Hybrid Composites of Polyamide-Imide and Silica
Applied to Wire Insulationby Masakazu Mesaki *, Yoshinori Tatematsu *2 and Hideki Goda *3

Aromatic polyamide-imide resins (PAI) based on Trimellitic anhydride andDiphenylmethane-4,4'-diisocyanete were modified by an oligo-alkoxysilane compound to obtain novel silane-modified PAI. Then, PAI-silica hybrid films were prepared by drying
and curing the silane-modified PAI. Increasing the silica unit content gives the hybrid films a higher Young's modulus and a higher tensile strength than the original PAI film, however, maximum
elongation is maintained. Furthermore, they have much lower moisture absorption than PAI film
under wet conditions. Moreover, this hybrid film has been found to have a different dielectric constant than the conventional film. This report describes properties of silane-modified PAI varnish,
which is used as an insulation layer for enameled wire.

ABSTRACT1.

INTRODUCTION

PAI dissolves in polar solvents such as N-Methyl-2pyrroridone, and it is widely used for insulation, such as amagnet wire insulation and polymer-coated copper foil for
circuit boards. Moreover, PAI is a heat-resistant resin,which can be molded. It is used industrially for its outstanding electrical properties and mechanical characteristics as an engineering plastic. Thus, PAI has bothprocessability and heat resistance, and its synthesis iscomparatively easy. That is why further improvements to
its various characteristics can be expected. PAI have thefeatures mentioned above, but their moisture absorption isstill not as low as other plastics. On the other hand, PAI isused as enameled wire insulation, because it exhibitsgood heat-resistance and moisture-resistance 1). However,its heat-resistance should not be compared with a polyimide resin, for example Kapton. Moreover, PAI has goodchemical stability against water, but water absorption ishigh, therefore, it cannot be used for materials that requirea stable dielectric constant.
We examined these various problems associated withPAI. We synthesized a novel silane-modified PAI prepared with an oligo-alkoxysilane compound. The novel
silane-modified PAI was cured to make a PAI-silica hybrid.
Some properties of the PAI-silica hybrid are better thanthose of conventional PAI. Comparing enameled wires
made from conventional PAI and PAI-silica, we found that
the PAI-silica hybrid has different properties from the conventional PAI. In particular, silica has an effect on Young's
modulus, maximum elongation rate, and glass transition* Polymer Engineering Center, Ecology and Energy Lab.
*2 Engineering Dept., Magnet Wire Div.
*3 Arakawa Chemical Industries, Ltd.

temperature of cured film.
This paper describes the developments and the properties of PAI-silica hybrid, and the properties of this enamel
when applied to magnet wires.

2.

SYNTHESIS OF PAI-SILICA HYBRID

The methods of adding silica powder to enamel are wellknown. The general method is to mix fine silica powder toa varnish for dispersal in a polymer film. In this case, as it
is often difficult to disperse silica and varnish, a surfactant
is used in combination. This method has a wide range ofapplications, because it is possible to use any kind ofpolymer solution. Hitachi Cable Company Japan has introduced an example of silica blended with polyimide for theinsulation of magnet wire. In this example, silica is added
to raise the heat-resistance of the polyimide.
We recently developed a new method of preparing PAI
silica hybrid. According to this method, it is possible tointroduce silica selectively to the polymer end of the PAI.
This method is superior to mixing silica powder to varnish, except for restrictions on the kinds of solvent and thepolymer. The film properties prepared by this method
were superior to the above-mentioned method.
The following are the fabrication hybrid of the PAI andsilica, and the properties of PAI-silica hybrid 2)~ 4).

3.

