In a world of fast-changing lifestyles, consumers expect products
to be lightweight, shatter-proof, aesthetically appealing, easy
to handle and offer good protection. The only answer to all these
pre requisites is plastics. Plastics have, in fact, revolutionized
the whole world in the last three decades. They have become part
and parcel in every walk of life.
Flexible packaging materials like polyethylene, polyvinyl chloride,
polypropylene and polyester films play a major role in almost all
the industries, especially the food-packaging sector. Biaxially
oriented polypropylene (BOPP) film is among the most favoured packaging
material across the industrial spectrum. The BOPP film suitable
for the food packaging industry is generally a co-extruded, heat-sealable
film. The sector also uses non-heat sealable films. Co-extruded,
heat-sealable films are used as a single web for packaging of products
unlike polyester films which cannot be heat-sealed on its own and
warrants a second web to be laminated.
MANUFACTURING BOPP FILMS
BOPP films are generally manufactured by the following processes:
a) Sequential Orientation:
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Stenter Process |
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Double Bubble Process |
b) Simultaneous Orientation
Since sequential orientation by stenter is the most commercially
employed process for the manufacture of BOPP films, the process
needs to be looked at in greater detail.
Homopolymer polypropylene is processed with various functional
additives like amides, ethoxylated amines, stearates etc, to reduce
the film's co-efficient of friction and increase the static charge
dissipation rate. It is also mixed with various fillers like talc
and calcium carbonate and pigments like titanium dioxide to provide
different shades to the film.
Polypropylene with the requisite additives, fillers and pigments
are melt co-extruded as thick sheet. With the help of one or more
satellite co-extruders, a random copolymer of polypropylene or other
terpolymers are melt co-extruded on either one or both surfaces
of the homopolymer sheet. The homopolymer core layer, which is generally
between 70-90% of the total thickness, provides the mechanical strength,
optical properties, reduction in density (in certain cases) and
other relevant characteristics. The copolymer skin layers, which
may contain the above-mentioned functional additives, may also contain
anti-blocking additives. These skin layers render the film heat-sealable
and improve its optical properties.
This sheet is later solidified by passing it over a chill roll
kept in a water bath. The cast sheet is then passed over several
heated, chrome-plated rolls, which pre-heat it. After achieving
sufficient heat, it is passed over two or more pairs of rolls in
which one set of rolls rotate at a speed faster than its preceding
rolls. Due to the difference in circumferential speed, the film
is oriented in the longitudinal direction. The orientation is generally
to the tune of 4-7 times.
The mono-oriented film is then clipped by clippers, which carry
them into a hot air circulated oven. The clippers are placed over
a parallel chain track. After sufficiently heating and transverse
direction orientation to the tune of about 7-10 times, the biaxially
oriented film is then annealed to retain the oriented stresses and
cooled to room temperature. While the unoriented gripped portions
are trimmed, the final film is corona treated to enhance surface
tension and wound in jumbo rolls .
PROPERTIES OF BOPP FILMS
Polypropylene (PP) has the lowest density of 0.905 g/cc of all
the plastics available till now. In a special grade of pearlised
BOPP film, the density is further reduced to 0.700 g/cc apart from
providing a pearly aesthetic look and barrier to light. Owing to
the low density of PP, the yield of the film is more than in other
films and thus economical. The biaxial orientation of PP enhances
various properties over conventional cast or tubular quenched PP
films. The mechanical strength, dimensional stability, stiffness,
non-terability, optical properties, barrier properties etc. are
greatly improved owing to the biaxial orienting process. Table-1
provides a comparative analysis of the property details of various
films.
ADVANTAGES OF BOPP FILMS
The main advantages of using BOPP films are:
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It provides a higher surface area per unit weight, which makes
it cost effective; |
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It has very low moisture transmission rate than other packaging
films, which improves the shelf life and retains crispness of the
food products sensitive to moisture; |
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It has a combination of excellent slip and optical properties.
The high gloss and excellent clarity provides good aesthetic appeal
whereas controlled slop properties make it suitable to perform well
on various form fill seal (FFS), overwrapping and converting machines
in order to improve or maintain high productivity; |
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High tensile strength, low elongation and thermal shrinkage
render it suitable for processing on high-speed printing, coating/laminating
and other converting machines. |
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It can be tailor-made to suit various customer needs, which
may be a combination of any optical/surface, optical/barrier and
optical/mechanical properties etc; |
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It is non-reactive or inert to its olefinic (hydrocarbon) nature,
which makes it suitable for safe use in the food and pharmaceutical
industries. It is also suitable for common methods of sterilization
practiced presently in the industry; |
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The co-extruded heat sealable films are preferred because they
provide sealability in addition to barrier and better aesthetics
without involvement of any additional coating or laminating operation,
making the package cost effective; |
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It is recyclable and does not emit hazardous gas while reprocessing. |
ROLE OF BOPP FILM IN FOOD PACKAGING
The food packaging requirements are very stringent. BOPP films
have many properties that are suitable and advantageous for packing
food materials. The basic properties for packing food materials
are as follows:
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The package should be able to protect the nutritional properties
of the food till final consumption. Therefore, it is desirable that
the package should have sufficient barrier to moisture, gas and
light. |
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The package should be cost-effective. |
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The package should be able to enhance the product's sales appeal
on the shelf and also give all the statutory information. Hence,
the pack should have good optical properties and printability. BOPP
films have excellent clarity, transparency and gloss. |
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