1Chapter 1

Introduction

John R. Wagner Jr 1 and Scott B. Marks 2

1 Crescent Associates, Inc.2 E.I. du Pont de Nemours & Co

Materials: A historical perspective

Today, we have many clear plastic packaging films. JacquesE. Brandenberger invented the first common clear film,‘Cellophane’ in 1908. Jacques was a Swiss textile engineerwho first thought of the idea for a clear, protective,packaging layer in 1900. He was seated at a restaurantwhen a customer spilled wine onto the tablecloth. As thewaiter replaced the cloth, Brandenberger decided that hewould invent a clear flexible film that could be applied tocloth, making it waterproof.

He experimented with different materials and triedapplying liquid viscose (a cellulose product known asrayon) to cloth, but the viscose made the cloth too stiff.His idea failed but he noted that the coating peeled offin a transparent film. Like so many inventions, theoriginal use was abandoned and new and better useswere found. By 1908, he developed the first machine forthe manufacturing of transparent sheets of regeneratedcellulose. By 1912, Brandenberger was making a saleablethin flexible film used in gas masks. He obtained patentsto cover the machinery and the essential ideas of hisprocess [1]. The rights to the production of cellophanewere then obtained by E.I. du Pont de Nemours & Co,who began to produce and refine further the process forproducing cellophane. Breakthrough improvements in-cluded adding a moisture barrier layer to the cellophane,in the form of a nitrocellulose coating. This allowed forbetter stiffness retention in the cellophane and

facilitated use of the film as an overwrap film for foods.This coating was then refined to make it heat sealable aswell, creating the first readily sealable transparentpackaging film. Later on, other coatings were applied tocellophane including PVdC (polyvinylidene chloride),which added oxygen barrier and moisture barrier to thecellophane, resulting in the original non-metal barrierfilm for food packaging.

Today, we have many polymers such as PE (poly-ethylene), PP (polypropylene), PET (polyester) and PS(polystyrene) that are used to produce clear films forpackaging. While these aforementioned polymers arecommonly used in monolayer format, they are also used inmultilayer films produced by coextrusion and/or lamina-tion processes. There also exists a broad variety of spe-cialty polymers and compounds which sometimes are usedmonolithic, but are more commonly used in coextrusions.

Polyethylene was first synthesized by the Germanchemist Hans von Pechmann who prepared it by accidentin 1898 while heating diazomethane. When his col-leagues Eugen Bamberger and Friedrich Tschirner char-acterized the white, waxy substance that he had created,they recognized that it contained long -CH2- chains andtermed it polymethylene [2].

The first industrially practical polyethylene synthesiswas discovered (again by accident) in 1933 by EricFawcett and Reginald Gibson at the ICI works inNorthwich, UK. Upon applying extremely high pressure(several hundred atmospheres) to a mixture of ethylene

Multilayer Flexible Packaging; ISBN: 9780815520214

Copyright � 2009 Elsevier Inc. All rights of reproduction, in any form, reserved.

and benzaldehyde, they again produced a white, waxymaterial. Because the reaction had been initiated by traceoxygen contamination in their apparatus the experimentwas, at first, difficult to reproduce. It was not until 1935that another ICI chemist, Michael Perrin, developed thisaccident into a reproducible high-pressure synthesis forpolyethylene that became the basis for industrial low-density (LD)PE production beginning in 1939 [2].

Subsequent landmarks in polyethylene synthesis haverevolved around the development of several types ofcatalyst that promote ethylene polymerization at moremild temperatures and pressures. The first of these wasa chromium trioxide-based catalyst discovered in 1951by Robert Banks and J. Paul Hogan at Phillips Petroleum.In 1953, the German chemist, Karl Ziegler, developeda catalytic system based on titanium halides and orga-noaluminum compounds that worked at even milderconditions than the Phillips catalyst. The Phillips catalystis less expensive and easier to work with, however, andboth methods are used in industrial practice.

