New Products Developed Using Environment-Friendly
Elastomer Rubber
by Toshio Miyahara *, Kouichi Tanabe *, Mitsuo Shimazaki *,Takuya Tanaka *2 and Takao Kobayashi *3

Elastomer rubber products are widely used in industry, making use of their manysuperior properties including flexibility, resilience, elasticity, and absorption of
impact and vibration. Another advantage of elastomer rubber is that the use of various additives
can result in enhanced functionality over the basic characteristics. Furukawa Electric produces
and markets a broad range of elastomer rubber-based products for electrical and electronicequipment and the fields of architecture and civil engineering--areas in which there has been an
increasing demand in recent years for products that are environment friendly--halogen-free andflame retardant. This paper reports on the development of a halogen-free flame-retardant tape toreplace conventional PVC tape, and halogen-free flame-retardant thermally conductive sheet.

ABSTRACT1.

NON-HALO (HALOGEN-FREE) F-CO
TAPE

1.1 Introduction
Because of its electrical insulating properties and flexibility, tapes made from polyvinyl chloride (PVC) have long
been in general use, both as insulation and in various protective applications. But since PVC includes chlorine, itcan, when incinerated, produce dioxins as well as chlorine
gas, and the lead used as a stabilizer constitutes a further
danger of environmental pollution. These problems havetherefore led in recent years to a strong, through not legislatively mandated, movement toward chlorine-free vinyl.
Specific examples include restrictions on the use of toxic
materials embodied in the Green Government Office
Policies of the Ministry of Land, Infrastructure andTransport, and the adoption by the Ministry of Health,Labour and Welfare of halogen-free cables.
For our part, Furukawa Electric has developed a flameretardant tape that is halogen-free and contains no lead orother heavy metals as an environment-friendly replacement for PVC tape.

1.2.1 Physical Targets
The most compelling advantage of PVC tape is that anyone with moderate strength can produce adequate elongation during wrapping, and, when finished, can easilytear off by hand. Thus any replacement must be able toprovide:1) Balanced wrapping force (3-6 MPa of tensile strain
produces an elongation of 10-30 %);2) Easy tearing off by hand (capable of being easily
pulled to the point of tearing without further elongation, and absence of whitening);
3) Suitable unwrapping force (tape peeling force vs.
adhesive characteristic) of about 2 to 8 N for 19 mm
width.
1.2.2 Thermochemical Targets
Chlorine-based products have flame-retardant and heatresistant properties, and the replacement products must
incorporate new techniques for flame-retardancy and heat
resistance.

1.2 Tape Development Targets
The main targets in developing a halogen-free tape toreplace the PVC product may be summarized as follows:

1.2.3 Electrical Targets
Since these products have been used in a wide variety ofelectrical applications the replacements should obviously
have insulation performance equivalent or superior to thatof PVC.

* Research and Development Dept., F-CO Co.
*2 High-Voltage Material Gr., Environmental Technology
Development Dept., Ecology and Energy Lab.
*3 Engineering Sec., Thermal Products Dept., Electronic
Components Div.

1.3 Materials Design
1.3.1 Tape Base Material
Since the base polymer for use in the tape must, first ofall, have workability of the same level as PVC tape, it
must exhibit stress-strain (tensile stress-to-elongation)
characteristics close to those of the PVC product. Second
is ease of tearing off by hand. This ease of tearing off is

1

extremely important in materials design. These points
were therefore taken into account in selecting the polymer, but while ethylene copolymers have properties comparatively close to those of PVC, tensile strength as single
polymers is too high so that there are problems with bothwrapping and tearing off properties. As flexible polymers,
rubber-based materials are good, but used alone the tapeitself lacks strength and thermal resistance, while elongation during tearing off is excessive. No single polymers
can satisfy the required performance, so that blending
techniques are needed, and it was decided to create ablend of rubber-based polymer combined with theincreased strength and resistance of olefin polymer andTable 1Comparison of tape workability by type of polymer.
Halogen-free tapes
PVC tape

Tensile strength
N/10 mmElongation (%)

Standard ofSingle-3-polymer

polymerblend~10 or30

20

50 – 100

310

600 or 150

workability490

10 – 40
250 – 600

ManualEasily torn

tearing off

by hand
Virtually

Whitening

unnoticeable

10

Strain (MPa)

8

6

4

ethylene copolymer that exhibits tensile strength equivalent to PVC yet can reduce elongation. The three types ofpolymer blends had different physical properties depending on the composition and also behaved differently during
tape manufacture, so a balanced composition was adopted.
Table 1 shows a comparison of the tape workability by
the type of polymer used, and Figure 1 shows the stressstrain curves for PVC and halogen-free F-CO tapes in therange of the wrapping procedure. Photo 1 shows results
for the ease of tearing off of the tape obtained duringwrapping.
Next, to provide flame-retardancy, the composition ofvarious flame-retardant agents was examined. Heeding
environmental restrictions to be implemented in future,
metal hydrates were incorporated, avoiding the use ofphosphorus-based materials, and rubber-based polymers
are suited to high levels of filling. Taking account of tapeflexibility, high levels of filling are not possible and with themetal hydrate only flame-retardancy is insufficient, thus
requiring the addition of an agent that enhances flame
retardancy. Buy adding a flame retarding agent and aflame retardancy enhancer in suitable proportions to takeaccount of flexibility it was possible to achieve a satisfactory level of flame retardancy--an oxygen index (OI) of 26.
The metal hydrate was selected with regard to the temperature conditions of tape manufacture. Figure 2 shows therelationship between the amount of flame-retardant agent
and the OI value.
A processing enhancer was also added to further
improve the mixing characteristics, together with an aging
preventive agent to improve the long-term performance.
A patent application has been filed for the tapedescribed.

