Fixed draw ratio

Problems related to the use of a guest dye can be reduced if the polymer contains a fluorescent chemical group. Gohil and Salem [70] took advantage of such intrinsic fluorescence to characterize the in-plane distribution of orientation in biaxially drawn PET films. In these experiments, the chain-intrinsic fluorescent label is due to the formation of dimers by two terephthalic moieties, exclusively within the noncrystalline regions. A comparison between sequential and simultaneous drawing along the MD and TD directions was undertaken for a fixed MD draw ratio of 3.5 and various TD draw ratios. The orientational order was characterized by two "orientation ratios" Rmd and RTD such that... 

A quantity often used in the description of drawing phenomena is the natural draw ratio. As can be seen in Fig. 24.10, simple drawing of a material with the given stress-strain curve results in a fixed draw ratio corresponding to point B on the curve. This is called the natural draw ratio. 

Thus, it seems to be of interest to examine the influence of stress-induced polymorphic changes on the microhardness. While in the case of f-PP two samples comprising the a or phase were characterized, here we wish to follow the microhardness behaviour during the a-j6 polymorphic transition caused by a mechanical field. For this purpose PBT has been selected as a suitable material because of its ability to undergo stress-induced polymorphic transition from the a (relaxed) to the P (strained) form. Bristles of commercial PBT with a diameter of about 1 mm were drawn at room temperature via neck formation (final diameter about 0.5 mm and draw ratio of 3.4) and thereafter annealed in vacuum at 200°C for 6 h with fixed ends (Fakirov etal., 1998). 

Optimum conditions for conversion to form I (as monitored by ATR spectra) were established by preliminary drawing runs using an in-line tubular furnace. It was ascertained that machine draw ratios of between 5 1 and 6 1 at draw rates up to 3m/min were required to achieve substantial conversion. A linear relationship between tube density and furnace temperature was observed for a fixed machine draw ratio of 6 1. Over the temperature range 115°C to 170°C the density varied between 1.7886 to 1.7988 g cm"" which corresponded (assuming complete absence of form II) to a percentage form I content of 38.3% to 41.8%. 

Draw resonance will occur when the resistance to extensional deformation decreases as the stress level increases. The total amount of mass between die and take-up may vary with time because the take-up velocity is constant but not necessarily the extrudate dimensions. If the extrudate dimensions reduce just before the take-up, the extrudate dimensions above it have to increase. As the larger extrudate section is taken up, a thin extrudate section can form above it this can go on and on. Thus, a cyclic variation of the extrudate dimensions can occur. Draw resonance does not occur when the extrudate is solidified at the point of take-up because the extrudate dimensions at the take-up are then fixed [171, 172]. Isothermal draw resonance is found to be independent of the flow rate. The critical draw ratio for almost-Newtonian fluids such as nylon, polyester, polysiloxane, etc., is approximately 20. The critical draw ratio for strongly non-Newtonian fluids such as polyethylene, polypropylene, polystyrene, etc., can be as low as 3 [173]. The amplitude of the dimensional variation increases with draw ratio and drawdown length. 

When a PP is extruded and taken-up at a high draw ratio and crystallized under a high stress and annealed, a lamella-stacked structure is formed. In this structure, the lamellae are oriented perpendicular to the extrusion direction and connected by the tie molecules. When such an extrudate is stretched in the extrusion direction, the lamellae open elastically with the tie molecules working as fixing points. Therefore, after stress removal, the initial shape and structure are restored. Since such a part shows a high elastic recovery after a deformation and the elastic modulus is nearly the same as that of the usually processed article, it is called a Tiard-elastic item. When the hard-elastic film or fiber is drawn beyond the yielding point, plastic deformation occurs, leading to void formation, and a microporous film or fiber can be obtained. 

In this work, the studies focused on the effect of draw ratio employed during cold drawing on the morphology development of the PP/PET microfibrillar blends and composites prepared at a fixed blend composition of 85/15. The tensile properties of MFCs prepared at different hot stretch ratios were reported recently [17,18]. However, the effect... 

Figure 21.4. Strengthening effect of nano-Si02 on PP filaments (a) drawing ratio is fixed at 15 and (b) content of nano-Si02 is fixed at 1 wt%...

Figure 16-11. WAXD photographs for stretched cellulose films at the desired drawn ratio (a) undrawn water-swollen film after dried at a fixed state (b) draw ratio 1.5 (c) draw ratio 1.7, and (d) draw ratio 2.0...

Drawing of any unoriented crystalline polymer in the presence of a swelling liquid yields very small draw ratios, X 2, and a completely different nonfibrous material. The orientation hardly increases. But the lamellae perpendicular to the strain are separated by holes which enormously enhanced permeability (Knudsen type viscous flow of gases). If such a deformed sample is dried with fixed ends it retains its extended shape and behaves like a hard elastomer with very large recoverable strain, up to a few hundred percent, but much better lateral mechanical strength than the conventional hard elastomers obtained by extrusion under extreme force and temperature gradient. ... 

P(3HB-co-8%-3HV) fiber was prepared by one-step-drawing procedure of amorphous fiber with small crystal nuclei at room temperature. Amorphous fibers were obtained by quenching of the melt-spun fibers of P(3HB-co-8%-3HV) into ice water. Isothermal crystallization of amorphous P(3HB-co-8%-3HV) fibers was held in ice water for a certain period to prevent rapid crystallization and to grow small crystal nuclei. One-step-drawing after isothermal crystallization was performed by the stretching machine at room temperature, and then annealed at 60 °C for 30 min in an autoclave for fixing the extended polymer chains. One-step-drawn fibers with isothermal crystallization for 24 h were opaque and maximum total draw ratio was ca. 10 times. 

Let us consider further the calculation of the same characteristics of a PCP network, stretched up to some fixed drawing ratio X, for example, X = 3. Assuming network affine deformation the value of R at A, = 3 () becomes [6] ... 

Fig. 2. DSC thermograms of gel-crystallized drawn UHMWPE (draw ratio is 80) recorded with heating rate of 1.25K/min in different conditions free fiber, m = 0.01 mg (curve 1) ditto, m = 0.4 mg (curve 2) fiber pressured in a capsule (curve 3) constrained fiber with fixed ends (curve 4). A and B represents peaks.

Bristles with diameter of about 1 mm were prepared from all materials by means of melt extrusion. These isotropic (according to X-ray tests) bristles were drawn at room temperature until the entire sample underwent neck formation, corresponding to a draw ratio A = 3.5 5. The drawn material exhibited a reversible deformation at room temperature of about 50%. The drawn and undrawn bristles were annealed with fixed ends in vacuum at various temperatures, T, for 6 h. 

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