Heat sealability of laminated films with LLDPE and LDPE as the sealant materials in bar sealing...

Heat Sealability of Laminated Films with LLDPE andLDPE as the sealant Materials in Bar Sealing Application

Cheng See Yuan,1 Azman Hassan,2 Mohd Imran Hj Ghazali,3 Ahmad Fauzi Ismail4

1Department of Thermal-Fluid, Faculty of Mechanical Engineering, Kolej Universiti Teknikal Kebangsaan Malaysia,75450 Ayer Keroh, Melaka, Malaysia2Department of Polymer Engineering, Universiti Teknologi Malysia, Skudai, Malaysia3Department of Mechanical Engineering, Kolej Universiti Teknologi Tun Hussein Onn, Johor, Malaysia4Department of Gas Engineering, Universiti Teknologi Malysia, Skudai, Malaysia

Received 22 June 2006; accepted 9 November 2006DOI 10.1002/app.25863Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The heat sealability of laminated films withlinear low density polyethylene (LLDPE) and low densitypolyethylene (LDPE) as the sealant materials was investi-gated. A laboratory heat sealer was used to study theresponse of laminated films to temperature, time, and pres-sure. Platen temperature was confirmed as primary factorin controlling heat-seal strength. Dwell time must be suffi-ciently long to bring the interfacial temperature to a desiredlevel. When the desired heat-seal strength has beenachieved, further increase of dwell time did not improvedheat-seal strength. Platen pressure had little effect above thelevel required to flatten the materials for good contact. Barsealing process window for each sample were developed.The optimum combination of platen temperature and dwelltime for each laminated film can be obtained in the respec-

tive process windows. Strength of heat-seal and its failuremodes are closely related. Plateau initiation temperatureclosely corresponds to the final melting point of sealantmaterials. Relatively higher platen temperature wasrequired to seal laminated films with lower thermal con-ductance. Required dwell time corresponds closely to theheat flow rate of bar sealing process. Laminated films madefrom extrusion lamination process provided lower level ofachievable heat seal strength when compared with the lami-nated films made from dry-bond lamination process. � 2007Wiley Periodicals, Inc. J Appl Polym Sci 104: 3736–3745, 2007

Key words: heat-sealing; linear low density polyethylene(LLDPE) film; low density polyethylene (LDPE) film; sealstrength; process window

INTRODUCTION

Bar sealing is the most widely used method to pro-duce a seal in most form/fill/seal machines.1 Thereare three parameters in heat-sealing process, namelythe sealing temperature, dwell time, and sealingpressure. Good seal will only be produced if thesethree parameters are in proper combination whenforming a seal.

In heat-sealing process, heat is applied to melt thesealing layer to a molten stage, or partially molten toeffect sealing. The acceptable temperature for heat-sealing process is not a single temperature setting asthe plastic can be sealed at partially molten, or acompletely melt conditions. Therefore, the acceptablesealing temperature is a range of temperature settingin which a good seal will be produced for as long asit is being made within this temperature range.Dwell time is the duration of time that the laminatedfilm is brought into intimate contact by the heated

bars during the heat-seal cycle. The optimum dwelltime setting is important to ensure enough heat isapplied to the sealant material, and no excessivetime is wasted which may reduced the productionspeed. The greater the heat flow rate, the shorter thedwell time required. Thus, heat flow rate is a signifi-cant variable which determine the dwell time of theprocess. During heat-sealing process, the heated por-tions of the packaging material (thermoplastic) needsto be held together so that the molten plastic willnot skid and resulting in a bad seal, wrinkle in seal,or corner leak. Thus, it is crucial that pressure isexerted continuously at right angle to the directionof the planes of the dies to press the heated packag-ing materials together throughout the entire heatingcycle.