By the end of the 1950s, both the Phillips and Zieglertype catalysts were being used for high-density (HD)PEproduction. Phillips initially had difficulties producing anHDPE product of uniform quality and filled warehouseswith off-specification plastic. However, financial ruin wasunexpectedly averted in 1957 when the hula hoop, a toyconsisting of a circular polyethylene tube, became a fadamong youth in the USA.

A third type of catalytic system, one based onmetallocenes, was discovered in 1976 in Germany byWalter Kaminsky and Hansjorg Sinn. The Ziegler andmetallocene catalyst families have since proven to be veryflexible at copolymerizing ethylene with other olefins andhave become the basis for the wide range of polyethyleneresins available today, including very low-density poly-ethylene and linear low-density polyethylene. These newcatalysts have allowed creation of some very specialpolyethylene resins. One such example is the polymerused to produce DSM’s ‘Dyneema’ fiber, an ultra highstrength material that competes with aramid based fibersin a variety of applications where high strength is needed,but not where high temperature resistance is alsoneeded. As it has always been, new materials find theiruseful niches in the market.

Until recently, the metallocene systems were the mostactive single-site catalysts for ethylene polymerizationknown – new catalysts are typically compared to zirco-nocene dichloride. Much effort is currently beingexerted on developing new, single-site (so-called post-metallocene) catalysts that may allow greater tuning ofthe polymer structure than is possible with metallocenes.Recently, work by Fujita at the Mitsui Corporation(among others) has demonstrated that certain salicy-laldimine complexes of Group 4 metals show sub-stantially higher activity than the metallocenes [2].

Dr Karl Rehn at Hoechst AG in Germany first poly-merized polypropylene in 1951 and did not recognize theimportance of his discovery. It was then rediscovered onMarch 11, 1954, by Giulio Natta [3]. Biaxially orientedpolypropylene (BOPP) was introduced in the 1960s.Initially, it was developed in a blown film process and thefirst commercial production was started by DuPont inIowa, USA. Eventually, other companies started to pro-duce BOPP in a cast sheet/orientation process whichmade the film production more economical. This changewas instrumental in replacing cellophane, as it was lowercost and more environmentally friendly to produce. AsBOPP usage gained in the industry, there was a corre-sponding decline in cellophane usage and production.Today, cellophane production is a mere fraction of what itonce was.

British chemists, John Rex Whinfield and JamesTennant Dickson, employees of the Calico Printer’sAssociation of Manchester, patented ‘polyethyleneterephthalate’ (also called PET or PETE) in 1941, afteradvancing the early research conducted by WallaceCarothers of DuPont. They saw that Carothers’ researchhad not investigated the polyester formed from ethyleneglycol and terephthalic acid. Polyethylene terephthalateis the basis of synthetic fibers such as generic polyesterfiber and specialty fibers such as Invista’s ‘Dacron’.Whinfield and Dickson along with inventors W.K. Birt-whistle and C.G. Ritchie also created the first polyesterfiber called ‘Terylene’ in 1941 (first manufactured byImperial Chemical Industries or ICI). The second poly-ester fiber was DuPont’s ‘Dacron’ (now a product ofInvista Corp.) [4].

Polystyrene was discovered in 1839 by Eduard Simon[5], an apothecary in Berlin. From storax, the resin of theTurkish sweet gum tree (Liquidambar orientalis), hedistilled an oily substance, a monomer which he namedstyrol. Several days later, Simon found that the styrol hadthickened, presumably from oxidation, into a jelly hedubbed styrol oxide (‘Styroloxyd’). By 1845, Englishchemist John Blyth and German chemist AugustWilhelm von Hofmann showed that the same trans-formation of styrol took place in the absence of oxygen.They called their substance metastyrol. Analysis latershowed that it was chemically identical to Styroloxyd. In1866, Marcelin Berthelot correctly identified the for-mation of metastyrol from styrol as a polymerizationprocess. About 80 years went by before it was realizedthat heating of styrol starts a chain reaction which pro-duces macromolecules, following the thesis of Germanorganic chemist Hermann Staudinger (1881–1965). Thiseventually led to the substance receiving its presentname, polystyrene.

