Why does the PPR water pipe have white edges on the end face when cutting?

When we cut PPR water pipes with tube scissors, we often see the phenomenon of white edges, just like the picture above. Many people say that there is a problem with such a water pipe, which is the reason why the material is not good and calcium powder is added. Is that the case? Today to give you a science popularization.

This phenomenon of white edges on the incision usually appears on colored water pipes, and it is not easy to see in white PPR pipes, not because the white PPR pipe does not have it, but the white pipe itself is white, so it is not easy to see that there is a white edge.

So, what is the reason for this white edge?

In fact, this phenomenon has a technical term in polymer physics, called "silver streak phenomenon".

Silver streak phenomenon is a unique property of polymers, which refers to the polymer under the action of tensile stress, in some weak places of the material stress concentration and local plastic deformation, so that on the surface or interior of the material perpendicular to the stress direction of the appearance of length of 100μm, width of about 10μm, thickness of about 1μm fine groove or "crack" phenomenon.

When we cut the PPR water pipe, when the blade cuts the water pipe in the longitudinal direction, it will also generate a tensile stress in the lateral direction, which causes the PPR material to produce a silver streak at the incision. To put it in layman's terms, when the PPR material is sheared longitudinally, there is a force on the horizontal side, which pulls the material a little, so that the pull produces plastic deformation.

What is plastic deformation? Plastic deformation is relative to elastic deformation, elastic deformation refers to deformation can be restored to the original state, plastic deformation can not be restored.

Because the part where the silver streak phenomenon occurs is different from the refractive index of the normal PPR material, it has a silvery-white luster under the illumination of light. To make an image analogy, glass is transparent, but broken into glass slag is the existence of white powder, which will be easier to understand.

One of the characteristics of the sites that produce the silver streak phenomenon is reversibility. It can disappear on its own after the vitrification temperature of the material, which is called "self-healing". So, for the water pipe cut that produces silver streaks, if you take a hot air gun and blow them for a while, the silver streaks will naturally disappear.

The presence of white edges in the PPR water pipe shear is a normal physical phenomenon. If your scissors are sharp enough, or if you cut fast enough, you can minimize the lateral stress pull during the down-cutting process, and the silver streak phenomenon will weaken a lot or even be invisible.