RTV silicone rubber composites reinforced with carbon nanotubes,titanium-di-oxide and their hybrid:Mechanical and piezoelectric actuation performance

Vineet Kumr,Anuj Kumr,Sung Soo Hn,Sng-Shin Prk,＊

a School of Mechanical Engineering,Yeungnam University,South Korea

b School of Chemical Engineering,Yeungnam University,South Korea

Keywords:Multi-walled carbon nanotubes Titanium dioxide Hybrid Mechanical properties Piezoeletric actuation

ABSTRACT

1.Introduction

The next generation rubber composites require the use of nanofillers to achieve high performance in flexible devices[1].In this case,RTV silicone rubber has been considered as a simplistic solution to achieve high performance of the composites due to its ease of processing,high flexibility,lower viscosity and high mechanical and electrical properties[2].In addition,the nature of nanofiller used in the preparation of composites is also an important factor in determining the overall properties of the composites[3].These nanofiller are carbon nanotubes(CNT)[4],graphene[5],carbon black[6]or titanium dioxide[7].The composites based on RTV silicone rubber and these nanofillers have been used in wide range of applications such as coatings[8],medical[9],actuation[10],energy harvesting[11],and strain sensing[12].

The properties of polymer composites depends upon various parameters such as(a)the type of nanofiller,(b)type of polymer,(c)dispersion of nanofiller in rubber matrix(d)polymer-filler compatibility and so on[13–30].The type of nanofiller involves as nature(e.g.CNT or TiO2)[13,14],shape(e.g.sheet or spherical or cube)[15,16],size(thickness,lateral dimension in＜100nm)[17,18],surface area(in different range)[19]and single hybrid nanofiller[20].Furthermore,type of polymer involves thermoplastic[21],thermoset[22],and elastomer matrix[23].Among them,the elastomer matrix comprises of diene rubber[24],or natural rubber[25],or different types of silicone rubber[26].Moreover,the homogenous dispersion of nanofiller in rubber matrix and polymer-filler compatibility favor the enhancement in properties of rubber composites[27]due to an efficient stress transfer in the composites[28].

The use of nanofillers in hybrid form promotes high mechanical and electrical properties due to their synergistic effect on the properties of the nanocomposites[31].This synergistic effect among interacting single fillers promote the combined characteristics of them into rubber composite with high performance without increasing filler loading in rubber matrix[32].There are few studies on synergistic effect of nanofillers in RTV silicone rubber.Kumar et al.demonstrated a synergistic effect of CNT-nanographite and CNT-carbon black(CB)hybrids in RTV siliconerubber-based composites[33].In this study,the elastic modulus(4.65MPa)for CNT/CB hybrid at15phr was higher than that of CB/nanographite hybrid(3.13MPa)and CNT or CB or nanographite as the only filler[33].These hybrid fillers promoted the higher mechanical and electrical properires as compared to the single filler species due to synergistic effect[33,34].There are studies on use of CNT hybrid with graphene nanoplatelets,expanded graphite in styrene butadiene rubber[35].These studies further deepens importance of synergistic effect in CNT based hybrid filler in rubber matrix.Further,there are some reports on showing the use of TiO2in polymer matrix for various practical applications,such as photocatalyst[36]or anti corrosion[37].However,the studies on the use of TiO2and its hybrid with CNT in RTV silicone rubber has not been established so far.

In the present work,rubber-based composites were prepared by using CNT,CNT-TiO2hybrid,and TiO2in RTV silicone rubber matrix.Here,the synergistic effect of CNT-TiO2hybrid was evaluated on RTV rubber nanocomposites in comparison with CNT or TiO2as single nanofiller.Before analysing of RTV rubber nanocomposites,the purity and BET surface area were measured to analyse their role in affecting properties of the rubber nanocomposites.For RTV rubber nanocomposites,the compressive and tensile mechanical proeprties were studied to analyse the effect of nanofiller reinforcement in RTV silicone rubber matrix.Furthermore,the actuator displacement analysis was also perfomed for their piezoelectric actuator applications.

