Biaxial orientations

Property Cast film Monoaxiall y oriented Biaxially oriented 

A large fraction of the polymers that are commercially produced are fabricated into films. Films are produced by stretching the polymer along orthogonal axes, machine direction (MD), and in the transverse direction (TD), resulting in biaxial orientation. XRD is often used to characterize this biaxial orientation. There have been a few attempts to systematically characterize this biaxial orientation. The one proposed by White and Spruiell [69] can be readily used in practice. In the previous section, it was shown that uniaxial orientation can be described by one parameter, the degree of orientation of the chain axis with respect to the fiber axis. 

Similarly, it can be shown that a minimal description of biaxial orientation requires at least two parameters shown below  

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BOPP films. See Biaxially oriented polypropylene films. 

Available as both film and sheet unless otherwise noted. All materials are available in PDA grades except for CTA, PVP, and PL EX = extrusion CL = calendering REG = regeneration CAST = casting BO = biaxial orientation. 

The basic methods for forming film or sheeting materials may be classified as follows melt extmsion, calendering, solution casting, and chemical regeneration. Of special note is the use of biaxial orientation as part of the critical manufacturing steps for many film and sheet products. 

Melt Extrusion. By far the most important method for producing film and sheeting materials reties on one or another of the various melt extmsion techniques (5). The main variations of melt extmsion are the slot (or flat) die-cast film process, the blown films process, and the flat die sheeting-stack process. These may be combined with one or more steps such as coextmsion wherein multilayer film or sheet is formed, biaxial orientation, and in-line coating (6). 

Biaxial Orientation. Many polymer films require orientation to achieve commercially acceptable performance (10). Orientation may be uniaxial (generally in the machine direction [MD]) or biaxial where the web is stretched or oriented in the two perpendicular planar axes. The biaxial orientation may be balanced or unbalanced depending on use, but most preferably is balanced. Further, this balance of properties may relate particularly to tensile properties, tear properties, optical birefringence, thermal shrinkage, or a combination of properties. A balanced film should be anisotropic, although this is difficult to achieve across the web of a flat oriented film. 

Oriented polypropylene film (OPP) may be classified as heat-set and non-heat-set, blown and tentered, coextmded and coated. Orientation improves the cold-temperature resistance and other physical properties. Heat-set biaxially oriented polypropylene film (BOPP) is the most widely used protective packagiag film ia the United States. It is used to wrap bakery products, as lamination pHes for potato and com chips, and for pastas and numerous other flexible pouch and wrapping appHcations. Nonheat-set OPP is used as a sparkling, transparent shrink-film overwrap for cartons of candy. 

Soft-drink bottles made from poly(ethylene terephthalate) (PET) are usuady made by stretch-blow mol ding in a two-step process. Eirst, a test-tube-shaped preform is molded, which is then reheated to just above its glass-transition temperature, stretched, and blown. Stretching the PET produces biaxial orientation, which improves transparency, strength, and toughness of the botde (54,56). A one-step process is used for many custom containers that are injection stretch-blow molded. 

Although PS is normally considered a rather brittle material, biaxial orientation imparts some extremely desirable properties, particularly in regard to an increase in elongation. Thus, the 1.5—2% of elongation normally associated with unoriented PS can become as high as 10%, depending on the exact conditions of preparation (267,268). 

Figure 3.5. Biaxial orientation of polymethyl methacrylate. Variation of impact strength with percentage stretch. (After Ladbury )...

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

Much of the success of the poly(ethylene terephthalate) bottle has arisen from the control of the biaxial orientation that occurs during manufacture to give a product both strong and of low gas permeability. 

Polymers can exist in a number of states. They may be amorphous resins, rubbers or fluids or they can be crystalline structures. TTie molecular and the crystal structures can be monoaxially or biaxially oriented. Heterogeneous blends of polymers in different states of aggregation enable materials to be produced with combinations of properties not shown by single polymers. 

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

Table 11.5 Comparison of cast, monoaxially oriented and biaxially oriented polypropylene film ...

Tubular film may be biaxially oriented by passing eooled film through a pair of nip rolls, reheating it to a uniform temperature and then passing it between a further pair of rolls. Between the two pairs of rolls a mandrel of air is introduced in order to expand the film. The proeess is similar to that shown for poly(vinylidene ehloride) in Figure 17.4. 

