Tension shrink film

Thermomechanical analysis in the tension mode can be employed in the production of films and fibers to follow the consistency of the product during manufacturing, and to characterize properties of the product, such as CLTE, thermal shrinkage and shrinkage force, and physical properties like Tg and T. All these properties are important, and they determine the end-use possibilities. For example, matching fiber-matrix expansion properties is necessary to avoid delamination of components, such as in tire cords. Shrinkage (i.e., irreversible contraction) is important to avoid in textiles for safe ironing, and it is also an important property in heat-shrink films (Jaffe et al. 1997). 

When a liquid film bursts, it retracts by the effect of its tension that is shown to the eyes in experiments that we must reserve for another chapter however Dupr6 treats the phenomenon by his methods, and comes to express the speed there of the shrinking film by the formula ... 

In order to reduce the tendency of the film to shrink oriented film may be annealed at about 100°C whilst under tension immediately after drawing. The film is often coated with another polymer sueh as a vinylidene ehloride-based copolymer. This both improves the barrier properties and improves the heat scalability. 

Curvature relates to the local change in interface area when an interface moves. The energy change per unit volume swept out by the interface is equal to the product of k and the interfacial energy per unit area 7. Normally, for fluids, 7 is independent of the interface inclination h in this case, the interface is isotropic. For example, a soap bubble has isotropic interface tension. If perturbed, a floating individual soap bubble will quickly re-establish its equilibrium form—a sphere of fixed volume. Such a soap bubble will also shrink slowly—the gas will diffuse out of the bubble because of a pressure difference across the soap film (AP = jk = /Rc). Thus,... 

Two examples may be used to illustrate the complexity of problems of this kind. When film is made by extrusion followed by casting on chill rolls there can be a tendency for the extruded web to shrink inwards towards the centre of the rolls—the phenomenon known as neck-in . The edge of film concerned becomes thicker than the rest. It has been found that more elastic melts, capable of keeping a tension in the direction of extrusion, are less liable to exhibit this fault. 

In 1744 Euler discovered the catenoid, the first non-planar minimal surface. This surface is readily realised by a soap film, spanning coaxial circular bounding wires. The film shrinks under the action of its surface tension, forming the minimal surface (Fig. 1.13). 

What Thompson had seen in his wine glass, and what others had noticed probably since the invention of the beverage, was evaporative convection driven by unbalanced surface tension. In a partially filled goblet of wine, a film of wine wetting the inside surface of the goblet will writhe, shrink into droplets, and run down. Thompson noted that the tensile force [surface tension] is not the same in different liquids. Thus it is found to be much less in alcohol than in water. He then explained the motions in the evaporating wine films in the following terms ... 

The structural capsules start to be formed in films subjected to deformation in liquids until some tension threshold. Microcracks and microvoids appear and are filled with the inhibiting liquid under tensile stresses exceeding the polymer flow limit. Capillary channels connecting these voids with the process liquid and with each other start to merge or open in the course of structural transformations but do not disappear fully. The liquid may move over the network of the formed channels beyond the polymer matrix limits or concentrate in some voids able under certain conditions to enlarge the manifold. Thermal treatment of the deformed film intensifies the relaxation processes in the polymer matrix, the film shrinks in the tension direction and the capillaries between voids link up densely, thus insulating liquid particles from each other. If the film is treated in the extended state, a more complex mechanism of microcapsule formation is realized [4]. Cl liberation from microcapsules is related to their ability to break spontaneously under residual... 

ASTM D283X, Shrink tension and orientation release stress of pla.stic film and. sheeting. 1995. 

Films deposited on a rigid substrate experience biaxial tensile stresses during drying because they cannot shrink in the plane of the film. The biaxial tensile stress develops during drying as a result of the capillary tension in the pore liquid as it stretches to cover the dried surface exposed by the evaporation of the liquid (see Chapter 5). The capillary tension in the liquid would cause the film to shrink except the rigid substrate prevents the shrinkage in the plane of the film. Films... 

Shrinkage. Standard pet and PEN films from the stenter process will shrink between 1 and 3% at temperatures above the Tg. For the high level of accuracy required by some electrical applications shrinkages of below 0.1-0.2% are required. In order to meet this the film is unwoimd and passed through a carefully temperature-controlled oven, with almost no tension in the film. The amount of residual shrinkage in the film after this process is typically less than 0.1% in the MD and the TD for temperatures up to 150°C for PET and 200°C for PEN. 

Films may contain colorants, stabilizers, or other additives, and may be coated for the improvement of performance properties (heat sealability, gas permeability, etc.). Films may be annealed (heat-set) to reduce the unstrained linear shrinkage and shrink tension upon exposure to heat. 

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