In electrical and electronic products, automotive components, home appliances, and industrial equipment, flame retardancy is often a mandatory threshold for plastic material selection. When purchasing materials, many manufacturing customers ask directly: “Can this material pass V-0?” or “Is a halogen-free solution available?” However, actual discussions reveal that many technical personnel do not have a fully consistent understanding of the specific meanings of UL94 ratings, the differences among test methods, or the effects of halogenated and halogen-free flame retardants on material properties.
This article begins with UL94 test methods and rating definitions, systematically reviews the differences among V-0, V-2, and HB, explains the advantages and disadvantages of halogenated and halogen-free flame retardants, and provides practical material-selection recommendations. Using the PC, ABS, PP, PA, PBT, and other materials we supply as examples, it will also help you establish a clear method for selecting flame-retardant materials.
I. What Are UL94 Flammability Ratings?
UL94 is a standard developed by Underwriters Laboratories for testing the flammability of plastic materials. It is also one of the world’s most widely used systems for evaluating flammability ratings. It primarily evaluates burning rate, afterflame time, dripping behavior, and whether the cotton indicator below the specimen is ignited after a vertically or horizontally positioned material is exposed to a flame.
The UL94 rating system includes several classifications. The most common are:
HB: A horizontal burning classification with the lowest level of flame retardancy.
V-2, V-1, and V-0: Vertical burning classifications with progressively higher levels of flame retardancy.
5VB and 5VA: More stringent classifications for plaque specimens, intended for demanding applications such as equipment enclosures.
For most injection-molded parts, HB, V-2, and V-0 are the three classifications encountered most often in routine material selection. Their differences are explained below.
1. HB Classification
HB is the lowest flammability classification in UL94. During the test, the specimen is positioned horizontally and ignited at one end. The primary criterion is whether the burning rate is below the specified limit; there are no stringent requirements for afterflame time or dripping.
Practical meaning:
HB materials can be ignited but burn relatively slowly. They are suitable for applications with low flame-retardancy requirements, such as ordinary structural parts, decorative parts, and internal non-current-carrying components. Standard unmodified PP, ABS, PMMA, and POM commonly have only an HB rating. If a product must meet V-2 or V-0, an HB material is not sufficient.
2. V-2 Classification
In the V-2 test, the specimen is positioned vertically. After ignition, the afterflame time, total afterflame time, and whether molten drips ignite the cotton indicator below the specimen are observed.
Key criteria:
The flame self-extinguishes within 30 seconds after each flame application;
The total afterflame time for five specimens does not exceed 250 seconds;
Flaming drips are permitted to ignite the cotton indicator below the specimen.
Practical meaning:
V-2 materials are self-extinguishing but may produce flaming drips during combustion. Many flame-retardant PP, PC, and ABS grades can achieve V-2. These materials are suitable for applications with moderate flame-retardancy requirements where dripping does not create a serious safety risk. Note, however, that V-2 permits dripping. Applications involving exposed live components often require V-0 or a higher classification.
3. V-0 Classification
V-0 is one of the most stringent vertical burning classifications. The main differences from V-2 are that V-0 does not permit flaming drips that ignite the cotton indicator and requires a shorter afterflame time.
Key criteria:
The flame self-extinguishes within 10 seconds after each flame application;
The total afterflame time for five specimens does not exceed 50 seconds;
Flaming drips that ignite the cotton indicator are not permitted.
Practical meaning:
V-0 materials self-extinguish rapidly and produce little dripping, or their drips do not ignite the cotton indicator, providing a higher level of safety. They are widely used in electronic connectors, relay housings, charger housings, power modules, and current-carrying internal components of home appliances. Achieving V-0 generally requires efficient flame retardants or materials with a degree of inherent flame retardancy, such as PC and PBT.
4. 5VA and 5VB
These two classifications apply to enclosure materials with extremely demanding flame-retardancy requirements. The tests evaluate more than vertical burning; plaque specimens must also resist burn-through. 5VA is more stringent than 5VB. Because the testing and material costs are relatively high, these classifications are used mainly for large electronic-equipment enclosures, building fire-protection components, and similar applications. They are less commonly specified for ordinary injection-molded parts.
