The short chopped fibers out of a chopper gun does not have binders to hold it together. The binders in CSM chemically change in the presence of water and cause blisters. The short fibers reduce wicking of water because of their length. Woven materials like cloth and roving can wick water along its entire length.
Steve C. D'Antonios said in his Ocean Navigator June 2004 article:
"We know today that the majority of osmosis problems originate from resin and glass fabric additives &mdash the water-soluble materials (WSMs) &mdash rather than insufficient wet-out of the fibers when the hull is laid up. Although poor wet-out can accelerate osmosis &mdash each glass filament that is not saturated with resin becomes a wick for water ingress into the FRP laminate &mdash it is of secondary concern. Short glass filaments, such as those used in chopped strand mat (CSM), tend to promote osmosis; however, as mentioned, it is really the emulsion binders found in this material that cause the problem. The short, wispy strands, which tend to poke through cured resin, are simply a vehicle for water molecules to reach the WSMs that lie within the laminate.
This theory is borne out by the fact that chopper gun-applied chop &mdash similar to CSM, but applied with a gun rather than in rolls &mdash is less likely to blister, once again citing the University of Rhode Island study. In spite of the fact that it, too, is made up of short, wick-like fibers, it lacks the binding agent found in roll mat. Because it is applied with a gun from a spool of material that passes through cutting or chopping blades, it requires no sizing to maintain its structure until laminated with resin.
It is ironic that chopper gun laminates &mdash frequently used in production boatbuilding and often looked down upon as machine- rather than hand-built laminates &mdash while perhaps not as sturdy as hand lay-ups, are less likely to fall prey to osmosis."
