Properties of recycled high density polyethylene from milk bottles

The effect of virgin, high-density polyethylene (HDPE)/recycled HDPE composition on the physical properties of the blends was investigated. The recycled HDPE was obtained from a postconsumer cycle of milk bottles. It was found that elongation at break was the mechanical property mostly affected by the content of recycled HDPE. Overall, however, the recycled HDPE from milk bottles was found to be a material with useful properties not largely different from those of virgin resin and thus could be used, at an appropriate concentration in virgin HDPE, for different applications.     

A three-parameter equation for molecular weight distribution of low-density polyethylene

A new three-parameter distribution function is proposed which fits best the experimental molecular weight distribution curves of branched lowdensity polyethylenes. The data were interpreted from GPC measurements, and a special computer program was utilized in order to derive the best values of the empirical constants a, b, and c.     

The environmental stress crazing of polycarbonate

The environmental crazing from a central hole in extruded polycarbonate sheets, using ethanol as a crazing liquid, was studied. A stress analysis was made to analyse the criteria for craze initiation and growth. The craze length was found to change linearly with the square root of time and the rate of crazing was found to vary exponentially with the applied stress (above a certain stress level). It was concluded that the principal strain is the controlling parameter for environmental craze initiation and growth.     

Evaluation of the performance of iron‐based oxygen scavengers,with comments on their optimal applications

Oxygen scavengers are commonly used in packaged foods in Japan and much less so in other developed countries, in spite of the advantages that they offer in maintaining quality and extending shelf‐life. The reason stems from the additional cost involved, and even more so because of the lack of sufficient technical information on their performance and the lack of understanding of how to apply them effectively. In the present study the performance of iron‐based oxygen‐scavenging sachets was evaluated. It was found that the actual scavenging capacity is much higher than the ‘nominal’ capacity provided by the manufacturers. Also, a significant distribution in the oxygen absorption capacity exists, even in the same scavenger type. The rate of oxygen scavenging was found to depend on the scavenger type and capacity. It was also found that in an atmosphere containing CO₂ (as in MAP applications) the iron‐based oxygen scavengers also absorb CO₂. Copyright © 2004 John Wiley & Sons, Ltd.     

Antimicrobial activity of biodegradable polysaccharide and protein-based films containing active agents

Significant interest has emerged in the introduction of food packaging materials manufactured from biodegradable polymers that have the potential to reduce the environmental impacts associated with conventional packaging materials. Current technologies in active packaging enable effective antimicrobial (AM) packaging films to be prepared from biodegradable materials that have been modified and/or blended with different compatible materials and/or plasticisers. A wide range of AM films prepared from modified biodegradable materials have the potential to be used for packaging of various food products. This review examines biodegradable polymers derived from polysaccharides and protein-based materials for their potential use in packaging systems designed for the protection of food products from microbial contamination. A comprehensive table that systematically analyses and categorizes much of the current literature in this area is included in the review.     

Mass transfer from and through packaging materials

The use of polymer films for food packaging requires a knowledge of their transport properties under varying conditions. The permeability to gases and water vapour of the packaging films at the specific conditions of use, i.e. temperature, relative humidity and pressure gradient, are needed for selecting the optimum packaging system to ensure the required shelf-life of the packed product. Migration from, and absorption by, plastic films also play an important role in their selection. The 0₂ and CO₂ permeabilities of three plastic films at four different temperatures were evaluated and shown to follow the Arrhenius relationship in the range tested. The effects of four thicknesses of one film on its permeability are also reported. Three methods to evaluate migration from polymeric films were compared using different weight/volume ratios. The extraction method recommended by the FDA, using n-hexane in a stirred flask, was found to be best. The shelf-life of Matzoth (Jewish Passover bread) packed in polyethylene (stored at 25°C) was found to be longer than the shelf-life of those packed in the higher barrier PVDC/PE. This was apparently due to the absorption of undesirable oxidation compounds, given off from the Matzoth by the PE, which were not absorbed at detectable levels by the PVDC/PE.     

