Answer to Question 1
One method that will accomplish this formation of an outer shell is to mix two resins that have different melting points. During the rotomolding operation, the lower melting resin will become sticky first and coat the inside of the mold, thus forming the outer layer of the part. Later, the higher melting material will coat onto the shell and form the inner material. Another method is slush molding which one resin is put into the mold and poured out after a thin film has formed on the mold. A second material is then added to complete the part.
Answer to Question 2
Normally thermoplastics such as polyethylene cannot be crosslinked. However, if a peroxide or some other free-radical-forming material is added to the polyethylene and the temperature is raised, some hydrogens in the polyethylene can be removed and free radicals formed in their place. These free radicals can then react with crosslinking agents containing carbon-carbon double bonds which have also been added to the mixture. These crosslinking agents bond to the free radical locations on the polyethylene. Because the crosslink agents have at least two reactive carbon-carbon double bond sites, they can react with more than one free radical location and therefore tie together multiple polyethylene chains. Increased heat is needed to give the molecules the mobility to move together to form the crosslink bonds.
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