Deliberately oriented polystyrene is available in two forms filament (mono-axially oriented) and film (biaxially oriented). In both cases the increase in tensile strength in the direction of stretching is offset by a reduction in softening point because of the inherent instability of oriented molecules. 

In the late 1970s several developments occurred causing renewed interest in poly(ethylene terephthalate) as a plastics material. These included the development of a new mouldable grade by ICI (Melinar) and the development of a blow moulding technique to produce biaxially oriented PET bottles. In addition there appeared a glass-fibre filled, ionomer nucleated, dibenzoate plasticised material by Du Pont (Rynite) (see Chapter 26). 

With the expiry of the basic ICI patents on poly(ethylene terephthalate) there was considerable development in terephthalate polymers in the early 1970s. More than a dozen companies introduced poly(butylene terephthalate) as an engineering plastics material whilst a polyether-ester thermoplastic rubber was introduced by Du Pont as Hytrel. Polyfethylene terephthalate) was also the basis of the glass-filled engineering polymer (Rynite) introduced by Du Pont in the late 1970s. Towards the end of the 1970s poly(ethylene terephthalate) was used for the manufacture of biaxially oriented bottles for beer, colas and other carbonated drinks, and this application has since become of major importance. Similar processes are now used for making wide-neck Jars. 

Structurally the difference between PEN and PET is in the double (naphthenic) ring of the former compared to the single (benzene) ring of the latter. This leads to a stiffer chain so that both and are higher for PEN than for PET (Tg is 124°C for PEN, 75°C for PET is 270-273°C for PEN and 256-265°C for PET). Although PEN crystallises at a slower rate than PET, crystallization is (as with PET) enhanced by biaxial orientation and the barrier properties are much superior to PET with up to a fivefold enhancement in some cases. (As with many crystalline polymers the maximum rate of crystallisation occurs at temperatures about midway between Tg and in the case of both PEN and PET). At the present time PEN is significantly more expensive than PET partly due to the economies of scale and partly due to the fact that the transesterification route used with PEN is inherently more expensive than the direct acid routes now used with PET. This has led to the availability of copolymers and of blends which have intermediate properties. 

The major advantage of film blowing is the ease with which biaxial orientation can be introduced into the film. The pressure of the air in the bubble determines the blow-up and this controls the circumferential orientation. In addition, axial orientation may be introduced by increasing the nip roll speed relative to the linear velocity of the bubble. This is referred to as draw-down. 

Example 4.3 A plastic shrink wrapping with a thickness of 0.05 mm is to be produced using an annular die with a die gap of 0.8 mm. Assuming that the inflation of the bubble dominates the orientation in the film, determine the blow-up ratio required to give uniform biaxial orientation. 

This is another method which is used to produce biaxially oriented blow moulded containers. However, as it involves injection moulding, the description of this process will be considered in more detail later (Section 4.3.9). 

In addition to orientation in one direction (mono-axial orientation), biaxial orientation is possible. This is achieved when sheet is stretched in two directions resulting in layering of the molecules. This can increase the impact strength, tensile strength and solvent cracking resistance of polymers and with crystalline plastics the polymer clarity may also be improved. 

Based on this analysis it is evident that materials which are biaxially oriented will have good puncture resistance. Highly polar polymers would be resistant to puncture failure because of their tendency to increase in strength when stretched. The addition of randomly dispersed fibrous filler will also add resistance to puncture loads. From some examples such as oriented polyethylene glycol terephthalate (Mylar), vulcanized fiber, and oriented nylon, it is evident that these materials meet one or more of the conditions reviewed. Products and plastics that meet with puncture loading conditions in applications can be reinforced against this type of stress by use of a surface layer of plastic with good puncture resistance. Resistance of the surface layer to puncture will protect the product from puncture loads. An example of this type of application is the addition of an oriented PS layer to foam cups to improve their performance. 

Plastic films represent the largest worldwide market for plastics with practically all extruded (6). They are used to meet different performance requirements particularly for its major packaging market. Worldwide just for biaxial oriented (Chapter 8) polypropylene consumption is about 5 /2 billion lb. Their use includes tape, food, tobacco, and confectionery. Thermoforming film (and extruded sheets) is a major processing technique producing all kinds of products. 

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