The benefits of powder coating wood and MDF
Wood/MDF Powder Coating options
Powder coating MDF and wood is a disruptive technology that is set to open new markets. Specifically, the many benefits of powder coatings, normally thought as applicable only to metals can now be conferred to wood, MDF (medium density fiber board) and other temperature sensitive substrates. Examples of these benefits are improved adhesion, chemical resistance, scratch resistance, improved durability and a significant lowering of volatile organic content (VOC). In addition, irregular shaped objects, edges and borders can be painted at a significantly reduced production cost. Instead of multiple layers, sanding and mechanical preparation, a lot less effort is required.
While there are many benefits to powder coating including greater chemical- and humidity resistance, scratch resistance, durability and longevity, cost efficiency, high end look, environmentally friendly (low VOC) and easy maintenance (STP Performance Coatings, n.d.). These advantages are realized by investing in the necessary equipment such as ovens and electrically charged spray guns and fluidized bed feeding systems to feed the powders to the applicator guns. Still, there are some risks involved in the actual coating process that must be carefully considered such as touch-up, the speed of application and the correct amount of powder to apply. In addition, the baking process has to be carefully controlled to minimize under- or over baking.
To develop wood and MDF powder coatings, a number of additional challenges are faced. Chief amongst these is the fact that unlike metal, wood, MDF and plastic are not conductive. Powder coating of these objects require enhancing the conductivity such as exposing wood to an initial source of heat to drive moisture (at least 8%) to the surface or by spraying a conductive primer onto the object that has to be powder coated (Lin, n.d.), (ONOYAMA et al., 2005). In addition, the chemistry associated with each conductive primer may differ considerably from highly filled conductive metal coatings, quaternary ammonium compounds to inorganic compounds that facilitate charge transport. Non-conductive parts can sometimes be preheated and sprayed hot with conventional powder, or an in-mold process can be applied in which a mold is electrically charged and heated, powder is applied and melted and then exposed to the part to be coated. Successive layers of resin can be applied and later mechanically smoothed.
While facilitating conductivity in non-conductive substrates can be achieved by highly sophisticated chemistries, the next significant hurdle remains the heat sensitivity of the object that requires coating. Wood, MDF and plastics cannot endure exposure to high temperatures or excessive exposure times to high temperatures. A major chemical process that affects all of these non-metallic surfaces remains oxidative degradation. Therefore, ways to address low curing powder coatings is of great importance.
The various types of low temperature powder coating options
Technology with regard to low temperature bake powder coatings for MDF, wood and other temperature sensitive substrates has grown substantially (Kevin Biller, 2019), (IFS Coatings, 2020). Consequently, the number of raw materials used to make the polyesters (excluding polyurethanes) also grow. Some of them are given below in Schematic 1. Note that some of the polyols featured here can be used when making polyurethanes.
Schematic 1. Di- and tri-functional polyols and acids used in the synthesis of polyesters.
In this document, epoxy-polyester hybrids will be the main technology explored based on recent advances in this area. However, some of the additional technologies that are available will be briefly discussed (Make & Debut, 2016).
Polyester – TGIC technology
As with epoxy-polyester hybrid technologies, acid-functional polyesters can be manufactured with the desired acid value and molecular weight that provides good shelf life with Tg’s above 50 ºC and acid values between 60 and 75 mg KOH.g-1. Note that a balance has to be achieved in terms of molecular weight and functionality for the polyester to be able to provide the advantages normally associated with highly durable polyester resins using TGIC as crosslinker (Fink, 2018). Polyester structures incorporating trimellitic anhydride, terephthalic acid as well as diols that provide flexibility such as 1,3 Propanediol (Lichang Zhou, Shelby F. Thames, Oliver Wendell Smith, Wyndham Henry Boon, 2003), (Thomas E RENO, Wenjing ZHOU, Carlos CONCHA, 2014) provide control over melting points and functionality
Schematic 2. Modified polyesters in terms of molecular weight and acid functionality are reacted with TGIC to yield a crosslinked structure.
Curing temperatures below 140 ºC are reported for these polyester-TGIC systems. However, while TGIC is making a come-back in ultra-low curing polyesters, the same cannot be said for systems based on Primid or hydroxyalkyl amide (HAA).
Schematic 3. An example of a hydroxyalkyl amide (HAA) or Primid crosslinker.
HAA systems cure at 160 ºC with the release of water since it is a condensation reaction. Unfortunately, the HAA systems do not perform as well as the TGIC cured systems when it comes to low temperature curing. It simply requires longer times to cure fully.
Polyurethanes are formed by reacting a polyol with an isocyanate such as Methyl Diphenyl Diisocyanate (MDI).
Schematic 4. An idealized reaction between MDI and a polyol of various molecular weight and functionality.
Polyurethanes are not well known for their low-curing ability but it is reported that a few niche polyurethanes (PUs) can cure below 145 ºC. Polyurethanes bring with them desirable properties such as a one-shot matte and dull matte finish, very good chemical resistance as well as UV resistance.
Acrylics, and specifically glycidyl methacrylate acrylates (GMA) offer a host of advantages including smoothness, high gloss, scratch resistance and outdoor resistance (Make & Debut, 2016), (ALMATEX Acrylic Resins, n.d.). Advanced technology allows the molecular weight, molecular weight distribution and polymer morphology of free radical polymers to be controlled with great precision. Controlled free radical polymerization is one such technology used to modify functional acrylic polymers. The functional acrylic polymers can react with diacids as depicted in Schematic 5 below.
