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Polymer International Polym Int 54:946955 (2005)DOI: 10.1002/pi.1794

Effects of injection-molding processingparameters on acetaldehyde generation and

degradation of poly(ethylene terephthalate)Shunahshep R Shukla, Elizabeth A Lofgren and Saleh A Jabarin

Polymer Institute, College of Engineering, University of Toledo, Ohio 43606, USA

Abstract: The acetaldehyde (AA) generation behavior of poly(ethylene terephthalate) (PET) has been

investigated in terms of its relationship to changes in various processing conditions. A single-cavity

injection-molding machine was used to prepare preforms in order to relate changes in barrel temperature,

screw shear rate, back pressure, cooling time and total residence time to levels of AA generated during

processing. Within the temperature range 280300 C, a 10 C rise in processing temperature causes AA

production to double. Shear rate increases from 20 to 40 m min1 result in 1321 % increases in AA

generation at temperatures from 300 to 280 C. Increased back pressures from 0 to 200 bar result in AA

concentration increases of about 1.2 ppm for each 50 bar pressure increase. The majority of this change

is caused by increased polymer residence time. Longer cooling times also increase overall cycle timesand result in higher levels of AA generation, at the rate of almost 7 ppm per additional minute at 290C

processing conditions. Apparent activation energies of 167 kJ mol1 were calculated for samples prepared

at various shear rates. These results are in agreement with literature values obtained under conditions of

static mixing and indicate that shear rate and plastication do not significantly affect reaction mechanisms

for AA generation.

2005 Society of Chemical Industry

Keywords:poly(ethylene terephthalate); processing; acetaldehyde; injection molding; degradation

INTRODUCTION

Thermal and thermal oxidative degradation ofpoly(ethylene terephthalate) (PET) have been the sub-

jects of many research papers.112 The specific effects

of various processing conditions, however, have not

yet been satisfactorily investigated, since most previ-

ous research topics have not included extrusion and

injection molding. The current work establishes rela-

tionships among several important processing condi-

tions during extrusion and injection molding and their

relative effects on acetaldehyde (AA) generation and

the degradation behavior of PET. Material changes

resulting from processing temperature, screw shear

rate, melt residence time, back pressure and cooling

time variations have been included in these evalua-

tions. The primary objective of this work has been

to utilize a single-cavity injection-molding machine

to provide a methodology for and investigations of

the various parameters affecting the production of

AA during injection molding of PET. These inves-

tigations can then be extended to provide a the-

oretical background for establishing simulation and

prediction models for multi-cavity injection-molding

systems.

EXPERIMENTAL

DryingAll evaluations were performed using Eastman PET

homopolymer resin with an intrinsic viscosity (IV)

of 0.73dL g1. PET is known to undergo hydrolytic

degradation at temperatures exceeding 110 C. To

avoid this, all resin was dried to moisture levels

below 50 ppm13 before exposure to other processing

conditions. A Conair dryer, with desiccant in the

circulating air loop, was used for this purpose. Drying

was carried out in two steps. A low-temperature drying

step at 102 C for 18 h was followed by a high-

temperature drying step at 150 C for 4 h. The final

moisture content was found to be less than 30 ppm, as

measured with a DuPont moisture analyzer.

Processing

The dried PET pellets were introduced into the

injection-molding machine with a K-loader automatic

loading system supplied with the drying unit. All

preforms were prepared with a 55ton, single-

cavity, reciprocating-screw Arburg injection-molding

machine. For all runs the injection pressure was set at

a constant value of 1900 bar, with a holding pressure

Correspondence to: Saleh A Jabarin, Polymer Institute, College of Engineering, University of Toledo, Ohio 43606, USA

E-mail: [email protected] affiliation: Ohio State University, Columbus, Ohio, USA

Contract/grant sponsor: PET Industrial Consortium

(Received 27 September 2004; revised version received 2 December 2004; accepted 15 December 2004)

Published online 28 February 2005

2005 Society of Chemical Industry.Polym Int09598103/2005/$30.00 946


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AA generation during PET injection molding

of 1700 bar. The melt flow rate for all shots was set at

40cm3 s1. The mold temperature was controlled by

circulating cooling water (1021 C) flowing through

the drilled channels in the mold cavity and core plates.

The mold cooling time for all except the residence

time runs was set at 25 s. Extruder barrel temperatures

were controlled by heater bands in the various zones

and monitored with temperature sensors. All extruderzones were set to the same temperature to give a

consistent temperature profile, for ease of analysis

and interpretation of experimental data. Tables 1 to 3

summarize the various processing conditions utilized

for preform production.

