Plastic packings characteristics

Product changes which may be related to the pack characteristics can reduce (or increase) the microbial effectiveness of a product. Examples include adsorption and absorption of preservatives with plastics, and the change of pH due to the passage of carbon dioxide through certain plastics. 

From here on this chapter expands the properties of various plastics, the means by which certain physical and chemical characteristics can be identified and quantified, together with the constituents which may be found in a plastic. The converting processes may contribute other factors relevant to the selection and clearance of a plastic pack. The chapter concludes by reference to sterilisation methods and references. (The next chapter places particular emphasis on the clearance of a plastic pack , and should be read in conjunction with this section.)... 

In general, plastics have a higher density when they crystallise due to the closer packing of the molecules. Typical characteristics of crystalline and amorphous plastics are shown below. 

Characteristically, glasses are brittle solids which in practice break only under tension. The ionic and directional nature of the bonds and the identification of electrons with particular pairs of atoms preclude bond exchange. This, coupled with the random nature of the atomic lattice, i.e. the absence of close-packed planes, makes gross slip or plastic flow impossible. 

The addition of silica fume to dry-mix shotcrete in proportions of 10-15% by mass of Portland cement substantially improves the adhesive and cohesive properties of the freshly applied shotcrete. Silica fume creates a very dense and sticky mix with an almost complete lack of bleeding. These characteristics of the material are attributed to the extreme densification achieved by packing of the ultrafine silica fume particles between the cement particles in the plastic (fresh) shotcrete. Practical ramifications of these characteristics include the observations which follow [107, 117-119]. 

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