PACKAGING OF DAIRY PRODUCTS

Module 2. Packaging materials

Lesson 9
FORMS OF DIFFERENT PLASTIC MATERIALS - 1

9.1 Introduction

In this lesson the topics related to different plastic materials like cellulose, polyethylene, polypropylene, polyester, polyamide etc are discussed.

9.2 Cellulose

Cellophane is produced from wood pulp, treated chemically, and cast into a film on heated rollers. Glycerol is added as a softener, and the film is dried on heated rollers. Higher quantities of softener produce more flexible films.

Art silk, technically known as Cellulose Acetate is well known under the trade name "rayon". It is cheap and feels smooth on the skin, though it is weak when wet and creases easily. It could also be produced in a transparent sheet form known as "cellophane".

9.2.1 Characteristics of cellulose

1. Plain, uncoated cellulose is a glossy, transparent film that is odorless, tasteless, and biodegradable.

2. It is tough and puncture-resistant although it tears easily.

3. However, it is not heat-sealable.

4. It has poor water and gas vapor barrier characteristics.

5. Plain cellophane is used for foods that do not require a complete moisture or gas barrier.

6. Most cellophane is sold coated either with nitrocellulose on one or both sides or with polyvinylidene chloride (Saran).

These coatings improve the gas barrier and heat-seal characteristics of cellophane.

9.2.2 Modified celluloses:- Thermoplastics

• Cellulose acetate

• Cellulose acetate-butyrate

• Cellulose acetate-propionate

Application – Thermoformed blisters, windows for cartons and rigid transport cartons.

9.2.2.1 Cellulose acetate

This is a cellulose plastic made from bark (CAC) and acetate anhydride in the presence of acetic and sulphuric acids. It has good clarity, gloss and abrasion resistance. It is very similar in properties to PS. The high gas and water vapour permeability can be utilized for wrapping fresh produce like tomatoes and grapes.
Cellulose acetate is resistant to aromatic hydrocarbons but is soluble in Ketones and esters and is decomposed by strong acids and alkalis, cellulose acetate is sensitive to moisture and is not dimensionally stable.

9.2.2.2 Cellophane

It is derived from natural cellulose which is a main substance of all plants. In natural cellulose glucose units may vary from 3000-4000. In 1982 a process was discovered by which cellulose could be obtained in a soluble form by treating with NaOH, followed by Carbon Di-sulphide (CS2). In fiber form it is known as Rayon and in film form it is known as cellophane.

9.2.2.3 Regenerated cellulose

This is essentially same as natural cellulose, but has a lower molecular weight and can not be heat sealed. The films are made from wood chips in which cellulose is taken in to solution and re-precipitated as a continuous transparent film. It is excellent barrier to gas but is sensitive and permeable to water vapour.

9.2.3 Water soluble films

9.2.3.1 Carboxy methyl cellulose(CMC)

This film is available as Sodium EMC. It is hydrophilic and is insoluble in cold water.
Methyl Cellulose: It is obtained by treating cellulose with NaOH and then by alkylation. It is highly resistant to oils and greases. It can be formed in to small pouches.

9.2.3.2 Polyvinyl alcohol

The film is obtained by hydrolysis of polymerized vinyl acetate. It is insoluble in hydrocarbons. These are used for wrapping candies, single application for coffee, tea, derived milk, cold drinks, detergents and insecticides. It is soluble in water. It is utilized for manufacture of film, sachets used to give controlled dosage in water.

9.3 Polyethylene

Fig. 9.1 Basic structure and characteristics of polyethylene

Polyethylene or polythene (IUPAC name polyethene) is a thermoplastic commodity heavily used in consumer products and food packaging applications. It is produced by polymerization of ethylene.

Polyethylene is a polymer consisting of long chains of the monomer ethylene (IUPAC name ethene). In the polymer industry the name is sometimes shortened to PE or is commonly called polythene in UK. The ethene molecule (known almost universally as ethylene), C2H4 is CH2=CH2. Polyethylene is created through polymerization of ethene. It can be produced through radical polymerization, anionic addition polymerization, ion coordination polymerization or cationic addition polymerization. Each of these methods results in a different type of polyethylene

9.3.1 Characteristics of polyethylene

• Polyethylene was first synthesized by the German chemist Hans von Pechmann in 1898. It contained long -CH2- chains and termed it polymethylene. The first industrially practical polyethylene synthesis was discovered in 1933 in England. The types of catalyst that promote ethylene polymerization at more mild temperatures and pressures are:

• Chromium trioxide based

• Titanium halides and organoaluminum compounds

• Metallocenes was discovered in 1976 in Germany

• The catalyst families have since proven to be very flexible at copolymerizing ethylene with other olefins and have become the basis for the wide range of polyethylene resins available today, including very low density polyethylene , and linear low density polyethylene . Such resins, in the form of fibers have begun to be used in many high-strength applications

9.3.2 Physical properties of polyethylene

• For common commercial grades of medium-density and high-density polyethylene, the melting point is typically in the range 120 to 130°C whereas for average commercial low-density polyethylene it is typically 105 to 115°C.

