Strain sensitive filled elastomers

Transcription

1 Strain sensitive filled elastomers Martyn Bennett Alan Thomas, Vineet Jha and James Busfield (QMUL) Smart elastomers Fillers Carbons Resistivity Nanotubes Overview

2 Smart Elastomers QTC s are well developed and effective materials They offer a versatile route to smart materials The opportunity to use a filler that is well used in rubber should not be dismissed Smart Elastomers Give and indication of strain (or pressure in tyres). Give and indication of temperature. Give an indication of extent of oxidation. Warn users of impending fatigue failure. Advise that a seal is leaking.

3 How might an elastomer be smart? Have embedded in it a device to measure strain (or pressure in say tyres). Change colour with temperature or strain. Have a change in another easily measured physical property with strain / temperature / fatigue state. Types of Fillers Today a wide range of different fillers are either widely used in commercial applications or are being developed, including: conventional carbon blacks more highly structured carbon blacks, carbon nanotubes, and silicas.

4 Fillers in Elastomers Used for over 100 years. Fillers improve mechanical properties such as strength, abrasion resistance and fatigue life. Fillers increase the modulus of the rubber. Some fillers such as carbon black are conductive. Most unfilled rubbers are insulators. Filled rubber Unfilled rubber Silica and Carbon Black CONTENTS SILICA CARBON BLACK Structure Polarity Affinity with Polymer Filler-Filler Interaction Polar Low High Non-Polar High Low Polymer-Filler Bonding Chemical Bonding (with Silane Coupling Agent) Physical Relation

5 Dynamic Behaviour Fletcher-Gent Effect Dynamic Behaviour

6 Properties of Typical Carbon Blacks 300nm MT carbon black particles: spherical shape N 990 HAF Filler 300nm N220 N nm Structure and surface area of fillers 1000 Printex OAN/cc/100gm MT N-330 N STSA/m 2 /gm

7 Size and Structure of the Two Carbon Blacks Used in this Study N330 Printex Mean primary aggregate size (nm) Mean carbon black particle (nm) Surface area/nsaa/m2/gm Oil absorption number/dbpab (cc/100 g) a NSA: (ASTM D ): nitrogen surface area. b DBPA: (ASTM 2414): dibutyl phthalate absorption. Percolation Threshold

8 Resistivity of filled elastomers Conducting fillers can make the rubber conduct above the percolation threshold. Filled rubbers can be used either as a conductor or an insulator depending on the application. In the conducting zone rubber can be used for antistatic applications..m Log ρ / Ω 2 1 Insulating Increasing surface area and structure Carbon black loading/phr Decreasing surface area and structure Resistivity vs carbon black loading of SBR filled with N330 black Experimental method to measure resistivity How does the resistivity change with strain? Four point contact method. Above percolation threshold filled elastomers exhibit Ohmic behaviour. Resistivity was used to compare the behaviour of different filled elastomer under strain. V A V / V I / ma

9 Behaviour of filled elastomers under strain 100 Ω Resistivity /.m ρ Ω.m Changes in resistivity of HAF(N330) filled rubber under strain* λ Extension ratio *K. Yamaguchi, J. J. C. Busfield, A. G. Thomas. Electrical and mechanical behaviour of filled elastomer. I. The effect of strain. J Polymer Science: Part B: Polymer Physics 2003, 41, Breakdown of filler agglomerate structure under strain HAF Fillers 300nm 3.0 loading unloading.m Extension ratio Log Resistivity / Ω Changes in electrical resistivity under strain for 50 phr N220 black Irreversible behaviour under strain

10 1 st Cycle N330 black at a filler volume fraction of 21%. log (ρ /Ω.m) Extension ratio Stress/MPa Unfilled Extension ratio Electrical Mechanical Printex: A high surface area and structure filler.m Log Resistivity / Ω Extension ratio 1st loading 1st unloading 2nd loading 2nd unloading Resistivity vs extension ratio of rubber filled with 10 phr of high surface area structure filler under tensile strain Electron Microscopy Images SEM / TEM

11 Cyclic Behaviour of Printex Ω Log Resistivity /.m cycle 1 loading cycle 1 unloading cycle 10 loading cycle 10 unloading cycle 20 loading cycle 20 unloading Extension ratio Stress/MPa 1.4 cycle cycle cycle Extension ratio Resistivity vs extension ratio of rubber filled with 10 phr of high surface area structure filler under tensile strain Tensile stress strain curve for NR filled with 10 phr of filler Carbon Nanotubes

12 But in reality they do not yet deliver Composites Sci. &Tech. (2007) Why? Poor stress transfer efficiency in MWNT. Due to telescopic sliding.

13 Poor Dispersion & Adhesion Entanglements and bundles make it difficult to reach an uniform dispersion. Good bonding between the matrix and the CNT is required to take advantage of the nanotube s strength. and no orientation Regev, Loos and Koning, TU-Eindhoven

14 Percolation Conductivity: 10 8 S.m -1 Percolation threshold below 1 wt% CNTs Strain sensor? PU / MWNT Suggests the mechanism is tunneling. ω = ω 0(1 + ε ) Carbon nanotubes Surrounding polymer Tunnelling distance

15 Temperature sensor? Resistivity [Ohm.m] PP / MWNT (1.5-5 wt.%) MWNTs/CO 1.8% Dr=1 165 MWNTs/CO 2.25%% Dr=1 165 MWNTs/CO 3% Dr=1 165 MWNTs/CO 5% Dr= MWNT Time [Seconds] Conclusions Typical elastomer filled with MT (N990) and HAF (N220) carbon blacks used widely in industry show temporary, irreversible changes in filler agglomerate structure under strain. As witnessed by temporary irreversible changes in conductivity behaviour under strain. Very high surface area and structure fillers (printex and carbon nanotubes) show reversible changes in filler agglomerate structure under strain. As shown by a reversible change in resistivity under strain. This reversible change in resistivity under strain has many potential industrial applications.