July 31, 2009
My mother always boils new glasses and plates. Says it made them stronger.
Just a couple of months ago I had a laboratory experiment in polymer making. The goal was to make urea-formaldehyde, which was mostly used as adhesives in plywood industries, for laminating, coating, molding, casting, lacquers, textile finishing, etc. Urea-formaldehyde is a thermosetting plastics or termosets, which is a class of polymers that are irreversibly cured (by heat, hence the name). After it is cured, it is resistant to acid and base, and cannot be melted nor re-shaped. It cannot, therefore, be recycled.
But the polymer used in kitchenware is not urea-formaldehyde, it is its brother, melamine-formaldehyde. Somehow, during the meetings with the module lecturer who is ever so obscure (although thankfully not annoying…he has the assistant who is already too annoying), we got into talking about kitchenware.
“What are kitchenwares made of?”
“No. Polymers. The ones you can put in a microwave.”
“That’s right. Now, what did you do before using them?”
“Um…” *contemplate on whether or not to answer “Clean them”, but then again, that sounds too stupid even for me*
“What would the elderlies do?”
Now he’s asking what my Grandma would do?
“Yes, that’s right. Do you know why?”
Apparently, it is to release the formaline in the kitchenware. In the process of making melamine-formaldehyde, melamine and formaldehyde are mixed, just like in the process of makin urea-formaldehyde, urea and formaldehyde are mixed.
And of course, there is always an excess of reagents. In this case, excess formaldehyde (a.k.a formalin, the chemical used to preserver biological specimen which includes dead people, will vaporized in heat. The formalin is trapped in the solid structure of the melamine-formaldehyde polymer, but will be release when the formalin turns into vapor and leaves the solid 3-D structure into the air.
This happens when heat is involved, say…when you put hot meal into your brand new melamine plate. The formalin released will then be absorbed by you. That will not do, will it?
That is why we boil new plate and glasses.
February 23, 2009
My first laboratory experiment of this semester:
An ejector, or injector, or steam ejector or steam injector is a device used to transport fluid (the suction fluid) using the motion of another fluid (motive fluid).
So an ejector works like this: A high-pressure motive fluid is flowed through a nozzle (preferably a convergent nozzle), and the nozzle converts the pressure energy of a motive fluid into velocity energy, and a low pressure zone is created in front of the nozzle, since the motive fluid is expanded (V>>> therefore P<<<). This low pressure zone then draws in the suction fluid.
After passing through the mixing chamber of the ejector, the mixed fluid expands and the velocity is reduced, resulting in the recompression of the mixed fluid (by converting velocity energy back into pressure energy).
The motive fluid may be a liquid, steam or any other gas. The suction fluid may be a gas, a liquid, a slurry, or a dust-laden gas stream.
The diagram below depicts a typical modern ejector. It consists of a motive fluid inlet nozzle and a converging-diverging outlet nozzle.
Water, air, steam, or any other fluid at high pressure provides the motive force at the inlet. The Venturi effect, a particular case of Bernoulli’s principle, applies to the operation of this device. Fluid under high pressure is converted into a high-velocity jet at the throat of the convergent-divergent nozzle which creates a low pressure at that point. The low pressure draws the suction fluid into the convergent-divergent nozzle where it mixes with the motive fluid. In essence, the pressure energy of the inlet motive fluid is converted to kinetic energy in the form of velocity head at the throat of the convergent-divergent nozzle. As the mixed fluid then expands in the divergent diffuser, the kinetic energy is converted back to pressure energy at the diffuser outlet in accordance with Bernoulli’s principle.
A bit confused? So am I… >.<