Advances in TPE SBC compounding for UL flame retardant cable applications.

The primary applications of TPE compounds requiring fire retardancy are in wire and cable insulation, jacketing and electronics devices. Styrenic thermoplastic elastomers (TPE-S) are used in a wide variety of fire retardant applications, including automotive, audio, battery, electrical junction boxes, submersible pump and other flexible cord applications that require a balance of electrical, thermal, flame-resistance and physical properties. In the past, many industrial product designers and manufacturers sacrificed TPE mechanical performance in order to incorporate flame retardants into products (ref. 1).

Key requirements of these applications include requisite UL ratings, mechanical property retention after heat aging, ease of processing and low temperature flexibility. Styrenic TPE-S compounds are particularly suitable for such applications. Formulations of flame retardant TPE-S to achieve UL 1581 VW-1 and method 1061 flame resistance require high levels of additives. However, a high level of additives tends to negatively impact the mechanical, physical and rheological properties of the FR compounds.

This article discusses Teknor Apex's recent development of a series of high performance RollS compliant flame retardant TPE-S compounds. The effort covers the manipulation of styrenic block copolymer (SBC) technology (ref. 2), combined FR additives technology (ref. 3) and fire science on the UL vertical bums. The use of these technologies enables the achievement of UL 1581 VW-1 and method 1061 in a compound with good properties.

Results and discussion

UL 94 vertical burn

The UL 94 test is designed to assess the flammability of plastics materials for parts in devices and appliances. The test method is a measure of ignitability and flame spread for polymeric materials exposed to a small flame. To assess the test, a bar shaped specimen of plastics 120 mm x 13 mm with different thickness is positioned vertically and held from the top. The thicknesses of the test specimen are 3.2, 1.6 or 0.8 mm. Surgical cotton is placed 300 mm below the specimen to detect combustible drips that will ignite the cotton. A Bunsen burner flame is applied to the specimen twice for 10 seconds. After each flame application, the time of self-sustained combustion is recorded. The second application of the flame follows immediately after self-extinguishment of the specimen from the first flame application. Table 1 summarizes the criteria for V0, V-1 and V-2 ratings. For example, a V-0 classification is given to a material that meets all of the following criteria: (i) It is extinguished in less than 10 seconds after any flame application; (ii) the total combustion time for the five specimens tested should not exceed 50 seconds; and (iii) no combustible drips occur (ref. 4).

The UL 94 rating is a discrete classification such as V-0, V-1 or V-2. There is an attempt to correlate the UL 94 ratings to the heat release capacity (ref. 5). However, it appears the numerical values of T1 and T1 + T2 are useful in the screening evaluation. Figure 1 plots 5T1, the sum of T1 for five tested samples according to the UL 94 procedure, versus the wt. % of a flame retardant. The sample thickness is 0.125 inch. Based on the criteria shown in table 1, the compositions that achieve 5T1 less than 50 seconds are candidates for further study. In this example, it requires more than 20 wt. % of flame retardant. Figure 2 plots 5(T1 + T2), the sum of T1 and T2 for five tested samples according to the UL 94 procedure, versus the wt. % of a flame retardant. Recall the sum of 5(T1 + T2) must be less than 50 seconds. In this particular example, it requires approximately 20 wt. % of flame retardant to meet the V-0 rating at 0.125 inch.

[FIGURE 1 OMITTED]

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UL 1581 wire and cable flammability tests

VW-1 vertical-wire flame test 

This is a small-scale test conducted on a completed wire construction. The sampled construction is 24" in length. The UL 1581 test method states that a vertical wire, cable or cord shall not convey flame along its length, and it shall not convey flame to combustible materials in its vicinity during, between or after five, 15 second applications of a standard test flame. The flame source is a Tirrill burner (similar to a Bunsen burner) with a heat output of approximately 500 w or 1,700 btu/h. The flame is applied for 15 seconds and is then reapplied four more times each time after the wire ceases to bum. If the sample burns longer than 60 seconds after any application, or if the indicator flag or cotton batting is ignited during the test, the tested cable or wire fails the test (ref. 6).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

1061 cable flame test

This is also a small-scale test conducted on a single 24" length of cable. A vertical specimen of the finished cable shall not convey flame along its length, and it shall not convey flame to combustible materials in its vicinity during, between or after one minute application of a standard test flame. The standard test flame is to be nominally 125 mm high and is to produce heat at the nominal rate of 500 w or 1,700 btu/h. The flame is applied for one minute, three times. The period between flame applications is to be 30 seconds, regardless of whether flaming of the specimen ceases of its own accord within 30 seconds of the previous application. If the indicator flag is burned over 25% or the cotton batting is ignited during the test, the cable fails the test. (ref. 6).

The VW1 and 1061 cable flame tests are affected by the wire and cable design, for instance, the insulation wall thickness, the jacket wall thickness and the number of insulated wires.

