Polypropylene Failure, Used in Field Joint 1

Is the PP homo or co polymer. They have different embitterment points.

Rapid cooling by quenching will induce some frozen in stress, but it will also reduce the level of crystallisation,.

Higher crystallisation will reduce elongation at break and impact strength.

Foamed mouldings are not so prone to frozen in stress.

Co polymer is tougher at low temperatures

Regards
pat pprimmer@acay.com.au
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Thanks for the attention P.
The first layer is an FBE, 2nd layer is copolymer adheseive to join the (difficult to bond) FBE to the 3rd layer of PP foam. I think the PP is homogeneous in nature if thats what you are asking. What does 'embitterment' point mean? i'm new to this stuff. (i have mechanical background!) So for the problem in hand, with very cold environmental temps,would you agree that by quenching with, say, warmer water, the level of crystallanity can be improved (compared to cold water quenching in very cold environments), thus elongation before yield can be increased, and brittleness reduced??

Liam

Sorry, I presumed you had more knowledge of polymers than seems evident.

By pp homo I meant homopolymer.

Plastics are not truly solid, but super high viscosity liquids like lead, tar or glass. They go through a number of second order phase changes, including one from a glass like brittle state to a more malleable but still apparently solid state, and from this state to a rubber like state and from that to a dough like state.

For some plastics this transition is sudden, but for others it is gradual, depending on molecular shape, and spacing and strength of various bonds.

When we say crystalline in plastics we actually mean semi crystalline, as with "crystalline" plastics, the crystals actually form in a section of a molecule with amorphous bits between. These crystals always float in an amorphous matrix. Also crystals are defined as more ordered regions vs less ordered. They never form highly ordered consistent crystals like water or carbon.

Raw materials suppliers should be able to provide data on elongation vs temperature for various grades that they supply. Although these tests will be done on perfect mouldings, and often are not achievable in the real world, they are still a good guide, especially if done to the CAMPUS standards, which restricts the scope to adjust test conditions toward best results.

PP copolymer is less crystalline, and is more resistant to stress whitening, has a lower embroilment temperature, and higher elongation at break, as well as lower tensile strength and and flex modulus.

Cooling rate will effect crystalinity, but to reduce stress whitening it in foam, it is a double edged sword.

Quicker quenching will give a less crystalline material with smaller less ordered crystals. This will give a tougher material with greater elongation. Slow cooling gives more crystalinity and larger more ordered crystals.

Quick cooling gives a greater difference in crystalinity between the surface and the core of the moulding, as the core still cools slowly due to the insulating properties of the foamed polymer. The greater difference in crystalinity also means a difference in mould shrinkage through the moulding, resulting in internal stresses that use up some of the potential strength and elongation of the moulding.

From what you say, my money is on using co polymer and quenching it as quickly as possible

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.