Junping Song ,Lianxiang Ma ,Yan He ,*,Haiquan Yan ,Zan Wu ,Wei Li 2 Qingdao University of Science and Technology,Qingdao 26606,China

2 Department of Energy Engineering,Zhejiang University,Hangzhou 310027,China

3 Department of Energy Sciences,Lund University,Lund SE-22100,Sweden

Keywords:Graphite Natural rubber Thermal conductivity Tire Polyacrylate

A B S T R A C T The rubber composites with good thermal conductivity contribute to heat dissipation of tires.Graphite filled natural rubber composites were developed in this study to provide good thermal conductivity.Graphite was coated with polyacrylate polymerized by monomers including methyl methacrylate,n-butyl acrylate and acrylic acid.The ratios between a filler and acrylate polymerization emulsion and those between monomers were varied.Eight types of surface modification formulas were experimentally investigated.Modification formula can affect coating results and composite properties greatly.The best coating type was achieved by a ratio of 1:1 between methyl methacrylate and n-butyl acrylate.The coating of graphite was thermally stable in a running tire.Filled with modified graphite,the tire thermal conductivity reached up to 0.517–0.569 W·m?1·K?1.In addition,the mechanical performance was improved with increased crosslink density,extended scorch time and short vulcanization time.

1.Introduction

When a tire is in use,dynamic deformations caused by loss of hysteresis of vulcanized rubber composites lead to thermal energy production.In addition,the internal friction at the molecular level between filler–filler, filler–rubber and rubber–rubber networks,and the friction between tire and ground are the other two ways for thermal energy generation.Part of the energy is dissipated into the circumference;however,most of the energy causes heat buildup in the tire,accompanied by increased temperature.High temperature will result in degradation of mechanical performance,acceleration of fatigue damage,oxidation and heat-degradation of the rubber,and most seriously,puncture of the tire.Therefore,in order to enhance the tire durability,much effort has been made to lower the temperature of a running tire.

There are three ways to dissipate the thermal energy.One is to enhance the heat exchange on the tire surface.The second is to decrease the heat generation in the tire.And the third way is to transfer the generated heat from inside to outside.Much research works on thermal conductivity of rubber composites have been done[1–8],how ever,there are rather few reports dealing with the applications in tires[9–11].Wang et al.[9]indicated that nano-zinc oxide could not only reinforce ethylene propylene diene monomer(EPDM)but also improve thermal conductivity significantly,and the composites could serve in dynamic conditions with longer expected service life.Das et al.[10]achieved a good dispersion of multi-walled carbon nanotubes in a rubber blend by a novel mixing approach and studied properties of the composite.The theoretically predicted thermal conductivities of isolated tubes could not be transferred into rubber-based composites in practice.The large surface area of the carbon nanotubes led to a strong phonon boundary scattering,which resulted in poor thermal conductivity of composites.Li et al.[11]studied the thermal conductivity of emulsion polymerized styrene–butadiene rubber(ESBR)vulcanizate filled with alumina,zinc oxide,carbon nanotubes and silicon carbide.The result showed that the thermal conductivity of ESBR vulcanizate filled with aluminaor zinc oxide increased nearly linearly with increasing loading when the filler loading exceeded 20 phr,where phr is defined as the grams of filler added into 100 g of matrix;The ESBR vulcanizate filled with carbon nanotubes had the highest thermal conductivity at a given filler loading in comparison with composites filled with alumina or zinc oxide.At a given loading of 100 phr,the ESBR vulcanizations filled with two different silicon carbide particle sizes of 1–3 μm and 5–11 μm at a mass ratio of 1:1 had the highest thermal conductivity in comparison with the vulcanizations filled with only one kind of silicon carbide.Natural rubber is widely used in tire industry.Enormous studies have been focused on the mechanical properties of natural rubber based compounds.How ever,few studies address thermal conductivity.The present authors[12–14]studied the thermal conductivity of carbon black filled natural rubber composites;how ever,thermal conductivity enhancement of the reinforcing filler was limited.

http://dx.doi.org/10.1016/j.cjche.2014.05.022

1004-9541/?2015 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Graphite,as one of the main forms of the sixth element in the Periodic Table of Elements,is abundantly available in nature.The π orbital distributed over the entire graphene sheet makes it thermally and electrically conductive.The thermal conductivity of graphite is about 209.34 W·m?1·K?1,which is three orders of magnitude of that of pure rubber.Therefore,if graphite filler is used in natural rubber with good dispersion,it surely can greatly enhance the thermal conductivity of the rubber.Wang[15]concluded that for tread rubber of radial tires,graphite could be used instead of silica,and it was possible to maintain a certain level of main properties of the compounds and vulcanizate.How ever,there are no detailed reports about graphite filled tire rubber for thermal conductivity improvement up to now.

