To stretch PP is to improve both its mechanical and optical properties. How does it work? Stretching, also known as orientation is a common technique used any many different PP applications such as straps, films, living hinges, bottles and fibers. Probably one of the largest volumes of oriented PP is knows as BOPP or bi-axially oriented polypropylene. As the name suggests, the PP is stretched in two different directions known as the machine direction (MD) and the transverse or cross direction (CD). If allowed to cool without stretching, the molecular chains in PP will align randomly. However, by applying an outside force to PP during cooling, the molecules tend to coil into a
shape; these helices then line up next to one another to form the crystals that give commercial polypropylene many of its desirable properties such as strength, stiffness, clarity and improved barrier properties. Alignment of the crystallizing cells gives OPP film its advantage over PP that was allowed to cool without stretching.
Stretching also occurs during fiber production as well. As the molten polymer leaves the mold, it is pulled away by hot air vents that stretch the fibers away from the mold at a high speed. As the fibers stretch, they shrink in size but gain a significant amount of strength as their molecules align within the polymer strand. It is easy to imagine how easily these strands might break due to impurities in the material or fluctuations in the manufacturing process. For these reasons, it is important to start with a material that has a very narrow molecular weight distribution (MWD). By using a NMWD material, makers can ensure the uniformity of the molecules being stretched. If there were long and short molecules in the PP fibers, then the fiber would break every time a short molecule was stretched beyond its limit. Currently, ExxonMobil is the market leader for producing NMWD material. PP3155E3 (MFR 36) is probably the most well known material on the market for this type of application. Their new material, PP3885E1 is also finding greater acceptance with those manufacturers whose machinery can handle the higher MFR (65).