Dynamic characteristics and processing of fillers in polyurethane elastomers for vibration damping applications

Polyurethane elastomers have the potential of being used to reduce vibrational noise in many engineering applications. The performance of the elastomer is directly related to matching the nature of the mechanical loss characteristics to the frequency and temperature dependence of the source of the vibration. Materials with a broad frequency response and good mechanical properties are desirable for situations were load bearing and isolation becomes an issue. Because automobile, and other related vehicles operate over a broad temperature range, it is desirable for the damping characteristics of the elastomer to ideally be independent of temperature and frequency. In practice, this is not possible and the creation of materials with a broad spectrum response is desirable. In this paper, the effects of various fillers on the breadth and temperature dependence of the vibration damping characteristics of a filled and crosslinked polyurethane elastomer are explored. The fillers studied are wollastonite, barium sulphate and talc. These materials have different shapes, sizes and surface chemistry and undergo different types of interaction with the matrix. The vibration damping characteristics were further varied by the use of a crosslinking agent. Data presented on the rheological characteristics indicate the strength of the filler-polyol interactions. Dielectric relaxation and dynamic mechanical thermal analysis demonstrate the way in which changes in the type of filler, concentration and amount of crosslinker lead to changes in the location and breadth of the energy dissipation process in these elastomers. The vibration damping characteristics of a selected material are presented to demonstrate the potential of these materials

Structure of laponite-styrene precursor dispersions for production of advanced polymer-clay nanocomposites

One method for production of polymer-clay nanocomposites involves dispersal of surface-modified clay in a polymerisable monomeric solvent, followed by fast in situ polymerisation. In order to tailor the properties of the final material we aim to control the dispersion state of the clay in the precursor solvent. Here, we study dispersions of surface-modified Laponite, a synthetic clay, in styrene via large-scale Monte-Carlo simulations and experimentally, using small angle X-ray and static light scattering. By tuning the effective interaction between simulated laponite particles we are able to reproduce the experimental scattering intensity patterns for this system, with good accuracy over a wide range of length scales. However, this agreement could only be obtained by introducing a permanent electrostatic dipole moment into the plane of each Laponite particle, which we explain in terms of the distribution of substituted metal atoms within each Laponite particle. This suggests that Laponite dispersions, and perhaps other clay suspensions, should display some of the structural characteristics of dipolar fluids. Our simulated structures show aggregation regimes ranging from networks of long chains to dense clusters of Laponite particles, and we also obtain some intriguing ‘globular’ clusters, similar to capsids. We see no indication of any ‘house-of-cards’ structures. The simulation that most closely matches experimental results indicates that gel-like networks are obtained in Laponite dispersions, which however appear optically clear and non-sedimenting over extended periods of time. This suggests it could be difficult to obtain truly isotropic equilibrium dispersion as a starting point for synthesis of advanced polymer-clay nanocomposites with controlled structures

Low coefficient of thermal expansion of thermoset composite materials

Selection of the correct resin – filler combination is important in achieving materials which have a low coefficient of thermal expansion (CTE). A study of nanosilica-modified resin, in combination with silica fillers, allows low levels of CTE to be achieved. In this study, a novel curing agent, ytterbium triflate, is reported. This curing agent provides a stable catalyst system which can be used to create the viscous composite mixtures but has the facility of effective cure over a relatively narrow temperature range from 70 °C to 100 °C. A series of formulations were examined based on incorporation of fillers with nanoscale silica particles into either the pure resin or a resin which contained nanoscale functionalized silica particles. The filler incorporation leads to a significant increase in the glass transition temperature as determined by dynamic mechanical thermal analysis. The CTE was observed to be lower below T g than above it. Changing the nanosilica particle size and distribution produced significant changes in the values. However, the CTE scaled according to the total silica content; and the values were in general lower than those calculated theoretically. The use of a highly functional o-cresol epoxy novolac demonstrated how increasing functionality raised the T g and lowered the CTE, but the use of too high a post-cure temperature reversed this trend. Very good results were achieved using 3,4-epoxycyclohexylmethyl, 3,4 epoxycyclohexanecarboxylate, which has a low viscosity and allowed high levels of silica to be readily achieved. This article indicates how the adjustment of the epoxy selected to be used as the base material and the type of silica particles used allows the values of T g and CTE to be modified in a composite material

Viscosity coefficients of nematic liquid crystals : II. Measurements of some nematic liquid crystals

