Pressurized Gas Foaming of Thermoin Tube “SN-UGA”
by Kojiro Inamori * and Masahiro Okadome *2

Thermoin tube SN-UGA is a copper tube covered with polymer foam that has
been developed as a part of air conditioning systems for large datacenters. As amain cover material, polypropylene (PP) was used because it shows superior heat resistanceand high thermal insulation. In the past, foaming of the PP covering of SN-UGA has been carriedout by a chemical method using azodicarbonamide (ADCA) as a foaming agent. But this method
has a disadvantage in that the ammonia generated as a by-product of ADCA decomposition
causes tarnishing of the copper tube. In this paper, we have investigated PP foaming with CO2.
CO2 has the effect of decreasing the melt viscosity and promoting coalescence of cells, but thiswas successfully overcome by using a tandem extrusion system and adding rubber component
with the PP. As a result, 5-fold expansion of PP foam was obtained.

ABSTRACT1.

INTRODUCTION

In the information-oriented society of today, the importance of electronic devices including integrated circuits israpidly increasing, and they are required to operate on alarge scale and at high speed. As a consequence, in large
datacenters, electronic devices such as computers are
generating more heat than ever, making it difficult to coolthe devices sufficiently using existing air conditioners.
Thermoin tube SN-UGA is a copper tube covered withpolymer foam that has been developed as a part of air
conditioning systems for large datacenters. Figure 1
shows a cross-section of SN-UGA and Figure 2 shows arooftop installation. Since SN-UGA is supposed to be
used for cooling very important computers, it has beendeveloped with special concern for security and fire prevention. For example, to increase heat resistance anddecrease heat conductivity, polypropylene (PP) foam isused as a raw material for the foam, and highly flame
retardant polyolefin with non-halogen material is used as
the raw material for the sheath. Tables 1 and 2 show theAnnealed copper tube
(JIS H3300 C1220T)
Thermal insulating material(Foamed polypropylene)

flame retardance and heat resistance properties of SNUGA. In this paper, we discuss the issues of existing SNUGA and the way to increase the expansion ratio of thefoam.

Figure 2
Table 1

Rooftop installation of thermoin tube SN-UGA.
Flame retardance properties of SN-UGA.

Oxygen index (JIS K7201)
Product name

Oxygen index

Covering material of SN-UGA made of flame retardant polyolefin

42

Covering material of existing product30

Thermal insulating material made of highly expanded polyethylene (for reference)

18

Incline burning test (JIS C3005)
Covering material(Flame-retardant EVA)

Figure 1

Cross-section of thermoin tube SN-UGA.

* Ecology & Energy Lab., R&D Div.
*2 Production Engineering Dept., Osaka Works, Metals Co.

Product name
SN-UGA4
Existing product
Reference Tube covered withhighly expanded
polyethylene

Burning time Burned length Burned(sec)(mm)
droppings
055

Exist

260

Exist—300(Entire length)

Exist

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Pressurized Gas Foaming of Thermoin Tube “SN-UGA”
2.0

Heat resistance properties of SN-UGA.
Shrinkage ratio of thermal
insulating material (%)

Shrinkage ratio of covering
material (%)

120°C, 24 h00130°C, 24 h00* The shrinkages were measured after annealing at 120°C and 130°C
for 24 hrs.

2BACKGROUND

2.1

Foaming Method

Amount of generated gas (mol)

1.0

N20.8

CO0.60.4
NH30.2
CO2200220240260280

Temperature (°C)

Figure 3

1.2

0.8

0.4

2.1.1 Chemical Foaming
In the past, SN-UGA has been produced by a chemical
foaming method using azodicarbonamide (ADCA) as afoaming agent. In this method, ADCA is thermally decomposed in an extruder, generating nitrogen gas to foam thepolymer. This method has the advantage that there is noneed to attach expensive devices to inject gas into theextruder, but it has a disadvantage in that the ammonia
generated as a by-product of ADCA decomposition
causes tarnishing of copper tube. (See Figure 3)

0180

1.6

Solubility (g-gas/g-polym.)

