PLASTICS AND THEIR ROLE IN FOOD PACKAGINGicpe.in/Plastics in Food Packaging/pdf/2-Final.pmd.pdf19 PLASTICS IN FOOD PACKAGING PLASTICS AND THEIR ROLE IN FOOD PACKAGING Polyolefins Polyolefins

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PLASTICS IN FOOD PACKAGING

PLASTICS AND THEIR ROLE IN FOOD PACKAGING

PLASTICS AND THEIR ROLEIN FOOD PACKAGING

Baldev Raj

Food Packaging Technology DepartmentCentral Food Technological Research Institute

Mysore 570 020 (INDIA)

Chapter 2

16


Chapter 2

17



Chapter 2

PLASTICS AND THEIR ROLEIN FOOD PACKAGING

INTRODUCTION

Food requires protection from variousenvironmental factors from the time of itsproduction till it is consumed. Hence,packaging is required to protect the food.The shelf life of packaged foods may rangefrom a few days to more than a year. Thus,the properties of packaging material musthave sufficient permanence to assure thatshelf life is not compromised. This is notto say that the package is entirely responsi-ble for shelf life retention. Foods deterioratewith time even with complete protection.Moreover, packages, with rare exceptionscannot improve the original quality of thefood itself. Plastics are one of the greatestinventions of the millennium. Thanks toplastics, there has been enormous develop-ments in the field of food packaging.

PRESENT SCENARIO

Globally, plastic industries have agrowth rate of 4 to 5 percent per annum,whereas, in India, it is 15%. Today, ourplastic industries are able to cater to theworld markets and India has acquiredexcellence to encash global opportunities.The consumption of basic plastics in Indiais about 3,915 Kilotons (KT) with polyole-

fins (polyethylene, polypropylene, etc.) atthe highest level of 2,640 KT followed byPVC at 850, PS at 210, ABS at 65, PET at 75and others at 75 KT. India is likely tobecome second or third largest plasticconsumer from the present ninth rank inthe world by the end of this decade. Thereare about 16 major raw material manu-facturers and 23,000 small scale processingunits, involving about Rs 27,000 crore ofrupees turnover. Plastic materials areproviding livelihoods to more than 3million people in the country.

The overall development of plasticindustry in our country is due to someuseful properties of plastics. They areversatile, light weight, non-corrosive,energy efficient, durable and user friendlymaterials. Their aesthetic appeal, abun-dance, and low cost are main factorsresponsible for industrial growth in India.The plastic industry is larger than those ofsteel and aluminum because plasticsenrich the quality of life by meeting the lifestyle needs of every segment in our society.Plastics are eco-friendly and preserve thenatural resources. India has become secondhighest economic growth region due torapid penetration of technology. The

18


Chapter 2

growth is driven by agriculture,communications, consumer goodspackaging, health care, infrastructure,transportation and infor-mationtechnology. Plastics play an important rolein fuelling the country's growth.

Plastics

Plastics are the designation commonlygiven to both thermosetting and thermo-plastic polymers, which are organicmolecules consisting of many repeatingunits linked together to produce long chainmolecules with high molecular weight(M.W.) in the range of 5,000 to 100,000.The repeating units are called �mono-mers�. Monomers are either aliphatic,which means they do contain long carbonchain, or aromatic, with a benzene ring asits part. In some cases, the polymer chaincontains more than one kind of monomer,called copolymer and frequently the chaincontains relatively short side branches.Some of the side branches may connect thelong backbone chains together to make thepolymer stiffer or more resistant tochemical attack. This phenomenon isknown as �cross-linking�.

Thermosets

Thermosetting polymers harden, or�cure�, when heat is applied. They cannotbe remelted, and so articles made fromthem must be fabricated by charging thepolymer ingredients to a mould, which isthen heated to produce a rigid article of therequired shape. Phenolics - phenol withformaldehyde, urea formaldehyde polymerwere among the first plastics invented.Both these materials have good resistanceto alcohols and solvents, and werefrequently used for bottle and jar caps in theearly days of plastics. They are used verylittle today in packaging. They are

basically used in closures and trays ratherthan primary food packaging structures.However, they are occasionally found inapplications where excellent resistance totemperature extremes is important. Otherthermosetting polymers include epoxiesand polyurethane.

Thermoplastics

Thermoplastics are the primary plasticsmaterials now used in packaging and fora host of non-packaging applications.They dominate the universe of plasticsbecause they can easily be formed into analmost limitless variety of shapes byheating them a number of times. The easeof fabricating plastics and the manyavailable fabrication methods are some ofthe most important reasons why plasticshave become such popular materials forpackaging foods and beverages. Fabri-cation has permitted the combination ofdifferent plastics to obtain uniqueproperties, and has also helped to produceplastics which can easily be handled onhigh speed packaging and printingmachines. The lower energy required tofabricate plastics has combined with thereduced weight of plastics to provideconsiderable economic advantage for manypackage systems.

Thermoplastics make up the greatestshare of plastic usage in food packagingbecause they can be rapidly formed econo-mically into any shape needed to fulfill thepackage function, and are especiallyamenable to recycling and waste-to-energyconversion. The principal families ofthermoplastics in food packaging are thepolyolefins (low density or high densitypolyethylene, polypropylene, etc.), poly-styrene, polyester, nylon, polycarbonateand vinyl polymers.

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Polyolefins

Polyolefins form an important class ofthermoplastics and include polyethylene,polypropylene and some other olefiniccopolymers which are among the mostwidely used food packaging plastics, in theform of films, mouldings, coatings,adhesives and closures. The variety oftypes and grades is steadily growing asmanufacturers find new compositions tosatisfy specific needs.

Polyethylene

Polyethylene is probably the polymerwe see most in daily life. It is the mostpopular plastic in the world. This is thepolymer that makes grocery bags, shampoobottles, children�s toys, and even bulletproof vests. For such a versatile material, ithas a very simple structure, the simplest ofall commercial polymers. A molecule ofpolyethylene is nothing more than a longchain of carbon atoms, with two hydrogenatoms attached to each carbon atom.

Sometime some of the carbons, insteadof having hydrogens attached to them, willhave long chains of polyethylene attachedto them. This is called branched, or low-density polyethylene, or LDPE. When thereis no branching, it is called linear

polyethylene, or High Density Polye-thylene (HDPE). Linear polyethylene ismuch stronger than branched one.

A molecule of linear polyethylene, orHDPE

A molecule of branched polyethylene, orLDPE

It is normally produced with molecularweights in the range of 200,000 to 500,000,but it can be made even with higher M.W.Polyethylene with molecular weights ofthree to six million is referred to as ultra-high molecular weight polyethylene, orUHMWPE. This can be used to makestrong fibers used in bullet proof vests.

Low Density Polyethylene(LDPE)

This is the largest volume singlepolymer used in food packaging in bothfilm and blow-molded forms. It is apolymer of ethylene, a hydrocarbon gas

Fig. 2.1. Polyolefin based foodpackaging materials

(LDPE/LLDPE/HDPE/HMHDPE/PP)

or

20


Chapter 2

available in large quantities as a by-product of petroleum refining and otherprocess. Polyethylene was first producedby Imperial Chemical Industries (ICI)Limited in 1933, under extremely highpressure on chain reactions. It wasproduced on a pilot plant scale that sameyear with full commercial scale productioncommencing in 1939.

The polymerization of ethylene canoccur over a wide range of temperaturesand pressures but most commercial highpressure processes utilize pressuresbetween 1,000 and 3,000 atmospheres andtemperatures between 100° and 350°C;higher temperatures causing degradationof the polyethylene.

The simplest structure for polyethyleneis a completely unbranched structure of�CH

2-. The vigorous nature of the high

pressure process leads to a great extent ofchain branching, with both short and longchains being formed. The branch chainsprevent close packing of the main polymerchains and this results in the productionof relatively low density polyethylenes(typically 0.91-0.94 g/cm3). Areas wherethe chains are parallel and closely packedare largely crystalline, while disorderedareas are amorphous. The crystalline areasare known as crystallites. When the poly-mer melt is cooled slowly, the crystallitesmay form spherulites.

The crystallinity of LDPE usually variesbetween 50 and 70%. The softening pointis affected by chain branching. Because thechains are unable to approach each otherclosely, the attractive forces between themare reduced and less energy (in form ofheat) is necessary to cause them to moverelatively to each other and thus flow. Thesoftening point of LDPE is just below

100°C, thus precluding the use of steam tosterilize it in certain food packagingapplications.