2.Experimental

2.1.Materials

RTV silicone rubber was purchased from KE441,Shin-Etsu to prepare rubber-based composites in this study.The curing in present work was performed by using hardener(CAT-RM,Shin Etsu).For RTV rubber composites,the nanofillers as multi-walled carbon nanotubes(MWCNTs)(CM-100,Hanwha Nanotech)and TiO2(Alfa Aeser)were used.In this paper,MWCNTs is further represented as CNT.

2.2.Preparation of composites

For the preparation of rubber-based nanocomposite,various nanofiller powders(e.g.CNT,CNT-TiO2,and TiO2)were added to RTV silicone rubber solution and mixed for10min to obtain homogenous nanocomposite solution.After this step,2phr hardener was added and mixed for1min for desired semi-solid RTV silicone rubber nanocomposite solutions.Before pouring it into moulds,moulds were sprayed by mould releasing agent and then the obtained semi-solid nanocomposite solutions were poured into the sprayed cylindrical and rectangular moulds to prepare specimen.Finally,moulds were manually pressed and kept for24at room temperature.The samples were designated as RTV-SR/CNT,RTV-SR/CNT-TiO2,and RTVSR/TiO2nanocomposites for different nanofiller contents(see Table1).

Fig.1.Schematic illustration of the preparation of RTV silicone rubber nanocomposite reinforced with CNT(a),CNT-TiO2hybrid(b),and TiO2(c)nanofillers.

Fig.2.SEM and corresponding EDX spectra of the MWCNTs(a)and TiO2(b)nanomaterials.

Fig.3.XRD patterns(a and b)and adsorption-desorption isotherms(c and d)of CNT and TiO2.

Fig.4.SEM image showing filler dispersion of CNT-TiO2(5phr)in RTV-SR matrix and their corresponding EDX spectra.Arrows in red color indicate CNT and yellow color indicate TiO2.

The schematic illustration of the preparation of RTV-SR/CNT-TiO2nanocomposite is shown in Fig.1.

2.3.Characterization techniques

The microstructure and purity was analysed by using field emission scanning electron microscopy(FESEM,S-4100,Hitachi)and energy dispersive x-ray spectrometer(EDX),respectively.X-ay diffraction(D8 Advance,Bruker)was used to study crystalline structure of nanofillers and rubber nanocomposites at a scan rate of10°min-1.BET adsorption isotherms were analysed by using BELSORP-max(BEL,Japan Inc.)to estimate the surface area of the nanofillers used at77K.The compressive and tensile mechanical properties were evaluated by using universal testing machine(UTM,Lloyd instruments)at the strain rate of2mm/min for compressive strain and100mm/min for tensile strain.For compressive properties,the size dimensions of cylindrical samples were 10mm in thickness and20mm in diameter.For tensile properties,the shape of the tensile specimen was dumble shaped with2mm thickness.Shore A hardness was analysied using Westop durometer.The actuation displacement was measured using laser sensor(Opto NCDT1302).For,this,the electrode was0.1mm thick and elastomer slab was1.0mm thick based on3M silicone rubber.The used electrode was based on2phr nanofiller(CNT,TiO2and CNT-TiO2hybrid).

3.Result and discussions

3.1.Microstructure and purity of the filler particles

The microstructure of CNT and TiO2were analysed using SEM,as shown in Fig.2.The SEM image of CNT reveals a1-dimensional(1-D)tube-like morphology.The diameter,lateral size,and aspect ratio of the CNT were measured using SEM image.It was found that the CNT diameter was approx.15–17nm and lateral dimension was in submicron range that lead to a higher aspect ratio of60–75.Further,the particle size of the TiO2was also estimated from SEM image and the particle size of TiO2was around17–21nm.Therefore,both fillers(CNT and TiO2)possess atleast one dimension below100nm and are supposed to very effective as nanofillers in improving the properties of RTV-SR nanocomposites with these characteristics.

