Multi Layer Composite Films

TheoryPresented by Shrikant AthavaleFor PG Students , SIES Nerul

On 06-11-2010

Introduction

• Film extrusion is one of the most important processes for plastics accounting for almost a quarter of all thermoplastics consumed

• Has enjoyed some periods of rapid growth in recent years, particularly within the packaging industry

• Film is defined as a sheet less than 250 μm in thickness

• There are two types of film extrusion: cast film and blown film

• Background of Flat Film Forming

• Dies

T-slot

Coat hanger

• Coextrusion

• Rolling/Cooling of Film

• Wind up

• Process dependent properties

• Model

• Uses

• Applications

TOPAS is an amorphous, transparent copolymerbased on the polymerization of ethylene and norborneneusing metallocene catalysts. Its propertyprofile can be varied over a wide range by adjustingthe chemical structure during polymerization. Thesenew materials exhibit a unique combination ofproperties whose performance benefits include:

1. High transparency and gloss2. Excellent moisture and aroma barrier3. Variable glass transition temperatures from 65°C up to

178°C4. High stiffness and strength5. Easy to extrude and thermoform6. Compatibility with polyolefins7. Excellent biocompatibility and inertness8. Resists acids and alkalis

The advantages of Multilayer Films are

Introduction• Cast film extrusion is a continuous operation of

melting and conveying a polymer in a heated screw-and-barrel assembly

• Polymer is extruded through a slit onto a chilled, highly polished turning roll

• Film is sent to a second roller for cooling on the other side

• Alternatively, polymer web is passed through a quench tank for cooling

• Film then passes through a system of rollers, which have different purposes, and is finally wound onto a roll for storage

Dies

• Most flat dies are of T-slot or coat hanger designs, which contain a manifold to spread the flowing polymer across the width of the die, followed downstream by alternating narrow and open slits to create the desired flow distribution and pressure drop

T-slot Die• The basic manifold for a sheet die is a constant

cross section or T-slot design• Relies on a large manifold area and a lip long

enough to create a large enough pressure drop to force the melt to the ends of the die

• Used when processing low viscosity plastics that are not thermally sensitive

Coat Hanger Die• Conventional constant-deflection die• Internal pressures cause the polymer to deflect

uniformly across the width of the die• Delivers more streamlined flow since there are

few areas where melt flow rates may slow and material linger long enough for polymer degradation to occur

Coextrusion• Most cast film lines today are coextrusion lines,

combining layers from as many as 7 extruders into the product through multi-manifold dies, or else single manifold dies with the aid of feedblocks

• When the materials enter the die, they are no longer confined individually within steel channels. They are in intimate contact with each other and in the fluid state

• Layers in the final film will be uniform if the adjoining materials have a reasonable rheological match and there is uniform flow distribution inside the die

• When layer distortion occurs it happens in the die, not the feedblock, during the transition or distribution in shape from the square incoming stream to the wide, thin film shape

Chill-roll assembly

Chill rolled cooling• The film extrudate leaves the die typically in a

downward motion• Extrudate is pulled away from die and onto a water

cooled roller (chill roller)• Air knife is sometimes used to ensure intimate contact

between polymer and roll• Die and chill rolls are positioned as close as possible

– e.g. gap for HDPE is approx. 13mm• Heat transfer to rolls : conduction

– Thermal conductivity of both polymer and roller material

• Chill rollers speed control final thickness, drawdown and neck-in effects

Contacting the chill roll

• Optimally, the molten polymer drops onto the chill roll and contacts tangentially

• The alignment or parallelism of this roll to the die is critical in relation to the falling film

• Whenever wrinkling of the film occurs on the casting roll surface, it is likely that the first roll must be repositioned

“Neck-in” effects• Hot film extruded is drawn to the colder

rollers– Shrinking at edges : Neck in leads to beading– Shrinkage at edge depends upon polymer melt

temperature and polymer itself

Chill rollers

• High gloss finish – chromium finish for highly polished roll

• Matte finish – metal rolls are sandblasted, acid etched, or machined

• Design Criteria: polymer cannot stick to rollers

• Thermal conductivity and conduction of heat from molten polymer to chilled rollers

• At least two or more rolls

Quench tank cooling• Difficult to control precise variables such as

water temperature• Good optical properties and low operating cost

compared to chill rolled cooling• Film defects are common

– Vibration and water movement• Less common today than chill

rolled cooling

Take-Off Rollers

• Linear rate extrudate is removed from die

• Controls the velocity of the roller system– Thickness control

• Require accurate and sensitive speed controls

• Multifunction to cool the polymer film

Rosato, Dominick. Plastics Engineering, Manufacturing & Data Handbook. “Ch.3: Extrusion”. Plastic Institute of America. Springer. (2001). Online access from Knovel.

