effects of temperature profiles through preform thickness on the properties of reheat–blown pet...

Advances in Polymer Technology, Vol. 17, No. 3, 237–249, 1998Q 1998 by John Wiley & Sons, Inc. CCC 0730-6679/98/030237-13

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Effects of Temperature ProfilesThrough Preform Thickness onthe Properties of Reheat–Blown PET Containers

G. VENKATESWARAN, M. R. CAMERON, and S. A. JABARINPolymer Institute and Department of Chemical Engineering, College of Engineering,University of Toledo, Toledo, Ohio 43606

Received: August 5, 1997Accepted: January 22, 1998

ABSTRACT: Nonuniform temperature distributions through preformsidewalls have been studied in relation to their effects on the orientation andfunctional properties of reheat-and-blown PET containers. Each temperatureprofile, through the preform thickness, was computed from processingconditions by solving the energy equation with radiation as the heating source.The computed temperature profile was verified by measuring the inside andoutside preform surface temperatures using infrared thermocouples. Bottleswere then blow molded with various temperature profiles and measurementswere carried out on samples cut from the sidewalls. Thickness distributions andaxial ratios were determined as were changes in mechanical, optical, and barrierproperties. It was found that optical anisotropy through bottle wall thicknesswas minimal when the inside preform surface was at a higher temperature thanthe outside surface. Densities of the bottle sidewalls were found to be higher forbottles blow molded at higher average temperatures and there were smallincreases in density for bottles blown with the preform inside surface at ahigher temperature than the outside surface. Haze measurements showed that,to obtain optically clear containers, bottles with an inside hoop ratio of 5.25should be blown with an inside surface temperature of at least 1007C. q 1998John Wiley & Sons, Inc. Adv Polym Techn 17: 237–249, 1998

Introduction

P oly(ethylene terephthalate) (PET) has beenwidely used for the production of rigid, light-

Correspondence to: S. A. JabarinContract grant sponsor: PET Industrial Consortium

weight, optically clear containers. The reheat-and-blow process is often the process of choice for mak-ing PET bottles on a large scale. In this process, apreform is first made by injection molding and isallowed to cool to room temperature. It is then re-heated by infrared radiation to the appropriate ori-entation temperature and stretch blow molded intoa bottle. Heating by infrared radiation depends on

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EFFECTS OF TEMPERATURE THROUGH PREFORM THICKNESS

the heater temperature, the absorption characteris-tics of the plastics, and the geometry of the heatingsystem. During the reheating step, there is a non-uniform temperature distribution in both the axialand hoop directions. Biaxial stretching results insubstantial amounts of induced orientation in thesidewall of the container. The amount of inducedorientation depends on molecular weight, stretchratio, stretching speed, and temperature. Preformsused for 2-L bottles are quite thick (about ),4 mmand therefore there is a significant difference be-tween the inside and outside hoop stretch ratios.This variation in stretch ratio along with the non-uniform temperature distribution causes orientationto vary through the thickness of the bottle wall. Be-cause functional properties such as modulus, en-ergy absorption, and permeability depend on theamount of induced orientation, there will be a radialvariation of properties through the thickness direc-tion. In this work, we have studied this radial vari-ation of properties resulting from differences in in-duced orientation or anisotropy through thethickness direction of stretch-blow-molded contain-ers.

Investigation of orientation and structure devel-opment in PET films and bottles has been the subjectof interest for the past couple of decades. Heffelfin-ger et al.1 studied the structure and orientation ofPET films in terms of trans and gauche isomers. Therelationships among molecular orientation, physicalproperties, and molecular weight of PET and theirdependence on orientation variables has been stud-ied by Jabarin.2 Bonnebat et al.3 studied uni- andbiaxial stretching of various polymer films and therelationship between stress and recoverable strain.The nature of stress–strain curves and the strain-hardening parameter for biaxially stretched PETsheets has been studied by Jabarin et al. Some of4,5

the earlier studies on stretch blow molding con-cerned structure development in PET bottles. Bon-nebat and coworkers6 studied the effects of resinmolecular weight on preform stretching behaviorand the development of orientation in bottles. It wasfound that resin molecular weight directly controlsthe natural draw ratio required to obtain a uniformbottle wall thickness. Detailed investigation of thekinematics of stretch blow molding and the struc-ture development in terms of crystallinity, orienta-tion, and morphology during stretch blow moldingwas done by Cakmak et al. They developed a cor-7,8

relation between refractive index and percent crys-tallinity for films and used it to determine the sur-face crystallinity of bottles.

