Polyethylene physical forms

Composition Acrylic acid modified high density polyethylene Physical Form Pellets... 

There are at the present time many thousands of grades of commercial plastics materials offered for sale throughout the world. Only rarely are the properties of any two of these grades identical, for although the number of chemically distinct species (e.g. polyethylenes, polystyrenes) is limited, there are many variations within each group. Such variations can arise through differences in molecular structure, differences in physical form, the presence of impurities and also in the nature and amount of additives which may have been incorporated into the base polymer. One of the aims of this book is to show how the many different materials arise, to discuss their properties and to show how these properties can to a large extent be explained by consideration of the composition of a plastics material and in particular the molecular structure of the base polymer employed. 

The combination of solid-state ATR-IR and solid-state NMR data supported the conclusion that the presence of crystalline material was responsible for changes in the dissolution profiles of the different lots. The results appear consistent with historical examples of changes in API physical form of solid, high molecular weight, polyethylene glycol dispersion formulations of amorphous indomethacin and griseofulvin (92-95). 

The crystalline plastics (basic polymers) tend to have their molecules arranged in a relatively regular repeating structure such as polyethylene (PE) and polypropylene (PP). This behavior identifies its morphology that is the study of the physical form or structure of a material. They are usually translucent or opaque and generally have higher softening points than the amorphous plastics. They can be made transparent with chemical modification. Since commercially perfect crystalline polymers are not produced, they are identified technically as semicrystalline TPs. The crystalline TPs normally has up to 80% crystalline structure and the rest is amorphous. 

The ability of plastics to float on the surface of a beaker of tap water at 20°C is related to their density at that temperature. The density of water at 20°C is approximately 1 g cm-3. If a small sample floats on the surface of the water, it has a density lower or equal to one at the same temperature if it sinks, it has a density greater than one. Polyethylene, polypropylene and polystyrene float on water while other plastics sink, Bakelite and casein having the highest densities. The flotation test is a rough method to identify plastics since results are dependent on the physical form of the plastic. Foams contain cells filled with air, so their densities will be lower than a solid block of the same type of plastic. 

Polyethylene provides an example of how crystallinity affects a polymer s physical properties. Free-radical polymerization gives a highly branched, low-density polyethylene that forms very small crystallites because the random chain branching... 

Electron microscopy permits the examination of morphological features ranging in size from lamellae up to spherulites. In most cases it requires that specimens be specially prepared to enhance contrast or stabilize their surface. With appropriate sampling and preparation techniques, electron microscopy can be applied to virtually any polyethylene sample regardless of its physical form or molecular characteristics. Scanning electron microscopy is used to view the surface morphology of specimens, while transmission electron microscopy is used to examine the fine... 

Extmsion of polyethylene and some polypropylenes is usually through a circular die into a tubular form, which is cut and collapsed into flat film. Extmsion through a linear slot onto chilled rollers is called casting and is often used for polypropylene, polyester, and other resins. Cast, as well as some blown, films may be further heated and stretched in the machine or in transverse directions to orient the polymer within the film and improve physical properties such as tensile strength, stiffness, and low temperature resistance. 

Most commercial polymers are substantially linear. They have a single chain of mers that forms the backbone of the molecule. Side-chains can occur and can have a major affect on physical properties. An elemental analysis of any polyolefin, (e.g., polyethylene, polypropylene, poly(l-butene), etc.) gives the same empirical formula, CH2, and it is only the nature of the side-chains that distinguishes between the polyolefins. Polypropylene has methyl side-chains on every other carbon atom along the backbone. Side-chains at random locations are called branches. Branching and other polymer structures can be deduced using analytical techniques such as NMR. 

In addition to the desired polymerization reaction, the dialcohol reactants can participate in deleterious side reactions. Ethylene glycol, used in the manufacture of polyethylene terephthalate, can react with itself to form a dialcohol ether and water as shown in Fig. 24.4a). This dialcohol ether can incorporate into the growing polymer chain because it contains terminal alcohol units. Unfortunately, this incorporation lowers the crystallinity of the polyester on cooling which alters the polymer s physical properties. 1,4 butanediol, the dialcohol used to manufacture polybutylene terephthalate, can form tetrahydrofuran and water as shown in Fig. 24.4b). Both the tetrahydrofuran and water can be easily removed from the melt but this reaction reduces the efficiency of the process since reactants are lost. 

Polyethylene terephthalate crystallizes very slowly into only one stable crystalline form, containing monoclinic unit cells. To maximize its physical strength, high crystallinities must... 

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