Crosslinking PE 2

[AMWS]

Hi,

I was under the impression that crosslinking a plastic would increase tensile strength and reduce elongation. I have recently tested some PE crosslinked at different doses and not crosslinked at all and the tensile and elongation properties were very close. The only difference I noticed was the higher the dose the more consistent the results were within a sample set. Is this correct?
I am trying to determine what is the proper level of crosslinking

Thanks

 

[btrueblood]

Crosslinked with what?

 

[AMWS]

E-Beam.

 

[btrueblood]

Hmm. I thought radiation techniques both cut the molecular chains (effectively lowering the molecular wt. of the PE) as well as promoting "crosslinking" across the cut sites. That might help explain your results?

 

[RichGeoffroy]

AMWS:

In general, your supposition is correct, however polyethylene is something of a different animal. Remember, polyethylene has a low Tg (Glass Transition Temperature), somewhere around -60[sup]o[/sup]C. Therefore, it should be a soft, gummy elastomer at room temperature. The reason it is a reasonably hard plastic is due primarily to its high degree of crystallinity.

Now, as you induce crosslinking, each crosslink branch acts as a defect in the chain, therein reducing the likelihood that the polymer chain can crystallize with adjacent chains. The higher the degree of crosslinking, the lower the degree of crystallinity becomes. So as you restrict the mobility of the polymer chains through crosslinking, you are subsequently reducing the stiffness induced by crystallinity --- a trade-off.

Polyethylene will, therefore, decrease in stiffness as one crosslinks the polymer until most of the crystallinity is lost. Then, the polymer will begin to get harder and stiffer, but you may have to get a very high degree of crosslinking to achieve significantly enhanced polymer stiffness.



Rich Geoffroy
Polymer Services Group
polyserv@cox.net