d. Film (PVC). The film is a thin, transparent, polyvinyl chloride material used as a separating film for a flat patch
to prevent the patch from adhering to the backup plate or supports. In pipe patching, it is used to cover the entire patch
and retain the activated resin around the patch. In this application, it is described as a retaining cover. Kraft wrapping
paper or Mylar can be substituted as retaining covers.
8-18. PATCH APPLICATION SEQUENCE. During the several steps in the application of the different patching
materials, the area covered gets progressively larger. FIGURE 8-9 illustrates the position of patching materials relative
to each other.
FIGURE 8-9. Sequence of Applied Patch Materials.
This buildup in patch length during applications must be considered initially in determining the length of patch to
be applied. Where practical, allow the patch to extend 4 to 5 inches on each side of the rupture. With a severed pipe,
the length of the patch must be considered to be longer than in the case of a simple pipe patch.
8-19. OUANTITY OF MATERIAL REOUIRED. After the decision has been made as to the type and size of the patch to
be applied and the pipe has been prepared, the following formulas can be used to determine the quantities of basic
materials that will be required.
WR x 2
Active resin (grams)
Woven Roving (in square inches)
Circumference of pipe
The width of WR = length of patch