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Temperature-Sensitive Packaging Insulation and Phase Change Options

The fast growing trends of globalized medicine and long distance research collaboration makes the

transportation of bio/pharma products an increasing concern and cost. In the past year, healthcare

companies’ concern about temperature related product loss has doubled, and less than half of the

companies believe they are successfully managing supply chain costs.1 As healthcare companies ship

temperature sensitive products across farther distances, they must heighten their consideration of payload

requirements and shipping options.

Opposing the effort to maintain temperature stability in the supply chain is the ever increasing costs

associated with transportation. Every option needed to maintain temperature increases the shipping cost.

Insulation adds to dimensional weighting and landfill waste, phase change material (PCM) adds to overall

weight and decreased payload space. Companies can easily end up with ridiculous packaging systems in

order to meet their payload transport needs. Think about how much volume in your typical package is

devoted to your payload vs. your insulation/PCM system.

Therefore, understanding commonly available insulation materials and PCMs is vital to effectively

addressing these concerns.

Insulation

The least expensive energy is the energy you never use

Insulation efficiency can be measured by thermal conductivity. Thermal conductivity measures the

ability of a material to transfer heat energy from one place to another. Its unit of measurement is Watts

per meter-Kelvin (W/mK), where Watts is heat energy transferred over a distance between pieces of

material (meter) at different temperatures (Kelvin). When less energy transfers across a material, the

result is increased temperature stability. Therefore, a lower thermal conductivity means better insulation.

Manufacturers alter and control properties affecting gaseous thermal conductivity, solid thermal

conductivity and radiative thermal conductivity in order to decrease a material’s overall thermal

conductivity. These properties can vary substantially even within specific material categories (i.e. 2 lb.

density polyurethane has a different thermal conductivity than 4 lb. density polyurethane).

1 “UPS 2011 PAIN IN THE (SUPPLY) CHAIN SURVEY”

http://pressroom.ups.com/pressroom/staticfiles/pdf/case_studies/2011_PITC_Executive_Summary_FINAL.pdf

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In regards to gaseous thermal conductivity, the transfer of heat through gas within the pore spaces of an

insulator, four characteristics should be considered. First, the presence of water within the material’s

pores will cause thermal conductivity to increase. Dry, still air, conducts at 0.025 W/mK, whereas water

vapor conducts at 0.6 W/mK and frozen water conducts at greater than 2 W/mK. Keeping water vapor

from saturating an insulator is thus crucial for low thermal conductivities. Second, large gas molecules

move more slowly and transfer heat more slowly. To illustrate, Hydrogen and Helium conduct at 0.15

W/mK, whereas CFC Freon-12 conducts at 0.01 W/mK. This is one of the reasons manufacturers used

CFC’s when producing poly foams in the past. Furthermore, since gas transfer occurs when two gas

molecules hit each other, less gas within the pores and smaller pores will both aid in lowering thermal

conductivity. No gaseous thermal conduction occurs when a gas molecule hits the inside of the insulation

material. Therefore the industry has experienced an increase in use of vacuum panels.

Solid thermal conductivity, which is the transfer of heat across a solid, is affected by material density

and structural shape. Less dense materials will have lower thermal conductivity. Conductance is also

inhibited in materials with “tortuous structures” and many “dead-ends”. Crystalline structures will

conduct heat more quickly than non-crystalline, amorphous or random structures Foam plastics, for

instance, have low solid conductivity (.02 W/mK), while aluminum has a relatively high solid

conductivity (250 W/mK). However, it is interesting to note that specific microscopic details can make

dramatic differences. For instance, diamonds can have thermal conductivities from 900-2000 W/mK.

Lastly, radiative thermal conductivity should also be reduced in an insulation material. An opaque

material will reduce thermal conductivity by preventing light energy from entering. In fact, reflecting

incident light will further lower thermal conductivity. The ability to absorb infrared light without re-

emitting it will also reduce thermal conductivity. In addition to preventing outside radiation from

entering the material, this ability will prevent heat energy from transferring internally via radiation. Some

manufacturers will introduce carbon black for example, to reduce the radiative transfer of heat internal to

an insulator.

