Multilayer extrusion

Current usage is almost entirely associated with the good adhesion to aluminium. Specific applications include the bonding of aluminium foil to plastics films, as the adhesive layer between aluminium foil and polyethylene in multilayer extrusion-laminated non-lead toothpaste tubes and in coated aluminium foil pouches. Grades have more recently become available for manufacture by blown film processes designed for use in skin packaging applications. Such materials are said to comply with FDA regulations. 

Chicago, II., 29th Aug-2nd Sept.l993,p.403-6. 6A MULTILAYER EXTRUSION DIE GEOMETRY FOR THERMOFORMED POLYPROPYLENE FOAM SHEETS AND BIAXIALLY ORIENTED FOAM FILMS Raukola J Savolainen A Tampere,University of Technology (TAPPI)... 

These equations are solved numerically under the assumptions of velocity, shear stress, and temperature continuity at all interfaces. They use the Sabia 4-parameter viscosity model (69), because of its ability to include the Newtonian plateau viscosity, which is important for multilayer extrusion, because of the existence of low shear-rate viscosities at the interfaces. 

Fig. 12.36 Capillary viscosity data and Sabia equation fit. [Reprinted by permission from H. Mavridis and R. N. Shroff, Multilayer Extrusion Experiments and Computer Simulation, Polym. Eng. Sci., 34, 559 (1994).]...

BLOW-MOLDED CONTAINERS. Initial evaluation of SELAR OH Plus in extruded blow molding containers was disappointing. Barrier improvements of only 30 to 401 were measured compared to the expected 300%.. The resolution of this discrepancy provided some valuable insights into the role of platelet orientation in barrier improvement and the effects of die swell in multilayer extrusion blow molding. 

Thermoplastic elastomers can be one of the components in multilayer extrusion. They can serve either to bond two dissimilar materials, for example, ABS and high Impact polystyrene, or as a layer with physical properties that contribute to the film properties, for example, a film with an S-EB-S center and a polypropylene outer surface. In this case they yield a flexible film with a nontacky surface. In other cases, multilayered films can be constructed to control gas permeability. 

Technology to successfully produce multilayer injection blow molded bottles is much newer than that to produce multilayer extrusion blow molded bottles. Heinz Inc. s ketchup bottle was the first U.S. example (Fig. 12.22). The use of PET bottles made by coinjection blow molding has grown rapidly. As is the case for coextrusion blow molding, the key is production of the parison. Once the multilayer parison is produced, the remainder of the process is essentially the same as for single layer materials. Even stretch blow molding can be used. 

Film Processing Methods Extrusion, coextrusion, and Saran F- lacquer solution films. Multilayer Extrusion Saran resins are used in combination with a myriad of other polymers in flexible and rigid multilayer products. Multilayer cast and blown film coextrusion processes for Saran can be used with all polyethylenes, polypropylenes, and nylons. 

We described a facile multilayer extrusion technique for fabricating these novel polymer film materials. The presence of a discrete nanolayered structure was verified by an AFM study. The AFM study also demonstrated the presence a distribution of layer thickness within the polymer film. The distribution is essential to controlling the bandwidth of the induced reflectivity. A broad band reflectivity was demonstrated in a layered material with a permanent index modulation of 0.0045. This wavelength dependence of the reflectivity showed a fine structure characteristic of dielectric stack with a distribution of layer thicknesses. 

Multilayer injection blow-molded containers have been available for a much shorter time than multilayer extrusion blow-molded containers, as development of the process was more difficult. However, these too are now available. A prominent example is multilayer polyethylene terephthalate (PET)-based ketchup bottles, which contain three layers of PET, and two layers of ethylene vinyl alcohol (EVOH) as an oxygen barrier. 

Multilayer extrusion provides the imique capability of producing layers of different resins to give superior fimctional properties. For example, an inexpensive resin can be used as the core of a three-ply extrudate, the outer plies being more expensive but also much thinner than if extruded alone (see Coextrusion). 

Economics. An advantage of coextrusion is the capability of combining layers of high performance resins with low cost resin layers to produce high per-formance/low cost composite structmes. The use of recycled and scrap resins in buried layers further improves economy. Multilayer extrusion economic considerations have been calculated (89). The effect of recycle on film properties, eg, tensile strength, impact, and elongation, depends on the degree of compatibiUty of polymers in the recycle layer. Often the tie-layer polymer acts as a compatibi-lizer for recycle. Recycle of incompatible polymers with different refractive indexes usually causes haziness and cannot be used when excellent optical properties are required. 

Multilayer extrusion is a technique developed at Case Western University by Professor Eric Baer. The technique involves the co-extrusion of different polymers and can achieve multilayer films without the use of solvents (Mueller et al. 1997). 

Streaks Straight lines in the machine direction at regular intervals along the width of the sheet in a multilayer extrusion Non-uniform temperature distribution in the cooling or calendaring rolls... 

Co-extrusion has gained importance for the production of blown and cast films. Co-extrusion allows the property profiles of various PE to be combined. The film properties can therefore be adapted to the particular demands with extremely economical material consumption. Tried and tested designs of multilayer extrusion dies permit the formation of extremely thin layers, even under difficult rheological conditions. 

The advantage in this type of multilayer extrusion is that polymers with widely different processing temperatures and rheological properties can be used. The major problem is that of generating satisfactory adhesion between the components. Usually the technique is only used for two polymers. 

The most often used die geometries in multiple layer extrusion are slits and tubular film dies. However, there are also cases in which profile dies are used. Some quantitative design work can be carried out in the case of flow through flat film and tubular dies. The factors that can be dealt with are flow rates and pressure drops required to provide a given thickness in a multilayer exttudate. The major problem in multilayer extrusion is the instability in the flow. This will be discussed in a qualitative fashion in Section 7.6.3. However, the design equations can at least be used to estimate whether the flow will be unstable. 

There are several other cases which are commonly encountered. Eor thin annular dies Eqs. 7.54 through 7.59 can be adopted directly by making appropriate replacements for W and h. Eor multilayer extrusion through an annulus in which the gap is too thick to apply the thin slit approximation, the appropriate equations are derived in Problem 7B.8. For more than two layers the equations for flow through parallel plates can be generalized as follows (Schrenk and Alfrey, 1976). Referring to Figure 7.51 we can obtain expressions... 

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