• Most Low Density Polyethylene (LDPE) , Medium Density Polyethylene (MDPE), and High Density Polyethylene (HDPE) grades have excellent chemical resistance and do not dissolve at room temperature because of the crystallinity.

• Polyethylene is classified into several different categories based mostly on its density and branching. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure, and the molecular weight.

• Polyethylene (other than cross-linked polyethylene) usually can be dissolved at elevated temperatures in aromatic hydrocarbons, such as toluene or xylene, or chlorinated solvents, such as trichloroethane or trichlorobenzene

9.3.3 Classification of polyethylene

9.3.3.1 High-density polyethylene (HDPE)

• HDPE is defined by a density of greater or equal to 0.941 g/cm3

• It also has a higher softening temperature (121°C) and can therefore be heat-sterilized.

• It has a low degree of branching and thus stronger intermolecular forces and tensile strength.

• It is stronger, thicker, less flexible, less transparent, and more brittle and has lower permeability to gases and moisture than LDPE.

• It is used in products and packaging such as milk jugs, detergent bottles, margarine tubs, garbage containers and water pipes. It is commonly used in the production of bags, as liners, and as an over wrap.

9.3.3.2 Medium-density polyethylene (MDPE)

• MDPE is defined by a density range of 0.926–0.940 g/cm3

• It can be produced by chromium/silica catalysts, Ziegler-Natta catalysts or metallocene catalysts

• It has good shock and drop resistance properties

• It is also less notch sensitive than HDPE, stress cracking resistance is better than HDPE

• MDPE is typically used in gas pipes and fittings, sacks, shrink film, packaging film, carrier bags, screw closures etc.

9.3.3.3 Linear low-density polyethylene (LLDPE)

LLDPE is defined by a density range of 0.915–0.925 g/cm3

• It is a substantially linear polymer, with significant numbers of short branches, commonly made by copolymerization of ethylene with short-chain alpha-olefins (e.g. 1-butene, 1-hexene, and 1-octene) and combines the clarity of LDPE and the strength of HDPE.

• It has higher tensile strength than LDPE, exhibits higher impact and puncture resistance than LDPE, Lower thickness (gauge) films can be blown compared to LDPE, with better environmental stress cracking resistance compared to LDPE but is not as easy to process.

• It is used in packaging, particularly film for bags and sheets. Lower thickness (gauge) may be used compared to LDPE. Cable covering, toys, lids, buckets and containers, pipees etc.

• While other applications are available, LLDPE is used predominantly in film applications due to its toughness, flexibility, and relative transparency.

9.3.3.4 Low-density polyethylene (LDPE)

• LDPE is defined by a density range of 0.910–0.940 g/cm3.

• It has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure as well. It has therefore less strong intermolecular forces.

• This results in a lower tensile strength and increased ductility.

• It is created by free radical polymerization.

• The high degree of branches with long chains gives molten LDPE unique and desirable flow properties.

• It is used for both rigid containers and plastic film applications such as plastic bags and film wrap.

• It is heat-sealable at low temperatures (~80°C).

• It is chemically inert, odor-free, and shrinks when heated.

• It is a good moisture barrier but a poor gas barrier. This selective permeability makes it a good choice of packaging material for such products as fresh meat, fruits, and vegetables.

• It is less expensive than most films and is therefore widely used for many packaging applications, including applications in shrink- or stretch-wrapping of products.

9.3.3.5 High molecular high density polyethylene (HMHDPE)

HMHDPE films have superior mechanical and barrier properties. They can be folded easily, have good stiffness and high initial tear resistance. They can be used in thinner gauges. This resembles paper in feel. It is white translucent. It has better barrier properties to moisture, air, oil and grease. It is used for wrapping meat, fish, dairy products, backery products and vegetables.

