Electrical applications for TPVs.

Thermoplastic elastomers (TPEs) are materials that consist of a blend of rubber and plastic phases. TPE materials have a good balance of rubber-like engineering properties but can be processed on conventional thermoplastic processing equipment such as extruders or injection molding machines. Within the class of TPEs, an interesting subset is the family of thermoplastic vulcanizates (TPVs). These materials consist of a fully-cured rubber phase contained in a continuous thermoplastic matrix. The continuous phase is most often polypropylene, while the cured rubber phase is usually EPDM. These materials have an even greater degree of rubber-like characteristics, such as superior sealing behavior denoted by low compression set properties. They are extremely flexible materials that have the feel of rubber. These TPV materials also have outstanding heat resistance, chemical resistance and cold temperature performance, in comparison to other TPE materials.

TPEs, in general, have outstanding electrical properties. This superior electrical performance has led to many commercial applications within the electrical wire and cable application area. They are noted for their flexibility, good heat resistance, and can be easily compounded with additives for flame retardancy or other more demanding performance requirements. TPVs have also the same superior electrical properties as other TPE materials. In addition, their superior chemical and heat resistance has led to wire and cable applications that traditional TPE materials could not penetrate. These materials are also compounded to meet specialty performance needs such as flame retardancy, wet electrical stability and additional heat resistance. Within wire and cable applications, these specialized performance requirements are often needed, combined in one specialized compound.

Electrical properties

Most thermoplastics and elastomers can be used as insulation materials in wire and cable applications. The electrical properties of dielectric constant, dielectric strength, volume and surface resistivity and power factor are all key properties that distinguish one type of insulating material from another. Additional performance criteria often needed in wire and cable include flame resistance, oil and chemical resistance, thermal and UV resistance, deformation, impact, cold temperature performance, smoke and toxicity, and wet electrical properties.

The dielectric constant is a measure of the ability of a material to store electrical energy. It is defined as the ratio of the capacitance of the dielectric material to the capacitance of air. In general, lower numbers arc preferred. TPV materials have dielectric constants in the range of 2.5 to 3.0. This value compares to polyethylene (2 to 2.5), PVC (4 to 6) and polyurethane (5 to 7). The dielectric constant depends on frequency, temperature and moisture content. The standard used in the industry to measure dielectric constant is ASTM D150.

The dielectric strength is a measure of the ability of an insulating material to resist electrical breakdown when voltage is applied across it. It is usually expressed in volts/mil. The dielectric strength of different materials should only be compared at equivalent thickness. TPV materials have a dielectric strength in the range of 1,250 volts/mil @ 20 mils to 725 volts/mil @ 60 mils. This property is measured using ASTM D149 as a standard.

Resistivity expresses the electrical resistance of a material. ASTM D257 is the standard test method. Usually, a good insulating material has a very high resistance value (>[10.sup.14] ohms). TPV materials have very high resistance values in the range of [10.sup.14] to [10.sup.16] ohms.

Power factor measures the power loss dissipated as heat due to the build-up and collapse of the electrical field within the insulation in AC circuits. The standard test here is ASTM D150. The power factor (or dissipation factor) is expressed in percentage. In general, the lower the number the better performing a material will be in an electrical environment. TPV materials have power factors in the range of 0.05 to 0.10%.

TPV materials offer good electrical properties along with other performance requirements that are often needed for applications in wire and cable. There are many special grades of TPV materials for these special performance requirements. Table 1 lists the electrical and special properties of specialized TPVs used in wire and cable applications. General purpose TPV materials are suitable for connectors, flexible cables and "dry" cable applications. The specialty TPV materials are more suitable for severe requirement applications such as tray cables, "wet or moist" cables or applications that require some degree of flame retardancy.

Regulatory agencies such as Underwriters Laboratory (UL), Canadian Standards Associations (CSA), Society of Automotive Engineers (SAE), American Society of Testing and Materials (ASTM) and the military have specifications written around product performance requirements.

