Blown Film

ExtrusionCantor

Kirk Cantor

Blown Film Extrusion

3rd Edition

Bonus companion software included

Cantor Blown Film Extrusion

Blown Film Extrusion

Kirk Cantor

3rd Edition

Hanser Publishers, Munich Hanser Publications, Cincinnati

Distributed in the Americas by: Hanser Publications 6915 Valley Avenue, Cincinnati, Ohio 45244-3029, USA Fax: (513) 527-8801 Phone: (513) 527-8977 www.hanserpublications.com

Distributed in all other countries by: Carl Hanser Verlag Postfach 86 04 20, 81631 Munich, Germany Fax: +49 (89) 98 48 09 www.hanser-fachbuch.de

The use of general descriptive names, trademarks, etc., in this publication, even if the former are not especially identi-fied, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. While the advice and information in this book are believed to be true and ac-curate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal respon-sibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with re-spect to the material contained herein.

The final determination of the suitability of any information for the use contemplated for a given application remains the sole responsibility of the user.

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© Carl Hanser Verlag, Munich 2019 Editor: Dr. Julia Diaz-Luque Coverconcept: Marc Müller-Bremer, www.rebranding.de, Munich Coverdesign: Stephan Rönigk Typesetting: le-tex publishing services GmbH, Leipzig Printed and bound by Hubert & Co GmbH und Co. KG Buchpartner, Göttingen Printed in Germany

ISBN: 978-1-56990-696-5 E-Book ISBN: 978-1-56990-697-2

The Author:

Prof. Kirk Cantor, Pennsylvania College of Technology, Williamsport, PA, USA

I am grateful for the many people that have supported my efforts to write this book and develop the software included. Funding for these projects was provided through a grant to the Plastics Resources for Educators Program (PREP) by sup-porters at the National Science Foundation. My colleagues at Pennsylvania College of Technology provided administrative support.

I am very thankful for my good friend and co-worker, Tim Weston, for his vision and leadership through PREP. Other PREP colleagues for whom I am thankful are Alex Bierly, who masterfully created all of the graphics for the simulator, and our other very talented artists, Mike Fleck, Matt Byers, and Craig Reitz.

Many thanks go to another close friend and mentor, Chris Rauwendaal, for not only helping with the text of this book, but for teaching me so much about extrusion over the years. I am thankful for the text review and insight given to me by Robert Krycki.

My friends at Hanser have been very helpful with the creation of the manuscript and artwork. Thanks especially to Christine Strohm for her years of encourage-ment and assistance.

Finally, I am most thankful for the support of my devoted family and the opportu-nity given to me by God. My lovely wife, Patsy, and my four beautiful daughters, Kristen, Caylee, Kelsey, and Shannon, have patiently endured my hours away and have even cheered me on to completion.

Kirk Cantor July 2018

Acknowledgments

Blown film extrusion is one of the most significant polymer processing methods. Several billion pounds of polymer, mostly polyethylene, are processed annually by this technique. While some applications for blown film are quite complex, such as scientific balloons (Figure 1), the majority of products manufactured on blown film equipment are used in commodity applications with low profit margins: grocery sacks, garbage bags, and flexible packaging (Figure 2). Consequently, sophisti-cated hardware, materials, and processing methods have been developed to yield film at very high output rates exhibiting both low dimensional variation and con-sistent solid-state properties.

Figure 1  A high altitude, scientific balloon being prepared for launch (National Aeronautics and Space Administration)

Preface

VIII Preface

Figure 2  Blown film extrusion is used to produce very high volumes of commodity products such as grocery and produce bags

Polymer chemistry and molecular structure are vital in establishing film proper-ties, but bubble geometry resulting from processing conditions is also significant. Molecular orientation and crystalline structure – controlled by bubble dimensions – affect properties such as tensile strength, impact toughness, and clarity.

As a manufacturing process, blown film is somewhat unique, even compared with other extrusion processes. Molten polymer generally exits the die vertically in the form of a freely extruded bubble reaching heights of 50 feet (15 meters) or more (Figure 3). Guides surrounding the bubble may limit its mobility, but it is still quite exposed to dimensional variation compared to the fixed extrudate in most other ex-trusion processes, which use vacuum sizers, calibrators, rollers, or other tech-niques. Depending on processing conditions, the blown film bubble has a shape freedom that allows almost any number of profiles within a designed range. Opera-tors must have a relatively high skill level to accurately obtain the required bubble geometry (i.e., the shape resulting in specified product dimensions and properties).

