The evaluation of plastic materials may include the determination of material flammability (burning characteristics), ignition characteristics from various thermal and electrical sources, electrical tracking and additional electrical characteristics, and analytical tests. Property retention following exposure to long-term elevated temperature (air-oven aging), water, ultraviolet light, cold, and other outside influences may also be evaluated.
Properties evaluated by UL include:
- Color Concentrates
- Components – Concentrates
- Outdoor suitability program for plastics
- UL 746A electrical and mechanical properties
- UL 746B thermal-aging program
- UL 746C file review
- UL 94 flammability
Materials for use in special environments are investigated for specific end-use product applications where requirements exist for the retention of properties after exposure to certain critical operating and/or environmental conditions. This includes exposure to various types of ultraviolet (UV) radiation.
Color Concentrates (QMQS2)
When an end-product manufacturer orders a Recognized plastic in a custom color, the Recognized plastics supplier must stop production, clean the machinery, and proceed with a special run for the production of this custom-color. This is a time-consuming and costly procedure that is typically reflected in the cost of the order. In addition, the Recognized plastics supplier may require a large minimum order unless the order is for a neat natural resin.
The color concentrate category (QMQS2) offers end-product manufacturers faced with these challenges the option of ordering the necessary amount of the Recognized neat natural resin while also obtaining a color concentrate Recognized for use with the Recognized plastic. By dry blending the two materials without exceeding the published maximum let-down ratio, end-product manufacturers can produce the contracted part, which is typically more cost-effective than ordering a custom Recognized plastic.
Recognition of a color concentrate for use with a vertically rated Recognized base resin is specific and requires the submittal of standardized test samples. The samples consisting of the color concentrate let-down into the Recognized base resin are tested to certify that the use of the color concentrate does not compromise the flame rating of the Recognized base resin. The Recognition of color concentrates requires submittal and testing for each Recognized vertically rated base resin.
Similarly, Recognition for use with HB-rated Recognized base resins involves the submittal of standardized test samples of the color concentrate let-down into a Recognized HB-rated base resin for testing. If this testing does not compromise the HB-flame rating of the Recognized base resin, the newly Recognized color concentrate has a generic Recognition which permits its use in any Recognized HB-rated base resin of that particular generic type.
Component Concentrates (QMLJ2)
A custom-compounding plastics company typically purchases a neat natural resin from a plastics supplier and enhances that formulation by introducing an additive concentrate package to generate a new product. This newly Recognized material is tested and published under the component-plastics category (QMFZ2). The component-concentrates category (QMLJ2) permits the Recognition of the additive concentrate package that was blended with the Recognized neat base resin to produce the newly formulated Recognized plastic.
Category QMLJ2 also permits the Recognized base resin to be shipped directly to the Recognized end-product manufacturing location. The Recognized concentrate package can be shipped directly to the manufacturer as well. Before molding, resin and package are dry-blended at the specific let-down ratio by the Recognized manufacturer. For this process, the manufacturer must be Recognized under the fabricated parts, Recognized molders category (QMMY2).
This method of supplying the base resin gives the end-product company an opportunity for large scale purchasing and shipment of the base resin. The Recognized concentrate can be ordered and shipped based on production schedules. This method also provides the plastics supplier with the capability of large-scale production runs of the base resin rather than custom compounding orders.
In publishing the Recognition for the concentrate, the Recognized concentrate package, Recognized base resin supplier’s name, file number and grade designation are noted on the Recognition card. The newly formulated Recognized material designation, file number and company name are also noted.
To ensure reproducibility of the Recognized concentrate package, identification testing consisting of infrared spectroscopy (IR), thermogravinetry (TGA), and differential scanning calorimetry (DSC) testing is conducted on the Recognized concentrate package let-down into its generic carrier. Periodic testing under UL’s Follow-Up Services program is conducted.
IEC Ball Pressure Temperature (BP)
IEC ball pressure temperature, in accordance with IEC 695-10-2, is expressed as the temperature (in degrees C) at which a 5-millimeter diameter ball causes a 2-millimeter diameter or less impression on the material surface.
A material considered suitable for outdoor use has gone through testing in accordance with UL 746C, Polymeric Materials, Use in Electrical Equipment Evaluations as follows:
- 1,000 hours of xenon-arc weatherometer conditioning
- Water immersion for seven days at 70 degrees C.
