Biaxial orientation effects

Biaxial orientation effects are of importance in the manufacture of films and sheet. Biaxially stretched poly(ethylene terephthalate) (e.g. Melinex),... 

Biaxial orientation effects are important in the manufacture of films and sheet. Biaxially stretched polypropylene, poly(ethyleneterephthalate) (e.g., Melinex) and poly(vinylidene chloride) (Saran) produced by flat-film extrusion and tentering are strong films of high clarity. In biaxial orientation, molecules are randomly oriented in two dimensions just as fibers would be in a random mat the orientation-induced crystallization produces structures which do not interfere with the light waves. With polyethylene, biaxial orientation often can be achieved in blown-film extrusion. 

There are other differences between cast, monoaxially oriented, and balanced biaxially oriented film. Typical figures illustrating these effects are given in Table 11.5. 

For a biaxially drawn polymer with orthorhombic symmetry, the chain axis is preferentially oriented with respect to the XtX2 plane of the sample, and the direction normal to the chain axis is also preferentially oriented. Infra-red measurements enable the determination of the quantities Pf20 and PL202 and P222, which define these orientation effects as discussed in case (iv) above. 

Biaxially oriented films, made by stretching in two mutually perpendicular directions, have reduced creep and stress relaxation compared to unoriented materials. Part ot the effect is due to the increased modulus, but for brittle polymers, the improved behavior can be due to reduced crazing. Biaxial orientation generally makes crazing much more difficult in all directions parallel to the plane of the film. 

Figure 14.22 plots the oxygen permeability of PET nonoriented sheets and biaxially oriented bottles as a function of the degree of crystallinity. Indeed, the effect of crystallinity is larger than that of biaxial orientation. But in practical terms, nonoriented sheets crystallize much more slowly than the biaxially oriented bottle walls, because deformation-induced orientation proceeds at higher rates at any temperature between Tg and Tm. That is, the schematically represented crystallization rate curve in Fig. 14.21 extends vertically upwards. 

Trogamid T can be processed without difficulty by the normal methods used for thermoplastics (5). In this respect it offers fewer problems than polyacetal or polycarbonate, for example. Some Trogamid T properties can be improved by biaxial orientation. The effects, however, are lower than with crystalline polymers. 

Danch et al. studied the effect of thermal history on the assignment of the glass transition event associated with the biaxial orientation of the smectic... 

Like other composite properties, the wear resistance is influenced by the orientation of reinforcing fibres. Sung and Suh found that with biaxially-oriented glass fibre and molybdenum disulphide in a PTFE matrix (Duroid 5813), the wear resistance was greatest when the highest proportion of fibre was normal to the sliding surface. The same effect was found with a graphite fibre/epoxy composite and a Kevlar fibre/epoxy composite. 

From Fig. 2a it is evident that as the test temperature raises from 0 up to 70 °C the resulting load-displacement curves display a decrease of the maximum load and an increase of the elongation at failure. On the other hand, as reported in Fig. 2b, when the displacement rate increases the load-displacement curves evidence an increase of the maximum load and, quite surprisingly, also the elongation at break increases. It is worth noting, that this rather unexpected rate effect on the load-displacement curves of notched samples has been already reported by Karger-Kocsis and co-workers for both biaxially oriented filled PET [7] and amorphous copolyester [9], and by Plummer et al. for polyoxymethylene tested at high temperature [14]. 

The effects of orientation on the mechanical properties of polymers at both small and large deformations depend on the mode of orientation, which determines the preferred average chain alignment. For example, the mechanical properties obtained after uniaxial orientation (which biases the chain end-to-end vectors in one favored direction) differ from those obtained by biaxial orientation (which biases these vectors in two favored direction). Furthermore, the mechanical properties obtained after simultaneous equibiaxial orientation (where orientation in the two favored directions is imposed simultaneously, at equal rates, and to equal extents) often differ from those obtained after sequential orientation in the two favored directions, as well as after orientation by different amounts and/or at different rates in those two directions. See Seitz [35] for a review of the effects of uniaxial and biaxial orientation on the fracture of polystyrene, which fails by brittle fracture or crazing, under uniaxial tension and impact. 

The effects of uniaxial and biaxial orientation can be quite different. Although many commercial processing techniques impose a biaxial strain on the polymer melt, we will deal here primarily with uniaxially deformed materials. 

Figure 6. Effect of biaxial orientation in polyethylene terephthalate). (Data taken from Ref. 47.)...

In the case of both EVAL EP-E105 and SOARNOL D resins, attempts to uniaxially orient to 2.5X were unsuccessful, as the films split and cracked upon drawing. Reducing the stretch rate and increasing the orientation temperature yielded no significant effect. As such, biaxial orientation of these two EVOH films was not attempted. 

The effect of orientation on amorphous barrier resins SELAR PA 3426 and XHTA-50A are shown in Table III. The measured oxygen permeability of 1.4 cc-mil/100 in 24 hr. atm for unoriented extrusion cast SELAR PA 3426 film is reduced by 19% upon uniaxial orientation, while the permeability of biaxially oriented films are 28% lower than that of extrusion cast film. Similarly, the decrease in oxygen permeability with 2X and 2.5X uniaxial orientation in XHTA-50A are 5% and 9% respectively, from the measured value of 3.1 cc-mil/100 in 24 hr. atm for unoriented film. The corresponding reduction in permeability with 2X x 2X biaxial orientation is 12%. 

The absorption correction (correcting for the effects arising from variations in irradiated volume as well as in beam path length within the sample) is important, and one way to ensure its accuracy is to obtain a reference sample having the same constitution and shape as the test sample but known to be isotropic and to make sure that the intensity measured with this reference sample is truly constant after the absorption correction is applied. From the intensity /( ) or /( , ) obtained after the absorption correction, the pole distribution r( ) or t( , d>), for a sample of uniaxial or biaxial orientation, respectively, is evaluated according to... 

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