The Importance of Thermal Transfer and Heat Retention in Designing Thermoformable Coextruded Film Structures Dan Ward NOVA Chemicals Corporation ABSTRACT Incomplete forming and excessive corner thinning are common problems in coex thermoforming films that result in loss of barrier and package integrity. Both problems can be avoided or minimized by designing coex structures that optimize thermal transfer and heat retention within the film and choosing interior polyethylene layers that soften at relatively low temperatures and extend or draw without neck-in. These studies demonstrate the importance of maintaining heat in coex films immediately before and during forming. By considering the thermodynamic properties of the individual polymer layers used in the exterior and interior layers of a coex film, heat loss can be minimized and final forming quality improved. The results provide practical guidelines for using standard simple laboratory tests such as DSC and elevated temperature tensile tests to help choose polymers and design coex structures for thermoforming. INTRODUCTION Thermoforming films are a rapidly growing segment of the flexible packaging industry as sophisticated coex structures displace more costly, less sustainable rigid packages. Thermoformed films for high-value food products such as meat and cheese have many demanding specifications that typically require specialty polymers such as polyamide (Nylon), EVOH, elastomer modified tie resins and specialty polyethylenes. Films with nine or more coex layers have been growing preferentially in thermoforming applications since these films offer more versatility and enable the manufacturer to displace more expensive materials with polyethylene. As the amount of polyethylene in thermoforming structures increase, additional care needs to be taken to choose the correct polyethylenes and locate them optimally in the coex structure. This will ensure that the overall film forms well and meet the physical property requirements while minimizing costs. Incomplete forming, excessive corner thinning and/or film breaks are common problems in coex thermoforming films that result in loss of barrier and package integrity. Raising the film preheat temperature can reduce these problems but can slow line speeds. In addition, thermoforming lines using conductive heating are often limited by low temperature sealants that will adhere to the heating plate at temperatures above 105oC. This maximum temperature limits the types of polyethylenes that can be used for the coex interior. Low Density Polyethylenes (LDPEs) are frequently used due to low softening temperatures. Most LDPEs however, will not provide sufficient strength to reduce the amount of nylon in the coex. Certain high-strength, linear low density PEs (LLDPEs) are more suitable to displace nylon thickness but have not performed well in standard forming structures; presumably due to higher softening temperatures and inconsistent gauge thinning related to the “neck-in” effect of many LLDPEs. Recent studies suggest that different types of LLDPE show less neck-in or gauge uniformity after extension(2). Certain types of octene LLDPE produced on a dual solution reactor platform showed relatively consistent gauge thinning in elevated temperature tensile test; comparable to LDPEs and other ethylene copolymers used in thermoforming. Other NOVA studies examining the dynamic surface temperature changes of monolayer and coex films in an impulse heat sealer found that the surface temperatures of different polymers drop at significantly different rates during the heating and cooling cycles(7). The time dependent surface temperature changes are directly related to the specific thermodynamic properties of polymers including specific heats, latent heats of crystallization, thermal conductivities, and others. These two studies suggest that certain LLDPEs may be less prone to excessive

thinning after forming and certain LLDPEs may form under standard conditions provided high film temperatures are maintained during the forming process. Furthermore, it should be possible to maintain critical temperatures and heats within coex structure by carefully selecting polymers with certain thermodynamic properties to be used in specific layers. The primary objectives of these studies were to:

• Identify polyethylenes that will perform well as interior layers in coex forming structures. • Determine predictors and guidelines for choosing thermoforming materials and optimizing coex

forming webs. • Identify key temperature related material properties that will help predict formability.

