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Processing Techniques For Plastic Films (Part 1) ABHAY MULAY Product Application & Research Centre Reliance Industries Limited Mumbai IPI,19.09.03
Processing Techniques For
Plastic Films (Part 1)
ABHAY MULAY Product Application & Research Centre
Reliance Industries Limited Mumbai
IPI,19.09.03
Index
Flexible packaging Overview
Blown film extrusion
∗ Upward blown
∗ Downward blown
Cast Film extrusion
∗ Uniaxial orientation
∗ Biaxial orientation
Calendering
Flexible Packaging Overview
Flexible packaging : Overview
Flexible packaging is the largest sector in plastics One of the fastest growing sectors, CARG ~ 20 %
per capita consumption of plastics in packaging 2001-02 2005-06 2.2 kg 3.7 kg
A growing demand due to: ∗ Growing population ∗ Urbanization & increasing “middle income” group ∗ Booming rural marketing ∗ Better convenience, need for small size packs ∗ Increase in “BRAND” awareness ∗ Entry of MNC’s
Flexible Packaging Sector : overview
45%
25%
20%
10%
LD/LLDPE PP HD/HMHDPE PVC
Total Film Market (02-03) ~1. 2 MMT
PE films: Largest share in flexible packaging
Polypropylene film : overview
3%
21%
77%
TQPP CPP BOPP
...largest contribution from TQPP films
PP Film Market (02-03) ~ 300KT
Processing Techniques
Processing Techniques - Film
The most commonly used processing techniques for the flexible packaging films are
− Blown film extrusion
∗ Upward blown
∗ Downward blown
− Cast Film extrusion
∗ Uniaxial orientation
∗ Biaxial orientation
− Calendering
Upward Blown Film
Blown Film Extrusion : Upward Blown
Technique predominantly used for PE films Process : ∗ Resin is plasticized in an extruder & pumped
through the annular die ∗ Molten extrudate in form of tube is inflated by air
into a bubble of desired diameter ∗ Cooling of bubble using air ring ∗ Haul off through collapsing boards & nip rolls ∗ Slitting & winding
Blown Film Extrusion : Upward Blown
Blown Film Extrusion : Upward Blown
Machinery details : Extruders : ∗ Single screw extruders ∗ L/D 24 : 1 to 28 : 1 ∗ Compression ratio : 2.5 to 3:1
Dies ∗ Bottom fed spiral mandrel dies ∗ Single layer / multilayer (co-extrusion)
Blown Film Extrusion : Upward Blown
Machinery details : Corona treatment unit : ∗ Corona treatment is a high voltage discharge used to
increase surface energy ∗ Treatment is measured using mixture of Ethyl cellosolve &
formamide solution ∗ Surface tension requirements :
− For solvent based lamination : 44 dynes / cm − For solvent based printing : 40 dynes / cm − For water based inks & adhesives : 48 – 50 dynes / cm
Winding : Center winding or surface winding
Key Processing variables
Melt temperature : ∗ Higher the melt temperature better is melt
homogenization ∗ Typical melt temperatures
− LDPE : 185 – 195oC − LLDPE : 205 – 215oC −HDPE : 215- 225oC −HMHDPE : 220 – 230oC
Higher melt temperatures also gives better optical properties & gloss
Key Processing variables
Cooling air temperature : ∗ Lower temperatures ( higher cooling rate) leads to
−Higher crystallization rate − Smaller crystal size −Better opticals −Better balance of properties for LDPE & LLDPE
∗ Internal bubble cooling (IBC) for higher outputs
Key Processing variables
Die gap : ∗ LDPE & HDPE
−Broad MWD, Lower higher shear viscosity −Narrow die gaps (0.8 – 1.0 mm) for better
properties ∗ LLDPE
−Narrow MWD, lower shear sensitivity −Higher die pressures leads to ‘shark skin’ effect −Use of wider die gaps (1.5 – 2.5 mm)
Key Processing variables
Blow up ratio (BUR) ∗ Ratio of bubble diameter / die diameter ∗ To be optimized for balance of properties ∗ LDPE /LLDPE : 2.25 to 2.75 ∗ HDPE / HMHDPE : 4 to 5
Neck height : ∗ Lock-in position for branched polymers (LDPE /
LLDPE) ∗ 12 to 15 inch for linear polymers (HDPE/HMHDPE)
Draw Down ratio : Die gap Film gauge x BUR
Key Processing variables
Applications : PE Blown Films
Downward Blown Film (TQPP)
Blow Film Extrusion : Downward Blown