Obviously, a container made of a multilayer flexi-ble-packaging material (laminated film) can be nostronger than the seal that holds it together.1 There-fore, acceptable heat-seal can be defined as a sealwhich has its strength greater than the strength of thepackaging material. This corresponds to the delami-nating or tearing mode failure in the peel test speci-men, where the laminated film was failed rather thanthe heat-seal fail in a peeling mode failure.

Correspondence to: A. Hassan ([email protected]).

Journal of Applied Polymer Science, Vol. 104, 3736–3745 (2007)VVC 2007 Wiley Periodicals, Inc.

If this requirement is not fulfilled, heat-seal of apackage will break open before the laminated film.Consequently, this would be a waste of using stronglaminated film which is usually more expansive, asthe package can easily be failed at the seal whensubjected to loads.

Several studies concerning heat-sealing parametersand the characteristics of the sealant material havebeen reported in literature. Theller2 was the pioneerresearcher of this technique. He studied the heatsealability of plastic film in bar sealing applications.He reported that the interfacial temperature anddwell time are to be the primary factors in control-ling heat-seal strength. Pressure normal to the sealsurface had little effect above the level required toflatten the web for good contact. Stehling andMeka3,4 had done a series of studies concerningheat-sealing process. The effect of heat-sealing pro-cess variables (seal bar temperature, dwell time, andpressure) on seal properties (seal strength, seal elon-gation, and seal energy) of polyethylene films wasquantitatively determined. Their study also showsthat to obtain the highest possible seal strength for agiven semicrystalline polymer, the required platentemperature can be estimated with the given dwelltime and interfacial temperature by finite elementmodel. Their results agree with Theller,2 where heat-seal strength is primarily controlled by sealing tem-perature and dwell time, rather than pressure. Theyhad also established the seal strength versus platentemperature plot. Several important heat-sealingquantities such as seal initiation temperature, plateauinitiation temperature, final plateau temperature,and plateau seal strength are identified in the plot.Tsujii et al.5 had investigated the effect of heat-seal-ing temperature on the mechanical properties andmorphology of oriented polypropylene (OPP)/castpolypropylene (CPP) laminated films. They reportedthat tensile strength of the seal was affected by theorientation of the films. While Yasuo et al.6 had car-ried out investigation on the failure criteria of theheat-sealed part of OPP and CPP heat seals made byimpulse type heat-sealing machine. They reportedthat seals were stronger in the transverse directionwhen compared with the machine direction. Studiesof the effect of corona-discharge treatment (CDT)on heat-seal strength was done by Owens7 andFarley and Meka.8 The former has reported that

corona-treated polyethylene films exhibit strong self-adhesion when joined together under conditions ofheat and pressure that give no adhesion withuntreated films. While, the latter found that the pri-mary effect of CDT on the heat-sealing behavior ofLLDPE films is a transition in the failure mode ofheat-seals from a normal tearing or inseparable bondto a peelable seal. They also found that CDTincreases the seal initiation temperature by 5–178Cand decreases the plateau seal strength by 5–20% asthe treat level, or wetting tension, increases from 31to 56 dyn/cm. The effect of material type on heat-seal strength was reported by Falla9 and Halle andDavis.10 The earlier reported that the coextrudedfilms having a heat-seal layer comprised of the ultralow density polyethylene (ULDPE) resins in the seal-ing layer of a coextruded film significantly increasethe heat-seal/hot tack range. The ULDPE resin pro-vides broader sealing range for pouch conversionand better physical properties in finished pouches.And the latter have shown that to obtain excellentheat-sealing performance from unlike seal layers ispossible by utilizing a unique family of linear ethyl-ene plastomers.

In this study, the heat sealability of laminatedfilms using linear low density polyethylene (LLDPE)and low density polyethylene (LDPE) as the sealantmaterial are investigated. The effects of various com-bination of platen temperature and dwell time to theprocess window of the laminated films are also stud-ied in view of to provide a guideline for the bar seal-ing user when setting up their form/fill/sealmachine.

