Flame Retardant Label Materials.. What s it all about? Dr. Jim Williams, CTO Polyonics, Inc. Flame retardant fabrics for curtains, bedding and other home use makes sense. Obviously flame retardant fabrics and uniforms for firefighters, astronauts, and even race car drivers makes sense as well. Flame retardant fabric, foam and other materials used inside trains, planes, and automobiles is also logical. But why the big push for flame retardant materials to be used inside consumer electronics, such as computers, cell-phones, and the like? Consider the wide array of batteries used in today s portable electronics. Photograph 1 displays a sampling of the their many shapes and sizes: Photograph 1. Batteries for portable electronic devices. We all know that they are low voltage devices, compared to electrical devices used at home, which rely on standard 110-220 volt electrical service. So, what s the big deal for 1 or 2 volt devices? News headlines from the past few years say it directly and succinctly: Toshiba recalling 830,000 laptop batteries as embarrassment for Sony grows iphone Catches Fire While Charging? Nokia says 46 million phone batteries could overheat Over 200,000 Wii battery recharging stations recalled Wal-Mart recalls 1.5 million DVD players Sony battery recall: 100,000 Sony Batteries Used in Laptops for Fire Hazard NOTE: A different recall than cited above What a different picture is depicted in Photograph 2a and 2b. Photograph 2a. Burning battery POLYONICS, INC 1
Photograph 2b. Result of a battery fire in the personal device Today s consumer products are constructed from a wide variety of plastic materials (more correctly polymeric materials or polymers), which enhance product durability, lower a product s weight, help decrease costs, and enhance aesthetics, among many other useful properties. Plastics are all around us, and will continue to be so in the foreseeable future. It is widely accepted by many people, that plastics don t burn. So what s all the fuss about? Contrary to this urban myth, when plastics are exposed to high enough heat, they may give off FLAMMABLE GASES which will burn. How high is high enough? It depends on the type of plastic. The fact is Plastics can burn..if they are heated hot enough to decompose them. The good news is that this ability to burn can be slowed down (or retarded), and in some cases quenched altogether. A whole field of science is dedicated to flame retardant plastics and polymer materials. Burning occurs when a flammable gas and oxygen are mixed in proper proportions, and then exposed to a flame source (or spark). If these gases (or vapors) are not present, or if oxygen levels are too low, then ignition cannot occur. One indication of a material s hazard level (i.e. ability to burn) is measured by what is known as its flash point The flash point of a chemical is the lowest temperature where enough fluid can evaporate to form a combustible concentration of gas. The lower the flash point is, the more the hazard of burning becomes. Table 1 shows the flash points of several materials, measured under normal ambient conditions: Table 1. Flash Points( F) of Selected Materials Material Flash Point ( F) Propane -76 to -117 Gasoline -45 Kerosine 100 to 162 Motor Oil 420 to 485 Polyethylene Film Not Applicable, Decomposition This means that motor oil must be heated up to the 420-485 F range, before it can be ignited with a flame. In general, plastic materials do not exhibit the hazards of liquid materials, because they do not give off these vapors (or fumes, known as volatiles ) under ordinary, everyday (or ambient ) conditions. But wait a minute, you say. we both know that we can burn polyethylene film. True, but you must remember that solids don t burn gases do. When you put polyethylene film POLYONICS, INC 2
into a flame, it starts to smoke (i.e. generates vapors or gases), and then the smoke ignites. Burning a plastic (or polymeric) material first means that gases must form. These gases must then mix with oxygen in the right proportion so that a flame will ignite the gaseous/oxygen mixture. In addition, the flame must also generate enough heat to continue to continue to decompose more of the polymer, by forming additional vapors to mix with yet more air, in order to continue burning. This continuous cycle is known as the Combustion Cycle. Combustion of Polymeric Materials The combustion of a polymeric material is a complex process involving a series of interrelated and/or independent stages occurring in the solid (or condensed) phase, the gas phase, and at the interface (surface) between the two phases. The most critical stage occurs at the fuel production stage, where an external heat source increases the temperature of the