A better understanding of the dynamic heat transfer properties of nylon, EVOH and different polyethylenes alone and in tandem will enable film designers to optimize heat retention in coex forming films and optimize cost and performance. MATERIALS AND METHODS Test Polymers: Several polymers from four different classes of polyethylenes were examined in preliminary monofilm forming tests. The four broad PE classes considered were: low density PE (LDPE) produced in a high pressure tubular reactor, butene or hexene copolymer linear low density PE produced in a gas phase reactor with Ziegler-Natta catalyst (ZN-LLDPE), Octene copolymer linear low density produced in a single solution reactor with Ziegler-Natta catalyst, and Octene copolymer linear low density produced in dual reactor-solution platform with Single-Site catalyst. One representative material from each class was chosen for additional monofilm and coex tests based on the best overall forming performance and consistency. The grades listed in Table 1, are manufactured by NOVA Chemicals Corporation and free of slip and antiblock additives. TABLE 1: Polyethylene Test Materials MATERIAL CODE

GRADE REACTOR TYPE

CATALYST MELT INDEX

DENSITY (g/cc)

LDPE NOVAPOL® LFy320-A

High pressure Tube

Peroxide 0.25 0.921

ZN-LLDPE NOVAPOL® PF-0118-F

Gas Phase Ziegler-Natta 1.00 0.919

ZN-VLDPE SCLAIR® FP112-A

Solution Single Reactor

Advanced Ziegler-Natta

0.90 0.912

s-LLDPE SURPASS® FPs016-C

Solution Dual Reactor

Single-Site 0.65 0.916

s-HDPE SURPASS® FPs016-C

Solution Dual Reactor

Single-Site 1.2 0.966

The forming and thermal transfer characteristics were compared to four other polymers commonly used in thermoforming structure presented in table 2:

Table 2: Non-PE and other materials used in monofilm and coex film tests MATERIAL CODE

GRADE SUPPLIER DESCRIPTION

Ionomer SURLYN® 1601 E. I. Dupont Sodium ionomer EVA ELVAX® 3165 E.I. Dupont Ethylene-Vinyl Acetate Copolymer. (%VA, MI) NYLON ULTRAMID® B40 BASF Polyamide Nylon 6 EVOH EVAL® E171-B EVALCA 44 mole% ethylene, vinyl alcohol copolymer Tie Resin Bynel® 41E710 E.I. Dupont LLDPE based tie resin Two additional materials used in coextruded films were SURPASS® HPs167-AB sHDPE manufactured by NOVA Chemicals Corporation and Bynel® 41E710 tie concentrate manufactured by Dupont. In coex structures containing Nylon or EVOH, 20 weight% of the tie concentrate was dry-blended with LLDPE. This blend was directly fed into the tie-layer extruder adjacent to EVOH or Nylon. Film Production: Monolayer test films with target thickness of 4 mils were produced on a Macro blown film line with 8 inch diameter die with 0.085 inch die gap, and a single 3.5 inch extruder. Target polymer melt temperature was kept at temperatures between 400o and 420o F. Blow up ratio (BUR) was 2.5:1 Multilayer (Coex) films with target thickness of 4 mils were produced on a 9-layer Brampton die with eight 2 inch extruders and one 2.5 inch extruder, a 13.8 inch diameter die, and 0.080 inch die gap. Target melt temperature was kept between 400 and 420o F. BUR was 2.5:1. 3-Layer coex films were produced by running the same material through the three outer, core and inner extruders. Thermoforming Evaluations: Thermoforming evaluations were run on a Hydrotrim® thermoformer. This unit shuttles the mounted film between an oven heating chamber and single cavity forming mold. Two types of thermoforming molds were used. A shallow tray with rounded corners and 2:1 draw ratio and a tapered cup with 3.36:1 draw ratio. Film surface temperature was measured using an Infrared gun and controlled by adjusting the oven temperature and heat soak time. Target surface temperature was 105o C for most tests. The oven soak time required to reach a 105o C surface temperature varied depending upon the film type. The thermoformability of films were judged according to four criteria.

• Completeness of cavity fill and/or film’s ability to maintain form after removal: “cell definition”. • Presence or absence of film breaks or blow-outs. • Uniformity or gauge reduction at critical points in the formed part.