Cone calorimetry testing

Cone calorimetry is a bench scale test developed at NIST (ref. 7). It is used to burn small samples for the evaluation of heat release rates, time to ignition, smoke generation and char formation. The basic principle, albeit empirical, exploits the observation that the net heat of combustion is proportional to the amount of oxygen required for combustion. Therefore, the investigation of the new FR TPE-S formulations requires the use of cone calorimetry testing.

Polymer/resin technology

Styrenic block copolymers (SBCs) are used for wire and cable applications. With the significant advances in hydrogenation technology, a broad range of hydrogenated SBCs is available. They are very compatible with polyolefins and mineral oils. Furthermore, due to the recent advances in polyolefin process and catalyst technology, a broad range of polyolefins can extend the service temperature range (refs. 8 and 9). The domain microstructure of SBC also affects the melt strength and processability (ref. 10). The combination of hydrogenated SBC rheology and polyolefin technology is the building block for high performance flame retardant compounds that possess a unique balance of properties, including excellent tensile properties and rheological characteristics. These properties are achieved while improving the flame retardancy to UL 94 V-0 rating, and also imparting good low temperature properties, good thermally aged properties and good dielectric properties.

Furthermore, blends of SBC and polyolefins can be developed for use where UV resistance, high service temperature (e.g., 105[degrees]C temperature rating), low service temperature (e.g., brittle point <-50[degrees]C) and processing stability are essential. Hydrogenated SBC based flame retardant TPEs can be formulated to cover a wide range of hardness ranging from durometer 50A to durometer 60D.

[FIGURE 5 OMITTED]

Flame retardants

There are several different categories of flame retardants. Halogen-containing flame retardants represent the most diversified class of flame retardants. A broad range of brominated and chlorinated flame retardants is commercially available. Fully brominated aromatics are generally used in resins with a relatively high processing temperature (refs. 11 and 12). Recent efforts in the development of new flame retardants have shifted toward phosphorous and other inorganic hydroxide halogen-free systems. In this presentation, the choice of polymers and a combination of flame retardant technologies results in an RollS compliant flame retardant TPE.

The effect on performance when combining FR technologies is a modification in rheology and burn characteristics with minimal effect on physical properties. The observed changes are demonstrated in figures 3-5. Figure 3 shows an increase in low shear viscosity with increases in FR ingredients. Figure 4 shows good stable char formation with a combination of FR ingredients. Finally, figure 5 shows a decrease in peak heat release rate with increasing FR ingredients, including:

* Eliminating dripping: As demonstrated in figure 3, the incorporation of combined FR technologies in YPEs increases the viscosity at low shear rates. This is translated to reducing drip under UL 94 vertical burning.

* Enhancing char formation and char integrity: Char formation and integrity are desirable for improved flame retardancy. The incorporation of combined FR technologies to flame retardant TPE compounds can promote char formation when exposed to a flame. Figure 4 compares the char of a conventional FR TPE-S to that of the new flame retardant TPE-S technology. The dark portion is char, and the white portion is ash or cracks in the char. The char for the combined FR technology is much thicker than that of a conventional FR TPE-S and again shows less cracking.

* Reducing peak heat release rate: Figure 5 shows the cone calorimeter peak heat release rate (PHRR) data of a conventional FR TPE-S versus the new combined FR TPE-S technology. The results show that the combined incorporation of ingredients reduces the peak heat release rate. It can also increase the flame out time.

Enhanced flame retardant TPE-S

Properties

The material technologies and their underlying principles discussed above are used to develop enhanced flame retardant TPE-S. These enhanced FR TPE-S compounds use an RollS compliant flame retardant. Specific examples are highlighted below. See table 2 for the typical properties of several enhanced flame retardant TPE-S compounds. The special features of these enhanced FR TPE-S systems are:

* RollS compliant;

* pass UL 94 V-0 rating at 0.060 inch;

* meet VW-1 and 1061 cable flame test, without dripping;

* brittle point below -50[degrees]C;

* exhibit good retention of tensile properties upon heat aging at 136[degrees]C for seven days, and also at 158[degrees]C for seven days; and

* show excellent electrical properties.

VW-1 and Method 1061 cable flammability tests and coil cords

Wire and cable extrusion trials verified these properties and also indicated good processability. These compounds are ideal for flexible cords, coil cords, robotics cables, power tools, high-flex cables, low temperature applications, and connector parts and components requiring a V-0 flame rating.

In VW-1 and 1061 cable flammability tests, the wire or cable must pass the criteria of retention of more than 75% of the indicator flag without dripping. Just for illustration, figures 6 and 7 show a burned wire and a burned cable that are made of EL- 1392B compound. The retention of the indicator flag upon VW-1 and 1061 cable flammability test is achieved without dripping. Figure 8 shows a coil cord made from EL1392B as insulation and also as a jacket. It has very good retract performance.

