High Temp Pet

Hot Fillable Containers Containing PET/PEN Copolymers and Blends

A. Ophir1, S. Kenig1, A. Shai2, Y. Barka’ai2 and J. Miltz*3

Abstract

Bottles and containers made of PET are not suitable for hot filling since the limiting

upper use temperature of this polymer is about 85C. In the present study the properties and

performance of bottles made from copolymers and blends of PET containing NDC groups

and manufactured by the Injection Stretch Blow Molding (ISBM) process were investigated.

These compositions possess advanced properties and can be used up to 95C. The properties

of these bottles were found to depend on their composition and microstructure. The glass

transition temperature, the degree of crystallinity and the induced strains that were measured

by differential scanning calorimetry and differential mechanical thermal analysis are reported.

It was concluded that NDC containing PET based copolymers and blends could be processed

by a one stage ISBM process into containers of improved properties and hot filling

capabilities.

KEYWORDS: PET, PEN, PET CPOLYMERS, POLYESTER BOTTLES

1. Israel Plastics and Rubber Center, Josepho Building, Haifa 32000, Israel.

2. LOG-Plastic Products Co., Ashdot Ya’akov Ichud, Jordan Valley 15155, Israel.

*3 To whom correspondence should be addressed, Dept. of Food Engineering and

Biotechnology, Technion-Israel Institute of Technology, Haifa, 32000, Israel.

E-Mail: [email protected]

1

Introduction

Polyethylene therephthalate (PET) containers are widely used for packaging of

carbonated and non-carbonated beverages, oils and other liquid drinks as well as for

packaging of various solid foods. Its high clarity, good mechanical and gas barrier properties,

lightweight as well as the ease of recycling, favors this material over glass and other polymers

for many food-packaging applications.

Many foods and drinks have to undergo heat treatments like pasteurization and sterilization in

order to kill pathogens and different spoiling microorganisms. Pasteurization is normally

carried out at temperatures below 100C while sterilization is carried out above 120C [1].

These two processes are time-temperature dependent, the higher the temperature the shorter

the time required for the destruction of the microorganisms. During heat treatment, beneficial

components of the food like vitamins, nutrients and flavor compounds are also destroyed.

However, the destruction of microorganisms is more temperature dependent than that of the

beneficial components [2]. Moreover, long heat treatments may cause undesirable changes in

odor and/or taste. Therefore, as far as the food is concerned, high temperature short time

(HTST) treatments are advantageous. One way to obtain a pasteurized liquid food (drink) is

hot filling followed by product cooling. The higher the filling temperature, the shorter is the

heat treatment and the product can be cooled sooner, with lesser damage. A heat treatment of

87-90C for about one minute is normally sufficient for the destruction of the pathogens,

before the product can be cooled [3]. Even heat set PET bottles, do not endure these

conditions and undergo deformations (primarily-shrinkage). Therefore, hot filling is usually

carried out mainly in glass containers. In recent years efforts have been made to modify PET,

and/or its processing conditions, in order to enable hot filling. Such containers would have

important advantages over those of glass: no breakage and significantly lower weight (lower

transportation costs) accompanied by appropriate gas and UV light barrier, recyclability and

multiple application possibilities [4]. Simplified filling lines with no cascade system for

thermal pretreatment of the empty glass bottles required for preventing thermal shock failure

provides the plastic bottles another major advantage. This may result in a considerable

economic advantage.

Recent years are characterized by a large increase in the use of hot-fillable PET, especially in

Japan [5]. This type of PET bottle is generally manufactured by a heat-set method using hot

molds in the stretch-blowing phase, as shown by McChensney and Chung [6]. However, in

2

order to achieve the morphology providing appropriate properties and thermal stability, using

this processing method for manufacturing of PET containers requires a reduction in the

output rate to 40- 55% of the nominal rate of a standard machine. Different reports deal with

heat-set PET products using hot molds [7-9]. The major concerns are the modulus (creep),

strength, and stress relaxation or thermal shrinkage affecting dimensional stability.

