Bengt Stenberg

PERFORMANCE OF ISOTROPIC MAGNETORHEOLOGICAL RUBBER MATERIALS

Magnetorheological (MR) rubber materials are the solid analog of magnetorheological fluids; hence, their rheological properties can be controlled by an applied magnetic field. If the particles embedded in the matrix are carbonyl iron, they have to be aligned by a magnetic field before the curing of the rubber, in order to achieve a substantial MR effect. However, the use of a crosslinked matrix allows larger particles to be used. We show that MR rubber materials with large irregular particles have a large MR effect although the particles are not aligned within the material. This is explained by the low critical particle volume concentration of such particles. A similar effect can be obtained for materials with carbonyl iron, if the particles are badly dispersed and thereby behaving like larger irregular particles. Besides the achieved level of dispersion, the rheological properties of the matrix material do not influence the MR effect.

Improving the magnetorheological effect in isotropic magnetorheological rubber materials

Magnetorheological (MR) rubber materials are the solid analogue of magnetorheological fluids; i.e. their rheological properties can be controlled by an applied magnetic field. However, the use of a crosslinked matrix allows larger particles to be used than in fluid matrices, and when large irregular particles are used alignment of the particles is not necessary for obtaining a substantial MR effect. We show that the absolute MR effect of isotropic MR rubber materials with large irregular iron particles is independent of the matrix material, and that the relative MR effect can be increased by the addition of plasticisers. Other ways of increasing the MR effect are to increase the magnetic field, although the materials saturate magnetically at high fields, or to use small amplitude strains. Finally, we show that the damping of isotropic MR rubbers is to some extent increased by an applied magnetic field, but that the increase is too small to be of any practical importance.

Chemiluminescence from oxidation of polypropylene: Correlation with peroxide concentration

Abstract The Chemiluminescence (CL) on heating in an inert atmosphere has been measured for samples taken at various times during oxidation of polypropylene (PP). Proportional relationships were obtained when the integrated emission (TLI) was plotted against peroxide concentration for all studied sets of parameters. Changes in melting temperature and molecular weight with ageing time were found to correlate with changes in the TLI. Oxidised PP was exposed to dimethylsulfide (DMS) for different periods with subsequent measurements of the CL and the peroxide concentration. The peroxide concentration has earlier been found to exhibit a two-step decay with DMS exposure time, indicating two fractions of peroxides with different reactivity. The CL was found to follow the same two-step decay, indicating that both peroxide fractions contribute to CL. The oxidative stability of PP, measured by CL induction times in oxygen, increases when the polymer is exposed to DMS.

Biotechnological possibilities for waste tyre-rubber treatment.

Every year large amounts of spent rubber material, mainly from rubber tyres, are discarded. Of the annual total global production of rubber material, which amounts to 16–17 million tonnes, approximately 65% is used for the production of tyres. About 250 millions spent car tyres are generated yearly in USA only. This huge amount of waste rubber material is an environmental problem of great concern. Various ways to remediate the problem have been proposed. Among these are road fillings and combustion in kilns. Spent tyres, however, comprise valuable material that could be recycled if a proper technique can be developed. One way of recycling old tyres is to blend ground spent rubber with virgin material followed by vulcanization. The main obstacle to this recycling is bad adhesion between the crumb and matrix of virgin rubber material due to little formation of interfacial sulphur crosslinks. Micro-organisms able to break sulphur-sulphur and sulphur-carbon bonds can be used to devulcanize waste rubber in order to make polymer chains on the surface more flexible and facilitate increased binding upon vulcanization. Several species belonging to both Bacteria and Archaea have this ability. Mainly sulphur oxidizing species, such as different species of the genus Thiobacillus and thermoacidophiles of the order of Sulfolobales, have been studied in this context. The present paper will give a background to the problem and an overview of the biotechnological possibilities for solutions of waste rubber as an environmental problem, focusing on microbial desulphurization.

Effects of the type of crosslink on viscoelastic properties of natural rubber

The viscoelastic properties of various crosslinked natural rubbers, NR, were investigated by mechanical spectroscopy. The glass transition temperature, Tg, was found to be dependent on both the crosslink density and the crosslink type. Higher values of Tg were obtained for sulfur-crosslinked NR than for peroxide-crosslinked NR at the same crosslink density. The greater influence of the sulfur content on Tg may be attributed to polysulfidic crosslinks and cyclic sulfide structures favored at high sulfur contents. Sulfur-vulcanized NRs with monosulfidic crosslinks, favored at relatively high accelerator/sulfur ratios, have properties more similar to the peroxide-cured NR with simple carbon(SINGLE BOND)carbon crosslinks covalent bonds, resulting in only small shifts in Tg. A qualitative analysis of monosulfidic crosslinks and polysulfidic structures was performed with 13C solid-state NMR spectroscopy. The storage modulus, E′, in the rubbery plateau region increased with increasing crosslink density. However, the crosslink type did not influence the moduli values as much as it influenced the Tg values. Different methods of detecting the crosslink density were also discussed.

