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Mechanical Properties of Filler-reinforced Polymers

Fillers are sometimes blended with rubbers and plastics to lower cost and to improve their heat stability and mechanical properties. They are also major components of many formulated polymeric products such as coatings, adhesives, sealants, and grouts. Other important benefits of fillers include greater electric resistivity, lower flammability, and improved UV resistance.
A thorough understanding of the effect of filler type and loading on the properties of a composite is crucial to find suitable fillers and the optimal loading. Typical filler loadings are usually in the range of 20 to 40%.

The properties of filler-reinforced plastics depend on the interactions between the (sub-micron) filler particles and the surrounding polymer matrix, which, in turn, depend on the polymer structure and particle surface chemistry. The properties of filled plastics are also affected by the size and shape (distribution) of the particles.

The mechanical behavior of composites can be quite complicated. To simplify the prediction of its mechanical properties, the particle-polymer system is sometimes replaced with an simple array of polymer (P) and filler (F) phases. In the case of a glassy polymer matirx, an isostrain model (ε_P = (ε_F) is often a good representation of the mechanical behavior of the composite. The average modulus is then simply a linear function of the filler loading:^1,2

 E_C = E_P (1 - φ_F) + E_F φ_F · MRF

where φ_F is the filler volume fraction, and E_i are the moduli of the composite, polymer and filler, respectively. The modulus reduction factor (MRF) accounts for the reinforcement effect of particles with larger aspect ratio. This factor reaches unity for spherical or flake like particles.

The effect of filler on the tensile strength is less well understood. Provided the particles are small (few microns), the tensile strength is nearly constant for low loadings and might even increase at higher loadings (20 to 25%) whereas dispersed larger particles reduce the tensile strength, probably because they act as defects which leads to localized stress concentration (see Griffith's criteria).

The effect of dispersed particles on the impact strength of a plastics depends on the particle size, and its number per unit volume. Poor dispersion and formation of aggregates will (dramatically) reduce the impact strength whereas well dispersed submicroscopic particles can even increase the impact strength. In the case of more or less spherical particles, the crack propagation is often retarded by blocking or pinning (tip blunting) which increases the impact toughness.

 

Fiber-Reinforced Polymers

Engineering plastics are often reinforced with carbon or glass fibers. Both chopped and continuous filaments are added. Continuous fibers often carry a large portion of the load whereas dispersed chopped  fibers increase the critical energy for crack propagation due to fiber bridging. For the crack to propagate, fibers have to be fractured or pulled out of the polymer matrix. Fiber-reinforcement improves also many other properties including stiffness, heat distortion temperature, tensile strength as well as creep and fatigue resistance.  

One of the most important benefits of fibers is improved fracture toughness. However, a significant increase in fracture toughness is not always observed despite an increased critical energy for crack propagation. In the case of soft fibers, a large portion of the released fracture energy is absorbed by the fibers that undergo large plastic deformation in the deformation and crack propagation zone whereas brittle fibers fracture without absorbing much energy. However, inorganic, brittle fibers provide much higher strength and stiffness than polymeric fibers and are therefore often more desirable fur use in engineering plastics. In the case of a brittle plastics reinforced with brittle fibers, a noticeable increase in fracture toughness is only observed if fiber fracture in the crack propagation zone is avoided. This is only the case when the fibers adhere not to strongly to the polymer matrix. Under certain conditions, these composites can have surprisingly high fracture toughness. The reason for this phenomenon is that a large portion of the stored elastic energy is consumed by pulling out fibers and by shear deformation of the polymer matrix parallel to the fibers which results in large frictional dissipation of fracture energy so that less energy is available to propagate the crack through the polymer matrix.

References

1. G. E. Padawer, N. Beecher, Polym. Eng. Sci., 10, 3, 185-192 (1970)
2. J.M. Adams, Clay Minerals, 28, 509-530 (1993)