Why is plastic so hard to repair when it breaks?


To be clear, I’m not asking how to repair it, I’m just trying to understand in general terms why it is so difficult to repair effectively when on paper it seems like typically an easier material to work with than wood or metal (both of which are fairly trivial to repair).

Flagging this post as chemistry because I suspect that’s part of the answer, but it might very well be a physics problem instead.

In: 5

Plastics have their stretchy/bendy properties because they’re made up of big tangled chains of complex molecules.

It’s kinda like trying to repair a broken piece of wood or better yet fabric.

Plastic is made from, and gets many of its useful properties, from the long chains of polymers that are knotted together. Polymers are just long, repeating molecules. Plastics are flexible because the chains aren’t often chemically attached to each other and can slide around, but are still strong because they are still physically intertwined (like thread in cloth but more knot-like).

When you damage plastic, you’re damaging those chemical bonds, creating lots of loose ends. Using heat or chemical agents to re-bond it is effectively just mashing those ends back together and hoping they get tangled enough to hold or fusing them mostly at random. Even if you do join the parts back together, the fused area is often stiffer, weaker, and more brittle than the surrounding material, and thus is prone to breaking and snapping again. It’s also very difficult to make visually seamless repairs, so in places where strength isn’t a factor its still awkward.

Plastics are made of long chains that are all tangled up together. If you separate or rip them (break it), you can’t really tangle them back up easily.

Think about pulling aparts a cotton ball/pad. It’s also made of fibres that hold together, and if you pull it apart, you can’t put it back together because the fibres won’t just mix back like putty. You’d need to re-mix all of them again from scratch.

When you break wood, you don’t repair it as such. It’ll never be the same. What you can do is use a glue to hold the broken bits together. Glue works well because wood is porous, as the glue sinks in and solidifies the wood and glue can become nicely tangled together making it difficult to get them apart. Imagine soaking up a sponge full of glue and letting it set. The sponge and the glue are 2 different things but good luck separating them.

If we repair metal, is assume you mean welding? That’s handy because we liquify it and let the structure reform. It takes a skilled person with the correct tools to this, and we kind of often want to as metal products are expensive. You can glue metals, but generally/normally you’re going to lose a lot of strength there because the glue isn’t going to sink into the surface and get a good bond.

There’s a huge range of plastics that behave very differently to each other. You can melt and re-form some plastics, you can buy plastic welders and use soldering irons. It’s kind of the premise of 3d printing. Normally plastic is cheap so we don’t care to do this. It’s not that it’s more difficult, it’s just not efficient. There’s also plastics that are ‘baked’. In the same way that you can’t repair a cake with eggs, flour and sugar, once that plastic part has been made its internal structure has changed forever. Plastics are not really porous, so glues don’t work as well as they would on the likes of wood, and because the structure has changed from the raw ingredients you can’t add more ingredients to re-bake the cake as it were.

Plastic is similar in it’s repairability to wood or metal. Wood can be glued and jointed and then disguised with paint or stain. Metal can be welded or soldered and then painted depending on it’s application. plastic while often appearing like any other plastic can be made of many different types of plastics and the key to repair is identifying what type of plastic. Almost all plastic repair involves heating, or a chemical reaction of solvents and glue to finish a repair. There are plastic welders to repair some plastics but cosmetically they may not be appealing when a bond is completed though functional. Why plastic isn’t repaired often comes down to cost. It’s often cheapest to simply replace it and dispose of the old part due to the low cost of the material.

Not a very good ELI5 but nobody has responded.