UL224 is one of the safety standards of the UNDERWRITERS LABORATORIES INC..It is a criterion for the evaluation of extruded insulation tubes and has become an important test standard for the heat shrinkable tube industry. There are several projects in UL224. Today we mainly introduce the flame retardant VW-1 often mentioned in heat shrink tubing. It is called “vertical burning test” in UL224.

A wedge is provided to hold the base of the burner so that the burner flame tube is at a 20° inclination to the vertical and the tube axis is in a vertical plane (see figure). The torch is attached to the wedge and the entire mounting is placed on an adjustable stand; the non-defatted medical cotton is placed over the wedge bevel and the base of the torch. The flame was simmered for 15 seconds at a 20° angle with a flame of a test torch for 15 seconds, and repeated 5 times.

If the test results meet the following requirements at the same time ,then pass the test:

A) After any time laboratory burner stops supplying fire, Within 60 seconds, the remaining flame will go out;

B) During and after the test, the cotton laid at the bottom is not ignited;

C) During and after the test , the area of the indicator sample burned or charred into char is less than 25%(Smoke or brown burnt marks that can be wiped off with cloth or fingers are ignored).

It can be seen that the test standard for the flame-retardant performance of the heat shrink  tubing to reach VW-1 is very strict and important. Next, I will take you to a deeper understanding of the flame retardant determinant of heat shrink tubing - flame retardants.

References---[UNDERWRITERS LABORATORIES INC. UL224 Standard]

                                                                                                  By Jeter Lee