The material is tested for flammability, mechanical impact and mechanical strength before and after exposure to these conditions. Test results may lead to one of the following outcomes:
(f1) — This footnote indicates that the material has met both UV and water exposure or immersion requirements as called out in UL 746C. The material is suitable for outdoor use with respect to exposure to ultraviolet light, exposure to water, and immersion in accordance with UL 746C.
(f2) — This footnote indicates that the material has only met or has been tested partially for UV or water exposure or immersion. The material was subjected to one or more of the following tests: UV, water exposure or immersion in accordance with UL 746C, where the acceptability for outdoor use is to be determined by UL.
Most materials covered by this program have been investigated with respect to retention of certain critical properties including dielectric, tensile and impact as part of a longtime thermal-aging program conducted in accordance with UL 746B, Polymeric Materials, Long Term Property Evaluations.
The end-of-life of a material at each test temperature in this program is assumed to be the time when the value of the critical property had decreased to 50 percent of its original (as received) value. If a material has been investigated under the thermal-aging program, the relative temperature index (RTI), in degrees C, shown is based on a comparison with a material which has acceptable service experience and correlates numerically with the temperatures above in which the material is likely to degrade prematurely.
If a material has not been investigated under the thermal-aging program, the RTI shown is based on the generic class of the material. A tabulation of the temperature indices according to the generic material class is included in UL 746B and is based on the past field-test performance and chemical structure.
More than one RTI may be appropriate for a given material depending on the property requirements for a specific application. In certain cases, higher temperatures are permitted if so specified by the end-use product Standard.
Relative temperature index (RTI) is the maximum service temperature for a material where a class of critical property will not be unacceptably compromised through chemical thermal degradation. This spans over the reasonable life of an electrical product relative to a reference material having a confirmed, acceptable corresponding performance-defined RTI.
Electrical RTI is associated with critical electrical insulating properties.
Mechanical impact RTI is associated with critical impact resistance, resilience and flexibility properties.
UL 746A Testing
UL 746A testing for electrical properties
The following tests determine the electrical properties of polymeric materials:
- Dielectric breakdown voltage strength (DS)
- Comparative tracking index (CTI)
- High voltage tracking rate (HVTR)
- High voltage, low current, dry arc resistance — D495
UL 746A resistance to ignition of polymeric materials
The following tests determine the resistance to ignition of polymeric materials:
- Hot wire ignition (HWI)
- High-current arc ignition (HAI)
- High voltage arc resistance to ignition (HVTR)
- Glow-wire ignitability test (GWIT, GWFI)
UL 746C, Polymeric Materials, Use in Electrical Equipment Evaluations revision modifications took effect on June 1, 2004. The following information summarizes the changes and provides additional guidance to help assist with product development.
Tests for Flammability of Plastic Materials for Parts in Devices and Appliances
There are two pre-selection test programs conducted on plastic materials to measure flammability characteristics. The first program determines the material’s tendency to either extinguish or spread the flame once the specimen has been ignited. The program is described in UL 94, Tests for Flammability of Plastic Materials for Parts in Devices and Appliances which is now harmonized with IEC 60707, 60695-11-10 and 60695-11-20 and ISO 9772 and 9773.
The second program measures the ignition resistance of the plastic to electrical ignition sources. The material’s resistance to ignition and surface tracking characteristics is described in UL 746A, which is similar to the test procedures described in IEC 60112, 60695 and 60950.
UL 94 Flame Classifications
There are 12 UL 94 specified flame classifications assigned to materials based on the results of these small-scale flame tests. These classifications, listed in descending order for each of the following three groupings, are used to distinguish a material’s burning characteristics after test specimens have been exposed to a specified test flame under controlled laboratory conditions.
- Six of the classifications relate to materials commonly used in manufacturing enclosures, structural parts and insulators found in consumer electronic products (5VA, 5VB, V-0, V-1, V-2, HB).
- Three of the remaining six classifications relate to low-density foam materials commonly used in fabricating speaker grills and sound-damping material (HF-1, HF-2, HBF).
