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additives feature | Flame retardants

This micro switch is

moulded using BASF’s


A3U40G5 25% glass-rein- forced PA 66

Evaluating chemistries In interviews, resin and additive producers discussed materials that are gaining ground in FRs. Vikram Gopal, technology and product marketing director at PA 66 resin and compounds producer Invista, focused on four. Melamine cyanurate, versions of which contain

nitrogen, is good where requirements include a UL94 V-0 rating, comparative tracking index (CTI) above 500 volts, and coloured parts. Melamine cyanurate, Gopal says, produces “intumescent and runaway effects” that remove fuel from a fire. The material, which is also economical, is generally limited to unreinforced resins. Aluminium diethylphosphinate, a metal phosphinate used with synergists, is similar in performance to melamine cyanurate but uses solid-phase charring, which minimizes fuel supply, for flame retardance. Gopal notes, however, that the chemistry increases corrosion in processing equipment and lowers mechan- ical properties compared with retardants such as red phosphorous and brominated polystyrene. Red phosphorous FRs meet UL94 V-0 standards and

Engine components, such as this thermo-switch control module for the Audi A7, must resist increasingly high continu- ous-use temperatures

CTI is in the range of 450-550 volts. The material uses charring and gas-phase scavenging (oxygen limitation). Red phosphorous is limited to black parts due to its colour incompatibility, is difficult to process as a result of corrosiveness, and poses a safety risk from phos- phine emissions in processing. While brominated materials are no longer specified

in certain applications, they remain viable in parts where they can still be used. Gopal says that FRs based on brominated polystyrene and synergists are especially effective. The chemistry uses gas-phase radical scavenging to minimize oxygen and reduces the intensity of exothermic reactions. Trade-offs include a low CTI, high specific gravity, high smoke density and,

not surprisingly, corrosive smoke. Other chemistries include three cited by BASF: melamine polyphosphate, which contains nitrogen; nitrogen/phosphorous compounds; and magnesium hydroxide.

Martin Klatt, head of plastic safety and regulatory

affairs at BASF, discussed these at AMI’s Fire Retard- ants in Plastics conference in Denver, CO, USA, last summer. He indicated that like any additive, selection of FRs involves trade-offs. Magnesium hydroxide, for example, has high thermal stability, good electrical properties, low smoke density and colour neutrality in parts. But it requires relatively high loadings, is high in density and affects mechanical properties. Some chemistries work better in PA 66 than in PA 6,

owing in part to the inherent flame-retardant properties of the former. Nitrogen/phosphorous compounds, which BASF supplies in 25% glass fibre-reinforced grades of its Ultramid PA 66, provide good toughness and electrical properties, and have no colour issues, Klatt said, though trade-offs include high loadings and processability issues. Suppliers also tout innovative chemistries as FR

options. FRX Polymers developed phosphorous-based polymeric formulations for PA 6. Maggie Baumann, marketing adviser, says the material, called Nofia, uses a catalyst and two monomers, bisphenol A and dimethyl phenyl phosphinate, to form under heat and pressure a homopolymer of polyphosphinate. The homopolymer is combined with diphenyl carbonate and building blocks to produce a polymeric FR. Nofia is used at loadings of 10-20%, or as a copoly-

mer at loadings of 20-50%. “An end-user can replace a copolymer with our material,” Baumann says. Benefits include permanent flame retardance.

“Because it’s in polymeric form, there is no migration” of component materials, she explains. Nofia grades are processable – they have high melt flow and high melt strength, and thus “can be extruded or injection moulded without dramatically rebuilding a polymer.”

22 COMPOUNDING WORLD | December 2012

breakdowns can occur that generate phosphoric acid and other signatures. A lot of suppliers are working at the molecular level or with additives to mitigate the release of acidic by-products.”


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