Polyesters, orientation

Window adhesives attach the elear window film to the rest of the envelope. The window film is usually polyester, orientated polypropylene or polystyrene. Formulated homopolymers or eopolymers are used as adhesives. 

Just as it is not necessary for polymer chains to be linear, it is also not necessary for all repeat units to be the same. We have already mentioned molecules like proteins where a wide variety of different repeat units are present. Among synthetic polymers, those in which a single kind of repeat unit are involved are called homopolymers, and those containing more than one kind of repeat unit are copolymers. Note that these definitions are based on the repeat unit, not the monomer. An ordinary polyester is not a copolymer, even though two different monomers, acids and alcohols, are its monomers. By contrast, copolymers result when different monomers bond together in the same way to produce a chain in which each kind of monomer retains its respective substituents in the polymer molecule. The unmodified term copolymer is generally used to designate the case where two different repeat units are involved. Where three kinds of repeat units are present, the system is called a terpolymer where there are more than three, the system is called a multicomponent copolymer. The copolymers we discuss in this book will be primarily two-component molecules. We shall discuss copolymers in Chap. 7, so the present remarks are simply for purposes of orientation. 

Mechanical Properties. Polyester fibers are formed by melt spinning generally followed by hot drawing and heat setting to the final fiber form. The molecular orientation and crystalline fine stmcture developed depend on key process parameters in all fiber formation steps and are critical to the end use appHcation of the fibers. 

When drawdown is high, the film may be uniaxially oriented and the properties of the final film isotropic. In the manufacture of strapping tape this effect is accentuated. If the cast or quenched film is to be used to feed an orientation tine, additional attention must be given to the amorphous—crystalline nature of the film ia the draw processes so that maximum strength can be achieved and uniform gauge and optical quality maintained. Slot casting is used for the orientation of these resins, polyesters, polyamides, and a variety of others. 

Poly(vinylidene chloride). Poly(viayHdene chloride) [9002-85-1] (PVDC), most of which is produced by Dow Chemical, is best known in its saran or PVC-copolymerized form (see Vinylidene chloride and poly(VINYLIDENE chloride)). As solvent or emulsion coating, PVDC imparts high oxygen, fat, aroma, and water-vapor resistance to substrates such as ceUophane, oriented polypropylene, polyester, and nylon. 

Extmsion of polyethylene and some polypropylenes is usually through a circular die into a tubular form, which is cut and collapsed into flat film. Extmsion through a linear slot onto chilled rollers is called casting and is often used for polypropylene, polyester, and other resins. Cast, as well as some blown, films may be further heated and stretched in the machine or in transverse directions to orient the polymer within the film and improve physical properties such as tensile strength, stiffness, and low temperature resistance. 

In 1975, the synthesis of the first main-chain thermotropic polymers, three polyesters of 4,4 -dihydroxy-a,a -dimethylbenzalazine with 6, 8, and 10 methylene groups in the aHphatic chain, was reported (2). Shortly thereafter, at the Tennessee Eastman Co. thermotropic polyesters were synthesized by the acidolysis of poly(ethylene terephthalate) by/ -acetoxybenzoic acid (3). Copolymer compositions that contained 40—70 mol % of the oxybenzoyl unit formed anisotropic, turbid melts which were easily oriented. 

It is difficult for dye solutions in water to penetrate synthetic fibers such as polyester, cellulose triacetate, polyamides, and polyacryUcs which are somewhat hydrophobic. The rate of water imbibition differs with each fiber as shown in Table 1 as compared to viscose (see Fibers, regenerated CELLULOSics), which imbibes water at the rate of 100% (1). The low imbibition rate is attributed to the high T obtained when the polymeric fibers are drawn. During this drawing operation the polymer chains become highly oriented and tightly packed, forming a stmcture practically free of voids. 

The less simple polymers (like the epoxies, the polyesters and the formaldehyde-based resins) are networks each chain is cross-linked in many places to other chains, so that, if stretched out, the array would look like a piece of Belgian lace, somehow woven in three dimensions. These are the thermosets if heated, the structure softens but it does not melt the cross-links prevent viscous flow. Thermosets are usually a bit stiffer than amorphous thermoplastics because of the cross-links, but they cannot easily be crystallised or oriented, so there is less scope for changing their properties by processing. 

Polyester fibers contain crystalline as well as noncrystalline regions. The degree of crystallinity and molecular orientation are important in determining the tensile strength of the fiber (between 18-22 denier) and its shrinkage. The degree of crystallinity and molecular orientation can be determined by X-ray diffraction techniques. ... 

Polar molecule A molecule in which there is a separation of charge and hence positive and negative poles, 183-185 dipole force, 237 orientation, 183 Polarimeter, 600 Polarity, 184-185 Pollutants, 6 Polyamide, 615-616 Polyatomic ion, 36,39 Polyatomic molecules, 654 Polyester A large molecule made up of ester units, 614-615 Polyethylene, 611-612... 

Quadraxially oriented (four directional layer) glass fabric-TS vinyl ester polyester RP sheet panels with a foam core and gel coating are used. Most of the panels are 3 mm thick with molded-in rib structure supports. Body skins are bonded to the chassis with a double-stick acrylic tape developed by 3M Co. as well as mechanical fasteners. Unlike most steel designs, no B-pillar structural component between the front and rear doors is required thus providing more interior space and easy entry since doors open in opposite directions. 

For this case, which is the fairly common situation of a biaxially oriented film, it is therefore necessary to obtain a total of thirteen second and fourth order coefficients. Later, results for polyester films will be discussed, where seven of these thirteen coefficients have been determined experimentally. 

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