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SPE Polyolefins Conference 2005Low Density and Linear Low Density
Polyethylene Presentation
Presented by J. BayleyNOVA Chemicals Corporation
Note: The content of this presentation is intended for basic learning, the content may not describe or encompass all aspects of materials and processes
Brief History of Polyethylene• PE synthesis discovered accidentally in 1932 by Imperial Chemical
Company (ICI) Scientists• First High Pressure LDPE plant built in 1939• In 1953, large advancements were made by Scientist Carl Ziegler,
inventor of a new catalyst system. A scientist named Giulio Natta also shares credit for this catalyst development
• Known today as the Ziegler-Natta Catalyst (Z/N), this catalyst facilitated polymer synthesis at lower temperatures and pressures -High Density Polyethylene (HDPE) materials were introduced soon after
• In the late 1970’s LLDPE materials were introduced to the market• Significant Catalyst advances since that time with the advent of
Polyethylene DesignationsPolyethylene is classified by density ranges, as defined by ASTM:• LDPE Type I 0.910 - 0.925 g/cc• MDPE Type II 0.926 - 0.940 g/cc• HDPE Type III 0.941 - 0.960 g/cc (Copolymer)• HDPE Type IV >0.961 g/cc (Homopolymer)
LDPE Tubular Reactors (Simplified)• A tubular LDPE Reactor is a long heat exchanger• Free Radical polymerization uses Peroxide initiators or Oxygen to
promote polymerization reactions• Ethylene is circulated through a compressor - the main
pressurization of the feed stream is accomplished by a hyper compressor
• Initiators are introduced at various points along the length of the tube - Zone temperatures are accurately controlled
• No backmixing takes place in the tubular system, residence time is limited/short
• The exothermic heat of reaction is removed via water jackets on the outside walls of the tube
• Upon exiting the reactor the material passes through medium pressure and low pressure separators (separates Ethylene from PE), PE moves to the extruder
LDPE Autoclave Reactors (Simplified)• Free radical type of polymerization uses Peroxide initiators typically• System utilizes a stirred cylindrical vessel• Ethylene feed gas and Peroxide are introduced to a compressor and then
pumped with Peroxide initiator into the stirred autoclave vessel• Proprietary designs baffle or partition the reactor into discreet zones
enabling control of molecular species and amount of LCB of polymer in these zones
• Backmixing does take place in the autoclave system• Walls of the autoclave unit are thick to accommodate high pressure -
Heat of reaction is removed by the introduction of fresh feed• Upon exiting the reactor the material passes through medium pressure
and low pressure separators (separates Ethylene from LDPE polymer)• Polymer enters the pelletization process to be pelletized
Comparison of High Pressure Autoclave and Tubular LDPE Manufacturing Processes
Information Autoclave Tubular
Length 20 ft Up to 1 mile
Internal Diameter 3 ft 1-3 inches ID
Rx Temperature Range (°F) 350-500 350-600
Pressure within Rx (PSI) 15000-30000 20000-50000
Initiator Types Organic Peroxide Organic Peroxide or Oxygen
Typical Polymer Conversion Ranges per pass
Approx. 22% (varies with product mix)
Approx. 35% (varies with product mix)
Back Mixing Capability Yes No
General Observation More precise tailoring of MW, MWD and Long chain branching (LCB)
Less capable of molecular tailoring and less uniform long
chain branching (LCB)
General Observation Comb-like LCB structure Root-like LCB structure
(Note: This table provides general information. Technology may exist that is not encompassed by or include in this table. The information is intended for basic learning purposes only.)
Softness Softer and more pliable than other PE types
Permeability Higher, due to long chain branching and lower % crystallinity
Clarity Available in high clarity for film applications - Improves clarity of LLDPE when blended with LDPE in low amounts
Processing Shear thins in extrusion - processes easily at lower amps and pressures relative to LLDPE or HDPE
Equipment Needs Screw/Die designed for LDPE required if extruding 100% LDPE
Melt Strength Much higher than LLDPE due to presence of long chain side branched molecules - (Important for film blowing, foam etc.)
Other Pros Less prone to melt fracture than LLDPE or HDPE
Suitability as a Blend Resin Good, commonly used, can be detrimental to physical properties-LDPE is generally blended to improve ease of extrusion, increase
melt strength or improve clarity of the end product
Shrink Properties Possesses desirable biaxial shrink properties for shrink film
Limitations Absolute physical properties lowest in class - extensional limitations or drawdown limitations exist - LLDPE and HDPE
can be drawn much thinner in blown or cast film processes
LDPE Applications• LDPE is still an important PE type• The unique attributes of LDPE due to LCB provide
desirable properties for some specific product applications• LDPE is used at 100% in some applications such as
conventional Shrink Film, Extrusion Coating, Wire and Cable Jacketing, LDPE Foam etc.
• LDPE is used as a property modifier in film and sheeting applications and is often blended with LLDPE (to improve clarity, processability, output rates, etc.)