The effect of process parameters and preheat andequilibration times on the properties of reheat-and-blown containers was investigated by Kim. The9,10

effects of infrared lamp temperature and other var-iables on the reheat rate of PET has been studied byShelby. He studied the wavelength dependence of11

the absorption characteristics of PET by changingthe lamp temperature and its effect on the preheat-ing time. Esser and others have reviewed the use12

of infrared radiation during the processing of plas-tics. There is little published information availabledescribing the effects of temperature profiles anddifferent blow temperatures on the properties of re-heat-and-blown containers. In the current work, thetheoretical radial temperature distribution wascomputed from process conditions. The apparenttemperature for a given theoretical temperatureprofile was also computed. Settings were thendetermined to obtain different temperature pro-files and bottles were stretch blow molded withdifferent radial temperature distributions. Blownbottles were then characterized using various tech-niques.

Theoretical Computation ofTemperature Profile

Heating by infrared radiation is often used instretch blow molding and thermoforming pro-cesses. Infrared heating is particularly useful as theenergy directly penetrates to the material interiorgiving more rapid and uniform heating than thatachieved with the slow process of heat conduction.There are two important aspects to the process ofheating by infrared radiation: (i) spectral emissionbehavior of the radiator; and (ii) spectral absorptioncharacteristics of the plastic.

A body at a high temperature emits radiation inall directions. The hemispherical total emissivepower of a blackbody radiating into vacuum isgiven by the Stefan–Boltzmann law:

4E 5 sT (1)

where s is the Stefan–Boltzmann constant (5.67 3and T is the absolute temperature of24 2 410 W/m K )

the source. The intensity of radiation emitted variesover a range of wavelength. The fraction of emis-13

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sive power within a wavelength (l) range is givenby:

F 5 F 2 F (2)l T2l T 02l T 02l T1 2 1 2

where

3h15 hF 5 1 2 dh (3)E02lT 4 hp 0 e 2 1

where and C2 is a constant. Tabulatedh 5 C /lT,2

values are available for the fraction of blackbodyemissive power as a function of lT. Polynomial13

expressions have been derived, relating the frac-tional emissive power to lT. The following expres-sion has been used in this work:13

2 415 1 h h h3F 5 1 2 h 2 1 202lT S4p 3 8 60 5040

6 8h h1 2 , h . 2 (4)D272160 13305600

As the temperature of the heater or blackbodysource is increased, the wavelength at which themaximum intensity occurs is shifted toward lowerwavelengths. Thus, by changing the heater temper-ature, one can change the amount of radiation emit-ted in a given wavelength band.

In the stretch-blow-molding process, the preformis heated to the appropriate orientation temperatureby radiation from a bank of infrared heaters. Thepreform sits on a revolving mandrel so that the heat-ing is uniform in the circumferential direction. Re-flectors are used so that optimal use of emitted ra-diation is possible. The amount of radiation incidenton the plastic surface depends on the geometry ofthe system and the arrangement of heaters with re-spect to the preform. To account for this, a factorcalled “view factor,” or “configuration factor” is in-troduced. It represents the fraction of the emittedradiation leaving the heater and reaching the pre-form surface. Thus, the amount of radiation incidenton the preform surface is given by where9I 5 I F ,0 o G

I0 is the amount of radiation emitted by heaters andis the view factor for the system. The exact com-FG

putation of the view factor is very difficult and dif-ferent approaches such as Monte Carlo simulationand contour integration are being used. In this12

work, the computation of view factor was simplifiedby modeling the heaters as a group of parallel stripsand computing the view factor from each one of

these strips to a differential element on the preformsurface.