Thermal conductivity is also not linear, especially in three dimensional volumes. As the thickness of your

insulator increases linearly, the surface area increases as the square. The overall heat input to a three

dimensional system is directly proportional to its surface area. For every doubling of insulation thickness

added to a container (outside), the heat transfer is increased by a factor that can be approximated by

48(dt+t2), where d is the interior dimension of your container assuming a cube and t is the thickness of

your insulation. It can be clearly seen that the heat input quickly swamps any gain made by increasing

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insulation thickness. Put another way, insulation closer to the interior space of your container provides

much more heat insulation protection than insulation on the outside. In addition, all insulation has a

maximum thickness beyond which it adds no value at all.

Thermal Conductivity of Common Insulators

EPS PU/ISO Fiber Batting VIP Thermal Conductivity

(W/mK) 0.07-0.05 0.03-0.02 0.07-0.05 0.007-0.003

Mechanical Notes - Rigid, impact resistant, and brittle - Flexible - Rigid

- Petroleum-based raw feed stock - Needs supporting structure - Requires the protection of

vacuum barrier

- PU/ISO more durable than EPS - May crush and lose insulating

efficiency

- Panels must fit tightly

- PU has problems with large

unpredictable void spaces

Relative Costs - Requires expensive molds unless

panels are used, which then introduces

seams

- May be least expensive per unit

volume

- Most expensive per unit

volume

- PU/ISO typically 2-3 times as

expensive as EPS

- Low performance may require

large container and heavy cooling

power, ultimately raising costs

- High performance may

make TOTAL package least

expensive option

Ease of Use - Simple box - Tricky box assembly - Simple box

- May require complex cooling power

pack-out

- Must take caution not to crush

material and cause gaps in

insulation

- Easy pack-out

Expanded Polystyrene (EPS) is produced from beads of polystyrene that expand and fuse together when

heated. EPS is an attractive insulation material because it is rigid, impact resistant, relatively inexpensive,

and box assembly is simple. However, EPS has a relatively high thermal conductivity. This is due

primarily to high gaseous thermal conductivity, but suboptimal characteristics in solid and radiative

thermal conductivity also reduce the effectiveness of the insulation.

EPS has a high gaseous thermal conductivity of ~0.6 W/mK, which accounts for the majority of thermal

conduction occurring through EPS. After blowing agents diffuse out, the gases left in the pores are air and

water vapor, which have high thermal conductivity. In addition, pore sizes are large enough to allow for

complete gas interaction among air and water molecules left in the pore.

The solid thermal conductivity for the base material, styrene, has a solid thermal conductivity of ~0.3

W/mK, which is typical of common of plastics. The polystyrene beads form regular geometries similar to

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crystalline structures. This type of structure allows for easy heat transfer along the structural path. The

density of EPS may range from 0.02 to 0.2 g/cc, though the most common density for EPS is 0.1 g/cc.

Although EPS is optically opaque, it does not absorb internal radiation. This allows for significant

radiative thermal conductivity. To counteract this, some manufacturers add carbon black to EPS to

prevent internal radiation. Overall, EPS, along with Fiber Batting, has a highest thermal conductivity of

0.07-0.05 W/mK.

From a thermal conductivity perspective, Polyurethane and Polyisocyanurate (PU/ISO), rigid foam

insulations, are produced in similar fashions. A complex mix of chemicals is combined under heat and

pressure in a mold. Reactions produce gasses that create pores and cause chemicals to form chains

(polymers). The process finally results in a rigid structure with many holes. PU/ISO is also attractive

because of its rigidity and impact resistance. When compared to EPS, PU/ISO may be two to three times

more expensive, but its overall thermal conductivity less than half. PU/ISO possesses a similar solid

thermal conductivity to EPS, with slight improvements in gaseous thermal and radiative thermal

conductivity.

Gaseous thermal conductivity is again the primary factor in heat transfer. Manufacturers have been forced

to switch blowing agents from CFCs (~ 0.01 W/mK) to gasses with higher conductivities (~0.1 W/mK or

worse), raising the gaseous thermal conductivity. The dominant gaseous heat transfer is due to molecules

larger and less conductive than air and water vapor. Since PU/ISO blowing agents tend to remain in pores

after production, diffusion is relatively slow. Once the blowing agents diffuse out, the remaining gases are

air and water vapor. Pore sizes are small enough to limit convection, but overall gaseous thermal

conductivity is still between 0.025 and 0.6 W/mK.

Affecting solid thermal conductivity,the PU/ISO pores form regular geometries similar to crystalline

structures, resulting in many thermal conductance paths. In addition, PU/ISO densities are in the range of

0.1 to 0.25 g/cc, commonly 0.2 g/cc. PU/ISO is the densest solid of the most common insulation

materials.