Gauge at specific points in the formed was measured using a table-top micrometer. Analytical Tests: Film thermoformability was correlated with several physical and thermodynamic properties of the test materials. A general summary of the temperature dependent polymer properties examined along with the test conditions is provided in Table 3:

TABLE 3: Test methods and conditions used to measure temperature dependent polymer properties: PROPERTY TEST DEVICE CONDITIONS COMMENTS Basic Thermal properties

Differential Scanning Calorimetry (DSC)

10o C/minute Melting points, heats of melting and freezing, thermal inflection points

Specific Heats DSC 10o C/minute Specific heats determined from slope of heating curve.

Time Dependent Surface Temperature Changes.

Type T Thermocouple, digital converter and heat sealer

Various seal jaw temperatures

Exposed thermocouple wire placed on film surface. Temperatures recorded at 0.01 second increments.

Tensile Properties Tensile Tester with Oven heating chamber

1”/minute separation rate Various temps.

MD and TD Tensile properties determined between room temperature and 95C.

RESULTS Monolayer Films Thermoforming Results: Test results for monolayer films are summarized in pages 8 through 12 of the presentation. Three test films: ZN-LLDPE, LDPE and Nylon did not form completely in vacuum forming trials at 105o C preheat temperature in either tray or cup tests. All other polyethylene type films; sLLDPE, ZN-VLDPE, Ionomer, and EVA formed completely in both the tray and cup molds without breakage but with varying amounts of thinning or gauge reduction. The EVOH film formed completely in the tray mold but showed excessive thinning and/or breakage in the deeper draw cup mold. Ionomer film had the best overall forming characteristics as judged by the most uniform gauge reduction and cell definition. The two octene copolymer LLDPEs both had good overall forming characteristics but slightly more thinning at the edges and pockets. While the EVA and EVOH monolayer films formed completely in the tray mold, corner thinning was higher for these films and cell definition was poorer; possibly due to shrinkage after the film cooled. Surface Temperature and Heat Retention Affects on Monolayer Film Thermoformability: Surface temperature measurements suggested that the completeness of forming is significantly influenced by a film’s capacity to retain heat during and after the forming process. Films that maintained surface temperatures such as Ionomer and ZN-VLDPE tend to form completely while films that dissipated surface heat quickly typically “freeze off” before the forming process is complete. One exception in these studies was the monolayer EVOH film that lost heat quickly yet still formed completely in the tray mold. A possible explanation for this may be a temperature differential between the film surface and interior; the film surface dissipates heat quickly while the film’s interior still retains heat and higher temperatures. As the film cools, the force required to stretch or elongate increases and the ductility of the polymer decreases. The “freeze off” phenomenon used to describe incomplete forming likely occurs when the (temperature dependent) tensile forces needed to elongate the film exceed the vacuum pressure. Many LLDPE films are prone to “neck-in” where the films do not thin uniformly during elongation and gauge reduction is sudden or erratic from point to point in the test specimen (5). Elevated temperature tensile testing suggests that certain sLLDPE films are less prone to neck-in or erratic thinning at higher temperatures (2). Many thermal points and thermal properties were reviewed as possible predictors of minimum thermoforming temperatures including peak melting points, vicat softening point, glass transition temperature, and others. The best predictor for the polyolefin films used in this study was the inflection point in the DSC cooling curve immediately before heat transfer reached steady state. (See chart 12 in