PVC tape2
F-CO tape
0
10

20

30

40

50

Elongation (%)

Figure 1

Stress-strain curves for PVC and halogen-free F-CO
tapes.

a) PVC tape

1.3.2 Tape adhesive
The tape requires an adhesive. Even if the adhesive layer
is thin, there must be sufficient adhesive strength andunwrapping force (if the adhesive strength is too great,tape-wrapping characteristics will deteriorate.) With generally available acrylic adhesives or butyl rubber adhesives,
the peeling force is weak (that is to say the tape unwrapping force is weak), so that the tape unwraps and tapewrapping work is difficult to perform. And there is a problem of blocking (adhesive residue on the back surface ofthe tape) that prevents their plain use. As a solution tothese problems we developed a proprietary composition
consisting of an acrylic adhesive base with a suitable

40

OI valueb) F-CO tape

30
20
10
0
30

c) Other tape

35

40

45

50

Flame-retardant content (wt%)

Figure 2
Photo 1

2

Tape parting results obtained during wrapping.

Relationship between amount of flame-retardantagent and OI value.

Furukawa Review, No. 22

2002

Table 2

Properties of non-halo F-CO tape.

Other thermally conductive materials
Thermally conductive sheets
Cooling fansHeat-pipes

60,000Item

Unit

Tensile strength

N/10 mm20

15 or more%

490

100 or more

Non-halo F-CO JIS C 2336 standard

Elongation
Adhesive force
Insulation breakdown
strengthN-10 mm0.7

0.5 or more

kV/mm

55

25 or more

Value (million yen)

50,000

9,000

8,300

40,0007,600

30,00021,0007,000

17,50014,50012,00020,000
8,800

9,800

11,0008,000

10,000Withstand voltage

kV for 1 min

5

5 or more
0

Volumetric resistivityΩ-cm

2✕10152✕1012 or more

Oxygen index (OI)

—

26

—

amount of a plasticizing agent.
Since field wrapping under winter conditions can involve
temperatures near the freezing point, PVC tape becomes
stiff and adhesive strength drops sharply. The tape developed in this work is better adapted to cold conditions,
greatly increasing the workability at low temperatures.
1.4 Characteristics of "Non-Halo" F-CO TapeIn addition to the physical and thermochemical properties
described above, the electrical properties of the halogenfree tape developed here were also satisfactory (seeTable 2).
1.5 Summary
It has been possible to develop as a replacement for PVC
tape a new type of fire-retardant tape that is halogen-free,does not use phosphorous and contains no lead compounds. In May 2001 it was announced in the press andwas put on sale. In addition to the standard black color, it
is also available in red, blue, white, yellow, brown andgreen, and is expected to be widely used in efforts toward
environmental protection.

2.

THERMALLY CONDUCTIVE SHEET"F-CO TM SHEET"

2.1 Introduction
As CPUs and other electronic computer components
improve in performance, they generate more heat, so that
effective cooling is an indispensable aspect of maintaining
their performance. A commonly used method of accomplishing this is to channel the heat to a heat sink, but for
the heat sink to conduct the heat properly a material having good thermal conductivity must be sandwiched into
the intervening space. The performance of this included
material is all-important.
2.2

Segmentation of the Market for Thermally
Conductive Materials
Figure 3 shows the projected market for various thermallyconductive products; significant growth is anticipated.
Thermally conductive materials may be categorized in
two main forms--sheets and greases, but in recent times
there have appeared intermediate materials such as gels

Figure 3

7,000

8,500

9,500

9,700

1999

2000

2001

2002

Projected market for thermally conductive products.

