Specific tests are required to determine the behavior of adhesive and labels when exposed to intense heat and/or flames. Flammability properties of materials tested in a horizontal orientation are usually less demanding than those in a vertical orientation. The test method is generally dictated by the rating the material has to meet, which is usually specified by the customer or OEM. 

The most common and well-known test method for automotive applications is the American FMVSS 302 (Federal Motor Vehicle Safety Standard), with ISO 3795 as the global equivalent. Becoming more prevalent with the introduction of electric vehicles is the UL® 94 test method, created with the intention to classify the burning behavior of plastics used in electric devices and appliances. The next sections will cover both.

The UL® 94 standard describes several variations to the method dictated by the types of materials being tested, their thickness and or rigidity, and their burning characteristics. Some of the variations and resulting ratings correlate with other test standards and ratings. 

The UL® 94 HB method was designed for materials such as untreated polyolefins. Unlike UL® 94 V (see below), this test is performed on specimens in a horizontal orientation. This test is less stringent than the vertical burn test. Results from this method yield a burn rate for a material – that is, how long does it take for material to burn a given distance. In order for a material to be classified HB, it must not have a burn rate exceeding 40 mm per minute over a 75 mm span for specimens having a thickness of 3 to 13 mm, or a burn rate less than 75 mm over a 75 mm span if less than 3 mm thick, and all specimens must cease to burn before reaching 100 mm.

This method is used to evaluate solid materials, having a maximum thickness of 12.7 mm, yet rigid enough not to shrink or distort when a flame is applied. Five test specimens for each material, 127 mm long by 12.7 mm wide, are supported in a vertical rather than a horizontal position. A flame from the burner is applied to the bottom of each specimen, held for 10 seconds and then withdrawn until the specimen extinguishes. 

The flame is then reapplied for an additional ten seconds. The length of burn or afterglow time is recorded after each flame application. It is noted if any of the specimen drips and ignites a wad of cotton placed below the specimen. A rating of UL® 94 V-0 is the best achievable rating. A material with this rating is considered flame retardant. Labeling materials less than 0.381 mm thick are unlikely to meet the requirements of "V" rating.

The introduction mentioned a third variation of burn testing. This is a modified method for testing very thin materials that may shrink or distort away from the flame, or are consumed up to the holding clamp due to their thinness. Test specimens are cut to 200 mm by 50 mm. 

A mark is made on the outside across the width of the sample, 125 mm from the bottom. The specimens are then tightly wrapped around a 12.7mm mandrel, secured with tape or wire at the top above the 125 mm line. The mandrel is removed and the specimen is secured by a clamp in a vertical position. The method then follows the same procedure as in the "V" protocol. This method is most appropriate for label materials less than 0.254 mm thick and designed with flame retarding features. 

Federal Motor Vehicle Safety Standard 302 is a flammability test designed for the automotive industry and is frequently required for materials used inside the passenger compartment and in the engine compartment. 

As we discussed above, UL® 94 is the standard for safety or flammability testing of plastic materials for parts in electronic devices and appliances. We would argue that it is far more suited for products such as laptops and smartphones than it is for materials in EV batteries, as it is not included in the scope of the standard. FMVSS 302 tests the burning behavior of materials used inside road vehicles. It is the basis for testing specified by most automotive OEMs and better suited for our solutions.

FMVSS 302 is very similar to UL® 94 HB. A sample is clamped in a horizontally shaped "U" fixture and is exposed to an open flame at one end. The flame is removed and the rate of burn is examined. The burn rate is calculated then expressed as the distance burned over time. A material is usually classified as self-extinguishing if it stops burning in less than 1 minute and the flame has traveled less than 100 mm. 

UL® 94 dictates that individual components test really well for flame retardancy. Which can in turn lead to a very costly design, development and production process. Given the fact that combining two flame-retardant components doesn’t necessarily lead to a flame-retardant combination, designers, converters and engineers are faced with a challenge when it comes to choosing the right approach. 

You can combine two VTM 0-certified materials, a high UL® 94 quality standard for flammability, but that doesn’t automatically result in an VTM O-for the bonded laminate. 

To put it another way, a pressure-sensitive adhesive applied anywhere within a car, from seat heating to the design of the battery pack, will never be exposed, but will always be bonded to another material. Meanwhile, FMVSS 302 states that the top 13 mm of anything has to be tested. To quote the standard; 

The perfect partner can help you find the balance you are looking for. The Avery Dennison EV Battery portfolio offers multi-functional solutions that draw from our expansive portfolio of pressure-sensitive tapes and adhesives. Our products meet OEM specifications for a wide range of applications. Beyond bonding means we also welcome the opportunity to collaborate with automotive OEMs and tier suppliers to develop custom solutions that address this and many other challenges. You’ll enjoy access to testing facilities and pressure-sensitive adhesive experts who understand the challenges engineers face. We can work together to produce one-of-a-kind products that give you the advantage you seek. 

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