EXPERIMENTAL

Laminated films

Three samples were used in this study, which arecommercial laminated films of various constructions,thickness, and sealant material uses in flexible pack-aging application. In Samples 1 and 2, LLDPE isused as the sealant material, whereas Sample 3 usingLDPE. Details on laminated film description aresummarized in Table I.

Samples 1 and 2 are manufactured from mono-layer films using the dry-bond process. In this tech-nique, a liquid adhesive (urethanes) is applied to

TABLE IDescription of Laminated Films

Laminated film’sdesignation

Construction description ‘‘outer layer/innerlayer(s)/sealant material (mm)’’

Overallthickness (mm)

1 PET/LLDPE(65) 792 PET/LDPE/F/PET/LLDPE(50) 993 OPP/LDPE/F/LDPE(20) 59

HEAT SEALABILITY OF LAMINATED FILMS WITH LLDPE, LDPE 3737

Journal of Applied Polymer Science DOI 10.1002/app

one substrate. It is then dried with hot air. The driedsurface is then adhered to a second substrate usingheat and pressure. Sample 3 was manufacturedusing extrusion lamination process. In this process, athin layer of LDPE was used to bond together twolayers of film and foil.

Melting point distribution

In this study, Perkin–Elmer differential scanning cal-orimeter, model DSC-7, was used to study the melt-ing point Tm of the sealing materials. Samples (agedat room temperature for at least 7 days) of 6 6 4 mgwere prepared and placed in standard aluminumpans, respectively. The samples were scanned at108C/min scan rate starting at 25–3008C under a he-lium purge gas. Results are summarized in Table II.

Heat flow rate of bar sealing process

The rate of heat conduction Q through a plane layeris proportional to the temperature difference acrossthe layer DT and measured thermal conductance Cof the laminated film (Lees’ Disc Apparatus). That is,

Heat flow rate, Q ¼ CDT (1)

Making of heat-seals

In this project, samples in strip form are preparedfor sealing, in which the sealant material was sealedtogether in the interface to simulate the fin seal formin practice as schematically shown in Figure 1. Lami-nated films are first cut into 15-mm wide strips byLorentzen and Wettre cutter, made in Sweden. Thiscutter ensured that clean-cut edges are produced toprevent premature failures in T-peel test.

Heat-seals were made in the laboratory using amodel HSG/ETK heat-sealer, made in Germany.This device clamps two pieces of filmstrips betweenflat, 10-mm wide heated metal bars. The tempera-ture, pressure, and dwell time of the sealing bars areadjustable. Microprocessor-programmed controllersmaintained and digitally indicated set temperaturefor each bar. Both bars were operated at the sametemperature in these experiments, and kept closedbetween sealing to minimize heat loss and tempera-ture fluctuations. The pressure was fixed at two bars

to simulate the most commonly practiced actualindustrial condition, whereas platen temperatureand dwell time varies. After the heat-seal was made,the sandwich structure was allowed to cool at ambi-ent condition.

Testing of heat-seals

After the heat-seals were produced, it was allowedto condition at room temperature for at least 40 h toachieve chemical stabilization. Aging of heat-sealwas necessary as the strength of seal may change intime, which may be due to the memory of polymer,or thermophysical properties of polymer as the heat-seal samples undergo melting and cooling processes.In practice, packaging goods are aged through stor-age before distribution. Thus, heat-seals made in thisstudy were aged for at least 40 h before tested.

The samples were then peeled apart at room tem-perature in tensile tester of model MICRO 350, usinga 100 N load cell. Each leg of the test specimen wasclamped in the tensile tester. The heat-seal area ofthe specimen are placed at approximately equidis-tant between the clamps. The specimen is alignedin the clamps so that the seal line is perpendicular tothe direction of pull (as shown in Fig. 2). The con-stant rate of loading 300 mm/min with initial jawsseparation of 25 mm was chosen as recommendedby ASTM F88–85. The maximum force required totear apart the seal, and failure mode of each pullwere recorded.

