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Product Application & Research CentreMumbai

Biaxially Oriented Polypropylene Film (BOPP)

Many of the unique characteristics of polypropylene enhance its utilization in the

application of film. The main application for polypropylene film are the packaging of

food, textiles and tobacco products.

Biaxially Oriented Film (BOPP)

Biaxial orientation is a process in which a continuous cast film is heated to bring it to a

stretchable temperature and thereafter it is stretched in machine and then in transverse

direction.

Biaxial orientation of polypropylene (BOPP) film is done by two methods

• Tenter frame process

• Double bubble process

Tenter Frame Process

Tenter frame process is a very expensive way of processing BOPP film. However it has

become popular due to higher production rate with very thin and wide width film (Fig. 1)

Extruders

Extruders with 150 to 200 mm dia screws having L/D of 26:1 to 30:1 are more popular.

Dies

600 to 1500 mm wide coat hanger type of dies are used

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The Tenter Frame Process

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Casting & Quenching

The molten polymer extruded from the die is placed against a chill roll or multiple chill

rolls. The most popular being casting onto a chill roll partially submerged in a water

bath. The chill roll and water bath temperatures are generally 20 to 25°C. Usually, these

chill rolls are 1000 to 2000 mm in diameter and approximately 1000 to 1500 mm wide.

The surface of the chill roll must be very smooth; pitting or irregularities on the surface

of the roller will influence the film quality. An air knife is used to force proper contact of

molten polymer against the casting roll. The cooled web proceeds towards the machine

direction orientation section.

Machine Direction Orientation (MDO)

The base sheet is alternatively heated by passing it over the heated rollers (145 - 150°C)

to achieve uniform heating of web throughout its thickness. These rollers are usually

smaller in diameter (450 to 600mm) compared to the casting drums (chilled rolls).

When the film reaches the necessary stretching temperature, it is passed over a series of

stretching rolls which are usually arranged in pairs. Each pair has a rubber nip roll to

prevent slipping of the web while stretching. These first set of nip rollers are run at a

speed similar to preheat rollers. The next set of rollers are operated at higher speeds than

the first set of nip rollers. Usually the stretch ratio is kept between 4:1 to 6:1.

From the stretching rollers, the film is passed on to a series of annealing rollers, which

are heated to a temperature, adequate to prevent shrinkage in the machine direction (MD)

of the web.

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Transverse Direction Orientation (TDO)

From the machine direction orienter, the film is guided on to the chain (Fig. 3),

containing clips (Fig. 4) which close on to the film. A rail guides the chain with clips to

preheating section, where the film is heated with hot air at 160°C, slightly below the

melting point of polypropylene.

As the film leaves the preheat section of the oven, the chain diverges quite rapidly to a

ratio of 8:1 to 10:1. The film is then passed on to annealing oven where it is maintained

at 155°C, to reduce the shrinkage of the film. Usually the tenter frame run at line speeds

of 200 to 300 meters/min.

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Winding

Edges of the film remain thick, as they are in the clips while the film is being stetched.

These edges are then trimmed off as the film leaves the tenter frame. The wide film is

slit into the required width by rotating knives and wound on to separate winders.

Double Bubble Process

The equipment for this process is small compared to that of tenter frame process (Fig. 5).

It is difficult to produce thin film in double bubble process. The output in term of

quantity and width of the film is also very low compared to that in tenter frame.

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Extruders

Generally 50 mm to 75 mm extruders are used

Dies

Circular dies of 150 to 200 mm diameters are more popular

Quenching

The extruded tube from the die is directly taken into the quenching bath (20-25°C)

collapsed between the nip rollers located at the bottom of the quench bath. The film is

then taken up to second set of nip rollers via a drying unit. The water is stripped in the

drying unit.

Preheating

The tube in its flattened state is heated (at 150 to 160°C) either by IR heaters or hot air.

The heated tube is inflated to its new dimensions, which is usually 5 to 6 times its

original diameter. With the third set of nip rollers, the film is stretched in the machine

direction at the ratio of 5:1 to 6:1

A set of cooling rings are used to cool the inflated bubble prior to being collapsed.

Collapsed tube is slit on both the edges and the web is then separated before winding.

Effect of Processing Variables on Film Properties

Film properties depend on different processing variables, as described below

1. Requirement : A : High modulus and tensile strength

B : Low shrinkage

A Tensile strength and Modulus can be improved by :

- Lowering the temperatures of chill roll and quench water

- Increasing the stretch roll temperature

- Increasing the line speed

- Increasing the stretch ratios

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B Lower shrinkage can be achieved by :

- Using higher stretching temperature

- Using higher annealing temperature

II Requirements : A : Low Haze

B : High Gloss

A Haze can be reduced by

- Lowering temperature of chill roll and quench water

- Increasing the temperatures of stretching and annealing

B Gloss can be improved by

- Using higher melt temperature

- Using chill roll, MD rolls and die having clean and smooth

surfaces

Repol Polypropylene Grades for BOPP Film

Repol Grade MFI

(g/10min)

Characteristics

HOMOPOLYMER

H029SG 2.9 Good processability and

low gels

RANDOM COPOLYMER

R070EY 7.0 Contains antiblock and

slip. Low heat seal

temperatures

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Applications of BOPP Films

Following are the important features of BOPP film which make it a material of choice for

flexible packaging :

• Low specific gravity which results in higher yield (m2/kg)

• Higher mechanical properties, which allow for thinner films to be made

• BOPP films have by far the highest moisture barrier of all plastic films. WVRT

values at 38°C and 98% RH of different film substrates are as follows :

20 microns OPP - 4.85 gms/sq.m/24 hrs

20 microns HDPE - 6.25 gms/sq.m/24 hrs

25 micron Cast PP - 10.00 gms/sq.m/24 hrs

25 micron LDPE - 15.00 - 20.00 gms/sq.m/24 hrs

10 micron PET - 45.00 gms/sq.m/24 hrs

• BOPP films have good barrier to flavours and aromas, particularly at the levels of

concentration normally encountered in packaged products.

