Ethylen methakrylsyre copolymer (Surlyn®) danner varme- forseglingslaget, som gør, at brevets to laminerede materialestykker kan varmeforsegles sammen, hvorved de danner et brev.
The ethylene methacrylic acid copolymer (Surlyn®) is the heat seal layer which allows the two laminate sachet stocks to be heat-sealed together to form the sachet.