Table 1.Shear rate and temperature variations

Screw rotation rate

Temperature (C)

Back pressure

(bar) (m min1) (rpm)

Cooling

time (s)

280, 290 and 300 0 20 183 25

280, 290 and 300 50 20 183 25

280, 290 and 300 50 25 229 25

280, 290 and 300 50 30 275 25

280, 290 and 300 50 35 320 25

280, 290 and 300 50 40 366 25

280, 290 and 300 50 50 366 25

Table 2.Back pressure variations

Screw rotation rate

Temperature

(C)

Back pressure


Cooling

time (s)

290 0 30 275 25

290 50 30 275 25

290 100 30 275 25

290 150 30 275 25

290 200 30 275 25

290 250 30 275 25

290 300 30 275 25

Table 3.Cooling time variations

Screw rotation rate

Temperature

(C)

Back pressure


Cooling

time (s)

290 50 30 275 40

290 50 30 275 35

290 50 30 275 30

290 50 30 275 25

290 50 30 275 24

290 50 30 275 23

290 50 30 275 22

290 50 30 275 21

290 50 30 275 20

290 50 30 275 19

290 50 30 275 18290 50 30 275 17

290 50 30 275 16

AA concentration in preforms

Preforms prepared under a variety of processing

conditions were evaluated in terms of residual AA

concentration. The preforms were prepared for gas

chromatography analyses by submersion in liquid

nitrogen, followed by coarse manual crushing. The

resulting preform pieces were then finely ground

under liquid nitrogen using a stainless steel Ika-Werk mill. The cold powder was immediately

separated using sieves to obtain particles in the

range 2040 mesh for analyses. Cold particles of

ground preform materials were weighed and loaded

into ATD-400 sample tubes for measurement of

AA concentration. These sample tubes were sealed

after loading. Care was taken to maintain ambient

temperatures of less than 18 C during grinding

and sample preparation, in order to minimize

loss of residual AA from the ground preform

materials.

Preform residual AA concentrations were measuredwith a Perkin-Elmer Auto System XL gas chro-

matograph (GC), equipped with a flame ionization

detector (FID) and utilizing a 30 mm 0.32mm ID

Stabilwax DA (carbowax) capillary column. The

GC oven was maintained at 60 C and the FID at

300 C. Residual AA was volatilized from ground pre-

form material with a Perkin-Elmer ATD-400. Samples

were held in the ATD oven at 150 C for 45 min, in

a constant helium purge. Released AA was held in

the ATD cold trap (30 C), packed with Tenax

GR (60/80) and Carbosphere (80/100), throughout

the sample heating cycle. After completion of sample

heating, trapped AA was injected into the GC col-

umn as a result of rapid heating (300 C) of the cold

trap. The areas of the AA peaks, with retention times

of about 2.3 min, were measured using Perkin-Elmer

Turbochrom software. The GC was calibrated with

known amounts of AA treated in a manner similar

to that of the ground preform materials, but held

at 250 C for 10 min. Concentrations of AA were

calculated as g AA(g PET)1 or ppm.

Melt viscosity studies

Melt viscosity studies were conducted in order toevaluate the degradation of PET as a result of

processing through the injection-molding machine.

All melt viscosity measurements were performed with

a Rheometrics cone-and-plate type viscoelastic tester

at oscillation frequencies from 0.1 to 100 rad s1 and

at a constant oscillation amplitude and temperature.

The gap between the cone and plate was maintained

at 0.05 mm throughout all rheological evaluations,

which were performed at 280C under a nitrogen

atmosphere. Preform pieces were vacuum-dried for

24h at 140 C before evaluation. Results have been

reported in terms of equivalent solution IV values,

obtained with a calibration curve relating melt viscosity

corresponding to zero shear viscosity to known IV

values.

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SR Shukla, EA Lofgren, SA Jabarin

Timetemperature measurements

The time temperature profile of the PET polymer

melt was monitored using an Omega fiber-optic

infrared sensor, following the procedure of Campbell

et al.14 The infrared sensor was installed in the nozzle

of the injection-molding machine and was placed flush

to the surface so as not to disturb the flow of the

polymer melt through the nozzle. This sensor is a non-contact temperature measuring device with a standard

response time of 50 ms. The fiber-optic lens collects

the infrared heat radiation emitted by the polymer

melt, and the radiation is then directed to a detector

cell. The detector emits a signal corresponding to the

intensity of the incident radiation, which is amplified

and linearized and then output to the cable connector.

RESULTS AND DISCUSSION

Preforms prepared under the conditions described in

Tables 1 to 3 were evaluated in terms of the AA thatwas generated during processing. This generated AA

remained in the injection-molded preforms and this

was analyzed to obtain residual AA concentrations.

The following discussions include the effects of various

processing conditions on AA concentrations in the

preforms and also the state of degradation in terms

of preform IV. The effects of shear rate changes were

monitored at temperatures from 280 to 300 C, with

back pressure and cooling time held constant. Back

pressure and cooling time influences were monitored

at 290 C and constant 30 m min1 screw rotation

speeds. Back pressure was changed from 50 to 300 bar

with 25 s cooling times held constant. Cooling times

were varied in the range from 16 to 40s with

constant back pressure values of 50 bar. The changes

in the cooling times were utilized for investigation of

residence time effects on AA generation.

