Restore brittle plastic - Boil it
08-26-2016, 12:29 PM
#16
Rennlist Member
Thread Starter
I can't find the emoticon for exploding head. Thanks for the explanation, I think 😄
08-26-2016, 01:46 PM
#17
Rennlist Member
Just to add to this... I put some old Rubber hoses and Intake (85/86) connectors in the Ultrasonic for 20 mins at ~100 degs and noticed they came out more pliable, lifelike like new, and clean 
08-26-2016, 02:17 PM
#18
OK, you asked for it....
The common test for a ductile material is to put a bar in a machine and pull on it, optionally with an "extensometer" to measure strain directly. You get a stress versus strain curve from which you can extract yield stress, ultimate tensile stress, ductility, and modulus (stiffness, the inherent spring rate).
You can also get a value for toughness – one flavor of toughness: the area under the stress-strain curve represents the energy required to break the bar. Energy is the controlling parameter in impact failure: a hammer must have this amount of kinetic energy in order to break something.
Something seriously brittle will break in the grips of such a machine, so they're tested in bending. Glass, ceramics, that sort of stuff. Or just compression, like concrete.
Most plastics are not ductile - you cannot bend a piece into a new shape. They have a low modulus, so they don't seem brittle – you don't fear them as you would a piece of glass – but strictly speaking, they are brittle. Rubber is brittle, too. They are tough, though, because the area under the stress-strain curve is large - it takes a lot of work to break them. Being compliant (the opposite of stiff), you can manhandle them without breaking them.
Toughness is often thought of as resistance to impact damage. Since a material can behave differently at high strain rates, you often need to test by impact. Steel is a huge example. The standard test uses notched bars at various temperatures. You whack them with a pendulum hammer; the rise of the pendulum after impact tells you how much energy was required. The rivets used for the Titanic may have had an unfortunate ductile-brittle transition temperature.
This is just the tip of the iceberg about "strength". You have hardness tests, high and low cycle fatigue tests, thermal fatigue resistance, corrosion fatigue tests, fatigue crack growth rate tests, fracture toughness (a crack is present), stress corrosion tests, creep rupture tests – tests for every sort of failure. I was involved in developing turbine blade alloys - the screening test was creep rupture - hang a weight on a bar to get about 20,000 psi, let it sit at 2000F, and see how long it lasts - about a week was good.
The common test for a ductile material is to put a bar in a machine and pull on it, optionally with an "extensometer" to measure strain directly. You get a stress versus strain curve from which you can extract yield stress, ultimate tensile stress, ductility, and modulus (stiffness, the inherent spring rate).
You can also get a value for toughness – one flavor of toughness: the area under the stress-strain curve represents the energy required to break the bar. Energy is the controlling parameter in impact failure: a hammer must have this amount of kinetic energy in order to break something.
Something seriously brittle will break in the grips of such a machine, so they're tested in bending. Glass, ceramics, that sort of stuff. Or just compression, like concrete.
Most plastics are not ductile - you cannot bend a piece into a new shape. They have a low modulus, so they don't seem brittle – you don't fear them as you would a piece of glass – but strictly speaking, they are brittle. Rubber is brittle, too. They are tough, though, because the area under the stress-strain curve is large - it takes a lot of work to break them. Being compliant (the opposite of stiff), you can manhandle them without breaking them.
Toughness is often thought of as resistance to impact damage. Since a material can behave differently at high strain rates, you often need to test by impact. Steel is a huge example. The standard test uses notched bars at various temperatures. You whack them with a pendulum hammer; the rise of the pendulum after impact tells you how much energy was required. The rivets used for the Titanic may have had an unfortunate ductile-brittle transition temperature.
This is just the tip of the iceberg about "strength". You have hardness tests, high and low cycle fatigue tests, thermal fatigue resistance, corrosion fatigue tests, fatigue crack growth rate tests, fracture toughness (a crack is present), stress corrosion tests, creep rupture tests – tests for every sort of failure. I was involved in developing turbine blade alloys - the screening test was creep rupture - hang a weight on a bar to get about 20,000 psi, let it sit at 2000F, and see how long it lasts - about a week was good.
You Sir take me way back,Have forgotten more than I could imagine .
08-27-2016, 12:52 AM
#19
Drifting
Cleaning things in an ultrasonic cleaner is tricky business sometimes. The various combinations of material being cleaned, wave frequency, and amplitude can produce some disastrous results. The layperson may think tossing all their jewelry in an ultrasonic cleaner is a good idea. Not so much if you have some rare blue tanzanite earrings. Tanzanite crystals resonate at the frequency of common ultrasonic jewelry cleaners...as amplitude of deformation grows within the crystal, the lattice ruptures along cleavage planes (like tempered glass breaking into little cubes). It's much like a woman's voice shattering a glass, but it will happen more easily if there are planes of weak bonding in the crystal - i.e. cleavage planes. There is a cleaner in a lab at the USGS nicknamed the "Cell Disruptor". It will reduce a pot roast to red goo in a short order.
08-27-2016, 11:02 AM
#20
Addict
Rennlist Member
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08-28-2016, 02:26 AM
#21
Drifting
Plastics do not have a crystalline type structure. They are long-chain hydrocarbon molecules that cross link- polymerize- at various points along the molecular structure. The position and number of the cross links determine the 'hardness/ rigidity' of the plastic. The polymerization process usually involves heat. Thermoset plastics need external heat. With catalytic plastics, the heat is supplied by the reaction between the catalyst and the resin. You can put enough MEKP in your cup of polyester resin to make is smoke. Ask me how I know. Boiling the plastic may partially decouple the cross links and allow the plastic dissipate 'accumulated stress'. You could use a different heat source, like a heat gun or an IR lamp, but how to control the temp? By using boiling water you limit the delta T and max temp. If 100 C. isn't high enough I guess you could use the wife's pressure cooker, just don't get caught. ;-)
08-28-2016, 08:16 AM
#22
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True, polymers do not have the neat crystallinity of metals and minerals, but colonies of organized structure can form where the long chains sit side-by-side for part of their length. The remaining lengths are like a bowl of spaghetti. A piece of plastic might be 50% good spaghetti and 50% spaghetti "bones".
Polymerization is the process by which smaller molecules - monomers - link up end to end. Cross-linking is a subsequent process, when it occurs. The most notable case is rubber, of course, where it is known as vulcanization thanks to Mr. Goodyear. It's also the "V" in "RTV". An interesting thing about a piece of rubber or cross-linked plastic is that it is a single molecule or, at least, it challenges the normal meaning of "molecule".
Polymerization is the process by which smaller molecules - monomers - link up end to end. Cross-linking is a subsequent process, when it occurs. The most notable case is rubber, of course, where it is known as vulcanization thanks to Mr. Goodyear. It's also the "V" in "RTV". An interesting thing about a piece of rubber or cross-linked plastic is that it is a single molecule or, at least, it challenges the normal meaning of "molecule".
08-29-2016, 09:34 PM
#23
Drifting
Well Said, ADK...It's called a 'macro-molecule'. The G.I.issue glasses are made of it. They typically break thru the bridge and NOTHING will glue them together, and I tried everything. Including plastic glues and solvents, industrial epoxy and aircraft grade Cyanoacrylates. NOTHING!!
The problem remains: Getting 30+ year old plastic parts off the car without breaking them. The electrical plugs under the rear bumper are shattered, have to figure out a way to replace them.
The problem remains: Getting 30+ year old plastic parts off the car without breaking them. The electrical plugs under the rear bumper are shattered, have to figure out a way to replace them.
