ENGINEERING PROPERTIES OF LAMINATES 5-35 



Foreign Inclusions in Laminates: Boat hulls are usually large and careless handling 

 during the lay up can cause the inclusion of scraps of wood, pieces of string, paper, dirt, 

 etc. These foreign inclusions can cause separation and wrinkles in a laminate and reduce 

 its mechanical properties. 



Good shop practice requires that a molding plant be kept as clean as possible to prevent 

 contamination of the laminates. 



Curing Shrinkage: During the curing process, the thermosetting resins used in rein- 

 forced plastics shrink in volume due to molecular crosslinking when passing from the liquid 

 to the solid state. This reduction in volume or polymerization shrinkage is more marked in 

 the case of polyester than epoxy resins. The resin forms a bond with the glass during the 

 curing process. Since the glass does not undergo any appreciable change in volume during 

 cure, shrinkage of the resin sets up compressive stresses in the glass and residual tensile 

 stresses in the resin. In addition, the interfacial bond between the glass and resin may be 

 subject to shearing stresses during the curing and bonding processes. 



Another type of internal stress occurring during cure is due to thermal shrinkage. The 

 curing process develops a considerable amount of heat in a laminate due to exothermic 

 chemical reaction. Resins have a considerably higher coefficient of thermal expansion than 

 glass. A differential thermal shrinkage between the glass and resin occurs when a laminate 

 cools after curing. The effect of thermal shrinkage is additive to that of polymerization 

 shrinkage in causing residual stresses in the glass and resin of the cured laminate. 



The residual stresses in the complex structure of a fiberglass laminate can affect 

 its short term or long term loading strength, fatigue strength and resistance to crazing 

 and weathering. 



Since these stresses and defects result from the curing process, they can be minimized 

 by selection of optimum curing methods. Excessively rapid cures or exotherms higher 

 than those required for proper cure should be avoided since they can cause damage to a 

 laminate during cure. Thick laminate sections are particularly subject to uneven cure and 

 thermal stresses. 



The assistance of resin and catalyst manufacturers should be obtained in order to 

 determine the optimum curing cycle for a molding process unless the fabricator has 

 established methods from the results of extensive shop practice. 



Environment 



Fiberglass reinforced plastics can be adversely affected by unfavorable environmental 

 conditions. Extended periods of water immersion and exposure to extreme weathering con- 

 ditions can cause some reduction in strength properties and these effects must be considered 

 when selecting a laminate for a specific application. 



Water Immersion: The immersion of a fiberglass polyester laminate in water for a 

 considerable length of time will result in some reduction in mechanical properties (18). The 

 degree of reduction is influenced by the types of finish on the fiberglass and the percentage 

 of voids in the laminate. High void content laminates have low wet strength retention. As 

 previously discussed, high void content or porosity can occur in laminates with fiberglass 

 contents exceeding the average fabrication range. 



