MATERIALS AND MOLDING METHODS 4-11 



Polyesters 



Polyester resins are formed by the reaction of polybasic acids and glycols, A wide 

 variation in properties can be obtained by the use of various basic ingredients and propor- 

 tions (5,7). The per cent elongation of the cured resin at rupture under tensile stress is 

 used here as the basis for polyester classification. 



Rigid Polyesters: These have higher physical strength properties than the more elastic 

 types. Because of their flexural stiffness, rigid polyesters are used in applications such as 

 smaller motorboat hulls having minimum framing where resistance to flexing under load is 

 important. Rigid polyesters are more brittle and have less resistance to impact than the 

 semi-flexible and flexible types. Cured non-reinforced rigid resins under tensile stress 

 have an elongation at rupture between 0. 5% to 3. 0%. 



Semi-Rigid Polyesters: These are compounded by the manufacturer to have greater 

 resiliency and more resistance to impact and to retain much of the strength properties of 

 the rigid type. A number of specialty polyesters, compounded for boat manufacture, fall 

 into this classification. Semi-rigid polyesters are preferred for larger boat hulls exceeding 

 16 feet in length. In large boats the flexibility of the shell is controlled by spacing of the 

 shell supports so that advantage may be taken of the improved impact resistance of semi- 

 rigid polyesters. Semi-rigid, non-reinforced polyesters have an elongation at rupture of 

 3% to 10%. Polyesters with properties similar to semi-rigid types have been made by blend- 

 ing mixtures of rigid and flexible types. A blend of 90% rigid and 10% flexible polyesters 

 has been commonly used in small boat construction. This mixture, which has been repre- 

 sentative of good practice, is an arbitrary compromise between the strength and stiffness of 

 the rigid type and the elastic flexible type (6). 



Many fabricators prefer the single component, semi-rigid type for use in boat manu- 

 facture since it does not require mixing of the rigid and flexible components. Manufactured 

 semi-rigid types have better resistance to aging than mechanical mixtures of rigid and 

 flexible types. 



Flexible Polyesters: These resins have an elongation exceeding 10 per cent at break 

 and are both flexible and elastic. Due to their flexibility, they cannot be used alone in boat 

 hull construction, but are often blended with rigid polyesters before molding. 



Isophthalic Polyesters: These resins are a recent development of interest which ex- 

 hibit faster curing time and somewhat better wet and dry properties than conventional 

 polyesters (16). In general, isophthalic polyesters are better suited to pressure molding. 



Self-extinguishing Polyesters: These resins are formulated to increase their resistance 

 to fire (7, 17). They will not support combustion when the flame is removed. 



Self-extinguishing polyesters are made from a number of compounds which are either 

 additives to, or integral parts of the chemical structure of the resin. Included among these 

 materials are chlorinated paraffins, tetrachloro phthalic anyhride, trichlorethyl phosphate, 

 chlorendic "HET" acid, chloro maleic acid, organic phosphonates, and chlorinated styrenes 

 (17). Antimony trioxide added to a polyester in small amounts, 1 to 5 per cent, is effective in 

 increasing its fire resistance, particularly when used in connection with a chlorinated 

 polyester. Some self-extinguishing polyesters have a high viscosity making them somewhat 

 difficult to handle. Compounds added to increase fire resistance, that are not chemically 

 bound in the resin, may reduce the physical properties and leech out upon exposure. The 

 effect of these modifying agents on the strength and other properties of a laminate should be 

 carefully investigated and calibrated prior to specific application. 



