AUTOMOTIVE

Dual Wall Heat-Shrinkable Tubing with Hot-Melt Inner
Layer
Satoshi YAMASAKI*, Taro FUJITA, Shinya NISHIKAWA, Yasutaka EMOTO, Ryouhei FUJITA,
and Shuji AZUMA---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Heat shrinkable tubing with a meltable inner layer (dual-wall heat-shrinkable tubing) protects joints and connections in wiring
harness connection with high adhesion, and has been widely used in electronics products and aircrafts. The application of thetubing has been extended to automotive wiring harnesses due to its easy handling and waterproof performance, and tubing thatcan be applied to complicated harness configuration is in high demand. To meet this demand, we have developed a new dual-wallheat-shrinkable tubing that shrinks at a low temperature and stays long. We optimized the shrink behavior and mechanicalproperties of the outer layer by controlling resin blending, and the flow and adhesion of the inner layer by applying the molecular
design method and polymer alloy technology.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: heat-shrinkable tubing, hot melt adhesive, electron beam irradiation, cross-linking, polymer alloy

1. Introduction
Heat-shrinkable tubing is a tube that shrinks in aradial direction when heated. It is widely used in the electronics, automotive, aerospace, and other industries forelectrical insulation and mechanical protection of wire
connections, corrosion protection of metal pipes, and manyother purposes. Since 1964, Sumitomo Electric Industries,
Ltd. has been manufacturing and supplying heat-shrinkabletubing under the trade name of SUMITUBE. In the 1980s,
we developed dual-wall heat shrinkable tubing consistingof a hot-melt adhesive inner layer and heat-shrinkable outerlayer, and started production and sales. When dual-walltubing is heated, the adhesive inner wall melts and flows to
conform to the object to be covered and thus protects theobject providing a water tight environmental seal. Due tosuch excellent and reliable sealing performance, dual-walltubing is widely used in electronic devices, aircraft, andmany other types of equipment.
In the field of automotive in particular, customer
demand for higher heat resistance (125°C) dual-wall tubing
is increasing.
This paper describes the development of a new typeof dual-wall tubing that can be used to protect automotive
wiring harness connections.

2. Background and Target of the Development
Conventional dual-wall tubing is used to protect oneto-one wire connections in general. The tubing can be
shrunk with hot air or other heating device (250-350°C)
without any difficulty. On the other hand, for protecting
automotive wiring harness connections, it is required tomeet the following four specific conditions:(1) Since the tubing is used to protect one-to-pluralwire connections having a large size gap, theshrinkage characteristics of the tubing must be
optimized so that it fits tightly to the irregularouter contour of the connections. Otherwise, theOuter layer:
heat-shrinkable tubing

Inner layer:
hot melt adhesive

Conductor

HeaterElectric wire

One-to-one
wire connection

Wire connection

HeaterOne-to-pluralwire connection

Size gap

Conventional heat-shrinkable tubing

Newly developed dual-wall tubing

Fig. 1. Schematic illustration of wire connections covered with conventionaland new dual-wall heat-shrinkable tubing

tubing will slip out of place and would not be able
to protect the connections from water immersion(Fig. 1) at a service temperature of 125°C.
(2) Shrinking the tubing to a temperature higher thannecessary will damage the insulation material ofwire such as polyvinyl chloride (PVC)*1. It istherefore necessary to shrink the tubing at a relatively low temperature.
(3) An automated shrinking machine is used toprocess a large quantity of wiring harness splices
with dual-wall heat-shrinkable tubing in order toenhance efficiency and eliminate any inconsistencies in the work. The process is required to minimize the heat-shrinking time and achieve highreproducibility.
(4) The tubing is required to have mechanical strength
high enough to protect the wire connections fromdamage due to physical contact with other components during vehicle operation (Table 1).
To prevent PVC wires from thermal degradation
during heat shrinking, the process temperature must be
controlled to 100°C or less. To meet the above requirement,
we measured the maximum heating temperature and time

