Flame retardants update: phosphorus expands its niche.

Lower corrosivity and smoke obscuration spur interest in phosphorus-based additives to replace or even accompany halogenated flame retardants.

Phosphorus-based flame retardants appear primed to break out of their traditional niche and penetrate a broader spectrum of thermoplastics and thermosets. Increasing market interest in reducing smoke obscuration and corrosivity are helping to create opportunities for phosphorus additives as alternatives or complements to standard halogenated compounds. Poised to exploit these opportunities are new-generation products designed to overcome some of phosphorus' previous drawbacks.

Smoke-toxicity concerns about brominated and chlorinated flame retardants emerged in the 1980s as one of the key initial factors driving the development of non-halogenated flame retardants, including phosphorus types, but most additive suppliers say that is now somewhat of a lesser issue. Explains Ray Mount, Monsanto marketing/technical-service technologist, "In many ways, smoke toxicity is the least important issue, as everything you burn will result in some toxics being released. Instead, two other issues are more important: acid corrosivity in smoke that results from halogenated flame retardants and can affect electrical systems; and smoke density that obscures vision, as smoke with halogens is much heavier than that with phosphorus-based materials."

Joe Green, FMC's principal scientist for phosphorus products, explains that phosphorus-based flame retardants can function in both the vapor phase and condensed (solid) phase, while purely halogen-based compounds are effective only in the vapor phase. "In the condensed phase, phosphorus can act as a char former or as a glassy phosphate layer," Green says. Char formation--enhanced by the intumescent (foaming) effect of some phosphorus additives when exposed to fire--serves to keep oxygen away from the combustible substrate, insulates the substrate from heat, and holds in combustion gases.

Another issue in favor of phosphorus-based additives is that no antimony oxide is required as a synergist. Says Monsanto's Mount, "Antimony oxide is difficult to handle and, especially in Europe, there is a trend to move away from it due to health and safety concerns."

Additive suppliers also note that including phosphorus in the formulation can help reduce levels of other FR additives that act as fillers (i.e., nonmelting at process temperatures), thereby improving physical properties of flame-retardant compounds. Unlike most halogenated additives, many phosphorus-based products come in liquid or low-melting solid form. According to Charles Ludwig, associate v.p. at Great Lakes Chemical, "You get some plasticizing with phosphorus-based systems that's advantageous, as they blend well in the extruder. We feel phosphorus can offer the greatest potential when used in combination with bromine in place of antimony oxide, and that it will, in certain polymer systems, act as a compatibilizer, flow enhancer, or processing aid."

Although some of these arguments are fostering substitutions of phosphorus for halogen systems, in many cases plastics processors are finding that combinations of the two are most cost-effective and help minimize the drawbacks of either. That helps explain why Great Lakes Chemical, a heavyweight in brominated additives, is now also moving into phosphorus compounds. "We see people more and more willing to experiment with phosphorus/bromine combinations," Ludwig reports. And FMC, a specialist in phosphate esters, has been getting more and more into bromine/phosphorus chemistry in recent years. FMC scientist Green views halogen/phosphorus as the "best combination" with the most potential, adding that FMC is now seeing a lot of interest from engineering thermoplastics formulators. The company is expanding its product line of bromine/phosphorus combinations aimed at styrenics, olefins and TP polyesters.

WIDER FIELD OF RESINS

Up to now, major applications for phosphorus-based flame retardants have been limited to a few areas. One is polyurethane foams, where phosphate/halogen systems have dominated. Says Monsanto's Mount, "There's a bit of movement currently to get halogen out of these systems. One driving force is the State of California's regulation Cal 133, which is a furniture test that incorporates smoke requirements." The regulation applies to furnishings used in public institutions. Akzo Chemicals plans to launch early next year a second generation of its Fyrol PBR brominated phosphate ester with improved performance in flexible foams.

Phosphate esters have been popular for many years as flame-retardant plasticizers in flexible PVC, itself a highly halogenated polymer. Dover Chemical recently introduced a liquid bromochlorinated paraffin/phosphorus system called DD-9122 for both PVC and urethane foam (see PT, July '92, p. 83).

Phosphorus compounds have also enjoyed long use in PPO/HIPS engineering thermoplastic alloys. And nylons, mainly in Europe, have seen substantial use of inorganic phosphates and red phosphorus, primarily for electrical applications (small connectors, relays, coils for tv and computer screens). Notes Monsanto's Mount, "Use of red phosphorus in nylon has been taking place in Europe and Asia. It really has not caught on in North America. Reasons are that it is a bit more hazardous to handle and it imparts a red color. We see limited potential in nylon because nylon requires higher processing temperatures--over 500 F--and many phosphates decompose or volatilize above 500 F. We quote 480 F as an upper limit for our ammonium polyphosphate flame retardants."

