Shaping secondary

Secondary shaping operations such as extrusion blow molding, film blowing, and fiber spinning occur immediately after the extrusion profile emerges from the die. The thermo- 

Secondary shaping operations take place when the extruded pellet exits from the equipment. These operations include  

In cast film extrusion a thin film is extruded through a slit. It passes through a chilled turning roller where it is quenched from one side. The film is sent to a second roller in order to cool the other side. Finally, the film is wound in a roll by passing through a final system of rollers (Fig. 3.10 Osswald and Hernandez-Ortiz, 2006). Film thickness depends on roller speed. 

In film blowing, a tubular film is extruded upwards. It is blown upwards, with air introduced below the die, into a larger tubular film which is then picked up by a pair of nip rolls that seals the bubble (Fig. 3.11 Han, 2007). An external stream of chilled air cools and solidifies the film at a certain point called the freeze line, where the temperature of the film is equal to the melting temperature. A feature of this process is that the film is stretched biaxially, improving mechanical properties. Tangential circumferential stretching depends on blow-np ratio, i.e. the ratio between the tubular film diameter after air introduction and the initial tabular film diameter. This parameter is determined by the pressure level within the bubble. Axial stretching depends 

10 Schematic of film-casting (from Osswald and Hernandez-Ortiz, 2006). 

Blow molding is a process borrowed from the glass industry and is used to produce hollow shapes. There are three types of blow molding process  

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Secondary shaping (such as film and blow molding, thermoforming)... 

Secondary shaping operation - Here a preform such as a parison or sheet is transformed into a final product using thermoforming or blow molding. 

The processing techniques dealt with in this section are all within the category secondary shaping . The starting material is obtained from a primary shaping process as an intermediate, for instance as a sheet, as a film, a block or a pipe. 

In this paper, we review primary and secondary shape selective acid catalysis with zeolites. Next, we discuss shape selectivity with metal containing zeolites.We conclude with a section that deals with future trends in shape selective catalysis. 

Another example of secondary shape selectivity is shown by John and co-workers (77,73). They found that the hydroisomerization/hydroeracking of n-hexane over Pt/H-mordenite is significantly inhibited by the presence of benzene. They also found a correlation between the aromatic size relative to zeolite pore size on the inhibition of the hexane reaction and the changes in isomer selectivities. Figure 4 illustrates the relation between the various aromatics co-fed and the n-hexane isomerization rates on H-mordenite. From Figure 4, it is shown that as the kinetic diameter of the aromatics is increased, the isomer formation rate appears to pass through a minimum. This result can be explained by considering the size of the zeolite pore and the kinetic... 

Acetaldehyde decomposition, reaction pathway control, 14-15 Acetylene, continuous catalytic conversion over metal-modified shape-selective zeolite catalyst, 355-370 Acid-catalyzed shape selectivity in zeolites primary shape selectivity, 209-211 secondary shape selectivity, 211-213 Acid molecular sieves, reactions of m-diisopropylbenzene, 222-230 Activation of C-H, C-C, and C-0 bonds of oxygenates on Rh(l 11) bond-activation sequences, 350-353 divergence of alcohol and aldehyde decarbonylation pathways, 347-351 experimental procedure, 347 Additives, selectivity, 7,8r Adsorption of benzene on NaX and NaY zeolites, homogeneous, See Homogeneous adsorption of benzene on NaX and NaY zeolites... 

Profiles. The types of profiles that can be produced are determined by the die design and the secondary shaping that occurs in the haul off of the extrudate. Flow in the die occurs by pressure flow only as indicated by Equation 6. 

Manifold of shapes (flat, cramped long parts with/without secondary shape elements)... 

Hydrogels are another class of polymers with shape memory properties. They are cross-linked polymers with a hydrophilic portion that has high affinity for water and a hydrophobic part that can be controlled by temperature variations. The cross-linked part is responsible for setting the permanent shape at an elevated temperature, whilst the hydrophobic part assumes a secondary shape at a specific or critical temperature. Heating above these temperatures completes the recovery (Liu et al., 2007). 

ABSTRACT. The amount of published work on molecular shape-selective catalysis with zeolites is vast. In this paper, a brief overview of the general principles involved in molecular shape-selectivity is provided. The recently proposed distinction between primary and secondary shape-selectivity is discussed. Whereas primary shape-selectivity is the result of the interaction of a reactant with a micropore system, secondary shape-selectivity is caused by mutual interactions of reactant molecules in micropores. The potential of diffusion/reaction kinetic analysis and molecular graphics for rationalizing molecular shape-selectivity is illustrated, and an alternative explanation for the cage and window effect in cracking and hydrocracking is proposed. Pore mouth catalysis is a speculative mechanism advanced for some systems (a combination of a specific zeolite and a reactant), which exhibit peculiar selectivities and for which the intracrystalline diffusion rates of reactants are very low. 