With the emergence and commercialization of dimethyl 2,6-naphthalene dicarboxylate

(NDC) monomer, new avenues were opened for the development of thermally enhanced

polyester resins. The key variables in controlling the properties of the NDC containing resins

are its material formulation and process parameters. In addition, the container’s design and

geometry are of a major importance in providing sufficient structural strength for top load

and hydrostatic pressure during hot filling at the elevated temperatures. The bottles have also

to withstand the vacuum collapsing effect associated with the volumetric change during

cooling.

Introducing NDC groups to the primary polyester (PET) results in improved thermal stability,

high strength and high gas barrier properties. The preferred properties of PEN (polyethylene

naphthalate) compared to PET are illustrated in Table 1. PEN has a higher glass transition

temperature, Tg, by 43C than PET (121C vs. 78C) and higher rates of deformational and

thermal crystallization. These advanced properties indicate the potential for manufacturing

containers of superior hot fill capabilities compared to heat-set PET [10]. The development of

the Injection Stretch Blow Molding (ISBM) process also assisted in obtaining containers of

improved performance. This process is based on injection molding of a pre-form first,

followed by a separate stage of stretch-blow molding of the pre-form either in a single or a

two-stage process. In the Single-stage process, a number of pre-forms (typically two to eight)

are injection molded, preconditioned to the required temperature, stretch-blown to the final

size, cooled and ejected on the same unit. A four-station rotary table system is generally

utilized.

In the two-stage process, the pre-forms are injection molded to a close tolerance using multi-

cavity and hot-runner tools. Cooling is rapid, thus preventing the PET from crystallizing. The

amorphous pre-forms are collected and can be stored indefinitely, before reheating and

blowing. In the second stage, infrared, radio frequency or electrical heaters are used to reheat

the pre-forms to above the glass transition temperature. This temperature should, however,

not be as high as to induce spherolitic-type, thermally induced crystallization before blowing.

The temperature profile can be adjusted to obtain an optimum thickness distribution. Biaxial

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stretching is then achieved by mechanical stretching (axial) using a telescopic mandrel, and

radial deformation by pneumatic inflation. During this mutual stretch-blow-molding step,

PET crystallizes with the molecules being bi-axially oriented and the c-axis of the crystallites

lying parallel to the stretch directions. The ISBM process allows greater overall production

flexibility.

In the present study, the properties of bottles prepared from various compositions of

PET/PEN (copolyesters and blends) by the ISBM process, were investigated. The effect of

material formulation and several processing variables on the bottle thermal-mechanical

properties and stability and on oxygen barrier properties was evaluated.

Experimental

Materials

The following polyester resins were supplied by the M&G Polymers and used for the

preparation of bottles from various compositions:

1) PET homopolymer -CLEARTUF 8406.

2) PETN-HIPERTUF 89010, a low-naphthalene content PET copolymer.

3) PENT-HIPERTUF 86017, a low-terephthalate content PEN copolymer.

The PET properties and compositions of the blends and copolymers are given in Table 2.

Methods

One liter bottles, weighing 52.5g were produced by a one stage ISBM on an AOKI 250 LL

machine equipped with a specially designed vented screw and with a capability to heat the

mold up to 170C.

Samples from each of the eight different types of bottles were evaluated by differential

scanning calorimetry (DSC- Perkin Elmer type 7a) and differential mechanical thermal

analysis (DMTA-type 7c) at a rate of 10C. The top loading resistance was measured by

filling the bottles with preheated water (90-92C) and compressing them (top-loading)

immediately in a Karl Frank (Germany) compression tester. The bottles were also tested for

the volumetric thermal shrinkage at elevated temperatures and for oxygen transmission rate.

The volume shrinkage was measured by weighing the empty, followed by ambient water-

filled bottle. Then, the same bottle was filled with hot water (90-92C), kept for 5 minutes,

4

emptied and cooled to room temperature, followed by refilling it with ambient water and

reweighing. The bottle was rapidly wiped and air-dried between steps. From the difference in

the water-filled bottle before and after the hot filling the volume shrinkage was calculated.

The oxygen transmission rate was measured in an Oxtran100 oxygen permeability instrument

(Mocon, Minneapolis, MN.) The effects of mold wall and cooling air (circulated internally)

temperatures on the resulting properties were also studied.

