PACKAGING OF DAIRY PRODUCTS

Module 2. Packaging materials

Lesson 12
NEWER FORMS WITH COMBINATION OF TWO OR MORE INGREDIENTS

12.1 Introduction

This lesson deals with topics related to newer forms of plastic materials involving multi-ingredients like coated films, copolymers, laminates, common and special purpose plastics, and current trend in use of plastic as packaging material.

12.2 Coated Films

Individual films are often coated with other polymers and/or aluminum foil to improve the barrier properties or to impart heat-sealability as follows:

12.2.1 Characteristics

• Nitrocellulose is coated on one side with cellulose film to provide a moisture barrier property while retaining oxygen permeability.

• A nitrocellulose coating on both sides of the film improves the barrier to oxygen, moisture, and odors and enables the film to be heat-sealed when broad seals are used.

• Polyvinylidene chloride coating is applied to cellulose, using either an aqueous dispersion (MXXT/A Cellulose) or an organic solvent (MXXT/S Cellulose). In each case, the film becomes heat-sealable, and the barrier properties are improved.

• A coating of Vinyl chloride or Vinyl acetate gives a stiffer film that has intermediate permeability. Sleeves of this material are tough, stretchable and permeable to air and moisture.

• A thin coating of Aluminum (termed Metallization) produces a very good barrier to oils, gases, moisture, odors, and light. Metallized film is less expensive and more flexible than foil laminates that have similar barrier properties, and it is therefore suitable for high-speed filling on form-fill-seal equipment.Cellulose, polypropylene or polyester is metallized by depositing vaporized aluminum onto the surface of the film under vacuum. Metallized layer is basically made up of an initial layer of spherical metallic particles which become embedded in the plastic material. Subsequent layers are then built-up of metal particles. Metallized polyester has higher barrier properties than metallized polypropylene, but polypropylene is finding more widespread use as it is currently less expensive.

• Metallized polyester laminates saves 15-20% in material costs besides saving an extra laminating process as compared with other laminated packaging material.

• Metallized polyester film is used for packaging WMP, Pan masala, nut powder, fruit drinks, coffee etc.

12.2.2 Coated cellophane films

About 80% of the total film manufactured is coated because plain cellophane is hygroscopic, not moisture proof and not heat sealable.

Nomenclature:

P = Plain uncoated film
T = Transparent
C = Coloured
M = Moisture proof
A = Anchored (Coated more firmly secured to the base film) hence more resistance to water under wet and humid conditions.
D = Darning (one side coated)
L = Low moisture vapour proof ness
X = Saran coated
Q = Opaque
S = heat sealable
Tw = twist wrap (toffees)
H = Resistant to blocking at high humidity.

• PT Film: Plain transparent film: It is flexible, strong, transparent and grease proof and can be handled on automatic machinery and can be printed easily. (It is a barrier against non-boring insects). But PT film is hygroscopic and permeable to water vapour but, impermeable to dry gasses except water soluble gases like CO2. It is used as a hygienic wrapping for goods where some interchange of moisture with the surroundings may be permissible or even desired. Used for wrapping bread, soaps, paper plates & cups.

• MST: Moisture proof & heat sealable. It is used where moisture proofness and heat sealability are required. It is used as direct wrap for bread, cakes, over wrap for cigarettes, cartons of tea bags and cartons of biscuits.

• MSAT: Anchored grade of MST film used extensively for food packaging particularly hygroscopic frozen food.

• MSC: Moisture proof, heat sealable and coloured. It is decorative in nature. Amber, yellow or orange film is used to absorb UV rays which cause rancidity in foods products that have a high fat content.

• LSAT: Low moisture, heat sealable. This film is used for packing food products that are sensitive to deterioration by excess moisture inside the package. It is used to pack certain bacons, crushed backed goods.

