Bi-axial stretching

Extruded films are sometimes bi-axially stretched, in particular PE and PP films, which gain in strength and transparency by biaxial orientation. The stretching takes place at elevated temperatures, but below the melting point. The edges of the film are gripped by a series of clips, which diverge in the transverse direction and simultaneously accelerate in the machine direction. 

Transmission Electron Microscopy (TEM) has been used to characterize aluminum thin films thermally evaporated (vacuum around 10 4 Torr) on Polyethyleneterephtalate (Mylar) and to correlate the crystallographic structure of the system Al/Mylar and the adhesion of the aluminum films. The adhesion of these films has been measured by a Peel test technique. For the polymer, an amorphous layer (t=12 nm) followed by a crystalline film have been observed on a Corona treated film and the opposite configuration has been found on a bi-axially stretched film. Some spherical precipitation ana interdiffusion zones have also be observed in the Mylar for the films which have the lower coefficient of adhesion (100 g/inch). The main conclusion is the augmentation of the adhesion of the aluminum film as the size of the grains decreases and/or as the microroughness of the Al/Mylar interface increases. 

Metal thin films deposited on polymers are widely used in various industrial domains such as microelectronics (capacitors), magnetic recording, packaging, etc. Despite much attention that has been paid in the recent literature on the adhesive properties of metals films on polyimide (PI)( 1 - 5 ) and polyethyleneterephtalate (PET)((L) it appears that a better knowledge of the metal/polymer interface is needed. In this paper we focus ourself on the relationship between the adhesion and the structural properties of the aluminum films evaporated (or sputtered) on commercial bi-axially stretched PET (Du Pont de Nemours (Luxembourg) S.A.). A variety of treatment (corona, fluorine,etc.) have been applied in order to improve the adhesion of the metallic layer to the polymer. The crystallographic... 

ALUMINUM FILM ONTO NON TREATED BI-AXIALLY STRETCHED POLYMER... 

Figure 1 Morphology and structure of a low adhesion (100 g/inch) aluminum film evaporated on non treated bi-axially stretched PET.(marked is 100 nm)...

Figure 3 "Semi crystalline" skin for the bi-axially stretched film and "Amorphous skin for the corona treated film.(marked is 12.5 nm)...

The main results obtained for the growth kinetics, the adhesion, and the structure and morpholy of aluminum thin films deposited on bi-axially stretched PET films are the following. (l)For a set of experimental conditions (flux, temperature, and polymer surface), the aluminum film is discontinuous up to 10 nm (island formation) and then become continue. The grain size always increases with the thickness of the aluminum Elm as opposeted to the adhesion which remains rather constant. (2)When the aluminum film is continuous and for one thickness of deposition, the adhesion coefficient increases when the grain size decreases. (3)We found an increase of the adhesion coefficient when the skin of the polymer is "semi-crystalline and when the polymer is pretreated with a corona discharge. (4)The best results (for the adhesion of Al/PET) are found for a polymer treated in a fluorine atmosphere and when the deposition of the aluminum on polymer is done by sputtering. 

The method of preparation of polymeric photocatalytic membranes based on bi-axial stretching of a polymer extrusion containing TiOj as a catalytic hller (b) was applied for production of PTFE membranes (Morris et fl/., 2004).The obtained membranes contained 2 wt% of li02 distributed uniformly within the membrane. The SEM photographs revealed that the structure of the membrane resembled a net in which PTFE strands were knotted at multiple regions. The photocatalyst particles were primarily accumulated in the vicinity of these polymer knots. 

Aramid films have been in development since the late 1990s by several Japanese companies including Toray, Teijin, and Asahi. As with fibers, aramid solutions can be extruded through flat dies to form films. The conventional wet process can be employed to produce unidirectional and bi-oriented films from isotropic 77j-aramid solutions. It is more complicated to obtain isotropic p-aramid films. It requires extrusion of PPTA/H2SO4 solutions, which are exposed to subsequent steam treatment, bi-axial stretching, drying and specific annealing [19]. p-Aramid films have many applications in electrical industry. 

Results confirm that truncated conical shapes are formed under plane strain conditions while truncated pyramidal shapes are obtained under bi-axial stretching in the corners and plane-strain conditions in the side walls. It is worth noting that the experimental values of fracture strains for the pyramids are not placed on the equal bi-axial strain ratio line with slope +1 in the principal strain space. In fact, although the onset of failure is located at corners of the pyramids, the values of fracture strains are somewhat deviating towards the plane strain direction. This also explains the existence of different values of fracture strains plotted at various locations of the first quadrant of the principal strain space. 

A newer development is the manufacture of bi-axially oriented bottles, in particular for PETP. The pre-form is injection- moulded and rapidly cooled in the mould it remains amorphous (PETP crystallises very slowly) by heating above its Tg (65°) it passes into the rubbery state, and can then be blown-up and, simultaneously, longitudinally stretched. The biaxial orientation thus obtained, accelerates the crystallisation, and, at the same time, results in a very fine crystalline texture, so that a thin-walled, strong, transparent and heat-resistant bottle is obtained. 

Orientation may be obtained deliberately by stretching the melt just before it is frozen mono-axial orientation is important for the production of fibres and filaments, and bi-axial orientation (that is, stretching both lengthwise and transversely to an equal extent) is essential for the qualities required of some... 

As mentioned before, it is customary to stretch polypropylene uni-axially or bi-axially in order to enhance mechanical properties, including impact strength. It is essential to use antioxidants or UVA for weather resistance. 

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