Heat transfer in shrink films

Abstract

Polyethylene is a thermoplastic polymer composed of a repeating unit (-CH2-CH2)n. It is the most consumed plastic due to its low price and ease of manufacture. It can be described by three basic characteristics that influence the processing and properties: density, molecular weight and molecular weight distribution. Principally, polyethylene is classified according to its density. The more used in the film industry are: low density polyethylene (LDPE), lineal low-density polyethylene (LLDPE), and high-density polyethylene (HDPE). Polyethylene films are produced by blow extrusion, usually from the pellets of LDPE. And one important application of these films is packaging. Shrink films are very used in the packaging industry. The film shrinks because a high degree of molecular orientation or internal stress was introduced into it during its manufacture. When the film is introduced in an oven and reheated to a temperature above its softening point the internal stresses are released, causing the film to shrink. However, these films need more studies to prevent the problems that appear during the shrink process. And this is the objective of this project: to improve the shrink process studying the heat transfer in the oven, knowing that there happen four phenomena: radiation and convection heat transfer between the oven and the film; conduction heat transfers between the film and the bottles and convection heat transfer between the bottles and the oven. The calculations have been made for four different formulations. One important fact in the shrinkage is the Blow up ratio (BUR). It is a number that refers to the ratio of a blown film bubble diameter to the extrusion die diameter. The higher the BUR, the larger the bubble diameter is in relation to the die. There are two ways to determinate the shrink-percentage. One method is by immersion of the film in an oil-bath at a chosen temperature. The other method consists in a sample introduced in a hot oven and subsequently cooled down to the room temperature. However, to calculate the shrinkage in the laboratory the samples have been heated in an oven to different temperatures (130ºC, 135ºC, 140ºC, 190ºC and 220ºC) during different times, with the objective to see the percentage of shrinkage in the machine direction (MD) and the transverse direction (TD). As previously mentioned, the orientation of the film is an important aspect to the final shrinkage. For that reason, the orientation of the films in the study has been measured by an infrared spectroscopy method. The finally results of the shrinkage, the heat transfer and the orientation of the films have been related to arrive to the conclusion that the shrinkage of the film is a combined result of the film orientation, the temperature of the oven and the melting temperature of the films. And also that the important heat transfer happen just in the first 1,9 seconds.