In addition to the high thermal conductivity of graphite,the graphene sheets can easily slide with each other due to the weak van der Waals forces between the graphite layers.Therefore,the graphite has a soft and lubricating nature,which attributes to the relatively poor reinforcing properties of graphite for polymer.Li et al.[16],Zhao[17]and Tang et al.[18]pointed out that the graphite as a filler in conjunction with carbon black incorporated in rubber could improve the mechanical properties.The thermal conductivity of the composites would be better than that of the composites using graphite as a filler alone.Thus,graphite in conjunction with kinds of carbon black can improve the thermal conductivity of natural rubber on the basis of maintaining a certain level of mechanical properties.

Graphite exists as a layered material in its bulk state.Because of the high surface energy,graphite particles are easy to come together to form aggregates.As a filler,in order to utilize graphite efficiently,it is necessary to separate and disperse its layers in a polymeric matrix.Broadly,two methods are adopted.One is to form graphite intercalation compounds(GIC)[19]by intercalating various atoms,molecules,metal complexes and salts between the expanded graphene sheets.Another is surface treatment of the graphite[16,20–22].

Propylene ester copolymer emulsion is an emulsion polymerization product of acrylate or methyl propylene esters with other vinyl monomers.At present,methacrylic acid ester copolymer emulsion,vinyl acetate/methyl acrylate copolymer emulsion,and styrene/methyl acrylate copolymer emulsion are widely used.Because the acrylic ester contains functional groups of ester,carboxyl and hydroxyl,it has strong polarity,and thus has good adhesive properties with various substances.In addition,the acrylic emulsion bears good weather resistance,heat resistance and oil resistance,and it is non-toxic,easily synthetic,of non-environmental pollution and so on,and thus it is widely used[23–25].If graphite particles are coated with polyacrylate in natural rubber,aggregation among the particles will be hindered for the reduction in potential energy of the surface.Therefore,the filler can achieve good dispersion in a rubber matrix,and then enhance the thermal conductivity of rubber.

The present study reported graphite filled natural rubber composites with good thermal conductivity.The graphite was coated with polyacrylate by the method of emulsion polymerization to achieve good dispersion in the rubber matrix and eight schemes of graphite coating were investigated.In order to obtain certain mechanical properties,carbon black N660 and acetylene black were incorporated as fillers together with graphite and the acetylene black was also good for thermal conductivity enhancement.The curing characteristics,thermal conductivity and mechanical properties of the rubber composites were discussed in detail.

2.Experimental

2.1.Materials

Natural rubber(GB1NR,China)was supplied by Hainan Natural Rubber Co.,Ltd.(Hainan,China).The graphite(industrial-grade)used in the study was microcrystalline graphite with an average diameter of 15 μm.Carbon black N660 with a specific surface area of 33,000 m2·kg?1and acetylene black with a specific surface area of 78,000 m2·kg?1were used.Emulsifiers OP-10 and sodium dodecyl sulfate(SDS,C12H25–OSO3Na),and monomers methyl methacrylate(MMA,C5H8O2),n-butyl acrylate(BA,C7H12O2),diethylene glycol diacrylate(DEGDA,C10H14O5),acrylic acid(AA,C3H4O2)and initiator potassium persulfate(K2S2O8)were analytically pure.

2.2.Surface modification

Polymerized acrylate monomers were used to coat graphite in order to improve its dispersion in the matrix and ensure good interface between the filler and matrix.The choice of monomers was very important for properties of polymer emulsion,thereby affecting the coating effect.MMA was used as hard monomer,BA w as used as soft monomer,and AA was used as function monomer.According to the ratios between hard and soft monomers,and between the filler and acrylate polymerization emulsion,eight kinds of surface modification schemes are listed in Table 1.