Shear rate and temperature

Preforms were prepared at screw speeds from 20

to 40m min1 using extruder barrel temperatures

of 280, 290 and 300 C. Figure 1 illustrates the

dependence of preform AA concentration on shear rate

at each processing temperature. Specific experimental

conditions are outlined in Table 1. It can be seen

that AA generation at all three temperatures appears

to increase linearly with the rate of screw rotation.

Gregory and Watson15 have estimated the steady-

state flow properties of PET melts as a function of

temperature, shear rate and IV over a temperature

range of 265 to 295 C and an IV range of 0.35 to

0.82dLg1. They found that over the shear rate range

of 50 to 1000 s1 the flow behavior of PET was nearly

Newtonian.

The following relationship can be utilized for

calculation of shear rate at any single point along

an extruder screw.

S=DN

19.1h(1)

Screw speed (m min1)

15 20 25 30 35 40 45Preforma

cetaldehydeconcentration(ppm)

0

10

20

30

40

50

60

280 C

290 C

300 C

y= 0.285x+ 36.6

y= 0.173x+ 16.8

y= 0.122x+ 9.06

Figure 1. Effect of screw speed on preform AA concentration at 280,

290 and 300 C.

whereSis the shear rate in reciprocal seconds, Dis the

screw diameter,h is the screw channel depth and N is

the screw rpm. Using this relationship, the maximum

shear rates experienced by the PET melt in the barrel

of the injection-molding machine were found to range

from 264 s1 at 40 m min1 to 132 s1 at 20 m min1.

Since these values fall within the shear rate range

specified by Gregory and Watson,15 the PET melt can

be assumed to have Newtonian behavior under the

experimental conditions.

The flow of the PET melt between the screw of the

injection-molding machine and the inner barrel surface

can be approximated by the flow of an incompressible

Newtonian fluid between two coaxial cylinders. The

following expression from Bird et al16 can then beapplied for the volume heat source Sv resulting from

the viscous dissipation:

Sv =

dvz

dx

2(2)

where is the viscosity of the fluid (which in the

case of a Newtonian fluid, does not depend on the

shear rate), vz is the velocity of the fluid and x is the

direction perpendicular to the screw or barrel surface.

Equation (2) neglects curvature effects.

For the steady-state velocity of a fluid, with constant

velocity in a slit, the velocity profile is linear and can

be represented by the relation:

vz =x

hV (3)

where V is the velocity of the screw. The rate of viscous

heat production per unit volume is:

Sv =

V

h

2(4)

where h is the distance between the screw and the

barrel wall for any single point along the screw and

hence is a constant. Now let /h2 be designated byK,

a constant. Therefore, Sv V2. In this case, the heat

948 Polym Int54:946955 (2005)


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generation due to viscous heat dissipation will increase

with the square of the velocity. This implies that

the temperature rise due to viscous heat dissipation

for the 40 m min1 case, T40, will be roughly four

times the temperature rise due to viscous dissipation

for the 20 m min1 case, T20, for the same screw

rotation time. Since the total screw rotation time for

the 40 m min1

case was only half as much as the totalscrew rotation time for the 20 m min1 case, for this

system, we can write:

T20 0.5(T40) (5)

The temperature rise due to viscous dissipation for the

20 m min1 case was only half as much as that for the

40 m min1 case. In a similar way, we can argue for the

observed temperature rise with shear rate for all data

points. Table 4 summarizes the predicted temperature

rise per unit volume due to shear heating alone. In this

case,T= T T0, where T is the final temperatureafter shear and T0 is the initial temperature before

shear. Note that in Table 4, t1 is the screw retraction

time or plasticating time for the 20 m min1 screw

rotation rate, Q is the energy generated due to the

viscous dissipation in J m3, m is the mass of the

polymer undergoing shearing in kg, and Cp is the

specific heat of the material in J kg1 K1.

The most important fact that emerges from Table 4

is that the temperature rise shows a linear dependence

on the shear rate. In order to further interpret the

data of Fig 1, the AA generation values must be

plotted versus barrel temperature with shear rate as

the constant parameter, and Fig 2 presents the datain this form. The first important observation that

can be made from this plot is that for every 10 C

rise in temperature, the AA generation rate nearly

doubles. Secondly, the effect of barrel temperature

on AA generation is far greater than the effect of

screw shear rate. These values for AA generation at

each shear rate can be seen to vary exponentially

with extruder temperature. Curves for all shear rates

give excellent exponential fits with R2 values of

0.98 or greater. In the case of data obtained at

30 m min1, y = 4 107 e0.0617x.