II. Halogenated vs. Halogen-Free Flame Retardancy: What Is the Difference?
1. Halogenated Flame Retardants
Halogenated flame retardants generally refer to bromine- or chlorine-containing compounds, such as decabromodiphenylethane, brominated epoxy resin, and tetrabromobisphenol A. During combustion, they release hydrogen halide gases that capture free radicals in the combustion chain reaction, thereby suppressing flame propagation.
Advantages:
High flame-retardant efficiency; V-0 can be achieved with a relatively low additive loading;
Relatively manageable effects on the material’s mechanical properties;
Lower cost and mature technology.
Disadvantages:
Produces large amounts of smoke and toxic, corrosive gases, such as HBr and HCl, during combustion;
Some halogenated flame retardants are restricted by environmental regulations; for example, RoHS restricts certain brominated flame retardants;
Products exported to the EU market may face more stringent environmental compliance scrutiny.
Common applications:
Traditional electrical and electronic products, home-appliance housings, cable jackets, and other applications, particularly where cost is a major concern.
2. Halogen-Free Flame Retardants
Halogen-free flame retardants mainly include phosphorus-based systems (phosphate esters, red phosphorus, and phosphonates), nitrogen-based systems (melamine and its salts), intumescent flame retardants (phosphorus-nitrogen combinations), and inorganic flame retardants (aluminum hydroxide and magnesium hydroxide). They provide flame retardancy through mechanisms such as char formation, intumescence and thermal insulation, and dilution of combustible gases.
Advantages:
- Less smoke, lower toxicity, and better environmental performance during combustion;
- Complies with RoHS, REACH, and the halogen-free requirements of certain major brands.
Disadvantages:
- Generally lower flame-retardant efficiency than halogenated systems, requiring higher additive loadings;
- More pronounced effects on mechanical properties, heat resistance, and electrical properties;
- Some phosphorus-based flame retardants may migrate to the surface at high temperatures, affecting appearance and electrical properties;
- Generally more expensive than halogenated flame retardants.
Application trend:
As environmental regulations and brand requirements become more stringent, demand for halogen-free flame retardancy continues to grow in electrical and electronic products, automobiles, and consumer electronics. In particular, halogen-free V-0 has become a basic requirement for many export orders involving connectors, chargers, power adapters, and similar products.
III. Effects of Flame Retardants on Material Properties
When selecting a flame-retardant material, it is not sufficient to consider only its UL94 classification. The side effects of flame retardants on other material properties must also be understood. The table below shows typical changes in several common materials after flame retardants are added:
| Material | Flame-Retardant Type | Effect on Mechanical Properties | Effect on Processing Performance | Other Considerations |
| PP | Halogen-free intumescent flame retardant | Significant reduction in impact and tensile strength | Reduced flowability; higher processing temperature required | High additive loading and increased density |
| ABS | Brominated flame retardant | Reduced impact strength and surface gloss | Narrower processing window; susceptible to degradation | Pay attention to RoHS restrictions |
| PC | Halogen-free phosphate ester | Slightly reduced toughness and heat resistance | Improved flowability but susceptible to hydrolysis | Thorough drying required |
| PC/ABS | Halogen-free phosphorus-based system | Some reduction in impact strength | Acceptable processing performance | Pay attention to phase separation and color stability |
| PA6/PA66 | Halogen-free red phosphorus/nitrogen-based system | Reduced tensile and impact strength; strongly affected by moisture absorption | May migrate to the surface at high temperatures | Generally available only in black/dark colors |
| PBT | Halogen-free phosphorus-based system | Reduced tensile and impact strength; electrical properties may be affected | Narrow processing window; susceptible to migration | For connectors, pay attention to the CTI value |
Key point:
The properties of flame-retardant materials depend greatly on the grade and formulation. Different grades with the same UL94 classification may differ significantly in mechanical and processing performance. When replacing a flame-retardant material, manufacturers must conduct a small-batch molding trial and complete performance validation rather than making a decision based only on the classification.
