Haven’t we all been frustrated at one time or another, trying to fix something made of plastic? You used a glue you had handy, and it seemed to work because the pieces were held in place nicely after the glue cured; but when you put a little pressure on the joint, POP! It fell apart. That’s because only certain glues bind certain plastics. The wrong glue is actually repelled by many plastics, Teflon being the best known. There are seven types of plastic but millions of variations among them. Manufacturers have made it easier to wade through all this by adding imbedded plastic ID codes on their products.
To repair anything made of plastic you must first exactly identify the type of plastic, then enter the type in our search boxes for materials 1 and 2 to generate the list of glues best suited for your job.
There are three ways to identify plastics, as follows:
1) Look for the plastic ID Triangle code imbedded, usually out of sight, on the bottom of a piece. That is the best and quickest way. But, if it is not visible,
2) You should contact the manufacturer’s customer service, and they will email you its specific type; and while that’s not immediate, we recommend it highly because manufacturers have to comply with EPA/OSHA regulations about plastics, making their answers often quick and always accurate.
3) You can also do a few simple home examination/tests that can help identify what you’re working with. This will only give you a likely, but not certain, answer. To identify plastics with certainty yourself, you need access to an analytic chemical lab, which will cost you far more than any glues you are buying.
Plastics Density and Burn test results [COURTESY OF RALEIGH CHARTER HIGH SCHOOL, RALEIGH, NORTH CAROLINA]
Density. Cut small (approximately 3- to 5-mm square) pieces of each sample. Determine the approximate density by the behavior in the fluids provided.
An efficient way for an analytical team to distinguish the polymers is to use water to make the first cut, then follow a flow chart. For example:
Copper wire flame test. Using tongs or forceps, hold a 5-cm length of copper wire in the flame of a gas burner. Touch the hot wire to a sample of polymer and place it back in the flame. A green flame indicates the presence of halogen such as the chlorine in PVC.
Burn test. Using clean forceps, briefly hold a 1- to 2-cm strip of each sample in the flame of a match, lighter, or gas burner. Observe the behavior of the material in the flame and after removing it from the flame. If the sample continues to burn after removal, blow out the flame and observe the condition of the material. After observations are complete, quench the residue in a beaker of water. BE CAREFUL! SOME OF THE MOLTEN PLASTIC CAN BURN YOU QUICKLY, AND SOME OF THE FLAME TESTS REALLY STINK. YOU NEED GOOD VENTILATION BECAUSE SOME FUMES CAN BE TOXIC ON REPEATED EXPOSURE.
|PETE||Melts and bubbles first; burns slowly with some black soot; pungent odor of acetaldehyde|
|HDPE||Burns rapidly and cleanly; drips flames; white smoke when extinguished|
|PVC||Melts; may burn, but extinguishes on removal from flame|
|LDPE||Burns rapidly and cleanly; drips flames; white smoke when extinguished|
|PP||Burns more slowly than PE; may drip flames|
|PS||Burns rapidly; large amounts of black soot|
You’ll notice we have no data on Plastic ID type 7 or 07 in the above, because it is the smallest group of plastics, amounting to less than 10% of all plastics, but even so, it contains several different kinds, all in category 7 or 07, of which polycarbonate, PC, is the largest group. This plastic melts quickly and drips, gives off black smoke and soot, and burns with an orange flame which goes out on removal from igniting flame, and which gives off a faint, sweet, aromatic smell.
Analytical teams identify polymers by their densities and their behavior in two burn tests. Other characteristics such as hardness (can a sample be scratched by a fingernail?) and flexibility should be noted. Hardness and flexibility are somewhat ambiguous, as they are affected by molecular weight, sample thickness, presence of plasticizers and impact modifiers among other considerations. Care in observation and a basic knowledge of polymer properties should lead to the correct identification of the unknown samples.