• MSADT: This film has a sealable moisture proof coating on one side only. The importance of this film is due to its high OTR. Since it can readily transmit oxygen under moist and humid condition it is suited as direct wrap for fresh meat, with the uncoated side inside the package. The constant supply of fresh oxygen maintains the blood pigment of meat and at the same time moisture proof coating prevents desiccation of meat.

• MASXT: It resists puncturing by hard sharp foods stuffs and offers very good performance at low temperatures and hence also known as winter film. It is used for nuts, hard candies, biscuits and small hardware.

• MXXT: Moisture proof, coated with saran both the sides. It is highly moisture proof even after creasing and printing and has a good resistance to fats, greases and oils. (The film is more dimensionally stable; i.e resists wrinkling and shrinkage caused by changing humidity conditions.) It is used for packing desiccated coconut, potato chips, dried foods, biscuits and some type of cheese.

• MSAQ: opaque grade used for light sensitive foods like cheese, butter etc.

• MTTW: A non heat sealable Nitrocellulose coated type of cellophane used for twist packaging of sweets.

12.3 Co-Polymer

At the time of polymerization we can polymerize more than one monomer to form polymers. The resultant polymer is called co-polymer. It has modified/superior properties those are not available in homopolymers.

Advantages of co-polymers:

1. No problems of exudation, migration and leaking associated with plasticizers.

2. Controlling co-polymerization and selecting right type basic monomer combination can produce tailor made Plastic materials.

12.3.1 Other additives

Many plastics contain very small amounts of additives such as plasticizers, antioxidants, lubricants, antistatic agents, heat stabilizers, and UV stabilizers. These are added to facilitate processing of plastics or to impart some desirable properties to the plastics. For example, plasticizers are added to soften plastics, thus making them more flexible and less brittle for use in cold climates or with frozen stored products.

12.4 Laminated Films

“Laminates are the combination of two or more layers of packaging films.” They are used when required properties are not achieved in single packaging material.
Laminates are used for:

1. Good water vapour barrier properties

2. Good gas barrier properties

3. Good grease resistance

4. Heat sealing facility

5. To provide strength to the base film

6. To improve toughness

7. To improve tear resistance

8. To improve abrasion resistance

9. To improve machinability

10.To improve printability

Lamination of two or more films improves the appearance, barrier properties, or mechanical strength of a package. Materials that can be laminated to each other include

• Plastic to plastic,

• Paper to plastic,

• Paper to aluminum foil, and

• Paper to aluminum foil and then to plastic.
  

Several methods can be used to laminate materials.

(A) Adhesive Lamination: The webs are bonded by means of a suitable adhesive which may be (i) Aqueous, (ii) Lacquer or (iii) Hot melt. The adhesive is applied to the web which is subsequently brought in to contact with another by passing through two rollers.

• Aqueous adhesives: Dextrin, Starch, Casein, Sodium silicate.

• Lacquers: Polymethanes, urea formaldehyde, nylons, vinyls.

• Hot melts: Wax and wax blends.
  

(B) Extrusion Lamination: A thermoplastic web is directly extruded on to a suitable substrate web. Extrusion coating could also be achieved. It is a cheap process. A thermal extrusion or co-extrusion can be used.

(C) Direct Heat Lamination: A thermoplastic web maybe bonded to a second one by heat and pressure without the use of an adhesive.

12.4.1 Characteristics of laminated films

• Laminated materials are used when high gas and moisture characteristics are required for a long shelf life.

• Laminated structures usually consist of an outer protective tougher layer, e.g., nylon or polypropylene, a middle high gas barrier layer, e.g., EVOH or PVDC, and an inner heat sealant layer.

• LDPE is commonly used as a heat-sealant layer because of its low melting temperature; however, it sometimes does not give a good seal with starchy or greasy food products.

• The choice of sealant layer for these food products is either EVA or Surlyn.