Table 4 shows that the temperature rise increaseslinearly with shear rate and Fig 2 shows that AA

concentration increases exponentially with barrel

temperature. This suggests that AA generation should

Temperature (C)

275 280 285 290 295 300 305Preforma

cetalde

hydeconcentration(ppm)

0

10

20

30

40

50

60

20 m min1

25 m min1

30 m min1

35 m min1

40 m min1

Figure 2. Effect of temperature on preform AA concentration at screw

speeds of 20 40 m min1 (At 30m min1, y= 4 107 e0.0617x).

increase exponentially with shear rate. In this case, data

was refitted to an exponential curve to obtain a very

slightly better fit. However, for any curve, over small

enough increments, the behavior can be approximated

as linear. For these analyses and within the domain

of the experimental conditions, changes in shear rate

and induced temperature rise are small enough for all

trends to approximate linearity. Since it makes very

little difference whether the linear fit or exponential

one is utilized, most of the data have been fit linearly,

with the knowledge that this treatment is not as

rigorous as one based on exponential dependence.

Figures 1 and 2 show clearly that, at all shear

rates, more AA is generated at higher temperatures.The effects of shear rate on this AA generation are,

however, not as obvious. As shear rates increase from

20 to 40m min1, absolute amounts of AA generated

increase by 2.4, 3.4 and 5.7 ppm at 280, 290, and

300 C, respectively. These changes seem to indicate

that, at higher temperatures, total AA generation is

increasingly dependent on shear rate.

The viscous dissipation expression, given in

Eqn (4), predicts that for the same velocity or screw

circumferential speed, for two different temperature

levels, the viscous dissipation should be greater when

the viscosity is higher. The viscosity of a polymermelt decreases with an increase in temperature. Thus,

290< 280 and, for the same velocity and geometry,

the viscous dissipation at 280C should be greater

Table 4.Predicted temperature rise at various shear rates

Sv = KV2 Sv = Kf(V1)

2

Screw

retraction timet Q = Svt

T/unit volume =

Q/(mCp)

K 20 20 K 1.000 20 20 1.000 t1 1.00 K1 1.00 K2K 25 25 K 1.563 20 20 0.800 t1 1.25 K1 1.25 K2K 30 30 K 2.250 20 20 0.667 t1 1.50 K1 1.50 K2K 35 35 K 3.063 20 20 0.571 t1 1.75 K1 1.75 K2

K 40 40 K 4.000 20 20 0.500 t1 2.00 K1 2.00 K2

K1 = K 20 20 t1; K2 = K1/(mCp); V1 = screw speed at 20m min1; f= a factor relating (V1)

2 to other screw speeds or values of (V)2;

= density (kg m3).

Polym Int54:946955 (2005) 949


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than that at 290 C. As the change in total AA gen-

erated at each temperature level depends directly on

the change in temperature, we would expect that the

dependence of total generated AA on the screw shear

rate should be higher at the lower temperature value.

This is because the contribution of viscous dissipa-

tion should be greater than at the higher temperature,

where the contribution of viscous heat dissipationwould be comparatively lower. The absolute increases

in AA values thus appear contrary to what one might

expect from first principles. In reality, this discrep-

ancy does not exist. On close inspection of the data,

it is found that the percentage increase in AA gener-

ation at 280, 290 and 300 C is 21.2 %, 16.9 % and

13.4 %, respectively, over the shear rate range from

20 to 40m min1. Thus, the relative dependence of

AA generation at higher temperatures becomes more

in accordance with first principles. The slopes of the

plots given in Fig 1 include the effects of temperature

as well as shear rate. At higher temperature levels,

the effects of changes in all processing parameters

on the absolute values of AA generated and on the

AA generation rates are magnified. Thus, for proper

interpretation, the data should be analyzed using per-

centage changes in AA generation. While at higher

temperatures absolute amounts of generated AA are

higher with increased shear rates, the overall percent-

age changes are lower.

In addition to shear resulting from screw rotation,

the PET melt is exposed to another source of shear.

As the polymer melt flows into the mold cavity, it

passes through a narrow gate, where it encounters

very high rates of shear. Under current experimentalconditions, the gate diameter is 4 mm. The expression

for wall shear rate w can be utilized, assuming simple

Poiseuille flow, to obtain:17

w =

3n + 1

4n

8V

D

(6)

where V is the average velocity of the fluid and D is

the diameter of the capillary. Assuming that n 1, we

may write:

w =32Q

D3 (7)

where Q is the volumetric flow rate.