LDPE is a tough, slightly translucentmaterial, which can be blown extruded intotubular film, or extruded through a slit dieand chill-roll cast, the latter process givinga clearer film. It has good tensile strength,burst strength, impact resistance and tearstrength, retaining its strength down to�60°C. While it is an excellent barrier towater and water vapour, it is not a goodbarrier to gasses.

LDPE has excellent chemical resistance,particularly to acids, alkalis and inorganicsolutions, but is sensitive to hydrocarbonsand halogenated hydrocarbons and to oilsand greases. These compounds areabsorbed by the LDPE, which then swells.Environmental stress cracking (ESC) is aphenomenon, which occurs when amaterial is stressed multi-axially while incontact with certain polar liquids orvapours, resulting in surface cracks or evencomplete failure of the material. Essentialoils and vegetable oils are capable ofcausing ESC but the effect can be greatlyreduced by using high molecular weightgrades of LDPE. One of the great attributesof LDPE is its ability to be fusion weldedto itself to give good, tough, liquid-tightseals.

LDPE is also used as a rigid packagingmaterial. It can be easily blow moulded intobottles where its flexibility enables thecontents to be squeezed out. It is alsowidely used in the form of snap-on caps,collapsible tubes and a variety of spoutsand other dispensers. The surface of polye-thylene containers can be treated withfluorine after blow molding to form a verythin polar, cross-linked surface, which

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decreases the permeability of the polye-thylene to nonpolar penetrants. It alsoeliminates the need for treating the surfaceby corona-arc discharge or flame tech-niques to improve printability properties.Polyethylene is one of the most inertpolymers and causes no hazard in normalhandling.

Linear Low Density Polyethylene(LLDPE)

The first production of LLDPE wasmade in a solution process in 1960.Attempts in the 1970�s to produce LDPE,either by low-pressure gas phasepolymerization or by liquid phase processsimilar to those used for producing HDPE,led to the development of LLDPE, whichhas a molecular structure similar to HDPE.However, it is virtually free of long chainbranches but does contain numerous shortside chains. It is manufactured as a resultof copolymerizing ethylene with a smalleramount of higher alkenes such as propene,1-butene, 1-hexene or 1-octene. Suchbranching interferes with the ability of thepolymer crystallization, and thereforeresults in LLDPE having a density similarto LDPE. The linearity provides strength,while the branching provides toughness.The term linear in LLDPE is used to implythe absence of long chain branches. It wasafter 1977 that LLDPE became available incommercial quantities.

A major feature of LLDPE is that itsmolecular weight distribution is narrowerthan that of LDPE. Generally, the advan-tages of LLDPE over LDPE are improvedchemical resistance, improved perfor-mance at both low and high temperatures,higher surface gloss, higher strength at agiven density and a greater resistance toenvironmental stress cracking. In film form,

LLDPE shows improved puncture resis-tance and tear strength. At a density of 0.92g/cm3, the melting points of LDPE andLLDPE are 95°C and 118°C, respectively.LLDPE commonly has a density of around0.92 g/cm3, with butene in particular, asthe comonomer. The superior properties ofLLDPE have led to its use in newapplications for polyethylene as well as thereplacement of LDPE and HDPE in someareas, specially in liquid packaging of milk,arrack, etc.

High Density Polyethylene(HDPE)

Prior to 1950, the only commercialpolymer of ethylene was the highlybranched polymer LDPE. The techniquefor making a linear polymer wasdiscovered by Nobel laureate Karl Zieglerof Germany in the early 1950�s. Zieglerprepared HDPE by polymerizing ethyleneat a low pressure and at ambient tempe-ratures using mixtures of triethylaluminumand titanium tetrachloride. Relatively lowpressure/low temperature polymerizationprocesses at 27-34 atmospheres and 100-175°C or 20 atmospheres and 85-100°C areused to produce HDPE.

HDPE possesses a much more linearstructure than LDPE and has up to 90%crystallinity compared to LDPE whichexhibits crystallinity as low as 50%.Although some branch chains are formed,these are short and few in number. HDPEfilm is stiffer and harder than LDPE anddensities range from 0.94-0.96 g/cm3. Itssoftening point is about 121°C, and its lowtemperature resistance is almost the sameas LDPE. Tensile and bursting strengthsare higher but impact and tear strengths areboth lower than LDPE due to the linearnature of the HDPE molecules. They tend

22


Chapter 2

to align themselves in the direction of flowand thus the tear strength of the film ismuch lower in the machine directioncompared to the transverse direction.

The chemical resistance of HDPE is alsosuperior to that of LDPE and, in particular,it has better resistance to oils and greases.The film offers excellent moistureprotection, a much decreased gaspermeability compared with LDPE film, butis much more opaque. Heat sealing isconsiderably more difficult compared toLDPE film.

HDPE film has a white, translucentappearance and therefore tends to competewith paper. HDPE is blow molded intobottles, Jars, jerricans, etc., for a variety offood packaging applications.

Polypropylene (PP)

Structurally, PP is a vinyl polymer andis similar to polyethylene except that thebackbone chain has a methyl groupattached to it on every other carbon atom.Polypropylene can be made from themonomer propylene by Ziegler-Nattapolymerization and by metallocenecatalysis polymerization.

Polypropylene can be made withdifferent tacticities. Most polypropylenewe use is isotactic. This means that all themethyl groups are on the same side of thechain, as shown below:

But sometimes we also use atacticpolypropylene. Atactic means that themethyl groups are placed randomly on bothsides of the chain as shown below:

This atactic PP is soft and rubbery innature. However, using special meta-llocene catalysts it is believed that we canmake polymers which contain blocks ofisotactic polypropylene and blocks ofatactic polypropylene in the same polymerchain, as shown below.

This polymer is rubbery, and makes agood elastomer. Because the isotacticblocks form crystals by themselves and arejoined to the atactic blocks, soft rubberytethers of atactic polypropylene will tie eachlittle hard clump of crystalline isotacticpolypropylene together.

PP can be blow moulded and injectionmolded, the latter process being widelyused to produce closures and thin walledpots and crates. The glass transitiontemperature of PP is placed between 10°Cand -20°C with the result that the polymerbecomes brittle as subzero temperatures are

�

Atactic polypropylene

isotactic block

atactic block

Isotactic polypropylene

Propylene Polypropylene

Zieglar-NattaPolymerization

or metallocenecatalysis

23



approached. Copolymerization with 4-15% of ethylene improves the strength andlowers the T

m and T

g slightly; such

copolymers are often preferred to thehomopolymer in injection molding andbottle blowing applications, and also finduse in shrink-wrapping where the lowermelting point is an advantage.

PP has a lower density (0.90 g/cm3) anda higher softening point (140°-150°C) thanthe polyethylenes, low water vaportransmission, medium gas permeability,good resistance to greases and chemicals,good abrasion resistance, and hightemperature stability, as well as good glossand high clarity, the latter two factorsmaking it ideal for reverse printing.

Non-oriented PP film is often referred toas cast PP (CPP) film because it is generallymade by the chill-roll cast process,although other methods can be used. PPfilm is a very versatile material being usedas a thermoformable sheet, in cast form forfilm and bags, and as thin, strong biaxiallyoriented film for many applications. Castand oriented PP are sufficiently different asthey do not compete for the same end use.The cost of cast PP is much lower than thatof oriented PP. The cast form has polye-thylene type uses while the oriented formhas regenerated cellulose type uses. CastPP use in food packaging is limited owingto its brittleness at below-freezingtemperatures, and it is generally notrecommended for use with heavy, sharp ordense products unless laminated to stron-ger, more puncture-resistant materials. Therelatively high temperature resistance of PPpermits its use as the sealant layer inretortable pouches where temperatures ofup to 130°C are encountered.

Polypropylene is one of those ratherversatile polymers. It serves double duty,both as a plastic and as a fiber . As a plasticit is used to make things like dishwasher-safe food containers. It can do this becauseit doesn�t melt below 160 oC. As a fiber,polypropylene is used to make indoor-outdoor carpeting, the kind that is usedaround swimming pools and miniaturegolf courses. It works well for outdoorcarpet because it is easy to make coloredpolypropylene, and the polypropylenedoesn�t absorb water like nylon.

