Recovery, elastic

Elastic recovery is the ability of a fiber to recover from its deformation after the removal of the apphed stress. As discussed in the previous sections, the deformation of a fiber includes both elastic and plastic components. The elastic deformation mainly is caused by the changing of chain length and angle tmder apphed stress, and it is recoverable. The plastic deformation is associated with the molecular 

Naturai Poiymer Fibers (inciuding regenerated fibers)  

For this particular fiber, the elastic recoveiy at the first cycle is relatively low. However, after a few cycles, the fiber becomes conditioned and the stretch and recovery curves tend to fall on a loop, i.e., 100% elastic recoveiy. This phenomoum is important for some applications, such as tire cords. However, not all fibers can have 100% elastic recoveiy in repeated cycles. 

Work Returned during Recovery Total Work done during Stretching 

The fiber industry has long been aware of PTT s good tensile elastic recovery [3]. Ward et al. [4] studied the deformation behavior of PET, PTT and PBT fibers and found die tensile elastic recoveries were ranked in the unexpected descending order of PTT PBT PET. Chuah [47] found that the PTT elastic recovery and permanent set nearly tracked that of nylon 66 up to 30% strain (Eigure 11.12). 

Jakeways et al. [69] addressed only the crystalline chain deformation to explain PTT s elastic recovery. The macroscopic deformation must also simultaneously involve tlie partially ineversible amorphous chain deformation. The higher the applied strain, tlien tlie more dominant was the irreversible amorphous deformation with deviation from affine deformation. 

Many semicrystalline polymers are polymorphic and exist in different crystal forms. When PBT fiber is uniaxially stretched [75], the contracted gauche-trans-gauche a-crystal chain is extended to a fully trans conformation of a y-crystal. Above 20% strain, tlie crystal form is 100% y-crystal with a longer c-axis triclinic cell dimension. Thus, it is reasonable to ask whether the 

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The elongation of a stretched fiber is best described as a combination of instantaneous extension and a time-dependent extension or creep. This viscoelastic behavior is common to many textile fibers, including acetate. Conversely, recovery of viscoelastic fibers is typically described as a combination of immediate elastic recovery, delayed recovery, and permanent set or secondary creep. The permanent set is the residual extension that is not recoverable. These three components of recovery for acetate are given in Table 1 (4). The elastic recovery of acetate fibers alone and in blends has also been reported (5). In textile processing strains of more than 10% are avoided in order to produce a fabric of acceptable dimensional or shape stabiUty. 

Fiber Immediate elastic recovery, % Delayed recovery, % Permanent set, %... 

Hardness is a measure of a material s resistance to deformation. In this article hardness is taken to be the measure of a material s resistance to indentation by a tool or indenter harder than itself This seems a relatively simple concept until mathematical analysis is attempted the elastic, plastic, and elastic recovery properties of a material are involved, making the relationship quite complex. Further complications are introduced by variations in elastic modulus and frictional coefficients. 

Optimum mechanical piopeities of the fibers are developed provided the precursor novolak filaments ate less than 25 ]lni in diameter to ensure sufficient diffusion of the formaldehyde and catalyst into the fiber. The individual fibers are generally elliptical in cross section. Diameters range from 14 to 33 )J.m (0.2—1.0 tex or 2—10 den) and fiber lengths ate 1—100 mm. Tensile strength is 0.11—0.15 N /tex (1.3—1.8 g/den) and elongation is in the 30—60% range. Elastic recovery is as high as 96%. 

At strains over 300% the stress occurs mostiy in the amorphous regions up to the point where the sample breaks. AH of the grades exhibit permanent set, and the curves of grades with a Shore Hardness of 55 and higher exhibit a yield point. This means that parts have to be designed for low strains to stay within the area of elastic recovery. Special grades of elastomer are available to provide hydrolysis resistance (194), improved heat aging (195), and improved uv-stabihty (196). 

Fig. 3. Effect of cross-link density where A represents tear strength, fatigue life, and toughness B, elastic recovery and stiffness C, strength and D,...

When a fiber is stressed, the instantaneous elongation that occurs is defined as instantaneous elastic deformation. The subsequent delayed additional elongation that occurs with increasing time is creep deformation. Upon stress removal, the instantaneous recovery that occurs is called instantaneous elastic recovery and is approximately equal to the instantaneous elastic deformation. If the subsequent creep recovery is 100%, ie, equal to the creep deformation, the specimen exhibits primary creep only and is thus completely elastic. In such a case, the specimen has probably not been extended beyond its yield point. If after loading and load removal, the specimen fails to recover to its original length, the portion of creep deformation that is recoverable is still called primary creep the portion that is nonrecoverable is called secondary creep. This nonrecoverable elongation is typically called permanent set. 

To improve the rheological properties and extend the very short working time, a simple polyester is kicluded as thinner. Mixing is easy, and dimensional change ki ak is less than 0.1% over several hours. Elastic recovery and reproduction of detail are exceUent. The elastomeric cycHc imine impression materials have a higher modulus of elasticity than the condensation siHcone or polysulfide mbbers, and are more difficult to remove from the mouth. The materials have relatively low tear strength and an equUibrium water sorption of 14% thus, polyether impression materials tear readily. Because of thek poor dimensional stabUity ki water, they should be stored ki a dry environment. 

Blends with styrenic block copolymers improve the flexibiUty of bitumens and asphalts. The block copolymer content of these blends is usually less than 20% even as Httie as 3% can make significant differences to the properties of asphalt (qv). The block copolymers make the products more flexible, especially at low temperatures, and increase their softening point. They generally decrease the penetration and reduce the tendency to flow at high service temperatures and they also increase the stiffness, tensile strength, ductility, and elastic recovery of the final products. Melt viscosities at processing temperatures remain relatively low so the materials are still easy to apply. As the polymer concentration is increased to about 5%, an interconnected polymer network is formed. At this point the nature of the mixture changes from an asphalt modified by a polymer to a polymer extended with an asphalt. 

Romanchenko and Stepanov (1981) recognized that spall in an elastic-plastic material can involve plastic release to the tensile failure stress followed by elastic recovery as stress relaxation at the spall plane proceeds. Thus, the... 

Coran and Patel [33] selected a series of TPEs based on different rubbers and thermoplastics. Three types of rubbers EPDM, ethylene vinyl acetate (EVA), and nitrile (NBR) were selected and the plastics include PP, PS, styrene acrylonitrile (SAN), and PA. It was shown that the ultimate mechanical properties such as stress at break, elongation, and the elastic recovery of these dynamically cured blends increased with the similarity of the rubber and plastic in respect to the critical surface tension for wetting and with the crystallinity of the plastic phase. Critical chain length of the rubber molecule, crystallinity of the hard phase (plastic), and the surface energy are a few of the parameters used in the analysis. Better results are obtained with a crystalline plastic material when the entanglement molecular length of the... 

In addition to dynamic vulcanization, the technological compatibilization technique was also adopted by Coran and Patel [34] to obtain thermoplastic vulcanizate having good mechanical integrity and elastic recovery. 

B-C instantaneous elastic recovery occurs whan load a removed. 

Elastic recovery Uniform coloration Defined splicing 1 5 PETP... 

It has been shown that the anisotropy depends on the orientation of the diagonals of indentation relative to the axial direction 14). At least two well defined hardness values for draw ratios A. > 8 emerge. One value (maximum) can be derived from the indentation diagonal parallel to the fibre axis. The second one (minimum) is deduced from the diagonal perpendicular to it. The former value is, in fact, not a physical measure of hardness but responds to an instant elastic recovery of the fibrous network in the draw direction. The latter value defines the plastic component of the oriented material. 

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