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Polymeric ﬂame retardants for reinforced thermoplastic and thermoset resins Jan-Pleun Lens, Xiudong Sun, Lawino Kagumba Polyphosphonates and phosphonate oligomers are a group of ﬂame retardants that have gained much attention in recent years. Produced under the brand name Noﬁa,1 they are now commercially used to ﬂame retard different resins such as polyesters, polycarbonate, polyurethanes, epoxies, unsaturated polyesters, and polyureas. Phosphonates can be used as the single ﬂame retardant but recently, it was also shown, that phosphonates can support other non-halogenated and halogenated ﬂame retardants. FRX Polymers has been screening many systems including replacement of antimony trioxide as the synergist in ﬂame retardant formulations that are based on halogenated ﬂame retardants. The present article discusses examples of the use of phosphonates in reinforced thermoplastic and thermoset applications. Flame retardants (FRs) inhibit ignition, slow the spread of ﬁre and protect critical infrastructure during a ﬁre event. They also can reduce smoke and toxic fume production that can lead to death in ﬁre. Over 60% of all FR plastic formulations are still based on halogen-containing FR additives, even though these materials have some severe, undesirable side effects, such as persistence in the environment, bio-accumulation in animal and human tissues, and cytotoxicity. Driven by EU legislation that bans the use of certain families of brominated FRs and prohibits the recycling of bromine FR-containing plastics among nonhalogen containing plastics, many companies and researchers are looking for alternatives. Furthermore, it is highly desired that FR agents do not migrate out of their host plastic, exposing humans to these often-toxic chemicals and diminishing the application’s FR function over time. Many non-halogenated FR alternatives are based on phosphorus. However, these are typically small molecules and, like traditional halogenated FRs, tend to migrate out of the host plastic over time. To prevent migration of FR agents, one can use reactive type or polymeric FRs. Reactive FRs can be copolymerized in the main chain or as pendant side groups of polymers. This approach generally requires the polymer manufacturing processes to be modiﬁed, which is often associated with high costs. A better approach is to blend polymers that need to be ﬂame retarded with polymeric FRs. As in any polymer blend, 1
FRX POLYMERS and NOFIA are registered trademarks of FRX POLYMERS, INC.
the polymeric FR will be entangled with the host polymer into a physical network, thereby imparting permanent ﬂame retardancy while maintaining most of the original mechanical and thermal properties of the host polymer.
Polyphosphonates Under the brand name Noﬁa, FRX Polymers is the ﬁrst company to commercially supply non-halogenated, non-migrating FR materials that are of low concern to human health and the environment. This technology is based on chemistry that allows the polymerization of a phosphonate monomer into unique oligomeric and polymeric FRs. Compared to halogenated FRs, recently introduced polymeric brominated FRs, and other phosphorusbased FRs, these polyphosphonates have clear advantages.2 The polymers from the phosphorus-containing monomer are made through a melt-based process and the chemistry offers the possibility to tailor the products to a wide range of compositions and MWs. Varying the reaction conditions (time, temperature, pressure, and catalyst), either oligomers or polymers of high MW can be produced. By using different co-monomers, the polymer properties of the polyphosphonates can be tailored to speciﬁc needs. In addition, by replacing some of the phosphonate monomer with diphenyl carbonate, various phosphorus-containing analogs of polycarbonate can be made with phosphorus content from very low levels to >10 wt%. 2
Reference to Specialty Chemicals article.
