Fillers debonds

Figures 8.51 and 8.52 show differences between the behavior of polypropylene filled with glass beads at different temperatures. In both cases, the debonding between filler and the matrix requires the lowest level of energy and confirms that this is the most likely mode of failure. The volume fraction of filler has little effect on debonding, cavitation, and yielding at 0°C. At -6()°C, yielding is improved by increasing concentration of filler. Debonding is initiated at the poles and begins plastic yielding in the matrix which ultimately leads to failure. Strain required to initiate failure is reduced when the filler concentration is increased. ...

Tschoegl s result is especially interesting in the light of a recent proposal by Shuttleworth (1968, 1969) that equilibrium polymer-filler debonding is responsible for decreased tensile strength at elevated temperatures. This is contrary to the viscoelastic mechanism of high-temperature failure of Halpin and Bueche (1964), which was developed in an earlier section. A possible resolution of the relative importance of the two proposed mechanisms could lie in the application of Tschoegl s experiment to carbon black- or silica-reinforced materials. 

Farris, R.J., Matrix/filler debonding in poljnner composites, Rheo. /. 12 (1998) 315-321. 

The second mechanism by which dispersants improve impact resistance is by reducing bonding between filler and polymer. When impact occurs, the filler debonds from the polymer to create multiple microvoids as each filler particle debonds. These voids or crazes help to adsorb energy. It is well known that crazing is a mechanism to improve... 

An important consideration is the effect of filler and its degree of interaction with the polymer matrix. Under strain, a weak bond at the binder-filler interface often leads to dewetting of the binder from the solid particles to formation of voids and deterioration of mechanical properties. The primary objective is, therefore, to enhance the particle-matrix interaction or increase debond fracture energy. A most desirable property is a narrow gap between the maximum (e ) and ultimate elongation ch) on the stress-strain curve. The ratio, e , eh, may be considered as the interface efficiency, a ratio of unity implying perfect efficiency at the interfacial Junction. 

Although a number of filler characteristics influence composite properties, particle size, specific surface area, and surface energetics must again be mentioned here. All three also influence interfacial interactions. In the case of large particles and weak adhesion, the separation of the matrix/ filler interface is easy, debonding takes place under the effect of a small external load. Small particles form aggregates which cause a deterioration in the mechanical properties of the composites. Specific surface area, which depends on the particle size distribution of the filler, determines the size of the contact surface between the polymer and the filler. The size of this surface plays a crucial role in interfacial interactions and the formation of the interphase. 

In spite of the imperfections of the approach, the reversible work of adhesion can be used for the characterization of matrix/filler interactions in particulate filled polymers. Debonding is one of the dominating micromechanical processes in these materials. Stress analysis has shown that debonding stress (a ) depends on the reversible work of adhesion [8], i.e. ... 

Fig. 6. Debonding of interfaces, effect of adhesion on PP/CaC03 composites filler CaC03, (pf=0.2,r=L8 pm...

Recently, stress analysis has been carried out for the determination of stress distribution around inclusions in particulate filled composites. A model based on the energy analysis has led to the determination of debonding stress [8]. This stress, which is necessary for the separation of the matrix and filler, was shown to depend on the reversible work of adhesion (see Eq. 16) and it is closely related to parameter B. 

Polypropylene, fracture toughness, elastic modulus, filler particle, crack resistance, debonding, cavitation, process zone, silica, calcium carbonate. 

Kim and Michler have observed the relationship between morphology and strain micromechanisms in cases of both rigid and elastomeric filler growth of voids, by cavitation or debonding [7,31]. Oshyman has reported a transition, at a certain fraction of filler, correlated to the evolution from macroscopic homogeneous strain to micromechanisms such as crazes. It is in fact a transition between independent mode and correlated mode of strain micromechanims [32]. 

The development of a criterion for debonding mechanism, which assumes that the debonding stress is proportional to the strength of adhesion and depends on the particle size of the filler, might explain the experimental observations on toughness. 

An air filled space created around the filler particle by incomplete wetting or debonding... 

Another interaction is responsible for the recovery of the material after it is subjected to stress. " Rubber bridging the neighboring particles of filler is an example. Some particles are connected through several rubber chains which makes their association more permanent and assures filler-filler contact. These filler-filler contacts are responsible for the recovery since, unlike chain-filler contacts, they store the strain energy which is then used in the recovery process. Chain-filler contacts can easily debond or rearrange in different location and this process does not result in recovery of the initial shape. 

Debonding (also called dewetting) is one mechanism of the failure of filler reinforced composites which are subjected to either continuous stress or fluctuating stresses. Debonding may also be used as a method of production for some of the materials discussed in Section 7.3. 

This equation shows that debonding stress increases with adhesion and filler fraction and decreases with particle size. Figure 7.27 shows the effect of particle size on prediction of yield stress based on the debonding simulated by an equation derived from Eq 7.27. Decreasing particle size increases the stress required for debonding. 

The rate of debonding decreases as the number of debonded particles increases and as the stress increases. The debonding constants characterize the interaction and the influence of neighboring particles. Their values depend on the filler concentration and on the adhesion of the filler to the matrix. The volume increase due to debonding is given by the equation ... 

The volume increase depends on the filler fraction and on the applied strain. This is confiimed in practice. Debonding correlates with loss of stiffness. The first part of the stress-strain curve (elastic stage) is related to the strains beyond which debonding occurs. In glass bead filled polypropylene, this strain was 0.7%. ... 

In mixtures of particles, the stress of debonding is not uniform. Higher stress is needed to debond from smaller particles." Adhesion is inversely proportional to the cube root of the diameter of the particles. " Experiments confirmed that large particle sized filler decreased the tensile strength of composites. The filler concentration effect is not linear. Up to a certain concentration, filler did increase the tensile properties but beyond certain level there is a reverse effect. This may relate to the interactions described in previous sections where the quality of bonding (weak or strong) depended on filler concentration. 

If there is perfect adhesion (no debonding), tensile yield stress increases as the concentration of the filler increases (Figure 8.6). Filler particle size is also important. As the particle size of the filler decreases, the curves become more steep and the yield stress increases along with concentration increasing. 

These remarks evaluate the effect of filler-related phenomena on failure of plastic materials. Several reasons for the failure ofplastics are filler related. They include delamination of laminated composite materials, debonding in particulate filled materials, stress cracking of filler particles, yielding, cavitation, and corrosion. 

Major results Figure 14.16 shows the effect of mean particle size of spherical silica (flame-fused synthetic silica) on acoustic emission. The emission increases with the particle size of filler increasing. The source of this increase in acoustic emission is thought to be related to the fracture of particles, debonding of particles from... 

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