RESULTS AND DISCUSSION

Heat-seal strength and failure modes

The peeling test measures the forces required to sep-arate a heat-seal after it is allowed to cool. The maxi-mum force per width value obtained in such a testis commonly defined to be the heat-seal strength. Allrepresentative values in this article are the average

TABLE IIMelting Point of Sealant Materials Determined

Using DSC

Sealant materials Material forms Melting point (8C)

LLDPE Monolayer film 104.6LDPE Pellet 88.9

Figure 1 Fin seal dimensions.

3738 YUAN ET AL.


of six. In addition, failure modes at each pulled wascarefully examined.

Similar to the results reported by previousresearchers,2,4–6 the heat-seal failure of the laminatedfilms was found greatly influenced by its sealingtemperature and can be discussed in three stages asfollowing:

Heat-seal made at temperature substantiallylower than the melting point of the sealant mate-rial was found to fail in peeling mode failure.Under this failure mode, disentangles and extri-cates chain ends from the opposite surfaceoccurred, in which the heat-seal bond was peeledapart (Fig. 3). This happened when the strengthof the seal is lower than the strength of the lami-nate structures. Hence, heat-seal strength of allsamples reported under this failure mode arelower than the strengths reported in other failuremodes.

The strength of heat-seal increased graduallywith sealing temperature. This is attributed to the

increased of thermal motion (micro-Brownianmovement) of chain segments with temperature,4

and therefore, deepen the original zone of diffu-sion and causes a greater peeling force requiredto peel it apart.

At temperature a few degree Celsius before thefinal melting point, seal strength increasedsharply. Sample failed in either peeling, delami-nating, or tearing mode. Samples may also failwith the combination of delaminating and tearingmodes failure. This is the transition region inwhich failure mode was then changed from peel-ing to delaminating or tearing mode failure, orcombination of both failure modes. Acceptableheat-seal strength was achieved at this stage.

When sealing was done at plateau initiation tem-perature or greater, all heat-seals are found to havefailed under delaminating (Fig. 4) or tearing modefailure (Fig. 5), or combination of both failure modes(Fig. 6). Delaminating mode failure involves tensilebreak of the heat-seal layer, which is thin and weak,followed by separation of the interlaminar bond.While, tearing mode failure was due to the weaken-ing of the seal at its edge as the material fused, orthe weakening of the laminate structures as a resultof interlaminar bond separation, plus the geometryof the peel test itself.

After the acceptable seal strength was achieved,no significant increase of seal strength with tempera-ture were found for Sample 3. In contrast, thestrength of the seal for Samples 1 and 2 continue toincrease gradually with sealing temperature.

The continuously increase of heat-seal strength inSamples 1 and 2 may be due to the decrease of amor-phousness in the LLDPE film. This is because, highertemperature setting deepen the molecular entangle-ment between polymer molecules. Consequently,

Figure 3 Sealing layer torn apart from each other.Figure 4 The laminated film separated from the sealinglayer.

Figure 2 Direction of pull are 908 to the seal in peeling test.



LLDPE chain segments become closely packed, yield-ing a higher degree of crystallinity in the molecularstructure. Therefore, higher strength is obtained sincepolymer with higher crystallinity offer greater physi-cal strength.11

Failure modes of heat-seal had a close relationshipwith its strength. As mentioned above, heat-sealfailed in peeling mode failure having relativelylower strength due to only a slight molecular entan-glement between polymer molecules in the interfa-cial zone. Therefore, heat-seal can easily be peeledapart before any failure occur to the laminate struc-tures which are relatively stronger than the heat-sealwith only slight molecular entanglement.

As platen temperature increased, deeper molecularentanglement between polymer molecules in theinterfacial zone occurred. Thus, the heat-sealstrength increased subsequently as the depth of mo-lecular entanglement increased.

