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Anisotropic rubber properties through oriented fillers.

There are several literature studies of the directional properties of fillers added into rubber compounds, and then oriented (ref. 1). The orientation is usually provided by a shearing action. This can be between the rollers of a tight nip on a two-roll mill or calender, or can also be due to extrusion through a die with an 1/d typically greater than 5.

Fillers that are not spherical can be oriented. The grain effect of rubber sheets, with alignment of polymer chains and carbon black filled rubbers, is well known, and papers exist in the literature to describe the anisotropic quantification of carbon black. In the case of nano-clay fillers, these are oriented and then exfoliated. There is a body of literature demonstrating the orientation of the particles by x-ray diffraction, but perhaps there has been no study of the anisotropy this delivers.

Most of the studies that have investigated orientation tend to look at the properties along the orientation, and across the orientation direction. The studies also concentrate on tensile properties. There are very few examples where the orientation angle is other than 0 or 90[degrees], or where dynamic viscoelastic properties have been presented. These are reviewed and referenced here.

This is of interest to the NGFE group of companies (NGF Canada Limited, NGF Europe Limited), as one-dimensional rubber impregnated chopped strand particles and two dimensional glass flake particles are commercially available.

Static properties of oriented fillers

Glass flakes are platelets of glass with a very high aspect ratio that provide a very interesting oriented static property. These were studied with a CR lining compound with 50 phr of N550 carbon black (ref. 2). Particles 600 x 600 x 3[mu] were added to the CR, replacing the same amount of carbon black. A silane treatment had been applied to the flakes for good coupling between the rubber and the filler particles. Once sufficient flake filler loading was attained (10 phr), the permeation was reduced. At 15 phr of glass flakes, a ten-fold reduction in water permeation was observed (figure 1). This was due to the development of a labyrinthine pathway, effectively significantly increasing the effective rubber thickness (figure 1). Commercially, this allows rubber compounds to be reduced in thickness for similar effective permeation resistance. This article also showed that the apparent tear strength was increased by the addition of the oriented platelets across the direction of tear.

The physical properties of the CR compound with flake replacing carbon black followed the reduction in black loading: The tensile strength dropped, the modulus dropped with not much change to the extension to break. One exception was the tear strength, which increased with addition of flake replacing carbon black. At first sight, this is unusual, as carbon black is very good as a crack stopper. The increase in tearing force was due to tearing the rubber compound in a perpendicular direction to the alignment of the fillers. This meant that the tear path was not simply the width of the specimen, but was the labyrinthine tear path around the glass flakes. The longer tear path caused the increase in apparent tear strength.

Dynamic properties of oriented fillers

The first studies of dynamic anisotropic properties of rubber were conducted in the 1930s. The same compounds with different filler orientations were tested on the St. Joe flexometer (ref. 3) and the Goodrich flexometer (ref. 4). In particular, aluminum silicate (kaolin clay [plates]) and magnesium silicate (asbestine [needles/ribbons]) were tested in different orientations. Formulation (phr): NR 100, sulfur 4.0, DOTG 1.4, gas black 9.0, filler 150. The heat build-up in the Goodrich flexometer was significantly higher for particles aligned in the direction of compressive deformation, and lower for particles aligned in the normal direction to the deformation (figure 3). The St. Joe flexometer sheared the samples horizontally, while in vertical compression. The horizontal load was increased as required to maintain the shear displacement. The compounds were tested with orientation of the particles for orientation in the x, y and z axes. The differences in applied loads and heat build-up changed significantly, whether perpendicular or parallel to the compression, and also changed whether particles aligned parallel to the compression were on their long axis or short axis. The orientation nomenclature is shown in figure 2.

The results (figure 3) showed that the hysteretic heat buildup was significantly altered by the orientation of the filler particles. The results for clay orientations 1 and 2 were similar, indicating that the short and long axes were similar. The results for asbestine orientations 1 and 2 were dissimilar, indicating that the particle shape was like ribbons or needles. Low hysteretic heat build-up was observed on both machines for the particles aligned normal to the applied vertical compression.

A more recent study was made by Roy, Bhowmick and De in 1992 (ref. 5). Six mm long carbon fibers of 0.7 mm diameter were used. These broke down during mixing to be about 0.2 mm long. Formulations (phr): ISNR-50 100, peroxide-40 0.7, antioxidant 1.5, carbon fiber 0 and 10. The DMTA dynamic properties (table 1) were studied for 0 phr and 10 phr of carbon fiber aligned parallel and perpendicular to the bending displacement. Their paper concentrated on the properties at Tg. The analysis presented here is of their data at working temperatures of rubber goods. The 10 phr of carbon fiber increased both the elastic (E') and viscous (E") moduli, relative to the unfilled compound.

A ten-fold difference was observed in elastic modulus between the fiber orientations. A three-fold difference with orientation was seen in the viscous damping. Similarly, relative to the unfilled compound, the tan 6 was increased by 60%, or decreased by 40%. This means that for this compound, the hysteresis can be tuned according to the application, with high damping in one direction, and low damping in the orthogonal direction. Interestingly, these properties showed very large changes with temperature.