The amount of radiation absorbed depends onthe absorption characteristics of the material. Theabsorption of radiation is due to the excitation ofmolecules capable of vibration and it increases therotational and vibrational energy of the moleculesresulting in an increase in temperature. Therefore,depending on the functional groups present, eachplastic will have a characteristic absorption spec-trum. PET is mostly transparent to wavelengths be-low and shows strong absorption bands2.1 mmaround and3.4 mm 7–8 mm.

If the total intensity of incident radiation is 9I ,0

then the intensity in a given wavelength band isgiven by where is the fraction of emitted ra-9I f , f0 i i

diation in a given wavelength band. Furthermore,if r is the reflectivity of the plastic (usually taken as5%), then the amount of radiation entering the plas-tic is given by Consider a small differ-9I (1 2 r) f .0 i

ential element of plastic of thickness Dx. The inten-sity of radiation decreases exponentially withthickness. The amount of energy absorbed in agiven wavelength band is then given by:

(1 2 r)9 2a x 2a (x1Dx)i iS 5 I f (e 2 e (5)i 0 i

Dx

The total energy absorbed per unit volume is ob-tained by summing up over all wavelength bandsand is given by:

(1 2 r)9 2a x 2a (x1Dx)i iS(x) 5 I f (e 2 e (6)O 0 i

Dx

The temperature profile through thickness can beobtained by solving the one-dimensional energyequation with the total energy absorbed [S(x)] as thesource term, along with appropriate boundary con-ditions.

2dT d TrC 5 k 1 S(x) (7)p 2dt dx

Equation (7) was solved using a finite-differencetechnique. The effects of curvature of the preformand the reheating of the preform from inside bytransmitted radiation were also included. The an-gular dependency of reflectivity was not consid-ered. The heat transfer coefficient was computed as-suming an air velocity of The transmittance5 m/s.

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curve for the PET preform was obtained using anIR spectrometer for the wavelength range0.33–2.5 mm.

Computation of ApparentTemperature

Infrared pyrometers have been used for the mea-surement of surface temperatures of the preform,prior to blowing. In the case of opaque materials,the temperature determined from the measured in-frared radiation corresponds to the surface temper-ature, because radiation almost exclusively comesfrom the surface. For semitransparent materials,such as PET, the measured radiation comes not onlyfrom the surface but also from inside the material.If there is a nonuniform temperature distributionthrough the thickness direction, then the tempera-ture obtained will correspond to some weighted av-erage of the distribution called the apparent tem-perature In this work, the method suggested(T ).ap

by Hajji and Spruiell was used to determine the14

apparent temperature that would be measured byan IR pyrometer, if the theoretically computed tem-perature profile was present. The apparent temper-ature was computed from the following ex-(T )ap

pression:

l2 (1 2 r )(1 2 t )l l bbI (T ) dlE l apl 1 2 r t1 l l

l 12 a (1 2 r )l l5 [exp(2a g)E E l2 2l 1 2 r t 01 l l2 bb1 r t exp(a g)]I (T) dy dl (8)l l l l

where (T) is the spectral intensity of a blackbodybbIl

at temperature T, and are the spectral normalr tl l

reflectivity and transmissivity of the sheet, and isal

the optical thickness of the sheet, which is the prod-uct of the absorption coefficient of the material andthickness of the sheet. In this work, was taken torl

be a constant and was measured as a function oftl

wavelength using a Fourier transform infrared spec-trometer in the wavelength range in which the IRpyrometer measures radiation (6.5–14.0 mm). Thevariable y represents the thickness direction. Equa-tion (8) was solved numerically using Simpson’srule by calculating the RHS and iterating the ap-parent temperature until the two sides of the equa-tion matched.

