The objective of flame retardant thermoplastics is to increase the resistance of a material to ignition and, once ignited, to reduce the rate of flame spread. The product does not become non-combustible, but the use of a flame-retardant additive may prevent a small fire from becoming a catastrophe.

In most cases, decomposition occurs via free radical chain reactions, initiated by traces of oxygen or other oxidizing impurities, which are trapped in all plastics during manufacture. The oxidative degradation of polymers usually proceeds via the formation of hydroperoxide groups whose decomposition leads to highly reactive species such as H & OH radicals and thus to chain branching. These radicals are responsible for flame spread in the combustion process.

 Ignition
The flammable gases formed by pyrolysis, mix with atmospheric oxygen, and reach the lower ignition limit and are either ignited by an external flame or, if the temperature is sufficiently high, self-ignite. The flash ignition temperature (FIT) and self-ignition temperature (SIT) are given in Table 2.

Table 2: FIT and SIT of Some Thermoplastics.

Ignition depends on numerous variables such as oxygen availability, temperature, physical and chemical properties of polymer. The reaction of the combustible gases with oxygen is exothermic and if sufficient energy is available, overrides the endothermic pyrolytic reaction and initiates flame spread.

Flame Spread
The exothermic combustion reaction reinforces pyrolysis of the polymer by thermal feedback and fuels the flame at an increasing level.
Another factor, which determines the extent of flame spread, is the heat of combustion of the polymer. The heat of combustion of various polymers are compared with those of cotton and cellulose in Table 3.
There is no correlation between the heat of combustion and combustibility of a material. An example of this is extremely flammable celluloid, which has a heat of combustion of only 17,500 kJ/Kg.
Concurrent with the extremely rapid gas phase reactions, various slower oxygen- dependant reactions also take place. These give rise to soot and carbon-like residues and take place partly in a condensed phase with glow or incandescence.

Burning Behavior of Polyolefins
Polyolefins burn hesitatingly at first with a small bright blue flame (PE LD and PE HD) and subsequently, with a bright yellow flame, which continues to burn after removal of the ignition source. The fire gases and smoke vapors smell of wax and paraffin. This odor is pungent in the case of PP. After the flame is extinguished a smell of dead candle remains. In the absence of oxygen, PE starts to degrade thermally at about 300 Deg C. In the presence of Oxygen, thermal degradation, thermal degradation sets in at 150 Deg c with the color changing from white through brown to black. PP undergoes thermal degradation more easily than PE particularly when oxygen is present.

Flame Retardant Polyolefins
The primary additives used to accomplish the objective of imparting flame retardancy to Polyolefins are halogens and phosphorus containing organic compounds. Antimony oxide is generally required as a synergist for halogen compounds. Inorganic compounds containing high concentration of water of hydration such as alumina trihydrate and magnesium hydroxide are also used. The type of flame retardant and quantity needed for polyolefin applications are largely governed by cost/performance ratio.

Manufacture of Flame Retardant Polyolefins
World over flame retardant polypropylene compounds are finding increasing use in injection molding and fiber applications. In this application note, we would focus only on FR/PP compounds for injection molding applications.
FR/PP compounds are best made on a twin – screw compounding extruder having a l/d of 40:1 and equipped with side feeder arrangement for dosing FR additives. Gravimetric dosing units are needed for accurate metering of FR and other additives in to the polymer matrix.
Tailor-made formulations can be prepared depending on end –user specifications.
For injection molding application, a typical temperature profile in the injection-molding machine is

This temperature profile is subject to change depending on the flow length/thickness ratio of the product and capacity of the machine. FR additives affect the metal surface in the hopper, barrel and screw of the compounding and molding machines. Hence it is recommended to purge the machines with high flow LLDPE/LDPE or with any commercially available purge resins.

Testing of Flame Retardant Plastics
Flame retardant Polyolefins are frequently designed to meet specific flammability tests. The laboratory tests used most frequently for thermoplastics are described here.

Ease of Ignition: Oxygen Index
Ease of ignition may be defined as the facility with which a material or its pyrolysis products can be ignited under given conditions of temperature and oxygen concentration. This characteristic provides a measure of fire hazard.
ASTM D 2863-77 describes the test protocol for measuring oxygen index.

The oxygen index test employs a vertical glass tube 60 cm high and 8.4 cm in diameter, in which a clamp at its bottom end holds a rod or strip specimen vertically. A mixture of oxygen and nitrogen is metered into the bottom of the tube, passing through a bed of glass beads at the bottom to smoothen the flow of gas, providing a specific environment for the sample. The sample is then ignited at its upper end with a hydrogen flame, which is then withdrawn. The sample then burns like a candle from the top down. (Figure 2). The atmosphere that permits steady burning is then determined.

The oxygen index or the limiting oxygen index, is the minimum percent of oxygen in an oxygen-nitrogen mixture that will just sustain burning for 2 inches or 3 minutes, whichever comes first.

