Analysis of the transversely isotropy, due to the production process, of carbon black filled SBR

Abstract

-Through tensile tests of uniaxial and planar type, was proceeded to the mechanical characterization of SBR rubber sheets filled with particles of carbon black. The analysis has allowed defining the elastic features, the storage modulus, the transversal contraction coefficient (both obtained from uniaxial testing) and the shear modulus (from the planar/pure shear tests). Due to the calendering process by which was obtained, the elastomer sheet it's shown transversely isotropic, with different mechanical behavior in the direction of processing with respect to the transverse direction. Therefore, it was essential to evaluate two different values for each storage modulus, depending that the sample was loaded in the direction of calendering or in that orthogonal to it. Index Terms-Transversal Isotropy, Hyperelasticity, SBR, Carbon Black, Calendering.

I. INTRODUCTION

The Styrene Butadiene Rubber (SBR) arises from copolymerization of Butadiene and Styrene. The latter, present to the extent of 20-25% of the copolymer, increases both strength and stiffness from SBR with respect to the butadiene rubber (BR) [ 1 ], [ 2 ]. Often, it is filled with carbon black, carbon charges of nanometric particles obtained from the combustion of hydrocarbons, the presence of which improves the mechanical properties and slows the aging. The SBR is the most usage rubber [ 3 ], [ 4 ], [ 5 ], [ 6 ], [ 7 ], [ 8 ], [ 9 ], being able to provide mechanical properties similar to those of natural rubber at a lower cost. The elastomer is used mainly where there is a need for high-friction surfaces, so that the 70% of the production is for tires tread [ 10 ], [ 11 ], [ 12 ], [ 13 ].

The mechanical characterization of a material consists in determining the current constitutive law between stress and strain [ 3 ], [ 14 ], [ 15 ], [ 16 ].

The characterization process is particularly critical for elastomers, due the large deformations that, for the same elastomer definition [ 17 ], [ 18 ], can tolerate without reaching the failure.

The aim of this experimental analysis is reached the mechanical characterization of styrene-butadiene rubber, improved in its quality by the introduction of fillers of carbon black (25%) in the blend [ 4 ], [ 19 ].

The analysis developed is focused on tracking of mathematical functions, which correlated stress and strain, and on evaluation, starting with these functions, of the storage modules assumed at the beginning of the rubber test.

Copyright c 2017 held by the authors.

Transversely isotropic behavior, caused by the rubber calendering process [ 20 ], has made more difficult the analysis. The phenomenon is well known in the literature, so that were already made attempts to characterize elastomers which have transverse isotropy [ 21 ], [ 22 ], [ 23 ].

Therefore, tests were performed at different loading directions, in order to assess the behavioral differences of the material between the direction of calendering and the direction orthogonal to it.

II. CONSTITUTIVE EQUATIONS

Most of engineering materials belongs to the category of simple materials, for which the stress tensor T is a function of the history of the position gradient F = r x [ 2 ]: T (x; t) = T (F (x; ); x0; t) 2 [0; t]

Among the simple materials, those elastic exhibit a stress state as a function of the single value of the deformation gradient:

T (x; t) = T (F (x; ); x0; t)

According to this constitutive relationship, stress does not depend on the deformation path (Cauchy elasticity), but it is not the same for the deformation work that it performs. The hyperelastic materials are, instead, conservative elastic materials (Green elasticity), i.e. for which, there being an elastic potential function ' = '(F), which can be derived from stress, the differential form of the deformation work is exact t = TI (1) (2) (3)

where TI := J TF t is the first tensor of Piola-Kirchhoff, with J = det F.

The simple assumption of linear elastic material is correct if you can overlook the time-dependent effects (such as sliding and relaxation) [ 2 ], [ 24 ], [ 25 ] and for small deformations. Circumstance, the latter, in which it is allowed the stressstrain relationship linearization. The linear stress-strain law is represented in classic form, tensorial, from equations Tij = P3h;k=1 CijhkEhk, which contain the 34 = 81 constants Cijhk of the elastic tensor di of order 4. Since the stress and strain symmetry of the tensors requires six independent components of the stress tensor as a function of the six independent components of the strain tensor, only 36 independent constants are needed to determine the linear elastic relationship. 2 x 3 2 (1) 3 66 y 77 66 (2) 77 := 666 xzy777 = 66 (3) 77 ;

6 (4) 7 64 xz 57 46 (5) 57 yz (6) 2 "_x 3

2 "_(1) 3 6 "_y 7 6 "_(2) 7 "_ := 6666 _"_xzy777 = 6666 ""__((34)) 77 7 7

7 64 _ xz 57 64 "_(5) 75 _ yz "_(6)

We can express the linear relationship in a matrix simplified using independent components vectors 2 x 3 2c11 6 y 7 6c21 6 7 666 xzy777 = 6666cc3411 64 xz 57 yz 64c51 c61

The linear elastic relationship is defined by 6 6 = 36 components of the C tensor of order 2 of elastic constants. Assuming the occurrence of a potential elastic (hyperelastic material), according to the Schwartz theorem, the elasticity tensors are symmetrical and the constants are reduced to 21 [ 3 ].