Oriented Polypropylene (OPP)

In recent years there has been a largeincrease in the use of oriented polypro-pylene (OPP) for food packaging. Widevariations are possible in the extent oforientation in two directions, leading to awide range of properties. However, biaxi-ally oriented (BOPP) film has a high claritysince layering of the crystalline structuresreduces the variations in refractive indexacross the thickness of the film and this inturn reduces the amount of light scattering.The blown tubular or high expansionbubble process, or the tenter frame processcan produce OPP.

BOPP film has a tensile strength in eachdirection roughly equal to four times thatof cast PP film. Although tear initiation isdifficult, tear resistance after initiation islow. Biaxial orientation also improves themoisture barrier properties of PP film andits low temperature impact strength. OPPfilm is not considered to be a gas barrier filmbut this deficiency can be overcome bycoating with poly (vinylidene chloride).(PVDC) copolymer. OPP films often havea stiff feel and tend to crinkle audibly.

24


Chapter 2

If heat sealing is required, PP is nor-mally coated with a lower melting pointpolymer because shrinkage tends to occurwhen highly stretched film is heated. PE,PVDC copolymer and acrylic polymers areused as fusible coatings for OPP film.Though PE is cheaper, PVDC copolymerconfers far better resistance to water vaporand oxygen permeability. Acrylic polymeradds no barrier properties to OPP.

Poly (Vinyl Chloride) (PVC)

Structurally, PVC is a vinyl polymer. Itis similar to polyethylene, but on everyother carbon in the backbone chain, one ofthe hydrogen atoms is replaced with achlorine atom. It is produced by the freeradical polymerization of vinyl chloride.

PVC was one of those odd discoveriesa hundred years ago, in Germany. It wasprepared using acetylene and hydrochloricacid in 1912 by one German chemist, FritzKlatte:

Poly (vinyl chloride) is the plasticknown at the hardware store as PVC. Thisis the PVC from which pipes are made, andPVC pipe is everywhere. The �vinyl�siding used on houses is made of poly(vinyl chloride). Inside the house, PVC isused to make linoleum for the floor. In theseventies, PVC was often used to makevinyl car tops. PVC is useful because itresists two things that hate each other: fireand water. Because of its water resistance,it is used to make raincoats and showercurtains, and of course, water pipes. It hasflame resistance too, because it containschlorine. When PVC is burnt, chlorineatoms are released and chlorine atomsinhibit combustion.

Addition polymerization of VCMproduces poly (vinyl chloride) (PVC). Fromthe structure of VCM, it can be seen thataddition of molecules to the growing chaincan take place either head to head, head totail or in a completely random manner.PVC polymerizing in either of the first twoforms would be expected to be crystalline,while those containing the randomarrangement would be amorphous.Generally, PVC polymerizes in the atacticform and thus is largely an amorphouspolymer.

Vinyl polymers and copolymers makeup one of the most important anddiversified groups of linear polymer. Thisis because PVC can be compounded toproduce a wide spectrum of physicalproperties. This is reflected in the varietyof uses to which it is put - from exteriorguttering and water pipes to very thinflexible surgical gloves.

It is the second most widely usedsynthetic polymer after polyethylene and iscommonly referred to simply as �vinyl�. A

Vinyl chloride Poly (vinyl chloride)

Free radicalvinyl

polymerization

Poly (vinyl chloride)

Acetylene Vinyl chloride

25



range of PVC films with widely varyingproperties can be obtained from the basicpolymer. The two main variables arechanges in formulation (principally plasti-cizer content) and orientation. The formercan give films ranging from rigid, crispfilms to limp, tacky, stretchable films. Thedegree of orientation can also be variedfrom completely uniaxial to balancedbiaxial.

Unplasticized PVC tends to degradeand discolour at temperatures close tothose used in its processing. So suitablestablizers have to be included in theformulation. The stabilizers used aregenerally the salts of tin, lead, cadmium,barium, calcium or zinc along withepoxides and organic phosphates, andthey must be carefully selected for non-toxic

applications. The amount of residual VCMpresent in the polymer resin or formedduring blow moulding must be kept withinthe legal limits set.

Approval by US Food and DrugAdministration of certain octyltin compou-nds for use in stabilizing PVC during blowmoulding of containers has expanded theuse of this polymer for food packaging.

Extremely clear and glossy films can beproduced having a high tensile strengthand stiffness. The density is high at around1.4 g/cm3. The water vapour permeabilityis higher than that of the polyloefins but thegas permeability is lower. UnplasticizedPVC has excellent resistance to oils, fatsand greases and is also resistant to acidsand alkalis.

To a large extent, the properties ofplasticized PVC depend on the type ofplasticizer used, as well as the quantity.For these reasons it is difficult to be veryspecific about the physical properties ofPVC and as such a wide range of plasti-cization is possible.

Films with excellent gloss andtransparency can be obtained provided thatthe correct stabilizer and plasticizer areused. Both plasticizer and unplasticizerfilms can be sealed by high frequencywelding techniques. PVC is inert in itschemical behaviour, being self-extinguish-ing when exposed to a flame.

Thin plasticizer PVC film is widely usedin supermarkets for the stretch wrapping oftrays containing fresh red meat andproduce. The relatively high water vapourtransmission rate of PVC prevents conden-sation on the inside of the film. Orientationfilms are used for shrink wrapping ofproduce and fresh meat.

Fig. 2.2. PVC based food packaging materials(Thermoformed/Clingwrap)

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Chapter 2

Unplasticized PVC as a rigid sheetmaterial is thermoformed to produce a widerange of inserts from chocolate boxes tobiscuit trays. Unplastized PVC bottles havebetter clarity, oil resistance and barrierproperties than those made from polye-thylene. However, certain solvents, notablyketones and chlorinated hydrocarbonssoften them. PVC bottles have madeextensive penetration into the market for awide range of foods including fruit juicesand edible oils. Today, PET bottles arecompletely replacing PVC bottles, due totheir poor impact resistance and taintingproblems due to stabilizers.

Poly (Vinylidene Chloride)(PVDC)

Pure poly (vinylidene chloride) homo-polymer yields a rather stiff film which ishardly suited for packaging purposes.However, when it is copolymerized with 5to 50% (but typically 20%) of vinyl chloride,a soft, tough and relatively impermeablefilm results. The Dow Company has beenmarketing these copolymers since 1940under the trade name �Saran�.

The properties of PVDC copolymer filminclude a unique combination of lowpermeability to water vapor, gases andodours, as well as greases and alcohols. Ithas good stress-crack resistance to a widevariety of agents. It also has the ability towithstand hot filling and retorting, makingit a useful component in multilayer barriercontainer. It is an important component ofmany laminates and is the best coating forregenerated cellulose. PVDC copolymerscan be sealed to themselves and to othermaterials.

By itself, the copolymer is frequentlyused as a shrink film since orientation

improves tensile strength, flexibility, clarity,transparency and impact strength. Aswell, gas and moisture permeability arelowered and tear initiation becomesdifficult. The shrink film can be heat sealedusing impulse sealing with teflon-coatedheating bars.

Ionomers

These were first discovered by Du Pontin the 1960s, and are prepared by co-polymerizing ethylene with a small amountof (1-10%) unsaturated carboxylic acid suchas methacrylic acid/acrylic acid using thehigh pressure process. These polymers arecalled ethylene acrylic acid copolymer(EAA)/ethylene methacrylic acid copoly-mer (EMA). Further, such copolymers areneutralized to varying degrees with thederivative of a metal such as sodiummethoxide or magnesium; ionize crosslinks which confers enhanced stiffness andtoughness (the puncture resistance ofionomer film is double that of LDPE film ofthe same gauge) on the material at ambienttemperatures.

One example of an ionomer is poly(ethylene-co-methacrylic acid). This polymeris a sodium or zinc salt (which provides theions) of copolymers derived from ethyleneand methacrylic acid, commerciallybranded as �Surlyn�.

In an ionomer, the nonpolar chains aregrouped together and the polar ionicgroups are attracted to each other. The ionicgroups would like to go off into a littlecorner by themselves, but since they areattached to the polymer chain, they cannotdo so. This allows thermoplastic ionomersto act in ways similar to that of crosslinkedpolymers or block copolymers.

