Powder Coatings: New Advances and Remaining Challenges Powder Coatings: New Advances and Remaining...
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Transcript of Powder Coatings: New Advances and Remaining Challenges Powder Coatings: New Advances and Remaining...
Powder Coatings: New Advances and Remaining Challenges
Dr Tom StrawAkzoNobel Powder Coatings
NEPIC INNOVATION DAY15 November 2011
What is a Powder Coating?01
What is a Powder Coating?
Market Size
Producers
Segmentation
Source: Akzo Nobel Powder Coatings Competitor Survey 2009
In-use performance of the cured coating is normally subject to international standards, specifying adhesion, impact resistance, behaviour on exposure to corrosive environments, UV resistance
Baking is required to allow the applied particles to melt, coalesce, wet the substrate; all this while the curing reaction advances to conversions ≥ 80% in around 10-20 minutes for an economic process (normally in the range 160-200°C).
And hence substrates should be conducting and earthed.
Curing reaction is typically a step-growth A+B type reaction with epoxy + carboxylic acid predominating, and β-hydroxy alklylamide + carboxylic acid being important (and problematic due to inability to catalyse and releasing H2O on cure).
Tg ≥ 50°C to ensure a solid powder that is storage-stable and transportable in the glassy state to minimise pre-reaction over a 2 year shelf life.
What is a Powder Coating?
A solid thermosetting coating applied by electrostatic spray and baked to produce a cured coating.High performance
Thermosetting coating cured above Tg∞. Coating does not vitrify prematurely.
Zero VOC Melt processed during manufacture.Electrostatically applied to the object being coated.Cured by heat.
Materials efficient
No diluents/solvent/water to be transported with the coating.Over-sprayed powder can be automatically recycled.Waste from the process is minimal, and can be disposed of as non-hazardous waste.
Toxicologically benign
No toxic pigments, minimal use of harmful materials. Any harmful materials are in the solid state, handled in an industrial environment and converted to non-hazardous forms in the cured films.
What is a Powder Coating?
VIDEO
The solid particles adhere to the substrate by electrostatic attraction. Charging of the particles is (normally) via passing through a corona discharge.
An alternate mechanism is tribostatic charging, where the particles are impinged against either PTFE or nylon liners inside the application gun, leading to the development of frictional charging.
What is a Powder Coating?
Particles feel the effects of:
• Aerodynamic forces: relatively long-range and influence particles with higher surface area:mass ratio (i.e. fines).
• Electrostatic forces: relatively short range and influence particels with higher surface area:volume ratio (a proxy for charge:mass).
Particles become adhered when the electrostatic force captures it from the airstream.
Electrostatic force
Aerodynamic force
+q
“image” charge
-q
charged powder particle substrate
x
80-100 kV
Intense electric field gradient polarises air molecules
Air is ionised
Powder coating particles moving through the ionised air pick up charge.
Technical Challenges 02
Technical Challenges – Low Bake
0 20 40 60 80 100 120 140 160 200180
100 %
60 %
80 %
40 %
20 %
0 %
En
erg
y co
nsu
mp
tion
/ %
Temperature / °C
Linear relationship: hotter oven = greater energy requirementVariables: insulation, weight of metal being coated, oven air changes, gas price…
Complex relationship: colder storage = greater energy requirement, but modified by stock size held, ambient climate/seasonality, distance from producer…
Cost for air-conditioned storage
Refrigeration
Insulation
Good storage
Cost for heating the oven
Savings from lower baking
Volatility in fuel prices lead to pressure on curing oven costs. Carbon footprint considerations also will drive reduction in curing temperatures.
The main driver for low bake, however, is access to new substrates…
Ambient cure
Technical Challenges – New Substrates
Substrates not painted today by powder coatings are either damaged by the heat of the curing process, or electrical insulators, or both. These present substantial barriers to further market expansion by powder coatings.
Traditional approaches to this problem tryto reduce the powder coating curing temperature to accommodate the demandsof the thermally sensitive substrate.
The problems we have to contend with are rheology and reactivity.
