Coatings for Automotive plastics*

Excerpt: Plastics are used mainly to make cars more energy efficient by reducing weight, together with providing durability, corrosion resistance, toughness, design flexibility, resiliency and high performance

HUMAN development is clearly linked to continuous improvements in the materials used every day. Entire stages of history have been named after the critical materials—Stone Age, Bronze Age, Iron Age, and now, the Age of Plastics.

The application of plastic components in the automotive industry has been increasing over the last decades. Nowadays, the plastics are used mainly to make cars more energy efficient by reducing weight, together with providing durability, corrosion resistance, toughness, design flexibility, resiliency and high performance at low cost.

Both durable and nondurable applications often require the plastics to be either coated or printed.

Designing of coating for plastic has many challenges & a coating designed for one type of plastic may or may not suit for the other one.

Plastics require coatings to enhance their function or aesthetic appearance.

Functional

  • To cover surface defects/ mold marks
  • To improve mar/ scratch resistance, weather resistance, moisture resistance, chemical resistance & also to preserve their mechanical properties like Impact , bending resistance & flexibility etc.

Aesthetic

  • To give uniformity in different parts during assembly (Colour, Gloss, image clarity etc.)
  • To get special effect & reduce glare (Matt or Semi glossy finish or to get soft feel etc. Some of the key factors needs to be considered while designing the coating for plastic are

Physical & Chemical characteristics of Plastic:

  • Polarity- ABS is more polar than PP, Hence no primer is required
  • Surface tension- Between 48 to 56 Dynes /cm (< 37 Dynes-Not ok)
  • Surface wet ability

Plastic properties to be studied for designing of coating

  • Coefficient of Thermal Expansion
  • Heat distortion temperature
  • Mould release agent used
  • Plasticizer and additive used
  • Solvent resistance
  • Modulus elasticity
  • Moulding Conditions-Mould Temp./Injection speed /Stress

Process Parameters & Products

  • Surface treatment for Plastics prior to painting / Ad pro/ Primer
  • Painting Process- Application method, flash off, Baking conditions

End requirements of coated object

  • Durability ,resistance properties , Appearance requirements , customer specification , end use of the part etc.

Paint chemistry used for plastics

  • Primer- 2K Polyurethane Acrylic based OR CPO based
  • Base coat - Polyester /CAB based OR Acrylic Based
  • Clear coat - 2K Acrylic Resin Based
  • Hardners - Aliphatic/ aromatic
  • Pigment selection - Heavy metal free pigments are selected
  • Solvent selection - HAPS free for interior coatings/Non alcoholic solvents for 2k system

To protect the environment & reduce the VOC emission, the plastics are now a day painted by Robotic electrostatic application which demands for use of conductive plastic. Such requirements are fulfilled by application of conductive primer coat prior to application of subsequent coats.

Surface treatments

Mechanical Abrasion:

Abrasion with sand paper improves adhesion by increasing surface roughness.

Solvent wiping:

It removes loosely held contaminants on the surface like dust, grease, Silicon based mould releasing agents etc. Mild etching by solvent also helps .

Chemical Treatment:

It removes loosely held contaminants on the surface like dust, grease, mould releasing agents etc.

Flame Treatment:

Used mainly for PP Substrate. Flame of Acetylene/LPG gas mixed with Air is used for this treatment.

This helps in providing oxidised surface by changing chemical structure of surface by controlled oxidation. Utmost Safety required. It is not suitable for Moulded 3 dimensional parts and Sheets having thickness less than 0.6mm

Removal of contaminants-Antistatic air Guns:

Air gun separates air molecules into positive and negative ions by means of electric field, & hence to remove static charge on plastic prior to painting

Future considerations

  • Lifecycle analysis data and recycling options of Plastics.
  • Plastics and polymer composites education in engineering programs.
  • To change Consumer perception of plastics and polymer composites as inferior materials.

Conclusion

Thus, the use of coated plastics is very important in the automotive market in which the performance demands are high, and their maintenance throughout the vehicle lifetime is paramount.

In short, plastics meet the challenges of an automotive industry whose demands are greater than ever. While motorists want high performance cars with greater comfort, safety, fuel efficiency, style and lower prices, society demands lower pollution levels and increased recovery at end of life. Continual innovation is a key feature in the use of durable plastics in cars.

Coating Industry is equally geared up to meet the demanding coating requirements for various types of plastics. Hence plastics & coatings for plastics will continue to remain in focus for coming decades & will be a challenge to designers, engineers & paint technologists to innovate new products and take car performance further.

Author Details

K. P. Kulkarni

Kansai Nerolac Paints Ltd.

Lower Parel, Mumbai