presentation). In forming trials where the pre-heat temperature was varied, a test film that maintained surface temperature above this inflection point typically formed fully with less gauge reduction. Once the surface temperature dropped below this point, incomplete forming, excessive thinning and/or film breaks were common. Thermoforming of Coextruded Films: The purpose of the coex film forming trials was to determine whether thermoformability of certain polymers could be improved by designing coex structures that maximize heat retention in the films. A specific goal was to identify structures and/or interior layers that maintain temperatures in excess of the cooling inflection point for LLDPEs. The first set of multilayer tests examined the effect of a sHDPE core layer between the outer skin layers of LLDPE. While the sHDPE material has a higher cooling inflection point (106 C) than the LLDPE test materials (86o to 103oC), it also has two desirable properties for interior layers of thermoforming films; low specific heat and higher thermal conductivity. Surface temperature tests the sHDPE core layer effectively slower surface temperature loss in LLDPE films. The results show that a thin (18%) sHDPE core layer improves thermoformability of sLLDPE and ZN-VLDPE film structures. A sLLDPE structure with thicker (48%) sHDPE core layer had poorer thermoformability than the monolayer sLLDPE; the force required to elongate this amount of sHDPE was probably in excess of the vacuum pressure. The sHDPE core layer did not improve thermoformability of the ZN-LLDPE. While the sHDPE core slowed heat loss of the ZN-LLDPE surface, temperature still dropped below the 103 C cooling inflection point. The second set of tests was completed on coex films similar to current commercial structures containing a nylon skin layer, two interior nylon layers that sandwich an EVOH barrier layer and different LLDPEs in the interior or tie layers. The purpose of these tests was to evaluate the effects of different LLDPE interior layers on overall thermoformability. The results suggest that the type and arrangement of LLDPE interior layers affects the formability of commercial structures. Specifically, structures containing sLLDPE or ZN-VLDPE had better thermoformability than structures containing ZN-LLDPE interior. There also appeared to be a synergistic effect of using sLLDPE on the interior sealant side and ZN-VLDPE on the interior skin side. The final set of tests examined structures containing sLLDPE interior and tie layer and zero, one or two interior layers of sHDPE. By necessity, the interior nylon layers were replaced with blends of sLLDPE and tie concentrate in these structures. The purpose of these studies was to determine whether the sHDPE interior layers could improve thermoformability by maintaining heat in the more traditional coex forming structure. The forming test results showed that a single sHDPE interior layer closer to the nylon skin had slightly better deep-draw formability. A sHDPE layer in this position may reduce heat loss in the nylon skin which effectively keeps temperatures higher throughout the entire structure. The absence of Nylon interior layers appeared to hurt the draw uniformity of the structure. Microscopic examination of the films’ cross-sections at various points suggested that the EVOH layers were continuous but thinned suddenly and significantly in the corners and edges. CONCLUSIONS Certain polymers used in thin gauge thermoforming films are prone to rapid heat loss during the forming process. The heat loss and corresponding temperature drop effectively increases the polymer’s tensile strength and lowers ductility which may lead to incomplete forming, excessive or erratic gauge reduction and/or film breakage. For polyethylene materials, overall film formability was better in these studies when the films surface temperatures were kept above the lower inflection point on the DSC cooling curve.

Heat loss in coextruded thermoforming films can be minimized by considering the specific thermodynamic properties and location of each material being used. Based on these studies, three key considerations or strategies for improving formability are using exterior layers with low convective cooling coefficients and using interior layers with low cooling curve inflection point. Incorporating thin interior layers with low specific heat and high thermal conductivities may further improve thermoformability by reducing heat loss although care should be taken to ensure that thermal transfer and physical properties are balanced. Certain material properties obtained from simple laboratory tests can be very helpful in predicting whether a polyethylene will form well in a coex forming films. Three key measurements are the cooling inflection point from DSC, the tensile elongation strength and degree of polymer neck-in from high temperature tensile testing, and dynamic surface temperature changes during polymer heating and cooling cycles. When considered together, these PE properties will enable the film manufacturer to choose grades and design films that form completely with uniform thinning and no breakage. ACKNOWLDGEMENTS The author acknowledges the significant contributions of many individuals who made this paper possible. The technical consultations and assistance of Alan Wang Chris Hung, Mike Li, Krista Madsen, Bill Wright* and others were essential for these studies. REFERENCES (1) Soroka, W., Fundamentals of Packaging Technology- 3rd Edition, Institute of Packaging Professional

