ENHANCING FIRE SAFETY WITH AN ADVANCED ZERO-HALOGEN POLYMER COMPOSITION FOR HEAT-SHRINK TUBING

ENHANCING FIRE SAFETY WITH AN ADVANCED ZERO-HALOGEN POLYMER COMPOSITION FOR HEAT-SHRINK TUBING

By: Sreeni Kurup, Principal Polymer Engineer, TE Connectivity, Swindon, UK

The development of an improved high-temperature, halogen-free polymer composition for wire and cable harness sleeving is critical for fire safety

Halogenated polymers have traditionally been used to make heat-shrink tubing that provides electrical and physical protection for wiring and cable harnesses. However, these polymeric materials are inadequate to meet fire safety requirements specifying “limited fire hazard” or for complying with environmental directives banning or limiting the use of halogen compounds that could produce toxic smoke if combustion occurs. Consequently, today’s major electronic and electrical product manufacturers desire halogen-free materials for cable jackets and tubing.

TE Connectivity (TE) has developed an advanced zero-halogen polymer technology for halogen-free heat-shrink tubing products that address weight, electrical, and fire-safety concerns. Branded as INSTALITE ZH-150, this advanced cross-linked polymer composition offers a high-temperature rating, fluid resistance, and mechanical performance similar to our market-leading DR-25 elastomer products, but with improved fire-safety characteristics. As a result, halogen-free ZH-150 products provide a compliant, high-performance alternative to using halogenated and non-halogenated flame retardants employed in conventional cable jackets and tubing materials.

There are two types of fire retardants used in tubing materials, both of which have different characteristics.

Polymeric materials are used extensively in everyday life due to their outstanding combination of physical properties and their ability to be processed into a wide variety of forms. However, most polymeric materials are composed of flammable organic material. This is unacceptable for many applications that require a flame-retarded product.

Traditionally, polymer flame retardancy is achieved by two principal methods.

1. a) Using Halogenated Flame Retardants

Bromine-based compounds are commonly used as flame retardant because they are very efficient. Brominated materials actively interfere with reactions in the gaseous phase of a fire via two processes:

• They chemically interfere and inhibit the exothermic oxidation reaction in the flame, which reduces energy generation that would feed the fire.
• They produce heavy bromine-containing gases that protect the polymer surface and hinder the flow of oxygen to the fire, thereby starving the fire.

The high efficiency of brominated flame retardants means that only a small amount of additive is needed to produce acceptable flame retardancy. As a result, the material can be flame retarded, but still maintain suitable physical characteristics with little reduction in mechanical performance and fluid resistance. Moreover, the high flame retardancy of brominated additives enables the polymer to be self-extinguishing, which inhibits flame propagation.

However, when polymers with brominated flame retardant additives burn, they produce large volumes of thick, toxic smoke. This is not suitable in applications where people must escape from a confined space.

Fluorineis a halogen found in polymers known as fluoropolymers. This type of material exhibits intrinsic flame retardancy and self extinguishes when removed from the heat source.  However, in a combustion situation, fluoropolymers can produce large amounts of highly toxic smoke and corrosive gases, which is unacceptable in occupied spaces.

1. b) Using Non-Halogenated Flame Retardants

Metal hydroxidesare halogen-free flame retardants that are widely used to meet fire-safety requirements in applications where people must escape from a confined space.

Metal hydroxides are non-toxic, readily available, and environmentally friendly.

The additive acts as a flame retardant by decomposing in a fire to produce metal oxides and water, both of which inhibit flame propagation. Metal hydroxide retardants help suppress a fire in several ways:

• They undergo a decomposition process in an endothermic reaction that removes energy from the fire.
• They produce water vapor that cools the surface of the polymer and dilutes the combustible gas concentration in the fire.
• They produce metal oxides that form a protective barrier on the surface of the polymer to protect the substrate from the flame.

Unfortunately, these mechanisms are not as efficient as brominated flame retardants. Therefore, non-halogenated flame retardants must be used in much higher concentrations. High additive concentrations have a detrimental effect on the performance of the polymer system, leading to reduced resistance to high temperature and reduced flexibility at low temperatures.

Nevertheless, in the critical area of fire safety, metal hydroxide additives can be used to formulate flame-retarded polymer systems. Products using this system can slow the spread of fire and generate low toxicity by-products with minimal smoke, giving people time to escape.

In application, metal hydroxide systems allow electrical designers to meet fire-safety requirements for applications where people have limited routes of escape, but with the tradeoff of reduced product performance.

An advanced zero-halogen cross-linked polymer tubing–INSTALITE ZH-150–overcomes the fire safety and performance issues of conventional polymer conventional used for heat-shrink tubing.

Based on the latest developments in material science, TE has formulated a proprietary, cross-linked polymer composition that exhibits excellent physical and fire-safety properties for a variety of heat-shrink tubing applications (Figure 1). Branded as INSTALITE ZH-150, this material provides mechanical and electrical protection similar to existing high-temperature tubing (Table 1). ZH-150 offers fluid resistance, low- and high-temperature performance (-75C to 150°C) and low smoke output in a fire. It is also lightweight, easy to install, and available in a wide range of dimensions (Table 2) to reduce installation time and cost. The critical advantage is that ZH-150 is a zero-halogen protection system that is suitable for use across multiple environments. Typical market applications include aerospace, military, marine, rail and mass transit wherever fire safety cannot be compromised.

Summary: ZH-150 tubing provides enhanced fire-safety and application advantages.

Because using halogenated additives has become less acceptable, many industry specifications now require zero-halogen systems. This makes ZH-150 tubing a practical alternative to conventional products. Moreover, ZH-150 tubing can meet the demands of applications that must withstand temperature extremes (-75°C to 150°C/-103°F to 302°F), while remaining flexible and fluid resistant.

By design, the best properties of ZHTM zero-halogen polyolefin and DR-25 elastomer are combined in the cross-linked polymer formulation used in ZH-150 tubing.

Designers find ZH-150 tubing offers valuable advantages that include:

• Flame retarded, zero-halogen material
• Operates to 150°C (302°F) continuous exposure
• Flexible at low temperature down to -75°C (-103°F)
• Resistant to many aerospace and military fluids
• Lightweight and very flexible
• Mechanically tough, rugged, and abrasion resistant

Workable heat-shrink parameters: Shrink ratio: 2 to 1; minimum shrink temperature: 150°C (302°F); minimum full recovery temperature: 175°C (347°F)

Table 1:A comparison showing  how ZH-150 incorporates  the best features of two existing
TE Connectivity products: ZHTM (zero-halogen) and DR-25 (high temperature, fluid resistant)

Table 2:Available dimensions of INSTALITE ZH-150 tubing