For a typical shot, 48 cm3 of material is injected into

the mold cavity in 1.2 s, and the volumetric flow rate

Q is thus 40 cm3 s1. The wall shear rate w can then

be computed from the expression:

w =32 40

0.403= 6366s1

This shear rate, through the gate of the injection-

molding machine, is much higher than rates encoun-

tered by the melt in the screw. Screw shear rates at

30 m min1 are 198 s1. As the polymer melt passes

through the gate, it experiences shear rates 32 times

higher than those in the screw. These high shear rates

cause shear heating of the polymer melt and produc-

tion of AA. The total quantities of AA measured in

preforms produced under the various experimental

conditions all include AA produced as a result of the

shear heating through the gate. Values obtained at dif-

ferent temperatures and as a result of changing screw

speed, back pressure and residence time would all be

reduced if AA generated by shear heating through thegate could be excluded.

The volume of polymer melt experiencing the very

high shear through the gate is relatively small, since it

includes only the layer flowing closest to the wall. A

cross section of the polymer melt would thus exhibit

a distribution of temperatures and AA concentrations.

These values would tend to be higher near the outer

polymer surface. Shear rate calculations have been

based on assumptions that have not taken into account

any distributions of shear rates or temperatures.

AA generation results include overall increases with

contributions from various sources. Future work is

anticipated to model and analyze the flow behavior in

cases of single-cavity and multi-cavity molds during

injection molding of PET preforms. This work should

also relate the generation of AA to differences in shear

heating and polymer flow.

Back pressure

The effects of injection-molding machine back

pressure variations were investigated at an extruder

temperature of 290 C,with a screw rotation rate of

30 m min1 (275 rpm) and a cooling time of 25 s, as

given in Table 2. The levels of AA generated, as a

result of back pressure variations from 0 to 200 bar,are shown in Fig 3. It can be seen that residual AA

concentrations increase linearly with increasing back

pressure, to give an overall increase of about 20 %

within the evaluated pressure range. Several different

mechanisms may contribute to this increase. These

contributing factors include: (a) a temperature rise

due to adiabatic compression of the melt; (b) increased

screw shear; and (c) increased residence time of the

PET in the injection-molding machine.

Back pressure (bar)

0 50 100 150 200 250

Preforma

cetaldehydeconcentration(pp

m)

15

20

25

30

35

y= 0.0232x+ 24.2

Figure 3. Effect of back pressure on preform AA concentration at

290 C.

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An approximation for the adiabatic temperature rise

in the PET melt can be calculated using the specific

volume data for a PET melt given by Zoller and

Bolli,18 the specific heat data for melted PET given by

Smith and Dole,19 and the expression:

PV= CpT (8)

At 290 C and, for example, 200bar back pressure,

V= 0.0144 cm3 g1 from Zoller and Bolli18 and

Cp = 2.0 4 J g1 K1 from Smith and Dole.19 After

substituting the values with appropriate units into

Eqn (8), the rise in temperature T is found to

be of the order of 0.14 C. This value can then be

utilized in the expression given in Fig 2, relating AA

concentration to temperature. The resultant increase

in AA concentration, from the 0.14 C higher exposure

temperature, was found to be about 0.2 ppm of total

increase. In the case of samples prepared at 290 C

and 50 bar back pressure,V= 0.00332 cm3

g1

andthe T is found to be 0.008 C. In this example,

the resultant increase in AA concentration is only

0.01 ppm, which is not a significant source of AA

during extrusion.

The screw rotation time increases by nearly 0.1 s

per shot for every 50 bar increase in back pressure.

Normally, while the machine is running, there are

about 6.5 shots in the injection-molding machine at

any given time. A 50 bar rise in back pressure can,

therefore, increase the residence time of the PET in

the screw by 0.65 s. The residence time experiments

are described in the next section and from those we

can infer that for every 50 bar increase in back pressure

an AA concentration increase of about 0.1 ppm can

be attributed to increased time of heating within the

screw.

The effects of screw shear rate at 290C can be

assessed from the slope of the AA versus screw speed

given in Fig 1. At a screw shear rate of 30 m min1,

the AA generation resulting from screw rotation alone

is estimated to be 5.18 ppm. The relationship between

screw rotation time and back pressure is given in Fig 4.

It can be seen that screw rotation time increases by

about 0.1 s per shot for every 50 bar increase in back

pressure. The screw rotation time at a 30 m min1

screw rotation rate and 50 bar back pressure is 2.14 s.

As a result, the total AA value of 5.18 ppm generated

by shear alone must be divided by the screw rotation

time of 2.14s to obtain about 2.4 ppm of AA per

second of screw rotation. For 0.1 s of screw rotation,

therefore, the AA generated by screw shear alone is

0.24 ppm. From the slope in Fig 3, the total increase

in AA generation resulting from a 50 bar rise in back

pressure is found to be 1.16 ppm. The 0.24 ppm of

AA generated by shear can now be subtracted from

the total AA generated, to obtain AA generation that

cannot be attributed to screw shear:

1.16ppm(total) 0.24ppm

(screw shear)=

0.92 ppm AA(other

than screw shear)

Screwr

otationtime(s)

1.0

1.5

2.0

2.5

3.0

y= 0.00196x+ 2.04

Back pressure (bar)

0 50 100 150 200 250

Figure 4. Effect of back pressure on screw rotation time at 290 C.