polymer, resulting in degradation of the chemical bonds and subsequent evolution of volatile, or gaseous, fragments (see Figure 1). These volatile fragments diffuse into the surrounding air to create a flammable mixture with the oxygen, which can then ignite at the right concentration and temperature. Flaming combustion can occur if the exothermic gas-phase combustion-reaction (BURNING) generates sufficient heat-energy, (vs other types of reactions) which transfers back to the solid phase/polymer surface. The ensuing heat further decomposes the polymer, thus producing more fuel, thereby maintaining the combustion cycle (Figure 1) Figure 1 The combustion cycle of organic polymeric materials. Non Flammable Gases And Char Oxygen COMBUSTION Products of Combustion Flammable H Gases E A T < Oxygen POLYMER Thermal Degradation HEAT There are several ways to interrupt the burn cycle: (1) Minimize the thermal decomposition of the polymer; (2) Quench the flame; or, (3) Reduce the heat transferred from the flame back to the decomposing polymer. Flame Retardants Successful strategies to reduce the flammability of a material involve breaking the complex combustion process at one or more stages to reduce the rate of combustion (which reduces the POLYONICS, INC 3
heat generated), and/or to change the mechanism. A flame retardant interferes with one or more steps of the combustion cycle, which include heating the polymeric material, subsequently degrading it to form volatile gases, and further combusting them. There are two ways to interrupt the burn cycle. One method, solid phase inhibition, directly involves the polymer substrate. In this method, a flame retardant inhibits combustion by forming a glass-like coating, or char, preferably of low thermal conductivity, on the surface of the material exposed to heat. The coating layer consists of highly cross-linked carbonaceous residue from the initial burning step that insulates the underlying polymer from the heat of the flame, thus inhibiting the production of new fuel. In addition, the retardant may also undergo endothermic degradation, i.e. absorb heat, which means that it absorbs the energy needed to maintain the flame. The second method of interrupting the flame cycle, vapor phase inhibition, involves changing the flame s chemistry. Most theorists agree that combustion progresses by means of chemically reactive free radicals which form as the polymer thermally decomposes. As they react, they generate additional heat from the heat of reaction. A flame retardant chemical, while it is thermally decomposing, may be transformed into a volatile free-radical inhibitor, which can deactivate the highly active combustion propagating radicals that result from de-polymerization during the combustion process, to quench the flame by breaking the chain reaction (known as chain termination ). Flame retardants are classified into two main categories: Additives, which are mechanically blended during formulation of the adhesive and coating, then coated onto the polymeric substrate as a coating or as an adhesive; and, Reactives, which are chemically bonded to the polymers (or as an integral part of the polymer backbone) used in the adhesives and coatings, either by copolymerization or by modification of the parent polymer. Although often cheaper and more widely used, additives can contribute more detrimental side effects than do reactives. For example, simple additives do not become an integral part of the polymer matrix, so the material s physical and mechanical properties may be adversely affected, including printability and chemical resistance. In addition, the additives may leach out of the plastic over time, either by blooming to the surface, by exposure to chemical or solvent washing, or by physical abrasion. Flame retardant additives used with synthetic polymers include organic phosphorus compounds, organic halogen compounds, and combinations of organic halogen compounds with antimony oxides. Inorganic flame retardants include hydrated alumina, magnesium hydroxide, borates, among others. The commercial market for flame retardants is presently dominated by compounds containing halogens notably bromine and chlorine. Most organic halogen compounds(those containing chloride or bromide) are vapor phase inhibitors that decompose to yield HBr or HCl (hydrobromic or hydrochloric acid, respectively), which quench propagating free radical reactions in the flame. Furthermore, in the solid state, some halogen acids catalyze char formation, particularly with poly-olefins. Combining antimony trioxide, Sb 2 O 3 or antimony pentoxide, Sb 2 O 5 (so-called synergists ), in combination with organic halogen compounds is an even more effective vapor phase free-radical inhibitor than the halogen alone. Antimony oxides react with the respective organic halogen compound producing antimony tri-halide, which carries halogen (chloride/bromide) into the flame, where it is released as hydrogen chloride (or bromide, respectively). These are the acids referred to earlier. The end product of antimony is thought to be antimony oxide in finely divided form in the flame. The use of halogenated compounds has one major drawback: they increase the amount of smoke and toxic decomposition products evolved during polymer combustion. The use of halogenated materials also gives rise to the additional hazard of strongly acidic gases, for POLYONICS, INC 4
