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Evaluation of new film formulations in orientation studies

更新时间:2016-02-14 10:02:02 来源:Packaging Films

Orientation of film substrates is used for a range of applications. On a molecular level, orientation is the alignment of polymer chains in the film in a particular direction. What causes the molecular orientation to occur is a force that acts on the polymer molecules to pull them in a specific direction and, then, the molecule is frozen in place, by quenching from a molten state. Relative levels of orientation in a film product can be defined and measured in a variety of different ways, ranging from x-ray diffraction to measurement of tensile properties.
 
In general, films that are used for packaging applications can be oriented in either one direction, or a uniaxial process or in two directions, a so-called biaxial film production process. The two orientation directions are termed the machine direction, or MD, and the transverse direction, or TD. The machine direction corresponds to the direction that the film moves through the machine from start to finish. The transverse direction is the direction perpendicular to the machine direction. It is usually the same as the width of the film.
During the investigation and development of new film formulations, it is often necessary to define the optimum orientation profiles, in terms of temperatures and stretch ratios. In this article, a general protocol for the establishment of such a profile is discussed and the key experiments are highlighted. This information will assist anyone who is working on new films for packaging applications.
 
 
Laboratory evaluations of new film formulations
When evaluating new film formulations, it is often necessary to work with relatively small quantities of materials. In this case, it is important to have an approach to define the stretching features on a laboratory scale. There are several pieces of equipment available to perform such tests. These include Accupull from Inventure Laboratories and Karo IV from Brückner. Both of these units can be used to perform the studies that are proposed in this article.
A typical series of experiments will involve the stretching of the new film formulation to some defined ratio, usually either 5x by 5x or 6x by 6x, at a predetermined temperature. The initial stretching temperature is defined by the chemical composition of the film and the thermal transitions of the polymers being used. The glass transition temperature or Tg of the main component in the polymer formulation is of the most significance. The glass transition temperature is the temperature at which a transition from a hard glassy state to a molten or rubber-like state occurs. As such, it represents the lowest temperature at which any significant amount of stretching can be done. Table 1 contains a summary of the glass transition values of some common polymers.
The initial stretching experiments should be carried out at a temperature above the polymer Tg. A reasonable temperature to start is 10 °C (50 °F) above Tg or Tg +10 °C (50 °F). However, it should be noted that an evaluation at a single temperature is not sufficient to define a stretching temperature “processing window”. In this context, the term processing window means those stretching temperatures that allow for stretching of the film with a very limited number of film breaks. Ideally, there should be a range of stretching temperatures for which there are no film breaks.
Extending the concept of a processing window, in the development of a new film formulation, it is not sufficient to know that the new formulation can be adequately stretched at a single temperature. This is because in an actual commercial film production operation, there are often temperature gradients involved and the ability tocontrol to a single temperature value is very unlikely. If, on the other hand, a range of acceptable stretching temperatures is defined, it will be possible to create more flexibility in the stretching temperature profile. Such flexibility is a requirement if frequent film breaks are observed when a particular stretching temperature is being utilised.
In this case, a typical adjustment is to modify the orientation temperature. A knowledge of the processing window allows for systematic choices to be made in terms of the level of changes that are required to move from a situation in which frequent film breaks are observed to one in which the film orientation process is running effectively.
The need for an understanding of processing windows is exemplified in work done by this author involving a homopolymer polypropylene material that was synthesised using a metallocene catalyst [1]. In this study, film samples were evaluated for their stretching features using a laboratory film stretching unit. When the stretching was carried out at a single temperature, high quality film with physical properties greater than those of standard biaxially oriented polypropylene film were obtained. However, when the stretching temperature was decreased by 5 °C (41 °F), frequent film breaks occurred. On the other hand, when the stretching temperature was increased 5 °C (41 °F), the film optical properties, particularly the haze, worsened significantly. These results indicate that films made using this particular type of polypropylene homopolymer variety have a very narrow temperature processing window.
The idea of a processing window is further elucidated in figure 1. That figure shows that there is an optimum temperature range for obtaining the required stretch ratio. At temperatures that are lower than that optimum range, many films breaks are observed during the stretching process. At temperatures above the optimum temperature range, properties of the film, such as opticals, will suffer. Ideally, the temperature processing window should be as wide as possible to ensure that film stretching can be accomplished in an effective way. It should be noted that there is a temperature processing window for each particular stretch ratio that is required.


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