0034-3617/Ó 2017 Elsevier Ltd. All rights reserved. https://doi.org/10.1016/j.repl.2017.11.016
1 Please cite this article in press as: J.-P. Lens et al., (2017), https://doi.org/https://doi.org/10.1016/j.repl.2017.11.016
Polyphosphonates are compatible with polyesters (PET), polycarbonate (PC) blends, polyurethanes, epoxies, unsaturated polyesters and polyureas and deliver FR performance plus some unique properties to these materials. Being polymeric, the polyphosphonates typically do not affect the properties of the host plastics and, in some cases, even improve them. The polyphosphonates are produced as transparent pellets with a phosphor content of about 10.5 wt% and a similar transparency to PC. The melt stability is such that they can be processed via melt spinning, blow molding, blown ﬁlm, cast extrusion, and injection molding. Depending on the polymer system, the application, and the speciﬁc FR requirements, preferred phosphonate grades and usage level can be recommended. Much research is focused on identifying optimized formulations where polyphosphonates are further combined with other FR agents for an enhanced – and sometimes synergistic – FR performance. In some applications the polyphosphonates can be the main FR agent; in others, they are used as a booster in systems where other FR agents are used as the main component. Polyphosphonates can be spun directly into ﬁber with other polymers like e.g. PET for ﬂame retarded technical textiles, carpets, wire and cable braidings, wigs, and hair extensions. Because polyphosphonates form transparent blends with many different engineering resins, they are also extremely suitable for applications such as thin transparent FR PET, PC, and TPU ﬁlms or sheets for electronic, mass transportation, and building and construction applications. Another application area is in glass ﬁlled resins or glass fabric composites, where phosphonates provide excellent adhesion to glass resulting in a range of additional beneﬁts over its FR properties.
Polyphosphonates in glass ﬁlled polycarbonate applications Polyphosphonates can make transparent blends with PC and show the highest ﬂammability rating at a thickness of 0.1 mm when tested under the Underwriters Laboratory protocol (UL VTM0 or V0). More interestingly, they also show very good performance in heat release tests that are being used for rail and aviation applications. For non-transparent applications, polyphosphonates have been evaluated in glass ﬁber (GF) reinforced PC. GF PC is an ideal engineering plastic with good electrical insulating and impact properties that ﬁnds increasing use in electronic equipment housings and covers, brackets and structural parts, computer parts, etc. Many of these applications require the materials to meet the ﬂame retardancy UL 94 V-0 rating. Current commercial non-halogenated ﬂame retardant 40–50 wt% GF PCs from major suppliers only meet V-0 at low thickness at the expense of reduced heat deﬂection temperature (HDT), especially when GF loading is high whereas the desired design area is to get a V0 rating at 0.8 mm or less at an HDT of 110 °C or higher (Figure 1). In addition, the ﬂow of these highly ﬁlled GF PCs is very low, which is a limitation for injection molding applications, especially for thin wall applications. Polyphosphonate and poly(phosphonate-co-carbonate) provide GF PC with excellent ﬂame retardancy with V0 rating at thickness as low as 0.8
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The UL94/V0 thickness versus the HDT of commercially available FR GF PC compounds.
mm while maintaining a high ﬂow, high HDT, and very good mechanical properties. A 27-mm twin screw extruder (TSE) was used to compound a variety of compositions of GP PC with polyphosphonates and its copolymers. The temperature proﬁle for the extruder was set at 200/260/285/285/285/290/290/290 °C starting from the feeding zone. The compounding was conducted at 10 kg/hour with a screw speed of 100 rpm. Both PC and the ﬂame retardants were pre-dried in a desiccant dryer to a moisture level below 200 ppm and mixed before fed into the feed hopper. Glass ﬁber was fed downstream from a side feeder. After adequate drying to <200 ppm moisture, the GF PC compounds were processed in an injection molding machine with