PU/ISO is optically opaque and does absorb some internal radiation, due to higher carbon content than

EPS. Ultimately, PU/ISO achieves a total thermal conductivity of 0.03-0.02 W/mK.

Fiber Batting Insulation is produced through extruding short fibers of glass, though mineral wool may

be used as well. Fiber batting is the cheapest of the common insulation materials. However, its drawbacks

include its flexible structure, tricky box integration, and high thermal conductivity. Due to its high

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thermal insulation, companies must compensate by increasing cooling power such as dry ice or phase

change material. This adds weight and volume to the containers, generating steep shipping costs. Its

thermal conductivity is 0.05-0.07 W/mK, which is the same as EPS.

Gaseous thermal conductivity is again the dominant factor in heat transfer, because the pore sizes of Fiber

Batting are large enough to allow for complete gas interaction. The resultant gaseous thermal conductivity

ranges from 0.025 and 0.6 W/mK.

The conductance of glass is generally ~ 1 W/mK, which is high compared to the other insulating base

materials. However, Fiber Batting densities are low (0.01 to 0.1 g/cc) and the short fibers create a tortuous

path and dead-ends, significantly inhibiting solid thermal conductivity.

Since Fiber Batting is not naturally opaque, manufacturers must add opacifiers to reduce radiative heat

transfer. Furthermore, internal radiation occurs within the Fiber. Fiber Batting is the cheapest material to

produce as an insulating material, but has a relatively high thermal conductivity.

Vacuum Insulation Panels (VIPs) are produced by encapsulating a rigid core material within a gas

barrier and evacuating the porous core to reduce or eliminate gaseous thermal conductivity. Large

differences in core material choices exist, from rigid monolithic foams to compressed powders. VIPs

present an appealing option because they are rigid, have simple box assemblies, and possess the lowest

thermal conductivity – ten times lower than EPS or fiber batting. VIPs are also the most expensive

insulator per unit volume, though high performance may reduce total package costs to lower than those of

other insulators.

VIPs remove all gasses from the core. As a result, the gaseous thermal conductivity is negligible. (Typical

range is from 0.001 to 0.01 W/mK, depending upon the core porosities and effectiveness of vacuum

barriers).

Since many core options are available, the solid thermal conductance of cores may range from 0.001 to

0.3 W/mK. Bulk densities are vary from 0.15 to 0.25 g/cc.

Manufacturers use opacifiers to prevent heat transfer through radiative thermal conductivity. Small pore

size inhibits internal radiation. In the end, VIPs have the lowest thermal conductivity of all the common

insulators. Its thermal conductivity is 0.007-0.003 W/mK, an order of magnitude lower than other

insulation options.

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Lastly, never underestimate the heat loss at the edges or seams of your container. Just as in a glass of

water with a hole in the bottom, the effectiveness of your insulation is only as good as the weakest or least

effective heat loss points. Typically this will be at the lid interface but sometimes it is around the entire

container along every edge. So an insulation’s thermal conductivity is only part of the story, always

consider the entire container as assembled and its effective overall thermal conductivity.

Phase Change Materials

Phase change materials (PCM) describe any substance that undergoes a transformation from one “phase”

of matter to another; solid to liquid (melting/freezing), liquid to gas (evaporation/condensation) or solid to

gas (sublimation/vapor deposition) in either direction. Energy is required to make these transitions, either

energy input (absorbed) or energy given off (released), and this quantity of energy is referred to as “latent

heat” of transformation. It is important to note that this energy is transferred without a corresponding

change in temperature so that as a PCM is changing phase, it is “at” its phase change temperature and not

above or below it.

Changing phases also provides a much higher amount of energy absorbtion/release than letting a material

heat up or cool down without a change of phase (heat capacity). In general for example, water has a

latent heat of 334 J/g when it melts whereas it only can absorb about 4 J/g for every degree Celsius of

temperature change. This is why it is more desirable to use frozen water to keep a payload cool than

liquid water; you would have to use more than 80 times the amount of liquid water to equal the energy

absorption of ice. And changing phase from liquid water to gaseous water (steam) or boiling absorbs or

releases over 2200 J/g of energy! No wonder steam drove the industrial revolution.