12.4.2 Laminated packaging material

A laminated packaging material suitable for use in flexible containers for potato chips, corn chips, and the like, have an excellent moisture resistance and sealing properties. It can comprise:

1. As an outer surface: a transparent sheet of nylon, polyester, cellophane, or polypropylene,

2. A coextruded laminate: A pigmented polyethylene and ethylene acrylic acid copolymer,

3. A metallic foil: Preferably aluminum, and

4. An inner surface: A heat-sealable polyolefin.

12.5 Aluminum Foils

Fig. 12.5.1 Advantages and disadvantages of aluminum foils

Advantages: It is a good barrier, grease proof, non sorptive, shrink proof, neither brittle nor soften at low temperature, odorless, tasteless, hygienic, non-toxic, sterile, corrosion resistant, head reflects, opaque to light, dead folding and light appearance.

Disadvantages: Low tear strength, corroded by strong acids and alkalies, heat sealing is not possible.

Foil thicknesses used

12.6 Current Trends in use of Packaging Materials

Presentation (graphics and material), cost, consumer friendliness (easy to open and use) and effect on environment are the four most important issues in packaging industry today. They are fulfilled by the following newer materials:

• To cut down the cost, the preformed containers are used for the dairy products such as cream and yogurt, curd, shrikhand etc.

• Lid machines are being replaced by form-fill-seal machines to produce and fill the polystyrene (or polypropylene) containers.

• Considerable improvement has been achieved in the lid materials:

i) The paper/polyester heat seal lidding and more recently metalized polyester peelable film laminate gives an even peel without tearing when lid is removed from a container.

ii) Proprietary structures such as MARDOCOTE (Saran: polyvinylidene chloride) are encroaching on PVDC because of better sealing properties and improved shelf life at lower cost in cheese packaging.

• There is also a switch from the thermo formed (Multivac) packaging using nylon / polyester laminate to flow pack type of systems incorporating polypropylene or polyester barriers often incorporating gas flushing in cheese packaging.

• High barrier structures/laminate based on polyester / EVOH / polythene and polypropylene / EVOH polythene are being developed for products like yoghurts and milk based desserts.

• EVOH (Ethylene Vinyl Alcohol) has excellent gas barrier properties in dry conditions, and such materials would be particularly suitable for aseptic packaging.

• EVOH is also more environment-friendly than saran, the alternate high barrier material.

• PVDC however continues to dominate in foods that come in retorted pouches. A typical laminate structure for such use might consist of polyester / polypropylene / PVDC / polypropylene.

• The developments in laminate (paper/polyethylene/aluminium/polyethylene) used for milk have been restricted to improvement in print quality, better glass lacquers, more rapid sealing characteristics, using different and thinner polymer films reduce costs mainly by down gauging.

• Until now, permeability in plastic food packaging was provided only by polyvinylidene chlorides (PVDC) and ethylene vinyl alcohol (EVOH) co-polymers. Recently, a third, complementary polymer, metaxylenediamine and adipic acid poly condensate (NMXD6) is under development and is likely to provide more flexibility.

• Similarly, HNR (High Nitrile Resins) also known as nitrile group produced by co-polymerization of acrylonitrile and methyl acrylate in 75:25 ratio, and amorphous nylon Selar PA are other barrier materials that can be used monolithically because their barrier properties are not adversely affected by moisture. In fact, the oxygen barrier of Selar PA increases with an increase in moisture (humidity). Both resins also offer clarity and rigidity providing for good aesthetic and structural properties.

• Depending on ethylene content and humidity, EVOH can potentially give the best cost / performance, followed closely by PVDC and NMXD6.

• EVOH and NMXD6 are very sensitive to moisture and therefore, must be used in a multi layer system of blends, as a sandwiched layer.

• PVDC is not moisture sensitive but is difficult to process and scrap reuse is still an issue. PVDC is, therefore, predominantly used as a thin layer in laminations and coating.

Table 12.1 Common plastics and their uses

Table 12.2 Desirable attributes of commonly used flexible materials



Table 12.3 Compatibility of various packaging materials to work as a layer in laminates