Crosslinked Polyethylene 1

[RichGeoffroy]

In a recent e-mail inquiry, a reader asked:

"We would like to use radiation to crosslink hdpe. Will increasing the dose make the melt point and hardness that much greater?"

In my experience, neither will occur.

First, the melting point is a function of crystalline packing --- the more uniform the packing, the higher the crystalline melting point of polyethylene. Melting point is not a function of molecular weight. The PE crystalline regions will melt generally at the same temperature as they did prior to irradiation. The electron beam, however, will penetrate the crystalline regions and result in the formation of crosslink sites within the crystallites which creates nonuniformities and lowers the crystalline packing efficiency. Thus, one might anticipate a slight reduction in melting temperature. However, if you were to run DSC (Differential Scanning Calorimetry) on the polyethylenes, I suspect that you might find very little significant change in the peak melting temperature, but rather a broadening of the melting range, i.e., melting would begin at lower temperatures.

As far as hardness, I originally felt that hardness and stiffness would increase with irradiation dose. To my surprise, the stiffness of polyethylene actually decreased with increased crosslink density. Amorphous polyethylene is an elastomer --- a soft and pliable polymer which is rather useless on its own. What makes polyethylene so useful is the fact that it can be polymerized in a specific way which allows the molecules to pack together and crystallize. It is the crystallization of polyethylene which yields its unique properties.

The crystallites act as a “hard” reinforcement within the rubbery matrix. As the electron beam radiation creates more crosslink sites within the crystal, the crystals become less uniform, reducing the amount of crystallinity, and eventually the crystallites break up into smaller entities. This process results in a significant reduction of the crystallinity, and a corresponding reduction in many properties such as hardness, stiffness, and strength.



Rich Geoffroy
Polymer Services Group
polyserv@cox.net

 

[Demon3]

Hi Rich,

I think you brought up some good points. I think we have to make a distinction here. I agree that if you were to irradiate then melt and recrystallise you'd expect to see a lower, broader melting peak by DSC. However, if you just irradiate I don't see how the packing and crystallinity can change because the structure is locked in place. I'd expect maybe when the solid PE is irradiated, the hardness may well increase.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[Compositepro]

I would think that the most desireable and observable effects of crosslinking PE would be be due to what happens in the amorphous regions. The crystallites are essentially already crosslinked by hydrogen bonding until they melt.

 

[Demon3]

YOu won't find any hydrogen bonding in PE. Did you mean nylon? Anyway, yes you'd only expect the amorphous region to change as the crystals are immobile plu oxygen can't get to them either.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[hoidap]

The boarder metling can be considered that starting point is sooner, and ending point is later. Crosslink density also plays a role here.
Could we image that XHDPE is a blend of xHDPE and uncrosslink-HDPE, so the melt of uxHDPE responds to the heat in the same way, if its content is large enough to melt, the XHDPE will melt?.
But i think the MFI will be diferrent.
Yamaguchi in JAPS, 86, 2002 reported that XHDPE is able to enhance the melt strength and strain hardening in elongational viscosity of HDPE.
For hardness, the respond may be claimed to the porous of XHDPE compared to compact crytaline HDPE.

 

[RichGeoffroy]

In an inquiry regarding the original post, a reader stated:

Rich,

This sounds counter-intuitive! Are these physical property observations the same for PE silane/peroxide cross-linking? Why then are products cross-linked for higher temperature applications, like pipe and cable? Does the cross-linking take place in the crystalline regions only in e-beam? Or do functional silanes also graft into the crystalline regions? Could you please enlighten me?In an inquiry regarding the original post, a reader stated:

All three methods of crosslinking are different. The original question was with regard to the effects of electron beam crosslinking on the melting point. In the case of e-beam, crosslinking takes place in the solid state. Due to the high energy of the radiation, it penetrates both the crystalline and amorphous states and crosslinking takes place in both regions. However, it is only the effects of crosslinking in the crystalline region which actually affect the crystalline melting point of the polymer. Crosslinking within the amorphous phase has no effect on the crystalline melting temperature, however, it does affect the physical properties of the polymer.

Crosslinking of the silane groups is also conducted in the solid state. However, because the reaction is moisture activated, I suspect that crosslinking is largely in the amorphous state, where the moisture can easily penetrate the polymer, with little reaction occurring in the crystalline phase. Silane crosslinking should increase intercrystalline tie molecule density as well as the viscosity of the amorphous phase, but should not affect the crystalline melting temperature. In this case, it is largely the amorphous phase of the polymer which is affected by crosslinking. It will tend to behave more like a vulcanized matrix for the hard, crystalline phase.

Peroxide crosslinking, however, is a different animal all together. The crosslinking reaction takes place in the liquid, molten state, prior to crystallization. The crystalline regions form on cooling. Crosslink sites will be in both the crystalline and amorphous phases. Those in the crystalline regions will disrupt the crystallinity and reduce the crystalline melting temperature (as compared to the uncrosslinked polymer), as well as reduce the size of the crystallites formed and percentage of crystalline material within the polymer. The amorphous phase will be tougher and stronger, and more capable of resisting intercrystalline separation which is the typical mode of long-term failure in polyethylene.

All that being said, all three forms of crosslinking significantly increase the molecular weight of the base polyethylene polymer, especially that which is present in the noncrystalline amorphous phase located between the crystallites. Very high molecular weight polyethylene does not flow well, but has significantly enhanced mechanical properties at elevated temperature, even though its crystalline temperature remains about the same. That’s why crosslinked polyethylene pipe and cable perform so well at elevated temperature --- not because they have higher melting temperatures.


Rich Geoffroy
Polymer Services Group
polyserv@cox.net