Experimental

MEASUREMENT OF SURFACETEMPERATURES

Measurements of surface temperatures weredone using infrared pyrometers. A pyrometer mea-sures the amount of radiation emanating from asource and produces a signal proportional to theamount of radiation received. The pyrometers werecalibrated using a blackbody source and also usingradiation from a PET preform. A Watlow heater/controller and a resistance temperature detector(RTD) were used for calibration. The temperaturesof the RTD and voltage signals from the pyrometerswere recorded using a Camile data acquisition sys-tem and a calibration curve was obtained. The volt-age signal received by the pyrometers due to radi-ation could then be converted to a temperature scaleusing the calibration curve. Both the inside andoutside surface temperatures of the preform, afterheating by IR heaters, were recorded as functionsof soak times using the Camile data acquisitionsystem.

BLOW MOLDING PARAMETERS

Bottles were blown with different radial temper-ature profiles, with the outside surface at a highertemperature than the inside and also with a re-versed profile (i.e., with inside surface at a highertemperature). PET preforms used were supplied byEastman Chemical Company and were injectionmolded from PET 9921w resin, which is a copoly-mer of PET containing cyclohexane dimethanol(CHDM). Preforms had an IV of 0.74 and were

in weight with an external diameter of55 gand thickness in the straight sec-29.2 mm 3.94 mm

tion. The 2-L blow mold used had a hemisphericalbottom. A lab-scale reheat-and-blow machine,equipped with a Sidel-type heater box, was used.The heating system consisted of 10 GTE Sylvaniaquartz lamps rated at with a peak filament1600 Wtemperature of at The speed of the2200 K 240 V.heater box could be adjusted to give different heat-ing times and a timer that controls the mandrel de-lay could be manipulated to give different soaktimes. A stretch rod pressure of and low-blow80 psiand high-blow pressures of respec-100 and 210 psi,tively, were used. Low-blow delay was and800 ms



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FIGURE 1. Blackbody radiation spectrumsuperimposed on the transmittance curve for PET.

low-blow and high-blow on times were 500 andrespectively.4000 ms,

PROPERTY EVALUATION OF BOTTLES

Thickness distribution and axial stretch ratio. A gridwas marked on the preform and the axial stretchratio was determined from the ratio between dis-tances from fixed points on the preform before andafter inflation. The bottles were cut open and thethickness distribution was measured along thelength of the bottle.

Mechanical properties. Mechanical properties oftensile specimens cut in the hoop direction were de-termined using an Instron tensile testing machine.Young’s modulus, tensile energy absorption (areaunder the load–elongation curve), yield strength,and percent elongation to break were measured ata crosshead speed of The die-cut dog-bone-2 in./s.shaped samples had gauge lengths of and0.878 in.widths of 0.187 in.

Optical properties. Refractive indices in the axial,hoop, and thickness directions were measured us-ing an Abbe refractometer. Percent haze was mea-sured using a Gardner haze meter.

Density and percent crystallinity. Density at 257Cwas measured using a density gradient columnmade of calcium nitrate solution and calibrated bysuspending beads of known density. The values for100% crystalline and amorphous PET used in thiswork were 1.455 and 1.333 respectively.3 15,16g/cm ,Percent crystallinity was calculated from the mea-sured density values. Percent crystallinity was alsodetermined, using a differential scanning calorime-ter (DSC), with a heating rate of 107C/min.

Barrier properties. Oxygen permeability at 237Cwas determined by measuring the transmission rateusing a MoCon oxygen permeability whole packagetester.

Shrinkage. Shrinkage measurements were carriedout using a modified Harrop model dilatometer.Samples were cut in the hoop direction using aThwing–Albert precision cutter. The cut sampleshad a width of and a gauge length of about0.1 in.

A heating rate of 47C/min and a nitrogen at-1.0 in.mosphere were used.

Transmittance curve. The transmittance curve ofPET was obtained using a Cary Model 17D spectro-photometer in the wavelength range 0.33–2.5 mm.A Fourier transform infrared spectrometer was usedto measure the transmittance of the PET sample in

the spectral range in which the IR thermocouple re-ceives radiation (6.5–14.0 mm).

