Understanding Main Governing Factors for Mechanical Properties of Short
Aramid-Fibers Reinforced Elastomers
Nadia Vleugels
1, W.K. Dierkes
1, A. Blume
1, D.J. Schipper
2, J.W.M. Noordermeer
1*1University of Twente, Faculty of Engineering Technology, Elastomer Technology and Engineering, P.O. Box 217,
7500AE, Enschede, The Netherlands
2University of Twente, Faculty of Engineering Technology, Surface Technology and Tribology, P.O. Box 217, 7500AE,
Enschede, The Netherlands
Phone +3153 489 2529, Fax +31 489 2151, e-mail: j.w.m.noordermeer@utwente.nl
1. Introduction
Research into the fundamentals of interfacial strength improvement of short aramid fibers in carbon black reinforced or silica reinforced elastomer compounds turns out to be very complex. Short fiber reinforcement depends mainly on: 1) fiber concentration; 2) fiber orientation distribution; 3) fiber length and distribution; 4) fiber-matrix interfacial strength and 5) properties of the matrix1,2, respectively on the interdependencies between all these factors.
Typical elastomer formulations contain many ingredients such as reinforcing and non-reinforcing fillers and curatives, with the fibers coming in addition, which interact either chemically or physically with each other. Many analytical tests are inconclusive in elucidating these interactions. Because of the need to investigate these interactions as well as the influence of processing, a systematic study was carried out making use of a simplified/basic gum-formulation. In particular, various publications3,4 showed sometimes large and otherwise small improvements in mechanical properties
when the interfacial strength was improved in short fiber reinforced elastomer compounds. These were mainly derived from elementary tensile tests, where the effect of fibers was most conspicuous at low strains as an increase in tensile stress vs. non fiber–containing.
As the basis for this study a gum-rubber formulation was chosen, derived from the silica–reinforced passenger car tire tread technology with silane coupling agents, normally employed to establish chemical coupling between silica and the elastomers. The formulation was stripped to it basics by taking out the butadiene rubber and the silica, or just adding a minor amount of silica and using the coupling agent as potential chemical binder
a) b)
Fig. 1 ─ Silane coupling agents used: a) TESPD
and b) NXT.
between the fibers and the elastomer solution-polymerized SBR. Following this procedure, the separation of the influences of the single ingredients and process conditions should be possible. A design of experiments (DoE) approach was chosen, the Taguchi method5 for a robust design. In this paper a conceptual way of graphing the responses is
employed, also proposed by Taguchi, which involves visualizing the effects and identifying the factors and their interactions which appear to be significant.
The fiber reinforced elastomer compounds contain a variety of ingredients and involve various processing steps. The fiber types employed are non-coated/virgin; and pre-treated with an epoxy coating. This second type of fiber was selected because of its possible reactivity6 towards silane coupling agents. Two commonly used coupling agents
were selected for this study, see figure 1: Bis-(triethoxysilylpropyl)-disulfide (TESPD) and S-(3-(triethoxysilyl)propyl)-octanethioate (NXT). These coupling agents were supposed to react with the epoxy groups
coming from the fiber coating and with the elastomer. The fiber concentration was varied between 5 and 15 parts per hundred elastomer (phr); the silica concentration from 0 to 10 phr. A proposed mechanism of the epoxy coating with for example the coupling agent TESPD is shown in figure 2.
2. Results
The fibers have little influence on the rheology, Mooney viscosity of the compounds for the amounts added. Nor are the vulcanization characteristics influenced by a large extent. The main effects of adding the fibers are seen in the vulcanized mechanical properties and the tribological/wear properties which are fundamentally changed. The order of importance for the mechanical properties is: Amount of fibers added > amount of curatives added > fiber orientation ≫ fiber-rubber interaction dependent on fiber type and use of coupling agent. Although it is tempting to think of the reinforcement by the aramid fibers in the classical sense as obtained with reinforcing fillers like carbon black or silica, this is fundamentally wrong. The specific surface area of the 14 μm diameter aramid fibers amounts to 0.2 m2/gr. This is to be compared with specific
surface areas in the order of 100 m2/gr for typical rubber reinforcing fillers:
3 orders of magnitude higher. Compared to those conventional reinforcing
Fig. 2 ─ Proposed reaction mechanism
of TESPD with epoxy coated fiber and elastomer.
fillers, there is practically no surface offered by the aramid fibers to interact with the polymer matrix more than a nanometrically small layer around the fibers. The reinforcement effect coming from the flexible aramid-fibers must rather be seen in the model-sense of reinforcement of concrete with steel where there is no adhesion necessary. Keeping this model in mind, the results in the present paper make sense. Even though this may be a little disappointing from a mechanical performance point of view, it explains the large effects found in the friction and wear properties observed.7
3. References
1 S. Fu, B. Lauke and Y Mai, Science and engineering of short fiber reinforced polymer composites, Woodhead
Publishing, (2009).
2 S. K. De and J. R. White, Short fiber polymer composites, Woodhead Publising, (1996).
3 C. Hintze, Influence of processing induced morphology on mechanical properties of short aramid fiber filled elastomer
composites, University of Dresden, PhD-Thesis (2012).
4 M. Shirazi, Aromatic polyamide short fibers reinforced elastomers, University of Twente, PhD-Thesis (2013). 5 D. M. Byrne and S. Taguchi, American Society for Quality Control, (1986).
6 P. de Lange, Composites Part A: Applied Science and Manufacturing, 32, 331-342 (2001).
7 M. Mokhtari, D.J. Schipper, N. Vleugels, J.W.M. Noordermeer, Tribology International, doi:10.1016/j.triboint.2016.01.010.
