Phase Change Materials
Base Materials for PCM Incorporation
Wire Heating Element
Click here for a discussion of a number of outdoor fabrics that were considered for the gloves. The key factors in fabric selection were heat transfer coefficient, breathability, weight, cost, and durability.
Different kinds of fleece fabrics were analyzed. We purchased four types of fleece: two-100% polyester, 80:20 Polyester:Cotton, and a 20:80 Polyester:Cotton. The fabrics were from 58-60 inches wide and ~3/8 of a yard’ is needed to make one pair of gloves.
100% polyester fleece is breathable, lightweight (good because alleviates bulkiness ordinarily characteristic of winter gloves), cost-effective (Prices noted below), and relatively easy to sew. We purchased one thick polyester that was nearly twice the price of the thinner fleece. The 80:20 and 20:80 poly:cotton are more of a sweatshirt-like material. Both are thinner than the polartec fleece fabrics and equally lightweight. We also purchased a pattern on sale (price noted below). There were other patterns, but this one looked the best and was most cost-effective. No other patterns were on sale.
There are some potential problems with the flannel lining. The fabric may wear out quickly where the wires, battery or thermistor edges make contact. The fabric is not water-proof or wind-proof. Thus, it was unclear as to whether or not it would prove to be an adequate material to wick away moisture from the skin, and/or act as an insulator in the glove. After discussion with Mrs. Galbraith (a seamstress), it was decided to use a thin 100% polyester fabric for the inner lining, which allows more breathability.
|Sweatshirt fleece||80% cotton, 20% poly||2 yds||$6.74/yd||$13.48|
|Burgundy Paisley Flannel||100% cotton||1 yd||$4.99/yd||$4.99|
|Blue Paisley Flannel||100% cotton||1 yd||$4.99/yd||$4.99|
|Navy/Green Plaid||100% cotton||1 yd||$8.99/yd||$8.99|
|Navy Fleece||100% polyester fleece||2 yds||$6.99/yd||$14.98|
|PPK6 Fleece (Solid)||80% poly, 20% cotton||2 yds||$6.74/yd||$13.48|
|Indigo fleece||100% polyester fleece||2 yds||$12.99/yd||$25.98|
|Thread||150 yds||$0.99/150 yds||$0.99|
Why not Gore-tex or other wind resistant materials?
After looking into the wind-resistant materials and talking to various technical engineers at MaldenMills Company, wind-resistant material was not used. This was because the material would have locked in too much moisture and heat and block outer air flow so much that the heating element would not be very effective. Further information, references, and links to fabric sites is located on the Fabric Background Information page.
Fabric for Phase Change Materials
We have used the same fabrics for the gloves with the phase change material. The same 100% polyester lining was used and pockets were sewn onto the lining where the PCM was inserted.
For our initial prototype we put a small piece of polydimethyl siloxane resin with octadecane microspheres incorporated into it. The phase change materials recrystallize when the gloves get cold, releasing their stored heat and warming the hand. The next step was to make a larger piece of this and test it out in the glove. Due to the difficulty of processing microspheres, crushed octadecane was put in PDMS for the final product.
Polyethylene glycol and octadecane are two common phase change materials that can have melting temperatures near the temperature of the human body (37 degrees Celcius). The exact melting temperature of the material depends on molecular weight, among other factors. Because they give off a significant amount of heat while freezing (they have a high heat of crystallization), phase change materials have recently begun to be incorporated into clothing. The basic idea is that the material melts while the individual is warm or exercising and then freezes when they get cold, releasing heat. Incorporating phase change materials into the glove heaters will allow the gloves to give off some heat without consuming power. Phase change materials are often incorporated into clothing by fabricating them into microspheres. Microspheres increase surface area, allowing heat to be given off more easily. These are then encapsulated to prevent leakage and sandwiched between two thin layers of fabric, close to the skin. Sometimes the microspheres are actually embedded into the fibers of the material while other times they are used in a coating. We tested a few different ways of incorporating the material into the fabric. The procedures and tests were performed and the results are shown on the results page.
Polyethylene glycol is low molecular weight polyethylene oxide- a crystalline, thermoplastic, hydrophilic polymer used for a wide variety of applications. We will be using polyethylene glycol with a molecular weight of 900g/mole manufactured by Fluka (Sigma-Aldrich), which forms a wax-like solid at room temperature. This molecular weight was selected because it has a melting temperature of 34.9 degrees Celcius. Using the DSC, the heat of crystallization for this material was found to be -150.8 J/g (DSC results). The density of PEG is 1.11 g/cm3. Because it is water-soluble the processing of PEG microspheres will have to be slightly altered. Another option is to try other methods of incorporating PEG into a fabric such as by coating it directly or embedding it in a polypropylene or polydimethyl siloxane base. PEG has been used as a phase change material for solar heat storage by incorporation into housepaints although the full effects of this have not been studied in detail (Lane, 1986).
