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  • 1.
    Azhdar, Bruska
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Determination of dynamic and sliding friction, and observation of stick-slip phenomenon on compacted polymer powders during high-velocity compaction2006In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 25, no 8, p. 1069-1080Article in journal (Refereed)
    Abstract [en]

    Dynamic friction, sliding friction, and the stick-slip phenomenon have been studied on compacted polymer powders during high-velocity compaction. It is particularly important from a practical point of view to distinguish the stick-slip mechanism and the sliding mechanism which occur concurrently. A practical experimental system has been successfully developed to study the dry frictional force and to measure the sliding coefficient between the polymer powder particles and the die wall during high-velocity compaction. Two new components have been introduced as relaxation assists to improve the compaction process by reducing the frictional forces. It was found that the relaxation assist device leads to an improvement in the polymer powder compaction process by giving a more homogeneous opposite velocity and a better locking of the powder bed in the compacted form with less change in dimensions. The subsequent movement of the particles can be reduced and the powder bed attains a higher density with a minimum total elastic spring-back. The relative time of the stick-slip phenomenon during the compacting stage is also reduced so that the time needed to transfer from an intermittent stick-slip state to a smooth sliding state is reduced and the powder bed slides smoothly. It was found that the dynamic, dry frictional force is intermittent (stick-slip mechanism) at low compaction rates but that at high compaction rates is becomes more smooth (sliding mechanism). Both mechanisms depend on the nature of the powder and on the compaction conditions. At the beginning of the compaction stage, the sliding coefficient decreases due to an increase in the radial to axial stress ratio until the maximum pressure has been reached. During the reorganization stage, more time is needed for large particles to move, rotate and slide due to their relatively large diameter and mass. As a result, the reorganization stage is extended and the stick-slip phenomenon is observed more with increasing particle size. Much better transfer of the pressure throughout the powder bed and less loss of pressure lead to a higher sliding coefficient due to the overall friction during the compaction process. It was found that the sliding coefficient is proportional to the density. A more homogeneous density distribution in the compacted powder and a smaller pressure loss during compaction has a major influence on the sliding coefficient and on the quality of the compacted material

  • 2.
    Azhdar, Bruska
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Determination of springback gradient in the die on compacted polymer powders during high-velocity compaction2006In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 25, no 1, p. 114-123Article in journal (Refereed)
    Abstract [en]

    A uniaxial high-velocity compaction process for polymer powder using a cylindrical, hardened steel die and a new technique with relaxation assist was tested with various heights. The influences of the relaxation assist device on the process characteristics are discussed. Two bonded strain gauges and a high-speed video camera system were used to investigate the springback phenomenon during the compaction process. It was found that the relaxation assist improves the compaction of the polymer powder by locking the powder bed in the compacted form. It is shown that the high-velocity compaction process is an interruption process and that the delay times between the pressure waves can be reduced by increasing the height of the relaxation assist device. The delay times between the pressure waves are also strongly dependent on the strain rate. If the height of the relaxation assist device is increased, the first gross instantaneous springback, and the total elastic springback, are reduced. In addition, the density of the powder bed is increased. The relative times of the compacting stage, decompacting stage and the reorganisation of the particles can be also controlled by altering the height of the relaxation assist.

  • 3.
    Azhdar, Bruska
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Development of a High-Velocity Compaction process for polymer powders2005In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 24, no 7, p. 909-919Article in journal (Refereed)
    Abstract [en]

    The High-Velocity Compaction (HVC) process for powder polymers has been studied, with a focus on the compactibility characteristics and surface morphology of the compacted materials, with and without relaxation assists, by increasing compacting quantity and direction. The basic phenomena associated with HVC are explained and the general energy principle is introduced to explain pull-out phenomena during the decompacting stage. Polyamide-11 powders with different particle size distributions have been compacted with the application of different compaction profiles, e.g. different energies and velocities. Scanning electron microscopy (SEM) and image computer board camera, (IC-PCI Imaging Technology) have been used to the study the morphological characteristics, the limit of plastic deformation and particle bonding by plastic flow at contact points, and pull-out phenomena. The relative green density is influenced more by the pre-compacting (primary compaction step) than by the post-compacting (secondary compaction step). The pressure and density distribution differences between the upper and lower surface are not uniform. Projectile supports or 'relaxation assists' are presented as a new technique to reduce pull-out phenomenon. Experimental results for different compaction profiles are presented showing the effect of varying the opposite velocity during the decompacting stage, and how to improve the homogeneous densification between the upper and lower surface and the evenness of the upper surface of the compacted powder bed by using relaxation assists.

