Mismatched Fibers
Developer: Dr. Mary R. Reidmeyer
Project for Grade Level: High School
Discipline: Materials, Physics, Chemistry
Topic Area: Glass properties, Thermal expansion, Softening points
Time Required:
Goals: Create a glass fiber with two dissimilar glasses and observe the result.
Objectives: Demonstrate that single glass fiber can be created with two different glasses.
Observe the behavior of the glass fiber when it cools.
Understand why this behavior occurs.
Materials: Glass rods of different glass compositions
Pliers
Bunsen burner
Miraflex® sample
Safety Precautions:
Follow the instructions. Wear proper safety equipment such as goggles and heat resistant gloves. Avoid contact with the heated rods. Dispose of the waste glass properly.
Procedure:
1. Set up the Bunsen burner according to the manufacturer’s directions.
2. Light the Bunsen burner and adjust your flame so that is medium high and blue.
3. Select glass rods A and B. Hold one in each hand. Rotate each so that the free ends point at each other and could be lapped parallel to your body.
4. Simultaneously move the rods into the flame at mid level in the center of the flame so that approximately one inch of each rod is being heated in the flame.
5. Continuously rotate the glass rods so that they are heated evenly.
6. The ends of the glass rods in the flame will soften and start trying to sag. At this point pull the rods from the flame and immediately overlap the heated ends. Have an assistant gently squeeze the overlap joint together with pliers so that the glass makes good contact and fuses together.
7. Return the jointed glass to the flame and heat the entire one inch overlap section to complete the bond between the two glass rods. Remember to continuously rotate the glass rods.
8. Remove the glass rods from the heat and cool slightly so that the one inch overlap section becomes rigid again.
9. Know move the overlap section back into the edge of the flame so that only a ¼ to ½” section is heated to the softening point. Again rotate the rods continuously while heating.
10. When you see and feel the glass in this section become very soft and flexible, remove the rods from the heat and immediately pulled both rods away from each other. Pull straight out to the sides so that a glass fiber is stretched parallel to your body. Be very careful not to twist the fiber and hold it straight until the glass hardens. The fiber should be at least 12-24” in length.
11. Lay the glass rods and fiber carefully on table top (avoid flammable materials). Cut the fiber free from the rods with the pliers. The fiber cools very quickly but the ends of the rods may still be warm.
12. Observe whether the fiber is straight or curves in a large arc.
13. Repeat steps 3-12 for glass pairs C-D and E-F.
14. If you encounter difficulties such as twisting, pulling a fiber from only one rod (wrong area heated), failure of the overlap, etc, just clean up the ends of the rods by cutting away undesirable material with the pliers. Then repeat steps 3-12 until you achieve an acceptable dual-glass fiber.
Observations and Questions:
1. What did the glass fiber do upon cooling?
2. Look at the table of glass properties. What difference in properties would explain the glass fiber’s behavior?
3. Speculate the on the behavior of the glass fiber as the properties of the two glasses differ to a greater degree.
4. What practical use could be made of this behavior?
Teacher’s Note:
Supplemental Material-
Vocabulary:
Amorphous- having no crystalline structure
Glass- an amorphous, ridge, inorganic, nonmetallic material that solidified from the molten state without crystallization
Hard glass- a glass having a high-temperature softening point and high viscosity at elevated temperatures
Soft glass- a glass having a relatively low softening temperature or which is easily melted
Softening point- the temperature at which a glass fiber of uniform diameter elongates at a specific rate under its own weight
Thermal expansion- the reversible or permanent change in the dimensions of body when heated
Thermal expansion coefficient- also known as the coefficient of expansion (COE)- the fractional change in the length or volume of a material per degree of temperature change
Background Information:
Different glass compositions produce glasses with different properties such as fusing temperature, strength, density, chemical resistance, optical properties and thermal properties including softening point and thermal expansion. Glass artists and technical glass fabricators heat and manipulate pieces of glass to create useful shapes. Often multiple pieces of glass are fused together in the process. It is essential that the glass rods or tubes used are compatible. If the materials do not have very similar properties, upon cooling the fused pieces will separate or fail catastrophically. Two properties of particular interest are the thermal properties, softening point and the thermal expansion coefficient.
When glass heats, it expands and when it cools, it contracts. Glasses with a high coefficient of expansion expand and contract more than glasses with a lower coefficient of expansion. When two glasses are joined while heated to their softening point, upon cooling they both will contract. To observe that contraction behavior of two compositions, a fiber is pulled from the joined glasses. If their coefficient of expansion is the same, the fiber will remain straight. If the coefficient of expansion is different, the fiber will curve. The glass on the concave side of the curve will be the higher coefficient of expansion glass since it contracted more. For a fiber, the greater the curvature, the greater the difference in coefficients of expansion. A very fine fiber drawn from borosilicate and Moretti rods would have the greatest difference in coefficients of thermal expansion and would actually curl.
The following is a table of properties for several glasses:
Glass / Type / COE / Softening PointBullseye / soda-lime / 90 x 10-7/°C / 700°C
Moretti / soda-lime / 104 x 10-7/°C / 565°C
Soft Glass / soda-lime / 86-90 x 10-7/°C / 630-700°C
Hard Glass- Pyrex / borosilicate / 32 x 10-7/°C / 820°C
A Bunsen burner that burns natural gas or propane and air is not sufficiently hot to soften borosilicate glasses. Only soda-lime based glasses can be soften in the Bunsen burner flame. A propane and oxygen torch would be necessary to soften the borosilicate glasses. The samples of glasses that you received in this experiment are all soda-lime glasses but contain varying amounts of other oxides that change the COE and softening point.
Owens-Corning Miraflex® Insulation
The ability to produce a dual-glass fiber with interesting properties was recognized and commercialized by Owens-Corning. Using the dual-glass fiber concept with glasses having different coefficients of expansion, they were able to produce soft, curly, virtually itch-free fiberglass insulation. Owens-Corning has applied for and obtained over seventeen patents for both the machinery designs to produce dual-glass fibers in large quantities and the process and methods associated with producing insulation glass products. Examples are US Patent No. 5,468,275 entitled “Apparatus Having Elongated Orifices for Centrifuging Dual-Component, Curly Glass Fibers” that details the design of the spinner head to produce the fibers. US Patent No. 5,616,525 entitled “Irregularly Shaped Glass Fibers and Insulation Therefrom” lists the compositions and coefficient of expansion of glasses suitable to produce a curly fiber insulation product from the spinner described in Patent No. 5,468,275. This curly, soft insulation is marketed under the name “Miraflex®” and is packaged in unique pink poly-wrap rolls. Mireflex® is being marketed as a user friendly product that is easy to transport, handle and install because it is soft and much less irritating than traditional fiberglass products.
University of Missouri-Rolla – Ceramic Engineering Department
http://campus.umr.edu/ceramics
StuMaterialsMismatchedFibers