Results and Discussion

The thermal and mechanical properties obtained for specimens cut from the ‘lower panel’

section of the bottle wall (see Fig. 1) are summarised in Tables 3 - 5. In these tables the

results of top loading tests and the bottles volume shrinkage at high temperatures as well as

the oxygen transmission rate (O2TR) for three different mold wall temperatures: 130, 145,

160C are also presented.

Maruhashi and Asada [11] showed, in an early investigation, that the properties of stretch-

blown PET products were poor when the mold wall temperature was kept below 130C, for

short-term heat setting. In the present study it was found that cooling air at temperatures

above 6C, while maintaining the mold wall temperature above 125C, for proper forming of

the molded bottle in a reasonable time cycle, could not be used because of wall sticking

problems.

The volumetric shrinkage of bottles made from the PET homopolymer, the NDC copolymers

and of two blends, at three deferent mold wall temperatures, is shown in Fig. 2. It is evident

that the volume shrinkage, at elevated temperatures, of the blends is higher than that of PETN

and PENT copolymers but lower than that of the pure PET homopolymer. The blend

75%/25% PET/PENT exhibited a substantial reduction in volume shrinkage, particularly at

higher temperatures. It is worthwhile to mention that higher mold temperatures are especially

critical in bottles made of PET/PENT blends for obtaining good barrier and good hot filling

properties. The results observed for the PET and NDC copolymers in the present study are in

accordance with the results reported by Koch and Jaksztat [3], who showed that in the

absence of heat setting, the thermal shrinkage depends largely on the draw ratio.

Crystallinity in the ISBM polyesters containers is achieved primarily via the mechanism of

strain-induced crystallization. By orienting the polymer below the isotropic crystallization

temperature, small size extended chain crystallites are obtained with a non-spherolitic

morphology. The magnitude of the strain required for crystallization varies proportionally

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with the molecular weight because of the relaxation processes. However, molecules of a

lower molecular weight are expected to crystallise at a lower extent of orientation, since the

crystallization rate varies inversely with molecular weight [9]. Heat setting using hot molding

conditions has been found to mitigate volume shrinkage. This is attributed to the thermal

effect on the over all crystallinity level and increased perfection of the strain induced

crystallites, while simultaneously facilitating relaxation of tensed segments in the amorphous

regions. Maruhashi and Asada [12] claimed recently that a biaxially stretched heat set PET

has a minimum shrinkage at 85C when the stretching is carried out under a high stretch rate

and a high temperature. The first facilitates higher crystallinity with self-heat generation

during rapid deformation, while the second causes the amorphous molecular segments to

relax.

The effect of mold wall temperature on the amorphous phase Tg and on the heat of fusion,

Hf, of the crystalline phase in the various compositions is presented in Figs 3 and 4,

respectively. The mold temperature was found to affect more the Tg of the PET/PENT blends

than that of the PET homopolymer and copolymers. Increasing the mold temperature resulted

in an increase in the Tg as well as in Hf (namely, increased crystallinity) of all compositions.

It should be emphasized that the increase in Hf of the blends was less pronounced. This

result may be due to the extent of trans-esterification taking place between PET and PENT

during melt processing as explained by Anderson Zachmann [13]. As a result of trans-

esterification, the Tg is shifted towards intermediate values between those of the two

polymers. Simultaneously, secondary thermal crystallization processes, in the amorphous

phase, is restrained by the formation of inhomogenuities in the molecular structure and the

crystallinity is inhibited [13].

Two factors may enhance the dimensional stability of oriented PET products; (a) An increase

in the Tg of the polymer that reduces the extent of molecular relaxation in the amorphous

phase, (b) The introduction of microscopic heterogeneity in the form of rigid crystallites that

hinder the movements of neighboring molecular segments in the amorphous regions. These

two factors can be achieved by copolymerization or blending PET with a partially miscible

semi-crystalline polymer having a higher Tg (e.g. PEN). In addition, such a polymer should

have a similar refractive index to that of PET, to maintain clarity.

