Tensile-Strength Tests 



The tensile strengths of the dried panels of plastic sheeting were 

 determined by the ASTM method described in Reference 6. In this method, 

 specimens 1 by 6 inches are cut from the panels. As shown in Figure 6, 

 the tensile-strength specimens are made narrower at the center than at 

 the ends and resemble a double bladed paddle. The specimens are gripped 

 at the ends by the jaws of the tensile-strength tester, and the loads 

 required to break the specimens are determined. The load values are 

 divided by the cross sectional areas of the panels at the site of the 

 break, and they are reported as pounds per square inch to break (Table 

 1). The strengths of the rope specimens were also tested on the tensile 

 testing machine. The specimens were gripped by the eye-splices and 

 loaded to failure. The data are reported in terms of pounds of force 

 required to break the rope specimens (Table 2) . 



EXPERIMENTAL FINDINGS AND DISCUSSION 



The fouling growth which accumulated in 2-1/2 years on the specimens 

 exposed partially buried in anaerobic mud was very scant (Figure 3) . In 

 contrast, the fouling which accumulated on the specimens exposed near 

 the surface of the ocean was extremely abundant (Figure 5) . 



After a 1 -year exposure (described in Reference 4) the cotton ropes 

 (Figure 7) and Manila hemp ropes (Figure 8) had deteriorated so severely 

 that the fibers could easily be torn apart by hand. The 1/4- by 6- by 

 12-inch Douglas fir panel that was exposed immediately above the bottom 

 sediment was riddled by Bankia and Teredo (molluscan borers) and also by 

 Limnoria and Chelura (crustacean borers) as shown in Figure 4. After 2- 

 1/2 years in the harbor, nothing remained of either the cotton or hemp 

 ropes or of the Douglas fir panels. 



Panels 



There were no significant differences in the amounts of water 

 absorbed by plastic panels exposed for 2-1/2 years in the anaerobic 

 bottom sediments and in the aerobic near-surface waters. Moisture 

 absorption by the specimens of nylon sheet was approximately 6% of their 

 dry weight. The panels of phenol-formaldehyde sheeting picked up 

 approximately 2% of their dry weight in moisture; and the polyurethane 

 and methyl methacrylate sheeting, approximately 1%. The other plastics 

 absorbed negligible quantities of moisture. 



The polymer that absorbs the most water would seem to have the 

 greatest opportunity to react with water or with substances dissolved in 

 it. However, inertness of the polymer molecules would also have to be 

 considered. In seawater, nylon might deteriorate more rapidly than the 

 other plastics tested; phenol- formaldehyde resin next, and so on because 

 they absorb more water. 