There's also some use of phosphorus-based compounds in thermosets, notably unsaturated polyesters for boats, tub/showers and electrical products. Aceto Corp. is concentrating chiefly on the latter field, according to marketing manager Leonard Laughton. The company imports from Europe a tris-chloropropyl phosphate called Cortose that reportedly has received National Fire Prevention Association (NFPA) approval for pultruded polyester electrical parts, where it's used in combination with alumina trihydrate. Adds Mount of Monsanto, "We also see some movement in epoxy resins and potting compounds that are used around cables in walls." Epoxy electrical applications are believed to be future growth areas where phosphorus additives could replace tetrabromobisphenol-A, which now dominates that market.

However, the biggest changes are expected to take place in use of phosphorus flame retardants in thermoplastics--polyolefins, styrenics, and engineering types--although some of this growth may depend on commercial release of some additive technologies now in R&D. A cautious view is voiced by Dan Scharf, Hoechst Celanese manager of technical & marketing services for polymer additives: "My view is that the technology to replace halogen systems in polyolefins is just not there yet. We don't have equivalent 'drop-in' technology for all applications. This is also true in styrenics. I don't think anything really exists that can effectively replace brominated systems."

Nonetheless, there's quite a bit of developmental work under way in non-halogen, phosphorus-based flame retardants for polypropylene, polyethylene and other olefinic polymers, chiefly for wire and cable. Albright & Wilson's product development manager, Preston White, ventures to predict a complete about-face in FR-PP formulations, where he believes the formerly dominant halogen systems are on their way out. Phosphorus additives are also making headway in PE, EVA and EEA wire and cable areas, he says.

PP is said to be one of the hottest areas of FR formulation development because PP is increasingly being used in place of more expensive engineering thermoplastics such as PET or nylon in electrical/electronic parts and functional housings for small appliances. Intumescent types of non-halogenated phosphorus additives have been successful in these new applications, says Great Lakes' Ludwig.

Mount from Monsanto says, "We are seeing some success with non-halogen phosphorus-based FRs for electrical connectors, boxes, and wire and cable in PP and PE. This is more so in PP as phosphorus performs better in this type of polymer." Mount says that while phosphorus does work in PE, higher loadings are needed. To achieve a UL 94V-0 flammability rating, loadings in the range of 30-35% by weight are required for PE, as compared with 20-24% for PP, he says.

Adds White from Albright & Wilson, "We see major potential in wire and cable, primarily in insulation for jacketing," noting that this market has already made a step away from halogenated systems by going to alumina trihydrate and magnesium hydroxide. He says Albright & Wilson is developing a second generation of phosphorus-based compounds that will be able to meet those needs at lower filler loadings.

ENGINEERING RESINS BECKON 

Most suppliers say they see an increased interest in engineering thermoplastics, where they expect growing use of halogen/phosphorus combinations rather than phosphorus-only systems. Says Ray Mount from Monsanto, "We see potential in PBT, but as with nylon, you have a very narrow window because of the higher processing temperatures required." Albright & Wilson, according to White, has active developmental programs with non-halogen phosphorus compounds geared toward PET and PBT, as well as thermoset polyesters.

On the other hand, Ludwig of Great Lakes doesn't see much potential in PBT/PET: "Liquids don't work well with crystalline materials--that's why many phosphorus-based compounds do not work well with nylon. The exception is solid red phosphorus, which works well with nylon 6 and 66." In amorphous engineering alloys, however, he reports that there has been some movement toward non-halogenated phosphate esters for PC/ABS and ABS/PVC.

Bill Coble, Akzo business manager for phosphorus chemicals, says his company is definitely looking to replace other flame retardants in engineering thermoplastics with non-halogen phosphorus-based compounds such as its new bisphosphate product, Fyrolflex RDP, launched about a year ago. While originally targeted toward PPE/PPO alloys, the company's developmental work indicates the bisphosphates offer advantages in other engineering resins, such as polycarbonate, PET, and blends such as PC/ABS. "I see real growth potential in engineering thermoplastics as we get more into recycling," Coble notes. "People will become concerned about halogenated FR systems at the higher temperatures required in reprocessing and the possible release of acids, gases, dioxins and furans." Coble adds that Akzo plans to increase capacity for bisphosphates "on a global scale" and to introduce new products of this type.

Styrenics appear to be a more problematic target for phosphorus additives. Ludwig doesn't see much potential in HIPS and ABS. "I don't see any existing product doing a good job. Currently available phosphate esters tend to overplasticize and degrade heat distortion. Perhaps a new generation of phosphate esters might be able to overcome that key problem of continuous-use temperature."

White from Albright & Wilson agrees: "Styrenics are one area where the economics is a big hurdle. Brominated diphenyloxide is very cost-effective. We're still a generation away from addressing that area."

NEW PRODUCTS TACKLE DRAWBACKS

As noted above, phosphorus-based flame retardants are not without their drawbacks, which depend on the type of additive and the application in which it is used. Most common problems are with thermal stability, plasticization, large particle size, blooming and migration problems that can cause cracking and crazing in parts under stress, and cost.

