Polyethylene barrier properties

P. Gestoso and N. Ch. Karayiannis. Molecular simulation of the effect of temperature and architecture on polyethylene barrier properties. J. Polym. Phys. B 112, 2008, 5646. 

Styrene-based plastics are used somewhat in blow mol ding but not as much as linear polyethylene and PVC. HIPS and ABS are used in specialty botdes, containers, and furniture parts. ABS is also used as one of the impact modifiers for PVC. Clear, tough bottles with good barrier properties are blow-molded from these formulations. 

Considerable amounts of polyethylene film are produced using coaxial extrusion processes in which two or more melt streams are combined in the die to produce extruded film of two or more layers of plastics materials. Layers in such a composite may be included, for example, to improve barrier properties, to enhance sealability or even simply to act as an adhesive between dissimilar layers. 

Because of the excellent gas barrier properties, EVOH is of interest as a packaging material. However, because of its high water absorption it is usually used as an internal layer in a co-extruded film, sheet, bottle or tube. For example, the system HDPE-EVOH-EVA may be used as a barrier film for packaging cereals, and the system polystyrene-EVOH-polystyrene for packaging coffee and cream, whilst the system polystyrene-EVOH-polyethylene has the additional advantage of heat scalability. 

Surface fluorination changes the polymer surface drastically, the most commercially significant use of polymer surface direct fluorination is the creation of barriers against hydrocarbon permeation. The effectiveness of such barriers is enormous, with reductions in permeation rates of two orders of magnitude. Applications that exploit the enhanced barrier properties of surface-fluorinated polymers include (1) Polymer containers, e.g., gas tanks in cars and trucks, which are produced mostly from high-density polyethylene, where surface fluorination is used to decrease the permeation of fuel to the atmosphere and perfume bottles. (2) Polymeric membranes, to improve selectivity commercial production of surface-fluorinated membranes has already started.13... 

Perhaps the most exciting developments are yet to feature in fruit juice packaging. PET can be laminated with other plastics, such as nylon and ethylene vinyl alcohol (EVOH), to give extremely good barrier properties, and polyethylene naphthalate (PEN) may enable production of a plastic bottle that can be pasteurised at high temperatures. 

The packaging material is a laminate made from layers of paper, plastic (polyethylene) and aluminium foil. The particular composition for a given product is chosen according to its specific requirements. This affords the best protection to each product and keeps it in peak condition until consumed. Polyethylene is laminated to the board to make it impermeable, and an aluminium foil (of 10 p.m or less) is added to increase the barrier properties against external hazards such as sunlight and atmospheric oxygen, which would cause deterioration of product quality. 

The new packaging system developed by Tetra Pak is known as the Tetra Brik , and is generally available in 1-1, 200-ml, and 250-ml sizes. The system is considered to be an alternative to metal and glass containers. The packaging material comes in roll stock form and is a 6-or 7-layer laminate. Polyethylene and aluminum foil offer the major barrier properties to the package. 

Probably the most widely reported information concerning petrolatum s effect on damaged skin was a 1992 article by Ghadially and coworkers.71 Newspapers across the United States reported the results of this study, which showed that, when applied to skin that had been damaged by acetone, petrolatum accelerated the recovery of the skin s normal barrier properties. It was noted that this is in contrast to materials that are highly impermeable to water vapor (such as polyethylene films) which hinder barrier repair. However, another publication has indicated that the repair of the permeability barrier in human skin is not delayed by occlusion.72... 

EVOH have superior gas barrier properties.80 89 They are often used as the internal layer in multilayer food packaging films, blow molded rigid containers, gasoline tanks for automobiles for a variety of purposes, etc. EVOH can be fabricated by the usual melt processing methods. The barrier properties of films decrease in the presence of moisture, so multilayer with protective polypropylene (especially biaxially oriented material), low-density polyethylene, nylons, or other moisture barrier films provides films or products that are useful even with liquids. The... 

For thin walled containers, PVC-U gives a combination of high clarity, inherent rigidity and good barrier properties. However, the use of PVC in this area has greatly decreased due to the changeover to polyethylene terephthalate (PET) for cost reasons. 

In this review article, an account will be presented of the diffia t methods by which high modulus materials have been produced from flexible polymers. Much of the discussion will be concerned with polyethylene, although comparable results have been obtained fot polypropylene and polyoxymethylene, and these will also be considered. The initial stimulus to this research came from the quest for high stiffness, but other properties have also been enhanced, including strength, thermal and chemical stability, and barrier properties. The present article updates and extends previous reviews of progress in this exciting new area of polymer science. 

The equilibrium sorption and diffusion coefficient were determined for toluene and tetrahydrofuran in oriented linear polyethylene pipe produced by hydrostatic extrusion The results, which are summarised in Table 4, show that the sorption is about a factor of ten less than that for isotropic material, and the diffusion coefficients are a factor of a hundred less. It is clear from these results, taken in conjunction with those for gases discussed above, that the improvements in barrier properties increase with molecular size. This can be attributed to the increased difficulty of penetrating the structure as the molecular size increases. 

A limited number of patents concern sPS blend which are not compatible. In these cases the properties that are described are functional ones and not related to the poor toughness of sPS. For instance, blends of sPS and partially saponified ethylene-vinyl acetate copolymers exhibit improved gas barrier properties (entry 11) small amounts of sPS added to polyethylene terephthalate) (although the patent actually claims a wide range of compositions) are useful to increase the polyester crystallization rate (entry 5). 

Stratum Corneum Structure. Reviewers agree that for most compounds the rate-limiting barrier properties of skin are located within the SC (2, 10, 12, 15, 16, 21), The texture and cohesiveness of this tissue are familiar to anyone who has ever peeled bits of it from sunburned skin large sheets of SC can be separated from skin (2). Such sheets look like used polyethylene film, and their resistance to water diffusion approximates that of Mylar film of similar thickness (16). However, each sheet is a mosaic made of individual cells (2). 

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