Plastics

[SeniorCitizen]

As i understand plastics there are 2 basic types , thermo - plastic and thermo - set . How would a common person determine which a piece is ? Can thermoset be welded ?

 

[StarTech]

How would a common person determine which a piece is ?

You look for and use the recycle codes.

Can thermoset be welded

Basic answer is No it can't be welded.

https://get-it-made.co.uk/resources/thermoset-vs-thermoplastic-whats-the-difference

A thermoset is a polymer that forms irreversible chemical bonds during the curing process. Unlike thermoplastics that can be melted and remould multiple times, thermosets (as their names suggest) are set in a permanent physical and chemical composition after the first curing process. This occurs as polymers in the material ‘cross-link’, performing an unbreakable, irreversible bond. This unique feature makes thermosetting polymers an excellent choice for parts that need to have excellent dimensional stability at elevated temperatures.

The critical difference between thermoplastics and thermosets is how the materials behave during the curing process. Thermosets strengthen when cured but form chemical bonds that make them impossible to remould. Thermoplastics do not form any chemical bond when curing, making them re-mouldable and recyclable. This three-dimensional bonding in thermosets makes them stronger and more heat resistant than thermoplastics.

Thermosets' ability to retain their strength and geometry when exposed to elevated temperatures set them apart from thermoplastics. Thermosets will often degrade before melting when exposed to excess heat. These properties mean plastics can be used as a low-cost replacement for metals in some applications.

In a nutshell, thermosets generally have greater physical properties than thermoplastics; however they cannot be remoulded and recycled.

Now with that most plastics I have come across on mowers have been either ABS or HDPE. ABS is both epoxy-able and weld-able. HDPE normally can only be welded; although, there is some very expensive bonding glue available (around $300 per oz the last time I checked)

As Bert mention before there are recycle codes on larger pieces of plastics which will help in determining what you have in front of you.

I use Permatex Plastic Weld product for the ABS and bondable plastics. The problem is there is only two colors currently available. Cream and Black. I had one customer with a SST16 that I bonded his hood together over ten years and the Permatex stuff was still holding when the hood designated from UV damage. I tried the JB weld version and it was a complete failure. Bonding of the JB Weld was a joke as I put together a hood and taped it overnight. The next morning I started removing the tape and it fell part like I never used the JB Weld. I got some Permatex version and barely manage to get most of the hood together before it was setting up. Now of course I have never gotten a JB Weld product to work for me so that didn't surprise me.

 

[nc10]

For example, polyethylene is a thermoplastic that can be converted to a thermoset. Polyethylene, LLPE or HDPE, milk jugs, plastic drums, polyethylene pipe is a thermoplastic. Those parts can be dissolved by some solvents, so it's possible to solvent glue (or weld) low temperature polyethylene piping. The part is only useful up to 140 F or so. Sometimes polyethylene is extruded and crosslinked as part of the extrusion process to make PEX tubing, or XLPE wire and cable coatings (the X refers to crosslinked in both names), making a thermoset version.

The crosslinking is (are?) actual chemical bonds which form as part of or after the extrusion process that connects different polymer chains together. That makes the resulting polymer much more resistant to heat, and more resistant to oil, kerosene etc, making it more useful for hot water applications, or for wiring that might be exposed to water, high temperatures, oil, kerosene, etc. But the crosslinking also means its not dissolved by solvents used in solvent glues/solvent welding techniques.

Not sure how a "common person" could tell just by looking, depends on the piece, I guess. Piping, wire coatings etc, usually have labeling that you can look up to know what material is used. There are lab methods that determine what a plastic is composed of, thermal methods that look at the heat resistance of a part, or analytical methods that can identify chemical structures present. If a piece has good thermal properties, ie you can use it 350F, its less likely to something that can dissolved in solvents used for solvent welding. If it gets soft at low temps (ie < 200F) probably something that can solvent welded.

Is there a particular piece or item you are looking at? If you could share what it is, might be able to tell you.

 

[bertsmobile1]

There are lots of tables showing how to identify unknown plastic products
Most involve melting or burning a small piece
They all burn ( or don't burn ) differently , some have thick black smoke some have grey -white smoke & some burn clean. Some melt before they burn some burn before they melt and most will give off a very identifable smell when burning
Some dissolve in petrol / acetone / thinners while others do not

 

[nc10]

How well something burns could be an indication that the plastic formulation contained flame retardants, and less about what the base plastic is. I'd expect most plastic parts from cars, appliances and electronics to contain flame retardants.


If you are burning pvc. You'll be releasing HCl acid. Better not to did this, but in a really well ventilated area if you do.

 

[SeniorCitizen]

I've given up trying to locate a ridiculous size code # .

 

[nc10]

I've given up trying to locate a ridiculous size code # .

Can you explain a little more? Not following the connection between thermoset, thermoplastic and ridiculous size and code number.

 

[SeniorCitizen]

Can you explain a little more? Not following the connection between thermoset, thermoplastic and ridiculous size and code number.

Read reply #2 again .

 

[bertsmobile1]

Thermo set plastics go hard & brittle when you heat them .
Bakalite is the most commonly known thermo set
I used a lot of thermo sets to hold microscope specimens
And no once they have been "set" they can not be welded but most con be glued.
Most are supplied as a mixture of powders that are heated in a mould under pressure .

 

[PTmowerMech]

I believe this, or something like it, is the emblem on thermoplastics. Especially the arrows in a triangular shape.