Results and Discussion

The energy equation was solved numerically us-ing a finite difference method. The blackbody hemi-spherical emissive power is a function of absolutetemperature and wavelength and is given byPlanck’s spectral distribution of emissive power.13

The blackbody radiation spectra computed for dif-ferent heater voltages of 190, 210, and (and220 Vhence different heater temperatures) are shown inFigure 1. Also shown in Figure 1 is the transmittancecurve for a 3.3-mm-thick PET sample obtained us-ing the IR spectrometer and corrected for surfacereflection. It can be seen that, by changing the heatertemperature, the amount of incident radiationin a given wavelength band can be changed. Thepenetration of absorbed energy through dimen-sionless thickness [radiation source term, S(x)]is plotted in Figure 2 for different heater voltages.Here, 0 represents the outside surface and 1 repre-sents the inside surface. The energy absorbed de-creases exponentially through thickness and theslight upswing seen at the inside surface is due toreheating of the surface due to transmitted radia-tion. The evolution of the temperature profile

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FIGURE 2. Penetration of absorbed energy throughthickness of the preform.

FIGURE 3. Evolution of temperature profile throughdimensionless thickness as a function of time.

TABLE ITemperature Profiles and Blow Molding Parameters

Sample No.Outside Temp.

(&C)Inside Temp.

(&C)Heater Voltage

(V)Heating Time

(s)Soak Time

(s)

1 80 90 190 24 492 90 80 190 21 153 90 100 210 23 454 100 90 210 21 155 100 110 220 25 426 110 100 220 23 15

through the preform thickness as a function of totaltime is shown in Figure 3. Initially,(heat 1 soak)the outside surface is at a much higher temperaturethan the inside and, as soak time increases, the pro-file stabilizes and at long soak times, the profile isreversed due to higher convection to the outside. Bymanipulating the heater voltage, heating time, andsoak time, different temperature profiles throughthe thickness of the preform can be generated. Thesettings required to obtain different temperatureprofiles are summarized in Table I.

The apparent surface temperatures were com-puted for each temperature distribution using themethod suggested by Haaji and Spruiell. The ap-14

parent surface temperatures were also measured as

a function of soak time using IR pyrometers. Figure4 shows the computed and measured apparent tem-peratures for a given heater voltage of and a190 Vheating time of The agreement was reasonably24 s.good, considering the number of assumptions madein solving the energy equation and the difficulty in-volved with surface temperature measurement. Af-ter the temperature profile had been ascertained,bottles with different temperature distributionsthrough the thickness direction were blow moldedand characterized.

The thickness distributions for bottles blownwith different temperature profiles are shown inFigure 5. Machine parameters, such as stretch rodvelocity and low-blow delay, which influence the



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FIGURE 4. Plot of computed and measured apparentsurface temperatures versus soak time.

FIGURE 5. Material (thickness) distribution for bottlesblown with different temperature distributions.

FIGURE 6. Axial ratio variation along length of thebottle.

material distribution, were kept constant. On the xaxis, the distance along the length of the bottle fromthe cylindrical bottom was plotted. A distance of upto represents the panel area on the bottle sur-9 cmface. The thickness distribution was uniform for bot-tles blown at the lower temperature end—that is,with 80–90 (outside temperature 807C and insidetemperature 907C) and 90–80 temperature profiles.This is because, for bottles blown at the lower tem-perature end, the natural draw ratio is lower result-ing in uniform material distribution. Bottles blownat the higher temperature end show more variationsin the thickness distributions in the sidewall por-tions of the bottles.

The variation of axial stretch ratio along thelength of the bottle is shown in Figure 6. The valuesshown are average values based on measurementsmade on three containers and the variation for eachprofile is within 3%. It can be seen that there is sub-stantial variation in axial ratios in the sidewall forbottles blown with different temperature distribu-tions. This variation in axial ratio is important be-cause the measured properties on the bottle side-wall depend not only on the temperature profilethrough thickness, but also on the axial ratio. Thisvariation in axial ratio must be considered, whiletrying to understand the effects of nonisothermaltemperature distribution on the mechanical andother properties of blown containers. The axial ratiowas also computed from thickness measurements

and geometry considerations, and the two were inreasonable agreement.

Figure 7 shows the modulus variation for bottlesblown with different temperature profiles. The xaxis gives the temperature profile (outside and in-

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FIGURE 8. Variation of tensile energy absorption withtemperature profiles.

