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How to Choose and Use Thermal Gap Fillers

( 11/07/2019 ) Written by: Dan Barber

Battery pack design isn't standard, so why should your gap filler be?

With the rapid growth of electric vehicle (EV) platforms, there are many variations of battery pack design. Liquid-cooled heat sinks are common, but the configuration can vary from single cooling plates to multiple cooling plates or even cooling ribbons, depending on the manufacturer and the vehicle footprint into which the battery packs must fit. Within the battery packs, the cells themselves can be either cylindrical, prismatic, or pouch type. Regardless of the design, efficient cooling is vital for all battery packs to extend battery life and vehicle range and to enable fast charging of depleted packs. Thus, manufacturers need a thermal management solution that is adaptable to their ever-changing requirements.

A common ingredient in all battery packs is the thermal interface material (TIM), used to make a thermal connection from the cells and/or modules to the heat sink. Increasingly manufacturers are choosing liquid-dispensed gap fillers over the more traditional thermal gap pads. We have published a number of studies discussing the technical and manufacturing advantages of gap fillers, which are dispensed as two-part liquids that, upon mixing, cure over time to a solid material. The advantages include superior thermal transfer due to their lower interfacial thermal impedance and their ability to conform to micro- and macroscopic surface irregularities, manufacturing flexibility and speed due to robotic meter-mix-dispense (MMD) application, and lower overall cost.

Because battery pack design and construction are not standardized, your requirements of a gap filler may also be unique. Choosing or developing the best gap filler to meet all of your needs requires consideration of a number of aspects, as shown in the figure below.

Performance

Performance of the finished product is usually the initial consideration.  The required level of thermal conductivity for the application, whether the material will be exposed to chemicals and/or moisture during use, and the need to supplement electrical insulation, mitigate vibration effects, and/or provide structural integrity all influence the properties required of the cured gap filler. These cured properties, especially material hardness and adhesive strength, will also affect the ability to repair, rework, and recycle the battery modules either during manufacture or at the end of life.

Assembly

Assembly of the battery packs places additional requirements on the uncured material, depending on the process equipment used and the required production throughput and resulting takt time. The same gap filler formulation must supply both the uncured and the cured properties, so the choice of raw materials and process are essential to meet these requirements. Local sourcing of raw materials and localized manufacture are essential to providing a reliable supply chain and a competitive price for the finished material.

Choosing an optimal gap filler can be a challenging task, so utilizing a materials expert to assist is a recommended approach. Our Application Engineers are trained in both the chemistry and the application of our products, including our CoolTherm® gap fillers, and their job is to understand customer requirements and applications in sufficient detail to permit proper choice of material. Below we describe some of the key properties of the cured and uncured materials that an Application Engineer considers when choosing a material that will best fit customer performance and manufacturing needs.

 

Gap Filler Chemistry Options

We specialize in a number of polymer chemistry types to match best our customers’ requirements. The two most relevant types for gap fillers are silicone and urethane polymers.

Silicones are generally mechanically soft materials when cured so that they exert low stress on delicate components. They have low adhesive strength, which means that some type of mechanical fastening will be needed to hold components in place; it also means that they are quite easy to rework, repair, and recycle. Their moisture resistance is good but their resistance to chemicals is poor. A number of LORD silicone-based gap fillers are also available with very low volatile content, which means they can be used without concern about other coating or painting processes.

Polyurethane gap fillers have better adhesion and chemical resistance than silicones. They can also be relatively soft and reworkable, although their higher adhesion may create challenges. Their high temperature limit (about 130°C) is not as good as silicones (above 200°C), but given that battery cells are generally limited to about 60°C the high-temperature limit is not a concern for either material. The main criteria for choosing between silicone and polyurethane gap fillers will therefore be the customer requirements for chemical exposure, adhesion, and reworkability.

Uncured Material Properties

Liquid-dispense gap fillers are generally two-part materials, called resin and hardener, that begin to cure (solidify) upon mixing. The materials are usually applied using automated meter, mix, dispense (MMD) equipment, in which the resin and hardener are dispensed by pressure at the correct volume ratio and blended through a static mix tip. The material can be dispensed manually using cartridges or robotically from pails or drums, and a wide variety of dispense patterns can be programmed based upon customer needs.

