Dariusz M. Bieliński

Ion beam modification of surface properties of polyethylene

Abstract Improvement of polymer tribological properties is an essential issue for their application when used as parts of machines or artificial human joints. Ion implantation is a very promising technique in this respect. This paper presents the application of energetic He and Ar ions for this purpose. The materials studied were low-density (LDPE) and high-density (HDPE) polyethylene and their blends. Atomic force and scanning electron microscopes were used for studies of surface modifications. Tribological tests consisted of friction coefficient and nanohardness measurements. Our results have indicated clearly that ion bombardment produces important changes in the polymer surface morphology and its internal structure. The related micromechanical parameters strongly depend on polymer density. For the pristine samples, the friction coefficient increases with decreasing polymer density. For hardness, opposite tendency was observed. A significant increase of hardness and friction coefficient was observed for HDPE subjected to ion bombardment. In contrast, ion bombardment does not produce significant changes of tribological parameters in 50–50 blend and LDPE.

Influence of surface-modified montmorillonites on properties of silicone rubber-based ceramizable composites

Ceramizable (ceramifiable) silicone rubber-based composites are modern elastomeric materials for fire protection application. The most important sector of economy using such materials is cable industry because there are special types of electric circuits that have to keep working in the case of fire. These kinds of composites can create ceramic phase protecting copper wire from melting under high temperature. When temperature increases, polymer matrix degrades (creating silica residue as one of the products) and mineral particles dispersed in silicone rubber matrix stick together creating stiff, durable, insulating and porous ceramic skin. In this paper, the influence of surface modification of montmorillonite with quaternary ammonium salts on ceramization of their silicone rubber composites is presented. Filler modification was studied, determining changes to its surface energy and thermal stability. Mechanical properties, flammability and thermal stability of composites were determined. Ceramization of the composites was discussed from the point of view of their mechanical properties and structure of ceramic phase after heat treatment, determined by compression stress tests, porosimetry and scanning electron microscopy adequately. Results show that type of modifier applied strongly affects properties of silicone rubber-based ceramizable composites before and after ceramization. Samples containing surface-modified montmorillonite produce significantly less heat during their thermal decomposition than composite filled with unmodified montmorillonite. Moreover, incorporation of montmorillonite modified with ammonium salt of linear organic chain causes the creation of nano-porous structure after ceramization. On the one hand, it facilitates heat insulation, but on the other hand, high total volume of pores adversely affects mechanical endurance of the ceramic phase.

Effect of carbon fibers on thermal properties and mechanical strength of ceramizable composites based on silicone rubber

Ceramizable (ceramifiable) silicone composites are one of the most important elastic technical materials produced industrially. These composites are commonly used to increase flame retardancy of electrical cables and to ensure integrity of electricity network during fire by their ability to create a continuous ceramic structure. In this paper, ceramizable silicone composites filled with different contents of carbon fibers were tested. The research was focused on the characterization of ceramic structure created during heat treatment of the composites and thermal properties of the composites. For this purpose, morphology (SEM) and compression strength of the ceramic structures were studied. To describe process of ceramic structures creation, TG/DSC analysis was done. These tests have demonstrated that, the increase in carbon fibers amount improves the mechanical properties of ceramic structure regardless of heat treatment conditions.

Thermal behavior of silicone rubber-based ceramizable composites characterized by Fourier transform infrared (FT-IR) spectroscopy and microcalorimetry.

Ceramizable (ceramifiable) silicone rubber–based composites are commonly used for cable insulation. These materials are able to create a protective ceramic layer during fire due to the ceramization process, which occurs at high temperature. When the temperature is increased, the polymer matrix is degraded and filler particles stick together by the fluxing agent, producing a solid, continuous ceramic phase that protects the copper wire from heat and mechanical stress. Despite increasing interest in these materials that has resulted in growing applications in the cable industry, their thermal behavior and ceramization process are still insufficiently described in the literature. In this paper, the thermal behavior of ceramizable silicone rubber–based composites is studied using microcalorimetry and Fourier transform infrared spectroscopy. The analysis of the experimental data made it possible to develop complete information on the mechanism of composite ceramization.

DIFFERENCES BETWEEN CRYSTALLIZATION OF LDPE AND IPP IN EPDM MATRIX

Supermolecular structure of blends made of low density polyethylene (LDPE) or isotactic polypropylene (iPP) and ethylene-propylene- diene rubber (EPDM) was studied with X-ray diffraction (WAXS and SAXS), differential scanning calorimetry (DSC) and positron annihilation spectroscopy. Experimental data reveals different behaviour of the plastomers in the elastomer matrix. The crystalline phase of LDPE is to some extent solvated by the amorphous EPDM, whereas addition of iPP enhances the degree of the blends crystallinity. This additional crystalline phase is of imperfect nature and probably originates from cocrystallization of propylene monomer units from EPDM onto iPP particles. LDPE recrystallizes in the blends with EPDM at a lower temperature, the higher the plastomer content. Contrary to this, iPP recrystallizes in the rubber matrix at higher temperature. The blends composition influences the morphology of the crystalline phase. Interlamellar amorphous layer thickness slightly increases with an increase of LDPE content, being simultaneously accompanied by a slight decrease of the crystalline lamella thickness. The long period values remain practically constant. In the case of iPP/EPDM blends, the crystalline lamella thickness increases with an increase of the plastomer content, whereas the interlamellar amorphous layer remains constant, which results in an increase of the long period. The reversed tendency, observed for high-filled samples containing ⩾ 50 phr of iPP, is likely to be associated with too many nucleation centres, finally reducing the spherulite size. Despite annihilation taking place in the amorphous as well as the crystalline phase of polymers, the positron lifetime data stays in good agreement with the postulated supermolecular structure of the blends.