SEI TECHNICAL REVIEW · NUMBER 86 · APRIL 2018 · 85

in the automatic shrinking machine and found that the
tubing can be heated to 135°C for one minute. When thetubing is heated to this temperature, its outer layer must
shrink completely, while the inner layer is required to flow
and conform to the irregular outer contour of the covered
object. At a service temperature of 125°C, on the other
hand, the tubing is required to show proper insulation andsealing performance without moving both the tube andadhesive out of position. The development target for the
Table 1. Difference in specifications between conventional and new tubing
Conventional tubing

New tubing

Use

Electronics, aircraft

Automotive wiring harness

Object to becovered

One-to-one wire connection

One-to-plural wire connection(size gap between both sides)
Resin with appropriate flowability andadhesiveness

Inner layer
Easy-to-process resin

Hard resin having appropriate shrink
characteristics

Outer layer
ShrinkingmethodUse of hot gun (manual work)
Small quantity

Shrinking machine(continuous shrinking)Large quantity

Shrinkingcondition

Heating to high temperature fora long time to ensure sufficientshrinkage

Since the object to be covered consists of
a PVC wire,
the maximum allowable heating
temperature is limited.
The shrinking time must be minimized toefficiently cover wire connections in bulk.

material to be used for the new tubing and its target characteristics are shown in Tables 2 and 3, respectively.

3. Development of Outer Layer Material Having
Optimal Composition
3-1 Manufacturing process of heat-shrinkable tubing
and principle of shrinkage
The manufacturing process of heat-shrinkable tubing
is shown in Fig. 2. As shown in this figure, the manufacturing process consists of three steps: extrusion, electron
beam*3 irradiation, and expansion. In the extrusion process,resin is extruded into tubes shape. In the electron beamirradiation process, the tubes are crosslinked. In the expansion process, the crosslinked tubes are softened by heating
and then internal pressure is applied to expand the tubes ina radial direction. Finally, the tubes are cooled and solidified as heat-shrinkable tubing.
The principle by which heat-shrinkable tubing shrinks
when heated is shown in Fig. 3. When an electron beam isdelivered to crystalline resin*4 consisting of both a crystal-

Electron beamirradiation process

Extrusion process
Table 2. Development target for new tubing material

Inner
layer

Raw material
Extrudedtube

Condition/item

TargetWhenshrinking

Shrinking by heating to
135°C for 1 minute

Percentage of heat shrinkage: 75%
min.*2Whenusing

No positional displacement
after exposure to
125°C for 1000 h

Percentage of heat shrinkage: 20%
max.*2ExtruderWhenshrinkingFluidization when heated to
135°C for 1 minute

Conformance to the irregular outer
contour of the objecton completionof shrinkage

Expansion process

When
using

No fluidization after exposure to
125°C for 1000 h

No fluidization

When
using

Adhesion to PVC, conductor, andouter layerHigh adhesion strength
at normal temperature

Relative adhesion force 70%
minimum of the metal-adhesion
strength of conventional tubing.

Outer
layer

Heatingzone

Tube

Coolingzone

Fig. 2. Heat-shrinkable tubing manufacturing method
Table 3. List of target characteristics of new tubing

Electricalcharacteristics
Chemicalcharacteristics

Target value
10.4 MPa min.

Tensile elongation

300% min.

Coefficient of elasticity

400 MPa

Piercing strength(V-shape blade/thrusting in)

450 N min.

Thermal shock
225°C × 4 h

No crack generation

Heat resistance
130°C × 7 d

No crack generation

Dielectric strength

15 kV/mm min.

Volume resistivity

1.0 × 1012 Ω･cm min.

Flammability

SAE J1128
Self extinction of flame within 70s

Original

Sealing
properties

125°C × 1000 hafter heat-aging
Oil resistance
Immersing in brake oil for 2 hLeakage current
0.25 μA max.

