A Guide to Polyolefin Extrusion Coating 6664.Pdf
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A GUIDE TO POLYOLEFIN EXTRUSION COATING Table of Contents Page What Are Polyolefins?.............................................................................................. 2 Effect of Molecular Structure and Composition on Properties and Processability .... 2 How Polyolefins Are Made ....................................................................................... 6 Polyolefins for Extrusion Coating ............................................................................. 7 LyondellBasell Works Closely with Processors ................................................................. 8 Shipping and Handling Polyolefin Extrusion Coating Resins .................................... 8 The Extrusion Coating Process .............................................................................. 8 Resin Handling/Conditioning ................................................................................ 8 Blending with Colorants and Additives ................................................................... 10 Substrate Handling and Surface Preparation ......................................................... 10 The Extrusion Coating Machine ............................................................................. 15 Start-Up of An Extrusion Coating Line ................................................................... 34 Guidelines for Start-Up ....................................................................................... 35 Shut-Down Procedures for Extrusion Coating Line ............................................... 36 Optimizing the Extrusion Coating Process ........................................................... 36 Process Variables Affecting Properties of Extrusion Coatings .............................. 37 Appendix 1: Common Coating Problems and Their Causes ................................ 39 Appendix 2: Formulas for the Extrusion of Polyolefins ......................................... 41 Appendix 3: Cleaning the Extruder and Its Parts .................................................. 42 Appendix 4: Metric Conversion Guide ................................................................... 43 Appendix 5: Abbreviations ............................................................................................ 45 Appendix 6: Glossary ........................................................................................... 47 Appendix 7: Test Methods Applicable to Polyolefin Extrusion Coating Resins and Their Substrates ............................................................ 51 Appendix 8: Trade Names for Products of LyondellBasell Chemicals ........................... 53 Index ................................................................................................................... 54 1 Ethylene copolymers, such as ethylene vinyl acetate (EVA) and H H ethylene n-butyl acrylate (EnBA) are What Are Polyolefins? made by the polymerization of ethylene units with comonomers, such as vinyl C = C Polyolefins are thermoplastic acetate (VA) and normal butyl acrylate resins polymerized from petroleum- (nBA), H H based gases. The two principal gases Polymerization of monomers are ethylene and propylene. Ethylene is creates a mixture of molecular chains the raw material for making polyethyl- of varying lengths. Some are short, ene (PE) and ethylene copolymer others enormously long containing Figure 1. Ethylene monomer resins and propylene is the main several hundred thousand monomer molecular structure ingredient for making polypropylene units. For polyethylene, the chains have (PP) and propylene copolymer resins. numerous side branches. For every Polyolefin resins are classified as 100 ethylene units in the molecular thermoplastics, which means that they chain, there are one to ten short or long H H H H H H H H H H can be melted, solidified and melted branches, radiating three-dimensionally again. This contrasts with thermoset around the polymer chain (Figure 3). resins which, once molded, cannot be C C C C C C C C C C Chain branching affects many polymer reprocessed. Most polyolefin resins for properties, including density, hardness, extrusion coating are sold in pellet flexibility and transparency, to name a H H H H H H H H H H form. The pellets are about 1/8 inch few. Chain branches also become long and 1/8 inch in diameter, usually points in the molecular network where somewhat translucent and white in oxidation may occur. In some Figure 2. Polyethylene molecular color. Polyolefin resins sometimes processing techniques where high chain. contain additives, such as thermal temperatures are reached, the resulting stabilizers, or are compounded with oxidation can adversely affect the colorants, antistatic agents, UV