Fillers' content influence on the mechanical properties of the glass mat reinforced polymeric composite.

Motoc Luca, Dana ; Teodorescu, Horatiu

1. INTRODUCTION

The world of thermoplastic composite materials, despite its trend toward nano scale, seems to leave some spaces to what was called 10 years ago "high-performance" advanced composite structures. Nowadays, these types of materials have shifted to "cost-performance" engineering composites, with a widespread use especially due to their advantages like: stampable products due to the polymeric material, excellent price-performance ratio, use of commercially available products, simple configuration of the manufacturing technology, etc.

In the present, ecological concern has resulted in a renewed interest in natural materials and issues such as recyclability and environmental safety are becoming more and more important for introduction of new materials and products. These lead to changes in the research interest, and with respect to the paper's subject to the replace of the non-renewable glass fibres with vegetable fibres (Garkhail, Heijenrath & Peijs, 2000). The data analysis methods, such as statistical investigation, have shifted as well, but still remaining one of the most used tool for experimental vs. theoretical predictions comparisons, even in case of the use of neural network implementation (Esfandiari, 2008).

Fiber composites are very sensitive to the compression loads (Schneider & Lauke, 2007). The behaviour of these structures under compression is much more critical than tensile or bending no matter their application area. This is the reason for which such testings are conducted carefully, the determination of the compressive modulus and strength being a little more difficult to predict in case the measuring devices are not equipped with proper sensitive transducers.

The present work focuses on the mechanical properties of the random E-glass fibres composites, reinforced with different volume fraction of various types of fillers. More specifically, the paper focuses on the experimental values determined in uniaxial tensile and compressive tests as a part of a more extensive work focused on these classes of composite materials (Luca Motoc & Soica, 2008). These experimental results provide useful information for design purpose regarding to the improvements that can be done with respect to their mechanical characteristics, especially when the structure is being employed in aggressive media environments.

The paper contains only a simple comparison of the experimental data retrieved with the theoretical values provided by the most comprehensive model from the literature, and this is due to restrictions implied (Jannerfeldt, et. al., 2001). The introduction of a new reinforcement efficiency factor depending on the fillers content will be left as a further study.

Due to their specificity, the structures approached are referred usually as SMC (e.g. Sheet Moulding Components) even the term may have another meaning, such as Soft Magnetic Materials. It's beyond to the subject of the present paper to debate on the terms' meaning, or to present the individual characteristics of each structure. Nonetheless, it is worth to mention that the Fe particles embedded along with the E-glass fibres mat into the polymeric matrix material exhibit relatively low magnetic properties. The previous mentioned are being on course to be developed, as a natural consequence of these experimental trials along with structures' composition modification.

2. EXPERIMENTAL ANALYSIS

2.1 Materials

In this study were used random, long E-glass fibre mats in a combination with a polymeric matrix from DSM Composite Resins (Switzerland), the latter being choose due to its availability and very good fibre wetting and impregnation properties. The fibres--MultiStrat[TM] Mat ES 33-0-25 supplied by Johns Manville, USA, were made up from multidirectional continuous E glass fibres, leading to a 60% volume fraction in the composites' panels. The fillers considered were CaC[O.sub.3] and Fe particles embedded into different volume fraction into the composite's matrix material (5 %, 10 % respectively).

2.2 Testing methods

The samples' mechanical properties were evaluated at room temperature, samples being shaped using a standardized form and subjected to tensile and compression loading conditions, all of them with a speed of 1 mm/min using a LS 100 Plus device from Lloyd Instruments Ltd. The results obtained applying a statistical analysis was considered as corresponding to the mean values.

3. RESULTS AND DISCUSSION

Both testing types lead to something that one may call at a first sight an "awkward" result. A deep looking and recalling the different types of materials involved as additives may solve the dilemma. The comparisons are being done with respect to the simple glass fibres mat and polymeric matrix composite materials, without fillers.