They are called transversely isotropic those materials, which have an isotropic response in the plane orthogonal to an axis, said axis of transverse symmetry. By choosing a base having the x-axis parallel to symmetry transverse

In this case, the elastic response is described by 5 independent parameters [ 26 ].

The constitutive equation can be formulated in terms of engineering constants [ 3 ], inverting the stress-strain relation 2 "x 3 2 x 3 6 "y 7 6 y 7 6666 "xzz7777 = C 1 66 z 77

66 xy77 64 xz57 64 xz 57 yz yz and

The elements on the main diagonal are the modules of normal deformability 1=Ei and tangential 1=Gij ; those that appear outside of the same diagonal such as the modules of transverse deformability vji=Ei. The symmetry of the previous matrix adds the condition vyx=Ex = vxy=Ey, which reduces to 5 the independent components. In the case of transversely isotropic material plates, which present the x or y axis of transverse symmetry, assuming the status of plane stress z = xz = yz = 0 and writing separately the equations: "z = x + ( z

vyz y) vyx Ex 1

Ey xz = 0 yz = 0

the constitutive relation can be narrowed to only three stress and three strain components in the x and y directions, becoming [ 27 ], [ 3 ]:

2 2 x 3 Ex 4 y 5 = 6 1 vyvxxyEvyyx

4 1 vxyvyx xy 0

vxyEx 1 vxyvyx

Ey 1 vxyvyx 0 where there is a symmetry condition vyx=Ex = vxy=Ey, whose effect is that only 4 of the 5 elastic constants appearing in the above matrix are independent.

III. PURE SHEAR IN PLANAR TEST

A valid constitutive model must properly describe the material behavior for each stress state. A single test as a simple uniaxial tensile test does not allow the construction of a reliable model in every situation. It then requires the programming of tests set that provide different stress mode.

The uniaxial tensile test is then flanked by planar tensile and equibiaxial tests [ 4 ], [ 19 ].

The planar tensile test replaces the pure shear test (torsion of cylindrical sample), not applicable on samples cut from rubber sheets. The central portion of the specimen is subject to a pure shear strain and stress [ 4 ], [ 24 ]. To prove this assumption, we consider an orthogonal axes system, formed by the loading direction, from the direction perpendicular to this plane in the specimen and the direction normal to the sample itself. Noted these directions, respectively, with the letters x, y, z (Figg. 1, 2):

The test consists to creating a stretching in the x-direction.

For implementation of the test (high ratio width/length of the sample), it is considered negligible the strain along the ydirection

y = 1 ) "_y = 0

Consequently, the incompressibility condition J = det F = 1 ) "_V = "_x + "_y + "_z = 0 for planar test is written [ 2 ]:

J = x z ,= 1 )

z = "_V = "_x + "_z , "_z = "_x 1 x

Therefore, assuming the incompressibility of the material, the coordinates of tensors F and V=1=2(grad v + grad vt)(strain rate) respect to the system of axes x, y, z are.,

F xyz =

2 x FD xyz = 4 0

0 2"_x V xyz = 4 0 0 being FD the right tensor of the deformation gradient. As can be seen from the following image (Fig. 3), relative to the circumferences of Mohr for the strain rate V, at each instant of the test, the strain occurs in pure shear mode.

The deformation in the z-direction is free, therefore in the absence of external loads in this direction is not generated stress ( z = 0). Moreover, from the generalized Hooke’s law "_y = _ y ( _ x + _ z ), being for incompressible materials

= 1=2 [ 3 ], for the y-direction stress is _ y = _ x = _ x=2. In conclusion, during the planar test, the stress tensor T follows the variation equation

Unlike from strain, a condition of pure shear not realized for stress. However, the deviator stress tensor T0 still takes a pure shear connotation (see Fig. 4):

h ddTt0 ixyz = 42 002_x 000 002_x 53 n and t are the orthogonal directions to the y-axis and forming angles of 45 with the x- and z-axis (Fig. 2). Since _nt = _ x=2, and being _ nt = 2"_x, the calculation of the shear modulus G is carried out using the formula

G = d nt = d nt 1 d x 4 d"x 20 mm wide and 180 mm long, with a gauge length of 120 mm.

In the planar tests, was adopted a rectangular specimen, 150 mm wide and 70 mm long, having a gauge length of 50 mm.

For the measurement of deformations of the specimen was adopted an optical method. In effect, the large deformations do not allow easy application of the classic method of the strain gages. The used solution is based on tracing a grid on the specimen surface, which allows to highlight the location of certain specific points. By known distances between various points of the grid in the absence of load, just evaluate their new position for load applied to achieve the deformation.

