polymers help deliver a solar future …€¦ · june/july 2012 polymers help deliver a solar...

June/July 2012

POLYMERS HELP DELIVER A SOLAR FUTURE

EUROPE’S TOP 50 PE FILM EXTRUDERS

BENEFITS OF WATER-COOLED BLOWN FILM

OUTLOOK FOR HEAVY DUTY PLASTIC SACKS

www.filmandsheet.com June/July 2012 | film & sheet eXtRUsiON 29

Water-cooling for blown film | process feature

Using water, rather than air, to cool blown film can create products with superior properties. Robert Armstrong, I-Hwa Lee and Karen Xiao explain some of the advantages

Water cooling of co-extruded blown film can create

differences in key physical properties, compared to

those produced using conventional air cooling, as we

have discovered through conducting a recent study.

the physical properties we have evaluated include

optics, barrier to oxygen and moisture, resistance to

biaxial deformation and a qualitative evaluation of

thermoformability on a typical horizontal form-fill-seal

(hffs) machine. We also looked at the effect of varying

water quench process parameters, such as water ring

position, water temperature and annealing temperatures.

the properties of water-quenched (WQ) films are

quite different to the air-quenched (AQ) films produced

by a typical upward blown film process. WQ films had

lower resistance to orientation, improved thermoform-

ability across a range of temperatures and reduced

haze, for example.

Our study was separated into three distinct parts.

Part i dealt with the properties of flat films; Part ii with

thermoformability; and Part iii with the orientability of

the films.

Downward coolingless than a decade ago, Brampton engineering

commercialised a new cooling system for blown film

applications in which the film is extruded downwards

and quenched using water instead of air – which

introduces different morphology into the film structure.

What is unique in this process is that the film is

quenched almost as soon as it passes through a water

ring. Once the film is cooled, it is still collapsed and

wound. the water-quench system minimises the time

Water-cooling brings benefits

to blown film

film & sheet eXtRUsiON | June/July 2012 www.filmandsheet.com30

process feature | Water-cooling for blown film

that the polymer melt spends in the crystal-growth

temperature range.

the crystallisation structure of films produced this

way will be very different. A ‘slowly cooled’ crystal

structure (typical of air quenching) is quite different to a

‘rapidly cooled’ crystal structure (from water-quench-

ing). Because very little time is spent in the ‘crystal-

growth’ temperature range, the film becomes much

more amorphous. the downward-extruded, water-

quenched system is a unique process that combines the

advantages of cast and conventional blown film

processes: it retains the improved clarity and thermo-

formability, and reduced curl of a cast film process; and

it also keeps the balanced orientation and the process

flexibility of a blown film process. this system has also

given blown film processors opportunities to use less

expensive resins to achieve the same or superior

properties. for example, in a typical thermoforming

structure, processors would commonly use a copolymer

polyamide or amorphous polyamide to improve

thermoformability and clarity. in a water-quench

process, these more expensive copolymers can be

reduced or eliminated completely.

Film structuresthe oxygen barrier and physical properties of co-extrud-

ed 100 micron, nine-layer structures were directly

compared using both a WQ and AQ co-extrusion process.

(the structures are shown in Table 1 on page 34.)

the materials used include a polyamide 6, a 38 mol%

ethylene-vinyl alcohol (eVOh-1), a modified 32 mol%

eVOh (eVOh-2), an octene-llDPe with a melt index of 1

and 0.918 g/cm3 density, a maleic anhydride-modified

Pe, and a polyolefin plastomer with a melt index of 1

and a 0.902 g/cm3 density.

the first structure (structure A) is a symmetric PA/

eVOh/PA co-extrusion and the second structure

(structure B) is an asymmetric co-extrusion with a

polyamide skin layer. for structure A and B, all materi-

als used in each structure were the same polymer from

the same production lot, so observed differences in

physical properties are directly related to the quenching

process. the third structure (structure C) is a symmetric

Pe/eVOh/Pe co-extrusion, with the eVOh either being a

38 mol% (eVOh-1) or a modified 32 mol% eVOh

(eVOh-2) which was expected to have lower resistance

to orientation than conventional 32 mol% eVOh. in all

cases the total thickness of the films was 100 microns.