temperature settings from 285 °C to 300 °C to produce test specimen (Table 1). Though it is known that PC and Noﬁa HM1100 are not miscible, interestingly, all compounds have a single Tg when analyzed with DSC, and the values are very close to the calculated Tg for two miscible polymers based on Fox equation. Thus, the data suggest that the GF or the sizing on the GF resulted in compatibilization of the two polymers to form a miscible blend. The GF used in Table 1 is Owens Corning 995-10P-4, but other GFs, such as PPG’s CHOPVANTAGE HP 3786, 3790 and 3540 have also been found to impart a single Tg in the blends. Another unexpected observation is the high ﬂow of these highly ﬁlled GF PC blends, which will greatly beneﬁt the injection molding of thin parts. From compositions A–E in Table 1 it is evident that a minimum amount of >1.4 wt% of P content is needed to achieve the V0 rating at 0.8 mm. Phosphorus enhances the rapid char formation with application of the ﬂame, preventing combustible gases from penetrating out of the polymer matrix and oxygen from advancing into the combustion zone, thereby extinguishing the ﬂame. At 2 wt% of P (B and E), both Noﬁa HM1100 and CO6000 achieved V0 rating at 0.8 mm with similar Izod properties, but the formulation with Noﬁa CO6000 (B) showed higher tensile properties. Comparison of E and F illustrates the effect of Teﬂon, which not only reduced drips but also shortened burning time. Comparison of E and G shows the effect of Joncryl 4300, a chain extender. The tensile properties were improved,
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PC LexanTM 141R (SABIC) Noﬁa CO6000 Noﬁa HM1100 PTFE (DuPont 6C) Joncryl 4300 (BASF) P content in total formulation Tg, °C by DSC Tg, °C Fox equation calc. HDT, 1.8 MPa, °C MVR, 300 °C/5.0 kgf UL [email protected]
mm tmax, sec t1+t2, sec Total drips #/drips ignited cotton # Un-Notched Izod, 1.6 mm, kg-cm/cm Notched Izod, 1.6 mm, kg-cm/cm Modulus, GPa TS, MPa Elongation at break, %
1.0% 136 138
2.0% 130 133
2.6% 127 129
1.4% 133 135
2.0% 128 130
47 Not Rated 42 >150 3/3 169 22 14 150 1.8
56 V0 7 33 0/0 174 22 14 160 1.8
77 V0 4 18 0/0 180 18 14 140 1.3
NA V-2 20 82 2/1 NA NA NA NA NA
2.0% 129 130 125 66 V0 6 15 1/0 158 22 11 120 1.5
18.7% 0.3% 0.5% 2.0% 129 130
58 V0 9 45 3/0 158 19 11 120 1.5
45 V0 6 28 0/0 169 21 12 130 1.7
and the ﬂow was reduced, although still very high compared to commercial FR GF PCs. The HDTs measured on selected formulations are above 120 °C, ﬁtting in the desired design area as depicted in Figure 1. In addition to FR GF PC, polyphosphonate and poly (phosphonate-co-carbonate) have also been found to be excellent non-halogenated ﬂame retardants for FR GF PET and PBT. As low as 10 wt% of Noﬁa HM1100 passes UL94 V0 at 1.6 mm in 30% glass ﬁlled PET, with no detrimental effect on mechanical properties. In FR GF PBT, Noﬁa HM1100 is used in combination with synergists such as melamine cyanurate to obtain V0/0.8 mm.
Polyphosphonates as ATO replacement in glass ﬁlled applications To address the issues of the persistence, bioaccumulation, and toxicity of low molecular weight halogenated FRs, a polymeric halogenated FR has been introduced to the market. Developed by Dow and made commercially available via three major suppliers of halogenated FRs, based on its polymeric nature, the material is promoted as a non-migrating FR and thus would not be released into the environment. However, many applications where these polymeric brominated FRs are used, as well as the low molecular weight brominated FRs, require the use of antimony trioxide (ATO) as a synergist. ATO is a heavy metal oxide and is a suspected carcinogen. It has recently been demonstrated that polyphosphonates can also be used in combination with brominated FRs as an ATO replacement thereby creating an all polymeric FR system. With ATO prices currently increasing worldwide, replacement of ATO has also become a high priority from an economic perspective for many users. The effectiveness of polyphosphonates as an ATO replacement has been validated in glass-ﬁlled nylon 66, PBT, and PET. Noﬁa FRs have a speciﬁc gravity of 1.2 and the speciﬁc gravity of ATO is 5.5. Thus, one must utilize up to ﬁve times more ATO on a volume basis to achieve the same effectiveness as Noﬁa FRs.
