effect of painting on the mechanical properties of injection-molded plastics
DESCRIPTION
Paint can have significant effects on the mechanical properties of plastic substrates. The selection of a paint system that is not properly matched to a given plastic substrate can lead to premature failure. While anecdotal accounts are plentiful, quantitative data regarding effects of coating on plastics is relatively scarce. This paper describes the effect of a high-solids acrylic topcoat paint, with either of two different adhesion promoters or no adhesion promoter, on the mechanical properties of four different plastic substrates: general-purpose acrylonitrile-butadiene-styrene (ABS), high-impact ABS, weather-resistant acrylonitrile-styrene-acrylate (ASA), and mineral-filled polyethylene ionomer/polyamide-6 blend.TRANSCRIPT
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Effects of Painting on Mechanical Properties of Injection Molded Plastics
David E. Palmer, P.E.BRP – Marine Propulsion Systems Division,
Sturtevant, WI
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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
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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
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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…
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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.
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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
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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
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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
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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.
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Chemical interactions
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• Swelling• Dissolution• Environmental stress cracking
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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
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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
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Substrates
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• General purpose ABS
• High impact ABS
• Weather-resistant ASA
• 10% mineral filled polyethylene ionomer/PA-6 blend
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Coatings
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• High-solids black acrylic topcoat
• Clear adhesion promoter (40% aromatic hydrocarbons)
• Black adhesion promoter (<2% aromatic hydrocarbons)
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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
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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.
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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).
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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).
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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.
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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)
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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.
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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.
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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.
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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