example, HCl and HBr, which can be liberated upon heating. These gases cannot only cause lung damage but can also corrode electrical equipment rendering it in-operative. As a result of these drawbacks, there has been increasing research to develop innovative, environmentally friendly, halogen-free flame retardants. These flame retardants, on the other hand, are both non-toxic and very effective. Thus, they are presently the most desirable class of flame-retardant additives; finally, they can be either organic or inorganic. This new generation of flame retardants act by solid phase inhibition, decomposing to form and to promote carbonaceous char formation. Moreover, in some, reaction cooling is accomplished by endothermic reaction, while a few can also act as vapor phase free radical inhibitors. The inhibition mechanism of the most common inorganic flame retardant, aluminum trihydrate (ATH), is well understood. When exposed to temperatures above 250 C, ATH loses water by dehydration. This reaction is strongly endothermic, consumes thermal and radiant energy from the flame, and slows the rate of thermal decomposition, or pyrolysis, of the substrate. In addition, the vaporized water acts as an inert diluent and cools the flame, reducing the heat flux back to the surface of the substrate. Following dehydration, the alumina residue itself acts as a coating on the surface of the substrate, insulating it from further heating. However, the drawback of using ATH is that a relatively large amount must be added for it to be an effective flame-retardant, which lowers physical and chemical resistance properties of the polymer surface. Commercially, flame retardancy of polymeric materials is still largely implemented by the use of additives because of cost. However, increasing research is now being directed toward the use of reactives (i.e. incorporation of the flame retardant material directly into the polymer backbone) to augment flame resistance(i.e. stop the polymer from burning) rather than just retardance, or slowing it down. It is thought that the incorporation of reactives will be more effective and longlasting, as well as being less detrimental to the material s original mechanical properties. Methods for Testing Flammability As a consequence of the complex nature and poor reproducibility of an actual fire event in the laboratory, there are many techniques for estimating the flammability of polymeric materials, depending upon the importance of different fire properties. These factors include: Ease of ignition, Rate of flame spread, Rate of heat release, Fire endurance, Ease of extinction, Smoke and particulate release, and Toxic gas evolution. For the purposes of this discussion we will focus on the generally accepted tests put forth by Underwriters Laboratories, or UL. UNDERWRITERS LABORATORY TESTS UL has categorized three major categories of Burn tests, commonly referred to as UL 94 Flammability Tests, as depicted in Figure 1 (Surface Burn, Vertical Burn, and Horizontal Burn): Figure 1. POLYONICS, INC 5
Given that hundreds of different polymeric materials are available for use in thousands of different ways and in different products, there is no single test that will suffice to measure the suitability of a given material for every use. However, UL has developed a classification system which rates the severity of three broad environmental classifications. A surface burn (UL 94 5VA and 5VB, respectively) tests the resistance of a material which is exposed to flame on one side only (i.e. a single surface). The vertical burns (94 V-0, V-1, and V-2) are more severe, since the burning area is at the bottom of the strip of material, allowing the heat to interact with the material above the flame, causing the generation of additional flammable gases (remember the combustion cycle, discussed previously). And finally, the Horizontal burn which measures the rate of burning along the length and width of the plastic sheet or film. Table 2 describes the specific characteristics a burning material must have for it to achieve its appropriate