Wire and cable companies submit their product to UL, CSA and other agencies for performance testing and approval. Several grades from one manufacturer of TPVs have been tested by UL and are recognized for suitability in specific applications. Table 2 lists some of these applications and relevant product specifications. UL standard 62 is the relevant standard for flexible cords and fixture wire. TPE and TPV materials fall underneath Class 36 of this standard. UL 1581 is a reference standard for electrical wires, cables and electrical products. CSA has comparable standards that correspond very closely to the appropriate UL reference.

The applications areas of wire and cable that require additional flame retardancy are very interesting. The flame tests of VW-1, horizontal and vertical, are two of the most commonly used flame tests for wire and cable products. These flammability tests are detailed in UL 1581. VW-1 horizontal is the less severe test. This test is a basic requirement for appliance wiring material and automotive primary wire. Usually, any material that is rated UL 94 V-2 or higher will pass this horizontal test in a construction. The VW-1 vertical test is more difficult to pass. Table 3 shows several TPV grades and how they perform in this vertical flame test. For this VW-1 vertical test, the grade should be rated at least UL 94 V-0 and needs to be used as the jacket of the wire and cable construction.

New developments

A previous work (ref. 1) discussed the development of a new line of non-halogenated TPE (NHTPE) materials that has been commercialized in Europe in application areas where traditional halogenated flame retardant materials are no longer allowed. These materials have exceptional flammability resistance, low smoke and low toxicity of off gases as detailed previously. These materials are now starting to gain wider acceptance within the U.S. market as domestic companies compete in the global marketing environment.

A new development of a heat stabilized, flame retardant, copper stabilized TPV was commercialized in 2000. Table 4 displays the properties of this material after heat aging for seven days at 158[degrees]C. This test is the short screening test used by wire and cable companies to test for the material's capability of having a temperature rating at 125[degrees]C. This material will be submitted to UL in various cable constructions and should be capable of obtaining a dry temperature rating of 115 to 125[degrees]C. The material is currently being tested by several companies for GTO cable and connectors applications for neon signs and lighting.

Another new development for TPV materials is the UL listing of a family of materials that pass the "f1" rating for the UL 746 C. This test combines UV, water and heat, and is needed for marine and various outdoor applications for electrical molded products. To date, only one colorable material has passed the UL f1 rating (ref. 2).

Applications

This section lists a few examples of commercial applications using TPVs in various combinations with other thermoset rubbers and thermoplastics. It is by no means an exhaustive list of current or possible applications. Each example is selected to describe one specific combination of materials (ref. 1).

Airfield lighting cable assemblies

In this application, flame retardancy is not required, but a combination of excellent electrical properties, flexibility, moldability and fluid resistance is desirable. The TPV material offers this combination of properties, as well as excellent weldability for optimal watertightness between transformer housing and cable jacket and between cable jacket and connector. No primers or adhesives are needed when the housing and the connectors are overmolded on the cable. This is a typical fit-for-purpose application which was successfully developed with various TPV grades. In this case, the TPV replaces polychloroprene.

Elevator cables

NHTPE material is used for the sheathing of flexible elevator cable applications, in combination with thermoset rubber for insulation. It provides the right cable flexibility in combination with halogen-free, flame retardant properties.

Flexible cords and cables

In several flexible cable applications, a TPV material is used for insulation and a NHTPE material for jacketing. This halogen-free, flame retardant combination provides optimal flexibility, electrical and mechanical properties.

In the U.S., a number of flexible cords applications use TPV materials listed in the UL 62 bulletin (ref. 2), meeting the requirements of SE, SVE, SJE, SVEO, SJEO, SEO, SJEOO, SEOO, SJEW-A, SEW-A, SJEOW-A and SEOW-A classifications.

Low voltage power cables

Several grades of NHTPE are currently used in specialty power cable applications requiring a combination of high flexibility, flame retardancy, halogen-free and suitable electrical properties.

Flexible mass transit cables

NHTPE grade, radiation crosslinked, is used in mass transit, rolling stock cable jacketing applications. It combines low temperature flexibility, appropriate heat and oil aging resistants with halogen-free, flame retardant properties.

Cable assemblies in damp, wet and dusty locations

A new TPV grade for bonding to nylons is overmolded on polyamide for the connector shown in figure 1. This grade provides excellent bonding with polyamide, without the use of adhesives.