27



In comparison with LDPE, ionomer/EAA/EMA films have excellent oil andgrease resistance, excellent resistance tostress cracking, greater clarity, lower haze,greater abrasion resistance and a highermoisture vapor permeability due to lowercrystallinity. Their disadvantages includetheir poorer slip and block characteristics.Ionomers are particularly useful incomposite structures to provide an innerlayer with good heat sealability. Laminatedor co extruded films with nylon orpolyesters are widely used for packagingmeat, cheese and vegetable oils. Theresuling ionic attractions stronglyinfluence the polymer properties.

However, ionomers are not crosslinkedpolymers, and are in fact a type of

thermoplastic called a reversible cross-linker. When heated, the ionic groups willlose their attractions for each other and thechains will move around freely. As thetemperature increases, the chains movearound faster and faster and the groupscannot stay in their clusters. This allowsfor a polymer with the properties of anelastomer and the processability of athermoplastic. These ionomers aresometimes known as thermoplasticelastomers.

EAA copolymer resin is normallyprocessed at melt temperatures rangingfrom 190 to 218°C in blown film equipment.The resin is available for use in blown andcast film extrusion and coextrusion equip-ment, designed to process polyethylene

Poly (ethylene-co-methacrylic acid), ionomer

NaOH

Ethylene

Methacrylic acid

Sodium salt of ionomer

28


Chapter 2

resins. They exhibit better properties incomparison with LDPE, e.g. EAA hasexcellent adhesion to coextruded nylon forsuperior structure reliability with high filmstrength and barrier integrity for productprotection. It has very good hot tack,sealability through contaminationespecially edible vegetable oils, and heatseal strength for strong, reliable heat seals.EAA has wide use in meat, poultry,seafood, and cheese packaging, edible oilsand other liquid product pouches.

Acrylates

Acrylates are a family of polymers,which are a type of vinyl polymer. Theseare of course, made from acrylatemonomers. Acrylate monomers are esters,which contain vinyl groups, that is, two-carbon atoms double bonded to each other,directly attached to the carbonyl carbon.

Some acrylates have an extra methylgroup attached to the alpha carbon, andthese are called methacrylates. One of themost common methacrylate polymers ispoly(methyl methacrylate) (PMMA).

Methacrylate

This little methyl group would make awhole lot of difference in the behavior andproperties of the polymer. Poly (methylacrylate) is a white rubber at room

temperature, but poly (methyl metha-crylate) (PMMA) is a strong, hard, and clearplastic.

PMMA is a vinyl polymer made by freeradical vinyl polymerization from themonomer methyl methacrylate.

As it turns out, how soft or hard apolymer is at a given temperature is deter-mined by what we call chain mobility thatis how well the polymer chains wiggle pastand around each other. If the polymerchains can slither and wiggle past andaround each other easily, the whole massof them will be able to flow more easily. Soa polymer, which can move around easily,will be soft, and one which cannot, will behard.

When it comes to making windows,PMMA has another advantage over glass.PMMA is more transparent than glass.When glass windows are made too thick,they become difficult to see through. ButPMMA windows can be made as much as13 inches (33 cm) thick, and they are stillperfectly transparent. This makes PMMA awonderful material for making largeaquariums, whose windows must be thickin order to contain the high pressure ofmillions of gallons of water.

Polyesters

Simple polyesters are derived fromcondensation of polyhydric alcohol and apolyfunctional acid and are sometimesdescribed as alkyds (from alcohol andacid). Each component needs a functio-nality (i.e., number of reactive groups suchas -OH, -COOH, per molecule of 2 to forma linear chain, while if one (or both)monomers have a functionality of at least3, cross-linkage can occur resulting in amuch more rigid 3-D lattice structure. Poly

Methyl methacrylate Poly (methyl methacrylate)

Free radicalvinyl

polymerization

29



(ethylene terephthalate) (PET) can beproduced by reacting ethylene glycol withterephthalic acid, although in practice thedimethyl ester of terephthalic acid is usedto give a more controllable reaction.Methanol is generated as the product of theexchange reaction and can be recovered forfurther use. Polyesters have hydrocarbonbackbones, which contain ester linkages,hence, the name ester. The structure in thepicture is called poly (ethyleneterephthalate), or PET for short, because itis made up of ethylene groups andterephthalate groups.

ethylene glycol, the reaction is calledtransesterification. The result is bis-(2-hydroxyethyl) terephthalate and methanol.

The bis-(2-hydroxyethyl) terephthalateis also heated up to a temperature of 270oC, and it reacts to give the poly(ethylene

terephthalate group ethylene group

Poly (ethylene terepthalate)Ethylene glycol

Dimethyl terephthalateEthylene glycol

Bis - (2-hydroxyethyl) terephthalate

Methanol

The ester groups in the polyester chainare polar, with the carbonyl oxygen atomhaving a somewhat negative charge and thecarbonyl carbon atom having a somewhatpositive charge. The positive and negativecharges of different ester groups areattracted to each other. This allows the estergroups of nearby chains to line up witheach other in crystal form that is why theycan form strong fibers.

In big plants where they make polye-ster with a compound called dimethylterephthalate, which is reacted with

terephthalate) and, oddly, ethylene glycolas a byproduct.

But in the laboratory, PET is made byother reactions. Terephthalic acid andethylene glycol can polymerize to make PETwhen the reaction mixture is heated withan acid catalyst. The product has acarboxyl group at one end and a hydroxylgroup at the other, so it can condense withanother molecule of alcohol and acid. Themolecules grow to a molecular weight ofup to 20,000.

PET is well known as a fiber (tradenames include Terylene, Crimpylen, Kodel,Dacron and Trevira) and as a film (tradenames include Mylar, Scotchpak, Celenar,Esterfane, Vindenem Melinex andHostaphane).

PET has a melting point (Tm

) of 267°C

and a glass transition point (Tg) between

67° and 80°C. PET films are most widelyused in the biaxially oriented, heatstabilized form. There are virtually noapplications for the material in its un-oriented form because, if crystalline, it isextremely brittle and opaque, and if

30


Chapter 2

amorphous, it is clear but not tough. In atwo-stage process, machine directionstretching induces 10-14% crystallinity andthis is raised to 20-25% by transverseorientation. In order to stabilize the biaxialorientation, the film is annealed (or heat set)under restraint at 180°-210°C, whichincreases the crystallinity to around 40%without appreciably affecting theorientation, and reduces the tendency toshrink on heating. Subsequent coatings areapplied to obtain special barrier properties,slip characteristics or heat sealability.

PET film�s outstanding properties as afood packaging material are its great tensilestrength, excellent chemical resistance,lightweight, elasticity and stability over awide range of temperatures (-60° to 220°C).This latter property has led to the use of PETfor �boil-in-bag� products, which are frozenbefore use (PET is usually laminated to orextrusion coated with LDPE and istypically the outside and primary supportfilm of such laminations), and as oven bagswhere they are able to withstand cookingtemperatures without decomposing.

To improve the barier properties of PET,coatings of either LDPE, PVDC copolymeror PVDCcoAN have been made. PET filmextrusion-coated with LDPE is very easy toseal and very tough. It can be sealed overpowders and some liquids, the integralseal will withstand sterilization by UV.Two-side PVDC copolymer coated gradesprovide a high barrier and a major specialapplication is the single-slice cheese wrap.

Metallization of PET films results in aconsiderable improvement in barrierproperties. A reduction in water vaportransmission rate by factor of 40 and oxygenpermeabilities by over 300 are obtained.

A fast-growing application for PET is

�ovenable� tray for frozen food andprepared meals, where they are preferredover foil trays because of their ability to bemicrowaved without the necessity for anouter board carton. These trays arethermoformed from cast PET film andcrystallized by heat-setting the trays whichprevents deformation during cooking andserving.

A relatively new polyester resin beingconsidered for co-extruded structures ispoly-(butylene terephthalate) (PBT) where1,4-butanediol is used instead of ethyleneglycol. It offers good gas barrier propertiestogether with good tensile strength,retaining these properties at elevatedtemperatures. PBT can be steam sterilized,thus making it eminently suitable for high-temperature applications such as hot-fillapplications and boil-in-bag products.Although PBT does not have good heat sealcharacteristics, it could be co-extruded withEVA to yield a heat sealable structure. Bothcast and blown co-extruded films have beenmade, and they can be used in the

unoriented form since their tensile strengthis better than unoriented PET.