Other studies have been made, also looking at transverse and longitudinal properties (refs. 6 and 7). The work of Clarke and Harris (ref. 1) examined intermediate cellulose fiber angles of 45 and 135[degrees], as well as 0 and 90[degrees] with respect to a directional shear. Formulation (phr): NRCV60 100, TMQ 1, zinc oxide 5, stearic acid, 2, sulfur 2, MBS 0.6, HMM Resimene 65% 3.1, N330 carbon black 45, Santoweb D cellulose fiber 0 and 10. The cellulose fibers were 1.5 mm long and 12p in diameter. Their results (figure 4, table 2) showed the largest difference existed between 45 and 135[degrees] fiber orientation. The differences in fiber orientation were not as large as those of Roy (ref. 5), perhaps masked somewhat by the 45 phr of carbon black (table 2).

Conclusions and discussions of further work

Each study of oriented filler particles has observed differences in mechanical properties for differences in particle orientation. It is possible to align fillers at any chosen angle, and so a fully tuneable dynamic response will become available. The damping behavior can be changed by the filler loading, the type of filler used, as well as the orientation of the filler. It is probable that high modulus carbon fibers will have a higher influence on elastic modulus than lower modulus glass fibers. The effect on damping properties has yet to be well studied. This could well allow new dimensions for the envelope of tire compound properties to allow different balances of traction, grip and rolling resistance to be achieved. Anti-vibration isolators can be optimized for damping in the required orientation only. NGFC and NGFE are suppliers of chopped glass fiber strands and carbon fiber strands, treated with RFL for good fiber to matrix adhesion. NGFC and NGFE are also suppliers of silane treated glass flake platelets. NGFC and NGFE are keen to partner with interested parties to compare the full viscoelastic effects of one-dimensional (linear) and two-dimensional (platelet) fillers in a multitude of orientations (figure 5).

This article is based on a paper presented at RubberCon, Manchester, U.K., May 2014.

References

(1.) J. Clarke and J. Harris, Plastics Rubbers and Composites, 2001, Vol. 30, No. 9, p. 406.

(2.) C.A. Stevens and D. W. Mason, Rubber World, August 2007, Vol. 236, No. 5, p. 36.

(3.) R.S. Havenhill, Physics, 1936, Vol. 7, No. 5, p. 179.

(4.) E.T. Lessig, Analytical Edition, Industrial and Engineering Chemistry, 1937, Vol. 9, p. 582.

(5.) D. Roy, A.K. Bhowmick and K. De, Polymer Engineering and Science, July 1992, Vol. 32, No. 14, p. 791.

(6.) L. Ibarra and C. Chamorro, Journal of Applied Polymer Science, 1992, Vol. 43, p. 1,805.

(7.) J.N. Das, N. G. Nair andN. Subramanian, Plastics Rubbers and Composites Processing and Applications, 1993, Vol. 20, p. 249.

by Christopher A. Stevens and David W. Mason, NGF Europe Limited

Table 1--DMTA results for 10 phr of carbon fiber at
different alignments (ref. 5)

Dynamic properties                E', MPa

                       20[degrees]C    100[degrees]C

0 phr                           2.5             2.1
10 phr transverse               3.3             2.4
10 phr longitudinal            25.2            10.9

Relative to unfilled       [E.sub.10]'/[E.sub.0]'

                       20[degrees]C    100[degrees]C

10 phr transverse               1.3             1.1
10 phr longitudinal            10.3             5.2

Dynamic properties                E", MPa

                       20[degrees]C    100[degrees]C

0 phr                           0.4             0.4
10 phr transverse               0.9             1.1
10 phr longitudinal             2.6             2.5

Relative to unfilled       [E.sub.10]"/[E.sub.0]"

                       20[degrees]C    100[degrees]C

10 phr transverse               2.1             2.8
10 phr longitudinal             6.1             2.7

Dynamic properties              Tan [delta]

                       20[degrees]C    100[degrees]C

0 phr                         0.170           0.189
10 phr transverse             0.275           0.471
10 phr longitudinal           0.104           0.099

Relative to unfilled        Tan [[delta].sub.10]/
                            Tan [[delta].sub.0]

                       20[degrees]C    100[degrees]C

10 phr transverse               1.6             2.5
10 phr longitudinal             0.6             0.5

Table 2--dynamic properties of 10 phr of cellulose
fiber versus orientation angle in NR with 45 phr of
N330 (ref. 1)

Fiber angle:    0[degrees]    45[degrees]    90[degrees]

G', MPa                2.5            2.6            3.6
G", MPa                0.4            0.5            0.6
Tan [delta]          0.172          0.188          0.167

Fiber angle:   135[degrees]      No fiber

G', MPa                5.1            2.6
G", MPa                0.9            0.5
Tan [delta]          0.179          0.188
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Author:Stevens, Christopher A.; Mason, David W.
Publication:Rubber World
Date:Mar 1, 2015
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