The purity of the fillers plays an important role in determining the properties of the composites[38].Therefore,the purity of both nanofillers were measured using EDX(FESEM,S-4100,Hitachi)and both nanofillers were found be extremely pure with＞96%for CNT and＞98%for TiO2.

3.2.X-ray diffraction and adsorption-desorption isotherms

Fig.5.Schematic illustration of the mechanism for an effective load transfer within RTV-SR/CNT-TiO2nanocomposites under(a)compressive strain and(b)tensile strain.

Fig.6.Compressive mechanical properties:Stress-strain curves for(a)CNT,(b)CNT-TiO2,and(c)TiO2,in the rubber composite.Compressive modulus against filler content(d)and reinforcing factor against filler loading(e)for CNT,CNT-TiO2,and TiO2.

The XRD results were obained for CNT and TiO2,and is presented in Fig.3(a and b).In Fig.3a,CNT shows a distinct peak(002)at around 2θ=25.3°which is characteristic feature of carbon nanomaterials like CNT or graphite.Similarly,there are several characteristic2θ peaks at around25.3°,37.8°,48.1°,55.1°,and62.5°corresponding to(101),(004),(200),(211),and(204)of TiO2[39],which correspond to its highly crystalline tetragonal structure(see Fig.3b).Moreover,XRD patterns show the absence of amorphous phases in both CNT and TiO2nanomaterials.

The adsorption-desorption isotherms were employed to study BET surface area of CNT and TiO2.A high BET surface area was measured for CNT(300m2/g),whereas low BET surface area for TiO2(165m2/g)was determined.Here,adsorption-desorption isotherms curves show the relationship between volume of gas adsorted and the partial pressure(Fig.3(c and d))and from these adsorption-desorption isotherms,it was observed that the volume of adsorbed gas was higher for CNT compared to TiO2as nanofiller.

3.3.Dispersion of CNT and TiO2in RTV-SR matrix

Homogenous dispersion of CNT and TiO2in RTV-SR matrix serves an important role in determining the properties of the rubber composites.Here,SEM microscopy was employed to determine the filler dispersion in the rubber composite(see Fig.4).The SEM images shows uniform dispersion of CNT and TiO2particles in the RTV-SR composite.Good wetting of CNT and TiO2particles by the silicone rubber in composite can also be observed clearly.Good dispersion and high polymer-filler interaction also justify the higher mechanical,and actuation properties,as described in further sections.Further,EDX was employed to determine elemental composition of the CNT-TiO2hybrid in RTV-SR composite.The EDX justifies presence of elements like C(for CNT),Si(Silicone rubber),Ti and O2(for TiO2particles).

3.4.Mechanical properties

The mechanical performance of the nanocomposite depends on the homogenous dispersion of nanofillers and efficient load-transfer from the matrix to the nanofillers within the polymeric nanocomposites[40]However,the agglomeration of the nanofillers in polymer matrix is a major challenge in improving mechanical properties of the nanocomposite[41,42].In this study,CNT or TiO2NPs demonstrated good load-bearing capability within RTV silicone rubber-based nanocomposites.Furthermore,CNT-TiO2hybrid exhibited synergistic effect of load-transfer mechanism[43]and resulted an enhancement of mechanical properties of RTV-SR/CNT-TiO2nanocomposite.A schematic illustration of the mechanism for effective load-transfer within RTV-SR/CNT-TiO2nanocomposite under compressive and tensile strains is shown in Fig.5.

