EFFECTS OF PAINT ON PLASTICS:

TEST –

DON’T

GUESS!Dave Palmer, P.E.Materials Engineer, BRP Marine Propulsion Systems Division

Mass Customization

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Paint vs. Mold-in-color

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Advantages of molded-in-color

Lower part cost

No secondary operations

Same color through thickness of part

Paint vs. Mold-in-color

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So, why paint plastics?

Volume may not justify a press run

Color/effect may not be available in resin

Improved environmental resistance

Aftermarket refinishing/repair

Materials and Coatings

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In general, coatings are applied to materials…

To change the material’s properties and/or performance in some way

Examples:

Color, appearance, aesthetics

Environmental resistance

Abrasion, wear, friction

Electrical conductivity

Thermal conductivity

Static dissipation

Materials and Coatings

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But sometimes…

Material properties change in unexpected ways

Example: Nickel plating on steel

(J.O. Almen, 1951)

Materials and Coatings

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Materials and coatings interact with…

• Each other• Their environment• Other materials and coatings

Example: Zinc-plated steel nut and nylon-

6,6 blowoff valve

Paint and Plastics

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Paints interact with plastics in three ways…

• Chemical• Thermal (i.e. cure cycle)• Mechanical

Chemical Interactions

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Solvents (including paint solvents) can cause …

• Swelling of the polymer• Dissolution of the polymer• Environmental stress cracking (ESC)• Changes in molecular entanglement

Example: ESC of ABS part due to threadlocking

compound

Thermal Interactions

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Curing of paint at elevated temperatures can cause …

• Accelerated solvent diffusion into plastic• Relief of molded-in stresses• Paint shrinkage (residual stress)• Thermal degradation of plastic

Mechanical Interactions

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Mechanical interactions depend on…

• Elastic mismatch• Interfacial strength• Film thickness

Figure adapted from Hu and Evans, 1989

How Can You Predict The Effects?

You can’t.

That’s why you test.

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Experimental Set-up

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Four plastic substrates…

Typically painted

Not typically painted

All have similar mold shrinkage (.006 - .008)

Substrates

General purpose ABS

High impact ABS

Weather resistant ASA

PA-6/PE ionomer blend (Surlyn)

Experimental Set-up

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Topcoat

Adhesion promoter

#2

Adhesion promoter

#1

Baseline (no coating)

Topcoat: 2K acrylic polyurethane

Adhesion promoter #1: 40% aromatic

hydrocarbons

Adhesion promoter #2: <2% aromatic

hydrocarbons

Testing

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• Tensile modulus• Tensile strength• Strain at break• Notched Izod impact strength

Baseline Results

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General

purpose ABS

High impact

ABS

Weather

resistant ASA

PA-6/PE

ionomer blend

(mineral filled)

Tensile

modulus

(MPa)2409 2004 2410 2195

Tensile

strength

(MPa)41 37 42 38

Strain at

break22% 5% 9% 38%

Impact

strength

(J/m)360 616 137 306

Tensile Modulus

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Tensile Modulus

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General

purpose ABS

High impact

ABS

Weather

resistant ASA

PA-6/PE

ionomer blend

Topcoat only + 0 0 -

Adpro #1 only 0 + 0 -

Adpro #1 w/

topcoat - - 0 -

Adpro #2 only - - 0 -

Adpro #2 w/

topcoat - - 0 -

Statistically significant effects at 95% confidence level

Tensile Strength

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Tensile Strength

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General

purpose ABS

High impact

ABS

Weather

resistant ASA

PA-6/PE

ionomer blend

Topcoat only 0 - 0 +

Adpro #1 only - 0 0 +

Adpro #1 w/

topcoat 0 - 0 +

Adpro #2 only 0 - 0 +

Adpro #2 w/

topcoat 0 - - +

Statistically significant effects at 95% confidence level

Strain at Break

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Strain at Break (Suryln)

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Strain at Break

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General

purpose ABS

High impact

ABS

Weather

resistant ASA

PA-6/PE

ionomer blend

Topcoat only - - 0 +

Adpro #1 only 0 0 0 +

Adpro #1 w/

topcoat 0 0 0 +

Adpro #2 only 0 0 0 +

Adpro #2 w/

topcoat 0 - 0 +

Statistically significant effects at 95% confidence level

Impact Strength

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Impact Strength (Surlyn)

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Impact Strength

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General

purpose ABS

High impact

ABS

Weather

resistant ASA

PA-6/PE

ionomer blend

Topcoat only - - 0 +

Adpro #1 only - - 0 +

Adpro #1 w/

topcoat - - - +

Adpro #2 only - - - +

Adpro #2 w/

topcoat - - - +

Statistically significant effects at 95% confidence level

Observations

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Styrenic polymers (ABS and ASA):

Uncoated → neckingCoated → crazing

Crazing = less efficient energy dissipation

mechanism(lower impact strength)

Observations

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PA-6/PE ionomer blend:Uncoated → necking

Coated → more stable necking

Shear bands shown by ruptured paint film

Conclusions

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Painting can have significant effects on mechanical strength of plastics.

These effects are complex and (at present) difficult to predict.

The best way to determine the effect of a given paint on a given plastic is to test.

Final Thoughts

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“One good test is worth 1000

expert opinions.” – Wernher von Braun

(attributed)

For More Information

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“Understanding the Effects of Paint on Plastics,” Plastics Technology, October 2014.

“Effects of Painting on Mechanical Properties of Injection Molded Plastics,” Society of Plastics Engineers ANTEC 2014.