Heating
condition

0Figure 4

2.1.3 Foaming Agents for Physical Foaming
In the 1980s, chlorofluorocarbons (CFCs) were used as
the foaming agent for physical foaming because they
offer remarkably high performance, such as dissolving
into polymer in large amounts, decreasing in temperaturewhen they vaporize, and so on. But as CFCs came to beeliminated because of their effect of depleting the ozone
layer, their replacement with CFC substitute or hydrocarbons such as isobutane has been accelerated. Recently,replacement with non-toxic gases such as carbon dioxide
or nitrogen has been moving forward.

12

34

Comparison of gas solubility in polystyrene at 373 K.

2.1.4 Carbon Dioxide as a Foaming AgentCarbon dioxide (CO2) is expected to be very useful forphysical foaming because it is non-toxic, non-flammable,
low in critical temperature, easy to get, and low in cost.
But as seen in Figure 4, CO2 has significantly lower solubility than CFCs. This means that it is difficult to obtain ahigh expansion ratio with CO 2, and special measures
must be taken to increase it.
Compared with nitrogen (N2), the solubility of CO2 ishigher (Figure 5). This means that it is easier to obtain ahigh expansion ratio with CO2 than with N2. On the other
hand, N2 has the advantage that it produces finer cells
than CO2. Nowadays, N2 is used mainly for injection foammolding, while CO2 is used for extrusion foaming. In thispaper, CO2 is used to increase the expansion ratio as
much as possible, and to avoid tarnishing of the coppertube.

Changes in gas composition of ADCA.

2.1.2 Physical Foaming
There is another foaming method called physical foaming, in which some kind of gas is injected directly into theextruder instead of being generated by thermal decomposition. This method requires special devices such as ahigh pressure pump and gas injecting port, but has theadvantage that no harmful by-products are generated.
There are many gases that can be used for physicalfoaming.

0Pressure (MPa)3Solubility (g-gas/g-polym.)

Table 22100Figure 5

510
Pressure (MPa)1520

Solubility of nitrogen and carbon dioxide in polystyrene melt.

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3.

POLYPROPYLENE FOAM MADE WITH
CO2

3.1 Improving Foaming Characteristics of PP
When CO2 is dissolved into a polymer, it has an interesting effect to it: as CO2 content increases, the viscosity ofthe polymer decreases due to its plasticizing effect. This
characteristic is useful in injection molding because it canfacilitate the extrusion of hard-to-extrude polymers. In
extrusion foaming, however, this is not always an advantage because lowered viscosity promotes coalescence ofcells, resulting in ugly large cells in the foam. Furthermore,
too large an amount of CO2 accelerates the dispersion of
CO2 to the air, resulting in lower expansion ratios.
One of the ways to overcome these problems is todecrease the melt temperature uniformly, because a lower
melt temperature results in higher melt strength, which
avoids cell coalescence and the dispersion of the gas tothe air. In this research, a tandem extrusion system isused to decrease the melt temperature. A schematic viewof the tandem system is seen in Figure 7. The first-stage
extruder has the role of completely mixing the gas withthe polymer, and the second-stage extruder has the roleof decreasing melt temperature. To suppress heat dissipation, the screw rotation rate of the second-stage extruder should be low. Blowing air or water just after the die
10-9

PolypropyleneCarbon dioxide
gas injection port

First-stage extruder

Flow direction of polymer

Second-stage extruder

Figure 7

Coppertube

Schematic of tandem extrusion system.

outlet were also considered as alternative methods ofcooling the melt, but these methods affected only the surface part of the melt.
3.2 Addition of Rubber Component
A rubber component was added to PP to provide threeadvantages, as described below.
3.2.1 Widening Process Window
As mentioned before, the viscosity of PP changes dramatically around the melting temperature, so that the process window for PP foaming is very narrow.
The process window can be widened by adding a rubber component to the PP because it makes the viscosity
curve of PP more gradual. Figure 8 shows the viscosity
curves of PP before and after the adding of the rubbercomponent.

