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?

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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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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).

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Time

Vis

cosi

ty softening, melt flow

mol

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eigh

t bui

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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%

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Thank You…