Oil resistanceImmersing in automatictransmission oil for 2 h

86 · Dual Wall Heat-Shrinkable Tubing with Hot-Melt Inner Layer

Crystalline region(Crystalline resin)
Non-crystalline region

Shrinkage by heating

Mechanicalcharacteristics

Condition
Tensile strength

Shape-memory effect

Item

Irradiation with electron beam

Crosslinking point(Crosslinked resin)(1) Expansion by heating(2) Solidification by cooling(Crosslinked resin after expansion by heating)Fig. 3. Principle of developing heat-shrinkability

line and non-crystalline regions, the resin molecules areconnected to each other in the non-crystalline region. As aresult, the resin transforms into crosslinked resin withcrosslinking points formed in the non-crystalline region.
The crosslinked resin is expanded by heating, and thencooled and solidified as thermally expanded crosslinked
resin. When the thermally expanded crosslinked resin isheated to a temperature equal to or higher than the melting
point of the crystalline region, the crystal melts andbecomes soft. As a result, due to the presence of the crosslinking points, the resin shrinks to the shape before expansion (shape-memory effect).(1)
3-2 Development of tubing material
The shrink temperature of heat-shrinkable tubing
depends on the melting point of the resin used for the outerlayer of tubing. From the viewpoints of cost, heat-shrinkability, and oil resistance, we selected polyethylene as abase resin. Polyethylene is economical and has excellent
extrusion characteristics. As a result of the investigation ofpolyethylene, it was found that its coefficient of elasticity
increases as its melting point increases as shown in Fig. 4.

125°C (percentage of heat shrinkage: 20% or less) or getout of position (Fig. 6).

After shrinkage(Before use)

Soft

600

125℃
Targetcoefficient of elasticity

135℃

HDPE

400

200

0
20

Fig. 5. Mechanism of positional displacement of tube when used

VLDPE
60

80

100

Conventional tubing
Coefficient of elasticity:
220 MPa

Newly developed tubing
Coefficient of elasticity:
460 MPa80
75％ Target percentage ofheat shrinkage when shrunk:

60
Pure HDPE
Coefficient of elasticity:
520 MPa

40
20％

20

80

90

Target percentage ofheat shrinkage when used:

100 110 120 130 140 150

Heating temperature for one minute (°C)

LDPE

40

Shrinkage force (inner stress)
that is imparted to the gap

Shrinkage force of tube
moves it leftward

0

LLDPE

Outer layer
Inner layer

Shrinkage force (stress inside)that is imparted to the gap

During use(125°C)

Percentage of heat shrinkage (%)

Coefficient of elasticity (MPa)Hard 800

Targetshrinkage temperature

Connection

Electric wire

100

HDPE: High density polyethylene
LLDPE: Linear low density polyethylene
LDPE: Low density polyethylene
VLDPE: Very low density polyethylene

Large size
gap

120

140

160

Fig. 6. Temperature-dependence of the coefficient of elasticity of newly
developed tubing (outer layer)

Melting point (°C)

Fig. 4. Melting point and coefficient of elasticity of various types ofpolyethylene

4. Development of Inner Layer Material Having
Optimal Composition

High-density polyethylene (HDPE)*5 was expected toprovide the tubing with high mechanical strength because
its coefficient of elasticity is higher than those of othertypes of polyethylene. However, it was found that HDPE
would not sufficiently shrink when heated to 135°C, a
temperature slightly lower than its melting point, for one
minute. In contrast, other types of polyethylene shrink
when heated to 125°C but soften at a service temperatureof 125°C, thereby producing internal stress in a radial
direction and sliding out of its intended position (Fig. 5).
To eliminate these shortcomings, we blended apolymer into polyethylene to optimize its melting point. As
a result, we developed a new polyethylene-based outerlayer material. The new material shrinks when heated to135° for one minute (percentage of heat shrinkage: 75% ormore), but does not shrink at a service temperature of

4-1 Inner layer material development
Polyamide resin was selected as the hot melt adhesive
to be used for the inner layer since this resin provides ahigh degree of freedom of design and exhibits a wide
variety of characteristics necessary for the inner layer.
(1) The flow properties of the resin were optimized bymolecular design. (2) The adhesion properties of the adhesive depends on the polarity of the material to which theadhesive is applied. Since polyamide resin itself does notadhere to polyethylene, the outer layer material (depending
on its application, the tubing will be used to cover polyethylene wires), we introduced a polymer alloy technology todevelop a new inner layer material.(2)4-2 Target adhesive flow setting
To determine the target viscosity of the hot melt adhesive to be used for the inner layer, several types of dualwall tubing were tested. For their outer layers, existing
resin compound was used, while for their inner layers,