In the late 1970�s, the benefits of biaxialorientation of PET were extended fromsheet film to bottle manufacture. As aresult important new markets have beenopened up, particularly for carbonated

Poly (butylene terepthalate)

Poly (trimethylene terepthalate)

31



beverages. The bottles are stretch-blowmoulded. The stretching or biaxialorientation is necessary to get maximumtensile strength and gas barrier, which inturn enables bottle weights to be lowenough to be economical. However, the useof stretch blown bottles is restricted upto60°C temperature, beyond this temperature,the bottles start shrinking.

napthalate group ethylene group

Poly (ethylene naphthalate)

Fig. 2. 3. PET based food packaging materials(films/laminates/containers)

strawberry jelly. PEN is so good at standingthe heat that it is not necessary to make thebottle entirely out of it. Just mixing somePEN with the PET gives a bottle that cantake the heat a lot better than plain PET.

Polycarbonates

Polycarbonates (PC) are polyesters of

unstable carbonic acid and have carbonate

linkages. They were originally produced

by the reaction of phosgene (COCl2) with

bisphenol A [2, 2� - bis (4-hydroxyphenyl)

propane]. Bisphenol A is still the most

commonly used phenol but diphenyl

Bisphenol A

Sodium salt of bisphenol A

Carbonate group

Polycarbonate gets its name from thecarbonate groups in its backbone chain. Itis called as polycarbonate of bisphenol Abecause it is made from bisphenol A and

carbonate has replaced phosgene.

Polycarbonate, or specificallypolycarbo-nate of bisphenol A, is a clearplastic used to make shatterproofwindows, lightweight eyeglass lenses, etc.

There is a new kind of polyester that iswell suited for jelly jars and returnablebottles. It is poly(ethylene naphthalate), orPEN.

PEN has a higher glass transitiontemperature than PET. That is the tempe-rature at which a polymer gets soft. Theglass transition temperature of PEN is highenough so that it can withstand the heat ofboth sterilizing bottle washing and hot

32


Chapter 2

phosgene. This starts out with the reactionof bisphenol A with sodium hydroxide toget the sodium salt of bisphenol A.

The sodium salt of bisphenol A is thenreacted with phosgene, a favorite chemicalweapon in World War I, to produce thepolycarbonate.

The polymer produced by this reactionis outstanding in its combination of hightemperature resistance, high impactstrength and clarity, retaining its propertieswell with increasing temperature. Chemi-cally, it is resistant to dilute acids but isstrongly attacked by alkalis and bases suchas amines. Its permeability to both watervapor and gases is high, and if appreciablebarrier properties are required, it must becoated. While it can be oriented, permeabi-lity does not decrease although its tensilestrength increases. It is not suitable as ashrink film since its rate of shrinkage aboveits heat distortion point is extremely slow.

Because PC is amorphous, it softens overa wide temperature range; the T

m of

bisphenol A polycarbonates is 220° - 250°Cand the T

g is 150°C. The melting point of

polycarbonates is decreased from 225° to195°C when the methyl pendant groups arereplaced by propyl groups. Thermoformingof polycarbonate film is readily carried outand deep draws with good mould detail areobtainable. Due to its high softening point,PC is also used in making feeding bottles

for infants. These bottles can be heatsterilized at high temperature.

A common application for PC isovenable trays for frozen foods andprepared meals where its low temperatureimpact strength adds durability andtoughness; it is commonly co-extruded withPET. The film has been used for boil-in-bagpacks and, when coated with LDPE, forskin packaging. Because of its good stabi-lity at high temperatures, uses such asretort pouch and for microwave oven cook-ware are envisaged. Vacuum metallizinggives good results; because of polycarbo-nate�s transparency, the finished productwill have a high gloss.

Polyamides (Nylons)

The early development of the nylons,the generic name for the family of syntheticpolyamides, said to be derived from acontraction of the names of two citiescoined by Du Pont between 1928 and 1937.In an attempt to circumvent the Du Pontpatents on nylon 6,6, German chemistsinvestigated a wide range of synthetic fiber-forming polymers in the late 1930�s. Thiswork resulted in the successful introdu-ction of nylon 6 (where a 6 carbon moleculecontained an acid group at one end and anamine group at the other) and betweenthem, nylons 6 and 6, 6 account for nearlyall of the polyamides produced for fiberapplications.

Nylons are one of the most commonpolymers used as a fiber. Nylon is foundin clothing all the time, but also in otherplaces, in the form of a thermoplastic.

Nylons are also called polyamides,because of the characteristic amide groupsin the backbone chain. These amidegroups are very polar and can hydrogen

Phosgene

33



Other nylons can have differentnumbers of carbon atoms in these stretches.It is usually made by reacting adipic acidwith hexamethylene diamine:

Another kind of nylon is nylon 6. It issimilar to nylon 6,6 except that it only hasone kind of carbon chain, which is sixatoms long.

It is made by a ring opening poly-merization from the monomer caprolactam.Nylon 6 does not behave much differentlyfrom nylon 6,6. The only reason that bothof them have come into existence is becauseDuPont patented nylon 6,6, so othercompanies had to invent nylon 6 in orderto enter nylon business.

Thus it can be seen that two differenttypes of nylon films are available based ontheir resin manufacture. One type is madeby a condensation of mixtures of diaminesand dibasic acids. These are identified bythe number of carbon atoms in the diamine,followed by the number of carbon atoms inthe diacid. The other type is formed by acondensation of single, hetero functionalamino acids, known as ω-amino acidssince the amino and carboxyl groups are atopposite ends of the polyamides. A singlenumber signifying the total number ofcarbon atoms in the amino acid is used forits identification.

Several nylons have been produced withchains greater than six-carbon atoms, asthey result in films with a lower meltingpoint and an increased resistance to watervapour. Again, the key to successfulcommercialization is to find cheap sourcesof the monomers.

In general, nylons are highly permeableto water vapour, the absorbed waterhaving a plasticizing effect which causes

Six carbon atoms

Six carbon atoms

Adipic acid

Hexamethylenediamine

Nylon 6,6

bond with each other. Because of this, andas the nylon backbone is so regular andsymmetrical, nylons are often crystallineand make very good fibers. The nylon in

the figure below is called nylon 6,6, becauseeach repeat unit of the polymer chain hastwo stretches of carbon atoms, each beingsix carbon atoms long.

g-Caprolactam

Nylon 6

34


Chapter 2

a reduction in tensile strength and anincrease in impact strength. Theirpermeability to oxygen and other gases isquite low when the films are dry. PVDCcopolymer coated nylons offer improvedoxygen and moisture vapour and UV lightbarrier properties. Odour retention isexcellent and the films are tasteless,odourless and nontoxic.

Biaxial orientation of nylon filmsprovides improved flex-crack resistance,mechanical and barrier properties. Thesefilms have new applications in packagingfoods such as processed and naturalcheese, fresh and processed meats, andfrozen foods. They are used in pouchesand bag-in-box structures. In some appli-cations, nylons compete with biaxiallyoriented PET. Although oriented nylonsoffer better gas barrier, softness andpuncture resistance, oriented PET offersbetter rigidity and moisture barrier. It ismostly used as a middle layer incoextruded films for oil packaging with acontact layer of EAA and LDPE as outerlayer.

Recently, a new amide based polymer,MXD-6, has been made from meta-xylenediamine and adipic acid. Biaxiallyoriented film produced from this polymeris used in several packaging applications;it has excellent thermal properties andprovides a good barrier to gases andmoisture.

Both the melting point and Tg of MXD-

6 are between those of nylon 6 and PET,but MXD-6 has better gas barrier propertiesthan nylon 6 and PET at all humidities,and is better than EVOH at 100% RH, dueto the existence of benzene ring in theMXD-6 polymer chain. Together with its

high clarity and good processability, theseproperties make MXD-6 nylon film suitableas a base substrate of laminate filmstructures for use in lidding and pouches,especially when the film is exposed to retortconditions.

Polystyrene

If ethylene and benzene are reactedtogether with a suitable catalyst, ethylbenzene is formed. Further, a process ofdehydrogenation produces styrene (PS).Polystyrene is made by the additionpolymerization of styrene.

