g. Plastics . 



(1) General . Chemically the term "plastics" is applied to a large 

 group of synthetic materials, including synthetic rubber, that are processed 

 by molding or forming into final shape. Plastics that are soft and pliable 

 at some stage in their production are formed into shape by the application of 

 heat and pressure. They are organic compounds that are transformed into 

 complex synthetic materials by chemical processes. They are high polymers in 

 that they consist of monomer atoms joined together into molecular aggrega- 

 tions called polymers. 



Plastics in general may be classified into two distinct groups, thermo- 

 plastics and thermosetting plastics. Thermoplastics soften repeatedly when 

 heated and harden when cooled. Thermosetting plastics go through a soft 

 stage only once. When hardened, an irreversible change takes place and they 

 cannot be softened again. Plastics can also be combined for a particular end 

 use, drawing together the best attributes of the blended components by 

 copolymerziation. The products are called copolymers. During the production 

 of plastics, additives such as plasticizers, fillers, colorants, stabilizers 

 and impact modifiers can be added. 



In addition to structural qualities, plastics are easily formable, 

 corrosion resistant, lightweight, wear resistant, energy absorbant, impact 

 resistant, flexible and ductile. A necessary consideration in the use of 

 plastics is that plastics will burn, some easily, others slowly and others 

 with great difficulty. 



(2) Geotextiles . Plastics in the form of geotextiles have an 

 important use in coastal structures, commonly functioning as filters in 

 drainage, shore and embankment protection structures. Geotextiles are a 

 relatively new material in the construction industry but have had a generally 

 successful experience record as filters in selected coastal structures over 

 the past 20 years . Substantial improvement in the design and materials 

 selection specifications has also occurred. 



The primary function of geotextiles when used as filters is to retain the 

 protected soil (prevent piping) and remain permeable to water without sig- 

 nificant head loss or the development of excessive hydrostatic pressure. To 

 function satisfactorily, the geotextile filter must have physical durability 

 and filtering integrity throughout the design life of the structure. In the 

 selection of a geotextile for a filter, the chosen fabric, in addition to 

 having required physical and chemical properties, should be of a kind and 

 finished form consistent with the site-specific requirements. 



Fabric construction is a predominant factor affecting performance. Woven 

 fabrics are commonly manufactured by crossing the yarns at right angles, 

 overlapping one over the other, the yarns being monofilament, multifilament, 

 mono-multifament or slit-film. Nonwoven fabrics include all materials not 

 woven or knitted. They consist of discrete fibers, which may be random or 

 pattern oriented in the fabric. The bonding methods described are needle 

 punched, heat bonded, resin bonded, and combination bonded. Combination 

 fabrics are produced by combining woven and nonwoven fabrics by one or more 

 bonding methods . . 



383 