The company I.G. Farben began manufacturing poly-styrene in Ludwigshafen, Germany, about 1931, hopingit would be a suitable replacement for die-cast zinc in

4

C H A P T E R 1 Introduction

many applications. Success was achieved when they de-veloped a reactor vessel that extruded polystyrenethrough a heated tube and cutter, producing polystyrenein pellet form.

In 1959, the Koppers Company in Pittsburgh,Pennsylvania, developed expanded polystyrene (EPS)foam [5].

Other polymers used in the packaging industry havehad similar stories of invention ranging from purelyaccidental discovery to hard wrought research.

Markets: A global economy

The dynamics of the industrialized nations post WorldWar II have lead to a plethora of applications for manypolymers from the mid-20th century continuing into the21st century. The flexible packaging industry has beenone significant part of this engine of growth and adoptionfor use in many applications has expanded into a globaleconomy, as many newly industrialized nations havebecome global players in trade.

Looking at recent years only in the USA, Table 1-1 [6]shows the flexible plastic packaging films market (inmillion lb) in the USA for three packaging films for theyears 2002 and 2007.

Fig. 1-1 plots these data and shows a trend line for thetotal film market. This exponential fit trend line calcu-lates a 4.5% growth rate from the 2002 to 2007 data. Atthis growth rate, the US flexible packaging film marketcould be over 20 billion pounds in 2020.

Selected data from the US census office for theNAICS code 26112 (Unsupported plastics packagingfilm & sheet mfg) is shown in Table 1-2 [7]. In addition tothese selected data, the total value shipments have beenadjusted to 1997 dollars using data from the Miseryindex web site [8].

Fig. 1-2 plots the data in Table 1-2 for employees, totalvalue shipments and the adjusted total value shipmentsto 1997 dollars versus year. The exponential curve fits,

calculate the total value shipment growth rate at 9.16%and the employee growth rate at 6.96%. When the totalvalue shipments are adjusted to 1997 dollarsthe calculated growth rate is 6.73%. This suggests thatthe market was growing at a 7% rate over this time periodand it closely followed the employee growth rate. If onewere to do a more rigorous estimate of future growth,one would have to have good historical sales and popu-lation data. Market growth is a function of market pen-etration, i.e. pounds of material used per person andnumber of people. Looking at global data one would findthat there are countries with minimal market penetra-tion, some with average market penetration and somewhere the value of flexible plastic packaging is highlyrecognized and are leaders in taking advantage of thebenefits flexible films give.

Table 1-1 Flexible plastic packaging films market in the USA:

breakdown of volume sales by material type for the years 2002

and 2007 (in million lb)

Flexible plastic 2002 2007

Packaging film type

Polyethylenes 7676.6 9584.3

Polypropylene 1160.3 1449.2

Thermo plastic polyesters 131.2 170.0

Total 8968.1 11203.5

Fig. 1-1 Flexible packaging film market in the USA – sales volumeversus year.

Table 1-2 Selected data from the US census office for the NAICS

code 26112 (Unsupported plastics packaging film & sheet mfg)

Year Companies Employees

Total valueshipments($1000)

Total 1997valueshipments($1000)

1997 131 15 428 3 638 911 3 638 911

1998 15 288 3 998 678 3 937 645

1999 16 447 4 813 330 4 638 284

2000 17 327 4 823 210 4 495 845

2001 16 875 4 782 978 4 335 645

2002 207 23 418 6 203 922 5 535 676

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Introduction C H A P T E R 1

Processes, materials, needs

Coextrusion is used in the following processes:

� blown film

� cast film (<10 mils or <250 mm)

� cast sheet (>10 mils or >250 mm)

� blow molding

� tubing and sheathing

� extrusion coating and laminating

� profile.