Styrene Polystyrene

Free radicalvinyl

polymerization

Syndiotacticpolystyrene

Atacticpolystyrene

35



gives it a particular high brilliance.Although acids and alkalis have no effecton it, it is soluble in higher alcohols, keto-nes, esters, aromatic and chlorinatedhydrocarbons and some oils. While beinga reasonably good barrier to gases, it is apoor barrier to water vapor. To overcome

Polystyrene is a vinyl polymer.Structurally, it is a long hydrocarbon chain,with a phenyl group attached to every othercarbon atom. Polystyrene is produced byfree radical vinyl polymerization, from themonomer styrene.

The polymer is normally atactic and isthus completely amorphous because thebulky nature of the benzene rings preventsa close approach of the chains. With theuse of special catalysts and polymerizationtechniques, isotactic PS has been preparedbut it reverts to the atactic form on melting.The T

g is in the range 90-100°C because of

the stiffening effect of the benzene ring.This, coupled with the amorphous natureof the polymer, results in a material whichis hard, rigid and transparent at roomtemperature. In this form, it is commonlyreferred to as crystal grade PS.

There is a new kind of polystyrene,called syndiotactic polystyrene. It isdifferent because the phenyl groups on thepolymer chain are attached to alternatingsides of the polymer backbone chain.�Normal� or atactic polystyrene has noorder with regard to the side of the chainon which the phenyl groups are attached.The new syndiotactic polystyrene iscrystalline and melts at 270 oC. This is thepolystyrene of the future.

Polystyrene is an inexpensive and hardplastic and it is also made in the form offoam packaging and insulation (Styrofoamis one brand of polystyrene foam). Clearplastic disposable drinking cups used forserving tea and ice creams are made ofpolystyrene.

Polystyrene makes a distinctly metallicsound when dropped onto a hard surface.It has a high refractive index (1.592), which

Fig. 2.4. Polystyrene based foodpackaging materials

(Disposable thermo formed trays and containers)

the brittleness of polystyrene, syntheticrubbers can be incorporated at levelsgenerally not exceeding 14% w/w. Therubbers act by restricting propagation ofmicrocracks formed during impact loading.Such a process increases the impactstrength and flexibility, but transparency,tensile strength and thermal resistance aremuch reduced. The chemical properties ofthis toughened or high impact polystyrene(HIPS) are much the same as those forunmodified polystyrene.

HIPS is an excellent material for thermo-forming. Because it is transparent, the useof radiant heat for thermoforming isinefficient and pigmented sheet is oftenused. It is injection molded into tubes,which find wide use in food packaging,despite being opaque. Crystal grade PS canbe made into a film but it is brittle unlessthe film is biaxially oriented.

The properties of polystyrene that makeit useful for many applications as a solid

36


Chapter 2

polymer also make it very desirable as afoam. PS foam has a high modulus, goodwater resistance, low moisture trans-mission, ease of fabrication and low cost.Closed cell foams have excellent thermalinsulating capability, low weight and goodcushioning characteristics. The combi-nation of those properties provides a widespectrum of products.

Copolymer of Sryrene: Polystyrene is alsoa component of a type of hard rubber calledpoly(styrene-butadiene-styrene) or SBSrubber. SBS rubber is a thermoplasticelastomer. Poly(styrene-butadiene-styrene),or SBS is a hard rubber, which is used forthings like the soles of shoes, tire treads,and other places where durability isimportant. It is a type of copolymer calleda block copolymer. Its backbone chain ismade up of three segments. The first is along chain of polystyrene, the middle a longchain of polybutadiene and the last seg-ment is another long section of polystyreneas shown in the picture:

Polystyrene is a tough hard plastic, andthis gives SBS its durability. Polybutadieneis a rubbery material, and this gives SBS itsrubber-like properties. In addition, thepolystyrene chains tend to clump together.When one styrene group of one SBSmolecule joins one clump, and the otherpolystyrene chain of the same SBS mole-cule joins another clump, the differentclumps become tied together with rubberypolybutadiene chains. This gives thematerial the ability to retain its shape after

being stretched. SBS is also a type ofunusual material called a thermoplasticelastomer. These are materials whichbehave like elastomeric rubbers at roomtemperature but when heated can beprocessed like plastics. Most types ofrubber are difficult to process because theyare crosslinked. But SBS and otherthermoplastic elastomers manage to berubbery without being crosslinked, makingthem easy to process into nifty usefulshapes.

Acrylonitriles and AssociatedCopolymers

The ready availability of acrylic acidderivatives from propylene has led to theiruse in numerous industrial polymerproducts. Acrylonitrile (sometimes referredto as vinyl cyanide, especially by those whoseek to raise fears about the safety of thepolymer as a food contact material since themonomer is carcinogenic), b.p. 70°C, isproduced mainly by the ammoxidation ofpropylene.

Polyacrylonitrile

The polyacrylonitrile polymer (PAN) is49% nitrile and is an amorphous,transparent polymer. It has a relatively lowT

g (87°C) and provides an outstanding

barrier to gas permeation, and exceptionalresistance to a wide range of agents. It isnot of commercial value in packaging, duelargely to its inability to be melt processed.It is therefore copolymerized with othermonomers that impart melt processability,thus making its desirable propertiesavailable in a packaging form.

Polyacrylonitrile is a vinyl polymer, anda derivative of the acrylate family ofpolymers. It is made from the monomer

Polystyreneblock

Polystyreneblock

Polybutadiene block

37



acrylonitrile by free radical vinylpolymerization.

Polyacrylonitrile is used for very fewproducts an average consumer would befamiliar with, except to make anotherpolymer, carbon fiber. Homopolymers ofpolyacrylonitrile have been used as fibersin hot gas filtration systems, outdoorawnings, sails for yachts, and even fiberreinforced concrete. But mostly copolymerscontaining polyacrylonitrile are used asfibers to make knitted clothing, like socksand sweaters, as well as outdoor productslike tents. If the label of some piece ofclothing says �acrylic�, then it�s made outof some copolymer of polyacrylonitrile.Usually, they are copolymers of acryloni-trile and methyl acrylate, or acrylonitrileand methyl methacrylate.

Styrene acrylonitrile copolymer (SAN) isa simple random copolymer of styrene andacrylonitrile. But ABS is more complicated.

It is made by polymerizing styrene andacrylonitrile in the presence of poly-butadiene. Polybutadiene has carbon-carbon double bonds in it, which canpolymerize too, which results in a poly-butadiene chain with SAN chains graftedonto it, as can be seen below.

Acrylonitrile Polyacrylonitrile

Free radicalvinyl

polymerization

AcrylonitrilePolybutadiene

SAN branches

Polybutadiuenebackbone

ABS

Poly (acrylonitrile-co-methyl acrylate)

Poly (acrylonitrile-co-methyl methacrylate

SAN

Styrene

Acrylonitrile butadiene styrene (ABS) isa stronger plastic than polystyrene becauseof the nitrile groups of its acrylonitrile units.The nitrile groups are very polar, so theyare attracted to each other. This allowsopposite charges on the nitrile groups tostabilize each other. This strong attractionholds ABS chains together tightly, makingthe material stronger. Also, the rubberypolybutadiene makes ABS tougher thanpolystyrene.

Poly (Tetrafluoroethylene) {Poly(Difluoromethylene)} - (PTFE)

The high thermal stability of thecarbon-fluorine bond has led to considera-

38


Chapter 2

ble interest in fluorine-containing poly-mers. They are set apart from other vinylpolymers because their monomers are theonly ones which need not bear anyhydrogen on the ethylenic carbons in orderto be polymerizable. PTFE was a chancediscovery by Plunkett in 1938. Today itaccounts for about 80% of the fluorinatedpolymer produced and is commonlyreferred to as Teflon.

Polytetrafluoroethylene, or PTFE, ismade of a carbon backbone chain, andeach carbon has two fluorine atomsattached to it.

Polytetrafluoroethylene is made fromthe monomer tetrafluoroethylene by freeradical vinyl polymerization.

stick cooking pans, and anything thatneeds to be slippery or non-stick. The bondbetween the fluorine atom and the carbonatom is so strong and stable that nothingreacts with it. Even when it gets as hot as afrying pan, not even oxygen will react withit. PTFE is also used to treat carpets andfabrics to make them stain resistant. Whatis more, it is also very useful in medicalapplications. Because human bodies rarelyreject it, it can be used for making implants.