  • 4.
    Azhdar, Bruska
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Kari, Leif
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Structural and vibroacoustics.
    Polymer-nanofiller prepared by high-energy ball milling and high velocity cold compaction2008In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 29, no 3, p. 252-261Article in journal (Refereed)
    Abstract [en]

    High-energy ball milling using comilling in a solid state by low-temperature mechanical alloying to prepare nickel-ferrite (NiFe2O4) nanopowders and ultrafine poly(methyl methacrylate) (PMMA), dispersing nanoparticles in a polymer matrix, and a uniaxial high-velocity cold compaction process using a cylindrical, hardened steel die and a new technique with relaxation assists have been studied. The focus has been on the particle size distributions of the nanocomposite powder during the milling and on the surface morphology of the nanocomposite-compacted materials after compaction with and without relaxation assists. Experimental results for different milling systems are presented showing the effects of milling time and material ratio. It was found that a longer mixing time give a higher degree of dispersion of the nanopowder on the PMMA particle surfaces. Furthermore, with increasing content of NiFe2O4 nanopowder, the reduction of the particle size was more effective. Different postcompacting profiles, i.e. different energy distributions between the upper and lower parts of the compacted powder bed, lead to different movements of the various particles and particle layers. Uniformity, homogeneity, and densification on the surfaces in the compacted powder are influenced by the postcompacting magnitude and direction. It was found that the relaxation assist device leads to an improvement in the polymer powder compaction process by reducing the expansion of the compacted volume and by reducing the different opposite velocities, giving the compacted composite bed a more homogeneous opposite velocity during the decompacting stage and reducing the delay time between the successive pressure waves.

  • 5. Jacobson, K.
    et al.
    Eriksson, Petter
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Reitberger, T.
    Stenberg, Bengt
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Chemiluminescence as a tool for polyolefin oxidation studies2004In: Advances in Polymer Science, ISSN 0065-3195, E-ISSN 1436-5030, Vol. 169, p. 151-176Article, review/survey (Refereed)
    Abstract [en]

    The oxidation of polymers such as polypropylene and polyethylene is accompanied by weak chemiluminescence. The development of sensitive photon counting systems has made it comparatively easy to measure faint light emissions and polymer chemiluminescence has become an important method to follow the initial stages in the oxidative degradation of polymers. Alternatively, chemiluminescence is used to determine the amount of hydroperoxides accumulated in a pre-oxidised polymer. Chemiluminescence has also been applied to study how irradiation or mechanical stress affects the rate of polymer oxidation. In recent years, imaging chemiluminescence has been established as a most valuable technique offering both spatial and temporal resolution of oxidation in polymers. This technique has disclosed that oxidation in polyolefins is non-uniformly distributed and proceeds by spreading. This review is the result of several investigations performed by the authors and other research groups where chemiluminescence has been used to study oxidative degradation of polyolefins, either as the main technique or as a complement to other techniques.

  • 6.
    Kari, Leif
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Eriksson, P.
    KTH, Superseded Departments, Polymer Technology.
    Stenberg, Bengt
    KTH, Superseded Departments, Polymer Technology.
    Dynamic stiffness of natural rubber cylinders in the audible frequency range using wave guides2001In: Kautschuk und Gummi, Kunststoffe, ISSN 0022-9520, Vol. 54, no 3, p. 106-+Article in journal (Refereed)
    Abstract [en]

    This paper applies a wave guide model to analyze the temperature influence on the dynamic stiffness of a cylindrical vibration isolator of natural rubber in the audible frequency range; covering 20 to 20 000 Hz and - 60 to + 60 degreesC. A nearly incompressible material model is applied with shear modulus based on a fractional derivative model, thereby reducing the parameter number needed to accurately fit measurement results. Employing the method of reduced variables enlarges the frequency range available from measurements. Of particular interest are the stiffness peaks and troughs, the low-frequency stiffness, the propagating and nonpropagating modes, and their dependence on temperature.

