Fundamentals of Fiber and Films: Formation, Processing, and other Characteristics

April 29, 2012

(written by: Willy Yanto Wijaya)

 

This writing is a summary of three lectures from Prof. Takeshi Kikutani and one lecture from Prof. Abdellah Ajji. Some of my own thoughts and comments, whenever available, are accompanied in the last part of each summary section.

 

Lecture 1: How to Make Fibers (Prof. Kikutani)

This first lecture covered mainly on fundamentals of fiber formation, i.e. formation of fiber shape and formation of fiber structure. In this lecture, one interesting example was shown, about polyethylene (-CH2-CH2-) which we normally know exist in the form of shopping bag plastic or garbage bag plastic. These plastics (made of polyethylene) typically have the strength around 0.2 GPa and modulus 2 GPa. However, if the same material of polyethylene is treated through gel-spinning and drawing, it can become ultra high modulus and strength fiber, having strength of 3 GPa and modulus 100 GPa. This example shows that characteristics or properties of polymers largely depend on the structure of the polymer, and structure of polymer can be adjusted by different method of processing (fiber formation).

In the fiber processing, macroscopically, the shape of fiber can be observed by naked-eye, and described as for example the fiber is thin and long. In addition, the actual structure of fiber can also be observed microscopically, described in the term of molecular orientation and crystallization.

Fiber spinning is a typical method for the formation of fiber shape. The steps in the fiber spinning include: fluidization, extrusion through tiny hole, elongation, solidification, and take-up. There are mainly two methods for fluidization: melt spinning (using heat) and solution spinning (using solvent). Several types of solution spinning include: wet spinning, dry spinning, as well as dry-jet wet spinning. Although more complex than the former two, dry-jet wet spinning can produce fibers with extreme properties such as Kevlar and Zylon.

For the microscopic fiber structure formation, two important concepts need to be taken into account are: orientation (alignment of molecules to fiber direction) and crystallization (three dimensional periodicity in the arrangement of atoms and molecules). In the alignment of molecular orientation, stress is the dominant factor if the molecules are in the melt condition, while deformation is dominant if the molecules are in the form of solid. For the process of crystallization, polymer has relatively slow crystallization rate compared to for example water to ice.

Lastly, this lecture also introduced unique fiber formation processes, for example combination of melt-spinning and drawing + annealing processes, as well as high-speed spinning. Some other methods were also briefly introduced: spun-bonding, melt blowing, flash spinning, and electro spinning. Fibers with engineered cross-section such as interference-colored fibers were also briefly discussed.

 

Comments: One particular interesting interaction that I noticed in this lecture is regarding close relation between crystallization and molecular orientation i.e. orientation-induced crystallization vs. crystallization-induced orientation. The question that arises is how significant one aspect influences the other one? Are they relatively in the same order? Or one effect is much more significant than the other? The next consideration is regarding the possibility to break-down the concepts of crystallization and molecular orientation into one-level deeper concept (a more fundamental description), whereby these two different concepts (parameters) will eventually be melted down and spun into a “new fiber” (parameter).

 

Lecture 2: Ultra-fine Fiber – Ultimate Processing (Prof. Kikutani)

The second lecture covered topics concerning ultimately fine processing of fibers, improvement of melt-spinning, fibers with complicated cross-sectional configurations, and various other discussions. Two main interests in ultimate fine processing are nano-fiber and engineered cross section fiber. The minimum size of ultimate fine fibers researchers are pursuing is mono-molecular fibers. Some methods to obtain mono-molecular fibers include molecular-fishing using AFM (atomic force microscopy) or carbon nano-tube. Some other methods such as controlled polymerization using mesoporous zeolites done by Prof. Aida of Tokyo University could produce poly-ethylene fibers with 30-50 nm diameter. Several other methods described in this lectures include: electro-spinning, flash-spinning, combination of spunbond process with melt blown technique.

The next issue conveyed in this lecture is about improvement of melt-spinning process, where surface tension and air-drag force are two important parameters governing the minimum fineness in direct melt spinning process.

Precise control of cross-sectional configuration was another topic talked in this lecture. Examples of this precise control of cross-sectional config are the Islands-in-a-sea type fiber (produced by Toray) and Petal-like hollow conjugate fiber (produced by Teijin). Next issue was about fibers with complicated cross-sectional configuration nano-structure. One interesting concept introduced is the parameter Complexity, which is defined as:

Complexity = L^2 / S,

where L is the circumference and S is the area.

Lastly, the interference-colored fiber was discussed, where eventually multi-layered structure was used instead of lamella-ridge structure.