Fig. 1-3 is a flow diagram of a coextrusion feedblock wherethree extruders are employed to produce a three-layer

asymmetrical structure. This particular design has thebarrier on the outside with a tie layer to provide adhesionto the core resin.

While a large majority of polymer extrusion is done inmonolithic form, the world of coextruding polymers hasbeen growing now for half a century. The whole conceptof polymer extrusion grew from the already existingtechnology for extrusion of processed foods, rangingfrom cereals to meats. Likewise, coextrusion was beingpracticed in the food industry before it was practicedusing molten polymers. Without realizing it, every readerof this manuscript has likely consumed some type ofcoextruded food in the form of candy, bakery items,cereals or processed meats.

Some considerations as to why we may wish tocoextrude polymers are:

� to bring the desired properties of differing materialsinto one structure

� to reduce emissions from solvents used in adhesivelamination processes

� to process multiple materials in one pass intoa structure to save process steps, cost and time.

Coextruding polymers is no simple matter. In the foodindustry, they have their compatibility issues and, in thepolymer industry, we take the complexity to a higher level.We must consider the three main scientific property arenasrelated to moving a polymer from solid form to liquid formand back to solid form of a differing shape, which are:

� thermal

� fluid

� mechanical [9].

Thermal considerations are important because theprocesses are non-isothermal. The science of fluid

Fig. 1-3 Flow diagram of a coextrusion feed block where threeextruders are employed to produce a three-layer asymmetricalstructure.

Fig. 1-2 Shipment value and number of employees versus year.

6

C H A P T E R 1 Introduction

flow is important because the fluids are non-Newto-nian viscoelastic with non-linear temperature de-pendent viscosities. These fluids have interfacialboundaries that depend on the fluids’ viscosity andnormal stress difference. Mechanical considerationsare important because the metal that contains thefluid flow is subjected to internal pressure forceswhich distort the metal. The metal has to be shapedproperly and has thermal mass and chemical re-sistance characteristics, which interact with theflowing fluids.

However, the benefits of combining materials incoextrusion far outweigh the challenges associated withsuch an endeavor. Coextrusion provides solutions tomeet product functionality. There is a variety of filmproperties that need to be taken into consideration whendesigning a packaging film. Some properties relate to thesurface characteristics while others relate to the entirethickness/body of the film. Surface characteristics typi-cally include coefficient of friction (COF), gloss, haze(surface induced) and sealability, as well as surface ten-sion and chemical receptivity, which are important formaterials to be printed. ‘Body’ characteristics includetensile properties, elongation, haze (internally induced),transparency, color, impact strength and a variety of otherparameters.

Additionally, packaging requirements may also includegas and aroma barrier, chemical resistance, puncture re-sistance, formability and shrink properties. In a worldwhere energy prices keep rising, the biggest challenge isto have a structure that includes all the desirable prop-erties in the most economical way. This requires thatprocessors have a good understanding of the materialproperties, as well as a good grasp of polymer rheology toachieve an optimal formulation especially whenattempting to coextrude [10].

In designing a coextruded film one has to consider theworld of needs including [11]:

� physical properties

B tensile, elongation, flexure, stiffness

B hardness, toughness, puncture, COF

B sealability, peelability, etc.

� barrier properties

B oxygen, moisture, oil, chemical

B aroma/fragrance, carbon dioxide, etc.

� additives

B colors, mineral fillers, COF modifiers, melt frac-ture inhibitors, scavengers, etc.

A typical coextruded structure will have a bulk layer,a barrier layer, a sealant layer and, often, an adhesive layerto join incompatible layers. Reclaim also has to be con-sidered and can be put back into its own layer or into thebulk layer [12].