IV. How Should You Select a Flame-Retardancy Classification and System for an Application?
1. Determine the Classification Based on Product-Safety Requirements
| Product Type | Recommended Minimum Classification | Description |
| Internal non-current-carrying structural parts | HB | No special flame-retardancy requirement |
| General home-appliance housings (no direct contact with live components) | V-2 | Moderate flame-retardancy requirement |
| Electronic connectors, relays, and switches | V-0 | V-0 and halogen-free are generally required |
| Charger and adapter housings | V-0 or 5VB | Depends on power rating and safety requirements |
| Automotive interiors (flame-retardant) | V-0 or compliance with FMVSS 302 | The automotive industry uses a separate standard |
| Building fire-protection components | 5VA or higher | Stringent fire-protection requirements |
2. Select a Halogenated or Halogen-Free System Based on Environmental Requirements
If the customer explicitly requires a halogen-free material, or if the product will be exported to the EU, Japan, or South Korea, give priority to a halogen-free flame-retardant grade.
If cost pressure is high, the customer requires only V-0, and the halogen-free requirement is unclear, both halogenated and halogen-free options can be offered for comparison.
For some applications, such as certain high-temperature nylons, halogen-free V-0 is technically difficult and expensive to achieve. Whether it is acceptable should be evaluated.
3. Consider Other Key Properties
Electrical properties: For connectors, relays, and similar parts, attention must be paid to CTI (comparative tracking index) and arc resistance. Halogen-free flame-retardant materials sometimes have a relatively low CTI, so grades must be screened specifically for this requirement.
Heat resistance: Flame retardants may reduce HDT. A heat-resistant flame-retardant grade is required for high-temperature environments.
Color limitations: Red phosphorus and certain nitrogen-based halogen-free flame retardants are dark in color, making light-colored or white products difficult to produce.
Blooming and migration: Some halogen-free flame retardants may migrate to the surface at high temperatures or during long-term use, affecting appearance and electrical properties.
V. Overview of the Flame-Retardant Materials We Supply
We primarily offer the following flame-retardant plastic resins to meet different classification and halogen-free requirements:
| Material | Achievable Flammability Classification | Halogen-Free Solution | Typical Applications |
| FR PBT | V-0 (halogenated/halogen-free) | Mature halogen-free solutions | Electronic connectors, relays, and coil bobbins |
| PC | V-0 and V-2 (primarily halogen-free) | Mature phosphate ester systems | Charger housings, power modules, and transparent flame-retardant parts (depending on grade) |
| PC/ABS | V-0 and V-2 (primarily halogen-free) | Halogen-free phosphorus-based systems | Laptop housings, home-appliance housings, and electronic-equipment housings |
| ABS | HB to V-0 (halogenated/halogen-free) | Halogen-free V-0 available, but with reduced performance | Home-appliance and electronic housings (cost-sensitive applications) |
| PP | HB and V-2 (V-0 is difficult) | Halogen-free intumescent systems can achieve V-2 | Internal home-appliance parts and flame-retardant modified PP products |
| PA6/PA66 | V-0 (halogenated/halogen-free) | Halogen-free V-0 available, with limited color options | Connectors, electrical components, and automotive components |
| POM | Generally HB; V-0 is extremely difficult | No mature halogen-free solution | Generally not recommended for applications with high flame-retardancy requirements |
| PMMA | Generally HB; transparent flame-retardant grades are rare | No mature transparent flame-retardant solution | General decorative parts and non-flame-retardant applications |
The information above is only an overview. The flame-retardancy classification, halogen-free certification, mechanical properties, and processing parameters of a specific grade should be confirmed using its TDS and actual test results. If you have specific flame-retardant material-selection requirements, please provide the application and certification requirements. We can help match a suitable grade and provide samples for testing.
VI. Further Reading: In-Depth Analysis by Material
To help manufacturing customers select materials more accurately, we will subsequently publish detailed articles on the flame-retardant properties of the following materials. Please follow us for updates:
- PC UL94 Ratings: V-0, V-2, and Flame-Retardant Polycarbonate
- ABS UL94 Ratings: HB to V-0 and Flame-Retardant Options
