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What are Gap Fillers?

( 06/14/2022 ) Written by: Timothy Vokes

Battery performance is crucial to the continued advancement of electric vehicle (EV) design. One of the fundamental challenges in creating a battery pack is the effective management of heat generated during the battery’s charge and discharge cycles.

A basic principle of physics is that heat can be transferred in three ways: 

  • Radiation - the outward propagation of infrared waves
  • Convection - the movement of molecules in liquids or gases from hot areas to colder areas
  • Conduction – the transfer of energy via direct contact with an adjoining body

When comparing the efficacy of each method, it is helpful to think of cooking as an example. Clearly, cooking food using solar radiation would be ineffective (assuming no devices were used to concentrate the solar rays). Similarly, cooking with a blow dryer, which uses a motor to force fluid motion, would be impractical. However, by placing a pan directly on a stove burner and using conduction, heat is transferred very efficiently, and food quickly cooks. 

Managing Heat in Electric Vehicle Battery Packs

The same concept holds true for batteries: transferring heat is best accomplished using conduction. We see evidence of this in our cell phone batteries, which depend upon radiation for their heat to dissipate into the open air. Without this happening, the phones can become excessively hot and will shut down. Fans rely on convection to cool the central processing units (CPUs) in computers. But again, the effectiveness is limited, as we see when computers get too warm while doing the hard work of, say, streaming videos or playing games.

In the past, car manufacturers have used EV battery pack designs that employ radiant methods of heat dissipation. The outcome is poor temperature distribution that results in hot spots and EVs that are limited in the performance they can offer. This makes these EVs disadvantaged in the marketplace. Most of the latest generation of EVs and upcoming prototypes, however, are turning to conductive, liquid-cooled methods for thermal management.

For optimal conductivity, every cook knows that a pan’s bottom must maintain close contact with the stove burner, and again, it’s a similar story for electric vehicle batteries: the battery pack must be in close contact with a heat sink. To the naked eye, these surfaces within the battery assembly appear to be quite smooth and in perfect contact. At the microscopic level, however, both surfaces have peaks and valleys that create roughness, entrap air, and prevent ideal contact (see figure).Gap Filler - Air (orange) caught between two substrates (grey)

Superior Heat Transfer Using Thermal Management Materials

Battery manufacturers use thermal interface materials (TIMs) to displace the air and fill in the gaps between the two substrates. The two most used TIMs are cure-in-place, liquid-dispensed gap fillers, and pre-cured thermal pads.

Our CoolTherm™ liquid-dispensed gap fillers are made specifically for the automotive industry. Liquid dispensed gap fillers will flow into microscopic spaces, allowing for looser tolerance for both design and manufacturing. Decreasing tolerances by just one millimeter can add substantial cost to the battery pack’s metal housing. However, manufacturers can save money by relaxing tolerances for those metal-to-metal connections and achieve the same amount of heat transfer by using gap fillers in the extra space.

Multiple Chemistries Available

With a diversity of chemistries available, our portfolio of gap fillers comes in silicone and urethane. Both the silicone and urethane gap fillers combine the thermal, electric, and mechanical properties of potting materials and the rheological (or flow) properties of an adhesive to create the qualities of a liquid-dispensed gap filler. However, each chemistry offers its own unique benefits to the applied product.

Compared to other chemistries, silicone is especially adept at protecting fragile electronic components due to its flexibility. Also unique to silicone, it functions in a wide range of temperatures, from -75°C to +200°C. 

Our silicone gap fillers include SC-1200 and SC-1500. Both are thermally conductive gap fillers that are two-component systems designed to provide excellent thermal conductivity for electronic applications while retaining desirable properties associated with silicones. Each of these silicone gap fillers also provides numerous benefits, including:

  • Low Stress – exhibits low shrinkage and stress on components as it cures.
  • Durability – composed of an addition-curing polydimethyl siloxane will not depolymerize when heated in confined spaces.
  • Environmentally Resistant – provides excellent thermal shock resistance.

On the other hand, urethanes excel in conditions that are cold to moderately hot (< 130°). Offering low viscosity, they cure as a soft gel to a semi-rigid material to protect stress-sensitive electronic devices from vibration and shock. Another plus, urethanes provide a great moisture barrier when cured.

Looking at our urethane options, we offer CoolTherm® UR-2000, CoolTherm® UR-2000 FST, and CoolTherm® UR-2002, all featuring high thermal conductivity and low viscosity. Each of these gap fillers functions as a two-component thermally conductive urethane system designed to provide excellent thermal conductivity for electronic applications. They also cure at room temperature to produce a flame retardant, compliant material.

Benefits and features of UR-2000 include:

  • Thixotropic Viscosity – maintains low viscosity during dispensing, with minimal flow after dispensing.
  • UL Rated – provides excellent flame retardancy; meets requirements of UL 94 V-0 standards.
  • Room Temperature Cure – suitable for curing at room temperature; may be heat cured (120°C) to expedite cure.
  • Durable – provides excellent adhesion to powder-coated and e-coated aluminum surfaces; excellent resistance to peeling from PET film and aluminum-coated PET substrates.

Meanwhile, the benefits of UR-2002 include:  

  • Thixotropic Viscosity – maintains low viscosity during dispensing, with minimal flow after dispensing.
  • Flame Retardant – provides excellent flame retardancy.
  • Room Temperature Cure – suitable for curing at room temperature and may be mildly heat cured (65°C) to expedite cure.
  • Reworkable – material compliance allows for reworkability of cured parts.

To help you meet your EV battery needs, we have a team of expert application engineers and technical experts who will work with you every step of the way. Custom tailoring our products for your unique needs, we have solutions to help you save money while improving EV battery performance. Have any questions? Let us know how we can help!

Timothy Vokes

Tim is a Senior Applications Engineer with LORD whose focus is on dispense strategies, manufacturing and material application process support for customers in the Electrification and Electric Vehicle market.

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