Press (2002) (2) Wang, X. and Boparai, M., “Identification of Thermoformability Indicators for Multilayer Films”

Accepted for SPE for 2010 ANTEC CONFERENCE (2010) (3) Ward, I.M Mechanical Properties of Solid Polymers – 2nd Edition, A Wiley-Interscience Publication

(1985) (4) L. Feng, M Kamal, “Crystallization and Melting Behavior of Homogeneous and Heterogeneous Linear

Low-Density Polyethylene Resins”, Polym Eng. & Sci., 45, 1140-1151 (2005) (5) Unwin, A.P., Duckett, R.A., Ward, I.M. Collins, T.L.D and Coates, P.D. “Supression of Necking in

Polyethylene” Journal of Applied Polymer Sci. 86, 3135-3147 (2002) (6) Raff, R.A.V. Encyclopedia of Polymer Science and Technology, Interscience, 1967. Copyright Wiley,

1967.

(7) Hung, C. and Li, M, Internal studies on heat transfer in Polyethylene Films

* Bill Wright is an employee of Optimum Plastics. All others are employees of NOVA Chemicals Corporation

The Importance of Thermal Transfer and Heat Retention in the Design of Thermoformable Film Structures

Presented by:

Dan WardTechnical Service SpecialistNOVA Chemicals Corporation

Thermoforming ChallengesThermoforming ChallengesIncomplete Forming Excessive thinning and corner

blowouts

2

PE is a critical performance component PE is a critical performance component in Coex thermoforming filmsin Coex thermoforming films

General Forming Film Structures for Meat and Cheese Packaging

7 Layers 9 Layers 11 Layers

% PE 44 62 67Cost 1.00 0.85 0.75

Polyamide (PA)

Tie Layer

PA

EVOH

Tie Layer

PE (Seal)

PA

Tie Layer

PE

Tie Layer

PA

EVOH

PA

Tie Layer

PE

PE

PE

PE

PE

3

Key Considerations for Thermoforming Key Considerations for Thermoforming Structures and Individual MaterialsStructures and Individual Materials

Thermal Transfer and Heat Retention during forming process

Complete Fill

Draw Uniformity (Consistent Thinning)

4

Study ObjectivesStudy ObjectivesIdentify polyethylenes that will perform well as interior layers in coex forming structures.

Determine predictors and guidelines for choosing thermoforming materials and optimizing coex forming webs.

Identify key temperature related material properties that will help predict formability.

5

Thermoforming Test MaterialsThermoforming Test MaterialsMaterial Code Supplier Grade Comments

ZN‐LLDPE –butene

NOVA Chemicals NOVAPOL® PF0118‐C Gas Phase – Z/N1 MI / 0.918 density

S‐LLDPE‐ octene NOVA Chemicals SURPASS® FPs016‐C Solution Reactor – ssc0.65 MI / 0.916 density

ZN‐VLDPE –octene

NOVA Chemicals SCLAIR® FP112‐A Solution Reactor – ssc1 MI / 0.912 density

Tubular LDPE NOVA Chemicals NOVAPOL® LFy320‐C 0.920 density, 0.3 MI

Ionomer Dupont SURLYN® 1601 Sodium Ionomer

EVA Dupont ELVAX® 3165 Ethylene vinyl Acetate18 wt % VA

EVOH EVALCA EVAL® E171‐B 44 mol % ethylene

Nylon 6 BASF Ultramid® B40 Film grade Nylon 6

6

THERMOFORMING EVALUATIONS THERMOFORMING EVALUATIONS Vacuum forming (Plug assist optional)Forming Temperature: 105 C target surface temperature measured by Infrared Tray 2:1 draw ratio, Depth = 3.5 cm. Length = 13.4 cm Width = 10.9Cup 3.36 : 1 draw ratio, Depth = 9 cm, top diameter = 2.75 cm , bottom diameter = 2 cm

