Solid-State Shear Pulverization
From the Preface
This book is the first extended look at a new and multifaceted polymer processing technology that has already been discussed in numerous articles. Called Solid-State Shear Pulverization (S3P), this innovative process produces polymeric powders with unique physical properties not found in the output of conventional size-reduction methods…. This technology, which utilizes a pulverizer based on a modified co-rotating twin-screw extruder…, has profound implications for both the creation of new polymer blends and recycling of plastic and rubber waste.
Unlike [earlier processes] where polymers are melted prior to pulverization, …pulverizing mixtures of polymers with the S3P process…does not involve melting. By contrast, S3P maintains polymers in the solid state and avoids the additional heat history that occurs during [other processes], which can be detrimental to the physical properties of pulverized materials. The research and development of the S3P technology…has grown significantly since 1990 from the development of a new plastics recycling process to a much broader polymer processing method that allows intimate mixing of polymers with very different viscosities, sold-state dispersion of additives, including pigments, and continuous production of powder with unique shapes and larger surface areas. Polymeric powders are of growing importance to plastics processors due to the increase use of plastics in various applications, such as rotational molding, powder coatings, and compounding, which require powder as the feedstock.
…[I]t has become clear that this process allows for in-situ compatibilization of dissimilar polymers by applying mechanical energy to cause chemical reactions. This aspect of S3P technology that we describe in this book should [be useful in] developing new polymer blends with the use of pre-made compatibilizing agents. In addition, it has been discovered that S3P efficiently mixes polymer blends with different component viscosities, resulting in the elimination of phase inversion. The S3P process directly produces blends with matrix and dispersed phase morphology like those obtained after phase inversion during a long melt-mixing process. This phenomenon is of practical importance because a long processing time is required by conventional melt-mixing to produce a stable blend morphology. S3P is also advantageous for producing thermoplastic or thermoset powder-coating compounds in a one-step process as opposed to a conventional multi-step operation that involves melt extrusion followed by batch grinding. The major capabilities of this new process can be summarized as follows:
o Continuous powder production from plastics or rubber feedstocks
o Blending of immiscible polymers
o Efficient mixing of polymers with unmatched viscosities
o Environmentally friendly recycling of multicolored, commingled plastics waste
o Sold-state dispersion of heat-sensitive additives
o Engineered plastic/rubber blends
Materials and processes well illustrated
The text is well illustrated with 60 photographs, micrographs, diagrams and others figures. Here is a small sampling of the captions of these figures. o Particle-size distribution for virgin LDPE powder made with PT-25 pulverizer o Optical photograph of virgin LDPE powder made with PT-25 pulverizer o Layout for a three-stage rubber pulverizer o Flow chart for powder coating production by conventional process and with new S3P technology o SEM image of pulverized virgin PP at 40X (first in series of SEM images of polymer powders) o Optical micrograph of melt-crystallized thin films of unpulverized virgin PP under polarized light o Log of viscosity vs. log shear rate for virgin HDPE after S3P processing o Gel permeation chromatograms (GPC) of polystyrene subjected to S3P processing
One of the several functions of Solid-State Shear Pulverization technology is recycling mixed plastic waste. This section of twenty full-color photographs and micrographs illustrates different processed materials, as well as the machinery and mixed waste used. Here is a small sampling of the photo and micrograph captions. o Resultant flake feedstock from granulation o S3P-made uniform powder from feedstock o Flake feedstock of post-consumer HDPE/PP blend (90/10 ratio) o Injection-molded test bar (with translucence) made from S3P powder without pelletization o Injection-molded test bar made from S3P powder without pelletization showing uniform color o Several test bars subjected to tensile testing showing exceptionally high elongation at break
Useful reference data in tables
More than 60 tables provide useful data in convenient form. Here is a small sampling of table captions. o Physical properties of virgin PP 8020 GU injection-molded from S3P-made powder (first in series of tables on physical properties of various plastics processed from S3P-made powder) o Sieve analysis of powder resulting from S3P of virgin LDPE 509.48 (one of series of tables on sieve analysis of polymer powders) o Melt-flow rate before and after S3P processing for virgin PS and two PP samples o Key physical properties of injection-molded post-consumer polyolefin blends pulverized by S3P process
Klementina Khait, M.S. Ch.E., Ph.D., is Research Associate Professor and Director of the Polymer Technology Center in the Department of Chemical Engineering, Northwestern University. Her industrial experience in polymer science and engineering includes work with Borg-Warner Chemicals and Quantum Chemical Corporation. She received her two advanced degrees, in chemical engineering and polymer chemistry, from the Technological Institute, St. Petersburg, Russia. Dr. Khait holds several patents and has published more than 50 papers in scientific and technical journals.
Stephen Carr, Ph.D., is Professor of Materials Science and Engineering and Chemical Engineering at Northwestern University. His industrial work includes work in polymer science and engineering with General Motors Corp. He received a doctorate in polymer science from Case Western Reserve University. He has been on the Northwestern University faculty since 1969.
Martin H. Mack is Vice President for R&D with the Berstorff Division of Krauss-Maffei Corporation. He holds an engineering degree from the University of Stuttgart. He has served for more than ten years on the Board of Directors of the Society of Plastics Engineers (SPE).
Table of Contents
All chapters end with a list of References.
1. Overview of Polymer Powders
2. Conventional Methods of Powder Production
o Size Reduction o Process Selection for Size Reduction o Micromechanics of Pulverization o Foundation for Scaling Laws o Other Considerations
3. Principles of Solid-State Shear Pulverization (S3P)
o Introduction o Review of Strain-Assisted Grinding of Polymers o Summary
4. Equipment for the S3P Process for Plastics and Rubber
o Machine Functions of the Twin-Screw Extruder o Modular Design of Barrel Sections o Modular Design of Screw Elements with Conveying, Melting, and Pulverization Functions o Conveying Elements o Kneading Blocks o Heat Transfer Calculations for Barrel and Screws o Screw Cooling o Process Examples o Commingled Plastics from Recycling Sources o Pulverization of Cured Rubber o Scale-up Considerations for the S3P Process o Summary
5. S3P Technology and Virgin Polymers
o Applications for Plastic Powders o Characterizing Powders o Summary
6. S3P Technology and Polymer Blends
o Compatibilization of Polymer Blends o The Strategy of Self-Compatibilization o The Mechanisms That Underlie the Operation of S3P
7. Value-Added Products Made from Recycled Plastics
o State of Plastics Recycling o S3P and Plastics Recycling o Physical Properties of Commingled Plastics o Characterizing Powders o Summary
8. Applying S3P Technology to the Recovery of Used-Tire Rubber
o State of Rubber Recycling o Characterizing Powders o Physical Properties of Tire Rubber Powder/Plastic Composites o Emerging Technologies in Tire-Rubber Recycling
Epilogue: Toward the Future