G  Love: Heated Gloves*

1. Summary: The goal of this project was to create a heating system for a pair of gloves that maintain a comfortable temperature and wearability. The main components of the glove are the heating element, battery, insulation, thermal switch and fabric. The heating elements extensively studied were high-resistance metallic wires and phase change materials. The glove design below displays the aspects of materials selection when utilizing either form of heating. This design supplies 1.7W of power to each hand. The resistive heating element selected was Ni:Cr high resistance wire and the phase change material selected was octadecane embedded in polydimethyl siloxane resin. Lithium Polymer Battery • Flat shape - dimensions: 65 x 36 x 6 mm • Located at the back of the hand. • Voltage supplied: 3.74V • Rechargeable • 4.4 Watt*hours • Compliments of Valence Technologies. Bi-Metallic Thermal Switch Integrated into the electric circuit is a bimetallic strip, containing two metals of different thermal expansion coefficients layered together. As the temperature rises, one of the metals elongates more than the other, forcing the strip to bend away from the contact and opening the circuit. Cooling releases the tension in the strip, and the circuit is closed. • Switch Temperature = 32C1 • Switches off heating circuit when switch temperature is reached, preventing overheating. • Compliments of Portage Electric Products, Inc. • Other thermal switches considered: polymer thermal switch, thermistor. Final PCM Glove Comparison GLove: Heated Gloves* Kristin Brodie, Jeff Colton, Colin Galbraith, Bushra Makiya and Tiffany Santos Amount of heat supplied to the hand by the heating element was determined by calculating the amount of heat needed to keep the hand comfortable when walking across the Harvard Bridge in -10°C weather with 15 mi/hr wind. Ni:Cr High Resistance Wire • 80% Ni, 20% Cr • Diameter = 0.40mm • Resistance = 0.0807 /cm • 100 cm of wire used in design • Wire temperature reaches 60C • Selected for superior mechanical properties compared to other wires (largest elastic region, see graph below). • Other high-resistance wires considered: 60Ni:16Cr:24Fe and 70Fe:19Cr:11Ni. Outer Fabric • 100% polyester fleece selected for its good insulating properties, breathability, and high heat transfer coefficient. • Melting Temperature = 260C • Heat transfer coefficient = 1.49 W/m2*K • Good breathability: hand will sweat minimally inside glove. • Other fabrics considered: 80/20 and 20/80 cotton/polyester blends. Teflon Tubing Insulation • Poly(tetrafluoroethylene): -(CF2-CF2)- • High Melting Temperature = 327C • Thermal Conductivity = 0.25W/m*K • Necessary for electrical insulation of wires and protection from corrosion. Phase Change Materials (PCMs) • A PCM is melted by body heat and the latent heat of fusion is stored. When subjected to a cooler environment, the PCM crystallizes, releasing the stored heat and thus warming the hand. • It is necessary to encapsulate the PCM into a base material such as polypropylene or high density polyethylene in order to prevent leakage when the PCM melts. Because extrusion facilities were not available, uniformly dispersing a PCM in either one of these materials proved to be very difficult. Therefore less efficient encapsulants had to be used. • PCMs considered: Octadecane in Polydimethyl Siloxane (PDMS) Resin and Polyethylene Glycol (PEG) in Polyethylene. • Octadecane C18H38 • Molecular Weight = 254.5g/mole • Melting Temperature = 27.2˚C • Crystallization temperature = 16.5˚C • (Measured by Differential Scanning Calorimetry) • Total Heat Capacity: 5g  283.5J/g = 1148J • Embedded in PDMS Resin: • Thermal Conductivity = 0.002 W/m*K • Polyethylene Glycol (PEG) • Molecular Weight = 190-210g/mole • Melting Temperature = 26.6˚C • Crystallization temperature = 9.8˚C • (Measured by Differential Scanning Calorimetry) • Total Heat Capacity: 7g  150.8J/g = 1056J • Heat sealed into a polyethylene bag. • Thermal Conductivity = 0.33 W/m*K • -(CH2-CH2)- • The PCM embedded in base material was then sewn into pockets in the inner lining of the glove according to the pattern below. Inner Lining Fabric • 100% polyester • Selected for softness and comfort. • Other fabrics considered: flannel (100% cotton). Final Gloves (left to right): Inner lining with battery and resistive heating element, inner lining with embedded PCMs, and outer glove. Octadecane in PDMS PEG in Polyethylene A comparison of the two encapsulation methods is shown above. Although polyethylene proved to be a better encapsulant due to the low thermal conductivity of PDMS (%reduction in efficiency = 35% vs. 51%), the overall heat capacity of the octadecane embedded in PDMS resin was greater (93.1J/g vs. 61.2J/g). This was due to the larger weight fraction of PCM in that sample and its higher latent heat storage capacity. Therefore octadecane was used in the gloves. In the future, it would be interesting to experiment with heat sealing octadecane in polyethylene. This could further increase the glove’s efficiency. Comparison of PCM and Wire Heating Elements: 5 grams of octadecane will give off the same amount of heat upon freezing per cycle (~1.7 W) as 100cm of Ni:Cr wire in ~14 minutes. *3.082 Materials Processing Laboratory, Undergraduate Team Project