For answers to questions not posted here or if you need additional information, please contact Prof. Dave Zumbrunnen (E-mail: zdavid@clemson.edu, Tel. 1-864-656-5625). |
§ How does smart blending differ from conventional blending or mixing processes?
The same laws of physics apply! However, in smart blending, they are applied such that fine-scale internal shapes of melt components or orientation and arrangement of solid additives (i.e., polymer blend morphology or microstructure) can be more controllably and deliberately formed directly in melt processing steps. Polymer melt components are more deliberately formed into prescribed small-scale shapes such as numerous thin layers, long fibers, sponge-like structures, platelets and ribbons, and small droplets. The characteristic smallest dimensions of these shapes can be less than 1 micron and may be only tens of nanometers. Actual sizes depend on polymer melt properties such as viscosities and surface tensions. Solid additives such as carbon black or inorganic nano-platelets can also be arranged controllably into desirable structures for property enhancement purposes. Examples first demonstrated in LAPM&T include aligned nanoplatelets, conducting networks in carbon black, and oriented nanotubes. With existing technology, such shapes or structures may arise only transitionally and locally in a blending device. In contrast, they form controllably over comparatively much larger volumes in smart blenders and are outcomes of progressive morphology (or structure) development. Progressive morphology development refers to the formation of specific structures in melts via sequential changes from one structure type to another. For example, a multi-layer melt can convert volumetrically to many other structure types with each type following another in sequence. Smart blending devices are specifically designed to promote progressive morphology development. Structure can be varied independently of composition so property optimization can be done. Fine-scale structures in extrusions can even be varied dynamically to give extrusions with graduated or periodically changing properties. In this manner, functional extrusions may be producible such as films that respond to environmental contaminants. Of course, extrusions with constant properties can also be obtained. For such extrusions, dynamic operation permits on-line property optimization.
§ Are smart blenders also called "continuous chaotic advection blenders" and "chaotic blenders?"
Yes, these terms have all been used to describe the machines developed in LAPM&T. We prefer the term, smart blender, because it denotes an ability to controllably form fine-scale shapes in polymer melt components or deliberately arrange solid additives into desired structural arrangements. However, the terms, chaotic advection blender and chaotic blender, are sometimes used where operative mechanisms are the focus.
§ Why is chaotic advection a necessary basis for smart blending methods? Also, what does advection mean?
In all blending processes, melt domains must be stretched. However, with smart blending methods, we also want to organize the melt domains in lieu of directly dispersing them as occurs in conventional equipment. Chaotic advection stretches and folds melt domains and can occur even in response to simple flow fields. Initially large melt bodies of immiscible plastics become converted to a multilayer arrangement. Such multilayers can be useful, but they are also a parent structure to a wide variety of other structures. The layers undergo changes in shape. When this occurs, changes can occur volumetrically inside smart blenders. Such transitions can be promoted by controlling stir rod motions in smart blenders. Consider, for example, the resultant structure if small ruptures form in all layers of a multi-layer melt once they become very thin. The ruptures are filled with melt from adjacent layers so previously isolated layers become interconnected. Upon extrusion, the properties of both the parent and resultant structure can differ and may find use in different applications. Generally, the characteristic sizes of small derivative structures relate to the parent layer thicknesses.
The term 'advection' denotes transport by motion. Thus, chaotic advection is a technically precise term in that fluid particles are transported chaotically. It was introduced by Professor Hassan Aref currently at Virginia Polytechnic Institute & State University in a seminal paper published in 1984 on general concepts of chaotic motions in passively advected particles. Subsequent work in chaotic advection was of a fundamental nature and focused typically on using chaotic advection as a model of mixing of identical fluids. Often, for example, fluids differed only with respect to their color. In the LAPM&T, the focus instead was on controllable formation of fine-scale structures in multi-component melts and particulate flows. Due to the multi-component aspects, in addition to being the birthplace of polymer blends and composites formed by chaotic advection, LAPM&T was also first to study interfacial effects in chaotic advection and thereby generalized prior and concurrent mixing studies to situations where different viscous fluids are of interest in lieu of identical fluids.
§ Has the application of smart blending to barrier films been considered?
Yes. Blends are producible with hundreds or even many thousands of thin, internal layers or other thin, high frontal area shapes like platelets and ribbons. Such shapes are direct outcomes of the stretching and folding characteristic of chaotic advection. Also, inorganic nano-platelets or other pancake-shaped particles can be oriented and selectively located volumetrically such that frontal surfaces act to impede permeation. All such structures in plastics can provide effective internal barriers to permeation. Both journal and conference papers are available on these applications. Note that methods are applicable to extrusions of essentially any form (film, pipe, rod, sheet) since blend structuring occurs in the smart blender upsteam of dies. Thus, tubes, pipes, and sheets with barrier walls are producible.
§ How can a process based on chaos be controllable? Are your results repeatable?
