Miscellaneous Mixing Techniques

Thermal spraying seems to be a viable solution to the processing limitations of conventional techniques for polymer nanocomposite manufacturing, such as the use of a high working temperature for melt mixing and solvents for solution mixing [175]. This is particularly true for the preparation of nanocomposite coatings. During a thermal spray process, materials, in powder, wire, or rod form, are heated, accelerated, and propelled by a high-temperature jet through a confining nozzle toward a surface. The individual molten or softened droplets impact, spread, cool, and solidify to form continuous coatings. Material heating, quenching, consolidation, and posttreatment are thus combined in a single step. The important benefits of using a thermal spray process for making polymer nanocomposites include the following: (i) powdered polymers are heated only to a temperature at which the particles are viscous enough to spread because of the high kinetic energy; (ii) good dispersion of nanoparticles is achieved by the high velocity; (iii) processing without the use of a volatile solvent is possible.

Schadler et al. employed a high-velocity oxy-fuel (HVOF) system to thermally spray SiO2/nylon 11 nanocomposite coatings [175]. The particles (7 and 12 nm in diameter, respectively) were pretreated to be either hydrophobic or hydrophilic, respectively. To avoid powder segregation in the jet due to differences in powder size and density, the nylon and silica particles were dry ball milled together for 48 h, resulting in a gradient shell of embedded silica particles near the surface of the nylon powder particles. By optimizing spray parameters such as nozzle design, spray distance, oxygen-to-fuel ratio, powder feed position, and substrate cooling, dense coatings with relatively uniform particulate distribution were achieved. Compared with neat nylon, scratch resistance improved by 30% and wear resistance improved by 55%. Silica particles with a hydrophobic surface led to better mechanical properties than those with a hydrophilic surface.

Recently Petrovicova et al. characterized the changes that the above silica reinforced nylon 11 underwent during thermal spraying and their influence on the final polymer morphology and coating performance [176, 177]. Agglomerates of silanated silica, on the order of 50 nm in size, were observed in the nanocomposites, whereas the agglomerates of untreated silica and hydrophilic silica were on the order of 100 nm. Reinforcement of the polymer matrix, for example, in the case of a 15 vol.% silanated silica-filled nanocomposite, resulted in an increase of up to 205% in the dynamic storage modulus (Table 10). At temperatures above the glass transition temperature of nylon 11, the storage modulus of a 15 vol.% hydrophobic silica nanocomposite was higher than that of the unfilled version by up to 195%. An increase in matrix crystallinity was assumed to be the predominant cause of the enhancement of the mechanical properties.

Unlike conventional dispersive mixing at a temperature above the melting point or the flow temperature of the polymer matrix, mechanical alloying, which compounds the components at room temperature through mechanochemical effects, is an effective route for producing polymer composites with sufficiently high interfacial adhesion. The process is a high-energy ball-milling one in which the repeated fracture and welding of powder particles, arising from ball-powder collision events, allows true alloy powders to be formed from mixtures of elemental powders [178, 179]. It appears to be very promising because it offers the possibility of mixing two ordinarily immiscible materials, and no coupling agents or other surface treatments are needed, in principle.

So far, very few results have been reported that deal with mechanical alloying of polymer nanocomposites. Hu et al. applied mechanical milling to the preparation of composites consisting of carbon nanotubes (20 nm in diameter) and an ultra-high-molecular-weight polyethylene (UHMWPE) [180]. Having been milled for 2 h, the mixture of carbon nanotubes and the micron powder of UHMWPE was homogeneous. At a filler loading of 1 wt.%, the impact strength of the composites increased by a factor of 43% compared with the value of the unfilled UHMWPE. Electron microscopic observations of the fractured surface indicated that a resin layer (15-20 nm thick) was wrapped around the nano-tubes, suggesting a good adhesion between the fillers and the matrix formed during mixing.

Table 10. Dynamic storage modulus G of thermally sprayed nano-composite coatings.

Silica content

Polymer

G at 30 °C

G at 70 °C

(vol.%)

Silica surface

matrix

(GPa)

(GPa)

0

D-60a

0.65

0.36

0

D-30b

0.86

0.35

5

Hydrophobic

D-60

1.15

0.86

5

Hydrophilic

D-60

1.58

1.07

5

Silanatedc

D-60

1.25

0.39

10

Hydrophobic

D-60

1.10

0.89

10

Hydrophobic

D-30

1.38

0.59

10

Hydrophilic

D-30

1.38

0.54

10

Silanated

D-60

1.29

0.61

15

Hydrophobic

D-60

1.39

1.06

15

Hydrophobic

D-30

1.83

0.61

15

Hydrophilic

D-60

1.31

0.99

15

Hydrophilic

D-30

1.79

0.56

15

Silanated

D-60

1.98

0.83

a D-60 denotes nylon 11 powders with a mean particle size of 60 ^m. b D-30 denotes nylon 11 powders with a mean particle size of 30 ^m. c Silanated silica means the silica surface was modified with the use of A-1100 y-aminopropyltriethoxy silane.

Source: Reprinted with permission from [177], E. Petrovicova et al., J. Appl. Polym. Sci. 78, 2272 (2000). ©2000, Wiley-VCH.

a D-60 denotes nylon 11 powders with a mean particle size of 60 ^m. b D-30 denotes nylon 11 powders with a mean particle size of 30 ^m. c Silanated silica means the silica surface was modified with the use of A-1100 y-aminopropyltriethoxy silane.

Source: Reprinted with permission from [177], E. Petrovicova et al., J. Appl. Polym. Sci. 78, 2272 (2000). ©2000, Wiley-VCH.

Chen et al. found that a-Fe2O3 nanoparticles (10 nm) were generated in the ball milling process of a Fe3O4/PVC system in the presence of air [181]. With the use of MóBbauer spectroscopy, it was revealed that PVC powders were partially degraded because of mechanical milling and became reactive on their surface. a-Fe2O3 nanoparticles were the products of activated PVC and Fe3O4. The authors also found that MóBbauer scattering, characteristic of interfacial interactions between Fe3O4 and PVC, appeared even in the case of argon protection.

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