Table 1Mass fraction of the components for the graphite modification

The surface modification of graphite was carried out as follow s:(1)water and emulsifiers OP-10 and SDS were mixed in a fl ask with four necks;(2)monomers were added to the mixture and emulsified for 20 min;(3)graphite was added in the mixture to emulsify for 40 min accompanied by very slowly stirring;(4)the mixture was heated up to 60°C followed by stirring for 20 min;(5)the emulsion was heated up to 80°C,then part of the initiator was put in,and the temperature was maintained for approximately 2 h for the reaction in the mixture;(6)the remaining amount of the initiator was added to continue the reaction for another 2 h;and(7)the mixture was cooled down followed by filtration and drying.

2.3.Sample preparation

The weight fractions of the components for rubber composites are shown in Table 2.

Table 2Sample composition

Both modified graphite and unmodified graphite were respectively incorporated in NR.Fillers,rubber and other addictives were mixed on an S(X)160A two-roll open rubber mixing machine.Vulcanizing was accomplished on a HS-100T-FTMO-2PT mold cure machine.The vulcanization temperature was 143°C,and the pressure was 10 MPa.The thickness of vulcanizate was about 2 mm.

2.4.Characterization

The thermal diffusivity and specific heat of samples were tested by the LFA447 nanoflash flash diffusivity instrument made by Netzsch Company(Germany),and the thermal conductivity can be achieved by automatic calculation,

Thermal diffusivity was determined by subjecting the front face of the small disk-shaped test sample to a short pulse of light energy while recording the resulting temperature rise at the back face of the sample.The temperature of the back face was measured with an IR detector.The output of the temperature detector was amplified and adjusted for the initial,ambient conditions so that the recorded temperature rise curve was the change in the sample temperature resulting from the firing of the flash lamp.

The temperature of the sample back face can be expressed mathematically as a function of several variables that were grouped into dimensionless parameters.These variables included sample geometry,thermal diffusivity,and the heat loss from the sample.The back face temperature rise was measured during the test.Thermal diffusivity was determined by comparing the measured data with the Cow an analysis model[26,27].The Cow an analysis model was a theoretical relationship between the dimensionless parameter

and the ratio R of two dimensionless temperatures obtained from the temperature rise curve,

Specific heat was tested by using a reference sample,which has the same size,thermal property and surface condition as the test sample,and know n specific heat.The reference sample used in this work was vulcanized rubber and the specific heat value was supplied by Netzsch Company.The specific heat of samples can be achieved by Eq.(4),

A microcomputer specific gravity material balance(GT-XB-320M,±0.001 g·cm?3)was used to measure the density of composites.

Before a test,samples were cleaned with alcohol,and left for the alcohol to evaporate,and then graphite was used to coat the surface evenly.

The temperature for measurement was varied from 30°C to 150°C.

An electrical tensile testing machine A1-7000M was used for measuring tensile properties and tear performance of the samples at a speed of 500 mm·min?1.The tensile test was performed according to GB/T 528-1998(China)and the tear performance was conducted according to GB/T 529-1999(China).

The morphology of graphite was visualized by JSM-6700F type of field emission scanning electron microscopy(SEM).

The quality variation of graphite with temperature was analyzed by a thermogravimetric analysis instrument TG209F1.In nitrogen atmosphere,the testing was conducted with a heating rate of 10 °C·min?1.Temperature changed from 0 °C to 750 °C.Before testing,each sample was washed follow ed by a drying process.

3.Results and Discussion

In the following tables and figures,0#denotes the composite filled with unmodified graphite,and 1#–8#denote the composites filled with 1#–8#modified graphite,respectively,as shown in Table 1.

3.1.Morphology of graphite

The morphology of modified and unmodified graphite is shown in Fig.1.It is observed that the apparent morphology of graphite before and after modification changes a lot.The surface of unmodified graphite is smooth and bright with clear edges of graphite layers,and the particles aggregate together.In contrast with that,the surface of modified graphite is coated with polyacrylate particles which distribute in the form of varying degrees of chains or network.Some of the particles are intercalated into graphite layers,leading to the expansion of the graphite layers.The edge of graphite layers is no longer clear and the aggregation of graphite lamellas is mitigated.