Controllability is a hallmark of smart blending technology. Smart denotes an ability to control structure formation. An ability to control also is an ability to repeat results. It is true that chaotic motions in smart blenders are the basis for the stretching and folding of melt domains leading to a variety of blend morphologies. These chaotic motions refer to individual fluid particles. However, minor and major component melts (such as polyethylene or nylon) consist of very large numbers of particles. If a minor component is injected into a smart blender, individual particles may undergo chaotic motion but the overall minor component and major component forms are reliably and repeatedly converted into alternately layers. That is, the blend morphology is a predictable and repeatable outcome whereas the precise locations of individual fluid particles cannot be predicted due to their chaotic motions. Where it is desired to build structures from individual particles such as nano-platelets, structural characteristics are similarly predictable and repeatable. Percolating networks, for example, can be constructed in melts in lieu of being outcomes of chance as occurs in conventional mixing. The physical properties of materials are determined by structure (e.g., polymer blend morphology) and not by the known locations of individual component particles so that smart blending can be used to optimize structure, composition, and properties. Methods are also conducive to forming functional structures and retaining them in extrusions.
§ Are smart (chaotic) blenders available commercially? Where can I buy one?
Smart blenders can be purchased commercially from machine licensees of Clemson University. They are available for various capacities and in both general-use and product-specific configurations.
§ I've heard before about chaotic mixing and see that you have used this term also to describe your work. Why are you now using the term, chaotic advection?
We used the term, chaotic mixing, in our early work dating to the early 1990s because it was more recognized by the chemical engineering and polymer processing communities. However, we now use the parent and more technically precise term, chaotic advection. Our focus is not on mixing per se, but on in situ structuring. Chaotic advection is used to organize melt domains at increasingly smaller size scales to form specific small scale shapes or arrangements among material components. By adopting the parent term, the structuring aspects of smart blending are clearer. Structure is broken down in mixing whereas structure is controllably evolved in smart blending based on chaotic advection. The term, chaotic advection, originated with Hassan Aref now of Virginia Polytechnic Institute & State University who wrote a seminal 1984 paper describing why and how chaotic motions in fluids can arise.
Although the purpose of smart blenders is to controllably form structures in melts, they can also be used to mix! However, mixing arises differently than in conventional mixing machines. To mix, a very fine-scale, multi-layer structure is formed in the smart blender. This multi-layer structure is subjected to additional refinement in the smart blender by chaotic advection until the numerous thin, tenuous layers either rupture or dissipate. The result can be a very well dispersed blend. Similarly, solid particles such as nano-platelets can become very well dispersed by arranging them in layers having thicknesses similar or less than the nano-platelets. Please note that one Clemson University patent application specifically includes aspects related to nano-platelets and other nano-solid additives.
§ How does your work differ from the work of others in the general area of chaotic mixing?
Our research and focus have differed greatly from that of other researchers in chaotic mixing who in almost every instance considered only fluids that are identical except for a color or other passive identifier. Their studies, which are useful, have elucidated mixing mechanisms and have been directed to creating mixtures normally consisting of droplets or dispersing additives for resin pellet production. In our work, fine-scale structure development among different types of fluids (polymer melts) is considered in response to chaotic advection as a route to obtain a variety of structured polymer blends and composites such as those shown in the tutorial on the home page. Material structure, composition, and properties are central. Polymeric materials, which may include solid additives, are literally assembled into a variety of fine-scale arrangements. Physical properties of extrusions and castings are optimized or functionality is imparted. Smart blending (or chaotic blending) has a materials science focus. Polymer blends and composites are regarded as composite materials where structure can have an important effect on properties. [Please also see the related preceding FAQ.]
§ Have you considered application of smart blending to injection molding?
We described in general terms application to injection molding but studies specifically focusing on injection molding have not been performed. In a recent 2005 paper in the journal, Polymer, the stability of blend morphologies is quantified. Results show that various blend morphologies persist such that retention in injection molding parts is possible. A novel sponge-like morphology at a small low density polyethylene composition in polypropylene was formed in the smart blender, for example, and was retained in extrusions after different melt residence times. Similar results were obtained for other morphologies.
§ I've heard some talk about smart blending concepts now being applied to tissue engineering. Can you explain?
We have demonstrated that solid particles such as carbon black can be arranged selectively into networks. Percolating structures, for example, are literally built in the melt in lieu of being outcomes of chance as in conventional mixing. The networks allow particle interactions such as electrical current flow. The LAPM&T has collaborated with medical and bioengineering researchers at the Medical University of South Carolina and Rutgers University to apply these concepts to similarly assemble living cells into networks. Cellular networks can be formed in advance or in tandem with injection into the body. By forming the cells into networks, cell interaction and the formation of capillaries for blood supply are promoted. It is hoped that tissue growth in the body or external to the body if not injected will occur even without the conventional tissue scaffold now used. A proposal to pursue this work was submitted to the National Institutes of Health and received positive reviews. Unfortunately, however, funds were not provided. A need for more preliminary data was given as a reason.
Where tissue scaffolds are used, smart blending may also find application. Polymer blend morphologies are producible that have desired characteristics.
§ Are the various small scale structures in your extruded plastics stable? Don't these break up over time?
The blend morphologies are retained in extrusions subsequent to cooling. They are persistent but can undergo changes if the extruded plastics are subsequently melted. Of course, plastic products are typically not heated to temperatures near their softening points when used for intended purposes. Also, keep in mind that all processes for producing polymer blends yield morphologies which are not in equilibrium with environmental conditions. For example, screw extruders subject polymer melts to various shear rates during processing which are absent subsequent to forming finished products. The blend morphologies that are obtained with screw extruders would also undergo changes if these products are subsequently melted. Thus, plastics produced in smart blenders do not differ in this regard from those produced with other blending devices.
More FAQs will be posted.