With respect to the tensile tests, seems that the increase of the mineral filler content lead to an improvement of the Young modulus. The same holds in case of the Fe particles embedded into the structure. But unfortunately, these types of composite materials are rather used on compression than traction.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The tensile-strain curves (not presented herein) exhibit some nonlinearity of the response. The nonlinear effect is the result of progressive failure due to various micro-failure modes, such as matrix cracking, fibre debonding and breakage. The composites failed in a brittle manner with a suddenly load drop.

The compressive moduli measured are higher than their tensile values, meaning the fact that no experimental errors were induced into the results. They differ from fillers to fillers, revealing an improvement in case of Fe particles along with the content increase, whereas a decrease for the mineral fillers, the same, along with the increase of their volume fraction content.

In figure 1 is being presented the experimental values retrieved for the Young modulus during the tensile tests conducted, whereas in figure 2 are being presented the results for the compressive tests. Figure 3 represent the variation of the most comprehensive theoretical model from literature used to approximate the stiffness of the composite having almost randomly oriented reinforcements. The model is known as Cox-Krenchel model it's written as, under the assumption of elastic fibers embedded into an elastic matrix material:

[E.sub.c] = (1 - [V.sub.f]) x [E.sub.m] + [[eta].sub.f] x [[eta].sub.fod] x [V.sub.f] x [E.sub.f] (1)

where [E.sub.c] is the modulus of the composite material, [V.sub.f] the fibre volume fraction, [E.sub.m] the modulus of the matrix material and [E.sub.f] the modulus of the fibers. The parameters [[eta].sub.f] and [[eta].sub.fod] are reinforcement efficiency factors related to the fibre aspect ration (length to diameter ratio) and the fibre orientation distributions, respectively. In the general case, both parameters range from 0 to 1. For random reinforced fibers the [[eta].sub.fod] parameter is set to 0.38 and for a fiber faction volume of 0.6 (this case) seems that the experimental retrieved value for the composite structure without fillers, in tensile tests, lead to an efficiency factor related to the fiber aspect ratio of 0.23 presumming that the length and diameter of the fibers are unknown values. Unfortunately, this theoretical model does not encompass the influence of some supplementary aided filler in the random dispersed fibres polymeric composite structure.

4. CONCLUSIONS

The tensile and compressive mechanical properties modification due to the presence of various types of fillers (e.g. metals, non-metals) embedded into different volume fraction into a random long glass fibres mat polymeric composite materials were studied with the aim of sizing their influence on the investigated parameter.

As expected, there is a significant effect of the fillers' volume fraction on the mechanical properties and no certain anticipated behaviour in the mechanical performance with the increase of this content was found.

The values will be used to tailor other types of composite structures, and special attention will paid due to the fact that a price-competitive character, usually aimed in industry for such materials, do not lead to the improvements of the mechanical performance.

The subject of the paper may seem to a be something that was already done, but it's opening a niche with respect to the needs of modifying the theoretical models in order to encompass the influence of various content of fillers embedded into the classical composite structures to improve their mechanical characteristics.

Acknowledgement

The research was supported from grant ID_135, 108/1/10/2007, CNCSIS, Romania.

5. REFERENCES

Esfandiari, A. (2008). The statistical investigation of mechanical properties of PP/natural fibres composites. Fibers and Polymers, Vol. 9, No. 1, 48-54.

Garkhail, S. K., Heijenrath, R. W. & Peijs, T. (2000). Mechanical properties of natural-fibre-mat-reinforced thermoplastics based on flax fibres and polypropylene. Applied Composite Materials, Vol. 7, 351-372.

Jannerfeldt, G., et. al. (2001). Matrix modification for improved reinforcement effectiveness in polypropylene/glass fibre composites. Applied Composite Materials, Vol. 8, 327-332.

Motoc Luca, D. & Soica, A. (2008). Mechanical behaviour of 3-phase polymeric composites subjected to static loading conditions. Available /rom:http://innomet.ttu.ee/daaam.

Schneider K. & Lauke B. (2007). Determination of the compressive properties of fibre-reinforced polymers in the in-plane direction according to ISO 14126. Part 2: A critical investigation of the failure behaviour. Applied Composite Materials, Vol. 14, 177-191.