This time, the theme is “chemical cracking.”
Even in the motorcycle industry, few people know about this.
It is a phenomenon where plastic cracks due to chemicals.
When lubricants penetrate areas under stress, the material becomes brittle and can crack.
I have personally experienced cracks in side covers and screens because of this.
In every case, the cracks formed around the bolt holes.
At first, I thought it was due to over-torque, but that actually wasn’t the case.
The root cause was the grease applied to make up for a poor fit.
Mechanism of Chemical Cracking
Why does a lubricant, which is supposed to reduce friction, end up destroying plastic? The reason lies in the molecular structure of the plastic itself.
When you look at plastic under magnification, long, string-like molecules are intricately entangled to form its shape. When stress is applied to areas like bolt holes, hooks, or pins, these molecular strings become stretched taut.
If a chemical like a lubricant penetrates that area, it seeps into the gaps between the molecules and unravels their entanglement. As a result, the material can no longer withstand the tension, and a sharp crack snaps open. This is the true nature of chemical cracking.
Incompatible Chemicals and Vulnerable Plastics
Among chemicals, acetone and benzene are particularly dangerous.
Depending on the type, silicone can also cause issues.
The common thread among these chemicals is their high potency, which means their side effects are equally strong.
In addition, plastics vary in their chemical resistance. Those classified as amorphous plastics are the ones to watch out for! You need to be careful with polycarbonate, ABS resin, and acrylic.
| PC Polycarbonate | Alkalis, aromatics, esters, and certain types of silicone grease | Highly transparent and durable, but extremely sensitive to chemicals. It risks cracking even from the penetration of silicone oil. |
| PS Polystyrene | Oils and fats, gasoline, citrus peels (limonene), and silicone additives | Because their molecular structure is fragile, they can easily crack from exposure to everyday oils. Even silicone grease can be dangerous depending on the types of additives it contains. |
| ABS resin | Thinner, benzene, ethanol, kerosene, and oils and fats | Be careful of residual solvents during painting, bonding, or cleaning. Areas around stressed screw holes are particularly prone to cracking. |
| POM Polyacetal | Strong acids, strong alkalis, and oxidizing agents | Highly resistant to solvents, but incredibly vulnerable to acids. It degrades and deteriorates rapidly when exposed to acidic cleaners or bleaching agents. |
| PVC Polyvinyl chloride | Acetone, esters, and aromatic solvents | Easily dissolved by solvents. If left in contact with rubber or other resins for an extended period, components can migrate, causing a sticky phenomenon known as “color transfer.” |
| PP / PE Polyethylene | Oxidizing acids , strong solvents (at high temperatures) | Boasts top-class chemical resistance among resins. While virtually invincible at room temperature, it has the characteristic of being difficult to bond or paint on its surface. |
Parts cleaner is bad news too!
Even though we have been talking about lubricants up to this point, we also need to be careful with degreasing cleaners!
Parts cleaners contain acetone and aromatic solvents, which were listed in the table earlier. The ingredients vary depending on the type, but since metal cleaners have a high degreasing effect, they are also highly aggressive.
In addition, some detergents contain acetone or benzene. However, when washing a car, you rinse it off with water at the end, so I don’t think you need to be that strict about it.
High-risk areas on motorcycles
So, which specific parts should we be concerned about?
In the case of ABS resin, as shown in this episode, applying grease to prevent seizing is dangerous.
Many are made of polycarbonate or acrylic.
If you use a strong solvent to remove bug stains, it can shatter starting from the base where it is bolted down.
It is often made of polycarbonate.
Therefore, excessive use of strong water repellents or cleaners can cause cracks in the moving parts.
There are very few people who can identify the exact type of plastic just by looking at it.
So, it’s important to be careful with the chemicals you use.
If oil or a strong solvent gets onto materials that are prone to chemical cracking, such as ABS or polycarbonate, degrease them with a neutral detergent and rinse thoroughly with water.
Also, if you want to prevent screws from seizing, you should carefully choose a silicone grease (be sure to check the ingredients) or a thread-locking agent that is safe for plastics.
Finally,
Even products labeled simply as “silicone grease” or “parts cleaner” come in many different types.
Their ingredients vary depending on the manufacturer.
That said, choosing chemicals while constantly referring to compatibility charts can be a hassle.
So, when using them on plastics, the easiest approach is to choose products labeled “low aggressiveness” or “safe for plastics.”
Chemical cracking can be seen as the downside of careful, detail-oriented work.
But at the same time, it can also come down to just a small lack of consideration.