UL 94 flammability designation. Table 2. UL 94 Flammability Ratings Description Surface Burn 5VA Burning stops within 60 seconds after five applications of five seconds each of a flame (larger than that used in Vertical Burn testing) to a test bar. Test specimens MAY NOT have a burn-through (no hole). This is the highest (most flame retardant) UL94 rating. Surface Burn 5VB Burning stops within 60 seconds after five applications of five seconds each of a flame (larger than that used in Vertical Burn testing) to a test bar. Test specimens MAY HAVE a burn-through (a hole). Vertical Burn V-0 Burning stops within 10 seconds after two applications of ten seconds each of a flame to a test bar. NO flaming drips are allowed. Vertical Burn V-1 Burning stops within 60 seconds after two applications of ten seconds each of a flame to a test bar. NO flaming drips are allowed. Vertical Burn V-2 Burning stops within 60 seconds after two applications of ten seconds each of a flame to a test bar. Flaming drips ARE allowed. Horizontal Burn H-B Slow horizontal burning on a 3mm thick specimen with a burning rate is less than 3"/min or stops burning before the 5" mark. H-B rated materials are considered "self-extinguishing". 94HB Horizontal Burn Test 94HB is generally accepted as the minimum flammability test a material must pass for UL recognition. Generally, a 94HB rating is assigned to a material if it passed any of the V tests. The 94HB rating is generally suitable for attended, portable, intermittent-duty, household appliance enclosures, such as a hair dryer. Refer to Figure 2 for the experimental setup of a typical horizontal burn test. A 1/2" x 5" sample is clamped on a ring stand. Marks are made on the sample 1" and 5" from the free end. A flame is applied to the sample for 30 seconds or until the sample burns past the 1" mark. The POLYONICS, INC 6
sample is allowed to burn until it stops or reaches the 5" mark. If the sample burns up to the 5" mark, a burn rate is calculated. If the sample stops burning before the 5" mark, the burn time and the length of the damaged section between the marks is reported. A material that is less than 0.118" receives a 94HB classification if it burns at a rate of less than 3" per minute or stops burning before the 5" mark. Three samples are tested. If only one of them fails, another set of three are tested. All must pass for the material to receive 94HB recognition. Figure 2. Experimental Setup for Horizontal Burn Tests 94V Vertical Burning Test Figure 3 depicts the setup for vertical burn testing. This vertical burn test has three flammability classifications: 94V-0, 94V-1, and 94V-2. These ratings are generally thought to be important for safety ratings of an unattended, portable, intermittent-duty, household appliance enclosure, such as a coffee maker. The left side of Figure 3 depicts POLYONICS, INC 7
the experimental setup for 94V flammability tests. During the test, the 1/2" x 5" sample of material is held in the vertical position, directly over a ball of cotton. A burner flame is then applied to the free end of the sample, for 10 seconds. Immediately after the flame is removed, and the flaming combustion stops, a second burn is done, for an additional 10 seconds. Two sets of five specimens each are tested. The following values are recorded. Duration of flaming combustion after the first burner flame application. Duration of flaming combustion after second burner flame application. Duration of glowing combustion after second burner flame application. Whether or not flaming drips ignite cotton placed below specimen. Whether or not specimen burns up to holding clamp. The side by side images below show the importance of the so-called burning drips. On the left hand side it shows what happens as the plastic melts, decomposes, and burns, simultaneously (i.e. a burning drip is formed). The right hand side is Polyonics XF-603 material that self extinguishes right away. Table 3 summarizes the criteria for each of the vertical burn 94V ratings: Table 3. Criteria Conditions for 94V Ratings 94V-0 94V-1 94V-2 Total flaming combustion for each specimen 10s 30s 30s Total flaming combustion for all 5 specimens of any set 50s 250s 250s Flaming and glowing combustion for each specimen after second burner flame application 30s 60s 60s Cotton ignited by flaming drips from any specimen NO NO YES Glowing or flaming combustion of any specimen to holding clamp NO NO NO 94VTM Vertical Thin Material Test A material can be too thin for the standard 94V test because it may distort, shrink, or flex during the burn test. There is another similar test for these thin (less than 0.010" ) materials. POLYONICS, INC 8