Common choices for the bulk layer are usually fromthe following groups [12]:

� PE (polyethylene)

B HD (high density), LLD (linear low density), LD(low density), VLD (very low density)

� PP (polypropylene)

B HoPP (homopolymer PP), CoPP (ethylene-propylene copolymer), Ter-PP (ethylene-propyl-ene-butene terpolymer)

� Acrylates

B EMA (ethylene methyl acrylate)

B EBA (ethylene butyl acrylate)

B EEA (ethylene ethyl acrylate)

B PMMA (poly(methyl methacrylate))

B EMMA (ethylene/methylmethacrylate)

� EVA(ethylene vinyl acetate)

B low %, med %, high % VA

� PS (polystyrene)

B HIPS (high impact PS), GPPS (general purpose PS).

The criteria that are important to consider are:

� cost

� adhesion

� flexibility

� softness

� stiffness, etc.

Resin selection criteria are:

� melt index – MW and MWD (physical propertiestensile strength, organoleptics)

� density – crystallinity (opticals, permeability,modulus – i.e. LDPE)

� molecular branching – (melt strength, processingparameters)

� comonomer content – all properties

� melt point – crystallinity (seal initiation temp.,thermoforming).

Following is a list of generic names for extrudable resins.When the brand/grade of a polymer is unknown, it can bereferred to in this fashion to help communicate betterabout the nature of the material [12]:

� Polyethylenes

B HDPE – high density polyethylene

B HMW-HDPE – high molecular weight HDPE

B MDPE – medium density polyethylene

B LDPE – low density polyethylene

B LLDPE – linear low density polyethylene

B VLDPE – very low density (linear) polyethylene(also VLLDPE)

B ULDPE – ultra low density polyethylene (alsoULLDPE)

B mPE – metallocene polyethylene, generic.

7

Introduction C H A P T E R 1

B mLMDPE – metallocene linear medium densitypolyethylene

B mLLDPE – metallocene linear low densitypolyethylene

B mVLDPE – metallocene very low (linear) density(sometimes referred to as a ‘plastomer’)

� Polypropylenes

B PP – polypropylene (generic indication whenminimal information is known)

B CoPP – copolymer polypropylene

B HoPP – homopolymer polypropylene

B Ter-PP – terpolymer polypropylene

� Acid copolymer resins

B ACR – acid copolymer resin, generic namefor EAA and EMAA resins

B EAA – ethylene acrylic acid copolymer, such asDuPont Nucrel�, Dow Primacor� and Exxon-Mobil Escor�

B EMAA – ethylene methacrylic acid copolymer,such as DuPont Nucrel�

B Ionomer – generic name for ionomeric copolymerresins, such as DuPont Surlyn�, and ExxonMobilIotek�

� Acrylates

B Acrylate – generic name for various acrylate copol-ymers, such as DuPont Elvaloy AC�, ArkemaLotryl�, Westlake EMAC�, ExxonMobilOptema�

B EBA – ethylene butyl acrylate

B EEA – ethylene ethyl acrylate

B EMA – ethylene methyl acrylate

B EMMA – ethylene methyl methacrylate

B EiBA – ethylene iso-butyl acrylate

B EnBA – ethylene normal-butyl acrylate

� Ethylene vinyl acetates

B EVA – ethylene vinyl acetate; examples: DuPontElvax�, AT Plastics Ateva�, Equistar Ultra-thene�, ExxonMobil Escorene Ultra�

� Polystyrenes

B PS – polystyrene

B EPS – expanded (or foamed) polystyrene

B HIPS – high impact polystyrene

B GPPS – general purpose polystyrene

� Polyvinyl chlorides

B PVC – polyvinyl chloride

B PVdC – polyvinylidene chloride. Most commonlyseen as a coating on a film, but there are alsoextrudable grades, such as Dow Saran� and SolVinIxan�

� Terpolymers

B EVACO – terpolymer of ethylene, vinyl acetateand carbon monoxide

B EnBACO – terpolymer of ethylene, normal-butylacrylate and carbon monoxide

B EiBAMAA – terpolymer of ethylene, isobutylacrylate and methacrylic acid

B (there are various other terpolymers in the industry)

