High-pressure extrusion

There is one piece of equipment that approaches the process engineer s dream of a machine that takes in raw materials at one end and finished product emerges from the far end. That machine is the high pressure extruder. Unfortunately, while high pressure extrusion can make crisp-bread it has a limited application in the bakery sector. 

FIGURE 14.7 Schematic presentation of the high-pressure extrusion apparatus. (From Schneider etal. (1995). With permission.)... 

Schneider, T., Sachse, A., Bossling, G, and Brandi, M. (1995). Generation of contrast-carrying liposomes of de ned size with a new continuous high pressure extrusion m bltiajtl.Pharm., 117, 1-12. 

Since the Navier s slip hypothesis of the last century, most experiments have failed to obtain positive evidence for a slip boundary condition on macroscopic scales in low molar mass liquids. However, Navier s notion of slip turns out to be extremely useful and convenient for the latest description of flow anomalies of highly entangled polymer melts including linear polyethylenes (LPE). The ability of a melt/solid interface to possess two profoundly different states as shown by Fig. 4a,b clearly reveals the potential role of interfacial slip in governing various melt flow phenomena in high pressure extrusion. Before reviewing recent experimental studies that have elucidated the molecular origins of different flow... 

As far as mechanical applications are concerned, the use of grafted MCM-41 materials represents a real improvement first, high pressure extrusions allow cyclic use without complete discharge of the operational device. Secondly, low intrusion pressure... 

High-pressure extrusion involves comparable changes. The product becomes hot and vapor bubbles are formed in it. The bubbles expand when the material leaves the extruder, and water vapor can readily escape. Upon cooling, a solid foam is formed, i.e., a glass containing many air cells. This is applied in manufacturing dry snacks. 

High-pressure extrusion i.e. jetting—widely used for hot melt systems where a jet is fired by a pressurised gun. 

Tab. 8.10 Technical information on some typical high pressure extrusion presses, (a) machine data, (b) briquette output (according to ZEMAC, Zeitz, Germany).

High Pressure Extrusion (Ram Presses, Extruders) Buhler AG... 

Liposomes have become known as one of the most versatile tools for the delivery of DNA, DNA-related, and many other therapeutic molecules [6,7,29], Liposomes are spherical vesicles that consist of an aqueous compartment enclosed in a phospholipid bilayer. If multiple bilayers of lipids are formed around the primary core, the structures that are generated are known as multilamellar vesicles (MLVs). MLVs are formed spontaneously by reconstitution of lipid films in aqueous media. Small unilamellar vesicles (SUVs) of specific size (100-500 nm) are produced by high-pressure extrusion of MLVs through polycarbonate membranes. SUVs (25-90nm) are also obtained by sonication of MLVs or larger SUVs, by detergent dialysis [62] and by many other, less important methods. Both hydrophilic and... 

Type of extrusion Low pressure extrusion Medium pressure extrusion High pressure extrusion ... 

Melt fracture is generally defined as surface roughness on the extrudate,but may include distortion of the entire body of the extrudate (Fig. 2.28). It is most often observed in high-pressure extrusions, like blown film. The appearances of various forms of melt fracture have led to names such as shark skin, orange peel, bamboo, and ripple. Unlike die swell, which is expected and accounted for in the die design, melt fracture should be avoided entirely. This defect is the result of incorrect die design, improper processing conditions, and/or poorly matched material properties. 

The crosshead die is also used for wire coating. In wire coating, a conductor passes through the hollow center of the core tube and becomes coated with polymer melt close to the die exit. The conductor may be a bare conductor or it may already have been coated with one or more layers of polymer. In wire coating, one distinguishes between high-pressure extrusion and low-pressure extrusion see Fig. 9.19. 

In high-pressure extrusion. Fig. 9.19, the polymer melt meets the conductor before the die exit. This allows for good contact between the conductor and the polymer. In low-pressure extrusion. Fig. 9.19, the polymer melt meets the conductor after the die exit. The polymer is tubed down over the conductor. Low-pressure extrusion is used when good contact between the wire and the polymer is not essential, for instance, when a loose jacket needs to be extruded over a coated wire. 

Size Control. Once the vesicles are formed, they will keep their size and size distribution. In order to reduce their size one has to disrupt the vesicle membrane, which is done by strong shear forces, eg by ultrasonic irradiation, ball milling, and high-pressure extrusion through membranes or slits ( French press ). Extrusion through polycarbonate membranes, which are available with pore diameters between 50 and 800 nm, allows a reduction and control in vesicle size and polydisper-sity to obtain small unilamellar vesicles with size distributions within 10-20%. Fast and direct preparation of small monodisperse vesicles in the size range of 50-200 nm is possible using inkjet printers (133). By appropriate choice of sample preparation method, it is thus possible to tailor the size of the vesicle between 50 nm and 10 fim with narrow size distribution. 

Explosive decompression High-pressure extrusion Effect of corrosion inhibitors Effect of hydrogen sulphide Steam/add resistance Methanol resistance... 

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