Jan Navratil

The influence of fat and monoacylglycerols on growth of spore-forming bacteria in processed cheese

Highly undesirable microbial contaminants of processed cheese are endospore-forming bacteria of the genera Bacillus and Clostridium. Survival of Bacillus subtilis, B. cereus, Clostridium butyricum and C. sporogenes was examined in model processed cheese samples supplemented with monoacylglycerols. In processed cheese samples, monoacylglycerols of undecanoic, undecenoic, lauric and adamantane-1-carboxylic acid at concentration of 0.15% w/w prevented the growth and multiplication of both Bacillus species throughout the storage period. The two species of Clostridium were less affected by monoacylglycerols in processed cheese samples and only partial inhibition was observed. The effect of milk fat content on microbial survival in processed cheese was also evaluated. The growth of Bacillus sp. was affected by the fat level of processed cheese while population levels of Clostridium sp. did not differ in processed cheese samples with 30, 40 and 50% fat in dry matter.

Hardness/microhardness properties of HDPE blends

This paper deals with utilization of recycled irradiated high-density polyethylene (rHDPEx) as a filler which was blended with non-modified high-density polyethylene (HDPE). Two blends were tested regarding the original state of the mixing components – HDPE granules/rHDPEx grit and HDPE granules/rHDPEx powder. Results show that the increasing amount of the rHDPEx, regardless its form, results in worsening both observed parameters – hardness and micro-indentation hardness.

Surface Layer Micro-Hardness of Modified LDPE by Radiation Cross-Linking after Temperature Load

The presented article deals with the research of surface layer ́s micro-mechanical properties of modified LDPE by radiation cross-linking after temperature load. These micro-mechanical properties were measured by the DSI (Depth Sensing Indentation) method on samples which were non-irradiated and irradiated by different doses of the β – radiation and then were temperature loaded. The purpose of the article is to consider to what extent the irradiation process influences the resulting micro-mechanical properties measured by the DSI method. The LDPE tested showed significant changes of indentation hardness and modulus after temperature load.

Effect of Particle Size of Recycled Polyamide 6 to Impact Toughness and Hardness

This work aims to investigate the effect of recycled material and effect of particle size on the mechanical properties of the recycled material. Tested material is polyamide 6. Every manufacture of any plastic parts produces certain amount of waste, such as defective injects or cold runner system from injection molding technology. When size of this material is reduced, particles of different sizes are made. From large particles similar to the original granulate material, to dust particles. This size variation can cause problems during subsequent processing. These particles of different size have distinct melting rate. To determine behavior of recycled material at room temperature. Impact toughness and hardness tests are performed.

Surface and Adhesive Properties of Low-Density Polyethylene after Radiation Cross-Linking

Radiation cross-linking gives inexpensive commodity plastics and technical plastics the mechanical, thermal, and chemical properties of high-performance plastic. This upgrading of the plastics enables them to be used in conditions which they would not be able to with stand otherwise. The irradiation cross-linking of thermoplastic materials via electron beam or cobalt 60 (gammy rays) is performed separately, after processing. Generally, ionizing radiation includes accelerated electrons, gamma rays and X-rays. Radiation processing with an electron beam offers several distinct advantages when compared with other radiation sources, particularly γ-rays and x-rays. The process is very fast, clean and can be controlled with much precision. There is no permanent radioactivity since the machine can be switched off. In contrast to γ-rays and x-rays, the electron beam can steered relatively easily, thus allowing irradiation of a variety of physical shapes. The energy-rich beta rays trigger chemical reactions in the plastics which results in networking of molecules (comparable to the vulcanization of rubbers which has been in industrial use for so long). The energy from the rays is absorbed by the material and cleavage of chemical bonds takes place. This releases free radicals which in next phase from desired molecular bonds. This article describes the effect of radiation cross-linking on the surface and adhesive properties of low-density polyethylene.