Properties of 40% GF PC Compounds with polyphosphonates.
In typical formulations, the end customer will start to experience a savings per part when using Noﬁa versus ATO, when ATO prices are higher than about $5.5/kg. (Figure 2). In 2017, ATO prices have been on the rise and have so far ﬂuctuated between $7.5/kg up to $9/kg. Customers who have converted to Noﬁa are typically saving between 2 to 3.5 cents per 100 ml part at these ATO prices. The polyphosphonates’ transparency is another key beneﬁt which is being explored in clear unﬁlled PVC systems while replacing ATO in PVC.
Phosphonates in thermoset applications For thermoset applications, reactive phosphonate oligomers are being used. In epoxy systems, phosphonates react with epoxy groups to form a crosslinked network resulting in high glass transitions temperature (Tg), excellent thermal and moisture resistance, and enhanced mechanical properties (Figure 3). Thus, in these systems, the phosphonate has a dual function of FR additive and hardener. In copper clad laminate (CCL) applications, epoxy-based laminates incorporating phosphonate oligomers as the FR and hardener also display a signiﬁcant decrease in the dissipation (loss) factor (Df) and have enhanced peel strength and modulus versus conventional halogen-free laminates that are based on 9,10-dihydro-9-oxa-phosphaphenanthrene 10-oxide.3 Phosphonate oligomers can also be incorporated into epoxybased glass/carbon ﬁber composites or in honeycomb sandwich panels used as structural components in aviation applications. The oligomers have good compatibility in both epoxy and phenolic resins, and can be directly dissolved into the epoxy resin at 80–100 °C or dissolved in a selected solvent like acetone for solvent-based systems. Reaction of phosphonates with epoxy resins is achieved using imidazole-type catalysts. The onset of the curing reaction is typically about 140 °C, with the optimal peak cure temperature at 180–190 °C. The cured composites pass 3 Development of Halogen Free, Low Loss Copper-Clad Laminates Containing a Novel Phosphonate Oligomer, IPC APEX EXPO S14-01, 2016.
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Nofia reactive flame retardant oligomers also act as hardeners in epoxy systems for copper clad laminates resulting in improved electrical and mechanical properties compared to other flame retardant systems.
Savings per 100 ml molded part when using Nofia HM1100 instead of ATO as a function of the ATO price.
vertical ﬂame tests UL94 V0 and 12-s burn test in accordance with C.F.R. Part 25.583. Like copper clad laminate applications, epoxy-based composites prepared using phosphonate oligomers have enhanced mechanical properties and very good heat resistance.
Commercial availability of polyphosphonates Polyphosphonates were ﬁrst discovered in the late 1940s and a signiﬁcant amount of research was done around the synthesis of these materials in the 1980s but due to technical difﬁculties and high costs they were never commercialized. During the last ten years, FRX Polymers optimized the synthesis of the phosphonate monomers, oligomers, and polymers and built the ﬁrst commercial plant for these materials in Antwerp, Belgium. Since
2014, it has produced a range of different grades that are sold worldwide. In Europe, FRX Polymers is collaborating with VELOX GmbH, a specialty chemicals distributor, operating out of Hamburg, Germany. Polyphosphonates are currently a highly versatile addition to the continuously growing spectrum of FR additives and are successfully being used in a range of different polymer systems and applications as an innovative, ecofriendly, permanent FR system, while simultaneously delivering a unique balance of properties. Much effort is being put into expanding the application area for phosphonates, both by FR producers and plastic compounders. Especially in combination with other FRs, the possibilities to custom design formulations that meet both FR and other non-FR requirements (mechanical, heat, stability properties) will greatly expand in the coming years.
4 Please cite this article in press as: J.-P. Lens et al., (2017), https://doi.org/https://doi.org/10.1016/j.repl.2017.11.016