The strong interdependence of process variables is another aspect of the process that requires a high level of operator skill and has led to extensive advancements in measurement and control techniques. There are many process variables – screw speed, nip speed, internal bubble air volume, and cooling rate (frost line height) – that influence bubble geometry and, as a result, film properties. An adjustment to any one of these variables leads to a change in several geometric characteristics of the bubble. For example, an operator may intend to only decrease film thickness by

IXPreface

increasing the nip speed; however, if no other control is modified, this adjustment will also create an increase in both frost line height and layflat width. Therefore, the proficient operator is aware of the influence of each process variable on all geometric characteristics of the bubble and can control more than one characteris-tic at a time.

Figure 3  A blown film extrusion line (Windmoeller & Hoelscher)

From hardware and materials through processing and properties, this book is in-tended to provide the reader with a comprehensive understanding of blown film extrusion through a useful balance of theory and practice. Included in this book are the answers to why effects occur the way that they do in the blown film pro-cess, so the reader can improve his/her ability to troubleshoot and improve sys-

X Preface

tems. At the same time, current practices and equipment are emphasized to keep readers up-to-date with the most productive and efficient technology.

The companion computer-based learning tool, The Blown Film Extrusion Simulator1, enhances the reader’s understanding. This software was developed specifically to teach blown film extrusion equipment operation and processing principles. The realistic graphic interface and intuitive operating techniques were designed to em-ulate actual processing methods, so learners can quickly move from the simulator to real production equipment. Throughout this book there are exercises (identified with the symbol ) using the simulator to complement the methods and princi-ples explained. It is intended that, when convenient, readers will take a break from reading the book and spend a few minutes with the simulator to enhance their understanding of the content. Before continuing to the next chapter, the reader may want to skip to Appendix 1 in Chapter 9 to learn how to download, install, and operate the simulator.

1 Available for download, together with the eBook Bonus, using your personal code at www.hanserpublications.com/ebookbonus.

Contents

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII

1 Materials for Blown Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Polyethylene (PE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.1.1.1 Low Density Polyethylene (LDPE) . . . . . . . . . . . . . . . . . . . . 21.1.1.2 High Density Polyethylene (HDPE) . . . . . . . . . . . . . . . . . . . 41.1.1.3 Linear Low Density Polyethylene (LLDPE) . . . . . . . . . . . . . 51.1.1.4 Medium Density Polyethylene (MDPE) . . . . . . . . . . . . . . . 61.1.1.5 Metallocene Polyethylene (mPE) . . . . . . . . . . . . . . . . . . . . . 6

1.1.2 Polypropylene (PP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.1.3 Polystyrene (PS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.1.4 Ethylene Vinyl Acetate (EVA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.1.5 Ethylene Vinyl Alcohol (EVOH) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.1.6 Polyvinyl Chloride (PVC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.1.7 Polyvinylidene Chloride (PVDC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.1.8 Polyamide (PA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101.1.9 Polyurethane (PU) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.2 Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.2.1 Antiblocking Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2.2 Antioxidants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2.3 Antistatic Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2.4 Colorants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.2.5 Lubricants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.2.6 Reinforcements and Fillers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.2.7 Stabilizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161.2.8 Tackifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

XII Contents

2 Polymer Rheology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.1 Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.2 Variables Influencing Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.2.1 Process Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.2.2 Molecular Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.2.2.1 Molecular Weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.2.2.2 Molecular Weight Distribution . . . . . . . . . . . . . . . . . . . . . . 262.2.2.3 Chain Branching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.3 Rheology in Blown Film Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.3.1 Inside the Extruder and Die . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.3.2 In the Bubble . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3 Extrusion Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.1 Extruder Hardware Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.1.1 Drive System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.1.1.1 Motor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.1.1.2 Speed Reducer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.1.1.3 Thrust Bearing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.1.2 Feed System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.1.3 Screw/Barrel System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.1.3.1 Screw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.1.3.2 Barrel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.1.4 Head/Die System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.1.4.1 Head Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.1.4.2 Adapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.1.4.3 Breaker Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.1.4.4 Melt Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.1.4.5 Die . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.1.5 Instrumentation and Control System . . . . . . . . . . . . . . . . . . . . . . . . 473.1.5.1 Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.1.5.2 Head Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513.1.5.3 Motor Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.2 Inside the Extruder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533.2.1 Solids Conveying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.2.1.1 Gravity-Induced Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.2.1.2 Drag-Induced Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.2.2 Melting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573.2.3 Melt Pumping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593.2.4 Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