Table 9.1 Properties of LDPE & HDPE

Ethylene copolymers

In addition to copolymerization with alpha-olefins, ethylene can also be copolymerized with a wide range of other monomers and ionic composition that creates ionized free radicals. Common examples include vinyl acetate (resulting product is ethylene-vinyl acetate copolymer, widely used in athletic shoe sole foams), and a variety of acrylates used in packaging and sporting goods.

9.4 Polyester

Polyester is a polycondensation product of ethylene glycol and terephthalic acid. The major polyester in the market place is polyethylene terephthalate (PET), marketed under the trade name Mylar.

• Polyester (Terylene) is a category of polymers which contain the ester functional group in their main chain. Although there are many forms of polyesters, the term "polyester" is most commonly used to refer to polyethylene terephthalate (PET). Other forms of polyester include naturally occurring cutin of plant cuticles as well as synthetic polyesters such as polycarbonate and polybutyrate.

• Polyester may be produced in numerous forms. For example, polyester as a thermoplastic may be heated and processed into different forms, e.g., fibers, sheets, and three-dimensional shapes.

• While combustible at high temperatures, polyester tends to shrink away from flames and self-extinguishes.

9.4.1 Characteristics of polyester

• The main characteristics of PET are its strength and toughness (can resist pressures of 50-60 psi used for soft drinks), its clarity, its good barrier properties to moisture and gases, and its high melting point.

• These characteristics make it an ideal packaging material for carbonated soft drink bottles and as a component of boil-in-bag food packages and retortable pouches ( max. used temp. 150°C).

• A more crystalline PET (CPET) is used to make “dual-ovenable” food trays that enable precooked food and entrees to be heated in a radiant oven or microwave without deformation of the packaging tray.

• It has good chemical resistance, soluble in benzyl alcohol, hot phenols and alkalis.

9.4.2 Uses of Polyester

• Polyester is the most widely used manufactured fiber.

• Polyesters are also used to make bottles, films, tarpaulin, liquid crystal displays, holograms, filters, dielectric film for capacitors, film insulation for wire and insulating tapes. In general they have extremely good mechanical properties and are extremely heat resistant.

• Thermosetting polyester resins are generally copolymers of unsaturated polyesters with styrene. Another important family is the group of vinyl esters.

• Unsaturated polyesters are commonly used as casting materials, fiberglass laminating resins, and non-metallic auto-body fillers.

• It is good material for metallization, Metalized PET is used in dairy industry for packaging of powders and processed cheese.

• It is also used for vacuum packaging of coffee and processed meat.

9.5 Polyamide (PA)

A polyamide is a polymer containing monomers of amides joined by peptide bonds. They can occur both naturally, examples being proteins, such as wool and silk, and can be made artificially, examples being nylons, aramids, and sodium poly(aspartate). Thus, Polyamide is made by the condensation polymerization of an organic acid and an amine. Nylon was the first purely synthetic fiber, introduced by Du Pont Corporation in 1939.

Subsequently polyamides 6, 10, 11, and 12 have been developed based on monomers which are ring compounds, e.g. caprolactam. Nylon 66 is a material manufactured by condensation polymerization. Nylon 6, 66 and 11 are most widely used as packaging films.

(a) Nylon 6- It is prepared from phenol. It is more flexible than Nylon 66 and has better grease resistance than Nylon 11. It can withstand dry heat up to 250°C and hence, it is used for roast-in-bags. It has high mechanical strength, high elongation, excellent abrasion and bursting resistance. Unsupported film is used for containing frozen foods, aromatic flavourings, fats & oils.

(b) Nylon 66- This has higher softening point i.e. 265°C.

(c) Nylon 11- It is manufactured from castor oil, undeconoic acid and ammonia. It softens at 125°C and is resistant to fats, oils and even concentrated alkalis and organic acids, but does not resist phenol and strong mineral acids.

9.5.1 Characteristics of polyamide

• Nylon is a clear, tough film with good mechanical properties over a wide temperature range (from –60°C to 200°C).

• It provides good gas and aroma barriers, but has poor moisture barrier properties.

• However, the films are expensive to produce, and they require high temperatures to form a heat seal.

• Nylons still remain important plastics, and not just for use in fabrics.

• Nylon is commonly used as the outer layer of laminated structures to add strength to the laminated structure.

• The barrier and mechanical properties of nylon can be enhanced through biaxial orientation to give biaxially oriented nylon (BON).

• In its bulk form it is very wear resistant, particularly if oil-impregnated, and is used to build gears, bearings, bushings, and because of good heat-resistance, increasingly for under-the-hood applications in cars, and other mechanical parts.

