EoW May 2010

technical article

Advances in TPE styrenic block copolymer compounding for UL flame retardant cable applications By Biing-Lin Lee, Darnell Worley, Phil Scadding, Ben Jones, Sachin Sakhalkar, Wilfred Giroux, Teknor Apex Company

Abstract Teknor Apex Company has undertaken research and development efforts in the TPE styrenic block copolymer compounding technologies to develop new RoHS-compliant enhanced flame retardant (FR) compounds. This paper will detail the performance of new compounds using different UL vertical burn test methods. EL-1392B (Shore A 86), EL-1934E (Shore A 82), and EL-1934F (Shore A 73) pass UL 94 V-0 rating at a thickness of 0.06". These compounds also pass UL 1581 VW-1 and method 1061 for cable flammability tests for insulation and jacketing. They 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. TPE compounds requiring fire retardancy are in wire and cable insulation, jacketing, and electronic devices. Styrenic thermoplastic elastomers (TPE-S) are also 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 [1] . Key requirements of these applications include requisite UL ratings, mechanical property retention after heat ageing, ease of processing and low temperature flexibility. Styrenic TPE-S compounds are particularly suitable for such applications. 1 Introduction The primary applications of

120mm x 13mm with different thicknesses, is positioned vertically and held from the top. The thicknesses of the test specimens are 3.2mm, 1.6mm and 0.8mm. Surgical cotton is placed 300mm 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 appli- cation of the flame follows immediately after self-extinguishment of the specimen from the first flame application. Table 1 summarises the criteria for V-0, 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, (iii) no combustible drips occur [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 [5] . However, the numerical values of T1 and T1+T2 are useful in the screening evaluation.

Formulations of flame retardant TPE-S to achieve UL 1581 VW-1 and method 1061 flame resistance require high levels of additives. However, high levels of additives tend to negatively impact the mechanical, physical and rheological properties of the FR compounds. This paper discusses Teknor Apex’s recent development of a series of high performance RoHS-compliant flame retardant TPE-S. The effort covers the manipulation of styrenic block copolymer (SBC) technology [2] , combined FR addi- tives technology [3] and fire science on the UL vertical burns. The use of these technologies enables the achievement of UL 1581 VW-1 and method 1061 in a compound with good properties. 2 Results anddiscussion 2.1 UL 94 Vertical burn The UL 94 test is designed to assess the flammability of plastic 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 shape specimen of plastic,

Table 1 ▼ ▼ : Rating/classification of UL 94 vertical burn [4]

Criteria conditions

V-0 V-1 V-2 <10s <30 <30

After flame time for each individual specimen T1 & T2 Total after flame time for 5 specimens for any conditions set (T1 + T2) After flame time, T2, plus afterglow time, T3, for each individual specimen after the 2 nd flame applications After flame or afterglow of any specimen up to the holding clamp Cotton indicator ignited by flaming particles or drops

<50s <250 <250

<30s <60 <60

No No

No

No No Yes T1: flame out time for first flame application; T2: flame out time right for the second flame application; T3: glow time after the second flame out.

60

EuroWire – May 2010