3.2.4.1 Distributive Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Contents XIII

3.2.4.2 Dispersive Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 623.2.4.3 Mixing Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3.2.5 Degassing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643.2.6 Die Forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

4 Hardware for Blown Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.1 Upstream Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4.2 Grooved Feed Throat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

4.3 Screws for Blown Film Extrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

4.4 Blown Film Dies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.5 Bubble Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

4.6 Bubble Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.7 Bubble Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

4.8 Collapsing Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

4.9 Haul-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

4.10 Winders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

4.11 Film Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

4.12 Line Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

5 Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1015.1 Process Variables vs. Bubble Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

5.2 Characteristic Bubble Ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

5.3 Process/Structure/Property Relationships . . . . . . . . . . . . . . . . . . . . . . . . . 107

5.4 Double Bubble Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

6 Coextrusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1116.1 Dies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

6.2 Interfacial Instabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

6.3 Product Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156.3.1 Breathable Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1156.3.2 Shrink Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1166.3.3 High Barrier Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

7 Film Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1197.1 Tensile Strength (ASTM D882) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

7.2 Elongation (ASTM D882) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

XIV Contents

7.3 Tear Strength (ASTM D1004, D1922, and D1938) . . . . . . . . . . . . . . . . . . . . . 123

7.4 Impact Resistance (ASTM D1709, D3420, and D4272) . . . . . . . . . . . . . . . . . 125

7.5 Blocking Load (ASTM D3354) and Coefficient of Friction (ASTM D1894) . . 126

7.6 Gel (Fisheye) Count (ASTM D3351 and D3596) . . . . . . . . . . . . . . . . . . . . . . 127

7.7 Low Temperature Brittleness (ASTM D1790) . . . . . . . . . . . . . . . . . . . . . . . 127

7.8 Gloss (ASTM D2457) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

7.9 Transparency (ASTM D1746) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

7.10 Haze (ASTM D1003) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

7.11 Density (ASTM D1505) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

7.12 Melt Flow Rate (ASTM D1238) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

7.13 Viscosity by Capillary Rheometry (ASTM D3835) . . . . . . . . . . . . . . . . . . . 132

8 Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1358.1 Extruder Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

8.1.1 Surging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1368.1.2 High Melt Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1378.1.3 Excessive Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398.1.4 Low Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

8.2 Film Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1408.2.1 Melt Fracture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1418.2.2 Thickness Variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1418.2.3 Die Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1468.2.4 Gels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1468.2.5 Low Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1478.2.6 Poor Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1498.2.7 Wrinkles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

9 Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1519.1 Appendix 1: The Blown Film Extrusion Simulator . . . . . . . . . . . . . . . . . . . 151

9.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1519.1.2 Download and Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1529.1.3 Running the Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1539.1.4 Worksheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

9.2 Appendix 2: Useful Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

This chapter covers the various materials extruded by the blown film process. It is divided into two broad categories: polymers and additives. Many types of polymers are extruded into blown film, but various grades of polyethylene comprise the vast majority. Additives are used in most operations to provide performance, appear-ance, processing, or cost benefits.

�� 1 .1� Polymers

The desired property set for any product determines the best material for the appli-cation. The most important properties, such as physical, mechanical, optical, ther-mal, and electrical, provide fitness-for-use in the final product. In addition to final product (solid–state) properties, processing properties are also very important in material selection. Ease of processing in blown film can be described by character-istics such as good thermal stability, high melt strength (outside of the die), rea-sonable head pressures, and no melt fracture (film surface imperfections). Finally, minimal cost is a key property.

There are numerous applications for blown film, but a very high percentage of film is used in commodity applications, such as packaging and bags. These products require a combination of performance, processing, and cost that make polyeth-ylene (PE) the ideal polymer for most applications. It is lightweight, water-resis-tant, has a good balance of strength and flexibility, and can provide some clarity. PE is easy to extrude and heat-seal and, perhaps most importantly, it is low-cost. In addition to these general properties of PE, it is a polymer that is very well under-stood scientifically, allowing for polymerization techniques that can be designed and controlled to yield specific property values over a very wide range. That is, PE grades that are much stronger than average, or much clearer, or much more flexi-ble, can be produced.

1 Materials for Blown Film

2 1 Materials for Blown Film

Within the broad family of polyethylene, there are several types that find applica-tion in blown film. There are, however, other polymers used as well. While a few of these others are used in monolayer specialty films, they are mostly used as part of coextruded multilayer films, where multiple extruders feed a single die that com-bines the various feed streams into a single, multilayer film. The following sections detail the many polymer types used in blown film extrusion.

