Film processing behavior

Because of the role of precursor structure on film processing behavior (consolidation, densification, crystallization behavior), the reaction pathways are typically biased through the use of the catalyst, which is simply an acid or a base. This steers the reaction toward an electrophilic or nucleophilic attack of the M—OR bond.1,63 Hydrolysis sensitivity of singly or multiply hydrolyzed silicon alkoxides is also influenced by the catalyst, which contributes to the observed variations in oligomer length and structure. Figure 2.3b illustrates... 

The oxidation products are almost insoluble and lead to the formation of protective films. They promote aeration cells if these products do not cover the metal surface uniformly. Ions of soluble salts play an important role in these cells. In the schematic diagram in Fig. 4-1 it is assumed that from the start the two corrosion partial reactions are taking place at two entirely separate locations. This process must quickly come to a complete standstill if soluble salts are absent, because otherwise the ions produced according to Eqs. (2-21) and (2-17) would form a local space charge. Corrosion in salt-free water is only possible if the two partial reactions are not spatially separated, but occur at the same place with equivalent current densities. The reaction products then react according to Eq. (4-2) and in the subsequent reactions (4-3a) and (4-3b) to form protective films. Similar behavior occurs in salt-free sandy soils. 

According to the end use application, PEs are processed by various techniques, which include injection moulding, blow moulding, rotomoulding, and film extrusion. However, since the bulk of the processed material is used as film in the area of packaging, the discussion in this chapter focuses mainly on processing behavior and the ultimate properties of tubular blown film. 

Chemical solution deposition (CSD) procedures have been widely used for the production of both amorphous and crystalline thin films for more than 20 years.1 Both colloidal (particulate) and polymeric-based processes have been developed. Numerous advances have been demonstrated in understanding solution chemistry, film formation behavior, and for crystalline films, phase transformation mechanisms during thermal processing. Several excellent review articles regarding CSD have been published, and the reader is referred to Refs. 5-12 for additional information on the topic. Recently, modeling of phase transformation behavior for control of thin-film microstructure has also been considered, as manipulation of film orientation and microstructure for various applications has grown in interest.13-15... 

Most often in these processes, compounds such as acetic acid, acetylacetone (acac, 2,4-pentanedione), or amine compounds are employed, since these compounds readily react with alkoxides.4M877 A typical reaction involves the formation of new chemical species that (hopefully) possess physical and chemical characteristics that are more attractive in terms of solution stability and film formation behavior. An example reaction between a metal alkoxide and acetic acid is shown below ... 

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

Miscellaneous additives, defoamers, matting agents 0-3 Affect processing behavior and appearance of finished film... 

The difference in processing behavior between rotors provided with straight blades and those with pitched blades is shown in Figure 6 by an application referring to solvent recovery from a polymer solution of low initial viscosity. The evaporation capacity of a thin-film evaporator equipped with straight blades decreases considerably as soon as the concentrated polymer reaches a viscosity between 1000 and 2000 P. The greatly increased mean film thickness that characterizes this vis-... 

Based on calculated pH and concentration profiles for the various chemical species in the liquid film, the process behavior can be understood. In particular, SO2 depletion close to the interface due to the very fast reactions causes a high enhancement of SO2 transfer. In addition, two different reaction regions can be discriminated in the liquid film. 

Conversion of the as-deposited film into the crystalline state has been carried out by a variety of methods. The most typical approach is a two-step heat treatment process involving separate low-temperature pyrolysis ( 300 to 350°C) and high-temperature ( 550 to 750°C) crystallization anneals. The times and temperatures utilized depend upon precursor chemistry, film composition, and layer thickness. At the laboratory scale, the pyrolysis step is most often carried out by simply placing the film on a hot plate that has been preset to the desired temperature. Nearly always, pyrolysis conditions are chosen based on the thermal decomposition behavior of powders derived from the same solution chemistry. Thermal gravimetric analysis (TGA) is normally employed for these studies, and while this approach seems less than ideal, it has proved reasonably effective. A few investigators have studied organic pyrolysis in thin films by Fourier transform infrared spectroscopy (FTIR) using reflectance techniques. - This approach allows for an in situ determination of film pyrolysis behavior. 

The blown film process has been studied analytically since the early 1970s (Table 24.1). The first analysis was proposed by Pearson and Petrie [3, 4], who followed a fluid mechanics approach. However, this model is restricted to Newtonian fluids under isothermal conditions. This first model has been modified several times to consider different aspects of the process, such as temperature variation and rheological behavior of the system. 

Isotactic PP has extremely good flow properties at a wide range of flow rates, and therefore good processing behavior. The melt flow index typically ranges from 0.5 to 50 g/10 min. Films, which can be produced by both blown and cast methods, can be oriented to provide improved optical characteristics and better strength. Because of the rapid crystallization of PP, blown films must be produced by either water quench or mandrel quench processes, unlike PE, which is cooled by air. 

Thermoset based composite laminates are generally produced bj Autoclave/Vacuum Degassing Lamination Process (38, 39). The characteristics of this inocess are shown in Fig. IS. In this process, pr eg plies of desired shape are laid up in a prescribed orientation to form a laminate. The laminate is covered with successive layers of an absorbent material (glass bleeder fabric), a fluminated film to prevent sticking, and, finally, with a vacuum bag. The mitire system is placed upon a smooth metal tool surface into an autoclave, vacuum is a Ued to the bag and the temperature is increased at a constant rate in order to promote the resin flow and polymerization. The autoclave process will be used along this section as a case study to describe the influence of the matrix characteristics on the processing behavior of hi performance conqmsites. 

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