Plastic Materials and Coating Technologies

In recent years, plastic materials have been preferred over conventional materials in numerous fields ranging from automotive/transportation to electronics, furniture, construction, toys and packaging sectors for the following reasons: • Lightweight, • Resistant to corrosion, moisture and many internal and external physical and chemical effects, • Economical, • Easy to design and manufacture in complex geometries, • Short production time, • Many are recyclable, • Paintable/colorable/aesthetic.

Major Plastic Material Types

Plastics can be examined in three main categories according to their chemical properties: 1. Thermoplastics 2. Thermosets 3. Blends and Alloys

1. Thermoplastics:

Plastic materials that melt with heat and are recyclable, the main ones being; LDPE low-density polyethylene and PP polypropylene homopolymer. PS : Polystyrene. PVC : Polyvinyl chloride. PMMA : Polymethylmethacrylate. ABS : Acrylonitrile butadiene styrene terpolymer. CAB : Cellulose acetobutyrate. TPO : Thermoplastic olefin elastomer Nylon 6/6. PPS : Polyphenylene sulfide. SAN : Styrene acrylonitrile. PA : Polyamide (nylon). PC : Polycarbonate. PPO : Polyphenylene oxide. PET/PBT : Polyethylene terephthalate/Polybutylene terephthalate. PAR : Polyarylate. PTFE : Polytetrafluoroethylene.

2. Thermosets:

Non-recyclable after polymerization, plastic materials with higher physical and chemical resistance due to being formed as a result of chemical reaction. They are divided into sub-groups such as polyester, epoxy and phenolic.

3. Blends and Alloys:

In the plastics industry, the use of mixtures or alloys of different polymers in varying proportions according to their thermal, mechanical and chemical properties is common. The main ones are; PPO/PS: High heat resistance, hardness, low moisture absorption and low cost. Nylon/ABS: Heat and chemical resistance, high impact resistance at low temperatures, low cost. Nylon/PE: High heat and chemical resistance, wear resistance. PET/PBT: High heat resistance, low cost, high gloss.

Plastic Material Selection

Selection of suitable plastic material for a specific purpose is made according to the following criteria: • Chemical resistance, • Impact resistance, • Processability/processing conditions, • Material and part cost, • Recyclability, • UV resistance, • Moisture and water resistance, • Heat resistance, • Paintability. Plastic materials are generally sensitive to UV rays, particularly to wavelengths between 290-350 nanometers and undergo photodegradation. The subject of this article is to improve the weak properties of plastic materials by processing them with appropriate surface preparation techniques and subsequently painting them with suitable paint systems.

Painting of Plastic Surfaces

Plastic materials are generally put into use in the industry following a painting process.

Reasons for Painting Requirements

• Covering mold defects, • Increasing UV resistance, • Improving scratch and physical/chemical resistance properties, • Imparting decorative characteristics, • Making surface appearance properties uniform.

Preparation of Plastic Surfaces for Painting

The two most important parameters in painting plastic surfaces are: • The surface preparation process specific to the material, • Selection of appropriate paint specific to the plastic type. Mold release agents based on oils, silicones or waxes used before injection on the plastic surface, stains from hand contact, dust and dirt, electrical charges on the surface, and microscopic cracks resulting from the process have very important effects on paint performance and appearance. During molding of polyolefinic plastic materials, the crystalline layer with variable thickness that forms on the surface during cooling of the material is called "Weak Boundary Layer" (WBL). This layer makes it difficult for paint to adhere to the plastic surface. For the paint to wet the surface during application and adhere to the surface, the surface tension of the paint must be similar to or preferably lower than the surface tension of the surface being applied. In principle, for a liquid to wet a solid, the surface tension of the liquid must be lower than the surface tension of the solid surface.

Table 1. Surface Tension Values of Commonly Used Solid Material Surfaces

Pre-treatments performed to enable painting of plastic surfaces and increase surface energies are called "surface preparation". Surface preparation techniques can be grouped into three main categories:

1. Mechanical Surface Preparation

•Plasma-Corona Discharge: Active atoms, molecules, ions, free radicals and free electrons are created on the polymeric surface in air atmosphere, the "weak boundary layer" on the surface is eliminated and the surface tension of the plastic is increased. •Glow Discharge: This is the process of applying plasma treatment in a vacuum environment of approximately 1 torr. This process is generally performed in an inert gas atmosphere such as argon or nitrogen. •Flaming: This method, which can be called flame treatment using gases such as methane, propane or butane in its simplest form, exposes the surface to thermal oxidation for less than 1 second.

2. Solvent Cleaning

Particularly on polypropylene and polyurethane surfaces, the "weak boundary layer" is eliminated by exposing it to trichloroethylene vapor and paint adhesion to the surface is ensured. This method is a risky and restricted method of use for health and environmental reasons.

3. Chemical Surface Preparation

The surface to be painted is exposed to the following solutions at 60-80°C for 1-2 minutes. •Aqueous potassium permanganate/sulfuric acid solution, •Aqueous chromic acid solution, •Aqueous peroxodisulfate solution.

4. Use of Adhesion Primers

Surface tension is increased by using CPO-based primer/adhesion layers. This method should be performed after necessary surface cleaning operations and elimination of the "weak boundary layer". While direct measurement of the surface tension of solid surfaces is not possible, it can be calculated indirectly by utilizing the contact angle created on the surface by a liquid droplet of known surface tension dropped onto the solid surface. δL = Surface tension of liquid δS = Surface tension of surface α = Contact angle

As the contact angle approaches zero, δL = δS and the wettability of the surface increases.

Selection of Appropriate Paint Systems for Plastic Surfaces

Considering the type of plastic surface to be painted and the conditions to which it will be exposed: •1K acrylic, •2K acrylic, •2K polyester, •UV, •Water and solvent-based paint systems can be used.

Table 2. Appropriate painting systems according to sectors and plastic material types

Muhammet Demirkıran Sales Manager Paints, Inks and Adhesives Group Kempro Kimyasal Maddeler ve Dış Tic. A.Ş.  

References: 1. "Basics of Coating Technology" by Dr. Artur Goldschmidt & Dr. Hans-Joachim Streitberger. 2. "Painting of Plastics" by Rose Ann Ryntz. 3. "Pigments, Paints, Polymer Coatings and Printing Inks" by R.A. Ryntz. 4. Organic Coatings: Science and Technology; by Zeno W. Wicks, 2007.