Polypropylene films have found wide acceptance in the packaging industry, especially as a food packaging agent, because of their superior physical properties. Polypropylene film, usually biaxially oriented, is characterized by high tensile modulus and stiffness, and excellent optical clarity and a certain degree of moisture resistance. However, polypropylene film is highly pervious to gases and air. Moreover, polypropylene possesses one major disadvantageous property, high inherent coefficient of friction (COF).
High COF complicates the processing of polypropylene film. Polypropylene film processing is impeded by poor transport, caused by its high COF, over rollers, guides and the like. In addition, high COF created film storage problems. Because of its high COF one layer of polypropylene film sticks to those above and beneath it, creating the problem commonly referred to as blocking.
This serious disadvantage of polypropylene film is well known to those skilled in the art. Thus, many proposed solutions to overcome this major deficiency have been proposed in the art. One such proposal has been to incorporate additives in the polypropylene resin processed to form the film. A favorite additive suggested in the art is fatty acid amides. These amides decrease the film's COF as they migrate to the film surface after heat treatment and aging.
Although this method has been used, actual COF is a function of the heat history to which the film has been exposed during shipping, storage and processing. As such, it is subject to wide variation. More significantly, the presence of fatty acid amides on the film surface oftentimes adversely affects the appearance of the film as manifested by decreased gloss and the presence of streaks. Another serious disadvantage of using fatty acid amides is the detrimental effect of fatty acid amides on polypropylene film surface wettability and adhesion. This adverse characteristic applies to coating, inks, adhesives and the like, especially in water based forms.
It is also known to coat polypropylene film with certain fatty acid amides to impart lubricating and antiblocking characteristics, as taught in U.S. Pat. No. 4,255,644. However, the application of such coatings by the film manufacturer is not particularly attractive because of the requirement that they be applied as solutions in organic solvents. Health and safety factors dictate against the in-plant utilization of organic solvents in coating processes undertaken during the film manufacture.
Other slip additives have been suggested to overcome the inherent problem of high COF in thermoplastic films. One such solution, as disclosed by U.S. Pat. No. 4,302,506, is the use of a latex coating containing stearamidopropyl- dimethyl-beta-hydroxyethylammonium nitrate and a crosslinkable acrylic copolymer. This coating has been advanced for use on polyester films.
Although this coating is alleged to improve slip properties without adversely affecting clarity in polyester films it is unnecessary to determine whether this coating would provide the same improved result on polypropylene film. It is known that such a coating is not in conformity with the requirements and regulations of the U.S. Food and Drug Administration for use in films which contact foods. As such a critical market for polypropylene films, as a food packaging agent, could not be exploited if such a material was added to polypropylene-containing film.
Yet another suggestion advanced in the prior art to reduce the blocking characteristic of polypropylene film is to add a finely divided inorganic material as a surface modifier. Such a modification not only provides a non-blocking surface having improved slip characteristics but is also independent of the heat history of the film. Moreover, the addition of such a surface modifier does not create the adverse optical and wettability effects associated with amide-modified films.
However, like the other proposed solutions, this proposed means of alleviating high COF creates new problems. Oftentimes, polypropylene film is laminated to other films. For example, glassine paper is commonly laminated to polypropylene film. Such a laminate, when provided as a thin surface layer containing finely divided inorganic material, exhibits significantly higher COF values than does the unlaminated polypropylene film. Such laminates are known to perform marginally at best on conventional form, fill and seal machines.
The above discussion reflects the need in the art for a new and improved polypropylene film characterized by improved anti-blocking characteristics and decreased COF. However, it is incumbent that the improvement in film slip property not correspond to the decline in other properties, typical of the solutions advanced in the prior art.