into a blow molding machine to produce another kayak. The Wave hull could not be ground up and rotomolded into a new hull.

Many automotive fuel tanks are blow molded UHMWPE, and even those specify that damaged fuel tanks are to be replaced, not repaired. From a manufacturer's perspective, I think you can understand this attitude. From Hobie's perspective, they would want any factory approved repair to have the same characteristics as the as-molded product.

BTW, olefins (PE and Polypropylene) are very difficult to bond to. Most times, treatment of the olefin surface prior to bonding is required. I have used specialized adhesives to hold things to fuel tanks until they are assembled to the vehicle and trapped in place, but these adhesives are relatively expensive and are not used for life of the vehicle attachment to the fuel tank. The most common attachment method is a technique called Hot Plate Welding, where the fuel tank and the part to be attached are heated to melting and then pressed to each other. This method of attachment has proven reliable for blow molded products and is, in effect, the process Carl describes above (melting the materials together).

The term "thermoplastics" refers to polymers that can be heated and formed multiple times. Olefins (if not cross-linked) are one type of thermoplastic. The right adhesive for each type of thermoplastic is often quite different, and what works for most thermoplastics rarely works for olefins.