Poly (Vinyl Acetate), or PVA

PVA is a vinyl polymer, made by freeradical vinyl polymerization of themonomer vinyl acetate.

TetrafluoroethylenePolytetrafluorethylene

Free radicalvinyl

polymerization

The non-stick nature of PTFE is due tothe presence of fluorine atoms in thepolymer, which repel other moleculescoming close.

In food packaging, PTFE finds wide useas a non-stick separating surface betweenthermoplastic films and the jaws of heatsealers. It is common to use a band of PTFE(often reinforced with glass fibers) oncontinuous heat sealers.

Polytetrafluoroethylene, better known bythe trade name Teflon, is used to make non-

Vinyl acetate Poly (vinyl acetate)

Free radicalvinyl

polymerization

Poly (vinyl acetate), or PVA for short, isone of those low-profile behind-the-scenespolymers. One place where PVA can befound is between two pieces of wood thatare glued together. PVA is used to makewood glues, as well as other adhesives.Paper and textiles often have coatingsmade of PVA and other ingredients to makethem shiny.

Ethylene-Vinyl Acetate (EVA)

Ethylene-vinyl acetate (EVA) copoly-mers with a vinyl acetate (VA) content of3-12%. They are similar in flexibility toplasticized PVC, and have a good lowtemperature flexibility and toughness.Their impact strength increases with VA

39



content and molecular weight. As the VAlevel increases, EVA becomes less crysta-lline and more elastic; as the crystallinitydecreases, the permeability to gases,moisture, fats and oils increases. Theabsence of leachable plasticizer provides aclear advantage over plasticizer PVC insome food applications. The addition ofantiblocking and /or slip additives reducessparkle and clarity, and increases haze.

EVA copolymers are not competitivecompared to other films because of theirhigh surface tack and friction, which makethem difficult to handle on conventionalprocessing machinery. However, they dohave three advantages over LDPE; the heatsealing temperature is lower, the barrierproperties are lower, and they haveexcellent stretch properties, the first 50% ofextension at room temperature beingelastic. Thus they find use as a stretch filmfor food packaging (particularly fresh meat)and cling-wrap purpose, and are likely toreplace PVC for stretch wrapping of foods.EVA is also used in co-extrusion processesfor the manufacture of multilayer films.

Ethylene-Vinyl Alcohol (EVOH)

By combining the processability ofethylene polymers with the barrierproperties obtained from vinyl alcoholpolymers, ethylene-vinyl alcohol (EVOH)copolymers offer not only excellentprocessability, but also superior barrier togases, odour, fragrances, solvents, etc. It isthese characteristics that have allowedplastic containers containing EVOH barrierlayers to replace many glass and metalcontainers for packaging food. EVOHcopolymers were introduced in Japan in the1970's and in USA in early1980�s.

EVOH resins are hydrolyzed copoly-

mers of vinyl acetate and ethylene. Thevinyl alcohol base has exceptionally highgas-barrier properties, but is water-solubleand difficult to process. By copolymerizingethylene with vinyl alcohol, the high gas-barrier properties are retained, andsignificant improvements are achieved inmoisture resistance and processability.

EVOH copolymers are highly crystallinein nature and their properties are highlydependent upon the relative concentrationof the co-monomers. Generally, as theethylene content increases, the gas barrierproperties decrease, the moisture-barrierproperties improve, and the resins processmore easily.

The most outstanding characteristic ofEVOH resins is their ability to provide abarrier to gases. Their use in a packagingstructure enhances flavour and qualityretention by preventing oxygen from pene-trating the package. In those applicationswhere gas-fill packaging techniques areused, EVOH resins effectively retain thecarbon dioxide or nitrogen used to blanketthe product.

Due to the presence of hydroxyl groupsin their molecular structure, EVOH resinsare hydrophilic and absorb moisture. Asmoisture is absorbed, the gas-barrierproperties are affected. However, throughthe use of multilayer technology toencapsulate the EVOH resin layer withhigh moisture-barrier resins such aspolyofleins, the moisture content of thebarrier layer can be controlled.

EVOH resins have high mechanicalstrength, elasticity and surface hardness,very high gloss, low haze, excellentabrasion resistance, very high resistance tooils and organic solvents, and provide an

40


Chapter 2

Table 2.1. Properties of major packaging resin films(Data based on 25 µm thickness)

LDPE LLDPE HDPE PP OPP PVC

Density (g/cc) 0.91� 0.918� 0.945� 0.91 0.90 1.22�0.925 0.923 0.967 1.36

Yield (m2/kg) 42.6 42.5 41.2 44 44 28

Tensil Strength (MPa) 1.5-5 3-8 2.5-6 30-40 20-30 4-8

Tensil modulus (MPa) 20.40 25 125 300 350 350Secant -600

Elongation at break (%) 200- 400- 200- 200- 50- 100-600 800 600 350 275 400

Tear strength 100- 150- 200- - 340 400-(elmendorl) (g) 200 900 300 700

WVTR g/m2/day at 14-18 14-18 5-7 7-9 4 30-4038°C & 90% RH gradient

O2 Permeability 7000- 7000- 1500- 2000- 2000- 300-

(cc/m2/day. atm 25°C) 8000 8000 2000 3000 2500 400

Haze (%) 5-10 6-13 3 3 3 1-2

Light transmission (%) 65 - - 80 80 90

Heat seal temperature 120- 105- 135- 95- 95- 140-range (°C) 175 170 155 150 150 170

Service temperature (°C) -55 to -29 to -40 to 5 to 5 to -29 to80 105 120 120 -120 65

excellent barrier to odours. When used asthe core of a multilayer material, theyprovide excellen3t performance.

EVOH resins are the most thermallystable of all the high barrier resins. Thisstability allows the regrinding and reuse ofscrap generated during processing backinto the package being produced. Many ofthe rigid packaging containers beingproduced today have regrind layerscontaining up to 15% EVOH.

Laminates/Co-extruded Films

As can be seen from Table 2.1, no singlepolymer possesses all the desirableproperties that may be required and, as therange of possible applications in the foodindustry is so diverse, there can be no suchthing as the ideal universal packagingmaterial. Optimum properties in eachindividual case can be achieved byrecourse to several polymers and even non-plastic materials such as aluminium or

41



paper, in combination. Such combinationsor laminates, consist of layers of individualmaterials on top of each other and bondedtogether by adhesives. Some of theindividual resins can also be co-extrudedto get the required barrier properties forbetter shelf life of the food products.

Some of the laminates/co-extruded filmsavailable today in the market for foodpackaging applications with their barrierproperties are shown in the Table 2.2.

ADVANTAGES OF PLASTICSOVER TRADITIONALPACKAGES

Many properties of plastics make themideally suited for food packaging. In mostcases, it is a combination of properties thatenables plastics to win the battle foradoption in comparison to other traditionalpackaging materials used in foodpackaging.

µPVDC IONO- PET PC NYLON OPS EVA EVOH

MER

1.6� 0.94�0.96 1.4 1.2 1.14 1.05 0.93 1.14�1.191.7

24 42.0 28.4 32.5 35 38 41.9 32.7-34.7

8-16 3.5-5 25-33 9-11 25.37 8-12 2-3 1.2-1.7

50-150 10-50 700 320 250-300 400-475 8-20 300-385

50-100 300-600 70-130 100-150 70-120 2-30 500-800 120-280

10-90 20-40 20-100 20-50 15-30 2-15 40-200 400-600

2-4 20-30 21 50-100 150 100-150 20-50 20-50

5-25 7000-8000 50-80 3000 25-40 4000-5000 10000 0-5.2

1-5 1-15 2 1 1.5 1 2-10 1-2

90 85 88 89 88 92 55-75 90

120- 105- 175 135-175 205-215 120-175 120-175 65-205 175-205150

-18 to -100 to -75 to -135 to -75 to -60 to 80 -75 to 65 -18 to 50135 165 150 140 205

42


Chapter 2

Density

Density ranges of 0.9 to 1.4 g/cc aretypical for most plastics used in packaging.This density range compares to about 2.6for glass, 2.7 for aluminum, 0.8-0.9 forpaper, 1.5 for polymer coated cellophane,and about 8 for steel. Although metals andglass have higher tensile strength/weightratios than most common plastics,thickness far greater than required to holdthe product from a strength point of vieware required to overcome brittleness (glass)or the tendency to bend and buckle(metals). Thus the required weight of therigid container made of plastic is several-fold less than for metal or glass.

