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BOPP Films in food packaging
 

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:

Stenter Process
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:

It provides a higher surface area per unit weight, which makes it cost effective;
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;
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;
High tensile strength, low elongation and thermal shrinkage render it suitable for processing on high-speed printing, coating/laminating and other converting machines.
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;
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;
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;
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:

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.
The package should be cost-effective.
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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