What is osmosis - Veneziani Yacht Paints

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1 OSMOSISGPR boatsGRP hulls are manufacturedusing a mould. The inside of themould is coated with anti-adhesionwax. At first, the gel-coat,corresponding to the outsidelayer of the hull, is applied. Thegel-coat consists of a polyesterresin containing pigments withthe desired colour. Several layersare applied with spray equipment.Subsequently, several layers of fiberglassmats are applied. Eachlayer is impregnated with catalysedpolyester resin. After completionof the curing period, the hullis removed from the mould.What is osmosisOsmosis is a phenomenon whichbecomes visible through the formationof blisters. The blistersare filled with water and localizedbetween the layers of resin in thehull. In the beginning, the blistersare small in size and restricted toa few areas of the hull. When thephenomenon is progressing, theblister’s size increases and eventuallythe whole area of the hull isaffected. The main cause ofosmosis is capillarity which enableswater molecules to penetrateinto the fiberglass laminate ofthe hull. Once inside, the waterreacts with the residual acidiccompounds in the laminate creatinga concentrated solution.More water is absorbed to balancethe concentration of the solutiontrapped inside the laminate(through osmotic pressure). Thiscauses the formation of the wellknown blisters.Causes of osmosisThe main causes of osmosis formationare the following:• Residues of non polymerizedresin• Water soluble contaminants inthe laminate• Micro-porosities in the gel-coator in the fiberglass mats due toair entrapment during the manufacturingprocess.Gel-coat with air bubblesGRPManufacturing resinSEA WATERCavities betweengel-coat and laminateCross-section of a hull showingair entrapment in the gel-coat,cavities and non polymerizedresin due to manufacturing defectsor excessive sanding ofthe gel-coatin the past.Air cavityNon polymerizedmanufacturing resinDeterioration of osmosis due toan increase of internal concentrationsand to water absorptionfrom the outside, whichcauses blistering.Damaged surface to be removed2
Phases of blisteringThe osmotic blistering on boat hullscan be divided into five differentphases. If you study the drawingshown here below and compare itwith the conditions of your boat’shull, it might be possible to carryout a preventive action suitable toavoid severe damage caused byosmosis.1) Formation of air blisters, oftenof small size, in the Gel Coat.2) Take up of water in the blisterscaused by the permeability of theGel Coat.3) Hydrolysis of the gel-coat resinas well as of the laminate resinand formation of a concentratedsolution.4) Increase of the blister size dueto osmotic pressure.5) Cracking of blister, caused bythe increasing pressure in the blisters.It is of paramount importance tochoose the right materials for anyrepair work. In the diagram shown1below the effectiveness of differentmaterials used in boat buildingis indicated.Formation of blisters1 Air blisters2 Water take up3 Formation of blisters4 Increase of blisters5 Cracking of Gel Coat3
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What is osmosis - Veneziani Yacht Paints

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