PEG solvents (Bailey, 1976):
Safety Information (from MSDS):
Disposal: Contact a licensed professional waste disposal service to dispose of this material.
Clean-up: Sweep up, place in a bag, and hold for waste disposal.
Response to Accidental Exposure: In case of contact with skin, immediately wash skin with soap and copius amounts of water. In case of contact with eyes, flush with water for 15 minutes and call a physician. If inhaled, remove to fresh air. If swallowed, wash out mouth with water and call a physician.
The complete MSDS for this material can be found here.
Octadecane has the chemical formula C18H38 and a molecular weight of 254.5g/mole. It is a hydrophobic material and has a melting temperature of between 28.5 and 29.3 degrees Celcius. Because of this melting temperature, it is often used as a phase change material in clothing applications. Our octadecane is also manufacured by Fluka and also forms a waxy substance at room temperature. Using the DSC, the heat of crystallization for this material was found to be -283.5 J/g (DSC results), which is somewhat higher than that of PEG. Its density is 0.77 g/cm3. This makes it difficult to centrifuge because it floats to the top and it is hard to remove water from underneath without losing a large quantity of material.
Safety Information (from MSDS):
Disposal: Dissolve or mix the material with a combustible solvent and burn in a chemical incinerator equipped with an afterburner and scrubber. Observe all federal, state, and local environmental regulations.
Clean-up: Sweep up, place in a bag, and hold for waste disposal.
Response to Accidental Exposure: In case of contact with skin, flush with water. In case of contact with eyes, flush with water for 15 minutes and seek medical advice. If inhaled, remove to fresh air. If swallowed, wash out mouth with water and call a physician.
The complete MSDS for this material can be found here.
Previous information on Phase Change Materials: Phase Change Materials
PDMS (polydimethyl siloxane) resin
PDMS (a viscous liquid) is mixed with a cross linking polymer. It sets overnight into a flexible, rubbery material. This flexibility is one of the major reasons it was selected as a base material for the octadecane microspheres- it will still allow the hand to move easily in the glove. PDMS is also extremely hydrophobic. This is important because it will not be affected by the possible presence of water or even water vapor. (this was a possible problem with the PVA coating that we got from microsphere fabrication which is water soluble). Additionally, the melting temperature of PDMS is much higher than that of the human body so that the resin will not melt when the glove is heated up. One problem may be its relatively low thermal conductivity (~0.002), which could slow heat release from the microspheres to the hand.
Site to Silicone information: http://www.elkaysiliconesindia.com/abresbottom.htm
Polypropylene is a hydrophobic, crystalline polymer with very high solvent resistance that can be made into fibers or thin sheets. Because it is highly crystalline, it has a high melting temperature and has proven to be very difficult for us to process.
-Water absorption: 0.03%
-Thermal Conductivity: 0.1-0.22 W/m*K
-High solvent resistance
The key properties desired for an insulating material for the wire heating element included high electrical resistivity, high thermal conductivity, high Tm, low water absorption, good ductility, and good weather resistance.
Teflon is a highly crystalline polymer with the chemical formula -(CF2-CF2)- and meets all of the above properties. Because it would be extremely difficult for us to process, we have bought already manufactured teflon tubing the same diameter as our wires. One disadvantage of teflon is that it is relatively inflexible, however the tubing is thin enough that this was not a problem.
-Resistivity: >1018 ohm*cm
-Tensile strength: 21-34 MPa
-Tm: 327 C
-Thermal conductivity: 0.25 W/m*K
-Water Absorption: < 0.01%
The key properties needed for a good temperature switch are reaction time, sensitivity, and cost. Two types of temperature sensors were considered for the gloves: bimetallic and polymer switches. Bimetallic and polymer thermal switches offer precise switching temperatures and quick response times. They are more desirable than thermocouples, which need a reference temperature, and thermistors, which do not have a large enough increase in resistance. Both bimetallic and polymer thermal switches are small, lightweight, and come in a variety of shapes.
Polymer thermal switches are comprised of a crystalline polymer matrix containing conductive particles. Below a critical temperature, the polymer matrix is dense enough to allow the conducting particles to form a continuous network. Above the critical, or "switching", temperature, the polymer expands and breaks the conductive particle network, opening the circuit. Upon cooling, the polymer contracts and the circuit is restored. Typical applications include over-current protection for sensitive electrical systems.