Even with the same ratio between graphite and acrylate polymerization emulsion,the modification results are different due to the different ratios between the monomers.As shown in Fig.1,the samples of 1#,3#,5#and 7#are the modification results of the graphite and monomers at a ratio of 10:1.The surface of 7#is almost fully coated with polyacrylate particles.Follow ed by the sample of 5#,the number of particles on its surface is less than that of 7#,whereas more than that of 1#and 3#,while the size of particles is larger than that of 7#and smaller than that of 1#and 3#.The same situation is found on the morphology of samples of 2#,4#,6#and 8#,which are the modification results of the graphite and monomers at a ratio of 5:1.The coating effect of 8#is the best,follow ed by 6#,2#and 4#.According to Table 1,the ratio between hard and soft monomers of 7#and 8#is about 1:1,so it can be inferred that when the ratio of hard and soft monomers is 1:1,the amount of resulting polyacrylate particles is the most.

When the ratio between the monomers is the same,while the ratio between the graphite and acrylate polymerization emulsion is different,the modification results are also different.It can be understood easily that the different amounts of monomers contribute to the different results.As is shown in Fig.1,the number of polyacrylate particles on the surface of 8#,6#,4#,and 2#is higher than that on the surface of 7#,5#,3#and 1#successively.Therefore,about the coating effect,8#is better than 7#,6#is better than 5#,4#is better than 3#,and 2#is better than 1#.

In general,the ratio between the graphite and acrylate polymerization emulsion and the ratio between the monomers will affect the coating results greatly.The ratio of 1:1 between hard and soft monomers results in the best coating effect.

3.2.Thermogravimetric analysis

The thermogravimetry(TG)curves(Fig.2)show that the mass change of modified and unmodified graphite is obviously different.For the sample 0#,as temperature increases,1.11%of mass is lost,which is probably caused by evaporation of the residual water in the compound.The mass loss for the modified graphite is much higher.It is surely that the substances on the surface of graphite make substantial contributions to the loss.

Fig.1.SEM pictures of modified and unmodified graphite(magnified 15,000 times).

Among the difference between modified and unmodified graphite,the mass change between different modified graphite filled composites is also different.The mass change of 2#is larger than that of 1#,that of 4#is higher than of 3#,that of 6#is larger than of 5#,and the mass change of 8#is greater than that of 7#.The different ratios between acrylate polymerization emulsion and graphite lead to the results.As is shown in Table 1,the ratio of the former is larger than that of the latter.The larger ratio is accompanied by more polyacrylate coating the graphite.This results in a greater mass change.

The initial thermal decomposition temperature of polyacrylate can also be seen in Fig.2.It can be inferred that the ratio between monomers is the main factor.As is depicted,the initial thermal decomposition temperature of 1#and 2#is about 380°C,3#and 4#about 370 °C,5#and 6#about 365 °C,and 7#and 8#about 361°C.One of the possible reasons is that the ratio between monomers affects the structure and properties of the polyacrylate.Besides,the higher the percentage of the soft monomer BA is,the longer the molecular chain of polyacrylate is.The longer molecular chain will reduce the strength and adhesion properties of the polyacrylate,and thus the initial thermal decomposition temperature decreases.According to Table 1,the percentage of BA in monomers of 1#and 2#is the low est,while 7#and 8#have the highest BA percentage.Therefore,the initial thermal decomposition temperature of 1#and 2#is the highest,while that of 7#and 8#is the low est.In addition,the ratio between monomers also affects the interaction and aggregation of polyacrylate particles.It will also have some effect on the initial decomposition temperature of the polymer.

3.3.Curing characteristics

The curing characteristics of modified and unmodified graphite filled NR composites are shown in Table 3.Compared to the sample incorporated with unmodified graphite,the curing characteristics of samples with modified graphite have the following changes of increasing the cross link density,which can be inferred from the value(MH?ML),increasing the highest moments,extending the scorch time,shortening the vulcanization time,and increasing the low est moments of 7#and 8#,while decreasing those of others.

3.4.Thermal conductivity

The uncertainty for the thermal diffusivity,specific heat,and thermal conductivity of all the samples is shown in Table 4.It is calculated according to the maximum standard deviation when the testing temperature changes from 30 °C to 150 °C,while the minimum standard deviation is 0.000.

