Surface modification with nano and micro particles
Not only functional polymers can be used as modifiers, but also particles. The processing is shown in figure 1.
Process during immobilisation of particles on the melt surface using injection moulding. 1. Application of particles on the mould surface, e.g. by spray-coating of a colloidal solution; in basic research work, particles are applied onto a substrate that is later be mounted in the mould. 2. Closing of the mould; 3. Injection of the melt and embedding of the particles on the melt surface; 4. During demoulding, the particles are transferred from the mould to the part surface.
Scheme of the created structure. The particle surrounded by a shell of a reactive functional polymer is connected to the plastic part surface by mechanical embedding as well as by chemical coupling. Parts of the polymer shell that is accessible can be used for functionalisation of the surface.
Example: Embedding of gold nanoparticles (AuNP)
Scanning electron microscopy images.
Top left: SEM image of AuNP (20 nm diameter) on substrate;
Bottom left: AuNP on plastic surface;
right: Transmission electron microscopy image of a thin cut (50 nm) perpendicular to the surface, and photography of the part.
The AuNP form a mono layer on the plastic surface, as before on the substrate. The arrangement of the particles is not altered by the melt flow. There are no shifts, vortexes or mixing. The surface roughness is smaller than the particle size. It is mainly determined by the substrate roughness. The high density of AuNP in the monolayer stains the whole part intensively red.
Catalytic activity of immobilised AuNP. Methyl orange is decomposed by NaBH4 under catalytic activity of AuNP. The activity of AuNP immobilised on the plastic surface is smaller then on the substrate surface due to partial embedding.
Example: Embedding of Silica nanoparticles (SiNP)
SiNP (200 nm diameter) embedded on a plastic surface
Competing processes during embedding of particles during moulding
The melt moves typically in a fontain flow during injection moulding. The rate vector on hitting the mould surface and thus, the particles as long as these are sufficiently small, is pependicular to the mould surface. The interfacial tension results in wetting of the aprticles by melt. At the same time, cooling of the outermost melt surface layer occurs on contact with the cold mould wall with extreme high rates. By this, the molecular state is practically solidified. Variation of these processes are of fundamental interest: Which process prevail under certain conditions? Can the embedding depth be controlled?
Possible applications of nano particle-modified layers
The nanoparticle layers provide typical properties of anorganic materials to the plastic surface. This is true for polarity, wettability and mechanical properties like hardness and abrasion. Special properties like electronic conductivity with transparency or a catalytic or an antibacterial finish may be achieved. Metal particles may be used to convert light to heat locally, which may be used for chemical structure formation. Some nanoparticles may be used as precursors for post-processing structure formation, like coupling to these particles.