Laminating gelcoats

Spray guns are available for gelcoat, chopper, saturator and flow coater, at outputs of 3.6-9 kg (8-20 lb)/min resin, 1.8 kg (41b)/min glass two-strand chopper, or 1.1 kg (2.61b)/min one-strand chopper, giving a total laminate deposition of 5.4 kg (12 lb)/min with two strands. Air consumption is 9-10 ft /min. For higher volumes, four- and six-strand choppers are also available, dispensing 17-19 kg/min resin and 33% glass content. 

Resin lamination should not be carried out at a temperature of less than 20C (68F). A catalyzed/promoted resin is heat-sensitive and catalyst levels should be adjusted to varying temperature as recommended by the manufacturer. With resin laminating, where space is limited, mold preparation such as releasing and gelcoating can be done in a tent made of polyethylene film (which can also be used as a warm area for curing resins in cold weather). 

Star cracking Over-thick gelcoat occurs when laminate receives reverse impact gelcoat should never be more than 0.4 mm (0.016 in) thick. 

Careful control over the thickness of the gelcoat is very important. Very thin gelcoats are much less effective. Choice of fibre finish is crucial and laminates containing powder bound CSM display lower water absorption and higher blister resistance than those made from emulsion bound CSM. The use of surface tissue immediately behind the gelcoat can significantly reduce the chance of blistering. 

A widely used method of laminate construction is by the contact moulding process or hand lay-up . In this, a layer of catalysed unreinforced resin, known as the gelcoat , is brushed or sprayed onto the mould and allowed to cure and then layers of reinforcing-fibre impregnated with resin are built up behind (Fig. 6.3). Variations of this include resin injection and vacuum-assisted resin injection in closed moulds. After removal of the cured moulding from the mould, the gelcoat forms the outer skin and as such performs the two functions of decoration and protection ... 

The use of fire retardant (FR) additives, and/or halogenated FR resins in gelcoats detracts from their weatherabiUty and for external applications it is good practice to achieve fire retardancy by using a highly FR laminating resin behind a non-FR gelcoat. 

Some GRP dome structures supported by metal frames were constructed in the mid-1970s for the Sharjah International Airport Terminal Building, United Arab Emirates and these were recently inspected. The cream, isophthalic gelcoat was complete but appeared dull when viewed close-up and a few blisters were seen. However, the laminate structure was sound with no signs of delamination. 

Conventional polyester gelcoats are sometimes used on epoxy laminates but care needs to be taken to ensure that a sufficient permanent bond between them has been obtained. Tie-coats are available for this purpose. 

B R Bogner and H R Edwards, The cost/performance advantages of iosphthalic acid based polyesters in gelcoated laminates . Paper 12-F, Proceedings 51st Annual Conference, SPI Composites Institute, SPI, New York, USA, February 1996. 

As the trapped solution becomes more dilute, it increases in volume and creates a hydraulic pressure within the laminate. This process can take many years, and in well manufactured structures it may never result in visible damage. But the internal pressure built up in the laminate may eventually manifest itself as blisters on the surface of the structure, usually echoing delaminations between the gelcoat and the structural laminate. This problem is of especial concern to the yachting fraternity, which has a signiflcant remedial treatment industry. Repairs can be expensive and unless the root cause is eliminated, remedial work will not completely solve the problem. 

Birley and co-workers [44,45] suggested that the stress at the gelcoat backing (structural laminate) resin interface was the most important factor in the initiation of blistering. Chen and Birley [46] classified blisters according to their origins ... 

Chem. Descrip. Organophilic smectite CAS 53320-8 0 EINECS/ELINCS 250476-2 Uses Clay for use in paints/coatings, adhesives, caulks, sealants, agriculture, inks, paper, cosmetics, personal care, refractory, foundry, rubber, waste treatment, greases, gelcoats, oil field, household care, laminates, unsat. polyesters... 

Chem. Descrip. Organophilic smectite CAS 53320-86-8 EINECS/ELINCS 258476-2 Uses For use in unsat. polyester laminates and gelcoat resins Claytone SO [Southern Clay Prods.)... 

A lightweight mat, not greater than 300 g/m may be used behind the gel coat in order to minimise the risk of air entrapment at the back of the gelcoat. This applies to both hand laminating and spray depositing techniques. 

When processes other than contact lamination are used, similar criteria will apply, i.e., elimination of air, temperature control and gelcoat bonding. 

If, unavoidably, the gelcoat must be left before laminating starts, then the mould should be protected to prevent contamination of the gelcoat. 

Glass fibre is laid on the gelcoat and wetted out with resin. It is important that the laminate adheres to the gelcoat and therefore the gelcoat should still be tacky when laminating commences (gelcoated... 

Jute sandwich composite wall panels have been developed using jute laminate as a face and plastic wood/honeycomb as a core material for prefabricated stmcture. The thickness of panels is ranged between 25 and 65 mm. The panels are lightweight, rigid, and easy to install with simple tools to an open frame work. During impact test, the panels withstand against 5 and 25 kg sand-filled leather balls after several blows. The face material coated with gelcoat on the outer surface is water resistant. 

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