Refer to the right side of Figure 3. An 8" x 2" sample is wrapped around a 1/2" mandrel, and then taped on one end. The mandrel is removed, leaving a cone-shaped sample that is relatively rigid. The two flame applications are 3 seconds each, instead of 10 seconds each. All of the other criteria from the 94V test apply, except that no specimens can have flaming or glowing combustion up to a mark 5" from the bottom of the sample. OTHER ASTM TESTS There are many other flammability tests and standards available. The most widely used ones are briefly discussed here. Oxygen Index Test (ASTM D 2863) One of the most widely used laboratory tests for evaluating the Ease of Ignition (or, Ignition Test) is the limiting oxygen index (LOI) (ASTM D-2863), a very convenient and reproducible test. This tests the minimum concentration of oxygen in a pure oxygen / nitrogen environment that is necessary for the sample to maintain combustion. The sample is hung vertically in a special chamber, and then ignited. The oxygen concentration is then reduced until the combustion is just maintained. The Limiting Oxygen Index (LOI) represents this percentage of oxygen which will maintain combustion. The higher the LOI value for a film or label means the less flammable that it is, i.e. it needs higher levels of oxygen to burn. If the oxygen level drops below the LOI, then the flame extinguishes, for lack of oxygen. LOI has become a standard test for rigid plastics and for fabrics and films. Although the LOI is often referred to as an ignition test, the ignition parameters are not rigidly controlled, and the sample needs to burn for at least 3 minutes after sustained ignition under the oxygen amount. It is also a measure of ease of extinction (i.e. extinguishing the fire). Flammability Test (ASTM D 568 for flexible plastics and D 635 for self-supporting plastics) ASTM D 568 supports the sample vertically, whereas D 635 supports the sample horizontally. A flame from a Bunsen burner is exposed to a plastic test bar for 30 seconds. The sample is allowed to burn until it either extinguishes itself or burns past a gage mark (100 mm) on the bar. If the sample does not burn past the 100 mm gage mark, time and extent of burning are reported. If it burns past the gage mark, and average burn rate, in centimeters per minute, is reported. Materials that do not burn to the gage mark are said to be self-extinguishing. Radiant Panel Test (ASTM E 162) A radiant panel is maintained at 670 C (1238 F) as a heat source to ignite a plastic sheet. The plastic sheet (152mm x 457mm, 6" x 18") is maintained at a set distance from the panel, with the top tilted at a 30 angle toward the panel. The rate of burning and the heat evolved in the burning are measured and combined to form a flame-spread index. Smoke Density Test (ASTM D 2843) This test measures the loss of light transmission through smoke produced from a burning plastic. A sample is burned inside of a special chamber. A light is passed between two photoelectric cell plates, and the light transmission is plotted against time. The area under this curve is the total smoke produced. There are literally hundreds of industry-specific flammability standards, ranging from mattresses, materials used for automobile interiors, label materials for wire and cable marking in aircraft and mass transportation, and many more. Different trade associations for different industries will give you access to pertinent fire and flammability standards which are appropriate for their respective and oftentimes unique requirements. A starting point list for internet searching is listed below: POLYONICS, INC 9
www.astm.org/standards/fire-and-flammability-standards.html An exhaustive listing www.nhtsa.gov National Highway Traffic Safety Administration www.nfpa.org National Fire Prevention Association.many how to guides standards Specific additional standards include FMVSS302 which details the requirements for materials used in the interior of automobiles; and BMS 1347J which specifies testing for label materials used by Boeing in wiring harnesses. Each large corporation involved in transportation, aerospace, and electronics (among many others) will refer to flammability standards which they require for materials used in products they manufacture. About the author: Dr. Jim Williams is Polyonics founder, Chairman, and Chief Scientist. His career spans more than 30 years of product and process development using identification technologies for marking customers products. He has specialized in marking technologies which will provide an image on a product, which will withstand the harshest industrial manufacturing environments and also will comply with customers requirements and specifications. For more information, email; jim.williams@polyonics.com. POLYONICS, INC 10