The Future of LDPE• Conventional LDPE has existed for many years and was
predicted to be replaced by LLDPE• LDPE future capacity growth is likely to be less than for
LLDPE, though demand continues to be strong for LDPE• LDPE is valued as performance modifier for extrusion
processing or to obtain desired physical properties such as clarity
• Manufacturers can be expected to push the boundaries of their processes and exploit existing technology, but significant advances in resin morphology are not widely expected to occur in this class of materials
LLDPE - General Information• Linear Low Density Polyethylene (LLDPE) is made by the
copolymerization of ethylene and a comonomer- (Example: Ethylene and Octene copolymerized - can be described as
an Ethylene-Octene Copolymer)
• LLDPE is composed of long linear molecules, the main polymer chain is composed of long strings of repeating Ethylene units - Short side chains (from comonomer) link onto the main polymer chains
• LLDPE typically has no long chain branching (LCB)• LLDPE materials are typically copolymers but terpolymers and
quatropolymers have also been made• LLDPE typically has a narrow distribution of main chain
molecule lengths (LDPE and HDPE tend to be broader)
Comonomer Type - Product Properties• Short side chain branching type influences product toughness -
(Example: Butene, Hexene, Octene)• Short side chains, like those made with butene comonomer are less
effective at disrupting chain folding• Longer side chains, like those formed with hexene and octene are
longer and result in superior physical properties• Z/N catalysts tend to have more difficulty than single-site catalysts in
placing comonomer on the longer chain (higher molecular weight) portion of the polymer thus more comonomer ends up on the shorter chains
• Comonomer addition levels are used to control resin density - (Example: Increased comomomer content increases short chain branch content and results in reduced resin density)
Prone to surface melt fracture in blown film and sheet extrusion-Process Conditions and Process aid additives are used to off-set this problemLimitations of LLDPE
High physical properties possible depending on comonomer used, catalyst used and molecular architecture- Very good elongational ability,
can be drawn down thinner as a film than LDPE, higher strength than LDPE permits downgaging
Strengths of LLDPE
Long linear molecules tend to orient highly in the machine direction-shrinkage as a result is more imbalanced relative to LDPEShrink Properties
Can be blended into LDPE where desired- Eg: Can be blended into shrink film to modify shrinkage propertiesSuitability as a Blend Resin
Much lower than LDPE due to NO long chain side branched molecules, only short chain branching generallyMelt Strength
Screw/Die designed for LLDPE required if extruding-Extruders, Screws, Dies and Air Rings need to be designed for LLDPE Equipment Needs
Stiff in shear during extrusion- Narrow molecular weight distribution, processes at higher amps and head pressures relative to LDPE Processing
Clarity not as good as for LDPE in most cases- LDPE can be blended to improve clarityClarity
Higher % crystallinity relative to LDPE-Barrier Properties dependant on part thickness and resin density to a large degreePermeability
Softer relative to HDPE but not as soft and pliable as LDPESoftness
• Metal based catalysts facilitate the reactions required to polymerize and convert Ethylene to PE
• Z/N catalyst is in common use today though modifications and improvements have been made
• Next generation catalysts known as single-site catalysts and Metallocene catalysts also exist and are used in the production of mLLDPE, sLLDPE and HDPE
*Note: Metallocene catalysts fall into the single-site catalyst family
• Every catalyst reaction site is the same, thus the molecules produced are more uniform
• Every polymer molecule contains the same amount of comonomer (can result in improved properties)
• Reduction in low molecular weight polymer component historically resulted in extrusion challenges largely addressed now by resin design and extrusion equipment improvements
• Metallocene catalysts are a subset of the broader single-site family
• Single-site catalyzed materials tend to have reduced low molecular weight grease levels
• Feed gases such as Ethylene, Butene or Hexene, Hydrogen etc. areintroduced to the fluidized bed in the base of the gas phase reactor
• Catalyst is introduced to the reactor• The exothermic heat of reaction is controlled by the fresh feed gas
circulation• High Rx throughput rates and low conversion rates per pass are
typically achieved - feed gases recycle through the reactor entering at the base and exiting at the top
• Granular PE product is produced in the reactor and intermittently discharged out of the reactor into a purge bin, hydrocarbons areremoved, granular materials conveyed to pelletization followed by pellet conveying to finishing area
• All aspects of reaction take place in solution• All raw ingredients including, Ethylene feed, Hydrogen, etc. are
dissolved into a solvent resulting in a solution composed of the raw ingredients required
• Catalyst is introduced to the reactor/s• Solution is introduced into one or more stirred autoclave reactors -
temperature, residence time and mixing are controlled• Polymer solution exits the reactor/s, solvent is flashed off in a
separator and returns to distillation• Polymer passes through a low pressure separator into an extruder• A devolatization extuder is used in some cases to remove residual
hydrocarbons while stripping vessels (post-extrusion) may also be used in some processes to accomplish this task
• NOVA Chemicals Internal Literature• Kirk Othmer Encyclopedia of Chemical Technology• American Plastics Council• Chem Systems (2003)• Commodity Thermoplastics (JP Arlie)• Judy Webb-Barrett (NOVA Chemicals)• Lan Nguyen (NOVA Chemicals)• Chris Foy (NOVA Chemicals)