and phase-changing materials, which are solid sheets at
normal temperature but change into grease as temperature rises.
2.3 Details of Materials Developed
From the physical attributes of flexibility and thermal conductivity, silicone rubber-based products are in overwhelming use. It has recently been found, however, thatwhen the temperature of sheets of such materials rises,
the low-molecular-weight siloxane they contain volatilizes
into the equipment. It is then redeposited on the contact
points of motors and relay switches and breaks down toform silicon dioxide, which acts as an insulator and contributes to point failure. This has created a strong demand
for siloxane-free thermally conductive sheets that can beused with no loss of flexibility or thermal conductivity.
The F-CO TM sheets that have been developed andmarketed by Furukawa Electric, though in the form ofsheets, use an acrylic base polymer and, containing nosilicone rubber, are free of LMW siloxane. They are also
halogen free and offer a high degree of thermal conductivity.
For a polymer material, acrylic rubber itself exhibits agood level of thermal conductivity, but high-conductivity
filling materials are added to improving performance, to adegree determined by the type and amount of filling material used.
Table 3 shows the thermal conductivity of several materials. Commonly used filling materials include aluminumoxide for ordinary applications, and high-cost boron nitride
and aluminum nitride for products requiring high thermal
conductivity.
F-CO TM sheets contain mainly magnesium oxide,
which has higher thermal conductivity than the aluminumoxide used in general-purpose products. Magnesiumoxide is highly water-absorbent, and when added to polymer in large amounts can cause stability problems. For
this reason it has been avoided in the past, but new manufacturing techniques have made it possible to produce a
product that is sufficiently hydrophobic as to be useful.
A number of patents have been applied for with respect
to these thermally conductive sheets. Table 4 shows acomparison of the features of F-CO TM sheet with conventional silicone-based thermally conductive sheet produced by a competitor.

3

Table 3

Thermal conductivity (W/m•K) of selected materials.

MineralsW/m • K

Fillers

W/m • K

Polymers

W/m • K

Diamond

2000

Boron nitride

Silver427

Aluminum nitride

210

Acrylic rubber

0.27

170

Chloroprene rubber

0.25

Copper

398

Gold

315

Magnesium oxide60

Silicone rubber

0.2

Aluminum oxide

36

Butyl rubber

0.1

Aluminum
Magnesium

237

Zinc oxide

25

Nylon 60.25

156

Carbon10

Polyethylene

0.22

Iron
SUS

80

Polyvinyl chloride

0.16

15

Polystyrene

0.12

From the standpoint of resistance to heat and cold, theproperties of the silicone rubber now in general use areoutstanding. Considering the operating conditions ofCPUs and other electronic components, however, such alevel of thermal properties is unnecessary. And with silicone rubber it is impossible to reduce siloxane production
to zero. We therefore decided to consider non-silicone
rubber products, and developed and brought to market amaterial based on acrylic rubber, which was second only
to silicone rubber in terms of heat resistance and had thehighest thermal conductivity of all the polymers in Table 3.
With respect to applicability to the heat-generating portions of electronic equipment, the need for a siloxane-freematerial is urgent, and it has already been used in gamemachines, TV sets, laptop computers and audio equipTable 4

ment.
The production of of F-CO TM thermally conductive
sheets--mainly for game machines--now exceeds 2 million
sheets per month. It is also under consideration for use incommunications equipment, automotive electronics, harddisk drives and DVD equipment.
Table 5 shows its characteristics.
2.4 Conclusion
F-CO TM sheets were brought to market in August 2001
and large numbers of inquiries have been received.
Currently available, in addition to the products alreadymentioned, are:1) Thermally conductive sheets to enhance the effect ofpreventing electromagnetic interference (EMI).
2) Thermally conductive sheets with strong adhesion on
both surfaces;
3) Low-rigidity thermally conductive sheets for applications requiring flexibility.
All of these sheets are halogen-free, and have flameretardant properties.
With the increasing need for electrical, electronic, communications and automotive equipment to be more compact, more functional, and more resistant to EMI noise,
these products are expected to contribute to the expansion of IT-related fields.

Comparison of features of F-CO TM sheet with competitor's product.
Property

F-CO TM

Competitor

Base material

Acrylic rubber

Silicone rubber

Heat withstand temp.

Approx. 180°C

Approx. 220°C

Cold withstandApprox. -40°C

Approx. -60°C

Good

Good

Thermal characteristics
Siloxane produced

No

Yes

Cost effectivenessExcellent

Good

Table 5Characteristics of F-CO TM sheet .
Insulating

Description

Standard

TypeStandard dimensions

2014

High thermal

adhesion

conductivity

N-500

EE

Thin film

Single-sided

Two-sided

Al-foil center layer,

adhesionadhesion

two-sided adhesion

EN500/50-1

EN500/50-2

A90/20-2

A250/100-2

T (mm)

1

0.5

1

0.5

0.5

0.09

0.25

W (mm)

210

190

115

200

200

240

240

L (m)

10

0.19

25

0.4

0.4

100

100

Thermal conductivity (W/m • K)

1.6

1.6

2.5

1.6

1.6

1.4

1.4

Thermal resistance (°C/W)

0.57

0.31

0.5

0.35

0.28

0.13

0.31

Volumetric resistivity (Ω-cm)

1✕10122.7✕1012

1✕10122.5✕1011

3.9✕1011

—

—

20

28

20

35

33

—

—

Adhesive force to Al (N/25 mm)

—

3.6

—

2.8

3

1.5

6.5

Flame-retardancy (by UL94)

V–0 equivalent

—

V–0

HB

HB

—

—

84

27

80

130

100

—

—

Insulation breakdown voltage (kV/mm)

30 % compression strength (N/cm2)

4

Single-layer

Furukawa Review, No. 22

2002