2.3. Results and discussion
2.3.4. Properties
0 5 10 15 20 25 0
20 40 60 80
Characteristic stress (MPa)
Glass bead content (vol%)
strength yield
AE VOLS
Figure 2.7 Comparison of the characteristic stresses determined in PA6/GB compo-sites. Symbols: () tensile strength, σT, () tensile yield stress, σy, () acoustic emission, σAE, () volume strain, σVOLS.
0 5 10 15 20 25 0
20 40 60 80
Characteristic stress (MPa)
Glass bead content (vol%)
strength yield
AE VOLS
Figure 2.7 Comparison of the characteristic stresses determined in PA6/GB compo-sites. Symbols: () tensile strength, σT, () tensile yield stress, σy, () acoustic emission, σAE, () volume strain, σVOLS.
2.3.4. Properties
The Young's modulus of the composites is plotted against the amount of filler or reinforcement in Figure 2.8. The comparison of micro- and nanofillers is very interesting and clearly shows the differences in their reinforcing effect. The stiffness of the composite is determined by the modulus of the components, their amount and by structure. Glass beads have large modulus, but their large size and debonding leads to moderate reinforc-ing effect. Wood flour has considerably smaller modulus, and the large size of the parti-cles as well as, their debonding and/or fracture also limits reinforcement. Glass fibers increase modulus quite considerably because of their large modulus and orientation. The effect of the two clays is also very interesting and somewhat contradictory. NaMMT leads to larger modulus than glass fibers, but only in a very narrow composition range. OMMT has a somewhat smaller reinforcing effect, but in a wider range of filler contents, because of exfoliation and better dispersion. Nevertheless, the composition dependence of the stiffness of silicate composites indicate structural effects, aggregation, incomplete exfo-liation, and the presence of large particles.
Tensile strength is plotted against filler content in Figure 2.9. Basically the same conclusions can be drawn from the correlation as in the case of modulus. Glass fibers reinforce PA considerably, while WF and GB hardly at all. The effect of the two silicate fillers is also more or less similar, but structural effects are much more pronounced here, as expected. Debonding is initiated very easily around the large particles of NaMMT and the probability of particle fracture also increases with increasing particle size. The exfo-liation and better dispersion of OMMT leads to larger strength, but the smaller interfacial interaction caused by organophilization and structural effects lead to the decrease of strength at larger filler content.
0 5 10 15 20 25
2 3 4 5 6 7
GF
WF NaMMT GB
Young's modulus (GPa)
Filler content (vol%)
OMMT
Figure 2.8 Composition dependence of the stiffness of PA6 composites containing the various fillers and reinforcements. Symbols () NaMMT, () OMMT, () GB, () WF, () GF.
The deformability of the composites is very important for their application as structural materials. Small elongation-at-break is usually accompanied by rigidity and brittle fracture. The deformability of the composites is plotted against filler content in Figure 2.10 in logarithmic scale to compensate for the large differences and the small values of GF and WF composites. The figure clearly shows that the large stiffness of the PA/glass fiber composites results in very small elongation-at-break values, and the de-formability of the wood flour composites is only slightly larger. Interestingly, considera-bly larger deformations are achieved in the case of GB and the two silicates, a further advantage of the latter. The use of layered silicate nanocomposites would be very advan-tageous indeed, because larger stiffness is accompanied by reasonable deformability, if larger extent of exfoliation could be achieved and structure could be controlled more pre-cisely and reproducibly in general.
Deformability usually gives some indication about fracture resistance, one expects larger impact resistance at large elongation-at-break. Fracture toughness measured by notched Charpy impact testing is plotted against filler content in Figure 2.11. Quite sur-prisingly, nanocomposites possess the smallest fracture resistance contrary to the indica-tion of Figure 2.10. Debonding leads to the yielding of the matrix and relatively larger fracture toughness for the microcomposites and the fracture of the glass fibers consumes additional energy and results in the increase of fracture resistance at large fiber contents.
Fracture resistance is very small for the nanocomposites at small silicate content for var-ious reasons like strong adhesion for NaMMT and larger interface for OMMT. At larger filler content the silicates behave like microfillers as a result of aggregation and insuffi-cient exfoliation. Uncontrolled structure is very disadvantageous after all and limits the application of layered silicate composites.
0 5 10 15 20 25
0 20 40 60 80 100 120 140 160
WF GF
GB
Tensile strength (MPa)
Filler content (vol%)
NaMMT OMMT
Figure 2.9 Tensile strength of PA6 composites plotted against their composition. Sym-bols () NaMMT, () OMMT, () GB, () WF, () GF.
0 5 10 15 20 25
1 10 100 1000
GF WF
GB
Elongation-at-break (%)
Filler content (vol%)
OMMT
NaMMT
Figure 2.10 Effect of composition on the deformability of PA composites containing various fillers and reinforcements. Symbols () NaMMT, () OMMT, () GB, () WF, () GF.
0 5 10 15 20 25
0 5 10 15 20
Impact resistance, a n (kJ/m2 )
Filler content (vol%)
GF
GB WF NaMMT OMMT
Figure 2.11 Notched Charpy impact resistance of the composites plotted as a function of filler content. Symbols: () NaMMT, () OMMT, () GB, () WF, () GF.
Deformability usually gives some indication about fracture resistance, one expects larger impact resistance at large elongation-at-break. Fracture toughness measured by notched Charpy impact testing is plotted against filler content in Figure 2.11. Quite sur-prisingly, nanocomposites possess the smallest fracture resistance contrary to the indica-tion of Figure 2.10. Debonding leads to the yielding of the matrix and relatively larger fracture toughness for the microcomposites and the fracture of the glass fibers consumes additional energy and results in the increase of fracture resistance at large fiber contents.
Fracture resistance is very small for the nanocomposites at small silicate content for var-ious reasons like strong adhesion for NaMMT and larger interface for OMMT. At larger filler content the silicates behave like microfillers as a result of aggregation and insuffi-cient exfoliation. Uncontrolled structure is very disadvantageous after all and limits the application of layered silicate composites.
0 5 10 15 20 25
0 20 40 60 80 100 120 140 160
WF GF
GB
Tensile strength (MPa)
Filler content (vol%)
NaMMT OMMT
Figure 2.9 Tensile strength of PA6 composites plotted against their composition. Sym-bols () NaMMT, () OMMT, () GB, () WF, () GF.
0 5 10 15 20 25
1 10 100 1000
GF WF
GB
Elongation-at-break (%)
Filler content (vol%)
OMMT
NaMMT
Figure 2.10 Effect of composition on the deformability of PA composites containing various fillers and reinforcements. Symbols () NaMMT, () OMMT, () GB, () WF, () GF.
0 5 10 15 20 25
0 5 10 15 20
Impact resistance, a n (kJ/m2 )
Filler content (vol%)
GF
GB WF NaMMT OMMT
Figure 2.11 Notched Charpy impact resistance of the composites plotted as a function of filler content. Symbols: () NaMMT, () OMMT, () GB, () WF, () GF.