Paper is a lightweight material, but itswet strength is so low that it cannot qualifyfor many foods packaging applicationsunless it is combined with plastic forreinforcement and waterproofing.

Ability of plastics to provide lightweightpackages, which is an important consumerconvenience and a way for processors toreduce shipping cost, is probably the largestsingle driving force behind plasticspenetration into the market for foodcontainers that were once the exclusiveprovince of metal and glass.

Break Resistance

In the thin gauges commonly used in

Table 2.2. Barrier Properties of Flexible Packaging Materials

Packaging materialsLaminates:PET/LDPELD//PA/EAAMET.PET/LDPEPET/HDPE-LDPEMET.PET/HDPE-LDPEPET/ LDPE/EAAHDPE/EAAPAPER/FOIL/ LDPEPET/FOIL/LDPEPET/PPPET/FOIL/PP

Co-extruded Films:LDPE/LLDPELDPE/ HDPEHDPE/ LDPE/HDPEHDPE/ LDPE/EAALDPE/BA/PA/BA/EAACPP/BA/PA/BA/EAA

Thickness (µm)

12/3737/25/5012/3712/11212/11212/25/2525/2540/9/3712/9/1012/5012/12/50

501201101109090

WVTR*

9.57.21.53.30.66.43.8nilnil3.5nil

92.02.02.15.44.1

OTR**

75854702701395nilnil6.5nil

40001500120015008580

* g/m2/24 h at 38°C and 90% RH gradient.** cc/m2/24h atm at 27°C and 65% RH.

43



packaging, rigid plastics containers resistbreaking far more effectively than glass.This, along with low density, is the majorreason why plastics have taken so manyrigid packaging applications away fromglass, which is otherwise an excellentpackaging material from the foodprotection standpoint.

Sealability

All flexible packages must be closed insome way, and the vast majority of them areclosed by heat-sealing, a process in whicha coating on the flexible substrate is heatedin the packaging machinery until it meltsand then is held in contact with the simi-larly coated opposite surface until the twocoatings solidify, as one layer. Polymercoatings and adhesives are universallyused to perform this function.

No other packaging material can matchplastics ability to create strong hermeticseals at low temperatures (100-250°C).Some resins have even been developed thatprovide an excellent seal in the presence ofliquids, greases and powders. EAA andoctene based LLDPE are excellent materialsfor sealability.

Fabrication Flexibility

All types of common plastics can bereadily converted into thin, strong and clearfilms. This means that for thousands offlexible packaging applications, metal andglass cannot be used and only paper,glassine, and cellophane can compete.Plastics are unsurpassed in the ease withwhich special shapes can be readilycreated. This attribute is particularlyimportant for rigid containers, whereimaginative design is constantly providingever more consumer friendly packages that

perform useful functions not yet fullycapitalized on this important attribute;future growth of plastics in rigid packagingwill depend more and more on designersand fabricators learning how versatileplastics can be in providing consumer-friendly packages.

Impermeability to Gases

Many food products are sensitive toattack by water vapour and oxygen. Forexample, crackers become soggy on high-humidity days, and fat in food turns rancidwhen exposed to oxygen.

Glass, metal and pinhole-free alumi-num foil (the latter a rarity) are totallyimpermeable to these two gases whichdeteriorate so many food products. Plasticsand polymer-coated cellophane rank wellbelow these materials but they are far moreimpermeable than uncoated paper. Evensimple, uncoated, homopolymer plasticssuch as polyethylene and polypropylenehave sufficient moisture barrier property formany applications. The early developmentof thin plastic barrier coatings applied tothese low cost films provided a moisturebarrier that was totally adequate in mostcases.

Incorporating an oxygen barrier in all-plastic flexible package can beaccomplished by including a barrier plasticlayer in a multilayer construction. In thecase of rigid containers, this is morecomplicated task, but recent developmentsin extruded barrier polymers and co-extrusion technology have allowed plasticsto penetrate the rigid packaging market forsome oxygen-sensitive food applications.Although all-plastic packages will neverhave the infinite oxygen barrier provided bymetal and glass, still they are now close to

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Chapter 2

providing enough oxygen barrier so that intime, this factor alone will not disqualifyall-plastic flexible and rigid packages fromcompeting for all the food packagingapplications.

Also, plastic with controlled permea-bility to gases becomes a positive attributein the design of packages for products suchas fresh produce, which continue to respireafter being harvested. Briefly, the shelf lifeof these products can be greatly extendedif they are packaged in a material which isselectively permeable to O

2 and CO

2 and

can thus maintain an optimum O2

/CO2

ratio in the atmosphere surrounding theproduct.

Plastic packages with controlled,

selective gas permeability are just

beginning to be used in food packaging,

but one day they will offer an important

way of preserving fresh food for periods farlonger than is now possible. This property

of selective permeation cannot be provided

by metal, glass and aluminium foil.

Light Control and PackageAppearance

Unmodified plastic films and sheets

range in appearance from crystal clear tohazy. Pigments or soluble dyes can be

added to produce total opacity in virtually

any colour or to produce transparent

coloured films and sheets. This enormous

versatility gives the package designer

freedom to create a package that clearlydisplays the contents. If the food being

packaged is sensitive to light-catalyzed

oxidation, as many foods are, pigmentation

or metallization can be used to screen out

light and to provide a background against

which artistically designed graphics are

revealed to their best advantage. No other

packaging material offers package

designers such a wide range of choices.

Many flexible packaging convectors are

successful mainly because they are expertsin exploiting plastics versatility in this area.

Tear and Puncture Resistance

This attribute gives plastics a major edge

over paper, cellophane and aluminum foil

in flexible packages. In addition, mostplastics films are more resistant than

aluminum foil to cracking when subjected

to repeated flexing.

Low Temperature Flexibility

Plastic films remain flexible at lowtemperatures, whereas cellophane starts toembrittle below about 4°C. This lowtemperature flexible characteristics ofplastics was major factor in their displacingmany flexible cellophane packages.

Metal Coating Receptivity

Very thin coatings of aluminum can bereadily vapour deposited on plasticsubstrates in high vacuum chambers. Theresulting attractive product is lessexpensive and sometimes functionallysuperior to aluminum foil, which is sealedinto flexible packages to provide a barrierto light, moisture, and oxygen.

Print Receptivity

The use of package to advertise anddescribe the product is an exceedinglyimportant food marketing tool. Paper andcellophane are marginally superior toplastic in their print receptivity, since someplastics require an inexpensive pre-treatment to develop this attribute whereasthe former materials do not. This diffe-rence no longer gives paper or cellophane

45



any significant advantage over plastic. Bycontrast, metal and glass containers aremore difficult to print and usually requireexpensive application of paper labels tocarry the graphics.

BIBLIOGRAPHY

Becker K, Koszinowski J and Piringer O(1983). Permeation von RiechundAromastoffen durch Polyolefine.Deutsche Lebensmittel-Rundschau, 79:257-266.

Brown WE (1992). Plastics in FoodPackaging Properties, Design andFabrication. Marcel Dekker Inc., NewYork.

Crosby NT (1981). Food Packaging Mate-rials (Aspects of Analysis and Migrationof Contaminants). Applied SciencePublishers Ltd, London

Jenkins WA and Harrington JP (1991).Packaging Foods with Plastics. Techno-mic Publishing Co. Inc, Lancaster,Pennsylvania.

Koszinowski J and Piringer O (1986).Influence of the packaging material andthe nature of the packed goods on theloss of volatile organic compounds-permeation of aroma components. In:Food Packaging and Preservation, ed:Mathlouthi M. Elsevier Applied SciencePublishers, London, pp. 325-340.

Paine FA and Paine HY (1982). A Hand-book of Food Packaging, The Council ofthe Institute of Packaging, Leonard Hill,Glasgow, UK.

Robertson GL (1993). Food Packaging:Principles and Practice. Marcel DekkerInc, New York.

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Chapter 2

Documents

PLASTICS AND THEIR ROLE IN FOOD PACKAGINGicpe.in/Plastics in Food Packaging/pdf/2-Final.pmd.pdf19 PLASTICS IN FOOD PACKAGING PLASTICS AND THEIR ROLE IN FOOD PACKAGING Polyolefins Polyolefins