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Re: Gels in HDPE

Hello,

> > I'm looking for a low-tech way to test for gels in HDPE. I have an > FTIR and a heated press that can make films for the machine on the > order of 2 to 3 mils thick, possibly thinner with some work. If I had > a polarized light source, I wonder if I could see and measure gels > with a 1000x or 500x microscope? > > Opinions please. > > Thank you, > > Bob

What size gel particles are you looking for? At a 1000x, you will be looking at an area that is less than 0.1 mm in diameter. There would have to be a lot of gel present, or you would have to examine a very large number of spots to see an occasional gel particle.

While I do not have experience with HDPE, in another polymer we extruded film about 10 mils thick, and we could see the gel particle by shining bright light through the film. You may want to try this approach on compression molded film of HDPE.

If the gel particles are large enough and sufficiently cross-linked, extruding through a screen-pack will trap them.

Dissolving the polymer in a solvent (hot xylene for PE?) and filtering through a filter, should also detect gel.

Regards,

Ernie

Reply to
Ernie
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You might also want to condition your light that you are working with polarization. If your put your sample between crossed polars and rotate them in unison, the gels, being amorphous, will always allow light to pass through then. The crystalline regions will go white to black or black to white every 90 degrees. Your can make your own setup or buy a polarizing microscope (or borrow one from a crystallographer or biologist).

John

Reply to
John Spevacek

Thanks for your thoughts, Ernie.

I'm looking for gels that could cause a 7 mil fiber to break in the orienting process, so they're pretty big gels. Probably on the order of 3 to 4 mil, so I wouldn't need 1000x to find them. Here's the part I was worrying about. I've read that under polarized light, gels show up in oriented film because these gels do not orient well. With this molded film, the body of the polymer would not be oriented either.

Now I've just talked to one of our pigment vendors and he thinks a little one inch diameter film like I'd get from the press is just not big enough. Said I'd need a square meter! of film to start counting gels.

Anyway, thanks for your help, Ernie.

Bob

Reply to
Robre

compression

Bob,

I have a lot of experience with plastics, but not much fibers fibers. If this was a broken plastic part, the first thing I would do is look at the area of the break. That is where the problem occurred, and that is where one can usually find the culprit (gel, dirt, large pigment particle, etc.). May be it is impossible to find the broken ends, but I sure would look for them. First examine the ends under a microscope, then use the FTIR to identify what is in the ends. As both you and John Spevacek mentioned, gel is likely to be amorphous.

If FTIR says, the ends are PE, then I would run DSC on the ends, and regular HDPE. Since the first heat cycle in the DSC can be very much influenced by thermal history of the sample, use the second DSC cycle (run sample to ~150C, cool to ~R.T. in the DSC, and record the second heat cycle for all samples) as characteristic of the material.

Good luck,

Ernie

Reply to
Ernie

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