Antimicrobial Activity of Natural Agents against Saccharomyces cerevisiae

The antimicrobial (AM) activity of starch‐based films coated with linalool, carvacrol or thymol against Saccharomyces cerevisiae in vitro and/or inoculated on the surface of Cheddar cheese was investigated. In solid medium using the agar diffusion method and in experiments involving the inoculation of the microorganism on the surface of Cheddar cheese, all the films containing these AM agents in coatings demonstrated an inhibitory effect against S. cerevisiae. The results suggest that the overall inhibitory effect of linalool, carvacrol or thymol increased significantly (p ≤ 0.05) with the concentration of each of the AM agents in the film coating and that the response is linear in the concentration range of 1 to 5% (w/w) of the AM agent. Thymol had the highest AM efficacy followed by carvacrol, whereas linalool had the lowest efficacy amongst the three systems. The zones of inhibition in the agar diffusion test method at 25°C for S. cerevisiae were found to be 7.6, 7.1 and 6.1 mm for thymol, carvacrol and linalool at 1% (w/w) loading and 13.2, 12.2 and 11.2 mm at 5% (w/w) loading of the AM agents, respectively. The death rates of S. cerevisiae on Cheddar cheese wrapped in films coated with thymol, carvacrol and linalool and stored for up to 28 days at 15°C were found to be 0.044, 0.043 and 0.038 per day at 1% (w/w) loading and 0.077, 0.073 and 0.063 per day at 5% (w/w) loading of the AM agents, respectively. Copyright © 2011 John Wiley & Sons, Ltd.     

Prospects for application of post-consumer used plastics in food packaging

The two most widely used polymers in packaging in recent years are polyethylene terephthalate (PET) and polyethylene (PE). The biggest fractions of these polymers are not re-utilized, in spite of the fact that they possess excellent properties even after their first application. The ban on using recycled polymers in food packaging applications and the lack of good value outlets for these materials causes them to end up in landfills. The high cost nylon, used in packaging primarily as high gas barrier laminates with PE, also finds its way to landfills. In this case, the reason is the difficulty of recycling different polymers that are incompatible. Thus, the Municipal Solid Waste (MSW) stream transferred to landfills contains many plastic packages. These packages are being blamed as a major pollutant of the environment in spite of the fact that all plastics contribute only a small percentage to the weight of the garbage in landfills. If proper and cost effective applications for the recycled polymers could be developed, the waste related to their disposal could be limited. In addition, the contribution of plastic packages to the environmental problem could be diminished. In the present paper, the possibility of sandwiching a contaminated PET layer between two layers of the virgin material was studied. The aim of the study was to determine whether such an operation could lower the migration level of contaminants from a multilayer structure (containing a recycled layer of PET) to values below the limits required by regulatory agencies. The diffusion coefficients (required to determine migration) of four organic liquids in PET were determined. As a result of the sandwiching operation, the amount of pollutant (toluene) migrating into the food simulant was reduced by two orders of magnitude. The properties of PE/nylon blends were also studied. It was found that the high gas barrier properties of nylon are preserved in the blend when proper processing conditions are used. Therefore, the recycled material could be used as a centre layer in a multilayer structure providing good gas barrier properties to this structure.     

Properties of Chemically Crosslinked Low-Density Polyethylene

Studies on the mechanical properties of chemically crosslinked polyethylene at room temperature and at 135°C are presented. The properties of unfilled samples are studied as function of the gel content and of loaded samples as function of the carbon black content. The mechanical behavior differs below and above the crystalline melting region, and conclusions cannot be carried from one range to the other.     

Changes in thin-layer susceptors during microwave heating

The dielectric and morphological changes of susceptors during heating in a microwave oven were studied. It was found that during the course of heating, cracks and holes were formed in the susceptors' surface and the degree of crystallinity of the polyethylene terephthalate (PET) film used was reduced, thus reducing the heat stability and heating capability of the susceptors.