Intermediate and Windup Steps• Slitter –

– cuts film edge to wanted width– Removes the “beaded” film– Scraps are typically recycled or reused

• Gauging –– Film thickness gauges– Allows for user feedback to change or alter processing

conditions• Surface treatment

– Plasma/Corona Treatment– Change surface energy

• Winders– Apply constant tension to film– Speeds of 2000ft/min– Diameters up to 5ft– Width up to 20ft

Winder for plastic film extrusion. Direct Industry. (2009) http://www.directindustry.com/prod/battenfeld-gloucester/winder-for-plastic-film-extrusion-20385-45141.html

Film Orientation• Uniaxial• High orientation in machine direction (MD)

– Polymer is being “drawn” by the cooling and take off rollers

– Polymer chain alignment • Low orientation in transverse direction (TD)• Critical draw ratio

– Parameter to control orientation• Biaxial orientation is possible by stretching

polymer in both directions• Critical for mechanical properties

Mitsubishi Polyester Film. Biaxial Orienation. (2009) http://www.m-petfilm.com/Europe/images/biaxial.jpg&imgrefurl

Process depend properties

• Polymer Density• Melt Index• Melt Temperature• Screw cooling• Screw speed• Extruder compound

efficiency

Rosato, Dominick. Plastics Engineering, Manufacturing & Data Handbook. “Ch.3: Extrusion”. Plastic Institute of America. Springer. (2001). Online access from Knovel.

Optical Properties

• Melt temperature (Tm)

– Lower Relative Tm: Hazy, lower elongation at break and tensile strength

– Higher Relative Tm: Glossy finish, higher elongation at break and tensile strength

• Cooling Rollers:– Optimal optical properties: 10°C less than

temperature need to melt polymer onto the rollers

– Must control within 2°C range

Common Polymers

• Polyolefins are the most widely-used plastics for film extrusion

• Polyolefins that can be extruded as monolayer and multi-layer film:– low density polyethylene (LDPE)– linear low density polyethylene (LLDPE)– high density polyethylene (HDPE)– ethylene copolymers

• ethylene vinyl acetate (EVA), ethylene methyl acrylate (EMA)

– polypropylene and propylene copolymers– thermoplastic olefins (TPOs)

Benefits

• Advantages of polyolefin films:– Ease of processing– Light weight– Good toughness and tear resistance– Flexibility (even at low temperatures)– Outstanding chemical resistance– Relatively low cost compared with other

plastics

Properties

• The basic properties of polyolefins can be changed using a broad range of chemical modifiers

• In addition, polyolefin-based films can be coextruded with various other polymers, including ethylene vinyl alcohol (EVOH), nylon, polyester barrier resins and adhesive tielayers, to produce multilayer films with special, high-performance properties

Forming webs } improved forming window and uniformitySoft shrink film } tough, stiff, soft shrink, halogen-free, polyolefinShrink sleeves and labels } high shrinkage and stiffness with low shrink forceSlip additive }non-migrating for reduced ambient and hot COFin polyolefin filmSealant films }additional stiffness, improved seal strengthand hot tackTwist wrap } high end clarity and gloss, excellent deadfoldBags/pouches }increased stiffness at room temperature and underhot-fill conditions, easy tear, retort performanceMedical trays } high moisture barrier, deep draw, clarityPaperboard coating } increased moisture barrier, reduced curlZipper closures }reduced warping and camber, increased openingforceBlister packs } high moisture barrier, deep draw, halogen-free

Critical Applications

Of multilayer Films

Properties

Optical propertiesTOPAS forms clear, colorless films which are findingutility in optical films for a variety of uses such as retardation,polarization, and brightness enhancement.Their light transmittance as shown in Figure 6 extendsthrough the visible spectrum into the near UV andexceeds that of polystyrene (PS), polycarbonate (PC),and nearly equals that of polymethyl methacrylate(PMMA). TOPAS films have low birefringence andvery low haze.