3.5.Compressive analysis

The compressive mechanical properties were studied in terms of compressive stress-strain curves(max35%)and are presented in Fig.6a(CNT),Fig.6b(CNT-TiO2hybrid),and Fig.6c(TiO2).The compressive stress-strain curves shows that the stress increased with increasing strain.However,the fall in compressive stress at3phr is possibly due more aggregation,whereas much increase at4phr is promisingly due to the transition in filler networking from specimen at3phr.At4phr,the higher amount of TiO2forms synergism and filler percolation with CNT filler particles leading to increase in compressive modulus.This behavior can also be witnessed in Fig.6d and e where the compressive modulus and reinforcing factor at3phr of CNT-TiO2hybrid is lower than at4phr and higher loading.In Fig.6a-c,at10% strain and5phr content,the compressive stress was0.29MPa for CNT,0.16MPa for CNT-TiO2hybrid,and0.1MPa for TiO2.At35%,this compressive stress was increased to1.53MPa for CNT,0.84MPa for CNT-TiO2hybrid,and 0.47MPa for TiO2.This increment could be due to an improved fillerpolymer interaction in the composite[44,45].In addition,the compressive stress increased with increasing filler volume fraction in the rubber composite.After the addition of filler particles,the increase in stress was possibly due to physical interactions induced by filler shape and content,and filler-induced stiffness in the composite[46].These filler particles interact and form cluster networks as well as facilitate interactions with polymeric chains and form temporary bonding among themselves[44,46].Also,the evaluation of clusters of filler particles can be observed and co-related with the mechanical behavior of the rubber composites[47].

Fig.7.Tensile mechanical properties:Stress-strain curves for(a)CNT,(b)CNT-TiO2,and(c)TiO2in the rubber composites.Tensile strength against filler content(d)and fracture strain as a function of filler content(e).

The compressive modulus(see Fig.6d)and reinforcing factor(see Fig.6e)were studied by increasing filler content in the composite.It was found that with an increase of filler content in composite,both compressive modulus and reinforcing factor were increased.The compressive modulus of unfilled rubber composite was measured as 2.18MPa,whereas it increased to6.8MPa for CNT,3.95MPa for CNTTiO2hybrid,and2.44MPa for TiO2at5phr loading.Similarly,the reinforcing factor was measured as1for unfilled rubber composite,whereas it increased to3.1for CNT,1.8for CNT-TiO2hybrid,and1.12 for TiO2at5phr loading.The increase in compressive modulus and reinforcing factor with the addition of filler volume fraction is due to reinforcing effect of the filler particles and filler-polymer interacting forces in composite[48].

3.6.Tensile analysis

For tensile stress-strain curves are shown in Fig.7a(CNT),Fig.7b(CNT-TiO2hybrid),and Fig.7c(TiO2).It can be noticed from the stressstrain behavior that the stress increased with increasing tensile strain for all rubber specimen tested.It was found that at10% strain and5phr filler,the stress of rubber composite specimen was measured as0.19MPa for CNT,0.14MPa for CNT-TiO2hybrid,and0.06MPa for TiO2.At fracture strain,the stress was increased to1.37MPa for CNT,1.33MPa for CNT-TiO2hybrid,and0.61MPa for TiO2.In addition,the tensile stress was also increased with increasing filler content in composites.For unfilled rubber specimen,the tensile stress was0.49MPa at80%strain.For filled rubber specimen,it increased to1.29MPa for CNT,0.94MPa for CNT-TiO2hybrid,and0.42MPa for TiO2at5phr loading.This is possibly due to an increase in filer content followed by the interactions and bonding between filler and polymeric chains in rubber matrix that cause resistance while tensile strain[49].

The tensile strength(see Fig.7d)and Fracture strain(see Fig.7e)were studied with increasing filler content in the rubber composites It was found that the tensile strength and fracture strain of the rubber composites were increased with increasing filler content.Similarly,the fracture strain was measured as112% for unfilled specimen and it decreased to90%for CNT,and increased to126%for CNT-TiO2hybrid,and120%for TiO2at5phr loading.This increament in tensile strength and fracture strain is possibly due to an efficient filler networking[20],and induced toughness by the filler particles in the rubber matrix[50].From mechanical analysis,it is concluded that CNT emerges as promising reinforcing filler in the rubber composites due to their significant contribution to achieve higher mechanical proeprties.It could be due to high aspect ratio(60–75)of CNT that forms filler percolative networks even at lower CNT content(approx.2phr)in rubber matrix[51].

Fig.8.Hardness against filler content in rubber composites for(a)CNT,(b)CNT-TiO2,and(c)TiO2.