Upward blown extrusion is difficult to be adopted for highly crystalline polymers like PP due to
∗ Slow crystallization rate
∗ Formation of bigger crystallites
∗ Leads to brittle & hazy film
Downward blown extrusion process is conventionally used for PP. The process is popularly known as ‘Tubular Quench’ Polypropylene (TQPP) process
TQPP Film
Process : ∗ The resin is extruded & the molten material in form of
tube comes out of the circular die ∗ Hot bubble is initially cooled & stabilized by air ring ∗ Tubular film is then passed through water cooling / sizing
ring ∗ Water flows downward through sizing ring keeping
uniform & continuous contact with film ∗ Cooling water temperature depends upon film thickness
& is between 8 – 20oC ∗ Film is subsequently collapsed, passed through drying
unit & finally rolled up
TQPP Process
Performance of TQPP vs. Other films
TQPP process is low capital intensive & economical
compared to Cast PP & BOPP
Compared to BOPP
∗ Tear strength is higher
∗ Lower shrinkage due to low orientation
Performance of TQPP vs. Other films
Compared to PE film
∗ Higher yield
∗ Better clarity
∗ Higher heat resistance
∗ Superior tensile properties
∗ WVTR & OTR better compared to LDPE/LLDPE & inferior to HDPE
Effect of resin parameters
Melt Flow Index (MFI)
∗ Higher MFI resin has poor melt strength & offers lower bubble stability
∗ Higher MFI resin gives better opticals, however gives poor mechanical properties
∗ Low MFI resin leads to difficulty in processing & poor opticals
∗ Optimum MFI range : 8 – 12 g/10 min
Effect of resin parameters
Low molecular weight fraction (LMWF)
∗ These are usually olegomers & atactic fraction
∗ Determined by % xylene solubles
∗ Higher levels of LMWF leads to die drooling, smoking
∗ Low levels leads to difficulty in processing
∗ Optimum levels of LMWF 2.5 – 3.5%
Effect of resin parameters
Type of resin ∗ Homopolymer & Terpolymer PP grades are
commercially used ∗ Films made from Homo PP grades have
−Higher stiffness −Higher SIT
∗ Films made from terpolymer PP grades have −Higher softness − Superior gloss − Lower SIT
Effect of processing variables
Desired film properties can be obtained by optimizing process variables ∗ Tensile strength of the film can be increased by
−Higher cooling rate − Lower quenching temperatures − Lower gap between die & cooling ring
Effect of processing variables
Clarity improved by ∗ Lower quench water temperature ∗ Higher flow of quench water ∗ Higher die temperatures
Gloss improved by ∗ Raise the die & barrel temperature ∗ Lower quench water temperature ∗ Higher flow of quench water
Advances in raw materials
Resin for high throughputs
significant improvement in throughputs
30 kg/hr 150 kg/hr
over stretching leads to bubble breakage /punctures at higher shear rates, ∗ extensional viscosity decreases ∗ melts becomes thinner (stress thinning of PP)
resin has to be designed for higher extensional viscosity
…resin has to be tailor made for high output lines
New grades of REPOL
New grades from RIL
REPOL H 080 EY REPOL H 080EG
Repol H 080 EY
PP homopolymer, with slip and antiblocking agents MFI : 8.0 g/10min
The grade is specially designed to provide: ∗ Good processability at high throughputs ∗ Good opticals ∗ Better mechanicals
Typical applications :
packaging of textiles/food products, multi-layer TQPP films
Repol H 080 EG
PP homopolymer, without slip and antiblocking agents
MFI : 8.0 g/10min The grade is designed to provide: ∗ Good processability on high through put lines ∗ Excellent clarity and gloss ∗ Good stiffness
Typical applications : packaging of textiles / food / other commodities
New applications for TQPP films
• Soft TQ textile bags & Liners
• Processed food packaging films
• Lamination films
• Soft Blister
• Stationery products
• Perforated films
Applications : TQPP films
New Developments
2 layer co-extrusion TQPP plant developed by M/s. Rajoo Engg.
