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The problems start to occur when the water molecules migrating into the GRP (fiberglass) encounter other chemicals inside the laminate, primarily water-soluble materials (WSMs) such as the emulsion binders used to hold the fiberglass mat together before it is moulded, or pockets of uncured or partially cured resins in the hull mould. The water molecules can have a chemical reaction with these substances, forming larger molecules of a new chemical, often acidic - which unlike the original small water molecules, can not carry on passing through the GRP. These larger molecules are then trapped. This is the point at which osmosis actually starts. The important parts are that the hull is not waterproof (it is a semi-permeable material), and that osmosis causes a low concentration fluid (water) to pass through the hull to join the higher concentration fluid (the chemical mix formed by the water plus WSM) inside the fiberglass laminate schedule. Pressure is thus built up inside the laminate. If this process takes place in a solid part of the laminate, there is usually no problem as the structure is strong enough to contain the pressure. If however it takes place on the boundary of a small air-bubble in the moulding, or at a point where layers of fiberglass are not properly bonded, the new chemical compounds slowly fill up the bubbles or the very small gaps between layers with liquid. Almost all mouldings have these air bubbles and small areas of poor bonding, although they should not. Ideally the resin should totally fill the gaps between the fiberglass strands, and every layer should perfectly bonded. This is extremely difficult to achieve with conventional moulding techniques. The process of osmosis in a fiberglass hull is very slow, unless the moulding is a poor lay-up schedule, and no matter how long it remains in water a typical fiberglass laminate cannot absorb more than about 2-3% of it’s own weight of water. If this osmosis (using the term in a correct manner) was all that happened, it would be a very minor problem. Even completely saturated with water molecules, a fiberglass laminated hull still retains most of it’s strength, although it does become slightly more flexible. Racers who want stiff hulls with the absolute minimum weight already mostly keep their boats ashore when not sailing, and for any properly built cruising boat 2% or so extra weight and a trace more flexibility in the hull-structure should not be a problem. Once again, if the air bubble simply filled with this acidic compound, the problem would still be relatively minor. However the nature of the osmosis process is that water molecules keep osmosing through the fiberglass laminate, and join the chemicals in the bubble, steadily building up hydraulic pressure. Eventually this causes the surface of the moulding to blister. These blisters are the typical sign of what I refer to as ‘osmosis’. When pierced these blisters will give off a small amount of chemical-smelling (usually vinegary) liquid - which is the juice built up inside the pressure-raised blisters. The term ‘blister juice’ is often used. This ‘blister juice’, which is usually acid, can break down the polyester. This breakdown process is known as hydrolysis, and causes a reduction in strength of the laminate. This is however normally very localized, and the moulding as a whole will still retain most of it’s strength despite blistering. Only if the blisters are very large, or very deep-seated, is this generally a problem.
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