Schematic 5. Reaction between a glycidyl methacrylate functional acrylic polymer and a diacid that can have various molecular weights. In the schematic, P1, P2, P3 and P4 are polymer fragments that may or may not be similar to the polymer sections that include the functional groups.
Since the acrylic technology is well-known in powder coating circles, the modified acrylics can also be used with modified polyesters bearing multiple acidic functionality. The acrylic polymer phase separates on a microscopic scale and causes an immediate matte finish.
Schematic 6. Reaction between glycidyl functional acrylic polymer and functional polyester. In the schematic, P1, P2, P3 and P4 are polymer fragments that may or may not be similar to the polymer sections that include the functional groups.
UV-curable ultra-low temperature systems
UV-curable powder coatings have been around for some time, spearheaded by the pioneering work of Vincent McGinnis (Mcginniss, 1979). Since McGinnis’ work the basic premise of having powders containing pendant, activated double bonds and epoxy functionality have remained the mainstay in terms of technology. Added to the mainstream technology are catalysts that have improved the reaction rate and lowering of the molecular weight of the functional polymer which could for example be a multi-functional organic acid functional polymer. The basic steps in UV-polymerization requires the powder to be heated after application to the substrate using electrostatic charging of the powder. Low temperatures of between 95 – 124 ºC can be applied. The heating can be achieved using infra-red lamps followed by irradiation with strong UV light. A free radical polymerization reaction is initiated that can crosslink the molten powder resin. Typically, two types of initiation mechanisms are encountered as shown in Schematic 7 and 8 below.
Schematic 7. Norrish type 1 photo-chemical initiation takes place by cleavage of molecular fragments into reactive radical species once uʋ-light has been absorbed.
Schematic 8. Abstraction of a hydrogen after the initial activation of benzophenone and benzophenone derivatives through enol formation and abstraction of hydrogen from solvent molecule such iso-propanol.
Ultra-low temperature curing powder coatings for wood and MDF
Polyesters and polyester hybrids is reported to be the most widely used chemistries presently being used and it is expected that this trend will continue into the future (Focus on Low-Temperature Cure Powder Coating Technologies - American Coatings Association, n.d.). Ultra-low curing (ULC) that can cure below 135 ºC require a careful blend of properties to ensure targeted low temperature curing while maintaining storage stability. Hence, the manipulation of molecular weight and functionality of the resins becomes critical. Epoxy-bearing phenolic resins are also well-known in the powder coating industry and this technology can be readily combined with modified polyfunctional polyesters. An example of the synthesis of an epoxy-novolac resin is given in Schematic 9 below.
Schematic 9. Idealized synthesis of epoxidized novolac phenolic resin by reaction with epichlorohydrin.
Once the epoxy-bearing polymer and the functionalized polyester resin is mixed, the low molecular weight of the polymers allow them to fuse and react to form a final crosslinked coating. Novolacs melt around 110 ºC while the modified polyesters can melt from 124 to 141 ºC, depending on the composition of the polyester and its molecular weight.
Schematic 10. Idealized mechanism of fusion of polymers carrying different functionalities to form a crosslinked network. The functionalities shown here include polymers with multiple organic acid groups (circles) as well epoxy-functional polymers (squares).
The hybrid epoxy-polyester powder coatings have good hardness, good scratch resistance and chemical resistance but their UV durability is relatively low. Typical areas of application that open up for this technology include office furniture and ready to assemble furniture.
The energy requirements for low-bake hybrids and how conventional powder coating can benefit
At the time of writing this document, Europe and the USA are experiencing unprecedented increases in fuel prices not seen since the 1970’s. Clearly a shift away from fossil fuels is now no longer a mere point for discussion, global warming and war has driven companies to investigate and introduce alternative technologies as a matter of urgency.
With regard to lowering energy requirements, the introduction of ultra-low curing technologies such as those introduced by Allnex allows companies to reduce energy costs significantly. It is reported in a recent publication (Fink, 2018), that reducing a powder coating oven temperature from 191 ºC to 163 ºC, realizes cost savings over 9 shifts of US$ 13.30 or 16.4%. For a single shift, 250 days per year, this amounts to US$ 3,325.
Similarly, (Fink, 2018) has been shown that less heating is required for heavy metal substrates. It is reported that the residence time in a conventional convection oven for a 5 kg metal plate can be reduced by 50% if a powder with a melting point of 200 ºC is replaced with one that has a melting point of 160 ºC.
New opportunities using ULC technology
Low curing technology is not only an essential development to lower energy costs The technology has also led to many more substrates that can be finished with powder coatings including medium density fiber board or MDF and plastic substrates. MDF normally undergoes a four-step process. After sanding, the MDF board is heated to about 121 ºC for 30 to 60 seconds to enhance the surface conductivity. The heated MDF object is then transferred to a spray booth where powder is applied. The coated pieces are then exposed to infrared radiation to melt and ensure curing of the paint. While ULC technology opens access to coat heat-sensitive substrates such as plastics, wiring, seals and electronic boards, a further advantage is to coat massive metal components, a task that has been daunting for powder coaters up until now due to the large amount of heat that has to be supplied to the metal in order for the resin powder to melt. Lower curing technology means massive metal objects can be coated faster thereby increasing productivity, throughput and lowering overall costs.
The main drivers for low temperature curing technologies are:
Lowering of VOC and hence improved and more sustainable environmentally friendly coatings.
Lower energy requirements which have become a cardinal contributor to costs in conventional powder coatings.
Opening of new market segments and new areas of applications such as MDF, plastics and other temperature sensitive substrates.
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