Diffusion behaviour of additives in polypropylene in correlation with polymer properties

Release of contaminants from polymer surfaces under condition of organized fluid flows

The use of polymers for water storage or distribution is closely monitored, especially with regard to the possible contamination with substances coming from the material's surfaces. Different standards are practiced across countries according to type of applied materials and such test methods are prevalently based on constant temperature conditions. However, these polymers systems could be located in diverse environment which does not necessarily provide constant conditions. Experimental findings show that exposure of liquid inside polymeric materials to specific temperature gradients, and consequently to emerging organized flows, can result in an accelerated leaching of undesirable substances from the solid surface. In presented work model steady-state and organized flow conditions are used to compare release of contaminates from polyethylene by measuring of surface tension, UV-Vis spectroscopy, FTIR, scanning electron microscopy and elemental analysis of polymer surfaces and water leachates. The pilot study shows that convective flow generated via temperature gradient significantly affects contaminant release in comparison to a steady state and mixing flow conditions.

Selection of antioxidants for capacitor grade polypropylene film: Insights into electrical performance of the oil-film system

High voltage power capacitors employ the oil-impregnated polypropylene film as the insulation. The swelling phenomenon might drive the antioxidants and small molecules within the film to migrate into the oil. It is necessary to comprehensively investigate the physical migration mechanism of antioxidants and their impact on the electrical performance of the oil-film combination insulation system and, consequently, formulate the proper selective prescription of antioxidants. Theoretical elucidation of the competitive interaction mechanism between the film and the oil in attracting antioxidant molecules was achieved through the calculation of inter-molecular binding energy, and the migration coefficient ηm was introduced to quantify the migration characteristics of antioxidants. Experimentally, the effects of antioxidants on the space charge distribution of the film, the dielectric properties of the oil, and the breakdown characteristics of both the film and oil were investigated. The experimental conclusions are consistent with theoretical analysis. The lamellar structure antioxidant molecules with ηm > 1 tend to migrate from the film to the oil, which results in increased dielectric loss and decreased breakdown strength of the insulating oil. In addition, the presence of phosphorus atoms in phosphite antioxidants contributes to a reduction in the breakdown strength of the film. For capacitor grade polypropylene film, in addition to the synergistic effect between different types of antioxidants on the thermo-oxidative stability, the structure of the antioxidant molecules and its influence on the electrical performance of the oil-film systems should also be taken into account.

Effect of high pressure processing on migration characteristics of polypropylene used in food contact materials

The migration of small molecular mass organic compounds from polypropylene (PP) copolymer films into food simulants during and after high pressure processing (HPP) was studied. An overlapping temperature profile was developed to isolate the pressure effect of HPP (700 MPa, 71°C, 5 min) from equivalent thermal processing (TP) at atmospheric pressure (0.1 MPa). Chloroform, toluene, methyl salicylate, and phenylcyclohexane were chosen as surrogate compounds, and were spiked into test polymer films at concentrations of 762-1152 mg kg^-1 by a solvent soaking technique. Migration (w/w) of surrogate compounds from loaded PP films into Miglyol 812 (a medium-chain triglyceride mixture) and 10% ethanol was quantified by headspace GC/MS during HPP and TP, and subsequent storage at 25°C for up to 10 days. HPP significantly delayed migration of the surrogates from PP into both food simulants relative to TP. The average migrations into Miglyol after TP and HPP were 92.2-109% and 16-60.6%, respectively. Diffusion coefficients estimated by migration modelling showed a reduction of more than two orders of magnitude for all surrogate compounds under high pressure at 700 MPa (AP' = 8.0) relative to equivalent TP at 0.1 MPa (AP' = 13.1). The relative T_g increase of PP copolymer under compression at 700 MPa was estimated as T_g+94°C. For 10% ethanol, average migrations after TP and HPP were 9.3-50.9% and 8.6-22.8%, respectively. During extended storage, migration into both simulants from HPP-treated samples was initially slower than that from untreated or TP-treated films. However, after 8-24 hours of storage, the differences in percent migration of selected surrogates were not significant (p > .05) among the treated PP films. Therefore, the physical changes of PP films that occur during HPP appear to be reversible with a return to their original dimensions and diffusion properties after decompression.

Determination of the mass transport properties of chemical additives in polypropylene material/simulated food system

The mass transport process (migration) of five additives from three different types of polypropylene (PP) films into selected food simulants was studied. The migration tests were carried out at different time-temperature conditions, and the concentration of additives in polymer matrix and food simulants were analysed by high-performance liquid chromatography (HPLC). With all data, the mass transport properties for migration kinetics (partition and diffusion coefficients) were determined. Results showed that the partition coefficient was affected significantly by the temperature and simulants' properties, whereas little affected by the types of PP film and molecular weight of substance. The polarity, structure and shape of substances can also have an influence in their partition between the polymer matrix and food simulants. Additionally, comparison results between the experimental diffusion coefficients and the calculated values by Piringer model suggested that the current migration model with the default modelling parameters for PP does not describe realistically the diffusion coefficients of additives. The calculated diffusion coefficients were greater than the experimental values, as a consequence, the migration of chemical additives will be overestimated. For more realistic migration calculations, more accurate modelling parameters in Piringer model should be established and the effect of food on migration should be high interest in future work.

Interference with spectrophotometric analysis of nucleic acids and proteins by leaching of chemicals from plastic tubes

Absorbance spectroscopy is routinely used to monitor the concentrations of nucleic acids and proteins within solutions and to assess changes in their structure caused by interaction with chemicals or other biomolecules. Biological samples used for such analyses are manipulated and stored in small microcentrifuge tubes (microtubes) composed of polypropylene plus several plastic additives. Here we demonstrate that normal handling of laboratory microtubes causes leaching of light-absorbing chemicals into biological samples that interfere with spectrophotometric measurements. The leached chromophores absorbed UV light strongly at 220 and 260 nm, which are the wavelengths normally used to detect and quantitate proteins and DNA. Some common laboratory techniques, including sonication and PCR, were particularly effective inducers of leaching. The magnitude of the increase in absorbance was dependent upon both exposure time and heat history, with greatest induction after tubes were warmed to temperatures at or above 37 degrees C. Mass spectrometry revealed that aqueous solutions stored in plastic microtubes accumulate a complex mixture of leached chemicals with molecular masses of 200-1400 Da. Leaching was ubiquitous among commercially available brands of microtubes, indicating a persistent source of error in biomolecule detection and concentration measurements.