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mechanical behavior, that is, its deformation and ow characteristics under stress. The mechanical behavior of a polymer can be characterized by its stress strain properties [Billmeyer, 1984; Nielsen and Landel, 1994]. This often involves observing the behavior of a polymer as one applies tension stress to it in order to elongate (strain) it to the point where it ruptures (pulls apart). The results are usually shown as a plot of the stress versus elongation (strain). The stress is usually expressed in newtons per square centimeter (N cm 2 ) or megapascals (MPa) where 1 MPa 100 N cm 2 . The strain is the fractional increase in the length of the polymer sample (i.e., L=L, where L is the original, unstretched sample length). The strain can also be expressed as the percent elongation, L=L 100%. Although N cm 2 is the SI unit for stress, psi (pounds per square inch) is found extensively in the literature. The conversion factor is 1 N cm 2 1:450 psi. SI units will be used throughout this text with other commonly used units also indicated. Several stress strain plots are shown in Fig. 1-10. Four important quantities characterize the stress strain behavior of a polymer: 1. Modulus. L=L. The resistance to deformation as measured by the initial stress divided by
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Fig. 1-10 Stress strain plots for a typical elastomer, exible plastic, rigid plastic, and ber.
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2. Ultimate Strength or Tensile Strength. The stress required to rupture the sample. 3. Ultimate Elongation. The extent of elongation at the point where the sample ruptures. 4. Elastic Elongation. The elasticity as measured by the extent of reversible elongation. Polymers vary widely in their mechanical behavior depending on the degree of crystallinity, degree of crosslinking, and the values of Tg and Tm . High strength and low extensibility are obtained in polymers by having various combinations of high degrees of crystallinity or crosslinking or rigid chains (charcterized by high Tg ). High extensibility and low strength in polymers are synonymous with low degrees of crystallinity and crosslinking and low Tg values. The temperature limits of utility of a polymer are governed by its Tg and/or Tm. Strength is lost at or near Tg for an amorphous polymer and at or near Tm for a crystalline polymer. An almost in nite variety of polymeric materials can be produced. The polymer scientist must have an awareness of the properties desired in the nal polymer in order to make a decision about the polymer to be synthesized. Different polymers are synthesized to yield various mechanical behaviors by the appropriate combinations of crystallinity, crosslinking, Tg , and Tm . Depending on the particular combination, a speci c polymer will be used as a ber, exible plastic, rigid plastic, or elastomer (rubber). Commonly encountered articles that typify these uses of polymers are clothing and rope ( ber), packaging lms and seat covers ( exible plastic), eyeglass lenses and housings for appliances (rigid plastic), and rubber bands and tires (elastomer). Table 1-4 shows the uses of many of the common polymers. Some polymers are used in more than one category because certain mechanical properties can be manipulated by appropriate chemical or physical means, such as by altering the crystallinity or adding plasticizers (Sec. 3-14c-1) or copolymerization (Sec. 3-14b, Chap. 6). Some polymers are used as both plastics and bers, other as both elastomers and plastics.
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TABLE 1-4 Use of Polymers Elastomers Polyisoprene Polyisobutylene Plastics Fibers
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Polyethylene Polytetra uoroethylene Poly(methyl methacrylate) Phenol formaldehyde Urea formaldehyde Melamine formaldehyde Polystyrene ! Poly(vinyl chloride) ! Polyurethane ! Polysiloxane ! Polyamide ! Polyester ! Cellulosics ! Polypropene ! Polyacrylonitrile Aspx pdf417 2d barcode drawerfor .net
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Elastomers, Fibers, and Plastics
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The differences between bers, plastics, and elastomers can be seen in the stress strain plots in Fig.1-10. The modulus of a polymer is the initial slope of such a plot; the tensile strength and ultimate elongation are the highest stress and elongation values, respectively. Elastomers are the group of polymers that can easily undergo very large, reversible elongations ( 500 1000%) at relatively low stresses. This requires that the polymer be completely (or almost completely) amorphous with a low glass transition temperature and low secondary forces so as to obtain high polymer chain mobility. Some degree of crosslinking is needed so that the deformation is rapidly and completely reversible (elastic). The initial modulus of an elastomer should be very low (<100 N cm 2 ), but this should increase fairly rapidly with increasing elongation; otherwise, it would have no overall strength and resistance to rupture at low strains. Most elastomers obtain the needed strength via crosslinking and the incorporation of reinforcing inorganic llers (e.g., carbon black, silica). Some elastomers undergo a small amount of crystallization during elongation, especially at very high elongations, and this acts as an additional strengthening mechanism. The Tm of the crystalline regions must be below or not signi cantly above the use temperature of the elastomer in order that the crystals melt and deformation be reversible when the stress is removed. Polyisoprene (natural rubber) is a typical elastomer it is amorphous, is easily crosslinked, has a low Tg ( 73 C), and has a low Tm (28 C). Crosslinked (moderately) polyisoprene has a modulus that is initially less than 70 N cm 2 ; however, its strength increases to about 1500 N cm 2 at 400% elongation and about 2000 N cm 2 at 500% elongation. Its elongation is reversible over the whole elongation range, that is, up to just prior to the rupture point. The extent of crosslinking and the resulting strength and elongation characteristics of an elastomer cover a considerable range depending on the speci c end use. The use of an elastomer to produce an automobile tire requires much more crosslinking and reinforcing llers than does the elastomer used for producing rubber bands. The former application requires a stronger rubber with less tendency to elongate than the latter application. Extensive crosslinking of a rubber converts the polymer to a rigid plastic. Fibers are polymers that have very high resistance to deformation they undergo only low elongations (<10 50%) and have very high moduli (>35,000 N cm 2 ) and tensile strengths (>35,000 N cm 2 ). A polymer must be very highly crystalline and contain polar chains with strong secondary forces in order to be useful as a ber. Mechanical stretching is used to impart very high crystallinity to a ber. The crystalline melting temperature of a ber must be above 200 C so that it will maintain its physical integrity during the use temperatures encountered in cleaning and ironing. However, Tm should not be excessively high not higher than 300 C otherwise, fabrication of the ber by melt spinning may not be possible. The polymer should be soluble in solvents used for solution spinning of the ber but not in dry-cleaning solvents. The glass transition temperature should have an intermediate vlaue; too high a Tg interferes with the stretching operation as well as with ironing, while too low a Tg would not allow crease retention in fabrics. Poly(hexamethylene adipamide) is a typical ber. It is stretched to high crystallinity, and its amide groups yield very strong secondary forces due to hydrogen bonding; the result is very high tensile strength (70,000 N cm 2 ), very high modulus (500,000 N cm 2 ), and low elongation (<20%). The Tm and Tg have optimal values of 265 and 50 C, respectively. [The use of polypropene as a ber is an exception to the generalization that polar polymers are required for ber applications. The polypropene used as a ber has a highly stereoregular structure and can be mechanically stretched to yield a highly oriented polymer with the strength characteristics required of a ber (see Sec. 8-11d).]
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