Co-extrusion of Homo PP & Random PP
Inner layer of random PP in co-extruded film offers superior sealing properties & low SIT
Co-extruded film can be used for lamination application PET/Adhesive/Co-ex PP film
Cast Film Extrusion (Uniaxial Orientation)
Cast Film Extrusion
Process ∗ Polymer is plasticized & homogenised in the
extruder ∗ Melt is passed through coat hanger die ∗ The extrudate comes out of a die as thin, wide
curtain of the film ∗ Molten film is quenched in water tank or onto a
chilled roll ∗ Finally film is corona treated, slit and rolled
Cast Film Extrusion
Cast Film Extrusion
Advantages of Cast Film over blown film ∗ Higher output ∗ Better transparency, gloss due to quenching ∗ Better stiffness ∗ Better thickness control ∗ Better mechanical properties in MD due to
unidirectional orientation
Advantages of blown film ∗ Lower machinery cost ∗ Width flexibility ∗ Balance of mechanical properties
Cast Film Extrusion
Machinery details : Single screw extruders ∗ L/D : 28 to 32 : 1 ∗ Compression ratio : 3:1 or more
Typical outputs in grooved feed extruders
Screw dia (mm) Output (kg/hr)75 140 - 16090 220 - 240120 350 - 400150 500 - 550
Cast Film Extrusion
Die : Coat hanger die
∗ uniform pressure drop due to triangular flow path
∗ Uniform material flow, uniform thickness
For multilayer co-extruded film
− Feedblock die
−Multimanifold die
Cast Film Extrusion
Chilled roll take-off
∗ Designed for maximum cooling efficiency
∗ Roll surface : Chrome plated, matt / mirror finished
∗ Roll width : 1200 mm to 3600 mm
∗ Roll diameter : 400 mm to 1200 mm
∗ Partially crystalline polymers : Cooling efficiency determines crystalline structure & opticals
∗ Lay on aids : Force film against surface of film roll ex. Air knifes & suction chambers
Cast Film Extrusion : Key variables
Melt temperature : ∗ Increasing melt temperature gives slight reductions in
haze ∗ Caution :
− Increased volatilization of additives in raw material −Odour issues − Volatiles condensation on chill roll surface affects
opticals ∗ Optimum melt temperatures
− LLDPE : 215 - 225oC − PP : 240 – 255oC
Cast Film Extrusion : Key variables
Chill roll temperature : ∗ Increasing chill roll temperature
−Deteriorate film clarity − Favours dimensional stability − Increases slip properties
∗ Decreasing roll temperature −Gives fine crystalline structure & excellent optical
properties −Due to build in stress, additional shrinkage on
storage ∗ Optimum chill roll temperature : 15 – 25oC
Polymer selection for Cast film
LDPE / LLDPE / Homo PP / Co PP
MFI : 3 – 10 g/10 min
Broad MWD resins gives lower neck-in
Higher the MFI, higher is clarity
Random PP with 1.4 to 3% ethylene gives softer films with low haze
Random PP also offer low SIT
Applications – Cast Film
Key applications of co-extruded cast film
Structure Special feature ApplicationLD + LLD /LD + LLD + HD /LD + LLD + PIB
* One side cling* Good clarity
Cling film, masking film
PP / PP filled / PP
* Low shrinkage* Smooth outer layer
Fast food trays
RCPP / PP /RCPP
* Sealability Industrial packaging
RCPP / TL / Nylon / TL / PP
* Good sealability* Transparency* Thermoforming property
Packaging of meat,Sausage & Cheese
Applications – Cast Film
Cast Film Extrusion (Biaxial Orientation)
Cast Film Extrusion : Biaxial orientation
Process :
∗ Biaxial orientation is a process in which continuous cast film is heated to bring it to a stretchable temperature
∗ Subsequently it is stretched in machine & then in transverse direction
∗ Commonly used polymers are PP & PET. The films are termed as BOPP & BOPET films respectively
BOPP Film Extrusion
Machinery Details : ∗ Extruders : 150 - 200 mm dia screws with L/D 26 to 30 : 1 ∗ Typical outputs : 1000 – 2000Kg/hr ∗ Dies : 600 mm to 2000 mm wide, coat hanger type ∗ Casting & Quenching : Molten polymer extruded from the
die is placed against chilled roll or multiple chill rolls
BOPP Film Extrusion
Machinery Details : ∗ Casting & Quenching :
− Popular method is casting onto a chilled roll partially submerged in water bath
−Chill roll & water temperature : 20 – 25oC −Chill roll diameter : 1000 – 2000 mm −Cooled web proceeds for MDO
∗ Orientation −Biaxial orientation of the film is done by tenter frame
process
BOPP Film Extrusion
Machine direction orientation (MDO) ∗ Cast film is heated by passing over heated rolls (145 –
150oC) ∗ When film reaches necessary stretching temperatures, it
is passed over series of stretching rolls ∗ Stretch ratio is kept between 4 : 1 to 6 : 1 ∗ Subsequently film is passed over annealing rolls to
prevent MD shrinkage
BOPP Film Extrusion
Transverse direction orientation (TDO) ∗ From MDO, film is guided onto the chain & is preheated
at 160oC ∗ As film leaves preheat section, chain diverges rapidly
stretching film in ratio of ~ 8 : 1 ∗ Film is passed in annealing oven at 155oC to reduce TDO
shrinkage Winding : Edges of the film are trimmed off. The film is slit and wound on separate winders.