� Grafted resins

B EVA-gMAh – ethylene vinyl acetate with a graftof maleic anhydride

B LLDPE-gMAh – LLDPE with a graft of maleicanhydride

B (therearevarious other grafted resins in the industry)

� Others

B PeelSealorEasyPeel–genericnamefor apeelable seal-ant resin of unknown brand and grade. Example brandnames include: DuPont Appeel�, Mitsui-DuPontCMPS�, Yasuhara Hirodyne� or Toyo Topco�

B TPS – generic name for ‘thermoplastic starch’resin. These are a new family of materials in themarket, for example, Plantic� TPS or DuPontBiomax� TPS.

Materials and barrier

When one talks about ‘barrier’, please note that there aredifferences in interpretation of what this relates to. Forsome it may be related to oxygen, while for others moisturebarrier may be more important. Barrier to other parametersmay also be of greater or lesser importance including carbondioxide, oils, acids, flavors, aromas, fragrances, solvents andother chemicals. Each polymer has its own unique charac-teristics and properties and, thus, will have differing ‘bar-rier’ performance. Which polymer or combination ofpolymers you may wish to coextrude will depend uponwhether you need a barrier for oxygen, water, oil, aroma orsomething else and to what degree for each property. Somecommon resins thatcanbeused forcertainbarriers are [12]:

� Nylon, polyamide, (PA)

B oxygen barrier, aroma barrier, and some oil barrier

� EVOH

B oxygen barrier, flavor/aroma barrier

� PET

B moisture barrier, some flavor/aroma barriersand some chemical barrier needs

� PVdC

B Good for moisture, oxygen, flavor, aroma,and some chemical

� HDPE

B moisture barrier

� PP

B moisture barrier

� ionomer

B oil barrier, some chemical barrier.

8

C H A P T E R 1 Introduction

When choosing one or more of these resins to coextrude, inaddition to the barrier properties, youmust alsounderstandthe criteria for flex durability, clarity and other physicalparameters related to each material and how it adds ordiminishes from the needs of the overall coextrusion.

Following here is some general information on Nylon.This is a material that is used broadly in coextrusions, buthas some unique properties related to processing, socannot be dropped into any extruder at will [12, 13].

Nylon or polyamide common types:

� Nylon 6: melting point (MP)¼220�C (430�F)

B The most common grade used in flexible packagingcoextrusions

� Nylon 66: MP¼250�C (480�F)

� Nylon 6/66: MP¼210�C (410�F)

� Amorphous Nylon

B used mainly in blends with Nylon 6 or 66, butoccasionally is used pure

� Nylon 11 and 12: MP¼180�C (360�F).

It is important to note that extrusion screws for Nylonare generally very different in design versus PP, PE andethylene copolymer screws. Thus, do not just try ex-truding Nylon resin through a screw not known to bedesigned for Nylon. Please consult with the equipmentmanufacturer for advice. Without doing so, you run therisk of damaging equipment including but not limited tothe screw, gearbox and drive motor on the extruder.

There are a number of extrusion grade Nylon resinproducers who supply globally:

� Honeywell

� BASF

� Bayer

� DuPont

� EMS

� Mitsubishi

� Ube, etc.

The features that one can obtain with Nylon are:

� good gas barrier

� good thermoformability

� good toughness, abrasion resistance, tensiles

� good optics

� good oil resistance.

The extrusion issues for consideration with Nylon are:

� crystalline melt point

� flow properties change rapidly from solid to melt andback to solid. SAFETY!

� moisture sensitivity

� various types, so need to choose extrusion tempera-ture profiles carefully

� ‘high’ extrusion temperature needed

� almost always processed in coextrusion.