Up to a certain level, when the strength of theheat-seal exceeded the strength of the interlaminarbond, the laminate structures separated from thesealing layer instead of peeling of the heat-seal, andleaving that portion and the other laminated filmintact (Fig. 4). Hence, heat-seal strength reported indelaminating mode failure was relatively higherthan the heat-seal strength reported in peeling modefailure.

In addition to the increase of heat-seal strength,the increase of platen temperature also increased thestrength of the interlaminar bond. However, undermarginal condition, uneven strengthening of inter-laminar bond on heat-seal area occurred. Laminatedfilms tested failed in the combination of delaminat-ing and tearing modes failure. In these failuremodes, the laminated film was first separated intomonolayer structures at the peel line. When come tothe area in which uneven strengthening of interlami-nar bond occurred, the monolayer structure brokesimultaneously after delaminating, due to theuneven tensile stress distribution (Fig. 6) at the heat-seal area.

When heat-seals are made at a higher platen tem-perature after obtaining the combine failure modes,the strength of the interlaminar bond has beenstrengthened evenly at the heat-seal area. Laminatedfilm tested was then failed in tearing mode failure.This is because the strength of the laminate struc-tures (monolayer films) are now relatively lowerthan the strength of the interlaminar bond and theheat-seal. Under this failure mode, the laminatedfilm broke just at the edge of the heat-seal (Fig. 5).Heat-seal strength achieved under this failure modeis the highest when compared with heat-sealstrength reported under other failure modes.

Effect of platen temperature and dwell time

Figure 7 shows the effects of dwell time on heat-sealstrength at different platen temperature for Sample1. At temperatures substantially lower than the melt-ing point of the sealant material, no effects of dwelltime on heat-seal strength is observed. The lowestactivated temperature of this sample was 1128C. Asmay be seen, heat-seal strength changes with dwelltime at this temperature setting. For instance, theheat-seal strength increased from 0.13 to 1.86 N/mmwhen sealed with dwell time setting of 0.1 and 1 s,respectively. This was due to the transient change inthe interfacial temperature with time. After 1 s, noeffect of dwell time on heat-seal strength was

Figure 5 Breaking of the laminated film just at the edgeof the seal.

Figure 6 Combination of delaminating and tearing modes.Figure 7 Graph of heat-seal strength versus dwell time atvarious platen temperatures for Sample 1.

3740 YUAN ET AL.


detected, because the steady-state interfacial temper-ature had been achieved.

At relatively higher platen temperature, the effectof dwell time on heat-seal strength was even greater.For example, at 1158C, 0.2 s was required to achievean acceptable heat-seal strength. However, this canbe achieved at 0.1 s when sealing is made at 1188Cor higher.

Similar results are reported for Sample 2. As maybe seen in Figure 8, acceptable heat-seal strengthwas achieved at 0.4 s, when sealed at lowest acti-vated temperature of 1188C. However, this can beachieved at a faster speed when sealing was made ata higher temperature setting.

Notable that the heat-seal strength of all samplewere slightly decreased at all temperature setting whensealed at 1 s or longer. This may due to the weakeningof the seals as it had been pressed in between theheated bar at the molten state, which reduces the thick-ness of the seals. This may also be caused by thedecrease of material properties when exposed to tem-perature at a relatively longer period of time. Thedecrease of heat-seal strength may not be significantthough, it is however, recommended that the dwelltime should not be set longer than 1 s as this will bringno beneficial effect on the heat-seal strength.

The study of heat-seal strength beyond 1 s ofdwell time is only of theoretical interest. In practice,the setting of dwell time will not be longer than 1 sas most of the packaging machine is operating atvery high speed.

Figure 9 shows the effect of heat-seal strength ondwell time at various platen temperatures for Sam-ple 3. As shown in the graph, heat-seal strength wasdependent mainly on platen temperature. Noincrease of heat-seal strength with dwell time wasdetected. This is because the relatively higher ther-mal conductance of the sample allowed the seal tobe made at shorter dwell time.