The flow properties of the gap filler materials (resin, hardener, and mixed) are critical for obtaining good dispense performance. In most cases, customers desire that the material flow easily during dispensing to maximize process speed but that the resulting bead maintains its shape after dispensing. The high-shear viscosity of the material is the main property that determines dispense speed and required pressure, as well as the size of the bead that can be obtained. To obtain good mixing of the resin and hardener in the static mixer, it is also best to have similar viscosity for the two materials before mixing.

Uncured silicone flow properties are not strongly temperature-dependent, so control is dependent entirely on formulation and equipment parameters, such as dispense pressures, transfer hose diameter and length, and static mixer size and configuration.  Uncured polyurethane materials have temperature-dependent flow properties, becoming significantly lower in viscosity at mildly elevated temperatures, which gives an additional measure of process control during application. Because dispense quality is dependent on both material and equipment parameters, it is highly recommended that the material supplier and MMD equipment supplier work together to optimize the dispense system.

Once the bead is dispensed, other flow properties control its ability to keep its shape. Low-shear viscosity, or yield stress, is the material’s tendency to flow under gravity, so a high value is better for minimizing sag. Also, the time required for the material to regain its yield stress after dispense is measured by its thixotropy. A material that takes too long to regain its yield stress may lose its bead shape, while a material with too short a time may be difficult to pump and/or dispense. Finally, the elasticity of the material can affect the shape and release of the material from the dispense head. A highly elastic material will be stringy (like honey or molasses) and may lead to problems obtaining a clean bead, whereas a material with low elasticity will release cleanly (like ketchup or toothpaste).

Yield stress and thixotropy also affect the settling behavior of the material, which can affect storage life; gap fillers are highly filled with thermally conductive particles, so a higher yield stress and shorter time to recover yield stress will minimize settling and help extend storage life. If properties are not optimal, it may also be necessary to recirculate material through the MMD lines to prevent settling of the filler particles.

Because the resin and hardener begin to polymerize immediately upon mixing, the rate of polymerization will also affect the assembly process. Upon mixing, the viscosity will immediately begin to increase; the working time is the time required for the material to double in viscosity, and it will affect the speed at which material must be dispensed and the idle time that is allowed for the equipment between parts. If the idle time is too long, it will be necessary to change the static mixer tips to allow for continued dispensing. The gel time is the time at which the material has polymerized to such an extent that it will no longer flow, although it has not fully cured and will not have much mechanical strength. A general rule of thumb is that handling strength of the parts is achieved after about twice the gel time. The desired gel time will vary by customer depending on how long is required before the part moves to the next process and whether any rework during manufacturing is desired.

 

Rheological Property

Effect on Customer Application

High-shear viscosity

  • Dispense speed
  • Dispense pressure
  • Size of MMD dispense lines
  • Size of dispensed bead
  • Adequate mixing of Resin & Hardener

Low-shear viscosity

(~yield stress)

  • Bead shape
  • Settling
  • Storage life

Thixotropy

(time to recover yield stress)

  • Bead shape
  • Settling (in storage and in MMD lines)
  • Storage life
  • Need to recirculate MMD lines
  • Adequate mixing of Resin & Hardener

Elasticity (i.e., is it “stringy”?)

  • Bead shape and clean release of bead

Working time after mixing

(2x viscosity)

  • Time between mix tip changes
  • Dispense speed
  • Bead shape

Gel time (time to no flow)

  • Time between mix tip changes
  • Handling strength of parts

 

Cured Material Properties

The ultimate properties of the material after curing are responsible for the material’s performance in use and include mechanical, thermal, and electrical properties. Mechanical properties such as hardness, tensile strength and elongation, and adhesion affect the strength of the material and its ability to withstand or mitigate vibration and thermal cycling. Materials with high hardness may provide greater overall mechanical strength, but they tend to be more brittle and can create more mechanical stress during thermal cycling or shock as compared to a softer material. Softer materials tend to have lower tensile strength and modulus and greater elongation, so that they exert lower mechanical stress on parts during thermal cycling but are less effective as adhesives.   