Equipment and procedurestwo 9-layer Brampton engineering blown film lines

were used in the study. Both consisted of 9-layer stacked

co-extrusion dies (sCD). the AQ line had two 2.5in

(63.5mm) and seven 2in (50.8mm) extruders, while the

water-quench line had nine 2.5in (63.5mm) extruders.

the AQ line has a standard internal bubble cooling (iBC)

system, while the WQ line has a water-cooling ring on

the outside of the bubble. Both lines also consisted of

Figure 3: Oxygen barrier OF FlaT Films

Figure 2: WaTer vapOur barrier OF FlaT Films

Figure 1: Haze OF FlaT Films

20

18

16

14

12

10

8

6

4

2

0

AQ

WQ

Haz

e (%

)

Structure A Structure B

15.5

4.1

17.8

7.4

Structure A is PA/EVOH/PA core with PE skinsStructure B is PA/EVOH/PA core with PA skin

90

80

70

60

50

40

30

20

10

0

AQ

WQ

WVT

R (m

g-m

m/[

m2 .d

ay])

Structure A


Structure B

40

63.6

51.6

80.2

16

14

12

10

8

6

4

2

0

AQ

WQ

AQ

WQ

OTR

(cc-

20um

/[m

2 .day

])

Structure A

20˚C/65%RH

30˚C/85%RH

Structure B Structure A Structure B


Water Quenched Blown Barrier FilmWith more 7 – 11 layer blown film installations worldwide than any other company, Brampton Engineering (BE) is the established world leader in barrier film coextrusion systems. BE’s conventional air cooled and AquaFrost® water quenched blown film lines operate in every continent producing barrier films used in food, medical and industrial packaging.

AquaFrost® lines deliver the clearest, softest, strongest and best thermoformable films in the market for the most demanding packaging applications. Available in up to 11 layers for maximum processing flexibility and successfully proven in markets worldwide for over 10 years, AquaFrost® is the original name for water quenched blown barrier film technology.

AquaFrost ®

11 -layer

PA

Tie

LLDPE

Tie

PA

EVOH

PA

Tie

PP

Tie

mLLDPE

Brampton Engineering Inc.World Leader in Blown Film Innovation 8031 Dixie Road, Brampton, ON L6T 3V1 Canadawww.be-ca.com



standard oscillating haul-off systems and double turret

winders for winding the films. Both die diameters were

450mm and the experiments were performed at 450kg/

hr or 1kg/hr/mm die circumference. the process

parameters investigated included water ring position,

water temperature and annealing temperature.

Batch forming tests were conducted using a ZeD

batch thermoformer (batch former), with eight samples

taken for each film condition. Draw depth was at 4cm

with a corner radius of 0.6cm. the oxygen transmission

rate (OtR) was then measured for each formed package.

for each package, the inside was purged with nitrogen.

the package was also controlled so that the relative

humidity (Rh) was 65% on the outside of the package

and 100% on the inside. the concentration of oxygen

inside the package was then measured over time.

in the multivac horizontal thermoformer (hffs

former) trials, the film was heated by both a top plate

and a bottom plate. in this experiment, the tempera-

tures of both plates were kept equal. the width of the

film was 325mm and either two or four pockets were

formed per set across each web. the bulk of the work

was conducted with the deeper draw four pocket

set-up, with the dimensions of 135mm x 125mm x

85mm. the heating time was kept at 1 second with 2

seconds of forming time while the temperature of the

heating plates was varied to identify the optimal

thermoforming window.

the instrumented biaxial orientation tests were

conducted using a Bruckner lab scale instrumented

biaxial orientation unit. film samples were prepared as

100mm x 100mm square sheets that were mounted in

the unit using same machine direction (mD) and

transverse (tD) orientation for all specimens. After

establishing conditions that allowed for orientation to

more than 3 x 3 ratio, all film samples were tested after

preheating at 100°C for 20 seconds. the orientation

speed was 1m/min in both the mD and tD. five repli-

cates of each film sample were oriented to 2 x 2 for

oxygen barrier testing. An additional three replicates

were tested to 3 x 3 orientation to determine the

resistance to deformation of each structure under

conditions that simulated the thermoforming process.

Data was collected every 0.1s from the time the test was

initiated. Key data was time and force in both mD and

tD. from the time and force data a set of stress-strain

and total stress-draw ratio results was calculated.

Flat film propertiesin Part i of the study, an evaluation of the haze, moisture

vapour barrier and oxygen barrier of flat films was

conducted. the overall lower haze – due to lower

crystallinity of the WQ structures – should also result in

lower barrier properties, and this is what has been

observed. films obtained from the AQ process gave lower

values of water vapour transmission (WVtR) in both the

symmetric and asymmetric structures. the OtR values

show a similar trend. the oxygen barrier was measured

under two sets of conditions: one with 65% relative

humidity (Rh) at 20°C and the second one with 80% Rh at

30°C. the difference in the OtR is magnified at the higher

Rh and temperature condition. As expected, the WQ films

gave higher OtR values – indicating a decrease in barrier

properties, compared to AQ films.

to investigate the effect of process conditions on the

final film properties, films were also obtained at various

water temperatures, water tank positions and annealing

temperatures. insignificant changes in OtR values were

seen, indicating that the process conditions have a minor

effect on the final film properties. Optical microscopy

and Wide-Angle X-Ray Diffraction (WAXD) analysis was

performed to qualitatively determine the difference in

crystalline structure of the AQ and WQ samples. the

results of this analysis indicated that the polyamide,

eVOh and Pe in the AQ samples has a higher degree of

crystallinity than their WQ counterparts.