Previous researchers have also confirmed that themain factors affecting heat-seal strength is the platentemperature. The results of Theller,2 who looked at the

effect of dwell time at constant platen temperature,indicated that the heat-seal strength is a strong func-tion of platen temperature and is not dependent ondwell time beyond 0.4 s for LDPE film sealed at 106and 1108C. In addition, Yasuo6 stated that tensile fail-ure which occurred inside the heat-seal is more sensi-tive to sealing temperature. Meka and Stehling3 alsoreported that heat-seal strength depends primarily onplated temperature and secondary on dwell time.

Effect of platen pressure

Meka and Stehling3 reported that platen pressurehas no measurable effect on heat-seal strength. Onlya small level of pressure was required to bring thesurface into contact. In this study, the effect of platenpressure on heat-seal strength was investigated onSample 1. The platen temperature of 1138C wasused. As may be seen in Figure 10, no significantchange of heat-seal strength with platen pressurewas found. These confirmed the results reported bythe previous researchers.

Heat-sealing curve

Heat-seal strength versus platen temperature curves atdwell time setting of 1 s are shown in Figure 11. Theheat-sealing curves for all samples were found having

Figure 8 Graph of heat-seal strength versus dwell time atvarious platen temperatures for Sample 2.

Figure 9 Graph of heat-sealing strength versus dwelltime at various platen temperatures for Sample 3.

Figure 10 Graph of heat-seal strength versus sealing pres-sure for Sample 1.



a similar shape to that of the idealized heat-sealstrength curve established by Stehling and Meka.4

Heat-sealing curve obtained on Samples 1 and 2exhibited slightly different with continuouslyincrease of heat-seal strength after the acceptableheat-seal was achieved. Therefore, no plateau regionreported as plateau seal strength is defined to be thestrength when reaches a constant value.

As shown in Figure 11, heat-seal began to bemade at a temperature which was found substan-tially lower then the melting point of the sealingmaterials. This temperature is called seal initiationtemperature, where a measurable but low level ofseal strength was achieved.

As platen temperature increased, seal strengthincreased gradually. At temperature a few degreeCelsius before plateau initiation temperature, sealstrength increased greatly for Samples 1 and 2. Con-trary, Sample 3 exhibited only little increase of itsseal strength. This is attributed to the low achievableplateau seal strength in Sample 3 that is made byextrusion lamination process. The reason is, in extru-sion lamination process, LDPE was used to bond to-gether other substances. However, the intrinsic adhe-sive affinity of LDPE (adhesive agent of extrusionlamination process) toward other substance is lim-ited.1 Therefore, laminated films made by extrusionlamination process, such as Sample 3, have relativelylow level of achievable plateau seal strength.

Plateau initiation temperature

For all samples, the temperature where the plateaubegins (Tpi) corresponds closely to the final meltingpoint (Tmf) of their sealant materials. This was shownin Figure 12, where the Tpi is plotted against Tmf forboth samples. The straight line in this figure is thelocus Tpi ¼ Tmf, and all points fall closely to thislocus.

Sample 3 that used LDPE as the sealant materialrequired Tpi to be set 188C higher then the Tmf,whereas, Samples 1 and 2, which use LLDPE as thesealing material, Tpi required to be set about 108Chigher then the Tmf.

Thermal conductance

Thermal conductance of each sample was measuredusing the Lees’ Disc Apparatus method. Results inTable III shows that Sample 3 have the highest ther-mal conductance, followed by Sample 1 the middleand Sample 2 the lowest.

Heat flow rate

As defined in eq. (1), heat flow rate for conductionstate at steady flow condition is proportional to theconductance of laminated films and heat force of theprocess. Assuming the outer surface temperature ofthe laminated film rise to the thermal condition ofplaten temperature at the moment of contact, and iskept at this thermal condition while the interfacialtemperature are at room temperature during heat-sealing cycle, and steady heat flow conditionoccurred, the heat flow rate calculated using eq. (1)for each sample at the plateau initiation temperaturesetting were as shown in Table IV.