The following diagram explains a bit about how a bimetallic temperature switch works:
There are a number of factors to consider when selecting a battery for any application. They include power consumption, energy density, capacity, lifetime, shape, weight, and cost. For the glove heaters the major parameters were that the battery be flat and relatively flexible and that it provides enough power to use the gloves for at least one day (6-8 hours) before having to be recharged. Rechargable batteries were selected because heating elements tend to consume a lot of power and disposable batteries would therefore have to be replaced every day.
The necessary power output of the battery was determined by the heat transfer calculation outlined here. Watch batteries give off much too little power for this type of application because generating heat requires a lot of power. Alkaline batteries were ruled out because they are not rechargeable. Liquid lithium ion batteries were ruled out because of their hard, inflexible casing. The best candidate for the glove heaters was a lithium polymer battery. Although even a lithium polymer battery will provide slightly less power than this calculation demands, they will still compensate for a significant amount of heat loss and therefore improve the overall comfort of the hand.
Lithium polymer batteries are rechargeable, flat, and relatively flexible. They can be manufactured in a wide variety of shapes and sizes. Because they are usually a higher voltage than alkaline batteries (3.6V compared to 1.5V), they can usually produce more Watt*hours, which is another benefit for the gloves. The energy density of lithium polymer is 200 Watt*hours/kg-three times that of nickel-cadmium batteries. Lithium polymer is also very lightweight, and the solid (or gelled) electrolyte eliminates the need for a metal casing. In addition, the batteries are very safe to use because there is much less danger of leakage. Lithium polymer batteries are constructed of a number of thin layers:
While the layers are often then rolled together as pictured here, the battery can also be kept flat as will be the case for our gloves. One potential problem is that they may be relatively difficult for us to obtain because they do not yet have a widespread market. Profesor Ceder obtained three lithium polymer batteries for us from Valence Technology Inc. There specifications are as follows:
Battery 1 (C43G):
Voltage: 3.76 V
Discharge Capacity: 0.754 Ah
Dimensions: 49mm x 35mm x 5.3mm
Battery 2 (C59D):
Voltage: 3.74 V
Discharge Capacity: 1.36 Ah
Dimensions: 88mm x 55mm x 3mm
Battery 3 (C65S):
Voltage: 3.74 V Discharge Capacity: 1.18 Ah
Dimensions: 66mm x 36mm x 5.5mm
There are three things that need to be considered from an engineer's standpoint with respect to the heating elements. The three things are:
The resistivity of the element is very important because the heat given off (power) will be determined by the equations:
Where i is the current, R is the resistance, r is the resistivity, is the length of the wire and A is the cross-sectional area. Since the current put through the wire will be a lot more difficult to change, r becomes an important factor. Also, having thin wires will help us increase the power.
Oxidation properties are important because oxidation causes defect motion changing the properties of the metal (specifically resistivity and ductility).
Ductility is important if the wire is kept within the elastic region of the stress-strain curve. If the wire is brought outside of this region, then fatigue will become a factor. It would be best in this situation to use a material that will be within the elastic region.
Nickel and iron alloys were the primary focus of our search. nickel alloys were looked at for their high resistivity properties, while iron alloys were looked at for their low cost.
Three wires were selected for comparison: stainless steel (70%Fe/19%Cr/11%Ni) with diameter = 0.404mm, 60%Ni/16%Cr/24%Fe wire with diameter = 0.38mm, and 80%Ni/20%Cr wire with diameter = 0.41mm. Because all of these wires have very high resistivities and the total resistance can be controlled by changing the length and cross-sectional area of the wire, resistivity was not a factor in our selection. Additionally, oxidation was not considered because the wire in the glove will be covered with an insulating material that will protect it from the atmosphere. Therefore the primary consideration in selecting the wire heating element was ductility. After conducting on each of the three wires, it was determined that the Ni-Cr alloy had the largest elastic region. Therefore, this wire was selected for the final glove design. References:
Bailey, F.E. Poly(ethylene) Oxide. Academic Press. New York: 1976.
Lane, George A. Solar Heat Storage: Latent Heat Material. CRC Press. Boca Raton: 1986.
PolySwitch" from Tyco Electronics
"Batteries." NASA. http://spacescience.nasa.gov/osstech/battery.htm
"Batteries in a Portable World." http://www.buchmann.ca/
Buchman, Isidor. "The Lithium-Polymer Battery: Substance or Hype?" http://www.powerpulse.net/powerpulse/archive/aa_080601a1.stm
Essex, David. "Solar Power, Batteries Included." Technology Review, August 17, 2001. http://www.technologyreview.com/articles/essex081701.asp
Interview, John VanderSande. February 27