Oscillating plate and rotational viscosity measurements are reported for a series of liquid crystals and include n-p-cyano-p-hexylbiphenyl (K18), 4-n-hepthyl-4′-cyanobiphenyl (K21), ethyl-cyclohexyl-ethyl-6-fluoro – n-propyl-biphenyl (I32), n-propyl-cyclohexyl-ethyl-6-fluoro – n-butyl-biphenyl (I43) and a n-pentyl-cyclohexyl-cyanophenyl : n-heptyl-cyclohexyl-cyanophenyl mixture. Rotational viscosity measurements were carried out over a temperature range from ambient to 90°C. Comparison of the values at a temperature of 5 K above the below the clearing point indicate an odd–even effect as the chain length of the hydrocarbon tail is altered. The principle viscosities η1, η2, η3 and η45were measured using an oscillating plate viscometer and the temperature dependences used to calculate the activation energies for flow in the various directions. The magnitude of the activation energy is shown to change with the length of the hydrocarbon chain. The incorporation of the cyclohexyl group imparts flexibility and reduces the activation energy flow, whilst the presence of the fluoro group increases the interactions between molecules, and this is reflected in higher values of the viscosity. The change of viscosity with alignment angle is explored for two of the systems studied and the fit to theory investigated. The Leslie–Ericksen coefficients are calculated for these systems and discussed in terms of changes in the molecular interactions

Viscosity coefficients of nematic liquid crystals : I. Oscillating plate viscometer measurements and rotational viscosity measurements: K15

Improvements in the oscillating plate and rotational viscosity measurements are described and data presented for 4-n-pentyl-4-cyanobiphenyl (K15). The influence of the magnitude of the magnetic field on the viscosity coefficients is explored and the limiting field which has to be achieved for field-independent values is identified. The field dependence of the viscosity coefficients is compared with theoretical prediction. Comparison of this data with earlier measurements indicates the limitations of the use of lower field strengths. Combining the data from the measured viscosities, the Leslie–Ericksen coefficients are calculated

Solvent effects on cure 1-benzyl alcohol on epoxy cure

The effect of benzyl alcohol and phenol on the cure of epoxy resins is reported. The epoxy resin, a blend of the diglycidyl ethers of bisphenol A and bisphenol F, was cured with 4,4'-methylenebis(cyclohexylamine) diamine. Measurements of the cure and vitrification times were obtained using a vibrating plate curometer. Mechanical properties were assessed using dynamic mechanical thermal analysis (DMTA). Time temperature transformation (TTT) diagrams were constructed. Benzyl alcohol lowers the viscosity, aids cure and plasticizes the final product. Phenol and triethylenetetramine [TETA] significantly enhance the rate of cure and have a catalytic effect

Positron annihilation lifetime spectroscopy and large molecule diffusion into a polyurethane matrix

Positron annihilation lifetime spectroscopy (PALS) measurements are reported on a three polyurethane (PU) materials created by the reaction of polymeric toluene diisocyanate with either ethylene glycol, 1,10-decanediol, or a silicone containing diol. Dynamical mechanical analysis indicated that the glass transition temperature of the PU’s were respectively 118, 95, and −40 °C. Whereas the ethylene glycol and 1,10-decanediol materials exhibited a glass–rubber transition, the silicone containing PU showed rubber characteristics over the temperature 0–180 °C. The PALS measurements on the silicone-based PU’s showed significantly larger voids dimensions than the other PU’s. Void collapse is observed to occur on the time scale of the oPs measurements. The permeation of dioctyl phthalate, 2-ethylhexylbenzyl phthalate, nonylphenol ethoxylate, isopropyl myristate, and oleic acid into a polyurethane matrix was measured gravimetrically. The silicone containing material at low temperatures exhibits relative simple permeation behavior however deviations from simple Fickian-type behavior are observed at higher temperature. Surprisingly, the ethylene glycol and 1,10-decanediol exhibited no significant absorption over a period of 5 months with the exception being nonylphenol ethoxylate. A comparison of the void sizes with the molecular dimensions for the lowest energy conformations of the permeants obtained using theoretical calculations indicate that for the silicone-based material the diffusion cross section for the permeants is larger than the available void size. The permeation process is considered to occur by a reptation type of motion of the permeants into channels created by the phase segregation of the flexible segments

Beta-phase formation in a crosslinkable poly(9,9-dihexylfluorene)

The synthesis of a crosslinkable poly(9,9-dihexylfluorene) derivative is reported. Vinyl ether moieties allow for photolithographic patterning, resulting in an immiscible polyfluorene network, which enables subsequent wet thin-film processing. The metastable β-phase morphology, inherent to most polyfluorene homopolymers, can be induced in this crosslinkable polymer. This is verified by optical spectroscopy, X-ray diffraction and by thermal analysis. The β-phase in polyfluorenes is of interest due to its superior optical and electrical characteristics compared to the amorphous phase. Here, amplified stimulated emission is studied. Surprisingly, the β-phase polymer exhibits higher emission thresholds than the amorphous analogue