Latent Heat of Common PCM

Latent Heat (J/g) Temperature (ºC)

Water/Organic Modifiers (ca. 100)-160 Control Room Temperature (abt. 22 )

Water/Organic Modifiers (ca. 100)-200 2-8

Water 330 0

Water/Salts 290 -10

Water/Salts 230 -22

Dry Ice 570 -78

Source for Water/Salts and Water/Organic Modifiers values: PCM Products, http://pcmproducts.net/Phase-Change-Material-Solutions.htm

Source for data in brackets: Verbal conversations with manufacturers and DSC data from Cornell University

Source for Water and Dry Ice: CRC Handbook of Chemistry and Physics

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What can easily be seen from the above table is that as soon as water is adulterated in the slightest it loses

its ability to absorb/release energy. So while it is sometimes necessary to stay away from the transition

temperatures of pure substances (water, dry ice, liquid nitrogen) as is the case for refrigerated payloads,

doing so comes at a very high cost – additional weight and volume associated with more PCM. It is

therefore very useful to consider the option of allowing a payload to “see” the temperature of a pure

substance PCM. While it may be unfavorable for a payload to “freeze”, since a PCM changes phase at its

phase change temperature and not above/below it, the payload may not freeze if it is in close proximity to

a PCM. In addition, many payloads have their own mass which may be significant enough to not freeze.

Remember that a payload will require an equivalent amount of energy to change phase (freeze) as well as

the PCM.

Lastly, it should be noted that the surface area of a PCM needs to be considered in all pack-outs. There

are many instances of packages arriving at their destination with the PCM still intact or partially frozen at

the same time as the payload temperature being out of range. Just because there is dry ice or partially

frozen PCM left in a package does not mean that the payload has maintained its desired temperature. All

containers have temperature gradients (differences in temperature from one space to another) and the heat

energy entering the container may not be absorbed by the PCM. In order for heat energy to be fully

absorbed by the PCM, enough of the PCM’s surface area must be exposed. If the PCM is a solid piece of

material, a ball or cube for example, with minimal surface area then it may not absorb all of the heat

energy entering the container. If the PCM is wet ice and it is melting, the center of a large gel-pack may

be frozen and insulated from absorbing heat energy by liquid surrounding it within the gel-pack. And

since the heat energy entering a container won’t wait around for the PCM to become available for energy

absorption, guess what is heating up (cooling down)? The payload…

An example: Some shippers will determine that their payload is at dry ice temperatures if they see any

dry ice left in their container. But imagine an extreme case: Place a piece of dry ice on a table and watch

it sublimate. What is the temperature just beside it? Room temperature. It will only sublimate at a rate,

or as fast, as its surface area will allow. If it is crushed up, it will sublimate much more quickly. Does

this mean that it has absorbed less heat energy? No, it has just absorbed that same amount of heat energy

more quickly. If this dry ice were inside your container, you would want it to sublimate as quickly as the

heat energy entering your container. Otherwise your payload will be absorbing it, and heating up.

The lesson is to match your PCM requirements to that of the effective insulation of your container.

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Conclusion

In order to develop the most thermally efficient packaging and shipping solution, it is imperative to know

and customize systems to a product’s specific requirements. Choosing the most efficient insulation and

PCM combination for a product is crucial for long-term savings as a result of extended shipping windows

and decreases in weight and size. However, values for thermal conductivity and its inverse, the R-Value,

as well as values for latent heat, provide only a starting point for understanding the effectiveness of an

insulation material. Ultimately, overall package design could negate even the best insulation and PCM

combination. When developing the optimal temperature sensitive shipping solution, companies must test

and measure real results of their own system under real shipping conditions. Taking the time to test,

improve, and develop shipping solutions will reduce shipping failures and costs, as well as extend

shipping windows. As the healthcare industry expands globally, these goals are increasingly imperative

to achieve.

About the Author

Robert Mendenhall, VP Business Development of American Aerogel. Robert and

three colleagues from Wesleyan University started working on aerogels in 1995 and

officially founded the company in 1999. Robert currently leads our business

development efforts and has worked on improving production techniques, enhancing

product qualities, and developing new applications. Prior to American Aerogel, Robert

held a fellowship in the Department of Chemistry at Wesleyan University and worked in the defense

industry at Physics International. He is the author and co-author of several patents, scientific peer-

reviewed papers and publications on a wide range of disciplines (packaging, thermal insulation, aerogels,

and instrumentation).