SEI TECHNICAL REVIEW · NUMBER 86 · APRIL 2018 · 87

various types of polyamide resin having different viscosities were used. After being shrunk by heating, each prototype tubing was removed from the covered object to checkfor conformance to the outer contour of the object and theflow properties of the adhesive in the usage environment.
The results revealed that, when the tubing is shrunk byheating, an adhesive having a melt viscosity of 550 Pa･s orless conforms to the irregular outer contour of the object tobe covered, and that an adhesive having a melt viscosity of
800 Pa･s or more does not flow out of the tubing. Based on
the above results, we introduced molecular design technology to develop and use an inner layer material whose
viscosity follows the curve shown in Fig. 7.

Viscosity (Pas)

10000

cantly increase as expected (Fig. 8).
Subsequently, we tested a polymer alloy (Fig. 9) madeby adding a small amount of olefin rubber to polyamide
resin. In this test, the polymer provided the polyamide resinwith a high adhesion to polyethylene while maintaining thehigh adhesion of the polymmide resin to PVC and metal.
The results of the transmission electron microscopy of thephase structure (Fig.10) revealed that the olefin rubber
dispersed finely on a nanometer order. We concluded thatan increase in the adhesion of the polymer alloy wasbrought about by an increase in the interfacial strength
between the polyamide resin and olefin rubber and anincrease in the resistance to stress concentration.

Object to be covered

Object to be covered

Polyamide
resin

Polyamide
resin

Target viscosity
550 Pa・s

1000

Target viscosity
800 Pa・s

100
90

100

110

adhesive

OlefinrubberPolyamide resin withsimply-added olefin rubber• Low interfacial strength• Stress is likely to concentrate.

120

130

140

Olefinrubberadhesive
Polymer alloy• High interfacial strength• Stress is not likely to concentrate.

Fig. 9. Difference in adhesiveness between polyamide resin with simply-added
olefin rubber and polymer alloy

150

Temperature (°C)
Fig. 7. Temperature dependence of the viscosity of newly developed tubing(Inner layer)

4-3 Development of material
Polyamide resin does not adhere to polyethylene. To
modify this intrinsic property, we added a proper amountof olefin rubber to this resin. The structure of this rubber issimilar to that of polyethylene. For polyamide resin witholefin rubber added, the adhesiveness of this resin to PVC
and metals decreased as the composition of olefin rubberincreased, and the adhesion to polyethylene did not signifi-

100nm
Fig. 10. Result of transmission electron microscopy of phase structure(Black: polyamide resin; white: olefin rubber)

Relative adhesiveness

120

Adhesion property to PVC/metal

100

Target
value

80

100
80

Adhesion property to polyethylene

Targetvalue

Polymer alloy

60

60

40

Polyamide resinwith simply-added
olefin rubber

20
0

120

Polymer alloy

Additive
Rate
5%

Additive
Rate
10%

Additive
Rate
20%

40
Polyamide resinwith simply-added
olefin rubber

20
0Additive
Rate
5%

Additive
Rate
10%

Additive
Rate
20%

Fig. 8. Dependence of the adhesiveness of polyamide resin on the additive rateof olefin rubber (with the adhesiveness of conventionally used resin
taken as 100)88 · Dual Wall Heat-Shrinkable Tubing with Hot-Melt Inner Layer

5. Evaluation Result for Prototyped Dual-Wall
TubingWe prototyped a dual-wall tubing (before shrinking:inner diameter = 5.8 mm; inner layer wall thickness + outerlayer wall thickness = 0.45 mm, after shrinking: innerdiameter = 1.3 mm; inner layer wall thickness= 0.65 mm;
outer layer wall thickness = 0.55 mm) consisting of the
newly developed inner and outer layer materials. When
shrunk to cover a one-to-three PVC wire connection using
a tube shrinking machine, the prototype demonstrated
sufficient waterproof and sealing performance. As a result

of the observation of a cross section of the tube, it was
confirmed that the hot-melt adhesive used for the inner
layer had completely conformed to the irregular outer
contour of the covered wire connection (Fig. 11).