FIGURE 7. Variation of modulus with differenttemperature profiles.

side surface temperatures) and the axial ratio at thelocation where the test specimen was cut. Thus, eachpoint represents values measured from bottlesblown with different temperature profiles. Themodulus remains more or less a constant and de-creases to some extent for bottles blown at thehigher temperature end. Modulus depends on thevolume fraction of crystals and the amorphous ori-entation or the fraction of taut tie molecules.17

Stretching at a higher temperature at the highstretch speeds encountered in bottle blowing resultsin higher cystallinity with an amorphous phase thatis less strained. These counteract each other to someextent and there was only a small decrease in mod-ulus in the temperature range studied.

Figure 8 shows tensile energy absorption for bot-tles blown with different temperature profiles. Theenergy absorption is low for bottles blown with alower average temperature—that is, for bottlesblown with 80–90 and 90–80 temperature profiles,and it increases as the blow temperature increases.The reason for lower energy absorption could betwofold. First, when stretched at a lower tempera-ture at high speeds, the amorphous orientation isquite high and the taut tie molecules are alreadyfully extended and the sample fails by chain rup-ture. Second, it was found that bottles blown at the

lower temperature end showed stress whitening orpearlescence, due to the presence of microvoids inthe sidewall. When such a sample is subjected to atensile test, these microvoids coalesce into a micro-crack resulting in premature failure. In the case ofbottles blown at the higher temperature end, thetaut tie molecules are more relaxed and percentelongation to break is higher resulting in higher ten-sile energy absorption before failure. It can also beseen that tensile energy absorption was higherwhen the preform inside surface was at a highertemperature prior to blowing.

The change in density in the sidewalls of bottlesblown with different temperature profiles is shownin Figure 9. It can be seen that bottles blown at thelower temperature end showed significantly lowerdensity. This is because PET structure is quite rigidand the chains do not have enough mobility at thelower temperature to align and orient themselves inthe stretch direction at the stretch speeds encoun-tered in bottle blowing. This results in formation ofdefects and microvoids that manifest as pearlesc-ence or stress whitening. Bottles blown at a higheraverage blow temperature showed higher density.In this case, the chains are mobile enough to be ori-ented in the direction of stretch and because thestretching speed is high there is not much loss inorientation, resulting in higher density. Also, bottlesblown with the inside surface at a higher tempera-



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FIGURE 9. Density of sidewall for bottles blown withdifferent profiles.

FIGURE 10. Crystallinity measured by density andDSC for bottles blown with different profiles.

FIGURE 11. Percent haze for bottles blown withdifferent profiles.

ture showed a slightly higher density comparedwith those blown with a reversed profile. The valuesof 100% crystalline and amorphous PET used are forhomopolymer PET. In this work, copolymer PEThas been used. Hence, the computed percent crys-tallinity values are not exact, but are valid for com-paring the values for different profiles.

Figure 10 shows percent crystallinity obtainedfrom density and differential scanning calorimetry(DSC) measurements for bottles blown with differ-ent temperature profiles. Percent crystallinity valuesmeasured by DSC were significantly higher thanthose obtained from density measurement, but theoverall trend was similar, with bottles blown athigher temperature showing higher percent crystal-linity. The differences in percent crystallinity valuesobtained from different techniques, such as x-ray,density, and DSC, have been studiedextensively and the higher values obtained18–20

in DSC can be attributed to the melting and recrys-tallization taking place during the course of thescan.

Figure 11 shows percent haze plotted for bottlesblown with different temperature profiles. Percenthaze has been defined as the percentage of trans-mitted light that is scattered at an angle greater than2.57 from the incident beam. The amount of haziness

increases as the scattering of light increases andscattering can be considered to arise from randomfluctuations in dielectric constant or refractiveindex. When voids are present, they act as very21

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FIGURE 13. Birefringence (axial) for bottles blown withdifferent profiles.