The modulus and hardness of a material is also highly dependent on its glass transition temperature, or Tg, which is the temperature at which a polymer undergoes a transition from a more rigid, brittle state (below Tg) to a more flexible, elastic state (above Tg). For most gap fillers a softer, more flexible material is preferred, so they usually have a Tg near the lowest expected in-use temperature so that the material is always in its softer, low-modulus state.

Adhesion is a critical property for gap fillers even if they are not being used as structural adhesives, because maintaining adhesion to the battery module and the cooling plate is essential to maximize thermal transfer. Adhesion is highly dependent on the substrates being connected, as well, so relevant testing (lap shear, peel test, and others) using customer-supplied substrates is recommended during evaluation and development of gap fillers.

Mechanical properties will also affect the ability to repair and/or recycle components. Harder materials with greater adhesion may make it difficult to disassemble parts without damaging them or to remove traces of material from the individual parts. It is therefore necessary to consider potential rework or repair concerns early in the design of the battery pack so that the mechanical structure and the gap filler material properties can be optimized together.

In addition to mechanical properties, cured gap fillers must also meet the design requirements for thermal and electrical properties. Thermal conductivity is a prime concern, as it will affect charge time (especially fast-charge capability), discharge time (for power & acceleration), and battery lifetime. Most customers require thermal conductivity of at least 2 W/m∙K, with some customers needing up to 4 W/m∙K. Materials are generally expected to be electrically insulating, with a high volume resistivity, and to have a high breakdown voltage to withstand potentially high voltages. Materials are also expected to be highly flame resistant, generally with a flame resistance rating according to UL-94 of V0.

 

Cured Material Property

Effect on Customer Product

Hardness

  • Stress on components
  • Recycling and reworkability
  • Mechanical strength
  • Shock & vibration performance

Tensile Strength/Elongation/

Young’s Modulus

  • Thermal cycling performance
  • Mechanical strength
  • Shock & vibration performance
  • Adhesion

Storage Modulus

  • Vibration transmission or damping
  • Mechanical strength

Glass Transition Temperature, Tg

  • Thermal cycling performance

Adhesive Strength

  • Thermal transfer
  • Shock & vibration performance
  • Recycling and reworkability
  • Need for mechanical fastening

 

Cured Material Property

Effect on Customer Product

Thermal Conductivity

  • Charge time (e.g., fast-charge)
  • Discharge time (e.g., power & acceleration)
  • Battery lifetime
  • Cooling system requirements

Electrical resistance (volume resistivity, breakdown voltage)

  • Allowable separation between cooling plate & batteries
  • Need for additional insulation materials

Flammability

  • Vehicle safety

Application/Dispensing

Once you've decided what you need… then we go into applying it. Material can be applied using handheld cartridges or automatic meter, mix, dispense equipment. MMD equipment is used to dispense gap fillers and/or adhesives from bulk containers. Due to the high volume/high flow rate applications that are commonly associated with LORD gap filler materials, automated MMD processes are used to minimize material waste and cycle times and reduce overall cost.

In Conclusion…

Developing and choosing the optimal gap filler for a particular battery design is complex. Balancing the required mechanical, thermal, and electrical properties is essential, and the formulation choices made to meet these cured properties will also affect the uncured properties and ability to dispense material during assembly. For example, increasing filler loading to get higher thermal conductivity will affect both flow properties and mechanical & electrical properties.) For these reasons, we strongly recommend engaging early with our Application Engineers and product development specialists, as well as engaging with vendors who will provide the MMD equipment. By innovating together, an optimal thermal management solution can be developed.

 

ABOUT THE AUTHOR MORE BY THIS AUTHOR
Dan Barber

Dan Barber received the B.S. degree in chemistry from Furman University and the Ph.D. degree in inorganic chemistry from the University of Virginia. Following postdoctoral work at Yale University, he taught chemistry at Oberlin College and Lafayette College. He began his industrial career in 2000 at KEMET Electronics working on ceramic capacitor technology development. He joined LORD Corporation in 2005 and is currently a Staff Scientist in Electronic Materials Technology. Dan has lead motor potting initiatives at LORD for 5 years and most recently is working on new product development of thermally conductive potting materials and gap fillers for motor and battery EV applications.

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