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.

1. Effects of Painting on
Mechanical Properties of
Injection Molded Plastics
David E. Palmer, P.E.
BRP – Marine Propulsion Systems Division,
Sturtevant, WI

2. Materials and coatings
QUESTION: Why apply a coating to a material?
2
Appearance
Abrasion/wear
Friction coefficient
Thermal barrier
Electrical conductivity
Corrosion resistance
Chemical resistance
UV resistance
Static dissipation
Optical reflectivity

3. Materials and coatings
3
QUESTION: Why apply a coating to a material?
ANSWER:
To change the material’s
properties and/or behavior

4. Materials and coatings
4
… is it reasonable to assume
that all other aspects will
remain unchanged?
QUESTION: If we change one aspect of a
materials’ properties or behavior…

5. Materials and coatings
5
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.

6. Materials and coatings
6
Effect of zinc-plated nut on glass-filled nylon-6,6
EDS of corrosion product
on surface of nut

7. 7
Paint and
plastics

8. Paint and plastics
8
QUESTION: Why paint plastics?
Mass customization
Color/effect not available in resin
Resin lacks UV resistance
Aftermarket refinishing
Touchup/repair

9. Paint and plastics
9
QUESTION: What are the effects of painting on
the mechanical properties of plastics?
•Mechanical interactions
•Chemical interactions
•Thermal effects

10. Mechanical interactions
• Elastic mismatch
• Interfacial strength
• Film thickness
10
Source:
M.S. Hu and A.G. Evans,
"The cracking and
decohesion of thin films
on ductile substrates,"
Acta Metallurgica
37:917-925, 1989.

11. Chemical interactions
11
• Swelling
• Dissolution
• Environmental stress cracking

12. Thermal effects
12
• Accelerated solvent diffusion into substrate
• Relief of molded-in stresses
• Shrinkage of paint (induces residual stresses)
• Thermal degradation of substrate

13. Experimental
13

14. Experimental design
• For four (4) different substrates, measure
mechanical properties in each of these six (6)
conditions:
14
No coating
Adhesion
promoter 1 only
Adhesion
promoter 2 only
Topcoat only
Adhesion
promoter 1 with
topcoat
Adhesion
promoter 2 with
topcoat

15. Substrates
15
• General purpose ABS
• High impact ABS
• Weather-resistant ASA
• 10% mineral filled polyethylene
ionomer/PA-6 blend

16. Coatings
16
• High-solids black acrylic topcoat
• Clear adhesion promoter (40%
aromatic hydrocarbons)
• Black adhesion promoter (<2%
aromatic hydrocarbons)

17. 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
17

18. Testing
18
• Notched Izod impact
ASTM D256-10 Method A
• Tension testing
ASTM D638-10
Polyethylene ionomer/PA-6 blend: 50 mm/min
All other substrates: 5 mm/min

19. 19
Results

20. Notched Izod impact
20

21. Tensile modulus
21

22. Tensile strength
22

23. Strain at yield
23

24. Strain at break
24

25. Supression of necking
25
High impact ABS
Black adhesion
promoter and
topcoat
High impact ABS
Uncoated
This effect was observed in all of the coated
ABS and ASA specimens.

26. Film cracking
26
This effect was
observed in all of
the ABS and ASA
specimens with
topcoat (regardless
of whether or not
they had adhesion
promoter).

27. Loss of adhesion
27
This effect was observed
the PE ionomer/PA-6
specimens with topcoat
(regardless of whether or
not they had adhesion
promoter).

28. Film adhesion
28
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.

29. 29
Conclusions

30. Conclusions
30
Rank ordering of effects:
Strain at break
Impact strength
Tensile modulus
Tensile strength
Strain at yield
Largest effects
Little/no effect

31. Conclusions
31
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.

32. Conclusions
32
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.

33. Conclusion
33
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.

34. Acknowledgements
34
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