Body Trim - Identifying/Painting/Repairing Plastic Parts
85VW2Applies To ALL
Group
50
Subject
Body Trim
Part Identifier N/A
Key Points
Identifying/painting and repairing plastic parts
Supercedes Product Circular, Group 50, No. 84-02 D
Number
85-04
October 11, 1985
Plastic Identification
Since many varieties of plastics are being used in the manufacture of our vehicles, proper identification of these plastics is of extreme importance when repairing or repainting these parts.
Automotive plastic parts are currently divided into two categories: Thermoplastics and Thermosets.
Thermoplastics
PP - Polypropylene EPDM - Ethylene propylene diene-mixed polymerisate
TPE - Thermoplastic elastomer based on EDPM ABS - Acrylnitrile butadiene styrene PC - Polycarbonate PE - Polyethylene PVC - Polyvinyl chloride PPO - Polyphenylene oxide (Noryl)
Thermoplastic parts soften when heat is applied, hence are the easiest of the two to repair.
Inner tender shields, some front spoilers and fender flares are examples of thermoplastic parts.
Thermosets
PUR - Polyurethane UP - Unsaturated polyester, better known as:
GFR-UP - Glassfiber reinforced unsaturated polyester
Thermosetting plastics harden (set) under heat, therefore, in most instances once a part is manufactured, nothing further may be done to alter its original shape.
Grills, Vanagon camper pop-top, and those parts reinforced by fiberglass are examples of thermosetting plastic parts.
In order to determine whether you have a thermoplastic or thermosetting plastic, hold a hot air welder an inch from the material for approximately ten seconds, if a softening or melt occurs you have a thermoplastic.
Parts formed from thermoplastics and some thermosets can be repaired in a multitude of ways:
1. Bent or misshapen pans can be reformed after the application of heat to make them pliable. Hot water, infrared lamps and heat guns can also be used to supply the necessary heat.
2. Some plastics (ABS, PC, PPO) can be repaired using solvents (Acetone or MEK (Methyl-Ethyl Ketone)).
3. Parts that have cracks or fractures can be refused by welding with a hot air welding torch..
Once the category of plastic is determined, there are two tests, flame and odor which are used to determine the actual plastic material used.
Test A
1. Remove a sliver of plastic from the back of the part.
2. Hold with pliers and ignite the plastic.
3. Observe the burning plastic.
4. To confirm the test, compare the odor and color of the smoke of the sample to the color coded rod used to reweld material. (See table page 2).
Test B
1. Soften or melt a small area on the hidden side of the material with the rod you think is correct. Press the materials together.
2. Hold the softened rod against the surface, briefly, then pull to test bond. Incompatible rod will pull away with no adhesion.
CAUTION
When using any material, or equipment to repair plastic parts, follow the manufacturer's instructions to achieve a proper repair.
Untreated "raw" plastic parts may be painted for use on a vehicle only with special attention to the problems of paint adhesion, flexibility, color match and service repair.
Successful painting of plastic parts is accomplished with the addition of two chemicals to the paint process:
1 - A bonding agent which is sprayed directly onto the untreated, "raw" parts (parts not primed or which have not been previously painted):
Part No. ALO 822 000 04 for EPDM Part No. ALO 822 100 06 for PUR
Note
When stock of Part No. ALO 822 100 06 has been used up, the new bonding agent for PUR, as well as for PC, will be Part No. ALO 822 200 10.
2 - An elastic additive, Part No. ALZ 011 000, which is mixed with 2K unicolor acrylic paint
CAUTION
Never add elastic additive to metallic base coats. The additive is to be combined with the 2K Acrylic Clear Lacquer on metallic paint applications.
The bonding agents will not adhere to PE and PP. These two thermoplastics are considered to be unpaintable.
No bonding agent is used when repainting plastic parts which have already been painted or parts supplied primed from the factory.
Depending on the type of plastic, the amount of elastic additive added to the spray coat varies from 30% for EPDM to 100% for PUR soft foam.
Increasing the quantity of elastic additive naturally reduces the covering capability which, in turn, leads to thicker coats. A thicker coat requires a longer drying time.
Painting procedure
The table on page 4 list the various methods of applying paint to different types of plastics.
Durability of the paint job relies on:
^ thorough cleaning of the surface and selecting the correct bonding agent
^ adhering to the specified mixing proportions
EXAMPLE:
500 ml 2K Acrylic paint ALN 769 . . . + 150 ml 30% elastic additive ALZ 011 000 650 ml "basic" paint to which 2K hardener ALZ 009 001 06 (slow) ALZ 009 002 06 (fast)
+ 325 ml is added in 2:1 proportions = 975 mi of paint ready for spraying
CAUTION
Put the elastic additive into the paint or clear lacquer first, then add hardener in correct (2:1) proportion.
Note
To prevent an abnormal amount of dirt in the paint when spraying, ground the spraying equipment properly. Most parts, especially PUR, tend to become electrostatically charged and tend to attract dust.
Parts painted off the car should be dried at about 50~C (122~F) to minimize the risk of distortion. The temperature should not exceed 70~C (158~F) for parts installed on the car. A lower drying temperature will also keep air bubbles (blisters) to a minimum.