[FIGURE 1 OMITTED]

Flexible control cables

A flexible, halogen-free, flame retardant control cable was recently installed in the new tunnel linking Sweden and Denmark. Both the insulation and the cable jacket are made out of NHTPE, silane grafted and moisture crosslinked.

Conclusions

TPVs are finding increasing acceptance in wire and cable applications because they offer cost/performance advantages in addition to the inherent ease of their thermoplastic processing. As specialty grades tailored to meet demanding electrical applications, the use of TPVs will continue to grow and replace traditional rubber or PVC cable systems in the wire and cable marketplace.

Table 1--TPV grades for electrical applications

Typical property                     Test       A       B       C

Hardness, durometer                 D2240     80A     87A     87A
Dielectric constant                  D150     2.3     2.3     2.3
Dielectric strength, v/mil           D149   1,440   1,610   1,640
Volume resistivity, ohm-cm           D150     all> [10.sup.14]
Dissipation factor, %                D150     .07     .06     .09
UL component rating, [degrees]C   UL 746B      90      90      --
UL W&C rating, [degrees]C, dry      UL 62      --      --     105
  105
UL W&C rating, [degrees]C, wet      UL 62      --      --      75
Flammability                        UL 94      HB      HB      HB
Heat stabilized                        NA      No      No     Yes

Typical property                      D       E       F       G

Hardness, durometer                 87A     45D     80A     85A
Dielectric constant                 2.5     2.5     2.5     2.4
Dielectric strength, v/mil        1,150   1,410   1,320   1,340
Volume resistivity, ohm-cm                all> [10.sup.14]
Dissipation factor, %               .09     .06     .05     .04
UL component rating, [degrees]C      --      --      --      --
UL W&C rating, [degrees]C, dry      105     105      --     105
  105
UL W&C rating, [degrees]C, wet       75      75      --      --
Flammability                        V-0     V-0     V-0    HB *
Heat stabilized                     Yes     Yes      No      No

* Although compound is only rated HB, formulation is flame retardant.

Table 2--insulation and jacketing specifications for insulation
and jacketing applications

Application           Insulation             Jacket

Flexible cord                87A       87A, 45D, FR

AWM wire            87A, 45D, FR       87A, 45D, FR
Tray cable               45D, FR            45D, FR
Submersible cable        87A, HS   80a, if required
Automotive wire     87A, 45D, FR
Low tension              45D, FR
Battery cable            45D, FR

Application                      Specification

Flexible cord                 UL 62, SEO, SEOW
                        SEOOWA, SJE, SV series
AWM wire                 UL, AWM style numbers
Tray cable                UL 1277, ICEA 82-552
Submersible cable   UL lists, submersible pump
Automotive wire              SAE J1128 GP, HDT
Low tension                 SAE J1128, GPT, HT
Battery cable                   SAE J1127, SGE

Note: FR denotes retardant and HS denotes heat stabilized.

Table 3--VW-1 flame results on wire made from TPV materials

TPV grade            Horizontal           Vertical
Thickness, mm   0.38   0.76   1.14   0.38   0.76   1.14

80A, V-0        Pass   Pass   Pass   Pass   Pass   Pass
85A, HB         Pass   Pass   Pass   Pass   Fail   Fail
45D, V-0        Pass   Pass   Pass   Pass   Pass   Pass
50D, V-2        Pass   Pass   Pass   Pass   Pass   Fail

Tests done on TPV insulation over 18 AWG conductor per UL 1581.

Table 4--heat aging performance of a new TPV
80A grade

Condition                  Tensile strength,    Elongation,
                             retained (%)      retained (%)

168 hrs. @ 136[degrees]C          107               93
168 hrs. @ 158[degrees]C          108               84

Test details, tested from 60 mil insulation over 14 AWG per UL 62
method.

References

(1.) Gustin, C and Pfeiffer, J.E.,: Proceedings of Rubber Division, April 2001.

(2.) Underwriters Laboratories, EL 80017 file number, (2000).

This article is based on a paper given at the October 2001 Rubber Division meeting.

Joseph E. Pfeiffer and Jeffrey D. Smola, Advanced Elastomer Systems, L.P., and Christian Gustin, Advanced Elastomer Systems NV/SA