FIGURE 12. Birefringence (hoop) for bottles blownwith different profiles.

efficient scattering centers resulting in a high degreeof percent haze. Bottles blown with the insidesurface at 807C and outside surface at 907C,show pearlescence due to the presence of voidsand, therefore, these bottles showed a high amountof percent haze. The amount of haze is very highwhen the inside surface is at 807C, for an insidestretch ratio of 5.25. The amount of haze decreaseswhen the inside surface temperature is at least1007C.

The refractive indices in the three directions weremeasured using an Abbe refractometer. The bire-fringence in the hoop direction is computed fromthe difference in refractive index in the hoop andnormal directions and is plotted in Figure 12 forboth inside and outside surfaces. The inside surfacealways has a higher birefringence than the outsidesurface, regardless of temperature profile, due to thehigher stretch ratio on the inside. The difference inbirefringence values between the inside and outsidesurfaces or the optical anisotropy through the thick-ness attains a minimum when the inside surface isat a higher temperature. Birefringence for the insidesurface increases with a reduction in surface tem-perature.

Figure 13 shows birefringence in the axial direc-

tion for bottles blown with different profiles. Thebirefringence values for the inside and outside sur-faces are quite close to each other with the valuesfor inside surfaces being slightly higher. This im-plies that the axial ratios for the two surfaces arealmost the same, with the inside surface having aslightly higher axial ratio. Miller marked a grid22

pattern on both the inside and outside surfaces of apreform and measured the hoop and axial ratios. Hefound that the axial ratio on the inside is slightlyhigher than that of the outside surface. It can be seenthat bottles blown at a higher average temperaturewith a 100–110 profile still showed a high birefrin-gence value in the axial direction. Birefringence isan average value measured over both crystallineand amorphous phases. Bottles blown at a highertemperature showed a higher fraction of crystallin-ity, and this compensates for the loss of amorphousorientation to a slight extent. A more important fac-tor is the axial ratio itself, which is very high (3.27)in this case. Thus, for birefringence in the axial di-rection for bottles, axial ratio in addition to temper-ature profile has to be considered.

It is known from the Lorenz–Lorentz equationthat there is a linear relationship between averagerefractive index and density. De Vries et al.3 andCakmak et al. developed correlations relating the23

average refractive index to the density of the sam-ples:



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TABLE IISurface Density Values Calculated from Average Refractive Indexa

Sample No. Profile

Inside Surface

De Vries Cakmak

Outside Surface

De Vries CakmakBulk

Density

1 80–90 1.3298 1.3335 1.3436 1.3513 1.35152 90–80 1.2534 1.2346 1.3480 1.3570 1.34913 90–100 1.3571 1.3688 1.3574 1.3693 1.35854 100–90 1.3289 1.3323 1.3502 1.3599 1.35795 100–110 1.3606 1.3734 1.3586 1.3707 1.36556 110–100 1.3563 1.3678 1.3565 1.3680 1.3627

expressed in grams per cubic centimeter.a Values

FIGURE 14. Oxygen permeability for bottles blownwith different profiles.

De Vries equation:2n 2 1

5 0.2471r (9)2n 1 2

Cakmak equation:2n 2 1

5 0.1904r 1 0.0746 (10)2n 1 2

where n is the average refractive index of the sam-ple. Because the refractive indices for both surfacesof the bottles have been measured, surface densitycan be computed from average refractive index val-ues, as a measure of anisotropy through the thick-ness of the blown container. Surface density valueswere computed using the above relations and theresults are summarized in Table II. The bulk den-sities, obtained with a density gradient column, arealso included for comparing the values. It can beseen that, for bottles blown with temperature pro-files 80–90, 90–80, and 100–90, the inside surfacedensity is less than or about 1.333, which is the den-sity of amorphous PET. Bottles blown with theseprofiles also showed stress whitening or pearlesc-ence on the sidewalls. Voids formed on the surfacecaused a lower average refractive index for thesebottles resulting in a lower surface density. Possibly,there is a correlation between the difference in bulkdensity and the surface density and the number andsize of voids present on that surface.

