EoW May 2010

technical article

2.3 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, compatible with polyolefins and mineral oils, is available. Furthermore, due to the recent advances in polyolefin process and catalyst technology, a broad range of polyolefins can extend the service temperature range [8, 9] . The domain microstructure of SBC also affects the melt strength and processability [10] . The combination of hydrogenated SBC rheology and polyolefin technology is a building block for high performance flame retardant compounds with a unique balance of properties including excel- lent 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 polyolefin can be developed for use where UV resistance, high service temperature (eg 105˚C temperature rating), low service temperature (eg brittle point < –50˚C), and processing stability are essential. Hydrogenated SBC-based flame retardant TPEs can be formulated to cover a wide range of hardness from Shore A 50s to Shore D 60s. 2.4 Flame retardants There are several different categories of flame-retardants, the most diversified class being of those containing halogen. A broad range of brominated and chlorinated flame-retardants are commercially avail- able. Fully brominated aromatics are generally used in resins with a relatively high processing temperature [11,12] . Recent efforts in the development of new flame-retardants have shifted toward phosphorous and other inorganic hydroxide halogen-free systems. In this paper the choice of polymers and a combination of flame retardant tech- nologies results in an RoHS-compliant flame retardant TPE. The effect on performance when com- bining FR technologies is a modification in rheology and burn characteristics with minimal effect on physical properties. The observed changes are demonstrated in Figures 3 , 4 and 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.

flame. The flame source is a Tirrill burner (similar to a Bunsen burner) with a heat output of approximately 500W or 1,700 Btu/h. The flame is applied for 15 seconds and reapplied four times, each time after the wire ceases to burn. 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 [6] . 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 com- bustible materials in its vicinity during, between, or after a one-minute application of a standard test flame. The standard test flame is nominally 125mm high and produces heat at

UL 94 5T1 versus wt% of Flame Retardant

Candidate composition

5T1 seconds

Wt% of flame retardant

Figure 1 ▲ ▲ : UL 94 5T1 versus wt% of flame retardant

UL 94 5(T1+T2) versus wt% of Flame Retardant

Series1

Candidate compositions

5(T1+T2), seconds

Wt% of flame retardant

Figure 2 ▲ ▲ : UL 94 5(T1+T2) versus wt% of flame retardant

Figure 1 plots 5T1, the sum of T1 for five tested samples according to the UL94 procedure, versus the weight % of a flame retardant. The sample thickness is 0.125". 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. 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". 2.2 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 of 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

the nominal rate of 500W or 1,700 Btu/h. The flame is applied three times, for one minute each time. The period between flame applications is to be 30 seconds; regardless of whether flaming of the specimen ceases 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 [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 the National Institute of Standards and Technology (NIST) [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 required the use of cone calorimetry testing.

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EuroWire – May 2010