2. Flammability Testing for Electrical and Electronic Materials UL94
The most widely accepted flammability performance standards for plastic materials are UL (94) ratings designed by Underwriter's Laboratories Inc., USA . These ratings are intended to provide an indication of a material's ability to extinguish a flame once ignited. Several ratings can be applied based on the rate of burning, time to extinguish ability to resist dripping and whether or not drips are burning.

Each material tested may receive several ratings based on color and/or thickness. When specifying a material for an application, the UL rating should be applicable for the thickness used in the wall section in the plastic part. The UL rating is "always" reported with the thickness. UL rating reported without specifying thickness is insufficient and can be misleading.

The UL-94 has four separate sections containing different test methodologies and these are described hereafter: -

UL 94 HB Horizontal Burning Test Procedure
In general, HB materials are not recommended for electrical properties except for mechanical and/or decorative purposes. Sometimes misunderstood: materials that are not meant to be FR materials) do not automatically meet HB requirements. UL 94 HB is, although the least severe of flammability classification has to be checked by testing

Test Protocol

The test places a sample 127mm (5.0 inches) long and 12.7mm (0.5 inches) wide in a horizontal position over a standard Bunsen burner. The test measures burn rate in mm/min or inches/min.

A sample, should not have a burn rate exceeding 76mm/min for thickness less than 3mm

A schematic of the test protocol is shown in Figure 3.

•  UL 94 V0, V1 & V2 Vertical Testing Procedure

The vertical tests (Figure 4) take the same specimens as are used for the HB test. Burning times, glowing times, when dripping occurs and whether or not the cotton beneath ignites are all noted. Framing drips, widely recognized as a main source for the spread of fire or flames, distinguish V1 from V2

Test Protocol

A sample 127mm (5.0 inches) long and 12.7mm (0.5 inches) wide is suspended vertically over a Bunsen burner flame. The thickness generally adopted is 0.8mm, 1.6mm or 3.2mm. The distance between the test specimen and cotton beneath is 300mm or 12 inches. A 20mm methane flame is applied for two, ten second ignitions. Different classifications are accorded depending on the burning characteristics of the material.

•  A V0 material may not burn for over 10 seconds after the removal of the flame. It may also not have flaming drips which can ignite the cotton placed beneath the test specimen
•  A material rated V1 may burn for a period not exceeding 30 seconds and not have any flaming drips
•  A material rated V2 may also not burn over 30 seconds, but may have following drips which ignite the cotton
•  A material which does not pass any of these categories is rated as " Failure "

UL94 5V, 5VB & 5VA Ratings

These are high performance ratings and are not considered here in this paper

Summary

A summary of the UL 94 rating categories is provided below: -

•  Flame Spread.
Flame spread, or the rate of travel of a flame front under given conditions of burning, is a measure of fire hazard. The spreads of flame along the surface of a material can transmit fire to more flammable materials in the vicinity.

The Underwriters Laboratories 25 ft.tunnel test developed by Steiner is perhaps the most widely accepted test for surface flame spread. The test requires a specimen 25ft long and 20 in. wide, mounted face down to form the roof of a 25 ft-long tunnel 17 ½ in. wide and 12 in. high. The fire source, two gas burners 1 ft from the fire end of the sample of select – grade red oak flooring would spread flame 19 ½ ft from the end of the igniting fire in 5 ½ Minutes +/- 15 sec. The end of the igniting fire is considered as being 4 ½ Ft from the burners, this flame length being due to an average air velocity of 240+/- 5 min. Flame spread classification is determined on a scale on which asbestos – cement board is zero and select-grade red oak flooring is 100.

•  Smoke Measurements.

Smoke or smoke density is defined as the degree of light obscuration produced from the burning of a material under a given set of combustion conditions. This characteristic provides a measure of fire hazard in that occupants have a better chance of escaping from a burning structure if they can see their way. ASTM E662-83 “Specific Optical Density of Smoke Generated by Solid Materials” is used to determine the smoke density characteristics of a material under controlled laboratory conditions.

The smoke density chamber test is used to determine the specific optical density of smoke generated within a closed chamber due to non-flaming pyrolytic decomposition and/or flaming combustion. The non-flaming mode employs an electrically heated radiant energy heat source with an irradiance level of 2.5W/cm^2. For the flaming mode, a six –tube burner, fuelled with a mixture of propane and air, is used in combination with the radiant heat to apply a row of equidistant flamelets across the lower edge of the specimen and into the sample trough.

Light transmission measurements are used to calculate the specific optical density, which is derived from a geometrical factor associated with the dimensions of the test chamber and specimen, and the measured optical density, a measurement characteristic of the concentration of the smoke. The photometric scale used to measure the smoke generated in this test is similar to the optical density scale for human vision.

(By Product Application & Research Centre, Reliance Industries Limited, Mumbai)