It has been shown that adiabatic compression at

50bar does not contribute significantly to AA

generation during extrusion. Temperature changes in

the melt are also very small under these experimental

conditions. Increased residence time in the melt state,

therefore, appears to be the major factor contributing

to additional AA generated under conditions of

increased back pressure. Residence time increases

include increased screw rotation times, resulting

from increased back pressure. The total rise in AA

concentration resulting from a 50 bar increase in

back pressure is 1.16 ppm. This value of 1.16 ppm

minus the 0.24 ppm resulting from screw shear leaves

0.92ppm AA generated primarily as a result of

increased time in the melt. This value includes the

0.1 ppm AA generated within the screw during theincreased time of rotation.

Residence time

Total polymer residence times were calculated using

the flight-volume data for the screw section of the

extruder, the volume of the melt reservoir, the volume

of the mold cavity, and the cycle time. These values

comprise the total residence time from the entrance of

the pellets to the feed section through the hopper,

until the time when the preform is ejected from

the mold cavity. As cooling times are increased

from 16 to 40 seconds, these changes result incumulative additions to all of the shots remaining

in the injection-molding system. Each cooling time,

therefore, increases polymer residence time by more

than the time required for one injection cycle. Figure 5

gives an overall schematic drawing showing the various

processes included in a complete injection-molding

cycle. This complete cycle is the total residence time

under evaluation.

Changes in AA generation as a result of varying

cooling times (ranging from 16 to 40 s) were monitored

at a processing temperature of 290 C. Back pressure

was set at 50 bar and screw speed held at a constant

30 m min1. Figure 6 shows the linear dependence

of this relationship, with increased cooling times

resulting in longer overall residence times and higher

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Machine cyclestart

Moldcloses

Plasticationunit moves

forward

Screw ramsforward / shot

injection

Packing

Plastication

Screw rotation

Coolingthe mold

Backwardmovement of

the plasticatingunit

Mold opens

Ejectoradvancement

Ejector retraction

End of cycle

Figure 5. Processes involved in a complete injection-molding cycle.

levels of generated AA. The linear relationship of

AA generation to total residence time has a negative

intercept, which corresponds to an apparent inductiontime of about 51 s before measurable AA is generated

during this total residence time. Possible explanations

for this induction time are discussed below.

Pellets do not melt immediately as they enter

the feed zone of an injection-molding machine.

Conventional models for most screws in extrusion

and injection molding assume that in the feed zone

there is a well-defined solid bed present in the screw

channels adjoining the melt pool. This solid bed

continues to diminish as it progresses towards the

metering zone, along the screw axis. In the case of the

equipment used for these experiments, the feed section

comprises about 60 % of the total screw length. If a

uniform gradient is assumed in the transition section,

the feed section could constitute more than 70 % of

Total residence time (s)

0 100 200 300 400Preforma

cetaldeh

ydeconcentration

(ppm)

0

10

20

30

40

50

y= 0.1143x5.872

Figure 6. Effect of total residence time on preform AA concentration.

the total flight volume. The total flight volume can

accommodate about 5.63 shots of polymer, therefore,

the volume of the material present in the feed zonewill comprise at least 3.9 shots.

It is difficult to estimate the exact solids volume

fraction in the barrel of an injection-molding machine.

The 51 s induction time, illustrated in Fig 6, results

in part from the volume fraction of unmelted solids

present in the feed and transition zones of the machine.

This induction time also includes cooling time during

which the injection-molded part remains in the mold

after it has been cooled to a temperature below which

AA is no longer generated. The time required for

complete ejection of the preform from the mold also

contributes to this induction time. It represents all the

time during which AA generation is not significant,even though polymer material is present within the

injection-molding machine.

Melt viscosity

Samples prepared under various experimental condi-

tions were evaluated in terms of melt viscosity using a

viscoelastic tester. The results corresponding to zero

shear viscosity were then converted to equivalent IV

values in order to monitor changes resulting from pro-

cessing variations. Measurements were performed on

preform materials prepared from 280 to 300C at

screw speeds of 20 to 40 m min1

. The results showthat the IV has very little dependence on shear rate

when samples are processed at 280 or 290 C. Samples

processed at 300 C show a slight decrease in IV with

increasing shear rate, as illustrated in Fig 7. Overall IV

values are slightly lower for samples exposed to higher

temperatures, with samples processed at higher shear

rates exhibiting the greatest temperature dependence.