polymer’s properties. stabilizers, etc. Density Polyolefin resins are a mixture of Effect of Molecular Structure and crystalline and amorphous structures. Molecular chains in crystalline areas Composition on Properties and are arranged somewhat parallel to each Processability other. In amorphous areas they are random. This mixture of crystalline and Three basic molecular properties amorphous regions (Figure 4) is affect most of the properties essential essential to the extrusion of good to high quality extrusion coatings. extrusion coatings. A totally amorphous These molecular properties are: polyolefin would be grease-like and have poor physical properties; a totally Figure 3. Polyethylene chain with crystalline polymer would be very hard • Average Molecular Weight side branches. and brittle. • Molecular weight Distribution HDPE resins have molecular • Crystallinity or Density chains with comparatively few side chain branches. Therefore, the chains These molecular properties are are packed closely together. The result determined by the materials used to is crystallinity up to 95%. LDPE resins produce polyolefins and the conditions have, generally, a crystallinity ranging under which resins are manufactured. from 60 to 75%, and LLDPE resins The basic elements from which have crystallinity from 60 to 85%. polyolefins are derived are hydrogen and carbon atoms. For polyethylenes, Density Ranges for Polyolefin these atoms are combined to form the Extrusion Coating Resins ethylene monomer, C2H4, i.e., two carbon atoms and four hydrogen atoms • LDPE resins range from 0.915 to (Figure 1). In the polymerization 0.925 grams per cubic centimeter process, the double bond connecting (g/cm³). the carbon atoms is broken. Under the • LLDPE resins have densities right conditions, these bonds combine ranging from 0.910 to 0.940 g/cm³. Figure 5. Crystalline (A) and with other ethylene molecules to form • MDPE resins range from amorphous (B) regions in long molecular chains (Figure 2). The 0.926-0.940 g/cc. polyolefin. resulting product is polyethylene resin. 2 • HDPE resins range from 0.941 to Table 1: General Guide to the Effects of Polyolefin Physical 0.955 g/cc. Properties on their Mechanical Properties and Processing. • The density of PP resins range from 0.890 to 0.915 g/cc. As Melt Index As Density Higher density, in turn, influences numerous properties (Table 1). With Characteristic Increases Increases increasing density some properties increase in value. However, increased density also results in a reduction of Chemical Resistance Stays the Same Increases some properties, e.g., stress cracking resistance and low temperature Clarity Increases Increases toughness. Elongation at Rupture Decreases Decreases Molecular Weight Atoms of different elements, such Extrusion Speed Increases Increases as carbon, hydrogen, etc., have Drawdown Increases Increases different atomic weights. The atomic weight of carbon is 12, and of Flexibility Stays the Same Decreases hydrogen, 1. Thus, the molecular weight of the ethylene unit is the sum of Gloss Increases Stays the Same its six atoms (2 carbon + 4 hydrogen) or Heat Resistance Stays the Same Increases 28. Every polyolefin resin consists of a (softening point) mixture of large and small chains, i.e., chains of high and low molecular Impermeability to Gases/Liquids Stays the Same Increases weights. The molecular weight of the polymer chain is generally in the Low Temperature Flexibiity Decreases Decreases thousands. The average of these is called, quite appropriately, the average Melt Viscosity Decreases Increases molecular weight. Mechanical Flex Life Decreases Decreases As average molecular weight increases, resin toughness increases. Stress Crack Resistance Decreases Decreases The same holds true for tensile strength and environmental stress cracking Tensile Strength at Break Decreases Increases resistance (cracking brought on when a polyolefin object is subjected to Resistance to Blocking Decreases Increases stresses in the presence of liquids such Stress Cracking Resistance Decreases Decreases as solvents, oils, detergents, etc.). However, because of increasing melt Tensile Strength at Rupture Decreases Increases viscosity, drawdown becomes more difficult as average molecular weight increases. Melt Viscosity Melt viscosity for polyethylene resins is expressed by melt index, a property tested under standard condi- high-pressure flow properties. There- molecular weight distribution” (Figure tions of temperature and pressure. Melt fore, MI (Table 2) must be used in 5). “Broad molecular weight index (MI) is inversely related to the conjunction with other yardsticks, such distribution” polyolefins have a wider resin’s average molecular weight: as as molecular weight distribution, to variety of chain lengths. In general, average molecular