• Nylons have high WVTR rates, but are very good gas barriers and hence used in laminates for vacuum packing.

• The transparency of nylon film is excellent.

• They have good grease resistance and mechanical strength.

9.6 Polypropylene

Polypropylene was first polymerized in 1954 by Giulio Natta. Polypropylene or polypropene (PP) (IUPAC- Poly (1-methylethylene)) is a thermoplastic polymer, made by the chemical industry by an addition polymer from the monomer propylene. Its other names are: Polypropylene; Polypropene; Polipropene 25 USAN; Propene polymers; Propylene polymers; 1-Propene homopolymer. Its molecular formula is (C3H6)x.

9.6.1 Characteristics of polypropylene

• Polypropylene (PP) is one of the lightest of all plastics.

• It has a melting point of ~ 165°C and density: Amorphous (0.85 g/cm3); Crystalline: (0.95 g/cm3).

• There are three general types of PP: homopolymer, random copolymer and impact or block copolymer.

• Two main types are made: Non-oriented or cast polypropylene (PP) and oriented polypropylene (OPP).

• Melt processing of polypropylene can be achieved via two general methods: 1) extrusion and 2) molding.

• Biaxially oriented polypropylene (OPP) is superior to PP in terms of clarity, impact strength and barrier properties to water vapor and gases.

• Common extrusion methods include production of melt blown and spun bond fibers to form long rolls for future conversion into a wide range of useful products.

• The most common molding technique for PP is injection molding. The types of molded parts made are almost limitless, but commonly include cups, cutlery, vials, caps, containers, housewares and automotive parts such as batteries. Blow molding and injection-stretch blow molding are techniques involving both extrusion and molding.

• Compared to LDPE and HDPE, PP is stiffer, tougher, and more transparent.

• It is rugged and unusually resistant to many chemical solvents, bases and acids.

• It also has superior gas and moisture barrier properties and higher heat resistance than LDPE and HDPE, which make it suitable for boil-in-bag and retortable products.

• It stretches, although less than polyethylene and it has low friction, which minimizes static buildup and makes it suitable for high-speed filling equipment.

• However, it is more brittle than polyethylene at low temperatures.

• Its resistance to oils & grease is better than polyethylene.

• One outstanding property of PP is its resistance to fatigue when fleshed.

Table 9.2 Properties of polypropylene

9.6.2 Applications of polypropylene

• A common application for polypropylene is as Biaxially Oriented polypropylene (BOPP). These BOPP sheets are used to make a wide variety of materials including clear bags. When polypropylene is biaxially oriented, it becomes crystal clear and serves as an excellent packaging material for artistic and retail products.

• It is used in a wide variety of applications, including packaging, textiles, stationery, plastic parts and reusable containers of various types.

• It is most commonly used for plastic moldings where it is injected into a mold while molten, forming complex shapes at relatively low cost and high volume, e.g. bottle tops, bottles and fittings.

• The large number of end use applications for PP is often possible because of the ability to tailor grades with specific molecular properties and additives during its manufacture. For example, antistatic additives can be added to help PP surfaces resist dust and dirt. Many physical finishing techniques can also be used on PP, such as machining. Surface treatments can be applied to PP parts in order to promote adhesion of printing ink and paints.

• Polypropylene has a typical peak melting point of ~160°C. Many plastic items for medical or laboratory use can be made from polypropylene because it can withstand the heat in an autoclave. Food containers made from it will not melt in the dishwasher, and do not melt during industrial hot filling processes. For this reason, most plastic tubs for dairy products are made of polypropylene and sealed with aluminum foil. After the product is cooled, the tubs are often given lids made of a less heat-resistant material, such as LDPE or polystyrene. Such containers provide the rubbery (softer, more flexible) feeling of LDPE with respect to PP of the same thickness.

• Rugged, translucent, reusable plastic containers made in a wide variety of shapes and sizes for consumers are commonly made of polypropylene, although the lids are often made of somewhat more flexible LDPE so they can snap on to the container to close it.

• Polypropylene can also be made into disposable bottles to contain liquid, powdered or similar consumer products, although HDPE and polyethylene terephthalate are commonly used to make bottles.

• PP can also be used to package soft bakery products and fresh produce because it is flexible enough to fit around irregular shapes.

• PP has been produced in sheet form and this has been widely used for the production of stationary folders, packaging and storage boxes. The wide colour range, durability and resistance to dirt make it ideal as a protective cover for papers and other materials. It is used in stickers because of these characteristics.