Visual appearance and scratch resistance of high performance thermoset and thermoplastic powder coatings

https://doi.org/10.1016/j.porgcoat.2012.09.024Get rights and content

Abstract

A comparative evaluation of electrostatic spray and ‘hot dipping’ fluidized bed to deposit two different organic paints belonging to the class of thermoplastic (PPA571, an alloy of acid modified polyolefins) and thermoset (TGIC-free transparent pigmented bronze polyester) powders was performed. Visual appearance of the investigated coatings was evaluated by colour, gloss and coating thickness measurements as well as by the determination of the surface morphologies. Micro-mechanical performance of the coatings was assessed by progressive load scratch tests.

‘Hot dipping’ fluidized bed is found a fast deposition technique as, after substrate pre-heating, it takes just few seconds to have the part completely powder coated. On the other hand, electrostatic spray deposition is a potentially selective coating technique, but it lasts longer (generally, 6–15 s) and, moreover, the coated parts must be post-cured for long time (at least, 15 min) and at high temperature (150–200 °C) to give rise to the formation of continuous films. Indeed, whilst ‘hot dipping’ fluidized bed is found particularly suitable for the deposition of thick and smooth thermoplastic coatings, electrostatic spray deposition is found the most viable technique to deposit thinner and highly scratch and wear resistant thermoset coatings.

Highlights

► ‘Hot dipping’ fluidized bed and electrostatic spray. ► Visual appearance evaluated by colour, gloss and surface morphology. ► Scratch response evaluated by progressive mode scratch tests. ► Electrostatically sprayed thermoset powders exhibit better scratch response. ► Thermoplastic coatings display better visual appearance but very singular scratch properties.

Introduction

Painting processes allow to deposit aesthetic and/or protective organic or hybrid organic/inorganic coatings onto a wide variety of metal and non metal substrates [1]. Painting processes can be subdivided in two macro-categories: (i) coating processes involving raw materials in the form of powders (i.e., powder coatings) and (ii) coating processes involving raw materials in the form of liquids (i.e., wet paints). Painting powders include both thermoplastic and thermoset resins, whilst wet paints include mostly solventborne, waterborne and high solid material formulations [2].

Although the market of powder coatings is rather limited (∼11% of world painting market share) if compared with the wet paintings, it still represents the fastest growing segment (+3.5% in US and Far East) and it involves ∼700 million tonnes of raw material annually sold [3]. In several manufacturing divisions (housing for consumer electronics, domestic appliances, automotive interiors, aerospace and aeronautic components, etc.), powder coatings have progressively replaced conventional wet paintings as their deposition process allows comparable performance with a somewhat competitive, safer and environmentally friendly technique [3]. In particular, powder coatings do not produce wet residuals which must be subsequently dismissed and all the painting materials can be virtually recovered and reused in the painting process. Moreover, during their baking process, there is no solvent to evaporate and, consequently, powder coatings have lower energy requirements and they entail extremely reduced emission of volatile organic compounds. Nonetheless, the economics and the performance of powder coatings are strictly related to the operating strategy and, in particular, to the choice of the raw materials as well as to the choice of the deposition and heating processes.

Powder paints are usually provided in the form of finely divided (∼20 μm) or coarser (∼150 μm) loose powders. They are composed of base resin (and cross-linking agents in thermoset resin), pigments, plasticizers, dispersants (flow promoters) and inorganic fillers [4]. Thermoplastic resins typically melt and flow at elevated temperatures whilst maintaining their chemical integrity. On the other hand, thermoset resins provide similar melting and flow properties (75–85 °C) whilst changing their chemical properties upon curing (140–200 °C). Both thermoplastic and thermoset powder paints are deposited in a dry process and, then, they are heated up to promote their flowing, levelling and full consolidation in a continuous film [5]. Alternatively, powder paints can be deposited on pre-heated substrates and exploit their heat capacity to melt, level and consolidate in the form of continuous film [5]. The thicknesses of the resulting coatings can vary in a wide range of few tens to few hundreds of microns [4], [5]. The application processes of powder coatings include: (i) electrostatic spray; (ii) hot dipping fluidized bed; and (iii) electrostatic fluidized bed [6]. Electrostatic spray uses a set of guns with a nozzle and an electrode at high potential to ionize an air flow and spray electrically charged powder paints on earthed substrates moving through a deposition booth [7], [8], [9]. The substrates are consequently coated with a layer of powder paints, which must be baked at relatively high temperature (150–200 °C) for rather long time (15–20 min) to flow and be fully consolidated up to give rise to a continuous film [10]. Fluidized bed is a deposition booth in itself and the parts to be coated are pre-heated at relatively high temperature (250–400 °C) and, then, dipped inside a ‘fluidized bed’ chamber in which the powder paints are taken in a ‘fluid-like’ state by means of an air flow [11], [12]. The pre-heated parts generally provide enough heat to the powder paint to allow their melting, levelling and full consolidation in the form of a continuous film. Alternatively, a short post-baking cycle (200 °C for few minutes) can be scheduled [13], [14], [15]. Electrostatic fluidized bed is a coating technique less spread in the industrial practice and it is an intermediate deposition technique, which involves a ‘fluidized bed’ chamber and a set of high potential electrodes inside it to ionize the fluidization air and transfer the electrical charge to powder paints when fluidized. Ionized powder paints can then coat earthed parts when dipped inside the ‘fluidized bed’ chamber [16], [17], [18]. After that, fully consolidation of the powder paints must be achieved by baking at relatively high temperature (150–200 °C) and for rather long time (15–20 min) as in the case of the electrostatic spray [19].