(Figure 1 shows the haze comparison for the

different film structures using the WQ and the AQ

methods, while Figure 2 and Figure 3 show the water

vapour transmission rates and oxygen transmission

rates respectively. All three are on page 30.)

Thermoforming resultsPart ii of the study looked at the forming characteristics

of AQ and WQ structures. the oxygen barrier of the

formed package of the WQ film is slightly lower than

that of the AQ packaging, indicating that the WQ film

has better thermoformability than the AQ film – so the

Water-cooling

extrudes

the film

downwards,

rather than

upwards



formed package of WQ film is better able to retain its

barrier properties. it is also worth noting that the shape

of the formed package from the WQ film was more

appealing than that of the AQ film, again confirming the

better package integrity.

three criteria were used to determine the formability

of the film on the multivac as a function of temperature:

depth and quality of the pocket or cavity draw; number

of cavities formed; and, the degree of cavity breaks. for

cavities that did form to full depth, a gauge distribution

across the pocket cross-section was measured.

Consistent with lower crystallinity, the WQ technology

allows barrier films to form more evenly at lower

forming temperatures, and over wider temperature

windows, than AQ films of the same construction. this

has been seen for nylon-encapsulated eVOh barrier

films with llDPe as the outer layers, and for nylon-

encapsulated eVOh barrier films with one outer layer of

PA6. the overall gauge distribution was more uniform,

and the thinnest areas at the corners and base of the

formed container were heavier for the WQ films than the

AQ films, because of the lower temperature forming that

is possible with water quenching. typically, the thinnest

areas are 12-15 microns when formed at 95-120°C, and

5-10 microns when formed at or above 130°C.

there were differences in gauge distribution of

formed cavities for films made by the two different

quenching methods. it was found that, for deep-draw

thermoforming applications, it would be necessary to

encapsulate eVOh with nylon. for air-cooled films with

the different barrier configurations, only nylon-encap-

sulated eVOh films showed a forming window with

temperature on the hffs former. the non-encapsulat-

ed eVOh films could not form, since 50% or more of the

cavities did not form or formed with breaks and film

blow-outs. the gauge distribution of the formed cavities

also showed that the corners and base were noticeably

thinner for the non-encapsulated eVOh films compared

to the nylon-encapsulated eVOh films.

Draw ratiosfor part iii of this study, three film structures (A, B and

C) were oriented to the same draw ratio at the same

strain rate under constant condition of preheating time

and temperature that allowed for orientation of all

samples. the preheating temperature of 100°C was

consistent with the lower operating temperature of both

the batch former and the hffs former. the preheating

time of 20 seconds was considerably longer than the

preheating time of the hffs former, but similar to that

of the batch former.

the oxygen barrier of the films oriented to 2 x 2 ratio

from 100 microns was evaluated using mocon 2/20 or

2/21 units at the elevated temperature and humidity

condition of 30°C and 85% Rh to magnify differences in

oxygen barrier caused by crystallinity of the polyamide

and eVOh.

the orientability (or resistance to deformation) of the

samples was evaluated by comparing the engineering

stress and strain curve, and more specifically the stress

at yield point and maximum stress. As a measure of

total resistance to deformation the total stress versus

draw ratio was estimated, and the modulus of tough-

ness (strain energy in deformation) was calculated

through to the target draw ratio of 3x3. (the key results

of the biaxial orientation tests, including the estimated

modulus of resilience and modulus of toughness for

each structure, are seen in Table 2 on page 36.)

