Film Processing Advances

Single-screw extruders are the preferred machines for plasticating and metering resin to downstream film processes. The extruder must provide a homogenous and stable extrudate at high rates and at the target discharge temperature and pressure. Moreover, gels must be at a low and acceptable level. Gels are defined as any particle that creates an optical defect in the film.

Understanding the physics relating to the development of blown film physical properties is potentially very important to both the industrial and academic practitioner. This chapter focuses on many of the processes that affect the development of physical properties during production of semicrystalline polymer such as polyethylene (PE) film. The physical properties of PE are frozen into the film as a result of physical crosslinks developed during crystallization. The crystallization of low-density polyethylene is investigated as received from the polymer producer and relative to the influence of a small amount (less than 6%) of high-density PE added to the asreceived resin as a nucleation agent. The process was evaluated by the use of simultaneous online SALS (small angle light scattering) and IR (infrared) temperature measurements to study structure development as influenced by nucleation during tubular blown film extrusion of LLDPE. As the concentration of the nucleation aid HDPE was increased, the observed scattering intensity decreased.

Flexible packaging is used in our everyday lives, such as in food packaging, medical packaging, and protective films. Multilayer coextrusion of up to 11 layers has become increasingly common over the past 15 years. There are many inherent advantages of coextrusion. First, with the ability to coextrude different materials into one single structure, different applications with various properties can now be achieved.

In blown film extrusion, the polymer is melted in an extruder and the hot melt is pumped through a die to form a thin-walled tube, which is simultaneously axially drawn and radially expanded. In most installations the extruders are horizontal and the blown film bubble is formed vertically upward as shown in Fig. 4.1.

Recently, extrusion products such as films and sheets have come to be used in various fields including industrial products, food packaging, daily life products, and so on, and their application has been expanding even further with the introduction of new types of polymer materials. The T-die casting method, the most widely used for flat film forming, gives high productivity and good uniformity of film thickness.

Drawing instabilities and rupture are a serious limitation in polymer fiber and film processing. Onset of these defects depends on the processing conditions, on heat transfer, and on the rheology of the polymer even if some may be encountered for Newtonian fluids. The main part of this chapter is devoted to the different forms of the draw resonance instability encountered in fiber spinning, cast film, and film blowing. The time-dependent equations are presented for the simplified situation of constant width cast film. Results of the two possible modeling strategies, linear stability analysis and direct simulation, applied to the different fiber and film processes are discussed.

The stretching of films constitutes a finishing process in which the mechanical properties, optical characteristics, and barrier properties are increased significantly. The improvements of the properties result from the orientation of the molecular chains due to the stretching as well as from the increase in the degree of crystallinity in the case of semicrystalline plastics.

There are two processes used to produce biaxially oriented films: the tentering process and the double bubble tubular film process.

The tentering process is divided into a sequential stretching method (a step-by-step stretching method) and a simultaneous stretching one.

Fibers, films, bottles, and various plastic products of three-dimensional shape are produced from polymeric materials through polymer processing technologies such as fiber spinning, fi lm casting and dra wing, tubular fi lm blowing, blow molding, thermoforming, injection molding, and so on . The most important characteristic of polymeric materials is that the properties of polymer products vary significantly depending on the higher-order structure developed in the products during polymer processing.

The double bubble tubular film process is a more economical way of producing biaxially oriented film when compared to the tenter biaxially oriented film process. This film manufacturing technique has been widely used to produce biaxially oriented films because of good shrinkability and excellent physical properties. For these reasons, double bubble tubular film (DBTF) is used as the shrink film for foods, PET bottles, electrical parts, and package film for retort foods.

In recent years, environmental problems have come into question in the packaging industry. These problems, such as dechlorination and waste reduction, have been a major focus in this industry. Waste reduction especially has been a serious problem. To achieve waste reduction, there is a rapidly increasing shift from a disposable bottle to a standing pouch for repackaging use in an effort to utilize resources effectively.

Polypropylene (PP) is an excellent resin from the viewpoint of it having a reasonable price, good physical properties, good recycling features, and so on. However, because it is a crystalline resin, it is rather difficult to obtain a highly transparent sheet, and it has had limited development with regards to usage when a high transparency is required.