Platelet structure

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

As surface area and pore structure are properties of key importance for any catalyst or support material, we will first describe how these properties can be measured. First, it is useful to draw a clear borderline between roughness and porosity. If most features on a surface are deeper than they are wide, then we call the surface porous (Fig. 5.16). Although it is convenient to think about pores in terms of hollow cylinders, one should realize that pores may have all kinds of shapes. The pore system of zeolites consists of microporous channels and cages, whereas the pores of a silica gel support are formed by the interstices between spheres. Alumina and carbon black, on the other hand, have platelet structures, resulting in slit-shaped pores. All support materials may contain micro, meso and macropores (see text box for definitions). 

Schematic representation of carbon filaments of different structure produced by metal-catalyzed decomposition of methane, (a) Platelet structure, (b) "herringbone" structure, and (c) ribbon structure. MP denotes a nanosized metal particle.

Figure 8. Platelet structures observed by weak beam dark field.

Mica is a reinforcing filler which, due to its platelet structure, increases the stiffness more than spherical particles do. 

Ultrastructural changes and giant platelets are seen in May-Hegglin anomaly. An alteration in the platelet microtubule system affects platelet structure, although platelet aggregation and interaction with vWF is normal. The lack of platelet... 

May-Hegglin anomaly Alteration in platelet micrombule system affects platelet structure platelet cytoplasm contains neutrophil inclusions (Dohle bodies)... 

Flake Zinc Pigments. Flake zinc pigments are used mainly as high-quality anticorrosive pigments in powder or paste form. Owing to their platelet structure they have a considerably higher surface area than spherical zinc dust particles. They can... 

The enzymes responsible for the platelet metabolism are distributed in different platelet structures, For example, the plasma membrane contains adenylate cyclase in contrast, phospholipase (PL) A2, diglycerol lipase, cyclooxygenase... 

Figure I Photomicrographs of the needle-like and platelet structures produced on alumina at 840°C.

Bentonite and attapulgite clays are used to modify viscosity of coatings. Mica clay has a platelet structure and can be useful in reducing permeability of paint films. 

The details of the structural characteristics of individual constituents in the various carbon deposits were obtained by examination of a number of specimens from each experiment in a JEOL 100 CX transmission electron microscope that was fitted with a high resolution pole piece, capable of 0.18 nm lattice resolution. Suitable transmission specimens were prepared by applying a drop of an ultrasonic dispersion of the deposit in iso-butanol to a carbon support film. In many cases the solid carbon product was found to consist entirely of filamentous structures. Variations in the width of the filaments as a function of both catalyst composition and growth conditions were determined from the measurements of over 300 such structures in each specimen. In certain samples evidence was found for the existence of another type of ca naceous solid, a shell-like deposit in which metal particles appeared to be encapsulated by graphitic platelet structures. Selected area electron diffraction studies were performed to ascertain the overall crystalline order of the carbon filaments and the shell-like materials produced from the various catalyst systems. 

Figure 8. Schematic rendition of a possible growth mechanism of the shell-like graphite deposits, where the catalyst material undergoes a wetting and spreading action with the platelet structures and the particle is progressively depleted in size as the reaction proceeds.

Gas and liquid permeability are influenced by the choice of filler. The platelet structure of mica or talc as a filler in paints and plastics decreases the transmission of gases and liquids. 

Mica, because of its platelet structure is a very useful filler. Its performance is improved by increasing the compatibility between filler and polymer. Silane modification is one simple and frequently used method. An alternative method involves a polymeric modifier which, in the case of polypropylene formulations, is polypropylene modified by maleic anhydride. Such modifiers act more as compatibilizers. They are added in small amounts to a system containing both mica... 

Talc is always an attractive subject of such studies due to its platelet structure. In thermoforming and compression molding processes of three resins (PP, HDPE, and PPS), each containing 20% talc, the talc particles were always parallel to the specimen surface, regardless of the resin used. Crystallites grew in a direction normal to the surface of talc particles and thus were perpendicular to the specimen surface. But in the case of unfilled HDPE, crystallites grew parallel to the specimen surface. There was no difference in crystallite growth direction in the case of polypropylene with and without talc. 

The physical properties of some fillers play a role in their function as stabilizers. A1(OH)3 undergoes endothermic decomposition which lowers temperature of material. Loss of water from MgiOH), may increase stability in some cases. In others, it may cause degradation. This is discussed below. The platelet structure of some fillers (e.g., talc or mica) contributes to an increased thermal stability because the degradation rate is increased as oxygen concentration increases. The structure formed by the platelets reduces the diffusion rate of oxygen. 

However, despite this progress, it is highly recommended that both PCS and LD be used simultaneously. It should be kept in mind that both methods are not measuring particle sizes. Rather, they detect light-scattering effects that are used to calculate particle size. For example, uncertainties may result from nonspherical particle shapes and from the assumption of certain parameters that are used to calculate the particle size. Platelet structures commonly occur during lipid crystallization [57] and have also been observed for SLN [11,40,58], The presence of several populations and other colloidal structures adds further difficulties. 

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