Use of Irradiated Polymers after Their Lifetime Period

This article deals with the study of the utilisation of irradiated HDPE products after their end-of-life cycle. Today, polymer waste processing is a matter of evermore intensive discussion. Common thermoplastic waste recycling—especially in the case of wastes with a defined composition—is generally well-known—and frequently used. On the contrary, processing cross-linked plastics is impossible to do in the same way as with virgin thermoplastics—mainly due to the impossibility of remelting them. The possibility of using waste in the form of grit or a powder, made from cross-linked High Density PolyEthylene (rHDPEx) products, after their end-of-life cycle, as a filler for virgin Low Density PolyEthylene (LDPE) was tested in a matrix. It was found that both the mechanical behaviour and processability of new composites with an LDPE matrix, with rHDPEx as a filler, depend—to a high degree—on the amount of the filler. The composite can be processed up to 60% of the filler content. The Polymer Mixture Fluidity dropped significantly, in line with the amount of filler, while the mechanical properties, on the other hand, predominantly grew with the increasing amount of rHDPEx.

Influence of Plasma Treatment on the Surface Properties and Strength of Bonded Joints at Selected Polyolefins

Bonding has experienced an enormous expansion in the various applications during the last few years in the field of material joining, due to which it is classified as a new joining technology, although it is, in fact, very old. Compared with the conventional joining methods (riveting, screwing and welding), bonding provides a new material combination possibilities and it allows us to obtain special shapes and properties, which can not be formed by conventional methods. To create a high-quality bonded joint, it is important to wet the bonded surface very well wetted by a wetting liquid. The wettability of the material is characterized by a contact angle of wetting, by which the surface energy is subsequently determined. For a high quality of the joint, the bonded material must have higher surface energy than the witting liquid (adhesive) [1-3]. This paper describes the effect of plasma surface treatment on the surface properties (surface energy, microhardness) of low-density polyethylene (LDPE) and high-density polyethylene (HDPE), and also on the final strength of bonded joints. The measured results indicate, that plasma surface treatment is very effective tool for improvement of surface properties and strength of bonded joints of HDPE and LDPE. The strength of bonded joints after plasma surface treatment was increased up to 350 % compared to untreated material. A similar trend was observed even for surface energy and microhardness of materials.

Material Properties of PA6/rPA6x Blend

This research paper suggests a possible solution for a growing problem of irradiated materials ́ waste processing. Recycled radiation crosslinked polyamide 6 (rPA6x) was used as a filler into non-modified polyamide 6 (PA6) in several concentrations. Influence of the amount of the filler on mechanical properties of resulting blends was investigated. Tensile test was carried out at two temperatures to determine mechanical behavior. Results show that there is a little difference at all observed material characteristics in comparison with non-modified PA6. According to the results it was proven that there is a possible utilization of such modified materials after the end of their service life but some other properties have to be examined.

Influence of Irradiation Doses on Mechanical Properties of Glass Fiber Filled PBT

The submitted paper compares mechanical properties of the hard surface layer of modified PBT filled by 35% of glass fibers. Hard surface layer was made by radiation cross-linking technology which allows polymer materials modification followed by the change of their end-use properties. The surface layer of polymer material is modified by β – radiation. When the polymer material is exposed to the β – radiation, it is possible to observe changes of the surface layer at applied load. The mechanical properties were measured by nanohardness test with using the DSI method (Depth Sensing Indentation).

Influence of Recycled Particle Size to Tensile Properties of Polyamide 6

The aim of this research is to show influence of particle size of recycled polymeric material to mechanical tensile properties. During grinding, the particles with various sizes are formed, from larger pieces to dust particles. This size variation can cause problems during subsequent processing. These particles of different sizes have distinct melting rate. That can cause an inhomogeneity of a melt and diverse structure of a solidified polymer. Several recycled mixtures with varied particle size after crushing were prepared. Tested polymer was polyamide 6 and the tests were performed at ambient temperature.