Deformation resistancefor structure A, resistance to deformation of the AQ

blown films was higher than the WQ films, as shown by a

higher stress at yield point and higher ultimate stress.

the total stress and the modulus of toughness was also

higher throughout orientation. Resistance to orientation

was greater in the mD than the tD for both methods.

there was an overlap between the mD resistance of the

WQ structure, and the tD resistance of the AQ structure.

the structure B films replicated the results of

Table 1: Experimental structures of 9-layer filmsStructure Layer 1 Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 Layer 7 Layer 8 Layer 9

A Pe Pe t PA eVOh-1 PA t Pe Pe

thickness (%) 15 8 8 15 8 15 8 8 15

B P t PA eVOh-1 PA t Pe t PA

thickness (%) 15 10 10 8 10 8 14 10 15

C Pe Pe Pe t eVOh-1 t Pe Pe Pe

thickness (%) 13 15 8 10 8 10 8 15 13

PA = polyamide 6EVOH-1 = 38mol% ethylene-vinyl alcoholEVOH-2 = a modified 32mol% EVOHPE = octene-LLDPET = maleic-anhydride modified PE P = polyolefin plastomer

structure A is PA/eVOh/PA core with Pe skins

structure B is PA/eVOh/PA core with PA skin

structure C is Pe/eVOh/Pe



structure A, with the total resistance to deformation,

stresses and modulus of toughness all being higher for

the AQ samples. Within the B sample set there were no

significant differences between samples, which is

consistent with results of the Part ii forming trials.

the structure C samples (eVOh-1 and eVOh-2)

oriented with similar characteristics – namely a low

resistance to deformation that was significantly lower

than both the structures containing nylon (A and B). the

Pe/eVOh/Pe structure C without nylon also exhibited

quite different resistance to orientation in the mD and

tD directions and also evidence of fibrillation of the

eVOh and/or Pe as variation in stress as orientation

progressed was much more dramatic than that

observed in orientation of the nylon containing films.

the oxygen barrier after orientation of the films was

evaluated under the worst case conditions of 30°C/85%

Rh to magnify any differences in barrier. the raw OtR

results were normalised to a value of permeability in

cc.20microns/m2.day at 30°C and 85% Rh (see Table 3).

the barrier of all films after orientation became quite

similar, with the barrier of the AQ films becoming lower

or the same and the WQ films becoming higher. this

suggests that the heating and then orientation of the

films created conditions of annealing, then strain-

induced crystallisation, that tended to produce similar

morphologies and thus barrier of the oriented films.

this would be consistent with the findings of the

forming and formed package barrier testing, which

showed the WQ films had superior characteristics and

barrier after forming due to higher average thickness

and less gauge variation.

Conclusionin the final part of the study, resistance to orientation of

the two films was compared. the WQ films had lower

resistance to orientation than the AQ films, which

explains the ability to thermoform the WQ films at a

lower temperature and achieve a more uniform gauge

distribution. this attribute, particularly the higher

corner and base thicknesses of the formed packages,

resulted in higher barrier properties for WQ films after

forming, even though the AQ films had higher oxygen

barrier in the unformed state.

these results are consistent with the lower crystalline

structures of the WQ films seen in Part i of the study.

the ability of nylon to improve the orientability of

eVOh was confirmed, with evidence that nylon prevents

fibrillation of eVOh (seen during biaxial orientation of

the structure C Pe/eVOh/Pe films). the properties of

the WQ films was quite different from the AQ films, with

the WQ films having lower resistance to orientation,

improved thermoformability across a range of tempera-

tures and reduced haze.

Conversely, the barrier of the WQ films to oxygen and

moisture was lower than that of the air quenched films

before forming.

the observed difference in key physical properties

was correlated with variation in polymer crystallinity

between the WQ and AQ films using optical microscopy

and WAXD.

the results of barrier testing of oriented films and

formed packages suggest that orientation at tempera-

tures below the recrystallisation temperature of the

eVOh tends to equalise crystallinity of the eVOh – and

thus the effective oxygen barrier of formed packages

becomes a function of average thickness and localised

variation in thickness of the eVOh layer, rather than the

original extrusion process.

About the authorsRobert Armstrong is at Kuraray America; i-hwa lee is at DuPont; and Karen Xiao is at Celgard – but was formerly at Brampton engineering.

Table 3: Oxygen permeability before and after orientation (30°C/85%RH)Structure A A B B C C

extrusion Process AQ WQ AQ WQ eVOh-1 eVOh-2

Unoriented 4.0 8.6 6.8 15.2 4.4 3.1

Oriented 6.2 6.5 7.0 6.2 4.1 4.2

Table 2: Summary of biaxial orientation test results (3 x 3 draw ratio at 100°C)Structure A A B B C C

extrusion Process AQ WQ AQ WQ eVOh-1 eVOh-2

Yield stress (mPa) 12.2 8.0 18.0 10.5 3.5 1.3

Ultimate engineering stress (mPa) 29.3 24.8 39.4 28.8 7.9 8.3

modulus of resilience (mPa) 0.96 0.43 1.1 0.64 0.20 0.17

modulus of toughness (mPa) 39.4 29.4 57.2 35.5 13.0 12.6

polymers help deliver a solar future …€¦ · june/july 2012 polymers help deliver a solar...

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