EXPERIMENTAL METHOD

3.1 Preparation of Varnish and Wires
Generally, PAI varnish is polymerized by Trimellitic
anhydride and Diphenylmethane-4,4'-diisocyanete in polarsolvents 5). The base polymer was made to react at an1

excess molar ratio of TMA (Scheme 1), because there arecarboxyl groups at the end of polymer chain to be modified with a silane. The oligo alkoxysilane compound wasreacted with the carboxyl groups. As a result, a silanemodified PAI with an alkoxysilane part at the polymer end
was created (Scheme 2).
An enameled wire was obtained by coating and baking
this silane-modified PAI varnish onto copper wire. During
baking, the alkoxysilane part of a polymer end is cured tosilica by a Sol-gel process. The PAI polymer chains areintermolecular cross-linked at the silica site of the chain
end. Through a series of these steps, the PAI-silica hybrid
insulation film was formed (Scheme 3).
We evaluated the magnet wires made from the conventional PAI and three kinds of PAI-silica hybrid. Table 1
gives the types of insulation layer of the magnet wires.
3.2 Evaluation
3.2.1 Wire PropertiesTable 2 gives the general properties of the magnet wires.
A conventional PAI varnish is used for the magnet wire
used in the comparison. Table 2 shows that the abrasion
resistance of PAI-silica hybrid wire is improved. However,a difference was not seen in electrical properties such as
breakdown voltage.

3.2.2 Film Mechanical Properties and Others
The insulation film of the magnet wire was evaluated after
removing it from the wire. The evaluation method is as follows. A current etches the conductor and only the sheathsurvives. The insulating sheath is taken from the wire.
This is the same method as that for making a tube for apinhole test. Then, elongation and other properties of thesheath are evaluated.
Table 1

Type of magnet wires evaluated.
TypeSymbol

Conventional PAI

C-AI

Silica contents 1.4 % PAI

14Si-AI

Silica contents 2.0 % PAI

20Si-AI

Silica contents 4.0 % PAI

40Si-AI

Table 2General properties of magnet wires evaluated.
C-AI

14Si-AI

20Si-AI

40Si-AI

Bare wire (mm)

1.000

1.000

1.000

1.000

Film thickness (mm)

0.034

0.035

0.035

0.035

Break down voltage (kV)12.213.3

13.5

13.6

Cut through temparature (°C)

420

425

435

435

Bi-directional abrasion (times)

210

245

280

310

Uni-directional abrasion (N)

2630

33

33

... Scheme 1... Scheme 2... Scheme 3

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Table 3 gives the mechanical properties. From thisresult, the insulation film of the PAI-silica hybrid wire has ahigher Young's modulus with the content of silica. In addition, the maximum elongation rate of the films is stable
and there is no change with silica content, even when themodulus is higher.
Furthermore, a Differential Scanning Calorimeter (DSC)
is used to measure the glass transition temperatures ofthese tubes. As a result, a difference in the glass transition temperature is observed with the content of silica. Inaddition, the energy gap at the glass transition is also different.
The moisture absorption of films was investigated.
Table 4 shows the results. The result shows that the moisture absorption changed with silica content.
3.2.3 Electrical Properties
The dielectric constant of the film from the wire was evaluated (Table 5). The dielectric constant decreases as silica
content increases. There is a large difference compared tothe conventional PAI. General data on the dielectric constant show that polyimide is about 3.5 to 4.0 and PTFE isabout 2.0 to 2.5. As a result of this evaluation, the dielectric constant was found to be higher than PTFE, but lower
than polyimide (Figure 1).

related to abrasion resistance.
The silica content is related to the improvement in theYoung's modulus, but it did not affect the flexibility of thefilm. It is considered that a difference exists in the crosslinking system of the polymer chain 6). The PAI chains ofthe PAI-silica hybrid are cross-linked through the silica
located at the chain ends, but its polymer does not make across-linkage at the middle of a polymer chain (Figure 2(A) and (B)). We think the flexibility of PAI was maintained
because movement of a PAI chain was not prevented.
Consequently, the glass transition temperature measured
using DSC may be influenced.
The moisture absorption of a film may depend on thesilica content of the film. Another possibility is that it may
be dependent on the location of entangled polymer
chains. It is impossible to judge which is correct fromthese data.
The PAI-silica hybrid shows a lower dielectric constant
than polyimide. It is known when silica particles are located in a polymer film, the dielectric constant decreases.
The amount of silica may influence the dielectric constant.
On the other hand, the influence of the cross-linkages of apolymer chain can also be considered.