Hoechst's Scharf says a key limiting factor is that many phosphorus flame retardants decompose or volatilize above 450 F, except for red phosphorus, which can be processed "somewhere above 450 F, but not that much." So, although thermal stability is not necessarily a problem in PP, PE, or thermosets like epoxies or polyester, it is a problem with higher-temperature polymers.

Albright & Wilson's White says, "We accept that there are thermal limitations, yet we feel there's a realm of phosphorus technology that we have not touched on yet, which will allow us to work in that high-heat region over 480 F. We really think the thermal stability factor is a false hurdle."

Great Lakes, Ludwig reveals, is now focusing its developmental efforts in phosphorus chemistry in two different directions. One is toward development of oligomeric-type phosphate compounds for use in higher temperature applications in styrenics and amorphous and crystalline engineering thermoplastics. The other area is a broader exploration of halogen/phosphorus synergism to provide alternatives to antimony oxide. "We can have bromine and phosphorus in the same molecule and we're looking at ways to do this for a full range of lower-end to higher-end thermal requirements. We're close commercially," he says.

Great Lakes is planning a new grassroots facility for production of non-halogenated phosphorus flame retardants. Initial products are three intumescent additives--CG 329, CN 1197, and NH1511--that have been commercially available on a more limited scale since 1986. They will be aimed at PP electrical parts and small appliance housings, olefinic wire and cable compounds, and such engineering thermoplastic blends as PC/ABS and ABS/PVC.

Albright & Wilson is working toward a second generation of its ammonium polyphosphates that will overcome problems of moisture sensitivity that adversely affect dielectric constant in wire and cable applications. "We know we have the flammability performance and we're aiming to develop products further to overcome these other problems," White says.

Monsanto's Mount concedes the drawback of larger particle sizes of phosphorus-based compounds, along with the fact that intumescent phosphorus systems require use of a catalyst, a blowing agent, and a char former. "These three things are needed for the phosphorus to work. It has been difficult to find the appropriate synergists that don't bloom or migrate." One answer to this problem may be Albright & Wilson's recently introduced Amgard EDAP, a nitrogen-phosphorus intumescent for PP that does not require any other additive to be effective and is available down to 5-micron particle size (see PT, July '92, p. 81). Other new intumescents that we reported in July include Hoechst Celanese's Exolit IFR 10, 15 and 23, based on ammonium polyphosphate, and Himont's Spinflam line of nitrogen-based compounds that use ammonium polyphosphate as a synergist. They're aimed at PE and copolymers, crosslinkable PE, TP urethane, PUR foams, nylon 6 and PBT.

Overcoming the cost disadvantage relative to decabromodiphenyl oxide and other halogenated additives is another key issue. Mount responds, "This is not a huge hurdle, as we're moving towards reducing the additive loadings required, which will in turn reduce the cost to the compounder."

Both Monsanto and Albright & Wilson say they expect to introduce new non-halogenated phosphorus-based additives next year. Monsanto's new entries--which the company says are neither phosphate esters nor ammonium polyphosphates--will be for flexible and possibly rigid polyurethane foam. Albright & Wilson's new products are organic triphenyl phosphites to be aimed at PET and PBT engineering plastics and fibers.

FMC's Green sees other obstacles to replacing halogenated additives with phosphorus in such applications as PC/ABS and PPO/HIPS blends. "Phosphate esters, for example, tend to fume off and condense, leaving deposits on molds that result in stress cracking. Also, properties are affected by overplasticization so that heat-distortion temperature drops significantly." FMC, he says, has a new brominated phosphate ester which, because of its higher molecular weight, does not volatilize and leave mold deposits. And, since this product is not a plasticizer, HDT reportedly is not significantly affected.

FMC has two developmental phosphorus/bromine flame retardants--PB 460 and PB 370--which it expects to launch by fall. PB 460 (4% phosphorus, 60% bromine) will target uses in polycarbonate, PET and PBT, alloys of these resins (like PC/ABS), and PET fibers and textiles. PB 370 (3% P, 70% Br) will be aimed at PP and ABS, as well as PP textiles/fibers. Besides high FR efficiency, key advantages of both additives are said to include melt blendability, thermal stability, and retention of polymer whiteness after processing.

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Aceto Corp., Lake Success, N.Y. Akzo Chemicals Inc., Dobbs Ferry, N.Y. Albright & Wilson Americas, Richmond, Va. Dover Chemical Corp., Dover, Ohio FMC Corp., Process Additives Div., Philadelphia Great Lakes Chemical Co., West Lafayette, Ind. Himont Inc., Functional Chemicals Div., Wilmington, Del. Hoechst Celanese Corp., Charlotte, N.C. Monsanto Chemical Co., St. Louis