Graph of heat flow rate versus dwell time at thelowest activated temperature (Fig. 13) shows thatthe dwell time of heat-sealing process correspondsclosely with the heat flow rate of the laminatedfilms. Samples with greater heat flow rate weresealed with shorter dwell time because heat flowwas faster in those samples.

Figure 11 Graph of heat-seal strength versus platen tem-perature at dwell time of 1 s.

Figure 12 Graph of plateau initiation temperature versusfinal melting point.

3742 YUAN ET AL.


Because heat flow rate is directly proportional toplaten temperature and thermal conductance of thelaminated film, thus in the case when the use of lon-ger dwell time is not permissible, a higher platentemperature setting can be used to achieve the sameflow rate. This is evident in Samples 1 and 2, whereTpi setting for Sample 2 was 38C higher than Sample1 when sealed at the same dwell time.

However, it is recommended that laminated filmswith higher thermal conductance is preferable as itallowed the use of shorter dwell time and lowerplaten temperature setting. The former have directimpact on the production speed and the latter haveless harmful effect on the product and uses lowerenergy in a long run.

Process windows of bar sealing application

Figure 14 shows the general process window of barsealing application. The process window can beapproximately described by several quantities illus-trated in the figure:

1. The left vertical border, this border indicatedthe shortest dwell time, which was determinedby the minimum possible time setting of thesealing machine. However, this do not limitsthe practical usage of the process window asalmost all the machines having the same limitof 0.1 s, and dwell time required in commercialproduction rate is commonly longer than 0.1 s.

2. The right vertical border, this border indicatedthe longest dwell time suggested in the processwindow. Longest dwell time of 1 s was selectedbecause all samples exhibited approximatelyconstant value or slight decrease of the heat-seal strength after 1 s dwell time (Figs. 7–9). Inpractice, dwell time required for commercialproduction rate is usually at the setting of 0.3–

0.7 s. Hence, 1 s of longest dwell time in thewindow is sufficient to provide adequate refer-ence for practical usage.

3. The lower border, the lowest activated platentemperature setting for each particular dwell timeis revealed in the lower border. Setting fall onthis border is the optimum combination of platentemperature and dwell time, because no exces-sive temperature was used under this setting.

4. The upper horizontal border, heat-seal can bemade in a rage of platen temperature setting,however, lower sealing temperature uses lessenergy, allow the package to be handle quicker,and have less potential effect on the containedproduct.11 Therefore, higher platen temperatureof 158C after the lowest activated temperaturesetting is selected to be the highest tempera-ture limit of the window. Combination of platentemperature and dwell time after the highesttemperature limit is not recommended becauseit is not efficient.

Figures 15–17 show the process windows of Samples1–3 respectively. All points drop inside the windoware combination of platen temperature and dwell timethat produced acceptable heat-seal. Points drop on thelower borderline were optimum combination of platen

Figure 13 Graph of heat flow rate versus dwell time atlowest activated temperature.

TABLE IVHeat Flow Rate Calculated at the Plateau Initiation Temperature

Laminated filmdesignation

Construction description ‘‘outer layer/innerlayer(s)/sealant material (mm)’’

Heat flow rate(1 � 105 W/m2)

1 PET/LLDPE(65) 1.2572 PET/LDPE/FOIL/PET/LLDPE(50) 1.3213 OPP/LDPE/FOIL/LDPE(20) 1.367

TABLE IIIThermal Conductance of Laminated Films Determined

Using Lees’ Disc Apparatus

Sealant materials Material forms Melting point (8C)

LLDPE Monolayer film 104.6LDPE Pellet 88.9



temperature and dwell time, because no excessivetemperature was used.