Fig.9.Actuation displacement against applied voltage for(a)CNT,(b)CNT-TiO2,and(c)TiO2.

3.7.Hardness of rubber composites

The hardness is a key factor in enhancing mechanical properties of the composites.The rubber composite with high hardness induces high mechanical stiffness in the composite.Therefore,the hardness was studied and presented in Fig.8a-c.

It was found that the hardness was increased with increasing filler content.The hardness for unfilled composite was measured as21and it increased to48for CNT,37for CNT-TiO2hybrid,and29for TiO2at5phr loading.Here,it can be concluded that the increase of filler content increases the hardness at all loadings for both fillers and their hybrids.The increase in hardness was highest for CNT-based rubber composites.It was due to higher reinforcing ability of filler that induces stiffness in composite at lower filler loadings[52,53].Another possible reason for higher hardness of CNT reinforced rubber composites was due to the formation of filler percolation threshold at small CNT content(approx.2phr)in the composite[51].

3.8.Piezoelectric actuation

The actuator needs continuous voltage input to output the mechanical motions.The description of actuator component was described in the characterization section above.The electrode was fabricated and sandwiched with the elastomer slab made of silicone rubber.In the present study,the inverse piezoelectric effect is elaborated.The voltage is supplied externally to the electrode and the mechanical displacement of the device is recorded.The actuation displacement vs input external voltage were studied and presented in Fig.9a-c for different electrode materials,such as2phr CNT,TiO2and their hybrid.

It was found that the actuation displacement increased as the voltage input amount increased.The actuation displacement was increased to 2mm for CNT,0.5mm for TiO2and1.6mm for their hybrid at10kV.Here,it can be concluded that the best actuation displacement was obtained by the CNT electrode based composites.It was possibly due to high electrical conductivity of CNT based composites that accelerates the transfer the input voltage in the electrode faster than that of TiO2and their hybrid.

4.Conclusion

In present study,the composites based on RTV silicone rubber as polymer matrix and CNT,TiO2and their hybrid as nanofiller was prepared using solution casting method.RTV silicone rubber was used due to its easy processing,easy curing and high mechanical and electrical proeprties.CNT,TiO2and their hybrid were used due to their high surface area,small particle size and high aspect ratio.BET surface area was 300m2/g for CNT and165m2/g for TiO2,whereas the particle size of CNT was12–15nm and TiO2was15–18nm.Further,the purity as confirmed by FESEM/EDX was measured as＞96% purity for CNT and＞98% purity for TiO2.Also,XRD patterns confirm high crystalline structure and absence of amorphous phases in nanofillers.The compressive modulus of the RTV-SR composites with CNT or CNT/TiO2hybrid was higher than that of TiO2.The compressive modulus was 2.18MPa for unfilled composite and increased to6.8MPa for CNT,3.95MPa for CNT-TiO2hybrid,and2.44MPa for TiO2at5phr loading.Similarly the tensile strength was0.54MPa for unfilled composite was and increased to1.37MPa for CNT,1.33MPa for CNT-TiO2hybrid,and 0.61MPa for TiO2at5phr loading.The fracture strain was90%for CNT,126%for CNT-TiO2hybrid,and120%for TiO2for unfilled composite at 5phr loading.The hardness was also determined as21for unfilled composite and it increased to48for CNT,37for CNT-TiO2hybrid,and29 for TiO2at5phr loading.Moreover,the piezoelectric actuation at10kV was2mm for CNT,0.5mm for TiO2and1.6mm for their hybrid at2phr loading in electrode of actuation set-up.

Declaration of competing interest

No conflicting interest is declared.

Acknowledgements

This study was supported by the Basic Science Research Program through partly the National Research Foundation of Korea(NRF)and BK21 PLUS4funded by the Ministry of Education(2017R1D1A3B03031732).Also,This paper was partly supported by Korea Institute for Advancement of Technology(KIAT)grant funded by the Korea Government(MOTIE)(P0002092,The Competency Development Program for Industry Specialist).