plasticizers, fillers, colorants, stabilizers, and impact modifiers. 

 Plastics which are hard and rigid or brittle at normal temperatures can be 

 made pliable and flexible by the addition of plasticizers. 



Fillers are normally added to both thermoplastics and thermosets to 

 enhance their processing, performance, or economics. For example phenolics, 

 without the addition of fillers, are hard and brittle, shrink in molds and 

 may crack. The addition of finely ground wood flour makes it easier to 

 mold and less costly. Powdered mica will enhance electrical resistance 

 while the addition of asbestos will improve heat resistance. Impact 

 resistance can be improved by the addition of chopped fibers such as 

 natural fibers, tire cord, rayon or glass. Colorants are easily added to 

 plastics although they are not usually necessary in coastal structures. 



Stabilizers are an important group of additive materials used to 

 increase the resistance of plastics to deteriorating influences of weather, 

 ultraviolet light, or radiation. Most plastics in their pure form do not 

 have a great deal of resistance to these environments. The addition of 

 stabilizers retards thermodegradation and oxidation. Materials with these 

 kinds of additives, in outdoor exposures, may have a design life of 30 to 

 40 years . 



Another important category of additives are the impact modifiers. The 

 inclusion of various fillers or plasticizers will increase the impact 

 resistance of plastics which are normally very brittle. 



e. Durability Properties . In addition to structural qualities, 

 plastics possess other desireable characteristics as a construction material. 

 They are easily formed, corrosion resistant, lightweight, wear resistant, 

 energy absorbent, impact resistant, flexible and ductile, and are used for 

 insulation due to their thermal and electrical resistance. Energy absorb- 

 tion and impact resistance vary with the different plastics. Rubber, being 

 a synthetic and not usually considered a plastic, can be formulated to have 

 a high degree of impact resistance within a large range of stiffness 

 characteristics . 



Fire is a necessary consideration in the selection of all structural 

 materials. Plastics will burn or disintegrate if exposed to fire or high 

 temperatures. Some will burn easily, some slowly, others with great 

 difficulty. Some will not support combustion in the absence of flame. 

 Improved fire resistance can be achieved by incorporating flame-retardant 

 chemicals into the molecular structure of the plastic materials. Phosphorus 

 and halogens have been effectively used for this purpose. 



2. Geotextile Filters . 



a. General . Because the most common use of plastics or geotextiles in 

 coastal construction is as a filter, that use is the predominant topic of 

 this section. These filters have been known as filter fabrics, construction 

 fabrics, plastic filter cloth, geotechnical fabrics, and engineering fabrics. 

 ASTM Joint Subcommittee D-18. 19/D-13.61 is developing test procedures for 

 evaluating these fabrics and has adopted the name of "geotextiles." Geo- 

 textiles are used in engineering as filters, materials separators, and 

 reinforcement for soils. These fabrics may be used in coastal structures to 



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