Understanding Flame-Retardant Thermoplastics

Understanding Flame-Retardant Thermoplastics

When it comes to flame-retardant thermoplastics, there’s a lot to consider: flammability tests, ratings, and regulatory requirements, and processability concerns, to name a few. And with many in the manufacturing industry retiring, there’s no guarantee a material expert will be available to provide you with the in-depth information about flammability requirements you may be looking for. With all this in mind, it’s easy to see how one could become overwhelmed in their search for the right flame-retardant material. Let’s break down the basics so you can be confident in your usage and selection of flame-retardant thermoplastics.

The PolySource Design Funnel

As mentioned in our previous blog post, Polyketone (POK) – A Cost-Efficient Material Option, we use a resource called the PolySource Design Funnel to help you figure out which type of plastic you need for your applications. The PolySource Design Funnel considers the physical, chemical, thermal, mechanical, and regulatory requirements to guide you to the product you need. In the case of flame-retardant thermoplastics, we continue to use the Design Funnel, paying close attention to the thermal, mechanical, and regulatory parts in particular.

UL Yellow Card Ratings

If you asked a resin supplier for clarification regarding flammability requirements, most would direct you to see their UL Yellow Card. But if you glanced at one, you might wonder what all the acronyms and numbers listed mean. The UL Yellow Card provides information about flame tests, electrical energy tests, thermal ratings, and UV resistance. Let’s take a closer look at some of these tests and what they entail, keeping in mind that UL ratings are “a la carte” and the costs add up.

UL 94 & UL 746 Tests

UL 94 tests are vertical or horizontal flame tests that are short-term and thickness-dependent. These tests look for flammability resistance to an open flame. There are six categories of rankings for UL 94 tests, ranging from least to most flame-retardant. The lowest rating, UL 94 HB, is a horizontal, self-extinguishing burn; the highest, UL 94 5VA, is a vertical burn that stops within 60 seconds without any holes burning through.

The UL 746A electrical energy tests look for flammability resistance to an electric arc or heated wire. Among the properties evaluated are HWI (hot wire ignition), HAI (high arc ignition), and a slew of short-term, thickness-independent properties. The results are compiled into performance level category (PLC) ratings. For UL 746A tests, the lower the PLC rating, the better.

Next up, we have UL 746B. These long-term tests provide RTI (Relative Thermal Index) ratings. RTI ratings are continuous use temperatures at which a plastic is estimated to retain > 50% of its initial values after 100,000 hours at that temperature, under zero load. This means there is no stress, pressure, or force.

The time it takes to collect the raw data for an RTI rating is between 500 to 5,000 hours. Linear regression analysis is used to plot the data to an extrapolated 100,000-hour result. Three RTI values can be assigned:

  • RTI Electrical, the retention of dielectric strength (electrical insulation)
  • RTI Impact, the retention of either tensile impact or Notched Izod impact
  • RTI Strength, the retention of tensile stress

It’s also important to note that any flame-retardant material with a V-rated grade must retain its V rating after the heat aging cycle.

RTI ratings are very costly to obtain, and many applications don’t need them. When long-term testing has not been conducted, a generic RTI is assigned to UL-listed materials.

The last UL standard to discuss is UL 746C. These long-term tests focus on outdoor suitability. They estimate 5-year UV exposure by simulating outdoor weather using either a carbon electrode UV or Xenon arc UV source for 700-1,000 hours. Samples are then immersed in water for 160 hours at 70° C.

Flame-retardant performance, tensile stress, and Notched Izod impact are measured before and after the exposure sequence. When the mechanical and flame-retardant performance is retained after both UV and water immersion, an f1 rating is assigned. However, if either the UV or water immersion performance is retained – but not both – an f2 rating is given. Additionally, just as with the UL 746B standard, materials with a V-rated grade must retain their V rating after testing.

Due to capacity constraints, these weathering tests can take as long as 8-10 months to complete.

Three Main Categories for Flame-Retardant Plastics

The three main categories for flame-retardant plastics are inherently flame-retardant, non-halogenated, and halogenated. As you decide which material to use, it’s essential to weigh the pros and cons of each type.

Inherently flame-retardant materials include PPS, PEI, fluoropolymers, PPSU, and certain PCs. These materials do not require nor contain external flame-retardant additives. Suitable for food contact, these are ideal for REACH (a European Union regulation) and RoHS (Restriction of Hazardous Substances Directive) compliance. A bonus is that these materials (except for PC) are low smoke generating, too. Drawbacks for inherently flame-retardant materials include the high cost per pound. Additionally, these materials can be a challenge to mold, and most fluoropolymers require expensive corrosion-resistant metal alloys for all surfaces in contact with the melt.

Non-halogenated flame-retardant additives are preferred for consumer appliances in Europe. These additives include phosphate esters, alumino-phosphate salts, amino-phosphate blends, red phosphorus, and magnesium and aluminum hydroxides. These materials are generally processing-friendly and don’t require much concern regarding machine corrosion. You’ll also find a lower material density in most cases. However, polyolefins require high loadings of non-halogenated flame-retardant additives, which degrade mechanical and processing performance. Supply constraints and high costs are negative factors to consider as well.

Lastly, there are halogenated flame-retardant additives to consider. Although these materials (including bromine and AO) are highly efficient and result in diminished plate-out and blooming, European regulators are likely to ban all brominated flame-retardant additives when suitable non-halogenated options exist. There are also issues with mold corrosion, machine wear, significantly increased material density, and smaller processing windows.

While there are many options for flame-retardant plastics, with the pros and cons in mind, you can make an informed decision that best meets your needs.

Regulatory and Processing Considerations

There are some regulatory and processing considerations that you’ll want to keep in mind when it comes to flame-retardant plastics. REACH targets specific chemicals, mixtures, and compounds. As previously mentioned, European regulators will likely ban all brominated flame-retardant additives when non-halogenated options exist. Brominated additives have been shifting to polymeric and oligomeric grades and away from “small molecule” types. RoHS targets specific additives and chemicals used in electrical and electronic devices. RoHS is aimed at end-of-line considerations and keeping hazardous materials out of landfills. The last regulatory consideration is California Proposition 65, commonly shortened to Prop 65. This California law aims to reduce or eliminate exposure to toxic chemicals that can cause cancer and birth defects. Prop 65 requires warnings on products containing these chemicals. One of these chemicals is antimony oxide, which is used together with halogenated compounds to create flame-retardant properties. However, it’s important to use your best judgment and keep in mind how extensive Prop 65’s list of dangerous chemicals is; alcoholic beverages, tamoxifen (a drug used to treat breast cancer), and cisplatin (a chemotherapy drug) also make the list.

In addition to the above regulations, there are also processability concerns that you’ll want to think about as you work with flame-retardant plastics. These include proper venting in the mold (don’t let this be an afterthought!), along with mold maintenance, cleaning, and purging. Other considerations include thermal stability concerns, drying flame-retardant polymers, and tooling, screw, and barrel materials and coatings.


With so many choices, there is no “one size fits all” answer when it comes to flame-retardant thermoplastics. There are many factors in determining what flame-retardant technologies will deliver the best design solution. With PolySource at the helm and the knowledge of flame-retardant thermoplastics in your back pocket, you can be confident that you will have the perfect products you need.


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