Formability Measurements:Forming = complete / incomplete filling of mold at all points.Thinning = Gauge measured at 16 points along three axes.Integrity = absence of blowouts

7

Monolayer Film Thickness Monolayer Film Thickness After 105After 105OO C Vacuum Tray FormingC Vacuum Tray Forming

MATERIALPocket

% of original gauge

LengthCorner

Width CornerAverage of 16

points

Ionomer 27% 29% 29% 42%

s-LLDPE 20% 26% 27% 48%

ZN-VLDPE 21% 33% 23% 52%

EVA 18% 36% 24% 47%

EVOH 20% 23% 20% 32%

ZN-LLDPE Incomplete Forming (60% fill)

LDPE Incomplete Forming (40% fill)

Nylon No forming ( 0% fill)

8

Formed Tray Film ThicknessFormed Tray Film ThicknessOriginal thickness = 4 milsOriginal thickness = 4 mils

0

10

20

30

40

50

60

70

80

90

100

Top Edge Bottom Sidewall Corner base Centerpoint

% Origina

l Thickne

ss

zLL (plug assist)

sLL (plug assist)

sLL (vacuum)

9

PE Dimensional Stability and PE Dimensional Stability and ““NeckNeck--inin”” PhenomenaPhenomenaFilm specimens after elevated temperature tensile elongationFilm specimens after elevated temperature tensile elongation

Typical Linear PE MaterialzLLDPE – Butene sLLDPE at <95C

LDPE, Functional CopolymerssLLDPE-Octene at > 95C

X-SectionFront View Front View X-Section

10

Surface Temperature vs. TimeSurface Temperature vs. Time95C Heat for 0.5 sec. Air Cooled95C Heat for 0.5 sec. Air Cooled

11

DSC Cooling Curves for LLDPEs and IonomerDSC Cooling Curves for LLDPEs and Ionomer

Best formability for LLDPE MonofilmsAt Temps. above inflection point

12

Improving Formability Through Coex Improving Formability Through Coex Structure DesignStructure Design

Hypothesis: Coex structures that maintain temperatures above cooling inflection point will form fully with better draw uniformity.

PE Interior: Cooling inflection point 10 C less than heat temp

Sealant: Low surface heat transfer coefficient

Core Layer: Low Specific Heat, high conductivity

Outer Skin: Low specific heat and heat transfer coefficient

Sealant

PE Interior Layer

Core Layers

PE Interior Layers

Outer Skin

13

Positives: HDPE requires less energy to raise temperature and efficiently transfers heat to skin layers keeping them above critical temperatures.

Negative: HDPE requires more force to form and more prone to brittle fracture. Higher Inflection temperature.

Multilayer Film Test Structure #1Multilayer Film Test Structure #14 mil A / B / A Coextrusion4 mil A / B / A Coextrusion

Material Inflection Tempdegrees C

Specific HeatW / J – g (23 C to inflect)

sHDPE (core) 106 1.96

sLLDPE 94 2.30

ZN-LLDPE 103 2.28

ZN-VLDPE 86 2.37

PE Skin Layers

sHDPE Core Layer

Low Cp + Thermal Cond.PE Skin Layers

14

Surface Temperature vs. TimeSurface Temperature vs. Time105C Heat for 0.5 sec. Air Cooled105C Heat for 0.5 sec. Air Cooled

15

Thermoforming Results Thermoforming Results 4 mil A/B/A Coex Films4 mil A/B/A Coex Films

Film Structure Layer Distribution (%) Tray Forming Cup Forming

s-HDPE 100% s-HDPE None None

s-LLDPE 100% s-LLDPEIncomplete

80% fillBurst

3.5 cm deep

s-LL / s-HDPE / s-LL 26% / 48% / 26% Incomplete

10% Burst

2 cm deep

s-LL/s-HDPE/s-LL 41% / 18% / 41%Full forming

Uniform drawFull forming

Bottom thinning

ZN-LL / s-HDPE / ZN-LL 41% / 18% / 41%Incomplete

40% Burst

2 cm deep

ZN-vLL / s-HDPE / ZN-vLL 41% / 18% / 41%Full forming

Uniform drawFull forming

Uniform draw

16

23 C (Room Temperature)

95 C (Forming Temperature)

17

Multilayer Film Test Structure #2Multilayer Film Test Structure #29 9 –– Layer Coex Films: Nylon Core and Skin LayersLayer Coex Films: Nylon Core and Skin Layers