Conclusions

A combination of the availability of new polymer materials and flame retardant technologies offers the ideal timing for the development of RollS compliant enhanced flame retardant TPEs. These new FR compounds extend the performance range beyond that of conventional FR TPEs. They pass UL 94 V-0 rating at 0.060 inch and meet VW-1 and method 1061 wire and cable flame tests without dripping. These are achieved with good low temperature flexibility and good retention of tensile properties upon heat aging, and also show excellent electrical properties. These new FR compounds are ideal for flexible cords, coil cords, robotic cables, power tools, high-flex cables, low temperature applications, and connector parts and components requiring a V-0 flame rating.

[FIGURE 6 OMITTED]

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[FIGURE 8 OMITTED]

The improved flame retardant capability of these SBC compounds is due to formulation techniques that modify melt viscosity and char formation.

This article was first published in the March/April 2009 issue of Wire and Cable Technology International

References

(1.) S. Sakhalkar, D. Worley II, B.-L. Lee, S. Daniels and J.-W Shin, "TPE product innovations designed to create new product opportunities for users, "SPE TPE Division, Topcon, 2007.

(2.) "Thermoplastic elastomers, a comprehensive review, "edited by G. Holden, H. Kricheldorf and R. Quirk, Hanser Publisher (2004), third edition.

(3.) www.albemarle.com.

(4.) UL 94, page 15, material classifications.

(5.) "Micro-calorimetry: The pyrolysis, combustion flow calorimeter," Fire Testing Technology Ltd. in http://www.fire-testing. com/html/instruments/micro.htm.

(6.) UL 1581,page 181 for VW-1 flame test and 1,061 cable flame test.

(7.) V. Babrauskas, in SFPE Handbook of Fire Protection Engineering, Second Edition, National Fire Protection Association, Quincy, MA. 1996.

(8.) Metallocene-Based Polyolefins : Preparation, Properties, and Technology (hardcover), edited by J. Scheirs and W. Kaminsky, John Wiley (2000).

(9.) T. C. Chung, T. C. Mike Chung , "Functionalization of polyolefins', "Academic Press (2002).

(10.) G. Holden and N.R. Legge, "Thermoplastic elastomers based on polystyrene-polybutadiene block copolymers," in Thermoplastic Elastomers, A Comprehensive Review, edited by N.R. Legge, G. Holden and H. E. Schroeder, Hanser Publishers, 1987.

(11.) "Flame retardant materials, "edited by A.R. Horrocks and D. Price, Woodhead Publishing Ltd., Cambridge, England (2001).

(12.) "Fire retardancy of polymeric materials," edited by A.F. Grand and C.A. Wilkie, Marcel Dekker, 2000.

by Biing-Lin Lee, Darnell Worley, Phil Scadding, Ben Jones, Sachin Sakhalkar and Wilfred Giroux, Teknor Apex

Table 1--rating/classification of UL 94
vertical burn (ref. 8)

Criteria conditions                          V-0     V-1     V-2

After flame time for each individual        <10s     <30     <30
  specimen T1 and T2
Total after flame time for five specimens   <50s    <250    <250
  for any conditions set (T1 + T2)
After flame time, T2, plus afterglow        <30s     <60     <60
  time, T3, for each individual specimen
  after the second flame applications
After flame or afterglow of any               No      No      No
  specimen up to the holding clamp
Cotton indicator ignited by flaming           No      No     Yes
  particles or drops

T1: Flame out time for first flame application; T2: Flame
out time for the second flame application; T3: Glow time
after the second flame out.

Table 2--typical property values of enhanced
flame retardant TPE-S

Property                           EL-13928     EL-1934E     EL-1934F

RoHS compliance                         Yes          Yes          Yes
Specific gravity                        1.24         1.32         1.30
Hardness (instant), durometer A        A 86            82           73
Hardness, delay, durometer A              83           78           68
Tensile strength, psi                  1,540        1,480        1,220
Elongation, %                            540          630          680
Tensile strength retention *              99           98          105
Elongation retention *                    95           92           95
Tensile strength retention **            100           97          104
Elongation retention **                   95           87           92
Brittle point, [degrees]C                -51          -59          -60
Melt index, #                             14           14           20
Dielectric constant, 1 mega Hz          2.42         2.41         2.40
Dielectric dissipation factor,         0.003       0.0032        0.003
  1 mega Hz
Dielectric constant, 1 kilo Hz          2.43         2.43         2.41
Dielectric dissipation factor,        0.0029       0.0043       0.0027
  1 kilo Hz
Oxygen index, % oxygen                    28           30           27
UL 94 ##                              V-0 @        V-0 @        V-0 @
                                  0.060 inch   0.060 inch   0.060 inch

* 136[degrees]C seven days aging; ** 158[degrees]C seven days aging; #
200[degrees]C 5 kg, g/10 min.; ## thickness 0.060 inch; DC =
dielectric constant; DF = dielectric dissipation factor