Ask Dr. Nick: Shrink Rate Data

Q: Do you have any data for shrink rates over time, for roto based products?

Dr. Nick: Interesting question. Nothing immediately sprang to mind, in terms of a prior published reference. But…

Clearly, shrink rates will be governed by the development of crystallinity within the PE.  So I looked for data concerning the development of crystal structures in PE.  There isn’t much that is quantitative.

I found a table (p268, Crawford & Throne, Rotational Molding Technology) that showed relative crystallization rates of various semi-crystalline polymers: PE scored 5000, whereas Acetal scored 400, PA6 scored 150, PP scored 20.  Units were µ/min – the observed speed of the crystal growth front, measured at 30ºC below the Melting Point of the polymer.  So crystallization of PE will be very fast, compared to anything else that a rotomolder might use.

This ties in with my practical understanding.  However, it’s well known that, even for PE, the development of full crystallinity is not instantaneous.  For example, my kayak customers always insisted on waiting 24 hours before they assessed shape retention with a new resin.

When I was doing developments in PP, my team recognized that we had to wait 7 days before full crystallinity had developed and we could make assessments on final physical properties.

I also found a statement (p104, Peacock, Handbook of Polyethylene), concerning the later growth of crystals after their initial, rapid, growth phase:

“As crystallization progresses, the rate of spherulitic growth decreases to a dependence upon the square root of time, indicating that growth is a diffusion-controlled process governed by the rate at which non-crystallizable sequences can diffuse away from the growth front.”

Fig 30 in the same book shows plots of Degree of Crystallization vs. Time for hydrogenated polybutadienes (this material is stated to be chemically analogous to branched LLDPE; the Degree of Crystallinity being achieved is similar to a typical film-grade LLDPE – approx. 20%).  The original reference is: ALAMO & MANDELKERN, 1991, Macromolecules 24:6480.  After an initial steep rise, the Degree of Crystallinity continues to rise as a shallow straight line against logarithmic time. 

I do recognize that this isn’t the exact answer you hoped for!  I’ll keep looking out for better information. 

If ARM Members or others reading this have input or data, please share them in the comments.

Dr. Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 30+ years experience in rotomolding, specializing in the fields of materials development and process control. In 2022 he was inducted in the Rotational Molding Hall of Fame. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.