Thermal Stability and Flammability of Styrene-Butadiene Rubber-Based (SBR) Ceramifiable Composites

Ceramifiable styrene-butadiene (SBR)-based composites containing low-softening-point-temperature glassy frit promoting ceramification, precipitated silica, one of four thermally stable refractory fillers (halloysite, calcined kaolin, mica or wollastonite) and a sulfur-based curing system were prepared. Kinetics of vulcanization and basic mechanical properties were analyzed and added as Supplementary Materials. Combustibility of the composites was measured by means of cone calorimetry. Their thermal properties were analyzed by means of thermogravimetry and specific heat capacity determination. Activation energy of thermal decomposition was calculated using the Flynn-Wall-Ozawa method. Finally, compression strength of the composites after ceramification was measured and their micromorphology was studied by scanning electron microscopy. The addition of a ceramification-facilitating system resulted in the lowering of combustibility and significant improvement of the thermal stability of the composites. Moreover, the compression strength of the mineral structure formed after ceramification is considerably high. The most promising refractory fillers for SBR-based ceramifiable composites are mica and halloysite.

Influence of Network Structure on Glass Transition Temperature of Elastomers

It is generally believed that only intermolecular, elastically-effective crosslinks influence elastomer properties. The role of the intramolecular modifications of the polymer chains is marginalized. The aim of our study was the characterization of the structural parameters of cured elastomers, and determination of their influence on the behavior of the polymer network. For this purpose, styrene-butadiene rubbers (SBR), cured with various curatives, such as DCP, TMTD, TBzTD, Vulcuren®, DPG/S8, CBS/S8, MBTS/S8 and ZDT/S8, were investigated. In every series of samples a broad range of crosslink density was obtained, in addition to diverse crosslink structures, as determined by equilibrium swelling and thiol-amine analysis. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to study the glass transition process, and positron annihilation lifetime spectroscopy (PALS) to investigate the size of the free volumes. For all samples, the values of the glass transition temperature (Tg) increased with a rise in crosslink density. At the same time, the free volume size proportionally decreased. The changes in Tg and free volume size show significant differences between the series crosslinked with various curatives. These variations are explained on the basis of the curatives’ structure effect. Furthermore, basic structure-property relationships are provided. They enable the prediction of the effect of curatives on the structural parameters of the network, and some of the resulting properties. It is proved that the applied techniques—DSC, DMA, and PALS—can serve to provide information about the modifications to the polymer chains. Moreover, on the basis of the obtained results and considering the diversified curatives available nowadays, the usability of “part per hundred rubber” (phr) unit is questioned.

Importance of the Surface Layer for Polymer Materials

The surface layer of polymers exhibits different composition, morphol ogy and structure, also in the nanoscale, from the bulk material. The paper demonstrates the clearest examples, discussing consequences of the gradient nature of the materials from the aspect of their exploitation (mainly hardness profile and friction). The ability of low molecular weight substances to migrate in a polymer matrix and the surface segregation occurring in polymer blends, are studied by FTIR, AFM, XPS, microindentation, microfriction, contact angle and DSC techni ques. It has been proved that the surface layer of polymer materials can be shaped during both compounding and process ing.

EFFECT OF POLYMER CHAIN MODIFICATIONS ON ELASTOMER PROPERTIES

ABSTRACT Considerable attention is paid to the influence of crosslink density and crosslink structures on the behavior of polymer chains and properties of elastomers. However, a very important parameter seems to be underestimated: the modifications to the polymer chains by curatives, formed by sulfur and fragments of accelerators. We draw attention to this important contribution to performance of spatial networks. The emulsion styrene–butadiene rubber samples, cured with tetramethylthiuram disulfide and sulfur (TMTD/S8) and zinc dialkyl dithiophosphate with sulfur (ZDT/S8), were studied. They were characterized in detail in terms of crosslink density and crosslink structures. Microscale techniques were used to obtain information about the behavior of the polymer chains: positron annihilation lifetime spectroscopy (PALS) to study the free volume structure and differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) to monitor the glass transition process. Properties such as static mech...

Ceramizable Composites for Fire Resistant Applications

The paper concerns composite materials made of silicone rubber matrix and ceramic fillers used as flame resistant coverings for electrical cables. Under fire, such materials must be able to form, relatively quickly, compact and stiff protecting coating, strong enough to maintain integrity of electrical circuit, even up to melting temperature of metal core. The residue of fired silicone rubber or silica filled elastomer exhibit a form of white powder. There is no evidence of solidification of silica particles, even after heating at 1100°C. However, the addition of some ceramic phases results in reaction with silica matrix (starting at about 900°C) producing a liquid phase, what facilitates particle binding. At lower firing temperatures (600°C) the problem of binding between the product of pyrolysis (silica) and filler is also present, what results in formation of fragile surface shield. The problem can be overcome by the addition of certain inorganic materials to the silicone rubber matrix. The paper discusses their influence on ability of silicone rubber composites, additionally containing glassy phase, wollastonite, mica, aluminium hydroxide, montmorillonite or calcined caoline, to ceramization.