Effects of Painting on Mechanical Properties of Injection Molded Plastics

David E. Palmer, P.E.BRP – Marine Propulsion Systems Division,

Sturtevant, WI

Materials and coatingsQUESTION: Why apply a coating to a material?

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Appearance

Abrasion/wear

Friction coefficient

Thermal barrier

Electrical conductivity

Corrosion resistance

Chemical resistance

UV resistance

Static dissipation

Optical reflectivity

Materials and coatings

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QUESTION: Why apply a coating to a material?

ANSWER:

To change the material’s properties and/or behavior

Materials and coatings

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… is it reasonable to assume that all other aspects will

remain unchanged?

QUESTION: If we change one aspect of a materials’ properties or behavior…

Materials and coatings

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Effect of nickel plating on fatigue life of steel

Source:J.O. Almen, "Fatigue, loss

and gain by electroplating," Product Engineering 22:109-116,

June 1951.

Materials and coatings

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Effect of zinc-plated nut on glass-filled nylon-6,6

EDS of corrosion product on surface of nut

Paint and plastics

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QUESTION: Why paint plastics?

Mass customization

Color/effect not available in resin

Resin lacks UV resistance

Aftermarket refinishing

Touchup/repair

Paint and plastics

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QUESTION: What are the effects of painting on the mechanical properties of plastics?

•Mechanical interactions

•Chemical interactions

•Thermal effects

Mechanical interactions• Elastic mismatch• Interfacial strength• Film thickness

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Source:M.S. Hu and A.G. Evans,

"The cracking and decohesion of thin films on ductile substrates,"

Acta Metallurgica 37:917-925, 1989.

Chemical interactions

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• Swelling• Dissolution• Environmental stress cracking

Thermal effects

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• Accelerated solvent diffusion into substrate• Relief of molded-in stresses• Shrinkage of paint (induces residual stresses)• Thermal degradation of substrate

Experimental design• For four (4) different substrates, measure

mechanical properties in each of these six (6) conditions:

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No coating Adhesion promoter 1 only

Adhesion promoter 2 only

Topcoat onlyAdhesion

promoter 1 with topcoat

Adhesion promoter 2 with

topcoat

Substrates

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• General purpose ABS

• High impact ABS

• Weather-resistant ASA

• 10% mineral filled polyethylene ionomer/PA-6 blend

Coatings

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• High-solids black acrylic topcoat

• Clear adhesion promoter (40% aromatic hydrocarbons)

• Black adhesion promoter (<2% aromatic hydrocarbons)

Sample preparation• Black topcoat:

45 µm dry film build

Cure at 93°C for 20 minutes

• Clear adhesion promoter:5 µm dry film build

Cure at 77°C for 20 minutes

• Black adhesion promoter:20 µm dry film build

Cure at 71°C for 20 minutes

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Testing

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• Notched Izod impactASTM D256-10 Method A

• Tension testingASTM D638-10

Polyethylene ionomer/PA-6 blend: 50 mm/min

All other substrates: 5 mm/min

Notched Izod impact

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Tensile modulus

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Tensile strength

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Strain at yield

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Strain at break

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Supression of necking

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High impact ABSBlack adhesion promoter and

topcoat

High impact ABSUncoated

This effect was observed in all of the coated ABS and ASA specimens.

Film cracking

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This effect was observed in all of the ABS and ASA

specimens with topcoat (regardless of whether or not they had adhesion

promoter).

Loss of adhesion

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This effect was observed the PE ionomer/PA-6

specimens with topcoat (regardless of whether or

not they had adhesion promoter).

Film adhesion

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For the PE ionomer/PA-6 specimens with either adhesion promoter (without topcoat), no loss of

adhesion was observed.

The specimen above experienced 165% elongation at break.

Conclusions

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Rank ordering of effects:

• Strain at break (most affected by painting)• Impact strength• Tensile modulus• Tensile strength• Strain at yield (least affected by painting)

Conclusions

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Coating effect on substrates:

For ABS and ASA substrates, strain at break and impact strength decreased with coating.

For mineral-filled PE ionomer/PA-6 blend, strain at break and impact strength increased with coating.

Conclusions

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Effects of adhesion promoters/topcoat:

The effects of the topcoat and the two adhesion promoters were different on different substrates.

When the adhesion promoters were used together with the topcoat, the effects were not additive.

Conclusion

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Matching a paint system to a plastic substrate should be done with great care.

Published data is scarce.

Testing is the best way to ensure a successful match.

Acknowledgements

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PolyOne – Richard Baril

SABIC – Dennis Yon

LTL Color Compounders – Don Hone

PPG Industries – Ron Kalski, Dave Nale, Cari Stein

Polymer Diagnostics – Keith Mathews, Jill Moran, Katlyn Hoffman

BRP US, Inc. – Bill Barth, Rich Smock, Mike Davenport, Ben Jones, Matt Coyne