Conventional tube

Newly developed tube
Initial position

Positional
displacement

Gap

Conductor

Outer layer
Inner layer

Electric wire

Outer layer

Inner layer

Positional displacement

No positional displacement

Fig. 12. Tube after being heated to 125°C(Newly developed tube was free from positional displacement.)

Inner layer

6. Conclusion
Electric wire

Outer layer

Electric wire

Conductor

Conductor

Fig. 11. Conformance of adhesive to irregular outer contour of wire connection

The physical characteristics of the prototype, which
are shown Table 4, demonstrate that it achieves the target
characteristics. Even after being heated to 125°C, the tube
did not exhibit the outflow of the adhesive or positional
displacement (Fig. 12).

Mechanicalcharacteristics

Electricalcharacteristics
Chemicalcharacteristics

Condition

Target value

ResultTensile strength

10.4 MPa min.

25 MPaTensile elongation

300% min.

550%

Coefficient of elasticity

400 MPa

460 MPa

Piercing strength(V-shape blade/thrusting in)

450 N min.

520 N

Thermal shock
225°C × 4 h

No crack generation

pass

Heat resistance
130°C × 7 d

No crack generation

passDielectric strength

15 kV/mm min.

20 kV/mm

Volume resistivity

1.0 × 1012 Ω･cm min.

9.0 × 1015 Ω･cm

Flammability

SAE J1128
Self extinction offlame within 70s

pass
pass

Original
125°C × 1000 hafter heat-aging
Sealing
properties

Oil resistance
Immersing in brake oil for 2 h
Oil resistance
Immersing in automatictransmission oil for 2 h

pass
Leakage current
0.25 μA max.

• ‌SUMITUBE is a trademark or registered trademark of Sumitomo ElectricIndustries, Ltd.

Technical Terms

Table 4. Evaluation results for newly developed tube
Item

We have developed dual-wall tubing that can be used
to protect one-to-plural automotive wiring harness connections. When heated to a relatively low temperature (135°C),
the new tubing conforms to the irregular outer contour of
the wire connection. In addition, this tubing does not
induce outflow of the adhesive or cause positional
displacement, and demonstrates high mechanical characteristics under a usage environment of 125°C. Due to its
excellent heat-shrinkability and sealing performance, the
new tubing is expected to find wide applications in theautomotive field.

pass＊1	Polyvinyl chloride (PVC): A synthetic plastic polymer
commonly used as an insulation covering for electric
wires at service temperatures of approximately 80 to
100°C.
＊2	Percentage of heat shrinkage: The percentage calculated
from 100 x (inner diameter before shrinkage – innerdiameter after shrinkage) / inner diameter before
shrinkage.
＊3	Electron beam: The flow of high energy electrons,
which impinge against a substance to induce achemical reaction by high electron energy.
Crystalline resin: Resin consisting of a crystalline＊4	region where molecular chains are arranged in anorderly way and a non-crystalline (amorphous) region
where molecular chains are arranged randomly.
High-density polyethylene (HDPE): A highly＊5	
crystalline polyethylene in which repeat units ofmoderate/low pressure-polymerized ethylene arelinked linearly with little branching.

pass

SEI TECHNICAL REVIEW · NUMBER 86 · APRIL 2018 · 89

References(1)	Satoshi Yamasaki, Shinya Nishikawa, “Nano-Composite Heat-Shrinkable
Tubing,” SEI Technical Review, No. 78, p. 68–72 (Apr. 2014)(2)	L. A. Utracki, “Polymer alloys and blends thermodynamics and rheology”

Contributors The lead author is indicated by an asterisk (*).

S. YAMASAKI*• ‌Assistant Manager, Energy and Electronics Materials
Laboratory

T. FUJITA• ‌Group Manager, Energy and Electronics Materials
Laboratory

S. NISHIKAWA• ‌Department Manager, Energy and Electronics
Materials Laboratory

Y. EMOTO• ‌Group Manager, Sumitomo Electric Fine Polymer,
Inc.

R. FUJITA• ‌Department Manager, Sumitomo Electric Fine
Polymer, Inc.

S. AZUMA• ‌Department Manager, Sumitomo Electric Fine
Polymer, Inc.

90 · Dual Wall Heat-Shrinkable Tubing with Hot-Melt Inner Layer