For the classic types of molecular shape-selectivity in zeolites, the reader is referred to the excellent review papers in literature [18-25]. In this paper we elaborate on the recently proposed distinction between primary and secondary shape-selectivity [26], and on the more or less abused concept of cage and window effects in cracl g and hydrocracking. In addition, some evidence available in literature for the speculative mechanism of pore mouth catalysis is presented. 

Santilli and Zones proposed to make a distinction between Primary and Secondary shape-selectivity [26]. Primary shape-selectivity is the result of interactions between a molecule and a micropore system. For mixtures of reagents and conditions of primary shape-selectivity, the rules for competitive adsorption in a sterically unrestricted environment apply. These are preferential adsorption of the molecule with the highest boiling point, and of (a)polar molecules on (a)polar zeolites. Secondary effects are present when one reactant interferes with the conversion of a second one in a way which does not occur in an unrestricted environment. Secondary effects can thus be superimposed on primaiy shape-selectivity. The following examples were given by Santilli and Zones [26] to illustrate the concept of secondary shape- selectivity. 

Namba et al. studied the cracking of octane on H-ZSM-5 in the presence of other alkanes [27]. The reaction conditions were such that the conversion of octane obeyed first order kinetics and that the coverage of the active sites was low. The octane conversion was not affected by the presence of 3-methylheptane or 2,2,4-trimethylpentane in the feed. 3-Methylheptane is expected to diffuse rapidly through the ZSM-5 pores, while 2,2,4-trimethylpentane is excluded from the pores. Secondary shape-selectivity does not occur with these two molecules. However, the octane conversion dropped sharply with increasing partial pressures of 2,2-dimethylbutane in the feed. This strong inhibition caimot be the result of primaiy shape-selectivity, since the competing 2,2-dimethylbutane molecule should not be selectively adsorbed over octane. The explanation is that the slowly diffusing 2,2-dimethylbutane molecules retard the diffusion and, consequently, the conversion of the octane molecules. 

In the conversion of a mixture of n-alkanes in absence of secondary shape-selectivity, the molecules with the largest carbon number inhibit the reaction of the less strongly adsorbed shorter molecules. Dauns and Weitkamp observed this phenomenon in the conversion of decane and dodecane on LaY type zeolite catalysts [28]. In this 12-MR zeolite, the conversion of dodecane is not affected by the presence of the lower boiling decane (it decreases from 64 to 60%), while the decane conversion drops from 47 to 20% (Table 1). Santilli and Zones studied the conversion of hexane and hexadecane on I M-5 and SSZ-16 zeolites [26]. The 10-MR zeolite ZSM-5 converts hexane and hexadecane to a similar extent in separate experiments (Table 1). In mixtures, the presence of hexane has little effect on the hexadecane conversion, but the latter inhibits the hexane conversion. However, on the 8-MR zeolite SSZ-16, hexadecane causes a slight increase in the hexane conversion, and hexane keeps hexadecane from reacting (Table 1). In the original work [26], no explanation for this secondary shape-selectivity effect has been... 

Quantitative treatment of shape-selectivity based on reaction/diffusion analysis has been applied to several reactions, including alkylation of toluene with methanol, isomerisation of xylenes and disproportionation of toluene [16,30]. It should be stressed that this diffusion/reaction analysis does not take into account eventual competition effects (secondary shape-selectivity) that may occur when reactants are mixed. 

Secondary shape selectivity occurs when one reagent affects the conversion of... 

This is the primaiy processing technique used to fabricate hollow plastic objects, particularly bottles, which do not need a very uniform distribution of wall thickness. It is a secondary shaping technique that inflates the preprocessed plastic (usually extruded) against the inside walls of the mold with a blow pin. In addition to extrusion blow molding, injection blow molding and stretch blow molding are commonly employed. With most polymers, especially when the product size is... 

Secondary shaping operations preforms are transformed into the final product using techniques such as thermoforming or blow molding. 

The shape memory behavior of an SMP makes it a very desirable material for use in biomedical applications. Thermally activated SMPs can be programmed and stored in a small secondary shape, and on introduction to the body and water plasticization, recover their large original shape (Beilvert et al., 2014). This property of SMPs can be harnessed for minimally invasive surgery and tissue engineering scaffolds (Beilvert et al., 2014). However, ceU compatibility of an SMP biomaterial needs to be extensively understood to determine its feasibility as a short-term or long-term implant and the impact of its SME on cells. 

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