The thermal conductivity of modified and unmodified graphite filled NR composites is shown in Fig.3.It is observed that the thermal conductivity of graphite filled rubber composites is three orders of magnitude below that of the isolated graphite.With modified graphite filling,the thermal conductivity increases.The maximum value of thermal conductivity reaches 0.569 W·m?1·K?1.The thermal conductivity of all the samples except 4#is not less than 0.54 W·m?1·K?1.Adopting polyacrylate coating graphite makes the thermal conductivity increase by 8.61%to 19.54%.

The reason for the great difference in thermal conductivity between the composites and isolated graphite is analyzed as follow s.The transport of heat in isolated graphite is dominated by phonons.When graphite is incorporated in a rubber matrix beyond the back-scattering effect,the further scattering effects,e.g.,inter facial boundary and defect scattering lead to a drastic reduction of thermal transport properties.In addition,the gap between graphite particles in the rubber matrix also hinders the transport of phonons through the graphite network.

Fig.2.TG curves of modified and unmodified graphite.

About the reason for the higher thermal conductivity of the composites with modified graphite than unmodified graphite,the possible explanation is that the graphite has good dispersion in rubber when coated with polyacrylate.This can build a bridge for phonon transport in the matrix;therefore,the thermal conductivity of modified graphite filled rubber compounds is larger than that of unmodified graphite filled rubber compounds.In addition,different modification formulas resulted in different dispersions of graphite in rubber,and thus the thermal conductivity of 1#–8#is different.The thermal conductivity increase in the modified graphite will be beneficial for the dissipation of the thermal energy in tires.

Table 3Curing characteristics of graphite filled NR composites

3.5.Mechanical performance

The mechanical performance of graphite filled NR composites is shown in Fig.4.Coating graphite with polyacrylate,the mechanical performance is improved.The tensile strength,tear strength,modulus at 100%elongation and modulus at 300%elongation are increased.The mechanical performance of 7#is the best,follow ed by 5#,3#and 8#.

The better mechanical performance is probably due to the good distribution of the filler in the rubber matrix and the improved interaction between graphite and the rubber matrix.Different modification formulas have great effect on the existence of corresponding graphitein the rubber;thereby the mechanical performance of the composites varies.The ratios of 1:1 between hard and soft monomers and 10:1 between graphite and acrylate polymerization emulsion achieve the best mechanical performance.

Table 4Uncertainty of the experimental data(p=0.99,n=6)

Fig.3.Thermal conductivity of graphite filled NR composites.

4.Conclusions

Fig.4.Mechanical properties of graphite filled NR composites.

Transferring heat from inside to outside is an important w ay for thermal dissipation of a tire.The aim of this study was to obtain one kind of rubber composite used in tires with good thermal conductivity.For this purpose,eight types of graphite coated with polyacrylate were conducted.It was observed that the modification formula for graphite had great effects on the coating results and the properties of the corresponding composites.The best coating effect was achieved by the ratio of about 1:1 between MMA and BA.TG curves of modified graphite proved that the polyacrylate coating on the surface of graphite was thermally stable if used in a running tire,and the ratio between monomers was the main factor for the initial thermal decomposition temperature of polyacrylate.Adopting polyacrylate coated graphite,thermal conductivity of the composites increased by 8.61%to 19.54%.The maximum value of thermal conductivity was 0.569 W·m?1·K?1and most of the thermal conductivity values were not less than 0.54 W·m?1·K?1.If the composites were used in tires,they surely can accelerate the cooling of tires.Coating graphite with polyacrylate improved the mechanical performance.The ratios of about 1:1 between MMA and BA and 10:1 between graphite and acrylate polymerization emulsion achieved the best mechanical performance.

Nomenclature

Cpspecific heat,J·kg?1·K?1

Cprspecific heat of reference sample,J·kg?1·K?1

Cpsspecific heat of sample,J·kg?1·K?1

d thickness of sample,m

MHmaximum torque,d N·m

MLminimum torque,d N·m

Mrmass of reference sample,kg

Msmass of sample,kg

Signalrsignal height of reference sample

Signalssignal height of sample

t50time at a half of the peak temperature in the temperature rise curve,s

Tc10 scorch time,s

Tc90 optimum cure time,s

α thermal diffusion,m2·s?1

λ thermal conductivity,W·m?1·K?1

ρ density,kg·m?3

θt50peak temperature rise at a time equal to t50

θ5（t50）peak temperature rise at a time equal to fi ve times t50