| Description and Operation General Although these days plastics can be produced in all colors and with a matt or gloss surface, painting is often necessary. NOTE:Manufacturer's limitations concerning the feasibility of painting certain components must always be observed. Reasons in favor of applying paint to plastic are: - Individual coloring, matching the body paint.
- More gloss and color brilliance through painting.
- Removal of production imperfections.
- Protection from atmospheric exposure.
Nowadays painting plastic presents no problems because the materials are known and matched to the paint. In order that the painter can use the correct painting materials, the type of plastic must first be correctly determined. To allow this, plastics are marked on the rear in accordance with the recommendations of the Association of Vehicle Manufacturers. Once the type of plastic is determined it is an easy matter to assign special paint recommendations, matched to that particular plastic. Unmarked plastics require knowledge of materials so that a correct choice of paint materials can be made and the component can be reliably painted. Plastic groups Thermoplastics When warmed these undergo a reversible transformation into a plastic deformable state and once cooled they maintain their shape. They consist of string-like (linear) or only slightly branched molecular chains. Thermosets Thermosets are hard and have the form of a close-meshed network in all directions. They do not undergo plastic deformation, are especially resistant to chemicals, are difficult to swell and are insoluble. At normal temperatures they are hard to brittle. At first the material does not undergo any change when heated, but when it reaches a critical point, the thermoset is totally destroyed. Elastomers Elastomers are characterized by high elasticity over a wide temperature range. They have properties like rubber or a sponge and after compression or distension they return to their original state. Types of plastic The plastics used in the automotive area: - ABS - Acrylonitrile butadiene styrene (polymer)
- PA - Polyamide
- PC - Polycarbonate
- PE - Polyethylene
- PP - Polypropylene
- PP/EPDM - Polypropylene/ethylene propylene diene copolymer
- PC/PBT - Polycarbonate/Polybutylene terephthalate
- PBT/PC - Polybutylene terephthalate/Polycarbonate
- PUR - Polyurethane
- GRP - Glass reinforced plastic
NOTE:PE and PP are plastics which cannot be painted, or can only be painted using special techniques. As well as the pure plastics, so-called 'blends' are also used. This means combinations of different plastics. If we were dealing with metals they would be called alloys. Plastic identification Normally the identifier is marked on the plastic components used in vehicle construction. One method to determine the plastic group is the sanding test. In this a place is chosen which will not be visible later, and the finger belt sander is used to sand the plastic. The plastic group can be determined using the pattern left by the sanding and the dust: - Thermosets produce a white dust.
- Thermoplastics smear and do not produce dust.
The plastic group can be determined by a sound test: - Degree of hardness - the higher-pitched the sound, the harder the plastic.
- Elasticity - the more muffled the sound, the higher the elasticity of the plastic.
Cleaning plastic Plastic components are manufactured using complicated moulds and presses or other highly engineered tools, mostly using an injection moulding process or reactive injection moulding process. In order to be able to remove the component from a particular tool, a separating agent is used, which in some cases adheres very strongly to the plastic. This separating agent on the plastic components must be completely removed before any surface coating is applied. Warm storage (tempering) before actual cleaning brings the following advantages: - The separating agent sweats out of the plastic.
- Tensions in the plastic are released.
- Air inclusions can be recognized and removed.
Intensively clean the item several times using a pad and fresh cleaning agent. NOTE:A single wipe, even with cleaning agent, is not usually sufficient in most cases. Clean textured components with the aid of a soft brush. After cleaning, it is absolutely vital that cleaning agent absorbed by the plastic should be expelled by tempering again. If the ventilation is good and the room temperature is about 20°C the solvent can be evaporated away by overnight storage. Painting new components It is absolutely vital that the substrate of an unpainted new component is free of separating agent. Paint can only be applied directly to very few plastics. The plastic must first be identified exactly and then worked with a repair system which is matched to the type of plastic. In most cases a plastic etch primer must be applied as adhesion base to all plastics which can be painted. NOTE:Plastics have a tendency to become electrostatically charged. This can easily cause contamination during painting. Special antistatic cleaning cloths prevent electrostatic charging. NOTE:Follow the paint manufacturer's guidelines during all work. The primer which has been applied to a primed new component can vary greatly. If no manufacturer's data is available, the composition and suitability for further working must be tested. Painted components with an already ascertained and intact paint coat present no problems for possible repainting. After sanding and careful cleaning with plastic cleaner or thinners, painting can be done directly. Unknown primer When dealing with unknown substrates it is important to carry out an adhesion test on the existing paint before any repainting is attempted. First of all a mechanical test must be carried out, for instance using a lattice cut and tear-off band. If the adhesion of the old paint is not acceptable, it must be mechanically removed and new paint finish applied. If the adhesion is acceptable, then an etch test is performed using 2-component thinners. If no etching can be detected in this test, application of the the paint finish can be started directly. Otherwise the old paint must be removed and a new paint finish created. With the help of universal or special plastic primers and with only a few materials complementary to those previously present anyway, the painter can now apply a long-lasting paint finish to all popular vehicle attachments made of plastic. Paint faults on plastic substrates NOTE:Paint faults are fully described in the chapter Paint Defects and Damage. The most common paint faults which can occur when painting plastic components and the methods of repair are briefly described. Discoloration Cause/damage pattern: - Plastic material is not suitable for painting.
- Incorrect bonding agent.
- Paint used not solvent resistant.
Repair of damage: - Repaint using suitable materials.
- Install a new unpainted component.
Softening Cause/damage pattern: - Substrate not carefully cleaned.
- Air humidity too high or working temperature too low.
- Drying time incorrect (too short).
- Materials for substrate not correctly matched to each other or not mixed correctly.
Repair of damage: - Dry out, sand, re-isolate and paint.
- Sand away faulty paint coats and re-apply paint finish.
Paint damage caused by detachment, poor adhesion Cause/damage pattern: - Insufficient paint adhesion between top coat and filler. The whole of the paint finish detaches from the plastic.
- Plastic not cleaned sufficiently, not or inadequately tempered.
- Unsuitable cleaning agent or materials used.
- Poor or lack of intermediate sanding.
- Paint finish underbaked or overbaked.
Repair of damage: - Sand away faulty paint coats and re-apply paint finish.
Paint damage caused by blisters, craters, sink holes Cause/damage pattern: - Painting on PUR plastic which was not painted in production.
- Surface of the plastic material too porous.
- Flash-off time not adhered to.
- Drying temperature too high.
- Moisture in plastic material.
- Layers too thick.
Repair of damage: - Clean the damaged area, sand, re-isolate and paint.
- Remove the paint layers and re-paint.
Crack formation Cause/damage pattern: - Overexpansion of painted PUR plastic components.
- Use of unsuitable paint materials.
- Paint materials not suited to each other or incorrect mixture ratio.
Repair of damage: - It is not possible to repair overexpanded PUR plastic components.
- On other plastics, sand away damaged layers, isolate and repaint.
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