 

Comments: In my opinion, the idea of complexity is quite interesting. The failure of producing lamella-ridge structure might also be due to its much higher complexity in its structure. Circle seems to possess the least value of complexity with value only 4*phi (which is even lower compared to even simple structure like square having complexity value of 16). The question is how high complexity that human can achieve so far in making structure (either fiber structure or any other structure)? And why nature can easily create structure with extremely high complexity, such as fractals (which have circumference^2 much larger than its area)? Can the mechanism of forming fractals be imitated in a consistent and reliable ways (methods) in the industrial applications?

 

Lecture 3: Films Processing, Characterization, and Analysis (Prof. Ajji)

The third lecture given by Prof. Ajji covered topics about cast film and biaxial film processes, analytical description of cast film process, and structural characterization for biax films. For the manufacturing of cast film, several merits to mention are: adjustable lip opening, thickness variability quite low (3%); however some demerits are that the temperature control (such as cooling step) is very vital, and require very large die.

In the analytical description of cast film processes, assumption such as isothermal Newtonian fluid case was taken. Newtonian fluid means that when the flow speed of the fluid changes, its viscosity does not change (for example water). Nevertheless, polymer is actually non-Newtonian fluid since its viscosity will change when the flow rate changes. From this analysis, it was understood that at a certain condition the width of the film will reach a certain limit while thickness continues decreasing when the speed increases.

Relatively similar to the case of fibers discussed in previous two lectures, the properties of film depend on its structure development which mainly affected by the process conditions and its material molecular structures. Cast films and blown films generally have low level of orientation, while biax films generally have high levels of orientation. Nucleated structures are also often encountered in blown films.

Lastly, structure characterization techniques were explained in this lecture: Birefringence, IR (Infrared Spectroscopy), and X-ray diffraction. Each characterization method has its own pros and cons. For example, birefringence is relatively simple to perform, but it can only see the average phases. IR is more complicated, but it can see the amorphous and crystalline phases. X-ray scattering can give complete probability distribution, however it can only handles the crystalline phase. It is also possible to combine, for example birefringence with the x-ray diffraction, to get certain properties of the films required.

 

Comments: The assumptions of isothermal deformation and Newtonian fluid were still fairly relevant when compared with the experimental results as discussed in the lecture. This holds for the cast film and biax films processing. Wondering if the films are to be made with the various methods in the fiber formation processes described previously, how far can these two assumptions hold? Further, can all those various processing methods for fibers be applied for manufacturing films? If not, what are the most significant parameters to affect the infeasibility of the applications?

 

Lecture 4: Fibers production and environmental issues (Prof. Kikutani)

Due to my attendance to an academic conference, I could not come to this final lecture. However, from the lecture materials, I can see that there is a very interesting discourse regarding impacts of different types of fibers production on the environment. At present (data as of 2006), the global production shares of fiber types are as follow: synthetic man-made fibers 55%, cotton 38%, cellulosic man-made fibers 5%, and wool 2%. The synthetic man-made fibers are derived from naphtha (portion of crude oil) which is not renewable and expected to be depleted one day. Therefore, if the shares of cotton or wool are to be increased to cover this, huge areas which can be used for food (agricultural) production will be sacrificed. And the case of the wool will be worst, one third of earth surface need to be converted into grassland for the sheep, if the annual fiber production of 68 million tones all to be covered by wool only. This is of course not plausible.

Therefore, cellulosic man-made fibers are expected to have bigger shares in the future. The feedstock for these cellulosic fibers can be obtained from sustainably managed forests, for example eucalyptus which is fast growing and requires water resources much less than cotton plantations.

However, to enable this kind of aim, efficiency in the production and use of fibers (such as reducing materials and energy consumption) are indispensable.

 

Comments: The discourse of promoting cellulosic man-made to have bigger future shares is very interesting. However, in my opinion, there are still several factors need to be considered, upon why at this present moment, cellulosic fibers’ share is very low, only 5%. One main factor I think is the cost. The costs of having labors to cut and transport the woods, the processing cost (energy consumption, etc) of the cellulose into fibers are still major challenges which prevent this type of fibers having big shares. For example, the production processes of textile products incorporate many steps: materials -> polymerization -> spinning -> drawing/annealing -> twisting -> dying -> weaving/knitting -> sewing; where many of these steps can be avoided (such as polymerization, annealing) in case the material is cotton. Therefore, indeed, the production efficiency and cost reduction are still major aspects have to be overcome in order to enable this type of fibers having bigger shares in the market.

Another important thing is that the intrinsic characteristic of the fiber material itself. For example, cotton and wool clothes have different “feel” and properties (such as heat transfer, humidity absorption). Will the wood clothes fit the feels of majority? Or should the cellulosic fibers better aimed at substituting the synthetics fibers in the production of PET bottles (at present more than 20 million tons annual production)?

Many questions still arise, but surely research on this cellulosic fiber and fibers in general, will play more and more important roles for the sustainable future.


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