Barrier propertiesTOPAS is being increasingly utilized as a barrier materialin packaging. Packaging must preserve the taste,flavor and composition of packaged foods and thecomposition of non-food items such as fragrances.It must prevent excessive amounts of oxygen, water,solvents, flavors, aromas and other gases or liquidsfrom leaving or entering a package. This has becomemore important as food packaging has moved awayfrom heavy, inflexible glass and metal containersto those made of plastic. Such packaging often involvessophisticated, multilayer structures containinghigh-barrier polymers like EVOH and PVdC. Thesestructures can be costly, require adhesive polymers,advanced and expensive processing equipment, andreduce the potential for recycling.TOPAS has one of the highest moisture barriers ofany polymeric material. While not considered a highbarrier to oxygen or other gases, it is a significantlybetter barrier than polyethylene and can be used inblends to modify oxygen, carbon dioxide and othergas transmission rates to target specific values such asthose required by fruits and vegetables.TOPAS has four to five times better barrier than LDPEover a broad range of permeants and does not requirean adhesive in combinations with polyethylenes.Blends with polyethylene having more than 70%TOPAS typically provide over 90% of the barrierof pure TOPAS. This can dramatically improve theperformance of a packaging film in preserving theoriginal characteristics of a package’s contents ormoderating the transfer or loss of aromas and odors asillustrated in Figure 8. TOPAS barrier layers can compensatefor the poor water vapor transmission performanceof commonly used oxygen barrier materialssuch as nylon and EVOH.

performance of a packaging film in preserving theoriginal characteristics of a package’s contents ormoderating the transfer or loss of aromas and odors asillustrated in Figure 8. TOPAS barrier layers can compensatefor the poor water vapor transmission performanceof commonly used oxygen barrier materialssuch as nylon and EVOH.

DeadfoldTOPAS has a high modulus which can be utilizedto produce a film with the deadfold characteristicsrequired by twist-wrapped candy. These applicationsoften require an expensive cellophane film. A filmhaving thin outer TOPAS layers (20 to 40% of totalthickness) and a PE core has excellent deadfold,clarity, recyclability and a high gloss surface withgood metallizability. These TOPAS-based twist filmshave proven to be processable on commercial highspeedwrapping lines designed for cellophane whereconventional polyolefin films fail.

Coefficient of frictionTOPAS can reduce the coefficient of friction (COF)to commercially acceptable levels in cast PE films.Blending in ~10% TOPAS by weight can producemedium-slip films useful for vertical form-fill-sealoperations (a COF of 0.2 to 0.4). A process melttemperature of no more than about 180°C is neededfor blends containing low Tg TOPAS grades, whilea higher process melt temperature of up to 240°Ccan be used for blends with high Tg material. TOPASresins are thus viable alternatives as non-migratory,non-contaminating slip additives when processed atrelatively low temperatures. Lower temperatureprocessing may affect haze, but does not affectprintability or sealing properties.

Chemical resistanceTOPAS is very pure because the metallocene catalystused in its production is filtered from most grades afterpolymerization. It also is extremely low in extractables,e.g. hexane extractables are 0.3% or less andash is nearly zero. It easily passes the EuropeanPharmacopoeia Section 3.1.3 extractable test forpolyolefins. It has excellent organoleptic properties.Tests with various food components have yieldedlower extractables and similar scalping levels tothose of standard PE resins.As a non-polar material, TOPAS is highly resistant topolar compounds such as water, alcohol and acetone.Like most polyolefins, it is less resistant to nonpolarmaterials. TOPAS should be tested against specificcompounds when chemical resistance is critical inan application.

Chemical resistance

Blown film extrusionTOPAS also performs well in blown film extrusionsystems. General processing recommendations givenfor cast film and the cast film starting temperatureprofiles given in Table 5 (PE-rich blends) and Table 6(Discrete TOPAS layers) can also be used for blownfilm. The key new variables are bubble stability andbubble collapsing. TOPAS has lower melt strengththan LDPE and the melt strength of other polymers inthe film structure will strongly influence bubble stability.Increased melt temperatures generally give thebest clarity with higher Tg grades. Bubble stability willdetermine the maximum possible melt temperature.Structures containing high levels of TOPAS or discretelayers of TOPAS can be stiff and cause challengesin achieving wrinkle-free layflat. In general, keepingthe film warmer in the collapsing area helps thecollapsing process. Collapsing equipment designedto handle stiff films such as nylons and HDPE willgenerally produce better results with stiff TOPASstructures. Additional equipment recommendationsare as follows:} Barrier mixing screws work best, where the advancingmelt pool is separated from unmelted pellets.Maddock mixing sections have proven effective.} A preferred screw has a screw L/D ratio (screwlength to diameter) of 24:1 or above and a lowcompression ratio for optimum melt homogeneity.A standard blown film tower can be used.} Grooved feed extruders are not preferred but maybe used with improved processibility grades suchas 8007F-400. Heated feed zones are sometimesrequired for good processability in grooved feedextruders.} Use typical spiral dies and die gaps of 1.5 to2.25 mm (60 to 90 mils).} Recommended blow-up ratio (BUR) is 2:1, butgood results have been achieved at 1.5:1 to 3.5:1.