All the aforementioned deposition techniques can allow the effective coatings of metal substrates at high production rate and reasonable costs. Nonetheless, the choice of raw material and coating process is of primary importance when performance (visual appearance, adhesion strength and wear resistance) of the coatings and economics of the process play a crucial role in the competitiveness of the manufacturing process. This is therefore the context in which the present investigation compares the suitability of the electrostatic spray and hot dipping fluidized bed to coat flat aluminium substrates with thermoplastic(PPA571, an alloy of acid modified polyolefins) and thermoset (TGIC-free transparent pigmented bronze polyester) powder paints, respectively. First, the visual appearance of the coatings was assessed by evaluating the colour, gloss and thickness as well as the roughness parameters achieved using the two different raw materials and deposition techniques. After that, the adhesion strength and the scratch resistance of the different coatings were checked by progressive mode scratch tests, using a Rockwell C conical indenter with a tip radius of 800 μm and a load in a wide range of 0–30 N.

The usage of the thermoplastic and thermoset powder paints together with the two different deposition techniques took to powder coatings with a wide variety of aesthetic and mechanical behaviour. In particular, hot dipping fluidized bed and thermoplastic powders allow to achieve thicker and softer coatings, whilst electrostatic spray and thermoset powders allow to achieve thinner, harder and wear resistant coatings. Analytical examinations and physical understanding of the experimental findings allow to rank among the different materials and deposition techniques, thus providing useful indications to powder coaters on how to best deal with the choice of the powder paints and coating processes.

Section snippets

Powder paints

Plascoat PPA571, supplied as a finely divided powder (95% less than 250 μm), was employed as raw material for the formation of thermoplastic coating (TP). It consists of halogen free blends of functionalized polyolefins including pigments and additives. It is specifically designed to provide a long lasting, tough coating for exterior applications to mild steel, galvanized steel and aluminium. PPA571 is safe and environmentally friendly with combustion fumes low in smoke and very low toxicity

Analysis of the coating processes

Visual appearance of an organic coating is basically related to colour and gloss. Colour and gloss are in turn related to material formulation, coating thickness and morphology. Coating thickness and morphology are related to material formulation itself, but also to coating process. Electrostatic spraying and ‘hot dipping’ fluidized bed are intrinsically different coating processes. They lead to coatings characterized by significant dissimilarity in both the thickness achievable and surface

Conclusions

The present investigation deals with the analysis of the visual appearance and scratch response of a class of thermoplastic (i.e., polyphthalamide) and thermoset powder coatings (i.e., polyester). Their visual appearance and scratch response are found to be strongly related to the intrinsic properties of the two materials, the way by which heat is delivered to the powder paints after as well as to the coating thickness achievable after electrostatic spray deposition and hot dipping fluidized

References (30)

  • K.D. Weiss

    Prog. Polym. Sci.

    (1997)
  • Q. Ye et al.

    J. Electrostat.

    (2002)
  • Q. Ye et al.

    Powder Technol.

    (2003)
  • S.S. Lee et al.

    Prog. Org. Coat.

    (1999)
  • M. Barletta et al.

    J. Mater. Process. Technol.

    (2006)
  • K.C. Leong et al.

    J. Mater. Process. Technol.

    (1999)
  • M. Barletta et al.

    Prog. Org. Coat.

    (2006)
  • M. Barletta et al.

    Prog. Org. Coat.

    (2007)
  • M. Barletta et al.

    Surf. Coat. Technol.

    (2006)
  • M. Barletta et al.

    Surf. Coat. Technol.

    (2006)
  • M. Barletta et al.

    Surf. Coat. Technol.

    (2006)
  • M. Barletta et al.

    Surf. Coat. Technol.

    (2007)
  • S.L. Zhang et al.

    Mater. Sci. Eng. A

    (2003)
  • H. Jiang et al.

    Polymer

    (2009)
  • S.J. Bull

    Surf. Coat. Technol.

    (1991)
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