PE 1 Layer

PE 2 Layer

Full tray forming

Full cup forming

DrawUniformity

sLLDPE sLLDPE ✓ No 40% ✓+ZN-LLDPE ZN-LLDPE no no/burst Poor

ZN-VLDPE sLLDPE ✓ No / burst ✓

sLLDPE ZN-LLDPE ✓ ✓ ✓++

OBJECTIVE: Evaluate effects of PE type in the interior layers on formability

Findings:1.Films with sLL and vLDPE interior layers had better formability than films with zLL.2.Synergistic forming effect when sLLDPE and vLDPE are used in separate interior layers

ZN vLLDPEPE 1

Nylon

EVOH

Nylon

PE 2

Nylon

18

Multilayer Film Test Structure #3Multilayer Film Test Structure #39 9 –– Layer Coex Films: HDPE Interior and Nylon Skin onlyLayer Coex Films: HDPE Interior and Nylon Skin only

HDPELayer

Full tray forming

Full cup forming

DrawUniformity

None ✓ No (60%) ✓Layer 1 only No (80%)

No / burst ✓-Layer 1 and 2 ✓ No / burst ✓-Layer 2 only ✓ No (80%) ✓-

OBJECTIVE: Determine whether thin interior layer of HDPE improves formability

Findings:1. HDPE in layer 2 may provide slight improvement. 2. Absence of core Nylon layers may hurt EVOH draw uniformity.

ZN vLDPE

sLLDPE

Layer 1 (HD or sLL)

sLLDPE

EVOH

sLLDPE

Layer 2 (HD or sLL)

sLLDPE

Nylon

19

Coex Structure Considerations Coex Structure Considerations for Thermoforming Webs for Thermoforming Webs

• Coex structures provide definite advantages over mono or 3-layer.• More consistent draw-down , fewer blowouts, better cavity fill.• Multiple thin layers generally form better than a single thick material layer.

• Maintain critical heat and temperature during the forming process• Interior layers with low specific heat and high thermal conductivity• Skin layers with low convective cooling coefficients.

• Balance physical property and thermal transfer requirements for optimal performance.

• Keep Nylon layers as thin as practical.   • Sealants with higher temperature resistance allow for better heat transfer.

20

Key PE Property and Predictors for Key PE Property and Predictors for ThermoformabilityThermoformability

Thermal Properties• Polymer cooling inflection point below heater or oven temperature.• Low specific heat (between room temperature and melt onset)

Elevated Temperature Tensile Properties• Dimensional stability without neck-in at forming temperature. • Absence of a distinct tensile yield point• Tensile yield below 5 MPa with consistent elongation force.

21

ConclusionsConclusionsA film’s thermoformability is influenced by its ability to retain heat during the forming process. Keeping individual layer temperatures above cooling curve inflection point is critical for complete forming and uniform draw.

Certain thermal/heat transfer properties coupled with high temperature tensile properties can be good predictors of thermoformability.

Films containing octene LLDPE produced by a solution process had the best overall thermoformability in these studies.

22

AcknowledgementsAcknowledgements

Alan Wang, Chris Hung, Bill Wright, and Kam Ho for technical input.

Mike Li, Greg Courtney and Krista Madsen for film production and testing.

23

Please remember to turn in your evaluation sheet...

Thank youPRESENTED BY

Dan WardDan WardPrincipal Technical Service Specialist

NOVA Chemicals Corporation,Calgary, AB, CanadaWarddr@novachem.com

24