Properties of uniaxially stretched polypropylene films

Canadian Journal of Chemical Engineering, Dec, 2008 by Farhad Sadeghi, Pierre J. Carreau

INTRODUCTION

Improving the strength of polymer films through stretching is a well-established technology. Film producers mostly rely on biaxial stretching to strengthen the films in both machine and transverse directions (TD) while the applications for uniaxial stretching (MDO for machine direction orientation) are more limited. Some of the applications are found in polyethylene terephthalate (PET) strapping, polyvinylchloride (PVC) food wraps, fibrous high-density polyethylene (HDPE) ribbons for weaving sacks, breathable hygienic films in diaper liners, self-adhesive labels, and polyolefin packaging and lamination (Schut, 2005). MDO is widely used for polyethylene and polypropylene (PP) films because of their numerous applications in packaging. A MDO unit normally includes a cast extrusion line, a slit die, a big drum roll for cooling, and multiple stretching units in sequence. Stretching is carried out in these units that can operate at different temperatures and stretching ratios. The MDO unit available at the Center for Applied Research on Polymers (CREPEC) contains two stretching zones as presented in Figure 1.

[FIGURE 1 OMITTED]

The MDO unit can be operated either in-line or off-line with extrusion and is controlled via variables such as: DR, drawing speed, drawing times (a film can be stretched many times), drawing temperature, and heat-setting conditions. The distance between the draw rollers was set at 5 cm.

The use of MDO units is growing, mostly for specific polymers. The process usually improves the strength and barrier properties, but there are some disadvantages for the produced films. For example, the MDO process improves the barrier properties of nylon and ethyl vinyl alcohol (EVOH) polymer, but it also makes the films brittle and of very low tear resistance in machine direction (MD) (Schut, 2005). For EVOH, MDO is applied selectively since any biaxial stretching deteriorates the barrier properties.

The stretching is usually performed at high temperature and results in a very highly oriented film that causes anisotropy. Nie et al. (2000) studied the morphology development during uniaxial and biaxial stretching of PP films using atomic force microscopy (AFM). They reported the formation of a fibrillar structure with thicker fibrils in the MD while much thinner fibrils connected the thick fibrils to create an overall strong crystalline network. Dez et al. (2005) investigated the influence of the stretching on the crystallinity and structure of biaxially oriented PP films. Their results confirmed the formation of the oriented crystalline fibrils during stretching. They also observed that the stretching ratio did not have a significant effect on the melting point of the stretched samples. Koike and Cakmak (2004) showed that an initial cross hatched of lamellae (typical morphology of PP films produced under low cooling conditions as shown by Sadeghi et al. (2005)) was firstly broken down into small crystal pieces during stretching and then joined to form microfibrils oriented in the MD. Koike and Cakmak (2004) observed an intermediate shish-kebab-like crystalline structure during stretching. The transformation from a shish-kebab-like into a fibrillar structure took place at a strain of 1.2 and the reported macrofibril size was 3-5 [micro]m in diameter.

Rettenberger et al. (2002) studied the effect of temperature on the stress-strain behaviour during stretching of PP. They found a typical ductile behaviour with a yield point, neck propagation, and strain hardening up to a temperature of 155[degrees]C. At higher temperatures instead of yielding the deformation was a quasi-rubber-like. This behaviour was also reported when the films were stretched at high strain rates (over 750 mm/s). They also observed that the non-homogeneity of the deformation reduced with increasing strain rate.

Srinivas et al. (2003) studied the cold stretching of a metallocene linear low-density polyethylene (LLDPE) and reported that the modulus and tensile strength were almost linearly improved with increasing DR (for DRs above 5). This DR dependence was also observed by Gould (1988) during the cold drawing of some polymers. For nylon and polyester, the tensile strength obeyed a strong linear relationship with DR, while for PP the graph revealed two regions: one with a steep slope in the small DR range of 5-7 followed by a second region with a smaller slope up to a DR of 10.

Bheda and Spruiell (1986) analyzed the light transmission properties of oriented PP films. Samples with higher orientation showed a greater overall light transmission. Bheda and Spruiell (1986) claimed that the light transmission was controlled first by surface roughness and second by the internal morphology of the films. Taraiya et al. (1993) examined the permeability of oriented PP films. They proposed that the primary factor that control the permeation of gas molecules and, hence, the barrier properties was the orientation of the amorphous phase. The flow of gas through the amorphous phase of a polymer film can be presented as P=DS, where P is the permeability coefficient, D is the diffusivity, and S is the gas solubility (adsorption of gas molecules on the film surface). The orientation of the amorphous phase influences both parameters, in particular the diffusivity. Taraiya et al. (1993) showed also that the oxygen permeability was reduced with decreasing draw temperature.