Process technology coextrusion

For the processing of metallic effect pigments in powder coatings, there are basically three possible technologies coextrusion, dry blending and bonding. 

The production of foamed films and sheets from polypropylene and polystyrene is discussed, with particular reference to packaging applications. Advantages of foamed materials for this application are examined, and the chemical and physical foaming processes are described. Extrusion technology for film and sheet by chemical and physical foaming processes is discussed, and recent developments in the coextrusion of multilayer packaging trays for the food industry are considered. 

Technologies, such as coextrusion and coinjection, allow PET and other plastics to package foods and other products.225 226> 227 Care must be taken to control the process so that the melt when blown will not have micro-voids in the container walls or will delaminate. Coextrusion and coinjection (or multilayer processes) are essential technique in the production of high performance BM products (Chapters 4 and 5). The parison or preform is coextruded with a number of different layers, each of which contributes an important property to the finished product. Increasingly, a mid layer may consist of recycled material which is encapsulated between inner and outer layers of virgin plastics. 

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. 

The economics of encapsulation is detailed in a previous chapter and should always be considered when selecting a process. The primary cost considerations are capital equipment and processing costs. In general, the atomization technologies are the least expensive to produce, followed by spray coating, coextrusion, and the emulsion-based technologies. 

Another consideration for the scale of a process is batch versus continuous. Batch processes limit production to the size and number of batches that can be produced in a given amount of time. Continuous production allows for uninterrupted operation, with the production rate controlled by the throughput of the equipment. The atomization and coextrusion technologies are the most common continuous encapsulation processes. Spray-coating processes are available as both batch and continuous. The emulsion-based processes are limited to batch processing, which is the basis for their increased operating costs over most of the physical processes. 

Blow molded parts demonstrate that, from technical and cost standpoints, BM offers a promising alternative to other processes, particularly injection molding (IM) and thermoforming. The technical evolution of BM, plus accompanying improvements and new developments in plastics, has led to new BM parts. With the coextrusion technology now established and the hardware in place, the variety of achievable properties can readily be extended by the correct combination of different materials (see Chapter 3). The potential for BM products includes much more than the simple bottles that have been made for many decades. Now the expertise and economics of the method are such that many ideas once deemed futuristic are much closer to realization (2, 96, 186-212). 

Coextrusion and lamination processes have been described extensively in the patent literature. The technology usually involves multilayer structures utilizing, as an adhesive layer, a compound of PVDF with another material, which provides the bonding to the substrate. In this way, the full benefits of a thin film of PVDF may be realized (121). Another approach using acrylic alloys with PVDF sometimes achieves adherence to various substrates without the tie layer necessary for the pure PVDF film (122). The full breadth of options available by these technologies to protect surfaces with weatherable PVDF surfaces has been reviewed (123). Surface PVDF layers on polyolefins has been a particular technology of interest with respect to suitable finishes in cost effective parts for exterior automotive and agricultural equipment applications (124). 

There are materials with low or no permeability to different environments or products. Barrier plastics are used with their technology not becoming more complex but more precise. Different factors influence performance such as being pinhole-fi ee chemical composition, crosslinking, modification, molecular orientation density, and thickness. The coextrusion and coinjection processes are used to reduce permeability while retaining other desirable properties. Total protection against vapor transmission by a single barrier material increases linearly... 

Multilayered films can be formed by various processes to create a structure with specific mechanical and physical properties. Microlayers of foam/film were formed by microlayer coextrusion technology [176], resulting in unique mechanical properties studied by optical microscopy. Cohen et al. [177] studied... 

Initially, multilayer coextrusion was considered as a new technology for creating PP nanocomposites with potentially well dispersed OMM. The three master batches were prepared and Irt down into the multilayer process in a 1 4 ratio with homopolymer PP so that the final samples contained between 2-5 wt% clay and a constant 5 wt% PP-g-MA compatibilizer. These samples all contained 20 layers and TEM analysis was used to determine the extent... 

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