Ethylene vinyl alcohol, commonly known as ‘EVOH’,has grown into a very useful barrier resin in the flexiblepackaging industry. Ethylene vinyl alcohol began its lifein the 1950s as an offshoot of research into polyvinylalcohol adhesive polymers. Ethylene vinyl alcohol wasoriginally developed and patented by the DuPontCompany, but the polymer did not have properties ofvalue as an adhesive. In the 1950s, there was no barrierfilm coextrusion market, so looking at the barrierproperties of the polymer was not even something ofconsideration. The technology for EVOH was eventuallysold to the Monsanto Company. Monsanto, in turn, latersold it to a Japanese corporation and, by this time,coextrusion was in swing and barrier films containingNylon were becoming prevalent. The oxygen barrierproperties of EVOH were studied and found to be veryinteresting if you keep the polymer dry, protecting itfrom moisture by other polymers in coextruded films[12, 13]. Today, there are three major suppliers ofEVOH:

� Kuraray Eval�� Evalca Eval� (subsidiary of Kuraray in the USA)

� Nippon-Gohsei Soarnol�.

Ethylene vinyl alcohol characteristics are:

� good oxygen barrier when dry

� almost always run in coextrusion

� needs tie layers (except to Nylon)

� moisture sensitive

� thermal sensitivity

B needs short, smooth flow paths when designingextrusions systems

� good optical properties

� high stiffness

� flex crack sensitivity.

Polyester has been previously discussed in thisintroduction with regards to its history.

There are many types of polyester and the mostimportant one for films/sheet is PET (polyethyleneterephthalate). Polyethylene terephthalate is typi-cally oriented to provide strength, clarity and bar-rier properties. Polyethylene terephthalate forextrusion coating is more challenging to processcompared to common polyolefins, such as PE and PP,and requires special equipment. Coextrusion makesit easier to run, but will require use of in-line dryersand proper PET screw designs [12,13]. Suppliersof film and extrusion coating resins in the USAwould be:

� Eastman

� DuPont

� M&G (formerly Shell).

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Introduction C H A P T E R 1

Polyester type polymers found in the packaging industryinclude:

� PET – generic extruded polyester. Could be mono-layer or in a coextrusion

� PET ext.ctg. – generic extrusion coating of polyester

� APET – amorphous polyester

� CPET – crystalline polyester

� PET or PETE – polyethylene terephthalate

� PETG – polyester copolymer with glycol

� PBT – polybutylene terephthalate

� PPT – polypropylene terephthalate

� PTT – polytrimethylene terephthalate

� PEN – polyethylene napthalate

� PLA – polylactic acid

� PGA – polyglycolic acid

� PHA – polyhydroxyalkanote.

Other resins that can be considered to provide a barrierof some type are:

� PAN – polyacrylontrile, sometimes written as ‘ACN’

� SAN – styrene acrylonitrile copolymer

� AN-MA –– acrylonitrile methyl acrylate copolymer(such as Innovene, Barex�)

� ABS – acrylonitrile butadiene styrene copolymer

� LCP – liquid crystal polymer

� COC – cyclic olefin copolymer, such as TAP Topas�or Mitsui Apel�

� PUR – polyurethane, extrudable type when seen ina coextrusion, for example

� SBC – styrene butadiene copolymer (such as Chev-ronPhilipps K-Resin�).

Materials and sealing

When assembling a coextruded film for the flexiblepackaging industry, most films require that they be seal-able. The most common seal method is thermal, usinga constant temperature sealer or a variable temperatureimpulse sealer. Other seal methods include:

� radiofrequency

� high frequency

� ultrasonic

� pressure sensitive.

Given that thermally created seals are the most common,following is an overview of what to consider when con-sidering a material for the sealing layer.

The sealant layer choice should be made consideringthe application needs, since each parameter does notneed to be maximized in order to perform in the givenend use. What is important is finding the appropriatebalance with properties such as:

� hot tack strength

� heat seal strength

� seal initiation temperature

� sealing speed

� coefficient of friction

� ability to seal through a given contaminant

� chemical resistance to the material to be contained

� economics, etc.

