Biaxial Stretching of Film

Principles and Applications

Mark T. DeMeuse

• Publisher: Woodhead Publishing
• Year: 2011
• ISBN:
  9780857092953
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Biaxial (having two axes) stretching of film is used for a range of applications and is the primary manufacturing process by which products are produced for the food packaging industry. Biaxial stretching of film: principles and applications provides an overview of the manufacturing processes and range of applications for biaxially stretched films.Part one reviews the fundamental principles of biaxial stretching. After an introductory chapter which defines terms, chapters discuss equipment design and requirements, laboratory evaluations, biaxial film structures and typical industrial processes for the biaxial orientation of films. Additional topics include post production processing of biaxially stretched films, the stress-strain behaviour of poly(ethylene terephthalate) and academic investigations of biaxially stretched films. Part two investigates the applications of biaxial films including fresh cut produce, snack packaging and product labelling. A final chapter investigates potential future trends for biaxially oriented films and orienting lines.Biaxial stretching of film: principles and applications is a valuable reference tool for a broad spectrum of readers, ranging from polymer and fibre engineers to electrical engineers. It will also be suitable for professionals in the food packaging and paper industries.

• A valuable reference tool for polymer and fibre engineers, electrical engineers and professionals in the food packaging and paper industries
• Provides a comprehensive overview of the manufacturing processes of biaxially stretched films and includes a discussion of their future applications
• Places emphasis on the technology as well as the different types of polymers used

Part I

Fundamental principles of biaxial stretching

1

Fundamentals of biaxial stretching and definitions of terms

M.T. Demeuse, Independent Consultant, USA

Abstract:

This chapter provides definitions of key terms which are involved with the process of biaxial orientation of films. The two common types of biaxial orientation processes which are currently used, simultaneous and sequential orientation, are described in detail and advantages and disadvantages of both processes are discussed. Polymers which are commonly used in the biaxial stretching process are mentioned as well as their common uses.

Key words

simultaneous

sequential

polypropylene

1.1 Introduction

Biaxial stretching of film is a common processing technology for the production of products in numerous applications, including food packaging and labels. There are many unique terms to the process which are not encountered in other polymer processes and a clear definition and understanding of the terminology is required to better understand the details of the events that occur during biaxial stretching. This chapter describes the fundamental aspects of the biaxial film production process and provides definitions of the key terms which are encountered.

2.1 Methods of biaxial stretching

Biaxial stretching of films is the process of forming hot plastic films in crossmachine directions, resulting in a stronger film. Another term commonly used for stretching is orientation. On a molecular level, orientation is the alignment of polymer chains in the film in particular directions. What causes the molecular orientation is a force which acts on the polymer molecules to pull them in a direction and then the molecule is frozen in place, as by quenching from a molten state. Relative levels of orientation can be measured in many different ways, ranging from X-ray diffraction to tensile properties. The simplest ways to measure film orientation are by measuring the shrinkage and tensile properties of the film. These measurements will give relative levels of orientation and can be correlated with the absolute measures of orientation, obtained from techniques such as X-ray.

The biaxial stretching process is performed in two directions within the film. The machine direction (MD) is the direction that the film moves through the machine from start to finish. The transverse direction (TD) is the direction perpendicular to the machine direction. It is usually the same as the width of the film.

Film can be biaxially oriented in two different ways, sequentially and simultaneously. In the sequential orientation process, the film is first oriented in the machine direction. This orientation can modify the crystallization features of the polymer. After the machine direction orientation, the film is stretched independently in the transverse direction. Owing to the changes in crystallization induced during the first orientation step, the temperatures used in the transverse orientation are usually higher than in the machine direction. Also, because of the different stretch ratios which are normally used in the two steps, the film physical properties are different in the two directions.

In the simultaneous orientation process, on the other hand, the film is stretched at the same time in both directions in a single step. Typically, the stretch ratios in both the machine and transverse directions are equivalent. This leads to the film’s physical properties being quite similar in both directions and more balanced than when the stretching is done in the sequential manner. Depending on the type of property profile that is desired in the final product, preference can be given to either the simultaneous or the sequential stretching approach.