BOPP Film : Double Bubble process
Equipment is smaller compared to tenter frame process Difficult to produce thin film Output in terms of quantity & width is very low Extruders : 50 to 75 mm Dies : Circular dies of 150 – 200 mm dia Quenching : ∗ Extruded tube from the die is directly taken into
quenching bath @ 20 – 25oC, collapsed between nip rolls ∗ Film passes to set of 2nd nip rolls through drying unit
BOPP Film : Double Bubble process
Preheating : Tube in flattened state is heated to 150 – 160oC using IR heaters Orientation : ∗ Heated tube is inflated to 5 – 6 times original diameter
(TDO) ∗ With set of nip rollers, film is stretched in machine
direction at ratio of 5 – 6:1 ∗ Cooling rings cools the inflated bubble before being
collapsed
BOPP Film : Double Bubble process
Effect of Processing variables
BOPP film properties depends on different processing variables Tensile strength & modulus can be improved by ∗ Lowering the temperature of chill roll & quench water ∗ Increasing stretch roll temperature ∗ Increasing the line speed ∗ Increasing the stretch ratio
Lower shrinkage achieved by ∗ Using higher stretching temperatures ∗ Using higher annealing temperature
Effect of Processing variables
Haze can be reduced by ∗ Lowering temperature of chill roll & quench water ∗ Increasing the temperature of stretching & annealing
Gloss can be improved by ∗ Using higher melt temperature ∗ Using chill roll, MD roll & die having clean surfaces
Applications : BOPP Film
Calendering
Calendering
Process of forming film/sheet by passing mass of molten material through successive pairs of counter rotating rolls
Process is commonly used for PVC, Rubber, & Non-woven
A typical PVC calender consist of 4 to 5 metal rolls arranged in various configuration
Each roll has control for temperature, speed, roll gap
For processing of PVC, 3 basic roll arrangements are used inclined Z, ‘L’ & inverted ‘L’
Calendering
Calendering line
Roll Configurations
Calendering : Roll configuration
L type calender can have 4 or 5 rolls of ~ 2 ft. dia & ~ 4 ft.
length
‘L’ type is commonly used for rigid PVC
Inverted ‘L’ type is commonly used for flexible PVC
Relative separatory forces for ‘Z’ type calender are more
compared to ‘L’ type calender. This leads to higher gauge
variation in transverse direction
PVC Calendering
Process involves following steps : ∗ Blending ∗ Fluxing ∗ Calender feeding ∗ Calendering ∗ Take-Off
PVC Calendering
Blending : Resin, Plasticisers, Modifiers, Fillers, Stabilizers, etc. are mixed together in ribbon blender. High intensity mixer gives better distribution compared to ribbon blenders Fluxing : The output of the blender, typically well dispersed, dry free flowing powder is fed into one of the fluxing equipment like ∗ Banbury / Intermix batch mixers ∗ Farrel continuous mixer ∗ Ko-Kneaders / Twin screw extruders ∗ Planetary extruders ( Rigid PVC)
PVC Calendering
Calendering feeding : ∗ The output for fluxing is fed to either two roll mill or
extruder strainer. Function of extruder strainer / two roll mill is − To maintain stock temperature − To act as a accumulator −Degassing − Straining of foreign particles
PVC Calendering
Calendering ∗ Output from two roll mill or extruder strainer is fed to
calender unit by a conveyor system ∗ Compound passes through metal detector system ∗ Conveyor feeds the PVC compound into 1st set of
calender rolls. The sheet is formed as the material passes through the rolls
∗ Each nip in calender stack reform the sheets & makes it wider, thinner & refinished
∗ For easy & faster release, last calender roll is kept at lower temperature & higher speed compared to preceding roll
PVC Calendering
Calendering take off :
∗ Hot, unsupported sheet from the last calender rolls is removed using driven rolls called ‘stripper rolls’
∗ Rolls are normally teflon coated to prevent sticking
∗ Subsequently sheet passes over embossing rolls, chilling rolls & winding station
PVC Resin characteristics
Resin for flexible PVC ∗ Suspension PVC with relative viscosity of 2 to 2.80
(K value 60 to 77) ∗ High MW resins are used when higher physical
properties are required or sheet is subjected to further heat (eg. Embossing)
∗ High MW resin have better UV resistance
PVC Resin characteristics
Resins for Rigid PVC ∗ Rigid PVC compounds are formulated with lower MW
resins ∗ Relative viscosity 1.85 to 2.0 (k value 55 to 60) ∗ Resin can be homopolymer or copolymer of VC & VA
Applications :PVC Calendering
R A I N W E A R
Thank You !