In many cases, a form/fill/seal machine used toproduce the heat-seal packages was set to run at adesired production speed, and so dwell time is vir-tually a given. Hence, process window become avery helpful guide to provide information aboutwhich platen temperature to be used to produce anacceptable heat-seal at the optimum condition. Forexample, suppose Sample 2 is to be used, and themachine is to be run at the dwell time setting of0.3 s, the optimum platen temperature setting wouldthen be at 1218C (Fig. 16). However, in some cases,high production rate is not required which might bedue to a lower market demand of the products.Thus the machine is designed to run at lower speedincorporated with a longer dwell time. In that case,

the combination of 0.5 s dwell time and 1188C platentemperature can certainly reduce the energy con-sumption by the entire process as lower temperaturesetting is used.

As illustrated in the bar sealing process windows(Figs. 15–17), a very short dwell time of 0.1 s can beused while combined with a very high temperature.However, care should be taken while selecting thematerial for the outer construction of the laminatedfilm, because the outer surface of the laminated filmwould be contacted directly to the heated bar. There-fore, materials of higher melting point should beconsidered to avoid damage of the seal area, whichcan occur if the heated bar temperature exceed themelting temperature of the material of the outer con-struction.

Figure 14 General process window of bar sealing applica-tion.

Figure 15 Process window of Sample 1 at 2 bar.



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For process window of Sample 3, the lower borderof the process windows was form by a horizontalline. This indicated that the acceptable heat-sealcould be obtained at any dwell time setting for aslong as the activated temperature level has beenachieved. As discussed above, shorter dwell timecan be used when higher heat flow rate is provided.But unlike other samples, the very short dwell timeof 0.1 s can be used at the lowest activated tempera-ture setting for this sample was attributed to its verygood thermal conductance. Therefore, laminatedfilms having higher thermal conductance providebetter combination of plated temperature and dwelltime, which helps to reduce energy consumptionin the heat-sealing process, by allowing lower platentemperature setting even though a very short dwelltime is used.

GENERAL CONCLUSIONS

The main conclusions that can be derived from thestudy are as follows:

1. Bar sealing process window for each sampleswere developed. Information about the opti-mum combination of platen temperature anddwell time for each individual laminated filmcan be obtained in the respective process win-dows. Thus, optimum machine setting can bepredicted when the laminated films are to beused in the real application and machine settingtime can be reduced.

2. The effect of bar sealing process variables onheat-seal strength of laminated film has beenquantitatively determined. Heat-seal strengthwas determined primarily by the platen temper-ature, and secondly by dwell time. Higherplaten temperature was required to seal lami-nated film having lower thermal conductance.However, shorter dwell time was required ifthe increase of the platen temperature hadincreased the heat flow rate of the process.Dwell times of the order of 0.3–0.5 s requiredfor commercial productions rates can beobtained by proper selection of the individualplated temperature. The heat-seal strength isnot significantly affected by pressure. However,

some level of pressure is necessary to bring twoheated laminated films into intimate contact toeffect sealing.

3. The achievable heat-seal strength was alsodetermined by the lamination process used toproduce the laminated film, and sealant mate-rial used in the laminated film. Laminated filmsmade from extrusion lamination process pro-vide lower achievable heat-seal strength whencompared with the laminated films made fromdry-bond lamination process.

4. Platen initiation temperature closely corre-sponds to the final melting point of the sealantmaterials. To achieve plateau seal strength,laminated films using LDPE and LLDPE assealant materials required platen temperature tobe set slightly higher than the final meltingpoint of the sealant materials. Outer construc-tion with higher melting temperature was pref-erable for laminated film that required to besealed using higher platen temperature to avoiddamage on the outer seal area.

5. Strength of heat-seal and its failure modes areclosely related. Heat-seal strength achievedunder tearing mode failure is the highest, fol-lowed by heat-seal strength achieved underdelaminating mode failure, and the heat-sealstrength achieved under peeling mode failure isthe lowest.

References

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Heat sealability of laminated films with LLDPE and LDPE as the sealant materials in bar sealing...

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