After adding
rubber component

Viscosity

2.2 Foaming of Polypropylene
Since polypropylene (PP) has a higher melting temperature than other general-purpose polymers like polyethylene or polystyrene, it shows greater thermal stability andmechanical strength. These advantages have attracted
attention, especially in the field of food packings. But it
has been difficult to increase the expansion ratio of PP
because its viscosity changes dramatically around themelting temperature. In other words, the process window
for PP foaming is very narrow. In this paper, an effort has
been made to widen the process window by adding
another polymer with PP.

Diffusion coefficient (m2/s)

Before adding
rubber component
10-10

Tm

Temperature (°C)

Figure 8

CO2 + PVAc
313.15 K

10-11

10-12
0Figure 60.1

0.2
0.3
Solubility (g-gas/g-polym.)

0.4

Diffusivity of carbon dioxide in polyvinyl acetate.

Dependence of viscosity on temperature before andafter adding rubber component.

3.2.2 Lowering of Flexural Modulus
It is desirable for thermally insulated copper tube to havea low flexural modulus to facilitate handling in construction. PP has excellent properties in terms of thermal resistance, but it has a high flexural modulus as well. Adding arubber component serves not only to widen the process
window, but also to lower the flexural modulus. The flexural modulus before and after adding rubber component isshown in Table 3.

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Pressurized Gas Foaming of Thermoin Tube “SN-UGA”

Table 3

Flexural modulus of polypropylene before and after
adding rubber component.

Conditions

Before adding (MPa)After adding (MPa)

0.1 MPa, 25°C

1200

650

These values were measured in unfoamed conditions.

3.2.3 Avoiding Large Cells
When PP is foamed by itself, non-uniform large cells
sometimes emerge in the foam. The reason is that themelt tension of PP is too low to keep the cells closed.
Adding a rubber component also has the effect of suppressing cell coalescence. Figure 9 compares the crosssection of foams before and after adding rubber component, and demonstrates that the number of large cells
decreases by the adding of a rubber component. Table 1
shows the properties of the SN-UGA expanded with CO2
obtained as a result of this work.

5.

CONCLUSIONS

Currently, the expansion ratio of foam for SN-UGA thathas been expanded with CO2 is not more than 5 times,
but a higher ratio is desirable because it contributes tolight weight, low material cost and improved thermal insulation properties. The simplest concept for increasing theexpansion ratio is simultaneously to satisfy two demands− to supply more gas and to suppress gas dispersion tothe air. Supplying large amount of gas can be achievedby increasing gas pressure, but suppressing gas dispersion is not easy. One of the ways to keep the gas inside
the foam is to cool the foam surface just after extrusionfrom the die, but this sometimes results in large cells andconsequently a rough surface. This shows that simply
lowering the temperature is not enough to assure a good
product. Therefore much is expected of high melt strength
PP (HMS) polypropylene, which offers high melt strength
even at high temperatures.

REFERENCES

Before adding

Figure 9Table 4

Properties of SN-UGA expanded with CO2.
Property

Expansion ratio

4.

After adding

Comparison of foamed PP before and after adding
rubber component.

1) E. Mitsui: Polymer Digest, 8 (1994), 42. (in Japanese)
2) Sangyo-chosakai: Kinzoku-zairyo-katsuyoh-jiten, (2000), 430. (inJapanese)
3) Y. Sato: Seikei-Kakou, 13, 2 (2001), 71. (in Japanese)4) M. Sugimoto, Y. Shimomura: Seikei-Kakou, 13, 2 (2001), 76. (inJapanese)

Performance
5-foldUpper temperature limit

130°C

Thermal conductivity

0.058 W/(m·K)

APPLICATION TO OTHER POLYMERSThe foaming method described above was also applied
to the production of polyethylene (PE) foam. Since PE isa polymer easier to highly expand than PP, it was problem-free to extend the application of this method to PE.
The properties of foamed PE are shown in Table 5. Since
PE has less thermal stability than PP, it should be used forinsulated copper tubes for general-water supply and hotwater supply systems.
Table 5

Properties of foamed polyethylene expanded with
CO2.
Property

Expansion ratio

Performance
5-foldUpper temperature limit

90°C

Thermal conductivity

0.058 W/(m·K)

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