Viscosity measurements were also taken to study

the effects of back pressure and cooling time during

injection molding. Figures 8 and 9 show that no

appreciable changes in IV occur as a result of back

pressure changes from 0 to 200 bar or for cooling

times from 15 to 25 s. During evaluations of back

pressure, the barrel temperatures were maintained

at 290 C, the screw rotation rate at 30m min1

952 Polym Int54:946955 (2005)


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Screw speed (m min1)

15 20 25 30 35 40 45

Intrinsicviscosity

0.60

0.62

0.64

0.66

0.68

0.70

0.72

0.74

Figure 7. Effect of screw speed on melt IV values at 300 C.

Back pressure (bar)

0 50 100 150 200 250

Intrinsicviscosity

0.50

0.55

0.60

0.65

0.70

0.75

0.80

Figure 8. Effect of back pressure on melt IV values at 290 C.

and the cooling time at 25 s. Cooling time effects

were monitored at the same temperature and screw

rotation conditions and a constant back pressure of

50 bar. Under these experimental conditions, samples

exhibit minimal changes in IV, while at the same time

showing increased AA generation. These results can

be explained according to Goodingss2 mechanism

of thermal degradation, which indicates that as

long as sufficient available free hydroxyl end-groups

are present, broken polymer links resulting from

degradation can repolymerize with the production

of equivalent amounts of AA and carboxyl end-groups. In this manner, changes resulting from thermal

degradation are offset by the repolymerization process,

with very little net loss in molecular weight or IV.

Only after the available hydroxyl end-groups have

been consumed (as a result of prolonged heat history),

will significant loss in IV be recorded. It can thus be

seen that changes in levels of generated AA are more

sensitive indicators of polymer degradation state than

are changes in IV values.

Timetemperature measurements of polymer

melt

Measurements of polymer melt temperatures were

obtained with a fiber-optic infrared sensor in

the nozzle of the injection-molding machine. The

Cooling time (s)

Intrin

sicviscosity

0.50

0.55

0.60

0.65

0.70

0.75

0.80

10 15 20 25 30

Figure 9. Effect of cooling time on melt IV values at 290 C.

time temperature profile of the polymer melt was

recorded in real time as tables, using Quattro Pro soft-

ware. These tables were then transferred to an Excelprogram for graphical analyses. Figure 10 gives an

example of data obtained using this method. Similar

measurements were performed as barrel temperature

setting were changed within the range 280 300C.

For all runs the injection pressure was set constant at

1900 bar, the holding pressure at 1700 bar, the cool-

ing time at 25 s, the mold cooling water was 15.6C,

the screw speed was set at 40 m min1, and the back

pressure was 100 bar.

The data represented by trendline 1 in Fig 10 were

collected as soon as all setpoint values on the injection-

molding machine read 285 C, after temperature

settings had been reduced from 290 C. Trendlines2 and 3 were, respectively, obtained after 10 and

21 additional shots had been made after the shot

represented by trendline 1. These trendlines illustrate

measured changes in melt temperature during various

stages of the injection-molding process, as well as

times required for temperature stabilization in each

process. It can be seen from these data that the melt

temperature of the polymer stabilizes at a far slower

rate then the barrel temperature of the extruder. These

results indicate that attainment of stable dynamic

conditions requires a considerable amount of time,

once setpoints are changed or after extruder start-up.Changes in the three trendlines of Fig 10 represent

various stages of the injection-molding cycle. The

recorded melt temperature rises and falls very sharply

at stages in the cycle that correspond to screw injection

induced compression and screw retraction induced

decompression of the polymer melt. About 1.2 s after

initiation of injection, the temperature reaches its peak

value due to adiabatic compression of the polymer

melt. The temperature falls slightly, during the holding

stage in the machine cycle. It then falls to the baseline

value within 1 s. In the holding stage, the temperature

of the polymer melt shows a steady decline as a

result of dissipation of frictional heat generated during

the injection stage, as the material flows past the

nozzle and gate and into the mold cavity. This heat

Polym Int54:946955 (2005) 953


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270

272

274

276

278

280

282

284

0 50 100 150 200 250 300

Time

1

23

Temperature(C

)

Figure 10. Temperature profiles measured in real time with an infrared sensor in the nozzle of the injection-molding machine (300 divisions in

x-axis = 60s).

is slowly dissipated to the surroundings, during the

holding stage. The decline also occurs because the

melt shot is thermally inhomogeneous, with the rear

of the shot, in the vicinity of the screw head, at the

lowest temperature.20 In addition to the temperature

changes from compression and decompression, a crest

in the temperature profile is observed immediately

after screw retraction. This crest corresponds to shearheating caused by the rotation of the screw. The

duration of the observed crest is very small compared

with the total cycle time since the screw rotates for only

a small fraction of the cycle time. The temperature rise

from shear heating is seen to be less significant than

the temperature rise from adiabatic compression of

the polymer melt.

