Table of Contents

1. How does HotVac work ?
2. Why is HotVac better than heating alone?

Answers

1. How does HotVac work ?

   Osmosis in glassfibre is usually caused by high moleculair weight alcohols (such as propylene glycol) which are added to polyester boatbuilding resins in small quantities during manufacture; usually as 'water scavengers' to help remove unwanted water of esterfication.
   
   These alcohols are strongly hygroscopic (i.e. water absorbing) in nature, and they migrate together under the influence of incoming moisture to form 'foci'; usually just behind the protective gelcoat. This in turn creates an 'osmotic cell', in which the gelcoat provides the semi-permeable membrane; and osmotic pressure is generated.
   
   This process will ultimately cause blistering in the gelcoat, which is recognised as "Osmosis", although the blisters themselves may take ten years or more to develop.
   
   Successful treatment of osmosis requires complete removal of glycols and other breakdown products, but with boiling points in the order of 200^oC, it will be seen that removal by normal means is next to impossible without completely destroying the laminate.
   
   The HotVac system overcomes these hurdles by using a combination of heat and high vacuum; so that glycol and other breakdown products can be vapourised and removal at temperatures which are not harmful to the laminate. This is best illustrated by the Vapour Pressure Curve for Propylene Glycol, shown in the graph.
   
   The HotVac system itself is comprised of a powerful vacuumpump, which is capable of creating a vacuum as low as 2 millibars absolute and special silicone rubber blankets of approximately 0.75 m² each. These blankets are fitted with seals around their outside to retain vacuum, and can be heated to temperatures of 100^oC or more as required.
   
   Being inherently flexible, the heated silicone blankets conform intimately to the shape of the yachts hull, and are simply applied after normal mechanical preparation as outlined here.
   
   Specially shaped blankets and small blankets are available for confined spaces, bows and unusual hull configurations.
   
   Each HotVac system can run up to four blankets at a time, and these are applied in sequence around the yachts underwater area until it is completely dried.
   
   Alternatively, the blankets can be applied to selected areas as required, or used for localised repairs.
   
   The blankets are usually applied for periods between four and eight hours at a time for complete drying, but can be used for longer periods on heavier lay-ups, or where drying is slow.
   
   The Hotvac system cannot work miracles, but it has proven extremely successful for drying 'problem' boats, and also provides some guarantee of turnaround times; which is especially important where working boats or charter yachts are being treated.
   
   It also provides new opportunities for repairing sandwich construction boats and other advanced composities, which may prove impossible or uneconomic to repair by more traditional methods.
   
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2. Why is HotVac better than heating alone?

   Under normal atmospheric conditions the vaporisation temperature of the moisture and other impurities is higher so that increased heating is needed to ensure they evaporate.
   The problem with applying this extra heat is that it can cause permanent damage to the hull at the same time.
   If safer levels of heat are used some of the harmful compounds will fail to evaporate, giving a less effective treatment and making osmosis damage more likely to recur once the hull is put back in the water.
   Source: http://www.centerlinecomposites.com

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