- The last three classifications are assigned to very thin films that are generally not capable of supporting themselves in a horizontal position (VTM-0, VTM-1, VTM-2). These are usually assigned to substrates on flexible printed circuit boards.
Horizontal versus Vertical Positioning
Specimens molded from the plastic material are oriented in either a horizontal or vertical position depending on the specifications of the relevant test method. They are then subjected to a defined flame ignition source for a specified period of time. In some tests, the test flame is only applied once – as is the case in the horizontal burning (HB) test – while the flame is applied at least twice in other tests.
An HB flame rating indicates that the material was tested in a horizontal position and found to burn at a rate less than a specified maximum.
The three vertical ratings – V2, V1 and V0 – indicate that the material was tested in a vertical position and self-extinguished within a specified time after the ignition source was removed. The vertical ratings also indicate whether the test specimen dripped flaming particles that ignited a cotton indicator located below the sample.
UL 94 also describes a method in which the test flame is applied for up to five applications in testing for a 5VA or 5VB classification. These small-scale tests measure the propensity of a material to extinguish or spread flames once it becomes ignited.
Difference in Test Methods and Criteria
When looking at flame ratings for plastic materials commonly molded to fabricate enclosures and structural parts/insulators found in consumer electronic products (5VA, 5VB, V-0, V-1, V-2 and HB), a material classified as 5VA or 5VB is subjected to a flame ignition source that is approximately five times more severe than that used in the V-0, V-1, V-2 and HB tests. Also, these specimens may not drip any flaming particles. Three of the remaining six classifications specified in UL 94 relate to low-density foam materials commonly used in fabricating speaker grills and sound-deadening material (HF-1, HF-2, HBF). The remaining three classifications are assigned to very thin films commonly used in flexible printed wiring boards, generally not capable of supporting themselves in a horizontal position (VTM-0, VTM-1, VTM-2). A flame rating of VTM-0 cannot be considered equivalent to a V-0 rating, as the test methods are quite different. Likewise, VTM-1 and VTM-2 cannot be considered equivalent to V-1 and V-2 respectively.
UL 746A Ignition Tests
In addition to flammability considerations, a material’s ability to resist ignition from electrical sources is another important factor that must be considered in the selection and evaluation of a material for use in electrical equipment. Possible electrical ignition sources in equipment include: overloaded (overheated) electrical conductors and components; arcing parts, such as the open contacts of switches and relays; and arcing at broken or loose connections (e.g., splices or terminals). Polymeric materials in direct contact with or in close proximity to overloaded or arcing electrical parts could ignite.
The three basic tests used to evaluate a material’s ability to resist ignition are hot-wire ignition (HWI), high-current (or high-amp) arc ignition (HAI), and high-voltage arc tracking rate (HVTR). Details regarding test criteria can be found in UL 746A, Polymeric Materials, Short Term Property Evaluations. The UL iQ™ Database tabulates the results of small-scale tests conducted on the materials.
The HWI test determines a material’s resistance to ignition when exposed to abnormally high temperatures resulting from a component failure such as a conductor carrying far more than its rated current. HWI performance is expressed as the mean number of seconds required to ignite a specimen when wrapped with an energized non-chrome resistive wire that dissipates a specified level of energy.
The HAI test determines a material’s ability to withstand electrical arcing either directly on or just above the surface of the plastic material. This can occur in the presence of open switch contacts or in the event of the failure of an electrical connection. HAI performance is expressed as the number of arc rupture exposures – using standardized electrode materials, geometry and electrical supply circuit – required to ignite a specimen when the arc occurs directly on the surface or a specified distance above the surface.
The HVTR for a material is expressed as the rate in inches per minute that a tracking path can be produced on the surface of the material under standardized test conditions. This test relates to the establishment of an electrically conductive path on the surface of a solid, insulated material as a result of electrical stress.
Another ignition test can be applied to measure a material’s resistance to ignition property. This test is the glow-wire ignitability test and is described in UL 746A and UL 746C, Polymeric Materials, Use in Electrical Equipment Evaluations. The test method is based on a test procedure that is documented in IEC 60695 and specified in numerous IEC end-product specifications including IEC 60335-1. The test is somewhat similar to the HWI test in that it measures a material’s resistance to ignition on application of a heated non-flaming source.