• Co-extruded BOPP film with good barrier and high transparency is heat sealable as

produced i.e. without having to be coated, avoiding additional operation. The other

major advantage is, being heat sealable on both surfaces, it is possible to seal

particularly on VFFS operations with better seal integrity and a much neater pack.

• BOPP films have outstanding optical properties and the gloss/transparency/sparkle

obtained are among the best available on any material.

• BOPP films have extremely high strength - particularly tensile strength, puncture

resistance, bursting strength, stiffness, tear strength and abrasion resistance.

• BOPP film packages also have a very crisp feel which make them definitely superior

to those made from flexible materials.

• BOPP films have very high stability / performance over a very wide range of

temperatures and environmental conditions. They are not susceptible to high relative

humidity like cellophane and hence retain their properties and dimensional stability

over the entire range of conditions normally encountered by packages.

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• BOPP films have very high chemical resistance to fats/oils - particularly of vegetable

origin, most chemicals and many solvents. The basic material (polypropylene) is

essentially a highly compatible with most packaged products. It is particularly

suitable for direct food contact.

• BOPP films have extremely high flex crack resistance. Polypropylene is the only

basic resin that has an "integral hinge" property, in that it can be flexed any number of

times without fracture. This property is extremely important in flexible pouches

during their transit and handling.

• BOPP films are eminently suitable for down stream operations. They are highly

amenable to all operations like printing, coating, lamination, metallisation and

pouching.

• BOPP film can be subjected to sterilization without impairing mechanical properties

and hence has wider packaging applications.

• BOPP film is heat sealable at high speeds using co-ex OPP or off-line coated film

• Due to the higher stiffness of BOPP film, higher packaging speeds (50-60 m/min) are

possible

• BOPP films can be vacuum metallised with high purity aluminium, to significantly

enhance barrier properties

Some of the typical applications of BOPP film are as follows :

(A) Plain film

Industrial tape (25-40 mic)• Pressure sensitive tapes

Lamination (10-20 mic)• Book covers, brochures, catalogues, shopping bags, carton boxes, cosmetic

boxes, restaurant menus, film to film laminates (PET/BOPP/POLY)

Food packing (20-40 mic)• Laminates, metallizing

Others (12-40 mic)

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• Tea chest lining, flower over-wrapping, textiles, cable over-wrappings, photoalbums, release film, biscuit inner-wraps, envelope windows, tear tapes,metallizing, stationery

(B) Co-Extruded film

Over wrapping (18-23 mic)• Cigarette, audio/video cassette boxes, cartons, blades, toffees and candies

General purpose (20-40 mic)• Lamination, snack food, bakeries, cosmetic, noodles, textiles, bread,

audio/video cassette, metallizing, twistwrap

Food packing (20-50 mic)• Lamination, food, chips

Super Seal (25-35 mic)• Powdery material, food grain, sugar

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BOPP Film

Trouble Shooting Guide

Tenter frame

Defect Source SolutionThickness variation - Adjust the die gap

- Adjust die heat temperatures- Check screen pack or filter, change if

necessary- Check air knife position- Look for air bubbles under the sheet

MDO - Increase temperature of preheat rolls- Decrease gap between stretching rolls- Decrease speed of MDO rolls

TDO - Adjust stretching section temperatures- Examine oven for foreign objects blocking

hot air nozzlesExtruderSurging

- Check melt temperature fluctuation andpressure fluctuations

- Increase back pressure by loweringmetering section temperatures

- Increase first transition zone temperatureSplits Foreign

particles in theresin

- Examine for gels or dirt- Discontinue putting reclaim or repelletized

pellets in main feed- Change filter pack

Ovens - Raise temperature of the preheat section ofoven

Water in theresin

- Change resin feed

Gauge Band - Find source of gauge variation and fixJerky chain - Lubricate rail and chainOil drops onfilm in oven

- Decrease oil flow to chain and rail andwipe off excess oil

Dirty clips - Wind base sheet on MDO winder andcheck chain

- Clean clips or remove faulty clip andreplace

Haze bandsfrom MDO

- Increase stretching heat of MDO

Non uniformmelt

- Adjust extruder heat profile

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Poor opticsHigh HazeLow Gloss

MDO Heats - Increase temperature of preheat rolls andstretching rolls if haze bands are present

Low extrusionheats

- Raise melt temperature

Slow cooling incasting section

- Decrease casting drum temperature- Decrease bath temperature

Water marks Water carry-over to MDO

section

- Remove water from web before MDO

Bubbles or Voids Resin airentrapment

- Change resin- Increase the transition zone temperature- Increase the back pressure

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BOPP Film

Trouble Shooting Guide

Double Bubble frame

Defect Source SolutionThickness variation Die : poor flow - Adjust the die gap

- Adjust the die temperaturesPolymer filter

unplugged- Check screen pack or filter, change if

necessaryUnlevel water - Adjust level of water ring or bathAir draft on

bubble- Seal air leaks in enclosure around bubble

Blow-outs or Bubblebreaks

Oven - Oven heats too low, raise temperature

Gels in film - Change filter- Change resin

Plate-Out marksfrom mandrel

- Wipe off mandrel- Lower melt temperature- Raise mandrel temperature- Change resins