MATERIALS AND MOLDING METHODS 4-13 



Accelerators 



Accelerators are usually added to polyester resins to initiate a cure at room tempera- 

 ture (7,21). These compounds are added to a resin in combination with catalysts to start a 

 rapid curing reaction without the application of heat. Cobalt naphthenate and dimethyl 

 aniline are two commonly used accelerators. Accelerators and catalysts will only work to- 

 gether in certain combinations or pairs. 



Resins used for contact molding of boats often contain an accelerator added by the manu- 

 facturer, and these resins require only the addition of a suitable catalyst to cure at room 

 temperature. A knowledge of the pot life of a particular resin mix is necessary to provide 

 adequate time for impregnation of the reinforcement between the completion of resin mixing 

 and the beginning of resin hardening. 



Stabilizers 



Stabilizers or inhibitors are compounds added to polyesters by the manufacturer to 

 prolong storage life (7,21). Since polyesters are active chemicals which will slowly set or 

 gel over a long period of time, depending on storage conditions, overage polyester should 

 not be used. If tertiary butyl catechol or hydroquinone is added in small amounts to inhibit 

 the gelling reaction, the catalyst quantity must be modified. 



The effective storage life of a polyester is largely determined by its inhibitor level and 

 the temperature at which it is stored. High ambient temperatures will result in shorter 

 storage life. 



FILLERS AND PIGMENTS 



Fillers and pigments are added to the molding resins to reduce shrinkage, minimize 

 crazing, lower material costs, impart color or opacity and to improve surface finishes (1). 

 Addition of excessive amounts of fillers can also increase resin viscosity to a point where it 

 could be difficult to work with. Laminates containing fillers may be opaque and may not be 

 readily inspected for internal flaws. Filled resins are often used as surfacing materials for 

 laminates. Commonly used fillers include diatomaceous earths, calcium carbonate and 

 aluminum silicate (22). 



The addition of approximately 3% of silica dioxide filler by weight to a polyester resin 

 will make it resemble grease in consistency. When these modified resins are applied to 

 vertical surfaces, they will not drain or run off. Filled resins of this type are called 

 thixotropic resins, and are available pre -compounded from various manufacturers (6, 7, 22). 



STIFFENER AND SANDWICH CORES 



When necessary to economically provide greater strength and rigidity to a hull, deck or 

 bulkhead, stiffeners or sandwich type construction may be used depending on the degree of 

 strength and rigidity desired (3, 4). In concept, the construction of a core stiffener and a 

 sandwich panel is identical except that the stiffener is proportioned to have a limited width 

 of flange and the sandwich panel is proportioned to have continuous facings throughout the 

 entire panel. The flanges of the stiffener and the facings of the sandwich panel are usually 

 thin high strength fiberglass laminates and the cores are thicker low density materials. 



