Surface and Coatings Technology
DENNIS MEINDERINK; KARLO J.R. NOLKEMPER; JULIUS BÜRGER; ALEJANDRO G. ORIVE; JÖRG K.N. LINDNER; GUIDO GRUNDMEIER
• Spray coating of ZnO tetrapods (ZnO TP/poly(acrylic acid)) homogenous nanocomposite films
• Improved adhesion is achieved for laminated PP foils.
• TEM and FE-SEM analysis of fracture modes
• Adhesion promotion is proven to be a result of both mechanical interlocking and macromolecular adhesion promotion.
Keywords: ZnO tetrapods (ZnO TP); Spray coating; Dispersions; Poly(acrylic acid) (PAA); Nanocomposite; Adhesion promoting films
Zinc oxide tetrapods (ZnO TP) have reached a high level of interest in the last years due to their broad fields of applications. Here, the combination of the adhesion promoting properties of both ZnO TP and poly(acrylic acid) (PAA) is demonstrated. A very rapid spray coating of water-based dispersions has been used to deposit homogenous thin nanocomposite films on poly(propylene) (PP) foils considered as model system. The characterization of ZnO TPs has been carried out by field-emission scanning electron microscopy (FE-SEM), high-resolution X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The increased shear strength values exhibited by PP/PP laminates with interfacial ZnO TP/PAA films could be attributed to mechanical interlocking of ZnO TP and the macromolecular adhesion promotion by PAA. FE-SEM imaging after shear tests indicated a failure in the core of the ZnO TP. The fast application method via plasma activation of the PP foils and spray coating of the composite films with low concentrations of 0.085 mM PAA and 8 mg/ml ZnO TP makes this technique rather interesting for industrial applications.
For the spray coating process, a conventional spray coater (ND-SP 11/4 Precision Spray Coater, Nadetech Innovations S.L., Spain) with vertical configuration and integrated hotplate was used. A mixture of nitrogen and the water-based dispersion was sprayed with a high viscosity liquid atomization spray nozzle (mini-atomized nozzle MMAE/MMA type, Nadetech Innovations S.L., Spain) made of 316 L stainless steel. Here, the following parameters were chosen at the spray coater: a hotplate temperature of 75 °C (±3 °C), a volume speed of 100 ml/min, a distance between the nozzle and the substrate surface of 12 cm, a movement speed of the nozzle of 1200 mm/min (i.e. 1 s for spray coating time on 1 cm2 PP foil), a system pressure higher than 6.0 bar of pure nitrogen and a pressure at the nozzle of 3.50 bar.
Before each spray coating experiment, the whole system was cleaned with at least 20 ml of ultrapure water (0.055 μS/cm, Ultra Clear TWF, SG water, Hamburg, Germany). Additionally, a stabilization time of 2 s to create a stable spray coating mist was set in the software (Nadetech Innovations S.L., Spain).