Trouble Shooting Guide

Orientation and shrinkageTOPAS orients readily at appropriate temperaturesabove its Tg. Its broad grade range (Tg from 65 to138°C) allows orientation and shrink temperatures tobe tailored to a process and a product. Shrink temperaturecurve steepness can be controlled preciselyby blending TOPAS grades, as illustrated in Figure 11.Shrinkage rates can also be influenced by blendingTOPAS with other polymers.TOPAS 8007F-04 and 9506F-04 have been monoaxiallyand biaxially oriented in flat (tenter, and machinedirection stretcher) and tubular (double bubble) processes.Orientation increases ductility, while addingsome stiffness. As a monolayer in biaxial processing,TOPAS orients best at about a 4x4 ratio. Higher orientationratios are attainable when TOPAS is a layer orcomponent of a multicomponent structure, e.g., skinlayers of TOPAS can be oriented at a 5x10 ratio inOPP tentering and other processes.TOPAS has high shrink recovery. In monoaxially orientedlabels, shrinkage of over 80% is possible. Itsshrink stress can be less than half that of competingmaterials, provided it is not oriented at too low a temperature.Inherent high dimensional stability is usefulin shrink applications by reducing film shrinkage belowits Tg, e.g., in warm storage conditions.

SealingHeat-sealing capability is usually specified by sealstrength, hot tack strength and seal initiation temperature.As it cools, TOPAS transitions from a rubbery to aglassy state to create a high-modulus material at 65°C(TOPAS 9506) to 78°C (TOPAS 8007). This adds significantseal strength at temperatures where PE has lowstrength and modulus.When blended with PE, TOPAS broadens the seal temperaturerange of many polyethylenes. Figure 12 showsa typical case where the seal range for pure LLDPE isextended and the seal strength is also noticeably higher.TOPAS often improves hot tack performance (strengthof the hot seal after cooling for 0.1 sec.) as much as100%. Hot tack strength is important in vertical form,fill and seal equipment where contents are droppedinto bags while the seal is still hot. The more robustsealing performance with TOPAS is valuable in a widerange of "real-world" packaging situations.When an all-TOPAS surface layer is used in a packagingfilm, the film can be sealed to itself much like a PEfilm. For example, TOPAS 8007F-04 seals to itself ata seal initiation temperature of 105°C (defined as sealtemperature where 8.8 N seal strength is achieved).The seal strength of the TOPAS-TOPAS seal is similarto that of LDPE and LLDPE. It also has good hot-tackstrength. The high modulus of TOPAS means the sealmay not be as tough as that of other materials, althoughthe seal will be hermetic.

SterilizationTOPAS can be sterilized by all common methods, includinggamma radiation, e-beam or beta irradiation,ethylene oxide and steam as shown in Table 10. It isrelatively stable to gamma irradiation at a dosage to5 Mrad. Its tensile strength, Izod impact strength andother mechanical properties show little change afterexposure and after 12 months aging. It does yellowslightly immediately after exposure, but this fades overseveral days to nearly the initial color.TOPAS 6013 can be steam sterilized, typically at121°C for 20 minutes. The procedure for sterilizingTOPAS calls for venting the steam with dry air above120°C. Drying should last about 60 minutes, depend-ing on size and thickness, to gain the desired claritybefore cooling to room temperature. For thermal sterilization,air pressure in the autoclave should match thesteam pressure.E-beam or beta irradiation causes TOPAS to yellowslightly. The color fades to the original color in about10 days. The intensity of the color shift is proportionalto dosage. TOPAS is unaffected by ethylene oxidesterilization and the temperatures this involves (above60°C in some cases).

PrintingPrinting is usually difficult on traditional PE film becauseit lacks sufficient modulus, is not thermallystable, and must be carefully dried in an oven to driveoff the ink solvent. In contrast, TOPAS provides a superiorprinting surface by improving flatness, increasingtemperature resistance and providing a glossy,high quality surface.TOPAS can also be blended with olefins to raise thermalresistance and modulus, which aids web handlingand reduces film elongation under tension forbetter print registration since adding as little as 10%TOPAS to LLDPE doubles film modulus. Like otherolefins, TOPAS films require pretreatment with coronaor plasma before printing. Their low moisture absorption,high modulus and heat resistance can overcomefilm processibility problems and deliver more consistentyields. Standard polyolefin ink systems are effectivefor TOPAS and TOPAS/PE blends.