The most common sealant layer resins range from highperformance to commodity performance. Inherentproperty differences in the polymers will affect thesealing performance and, since there is such a broad rangeof polymer grades in each family, it is best to consult withpolymer suppliers to match your given needs to theperformance of a given resin such as:

� ionomers of acid copolymers

� acid copolymer (EAA or EMAA)

� mVLDPE (very low density)

� EVA or EMA blends with LLDPE

� CoPP/TerPP, EVA, EMA, LLDPE, mLLDPE

� LDPE or PP.

Bringing things together

When assembling coextrusion structures, some materialswill bond to other materials. However, the adhesion levelmay be insufficient for the durability needs of the givenapplication. In order to create a more robust structure, it isrequired to use a material designed specifically to bond onematerial to another with improved interlayer adhesion.The family of polymers that will work as a ‘facilitator’ inthis case is commonly called ‘tie resins’. They are designedto act as glue and provide adhesion between the in-compatible layers. These ‘coex tie’ resins are designed toprovide both mechanical and/or chemical bonding toother polymers when in a molten state. The adhesivestrength obtained will be a function of the tie resin for-mulation and the coextrusion process parameters. Itshould be recognized that the extrusion process can havea significant impact in generating interlayer adhesion. Forexample, taking the same structure from a blown filmprocess to a cast film process using the same resins, usuallywill not translate into obtaining the same interlayer ad-hesion. This is a complex topic for another discussion [12].

To achieve higher bond strengths than attainable witha base polymer, resins are formulated with a variety ofmaterials including grafted functional groups and/or arecompounded with an array of modifiers. Proper choice ofthe correct ‘tie’ resin is based on many variables in-cluding, extrusion processing parameters and materialend use parameters. Your ‘tie’ resin supplier can providehelpful recommendations [12,13].

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C H A P T E R 1 Introduction

Base resins typically used for tie layers are:

� LDPE, LLDPE, HDPE

� EVA, PP

� acrylate copolymers

� acid copolymers.

Common modifiers used are:

� maleic anhydride

� olefinic tougheners

� tackifiers

� rubbers.

Some tie resin suppliers include:

� DuPont

� Mitsui

� Equistar

� Mitsubishi

� Arkema

� Rohm & Haas

� Dow

� DSM and several others.

When selecting a coextrudable adhesive resin, it is alwaysuseful to choose the correct resin for a given process and/orend-use application. If an EVA resin is selected caremust be paid to temperature limitations of the extru-sion process since EVA should not be processed over235�C (455�F). For all resins, the tie layer rheologymust match with adjacent materials to assure goodlayer uniformity with no ‘interfacial instability’. Somematerials have a minimum temperature that is neces-sary to generate adhesion via chemical and/or me-chanical interaction. Extrusion ‘process time’ is also animportant factor and related to the time between thepolymers exiting the die and solidification. This relatesto the ‘cooling’ and ‘drawing’ of the polymers and in-ternally induced stresses during production. Coex-trusion feedblock and die design are also influential inhow a ‘tie resin’ may be able to generate interlayeradhesion [12].

This book is an attempt to provide a snapshot ofmultilayer flexible films and where they are used. Likeany still picture, this book cannot describe the future andthe innovative uses, applications and film designs thatwill drive these products to fill the needs of consumers.

References

1. http://en.wikipedia.org/wiki/Cellophane

2. http://en.wikipedia.org/wiki/Polyethylene

3. http://en.wikipedia.org/wiki/Polypropylene

4. http://inventors.about.com/library/inventors/blpolyester.htm

5. http://en.wikipedia.org/wiki/Polystyrene

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11. S.B. Marks, Material selection for coextrusion. SPE Extrusion Division TopCon 2008 Cincinnati, OH, Oct (2008) 21–22.

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Introduction C H A P T E R 1