Molding machines have been developed to mold liquid polymers and tech- 

 nology is now being offered for injection molding of ultrahigh molecular 

 weight polyethylenes. Systems for extruding polypropylene using water cool- 

 ing of the bubble to obtain good toughness and high clarity are being de- 

 veloped. A new development in injection molding is making a solid skin of 

 one type of plastic and a foamed core of another. The manner in which 

 plastics are made impacts on the physical properties of a material such as 

 impact resistance, flexural strength, and heat distortion, permitting these 

 properties to be substantially improved with improved processing capability. 

 The improvement of physical properties will provide for a longer service life 

 of the present use of both rigid and flexible plastics. As flexural strength, 

 resistance to impact and heat distortion are increased, plastics may find an 

 increasing use as structural members. 



(2) New Resins. Polyester resins form a large family of resins. In 

 the manufacture of resins, three basic controls (i.e., density or degree of 

 crystallinity, molecular weight, and molecular weight distribution), result in 

 a great variety of resins. The abundance of glycols and dibasic acids de- 

 veloped from petroleum intermediates provides a wide latitude in designing 

 polyester resins to meet specific requirements. Unsaturated polyesters can 

 compete with epoxies, phenolics, and other plastics in electrical, physical 

 and mechanical properties. These resins predominate in applications requir- 

 ing corrosion resistance. For example, nonair- inhibited types are used as a 

 material in boat hulls, bouys, and decks, and for coating wood, concrete, 

 metals, and other structures. High temperature resistant resins such as 

 linear aromatic polyesters represent another new development. This particular 

 polymer also has a high resistance against most organic solvents. 



Polyethylene has a very simple molecular structure, but it is capable of 

 almost infinite variation and modification. The most recent development has 

 been in the very high density polyethylene resins that result in a hard 

 crystalline character. These developments may result in improved properties 

 such as impact resistance, tensile strength, and abrasion resistance for use 

 in bouys, fenders and bumpers, and unreinforced pipe. 



(3) Resin Modifications by Additives . Virtually all resins will 

 have different properties due to the incorporation of additives. Antioxi- 

 dants are used to prevent degradation of resins at high temperature. Ultra- 

 violet stabilizers prevent deterioration in atmospheric exposures. Fillers 

 are used for their reinforcing properties, such as the use of chopped glass 

 fibers to increase strength and stiffness. Air may be considered a filler 

 when injected into a resin during processing to produce a cellular or foamed 

 plastic. 



Dispersion resins are fine particle resins which can be dispersed in 

 plasticizers to produce liquid systems that are essentially 100 percent 

 solids. These systems are used in the manufacture of protective coating and 

 paint systems. Many of these systems are used to coat, impregnate or saturate 

 fabrics and yarns as well as to coat paper, and leather. 



New developments in the use of plastics in coastal structures will be 

 continuous for many years to come since a large variety of plastic resins are 

 available, their molecular structure can be rearranged to form new plastics, 



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