The results obtained after establishment of stable

dynamic extrusion conditions at temperatures from

280 to 300 C indicate that measured temperatures

were generally lower than set temperatures. The

peak temperatures corresponding to screw injection

induced compression were usually 5 6 C lower

than the set temperatures. Baseline temperatures

representing polymer in the melt reservoir were

about 1214 C lower than set temperatures. These

differences may have occurred because the screw

rotates for less than 10 % of the total cycle time.

In commercial melt cavity machines, the screw rotates

for 7080 % of the total cycle time. For commercial

extruders, levels of shear heating are higher and melt

temperatures are much closer to barrel temperatures.

Activation energy for AA generation

The process of melting and injection molding a

polymer material in an extruder usually involves a

distribution of residence times rather than a single

value. It is therefore appropriate to use the total

amount of AA generated during the production of

an injection-molded part, rather than the rate of

AA generation, for calculating apparent activation

energies. Total levels of AA (AT) generated at

various processing conditions were thus used for these

calculations. Figures 1 and 2 show total AA values

obtained for samples processed with screw speedsfrom 20 to 40 m min1, at temperatures from 280 to

300 C, with 50 bar of back pressure, and 25 s cooling

times. Sample sets processed at each screw speed but

at three different temperatures would have equivalent

processing exposure times. Since AA generation is a

zero-order reaction,8 values for total generated AA at

each screw speed were plotted as functions of their

processing temperatures to prepare Arrhenius plots

according to:

AT k = AeE/RT (9)

orlnAT = lnA E/RT (10)

where E is the activation energy and k is the rate

constant for AA generation during injection molding.

The gas content R is 8.314JK1 mol1 and T is

the absolute temperature (K). Figure 11 shows the

Arrhenius plots prepared with the AA generation

data. The slopes of lines through these data were

used according to slope = E/Rto obtain the apparent

activation energy values shown in Table 5.

The results given in the table indicate that these

changes in screw rotation rate have little effect on

the values of activation energy for AA generation.

The average apparent activation energy value of

167 kJ mol1 is in agreement with those given by

954 Polym Int54:946955 (2005)


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1/T(K1

)

0.00174 0.00175 0.00176 0.00177 0.00178 0.00179 0.00180 0.00181 0.00182

lnk

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

20 m min1

25 m min1

30 m min1

35 m min1

40 m min1

Figure 11. Arrhenius plot of total generated AA present in

injection-molded preforms (lnk)versusreciprocal processing

temperature (1/T).

Table 5.Activation energy values for AA generation

Screw rotation rate(m min1) Activation energy(kJmol1)

20 175

25 164

30 163

35 173

40 159

Goodings (149 kJ mol1),2 Jabarin (159 kJ mol1)13

and Halek (157 kJ mol1).8 The similarity of these

results indicates that shear rates, present in the screw

section of the injection molding machine, do not have

a significant influence on the basic mechanism ofAA generation reactions previously investigated under

conditions of static mixing. Data from the current

experiments fit the zero-order kinetics proposed by

Halek,8 with the enhanced mixing provided by

variations in shear rate and plastication causing no

appreciable changes in reaction mechanisms.

CONCLUSION

AA generation behavior of PET has been investigated

in terms of its relationship to changes in various pro-

cessing conditions during injection molding. Levels of

AA in preforms were found to increase with increasing

processing temperature, shear rate, back pressure and

overall residence time. Specific conclusions are given

below:

1 Within the temperature range from 280 to 300 C,

a 10 C rise in processing temperature doubles the

AA concentration in an injection-molded preform.

2 Shear rate increases from 20 to 40 m min1 result

in 1221 % more generation of AA at temperatures

from 300 to 280 C.

3 Back pressure increases from 0 to 200 bar result

in AA concentration increases of about 1.2 ppm

for each 50 bar pressure increase. The majority of

this change occurs because of increased residence

time.

4 Longer cooling times increase overall cycle time

and thus result in higher levels of AA generation, at

the rate of almost 7 ppm per additional minute at

290 C processing conditions.

5 Apparent activation energies of 167kJ mol1 were

calculated for samples prepared at various shear

rates. These results are in agreement with literature

values obtained under conditions of static mixingand indicate that shear rate and plastication do

not significantly affect reaction mechanisms for AA

generation.

6 Measurements obtained with an infrared sensor in

the injection-molding machine nozzle indicate that

the measured baseline melt temperature is generally

lower than the set barrel temperature and that

attainment of stable dynamic conditions requires

a considerable amount of time.

7 No appreciable changes in material IV values were

recorded at 290 C processing temperatures in

response to variations of shear rate, back pressure

or cooling time. An explanation for this behavior

is that as long as excess hydroxyl end-groups are

present, broken polymer links can be reformed with

the production of equivalent amounts of AA and

carboxyl end-groups.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the members of

the PET Industrial Consortium for their support of

this research.

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