1 .1 .1� Polyethylene (PE)

Polyethylene (PE) is the simplest polymer from a chemical standpoint. It is polym-erized from ethylene monomer and consists of a carbon chain backbone with two hydrogen atoms bonded to each carbon atom (Figure 1.1). Individual molecules, or chains, may reach lengths of hundreds to tens of thousands of carbon atoms. PE chains may be very linear or may be branched, depending on how the polymer was synthesized. There are many synthesis techniques for polyethylene [1].

H H H

C C C

H H H

H

C

H

~~

Figure 1 .1  A short section of a polyethylene molecule

Blown film processing methods for PE vary somewhat depending on the grade. Differences will be discussed in the following sections. However, one important similarity is that all PE grades have a high value of specific heat. Specific heat is a measure of the energy required to raise a unit mass of material one degree in tem-perature. If a polymer has a high value of specific heat, this means that heat re-moval from the melt is relatively slow. PE has a specific heat of approximately 2 kJ/kg·K compared to approximately 1 kJ/kg·K for most other polymers. This is the reason why cooling towers for blown film polyethylene are very tall. It takes time to re-move enough heat from the two layers passing through the nip rollers to prevent them from sticking together (sometimes called blocking).

1 .1 .1 .1� Low Density Polyethylene (LDPE)Polyethylene is often categorized by its density, a measure of the mass per unit volume (e. g., g/cm3 or lb/in3). When any type of polymer cools from the melt state, some of the chain sections may organize into highly ordered, more dense crystal-line regions (Figure 1.2). This will occur with sections of molecules possessing long, repeating patterns. In sections containing irregular patterns, such as branch points or chain ends, crystallization does not occur and these regions are called

31 .1�Polymers

amorphous (disordered). All PEs generally crystallize to some degree. Some poly-mers, such as commercial polystyrene, are completely amorphous due to hindrance of the entire molecular structure to crystallization.

Figure 1 .2  Polymer molecules are shown here in ordered, crystalline regions and disordered, amorphous regions

Low density polyethylene (LDPE) is synthesized in such a way that a highly branched polymer is formed. It consists of short chain branches (less than six car-bon atoms long) and long chain branches (almost as long as the length of the back-bone). Branch points along the chain serve as disruptions to the order of the sys-tem and prevent local crystallization. The lower level of crystallinity results in lower density. LDPE is generally in the density range of 0.910 to 0.925 g/cm3.

LDPE tends to be relatively easy to process. Compared to other PE grades, it melts at a relatively low temperature (220 to 240 °F, 105 to 115 °C) and does not require as much extruder motor power. LDPE blown film grades are moderately high in viscosity, but the wide range of branching yields a fairly wide processing window and high melt strength in the bubble. This leads to a stable bubble that can be run with a low frost line height (pocket bubble, or bowl or pear shape, Figure 1.3). LDPE heat-seals very easily.

4 1 Materials for Blown Film

Pocket bubble Long stalk bubble

Figure 1 .3  Two types of blown film bubbles

The properties of LDPE blown film can be characterized as tough and flexible. The toughness is derived from a good combination of strength and elongation, particu-larly when processed with high machine and transverse direction orientation. Flexibility results from low crystal content. LDPE bags provide a soft feel compared to the crinkly feel of HDPE bags. However, LDPE is not as stiff or strong as HDPE.

1 .1 .1 .2� High Density Polyethylene (HDPE)High density polyethylene (HDPE) is synthesized by a method very different from that used for LDPE. As a result, very linear chains are produced. In fact, HDPE is generally polymerized with a small amount of comonomer that leads to a few short chain branches placed intentionally along the main chain to make the polymer easier to process (Figure 1.4). A high degree of linearity results in a high percent-age of crystallinity (i. e., high density). HDPE is generally in the density range of 0.941 to 0.965 g/cm3.

Processing HDPE is somewhat different than processing LDPE. Because of its higher degree of crystallinity and more consistent molecular structure, HDPE melts at a higher temperature (265 to 275 °F, 130 to 135 °C) and has a narrower processing window. It also requires higher screw torque, hence more motor power. To promote good solids feeding via high barrel/pellet friction, grooved feed throats are often used.

Figure 1 .4  Schematic representation of an HDPE molecule showing high linearity with a small amount of short chain branching

One of the most obvious differences between processing HDPE and LDPE is that HDPE is usually run with a high frost line height (long stalk or wine glass shape, see Figure 1.3). The frost line height is generally on the order of eight to ten times the die diameter. HDPE tends to have lower melt strength than LDPE because of a