Polymer Temperature /°CPET 160Nylon 6 149acetal copolymer 141Polypropylene 135TPO 130Polycarbonate 125PC-ABS 110PPE/Noryl 110Nylon 99acrylic 90HDPE 88ABS 85Polystyrene 84acetal 80PVC 71LDPE 71Polyamide 70
Review of various published heat distortion, softening, and glass transition temperatures for commonly-used polymers
Low Bake Powder Coatings I – Rheology
Conventional Formulations
Normal polymer formulation allows reduction of melt viscosity at a given temperature (e.g. substitute aromatic diacids for aliphatic), BUT with a Tg penalty.
Refridgeration of powder is not desirable and off-sets any benefits of low bake products.
Dendritic Materials
e.g. Boltorn H2O, ex. Perstorp
Temperature
Vis
cosi
ty
Tg≥
50°C
Storage stability
With TiO2Clear
Semi-Crystalline Materials
Our extrusion production process does not lend itself to incorporation of semi-crystalline materials. The quenching process does not give time for recrystallisation. The resulting product is difficult to grind.
Flow and levelling
We’re no longer talking about powder coatings…
Low Bake Powder Coatings II – Reactivity
Time
Con
vers
ion
Curing160-200°C
Extrusion110-120°C
Storage(upto 2 years)
T=120°C
T=180°C
Minimise pre-reaction
Develop properties on cure
Time
Con
vers
ion
T=120°C
T=140°C
Extrusion110-120°C
We now have appreciable conversion during the production process – probably even gelation!
Catalysed system
How to retard (pre)reaction during production?• b-HAA + -CO2H reaction liberates H2O, and the pressure in the extruder can
retard this reaction (but the reaction is too slow for low bake).• Addition of catalyst late in the extrusion process could delay pre-reaction (but
adequate dispersion of the catalyst then becomes an issue).• “Latent” catalysts are typically mentioned at this point, but thermo-latent will not
work when production and cure temperatures are matched.• Photolatent bases[1,2] could be triggered during the application process, but be
inert during production.• Autocatalytic cure mechanisms (e.g. epoxy homopolymerisation) give a more
“S-shaped” conversion-time profile, and may be more amenable to production.1 Carroy, A., et. al., Prog. Org. Coat., 68(1-2), 2010, 37-412 Meier, M, et. al., Paint & Coatings Industry, 2009.
Alternate Approaches03
Separation of melt/flow and cure into two separate processes
• Degreasing for metal• Sanding for wood/pre-coated metal• Pre-heat if not fully conductive, or spraying of conductive solution.
Substrate Preparation
Alternate Approaches I – Radiation Cure
For powder coatings on wood, methacrylated unsaturated polyesters represent current state-of-the-art. Substrates are normally MDF, but plywood and some natural woods are possible (though the coating is normally then textured to hide defects).
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O
O
O
O
O
O
O4X
For powder coatings on metal, acrylated polyesters are included. This reduces the cross-link density and minimises adhesion issues. The main problem is obtaining through-cure to get adhesion while maintaining opacity in the visible region. Typical applications are pre-assembled parts (e.g. ball bearing assembly including lubricating grease).
O O
OO
Time
Vis
cosi
ty softening, melt flow
mol
ecul
ar w
eigh
t bui
ld
UV off UV on
In order to be able to offer differentiated solutions to these and other challenges, the World Wide RD&I Powder Coatings Teams have been supported by an on-going multi-million pound investment on Felling site: Buildings, Equipment & Personnel
New state-of-the-art Polymer Laboratory
Miniature resin plants for novel resin systems
Fully automated 50L Resin Pilot Plant for Scale-Up trials
Powder paint application line for product testing and customer support
Summary04
Summary
• Powder coatings represent a “green” finishing solution, with scope for further expansion.
• New markets are likely to include “difficult” substrates, damaged by heat and possibly non-conducting.
• Conventional step-growth cross-linking is limited by the constraints of being a powder coating:
• Alternative technologies such as UV cure require further developments in polymer and application technology to overcome the current limitations.
• Novel approaches are also being investigated with the aid of new investment in facilities and personnel.
Storage stable powder Tg≥50°C, tstorage≤2 years
Manufactured by extrusionT~120-140°C, tresidence~60 s
Good flow at low bakeh~50 Poise at T~140°C
High degree of conversion at low bakeT~120-140°C, t~10 mins, p≥80%
VS
Thank You…