Figure 14 shows the oxygen permeability of bot-tles blown with different temperature profiles. AMoCon oxygen permeation whole package testerwas used to measure the oxygen transmission rateand the permeability was computed by taking intoaccount the surface area and thickness distribution.The results are summarized in Table III. It can be

seen that permeability is not significantly affectedby temperature profiles. Bottles blown at a higheraverage temperature showed a slightly lower per-meability due to higher percent crystallinity. Thepermeability of an amorphous PET film has beenreported to be Thus,3 2 90.0699 cm mils/in. day atm.the bottles showed about 40% reduction in perme-ability over that of an amorphous film. In this typeof measurement, the contribution due to amorphousregions in the neck area is also included. Oxygenpermeability of samples cut from the sidewall re-gion also showed the same trend, with bottlesblown at higher temperature showing a small de-crease in permeability.

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TABLE IIIOxygen Permeability for Bottles Blown with Different Temperature Profiles

Sample No. ProfileTransmission rate

(cm3/day atm)o Ai /li

(in.2/mils)Permeability

[cm3(mils)/in.2 day atm]

1 80–90 0.357 8.22 0.04342 90–80 0.343 7.95 0.04313 90–100 0.349 8.77 0.03984 100–90 0.336 8.52 0.03945 100–110 0.367 8.64 0.04246 110–100 0.332 8.60 0.0386

FIGURE 15. Plot of shrinkage onset temperature andtotal shrinkage (at 122& C) for bottles blown with differentprofiles.

The onset temperature for shrinkage and the totalshrinkage are shown in Figure 15 for bottles blownwith different temperature profiles. It can be seenthat the onset of shrinkage occurs at a lower tem-perature for bottles blown at the lower temperatureend (OI-8090 and OI-9080). The bottles blown at thelower temperature have an amorphous phase thatis highly strained and has a lower percent crystal-linity as the molecules do not have enough mobilityto align perfectly in the stretch direction. The poly-mer chains oriented at lower temperature thereforestart relaxing earlier compared with bottles blown

at higher temperature. The total amount of shrink-age also decreases as the blow temperature in-creases, due to lower levels of extended moleculesand also because of higher crystallinity. The crys-tallites act as crosslink points preventing the ex-tended chains from returning to the random state.The amount of orientation varies through samplethickness and therefore the measured values areaverage values.

Summary

The temperature profile through the thickness ofthe preform was computed by solving the energyequation with radiation as the source term and ap-propriate boundary conditions. The temperatureprofile was verified experimentally by measuringthe surface temperatures using infrared pyrometers.Bottles were blow molded with different tempera-ture profiles through the thickness direction andproperty evaluations were carried out. Results showthat the blow temperature and the temperature pro-file can be selected depending on the functionalproperty to be optimized. The results can be sum-marized as follows:

1. No significant change in modulus is observedby reversing the profile, but the modulus de-creases to some extent for bottles blown athigher average temperature.

2. Tensile energy absorption is found to behigher for bottles blown at a higher tempera-ture, and there is an increase when the inside



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surface is at a higher temperature. Bottles ex-hibit pearlescence when the inside surfacetemperature is less than 1007C, for an insidestretch ratio of 5.25.

3. The anisotropy through the thickness direc-tion is minimal when the inside surface is at ahigher temperature. The inside surface bire-fringence in the hoop direction is alwayshigher than the outside surface birefringence,regardless of temperature profiles. Birefrin-gence in the axial direction depends on mate-rial distribution in addition to temperatureprofiles.

4. Surface density values, calculated from aver-age refractive indices, correlate well with pear-lescence.

5. Density of bottle sidewalls is higher for bottlesblown at a higher temperature, as moleculeshave enough mobility to align and orientthemselves in the stretch direction. Percentcrystallinity, measured using DSC, shows thesame trend as that obtained from density mea-surements.

6. Permeability is not significantly affected bytemperature profiles and bottles show about40% reduction in permeability compared withthat of amorphous PET film.

7. The onset of shrinkage is higher for bottlesblown at the higher temperature end and thetotal shrinkage is less when the average blowtemperature is high. No significant differencesin shrinkage values are apparent by reversingthe profile.

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effects of temperature profiles through preform thickness on the properties of reheat–blown pet...

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