For image acquisition, carried out at a constant rate, it used a digital camera Basler acA1300-30gm. It is equipped with a Sony CCD sensor ICX445 that provides 8-bit greyscale images at a maximum frame rate of 30 per second, with a resolution of 1280x960 px.

To provide an accurate strain measurement, the Vision

IV. EXPERIMENTAL TESTS Builder AI software of National Instruments acquires and The tested material is SBR filled with 25% of carbon black. processes images through the Vision Assistant command. At The tests were performed on specimens cut from a sheet first a convolution filter is applied to emphasize the grid. having a 3 mm thickness. These were carried out with a test In fact, the alteration of the grid, caused by the specimen machine Zwick/Roel, model Z100, equipped with 1 kN load stretching, reduces the brightness and thus the contrast with the cell, to imposed deformation, while maintaining constant the background, making difficult the identification of the points by speed of the movable crossbar. the software [ 33 ]. Finally, a median filter removes the image The uniaxial tests were performed on rectangular specimens, residual noise.

By the stress and strain values, the relationships of various functional dependencies between the physical quantities involved is obtained. The storage modules were determined from the mathematical derivation of these functions.

The uniaxial testing allows to express both the normal stress as a function of the normal strain in the vertical direction (loading direction), and the normal strain in the horizontal direction (orthogonal to the loading direction) as a function of vertical strain. From the first of these applications the normal storage modulus E = d =d" [ 24 ] is obtained, from the second application the transverse contraction coefficient = d"o=d" .

As already verified during a previous study on a silicone, developed at the University of Catania [ 25 ], a small strain rate, lower than ASTM standards, could rise viscoelastic relaxation phenomena in elastomers (decreasing of stress level compared to the strain achieved). Increasing the execution test rate, this aspect can be kept under control, drastically mitigating the effects. As a consequence of this, the stressstrain curve is higher for the higher test rate, then the rubber manifests an increase of stiffness and mechanical resistance

Fig. 9. Uniaxial Test: Normal Stress vs Normal Strain. to the growing of the test speed. A decrease of the failure strain is associated. It also occurs a different location of the sample crack, depending on the execution test rate. At the ends, near the gripping point, for tests with crossbar that moves 5 mm/min; at the center of the specimen for the tests with speed of 60 mm/min.

In all performed tests, the value of Young’s modulus E at the O(0; 0) ranged between 6.45 and 7.70 MPa, comparable with values reported in the literature for PDMS filled by carbon black (25%) [ 34 ]. The stretching at failure is much lower than the average obtained in the styrene-butadiene rubber [ 34 ].

In addition, at the O(0; 0), the Poisson’s ratio is greater for the high-speed tests (0.460 and 0.490) compared to those at low speed (0.360 and 0.387). For the same rate of the crossbar, instead, the tested elastomer shows a greater Young’s modulus and Poisson’s ratio in the transverse test, but a lesser failure strain. It is interesting to observe that for faster tests the coefficient , evaluated by means of real strains, assumes constant value and close to the theoretical limit value of 0.5, maximum value for isotropic materials [ 26 ], which characterizes the incompressibility, during the entire test. This statement is true only for the Hencky’s tensor. Adopting instead other tensors of deformation, it decreases during the experimental test, being equal to about 0.5 only at the beginning of the test Fig. 10. Uniaxial Test: Horizontal Strain vs Vertical Strain.

Fig. 11. Planar Test: Normal Stress vs Normal Strain. and reaching to several lower values for large deformations [ 8 ].

The shear modulus G = dd = (d x)=(d"x)=4 is calculated based on information extracted in the planar test.

From the analysis carried out, the module G at the origin is 2.04 MPa for longitudinal test and 1.94 MPa for the transverse direction to the calendering.

For each curve a high correlation coefficient value, greater than 0.995, has achieved , demonstrating the low-degree polynomial expressions are well suited to describing the relationships between the treated physical quantities [ 35 ], [ 36 ]. In uniaxial test, the horizontal-vertical true strain function is well represented by a linear expression. Instead, the appropriate polynomial relationship in order to describe the trend of the stress as a function of vertical strain is of 4th order, both in uniaxial and planar tensile tests.

VI. CONCLUSION

The uniaxial and planar tensile tests on specimens of SBR filled with carbon black are carried out.

Due to the high deformability, the material under stress, undergoes marked deformation. In these circumstances the classical methods of measuring deformation fall in default and require alternative assessment criteria which take into account the highly hyperelastic material characteristics. Then, they applied optical methods for overcoming this limit to characterize the elastomer.

The collected data were analyzed to determine the main elastomer storage modules. Different responses in relation to the different direction of load application are obtained, studying anisotropy expressed by the material due to the production process of calendering elastomeric sheet. Given the high variability of the materials properties, the results were compared with those indicated in the literature for the styrene-butadiene rubber filled with carbon black. The normal storage modulus (stiffness) and the transverse contraction coefficient (Poisson’s ratio) of the SBR as a function of the filler percentage have appeared, in accordance with the data available, lower for the tests with load applied in the calendaring direction.

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