Tuneable PTC effect in polymer-wax-carbon composite resistors

Purpose: The resistivity dependence on temperature of composite resistors made of carbon fillers dispersed in an organic matrix are known to be strongly affected by the matrix thermal expansion. High positive temperature coefficient (PTC) effects, i.e. essentially switching from resistive to quasi-insulating behaviour, can be caused by phase changes in the matrix and the assorted volume expansion, a behaviour that has been previously shown with both simple organic waxes and semicrystalline polymers. However, waxes become very liquid upon melting, possibly resulting in carbon sedimentation, and tuneability of semicrystalline polymers is limited. Approach: We therefore study a ternary polymer-wax-conductor (ethylcellulose-octadecanol-graphite) composite resistor system, where polymer and wax fuse to a viscous liquid upon heating, and re-solidify and separate by crystallisation of the wax upon cooling. Findings: It is shown that with appropriate formulation, the resulting resistors exhibit strong PTC effects, linked with the melting and crystallisation of the wax component. The behaviour somewhat depends on sample history, and notably cooling speed. Limitations: The phase equilibria and transformation kinetics of the polymer-wax system (including possible wax polymorphism), as well as the exact mechanism of the conductivity transition, remain to be investigated. Originality/value: As many compatible polymer-wax systems with different melting/solidification behaviours are available, ternary polymer-wax-conductor composite PTC resistors allow a high tuneability of properties. Moreover, the high viscosity in the liquid state should largely avoid the sedimentation issues present with binary wax-conductor systems.

Ludivine Ammon, Caroline Jacq, Thomas Maeder, Peter Ryser

The resistivity dependence on temperature of composite resistors made of carbon fillers dispersed in an organic matrix are known to be strongly affected by the matrix thermal expansion. High positive temperature coefficient (PTC) effects, i.e. essentially switching from resistive to quasi-insulating behaviour, can be caused by phase changes in the matrix and the assorted volume expansion, a behaviour that has been previously shown with both simple organic waxes (low temperatures) and semicrystalline polymers (higher temperatures, used commercially in resettable fuses). However, waxes become very liquid upon melting, possibly resulting in carbon sedimentation, and tuneability of semicrystalline polymers is limited. In this work, we therefore study composite resistors with a mixed polymer-wax matrix, the polymer imparting high viscosity in the liquid state and a variety of waxes allowing a high tunability of the behaviour. It is shown that the resulting resistors exhibit strong PTC effects, linked with the melting and crystallisation of the wax component. Keywords: thick-film resistors, polymer-carbon composites, PTC effect, resettable fuses

IMAPS Poland Chapter2013

Metallocene Catalyzed Polyolefins For Improving Thermal And Mechanical Properties Of Bitumen Blends

Jan-Anders Månson, John Christopher Plummer, Chiara Spadaro

Bitumen is a complex viscoelastic material used as a binder in applications ranging from road paving to waterproof membranes for roofing [1]. In such applications, bitumen is often blended with a polymer in order to improve its elasticity. The resulting polymer modified bitumens (PMBs) also generally show improvements in thermal and fatigue cracking resistance, and temperature susceptibility [2, 3]. A wide range of polymers have been used in PMBs, including thermoplastic elastomers, thermoplastics and thermosets [1], but among the most successful are styrene-butadiene-styrene (SBS) block copolymers, in which the polystyrene blocks provide cohesive strength and the rubbery polybutadiene blocks provide elasticity [3]. Because SBS is relatively expensive, the possibility of mixing bitumen with other polymers remains of interest [2-4] and metallocene-catalyzed linear low density polyethylenes (m-LLDPE) have recently been discussed as a possible alternative, combining low cost and elasticity with improved storage stability compared with conventional polyolefins [5, 6]. In the present study, PE/octene and PE/butene based m-LLDPE copolymers with different melt flow indices, glass transition temperatures, melting temperatures and degrees of crystallinity have been used to modify a roofing grade gel bitumen. The morphology, the thermal behaviour and the viscoelastic properties of the PMBs have been studied in the composition range 5 to 50 wt% m-LLDPE. In the case of the butenebased copolymer, fluorescence microscopy indicated a continuous m-LLDPE-rich phase matrix containing discrete asphaltene-rich domains at all polymer contents, although at 5 wt% of the octene-based copolymer; the continuous phase remained asphaltene-rich, and there was some evidence for phase inversion, the discrete m-LLDPE-rich domains containing additional asphaltene-rich domains. It follows that the melt rheological response of the PMBs was dominated by the m-LLDPE, but the excellent flow characteristics of this latter resulted in relatively modest increases in shear viscosity compared to those obtained with equivalent concentrations of SBS. Significant swelling by the maltene-rich components of bitumen led to a decrease of the melting point of both types of m-LLDPE, although the degree of crystallinity relative to the polymer content increased somewhat at low m-LLDPE concentrations. Time temperature superposition was used to provide an indication of the dynamic viscoelastic response in shear over a wide range of frequencies. The pure bitumen exhibited liquid-like behaviour over the entire temperature and frequency range investigated, the loss modulus, G’’, generally being significantly higher than the storage modulus, G’. However, whereas blending with the relatively crystalline octene-based m- LLDPE resulted in more elastic behaviour (G’ greater than G’’) the less crystalline butene-based m-LLDPE showed a crossover in G’ and G’’ for all the concentrations investigated. The crossover frequency was found to shift towards lower frequencies as the m-LLDPE content increased, suggesting it to be possible to fine tune the viscoelastic response, depending on the required end-properties. In view of the relative stability of the PMB microstructures referred to above, these results confirm the promise of m-LLDPE as additives to gel bitumen, and suggest there to be considerable flexibility in the range of m-LLDPE contents that may be envisaged, without compromising processing requirements. The consequences for applicative properties will be discussed.

2010

Thermal and dynamic mechanical properties of blends of bitumen with metallocene catalyzed polyolefins

Jan-Anders Månson, John Christopher Plummer, Chiara Spadaro

A high penetration grade bitumen has been blended with up to 50 wt% of two different grades of metallocene catalyzed linear low density polyethylene (m-LLDPE) in order to investigate the potential of these and similar copolymers as a substitute for styrene butadiene styrene triblock copolymers in polymer-modified bitumens (PMB). A continuous polymer-rich phase was observed at m-LLDPE contents as low as 5-10 wt%, along with a significant decrease in the effective glass transition temperature of the PMBs with increasing polymer concentration, suggesting benefits for low temperature flexibility. The m-LLDPE-based PMBs also showed relatively low dynamic shear viscosities up to high polymer contents in the range of temperature and shear rate corresponding to typical PMB processing conditions. However, the presence of bitumen in the m-LLDPE-rich phase led to a significant reduction in the melting points of the m-LLDPE, and softening of the PMBs at temperatures as low as 40-50 degrees C, depending on the composition and the melting point of the pure polymer. PMBs based on the m-LLDPE with the higher melting point remained fully elastic in this temperature range, but at the expense of increased crystallinity and a higher glass transition temperature, which limit improvements in low temperature flexibility. On the other hand, the potentially broad composition and property windows associated with m-LLDPEs suggest considerable scope for the fine tuning of PMB properties by using combinations of different m-LLDPEs and/or other polyolefins as a means to optimize performance.

Springer US2011

Metallocene Catalyzed Polyolefins For Improving Thermal And Mechanical Properties Of Bitumen Blends

Jan-Anders Månson, John Christopher Plummer, Chiara Spadaro

2010