Synthesis and evaluation of hyperbranched phenolic antioxidants of three different generations

Abstract Antioxidant-modified Boltorn® of 2nd, 3rd and 4th generation were synthesised. 3-(3,5-Di-tert-butyl-4-hydroxy-phenyl)-propionic acid was the active group attached to the three different hyperbranched polyesters. The syntheses were successful with high degrees of substitution. The antioxidants were evaluated in squalane and in polypropylene (PP) films using differential scanning calorimetry (DSC) to determine the oxidation induction time (OIT). They were compared to the commercial antioxidant Irganox 1010, which was added in an amount of 0.1 wt.%. All the synthesised antioxidants were superior to Irganox 1010 in squalane but did not contribute much to the stability of PP. The conclusion was drawn that the low antioxidative performance in PP is due to low mobility in combination with low solubility of the hyperbranched antioxidants.

Crosslinking of siloxane elastomers

Abstract Silicones are widely used where elastomers come in contact with food and pharmaceuticals. Different curing methods can be applied for crosslinking this elastomer. Ageing tests clearly show that peroxide-cured silicone elastomers should be avoided in view of the fact that post-curing occurs if they are sterilized. This will result in a change in properties over time. Dicumyl peroxide is frequently used in the crosslinking of elastomer materials. Products that can be extracted from the elastomer after crosslinking with dicumyl peroxide include cumenalcohol and acetophenone. No organic compounds could be extracted from the platinum-cured elastomer. Peroxide-cured silicone elastomer shows an increase in Young’s modulus by approx. 50% after 100 sterilizations whereas an elastomer cured with platinum shows a modulus that is almost constant. An addition of a curing system like platinum has the advantage of no peroxide handling. Furthermore, the platinum curing systems have the advantage of faster curing than peroxide curing. The uses of peroxide cured siloxane elastomers are based on old routines and the use of platinum-cured elastomers is recommended for the future.

Imaging chemiluminescence instrument for the study of heterogeneous oxidation effects in polymers

Abstract An imaging Chemiluminescence equipment used for studies of diffusion controlled oxidation effects in polymers is presented. The design involves an imaging photon counting system mounted on a cylindrical oven with a controlled atmosphere. The overshoot on heating is less than 2 °C and the temperature during isothermal conditions is maintained at ±0.4 °C, thus, a stable thermal environment is attained. To point out possible applications, the monitoring of oxidation profiles from aged nylon specimens and oxygen-barrier properties of an inorganic coating by chemical plasma deposition on a hydroxyl terminated polybutadiene substrate are presented. The results demonstrate the feasibility of studying diffusion controlled oxidation and also the possibility of the technique to estimate the function and durability of surface treatments on elastomers.

Comparison of dynamic mechanical measurements and Tg determinations with two different instruments

Abstract The results of dynamic mechanical measurements in tension, made by using two commercial dynamic mechanical instruments, a DMTA (Polymer Laboratories) and a DMA (Perkin Elmer), are compared and discussed. Differences in Tg values of approximately 10°C for an unfilled and a carbon-black-filled natural rubber (NR) were obtained, the higher temperature being associated with the DMTA instrument. For an MDPE material, temperature differences of approximately 14°C for the γ transition and approximately 8°C for the α; transition were noticed. These temperature differences were apparently due to temperature calibration errors and show the need to have an acceptable calibration standard for this type of measurement. The modulus values showed good agreement except for very stiff materials such as NR at temperatures below Tg, where the modulus values obtained with the DMTA were lower than those obtained with the DMA instrument. Incorrect sample geometry giving too stiff a structure could explain this low modulus. Different ways to characterize Tg by thermal analysis are also compared and discussed, with the emphasis on frequency-dependence.

Oxidation profiles of polyamide 6,6 studied by imaging chemiluminescence and FTIR

Abstract Injection moulded test specimens of polyamide 6,6 have been subjected to ageing in ambient atmosphere at temperatures ranging from 100 to 130 °C for different times. Skin-core oxidation was observed and evaluated by infrared spectroscopy in terms of carbonyl index depth profiles and by imaging chemiluminescence as peroxide depth profiles. The oxidation depth profiles determined by the two techniques showed good agreement with respect to ageing time and temperature. The oxidation depth was shown to decrease with increasing ageing temperature. The results clearly indicate a diffusioncontrolled oxidation process.