5.
DISCUSSION

The result shows the silica content and the intensity of thefilm are correlated. Because the correlation of the silica
content and the Young's modulus of the film is especiallyclear, it can be presumed that the silica content is also
Table 3Mechanical properties of insulation films.
C-AI

14Si-AI

20Si-AI

40Si-AI

Tensile Strength (MPa)

89.2

96.1

99.0

103.0

Elongation (%)

22.1

21.6

20.9

21.8

Young's modulus (GPa)

2.68

3.19

3.25

3.42

Table 4

The newly developed PAI-silica hybrid film (tube) prepared from magnet wire has an enhanced Young's modulus compared to conventional PAI film. This results inenhanced properties for unidirectional abrasion and bidirectional abrasion, thereby providing high windability. The
improvement in Young's modulus has contributed toimproving the abrasion resistance of the magnet wire.
The moisture absorption of the film may depend on thesilica content of the film. A 2 % silica content minimizes
water absorption.
The dielectric constant is influenced by the quantity ofsilica.
To protect the insulation layer from corona, various
investigations are performed by adding inorganic powder
to enameled wire insulation 7). It is necessary to add about
10 to 30 % of inorganic powder to insulation resin to pro-Glass transition temperature and moisture absorption of insulation films.

Tg (°C)
Moisture absorption at
20 % RH-24 h (%)
Moisture absorption at
95 % RH-24 h (%)

Table 5C-AI

14Si-AI

20Si-AI

40Si-AI

285.4

294.6

301.5

300.9

1.8

5.6

2.1

2.6

2.1

2.2

2.4

3.2

Dielectric constant of insulation film.

Dielectric constant at 1 kHz 23°C

4.

CONCLUSION

654

Epoxy

Dielectric constantat 1 kHz-23°C

14Si-AI

20Si-AI

40Si-AI

3.9

3.7

3.3

3.0

PolyimidSi14-AI
Si20-AI

3PC

Si40-AI

2PTFE
100C-AI

C-AI

Phenol

100200300400Glass transition temp. (°C)

Figure 1

Relationship between glass transition temperatureand dielectric constant of various polymers.

3Silica
SilicaPAICross Link Point

Cross Link Point
PAI(A) Conventional PAI

Figure 2

Silica(B) PAI-silica hybrid

Cross-linking model of conventional PAI and PAI-silica hybrid.

tect the insulation from corona. When silica is formed thecombination rate is 4 % at maximum in the examined silica hybrid PAI. It is thought to be possible by combining
the method of adding inorganic powder for this silica
hybrid PAI and others to protect the insulation from corona.
The magnet wires using the PAI-silica hybrid, which has
improved abrasion resistance, are equivalent to conventional PAI wires in other properties, thus application to thesame areas as before is possible. Furthermore, application to other areas, which efficiently employ the different
low moisture absorption characteristics and dielectric constants, is also possible.

References
1) Aya et.al. Plastic Age, 29 (No4), (1983), 116 (in Japanese)2) Imai et.al. Kobunshi Kako, 39, (1990), 438 (in Japanese)
3) Kakimoto. M et.al. Polymer Journal, 24, (1992), 1074) Wang. S et.al. Macromolecule Report, A31, (1994), 411
5) Imai et.al. Kobunshi Kagaku, 29, (1972), 182 (in Japanese)
6) Tsubokawa et.al. Selected Papers on Polymer Science, 44 (No5),(1987), 389 (in Japanese)
7) McGregor, et.al. U. S. Patent Document, 6100474, (2000)

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