There are two primary industrial processes for biaxial stretching of film, the double-bubble method and the tenter method. In the double bubble method, a circular die is used from the extruder to form a thick walled tube of polymer. This is then blown under air pressure, orienting the film in the transverse direction. At the same time, an equal orientation in the machine direction is achieved by adjusting the speed at which the tube is pulled downwards and collapsed. The double bubble method produces film which is balanced in the sense that it has the same mechanical properties in both the machine and transverse directions. The bubble method is an example of simultaneous orientation.

In the tenter production method, polymer is extruded as a sheet directly onto a chilled chrome roller. The film is then passed through a stretching unit by rollers moving faster than the rate at which the material is extruded. This orients the film in the machine direction. Film can typically be oriented up to ten times in the machine direction.

The film is then fed into a tenter frame for transverse direction orientation. In the tenter, the film is gripped along each edge by clamps that are attached to moving chains. These move outwards to stretch the film in the transverse direction. Typical stretch ratios depend on the polymer being used, but ratios as high as 10–12 times are possible with some polymers.¹ The tenter method is an example of sequential orientation.

The initial application of tenter frame technology was not in plastics but in fabric. The tenter frame was used to treat and dry the fabric while maintaining the fabric width by clips which ran on two parallel chains. It was only after the successful implementation of the technology to fabric materials that it was extended to the film stretching process.

There are certain benefits to bubble film versus tenter film. Among these are balanced orientation and excellent shrink tightening properties. Bubble film also offers improved film cutting, including die cutting. It provides access to both thin and thick film technology, down to 15 microns and up to 250 microns in thickness. Finally, owing to the high orientation level in the machine direction, bubble films offer high MD stiffness, high MD tensile strength and low MD elongation, important factors for conversion.

1.2.1 Biaxially oriented polypropylene (BOPP)

One of the primary polymers which is used in the biaxial film stretching process is polypropylene (PP). Film which is produced from PP in the biaxial orientation process is usually designated BOPP (biaxially oriented polypropylene). BOPP films are used in food packaging, cigarette package overwrap, labels, adhesive tapes and a variety of other applications.

A typical BOPP production line consists of the casting sheet extrusion, biaxial orientation, after treatment, rolling, cutting and automatic control. The focus of this discussion will be on the biaxial orientation portion of the line. Other aspects of the production line have been discussed elsewhere (see, for example, Jenkins and Osborn).²

Some of the features of BOPP films are:

• high tensile strength that facilitates high-speed conversion;

• high gloss and clarity;

• good puncture and flex-crack resistance over a wide range of temperatures;

• good barrier to water vapor;

• resistance to oils and greases;

• not affected by moisture and does not wrinkle or shrink with environmental changes.

Many of these features are important in the use of the BOPP film in the applications previously mentioned. More specific details of these features and how they impact the uses of the film will be provided in later chapters of this book under the sections on the various applications which are discussed.

1.2.2 The tenter frame biaxial stretching process

Very little technical information about the tenter frame biaxial stretching process has been published in the open literature. Typical machine parameters that can be adjusted to control molecular orientation are stretch ratios, both MD and TD, strain rates and stretching temperatures. The strain rate is adjusted through the line speed, which varies from tens of feet per minute up to speeds greater than 1000 feet per minute (300 m/min) for commercial production lines. The degree of crystallinity, crystal morphology, and degree of orientation in the stretched film depend on these processing conditions. Both off-line experimental studies of biaxial orientation³ and on-line techniques⁴ have been applied to understand the evolution of the film microstructure during the stretching process. An in-depth discussion of these results applied to BOPP film is provided in Chapter 4.

In the case of the double-bubble film production process, very little prior research is available in the literature. While it appears very similar to the more traditional melt blowing process, there are differences between the two processes that lead to different film properties. Those differences are:

• cooling of the extrudate – the limiting of crystallization and spherulitic growth provides the film s...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Part I: Fundamental principles of biaxial stretching
• Part II: Applications of biaxial films
• Index