  • 7.
    Kari, Leif
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Eriksson, P.
    KTH, Superseded Departments, Polymer Technology.
    Stenberg, Bengt
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    On the temperature dependence of vibration isolators in the audible frequency range2000In: International Congress on Sound and Vibration, 2000, p. 2947-2954Conference paper (Other academic)
  • 8.
    Kari, Leif
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Eriksson, P.
    KTH, Superseded Departments, Polymer Technology.
    Stenberg, Bengt
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Properties of rubber isolators in the audible frequency range2000In: International Rubber Conference, 2000Conference paper (Other academic)
  • 9.
    Kari, Leif
    et al.
    KTH, Superseded Departments, Vehicle Engineering.
    Lokander, Mattias
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Structure-borne sound properties of isotropic magneto-rheological rubber2002In: Kautschuk und Gummi, Kunststoffe, ISSN 0022-9520, Vol. 55, no 12, p. 669-673Article in journal (Refereed)
    Abstract [en]

    The dynamic properties of isotropic magneto-rheological rubber are examined through experiments within an audible frequency range covering 100 to 1250 Hz while applying an external magnetic field of 0 to 0.8 T. The shear modulus for this manufacturer convenient material, consisting of dispersed irregular, micrometer sized iron particles in a nitrile rubber matrix, is found to depend strongly on both frequency and magnetic field, while the loss factor is almost independent of those factors. Magneto-rheological rubbers applied to vibration isolators promise to have more functionality than conventional isolators while rapidly and reversibly change rigid body and stiffness peak frequencies.

  • 10.
    Lokander, Mattias
    et al.
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Reitberger, Torbjörn
    KTH, Superseded Departments, Chemistry.
    Stenberg, Bengt
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Magnetorheological Rubber Materials: The role of Reinforcement and FrictionManuscript (Other academic)
  • 11.
    Lokander, Mattias
    et al.
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Reitberger, Torbjörn
    KTH, Superseded Departments, Chemistry.
    Stenberg, Bengt
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Oxidation of Natural Rubber based Magnetorheological Elastomers2004In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 86, no 3, p. 467-471Article in journal (Refereed)
    Abstract [en]

    The Theological properties of magnetorheological (MR) materials can be changed continuously, rapidly and reversibly by an applied magnetic field. Solid MR materials consist of magnetically polarisable particles, generally iron, in an elastomer matrix. The high iron concentrations required (about 30% by volume) in order to get a substantial magnetorheological effect should influence the long-term stability of the materials. In this paper, the oxidative stability of natural rubber-based magnetorheological elastomers has been studied by chemiluminescence and oven ageing. The results show that the oxidative stability of natural rubber decreases dramatically when large amounts of iron particles are incorporated in the matrix. This is probably due to the large amounts of oxygen on the surface of the particles. Conventional antioxidants can be used to prolong the lifetime of magnetorheological elastomers, but in order to get acceptable lifetime of the materials a careful selection of the antioxidant system has to be made.

  • 12.
    Lokander, Mattias
    et al.
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Improving the magnetorheological effect in isotropic magnetorheological rubber materials2003In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 22, no 6, p. 677-680Article in journal (Refereed)
    Abstract [en]

    Magnetorheological (MR) rubber materials are the solid analogue of magnetorheological fluids; i.e. their theological 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.

  • 13.
    Lokander, Mattias
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Magnetorheological devices2006In: Elastomers and Components: Service Life Prediction - Progress and Challenges, Woodhead Publishing Limited, 2006, p. 165-169Chapter in book (Other academic)
    Abstract [en]

    The mechanical properties of magnetorheological materials can be changed and controlled by an applied magnetic field. This makes the group of materials interesting for damping applications and vibration control. Most of the work done so far has been performed on magnetorheological fluids, which are suspensions of magnetically polarizable particles in a carrier liquid. For some applications it would be preferable to use a rubber or a gel instead of the liquid. Such magnetorheological solids would not have any problems of sedimentation of particles, the devices could be made smaller and cheaper since there would be no need of a container for the fluid, and the changes in the mechanical properties could be expected to be faster than in a liquid. However, magnetorheological solids work only in the pre-yield region while the fluids typically work in the post-yield region, so the two groups of materials are therefore more complementary than competitive. Examples of rubbers that are interesting for magnetorheological applications are silicone, natural rubber and nitrile rubber. The addition of iron particles to the rubber will influence the service life of these rubber devices. For optimal function of the devices, prediction of service life is necessary. Such predictions are also necessary for magnetorheological devices to become commercially successful.

  • 14.
    Lokander, Mattias
    et al.
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Stenberg, Bengt
    KTH, Superseded Departments, Fibre and Polymer Technology.
    Performance of Isotropic Magnetorheological Rubber Materials2003In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 22, no 3, p. 245-251Article in journal (Refereed)
    Abstract [en]

    Magnetorheological (MR) rubber materials are the solid analog of magnetorheological fluids; hence, their theological 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 theological properties of the matrix material do not influence the MR effect.

1 - 14 of 14
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