The effect of dwell time and processing temperatures on the degree of transesterification in

PET/PENT blends was studied using a batch Brabender mixer and chracterized by DMTA.

Results indicated that a significant increase in the level of transesterification took place in the

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70/30 blends, even during short dwell times. This result elucidates the reduction in the overall

properties of this blend, in spite of the considerable increase in Tg, due to the decrease in

crystallinity as evident by the low Hf .

The relationship between thermal shrinkage and molecular or crystallisation rearrangements

taking place when oriented products are heated above the Tg is not completely understood. It

was reported [14] that in neat PET, thermal shrinkage could be correlated with the peak

crystallization temperature, Tc. Tc vanished altogether for samples exhibiting a low level of

shrinkage. This implies that enhanced dimensional stability of drawn PET is associated with

the increased level of crystallinity acquired through strain-induced crystallization (increased

Hf), followed by heat setting. However, in the case of blends the morphology of the drawn

samples may also play an important role, in addition to the degree of crystallinity.

Figure 5 shows the Storage Modulus of PET, the copolymers and the blends. It is evident that

the PENT copolymer possesses the highest and PET the lowest modulus while the PETN

copolymer and the blends possess values in between these two. It can also be seen that the

storage modulus was found to increase with the mold temperature for all compositions

(probably due to an enhanced crystallization) except of the PENT copolymer.

Figure 6 depicts the effect of mold wall temperature on the oxygen transmission rate (O2TR).

The decrease in oxygen permeability with the increase in mold temperature is in agreement

with the increase in Tg and the degree of crystallinity when the mold temperature is

increased, in agreement with the trend in the change in modulus with mold temperature. The

75/25 blend and PENT copolymer had the best barrier performance which could be

correlated also with the lowest shrinkage (greater structural stability).

Conclusions

Improved thermal and mechanical properties for hot filling could be realized when using

NDC containing PET based copolymers and blends using the ISBM process. The properties

of both types of materials, PETN copolymer and PET/PENT blends, in a one-stage ISBM

made containers, were found to depend on the polymer composition as well as on the

microstructure that could be controlled by the process variables. The availability of PET/PEN

copolymers and blends may open opportunities for providing a range of new materials for the

production of a one stage ISBM containers with improved physical properties and suitable for

hot filling.

7

References

1. Potter, N.N. and Hotchkiss,J.H. Food Science. Fifth Addition. Aspen, Gaithesburg, MD

(1998).

2. Reid, D.S. and Harris, L.J. Microorganisms and Microbial Toxins. In: Jackson, L.S.,

Knize, M.G. and Morgan, J.N. (Eds). Impact of Processing on Food Safety, 459,9-21

Kluwer -Academic/Plenum Publ., New York (1999).

3. M. Koch and W. Jaksztat, Konstoffe Plast Europe, 85, 1323 (1995).

4. V. Venet, Plast Europe (Hanser), 1, 61 (1993).

5. R. Po, E. Occhiello and G. Giannotta, Polymers for Advanced Technologies, 7, 365

(1996).

6. C.E. McChensney and T.S. Chung, SPE ANTEC Tech. papers, 29, 816 (1983).

7. C.M. Roland, Polym. Eng. Sci., 31, 849 (1991).

8. S.K. Sharma and A. Misna, J. Appl. Polym. Sci., 34, 2231 (1987).

9. L. Mascia and Z. Fekkai, Polym. Networks Blends, 2, 197 (1992).

10. M. Heinrich, Amoco Chemical Company, “Naphthalote-Based Polyesters: A Case

Study”,

Bev Pak’94, April 11-12, (1994).

11. Y. Maruhashi and T. Asada, Polym. Eng. and Sci., 32, 481 (1992).

12. Y. Maruhashi and T. Asada, Poym. Eng. and Sci., 36, 483 (1996).

13. E. Anderson and H.G. Zachmann, Colloid Polym. Sci., 272, 1352 (1994).

14. Z. Fekkai, PhD Thesis, Loughborough University, UK (1991).

8

Table 1: Comparison Between Properties of PET and PEN*

Property / Polymer PET PEN

Elastic Modulus, GPa 11.8 17.6Tensile Strength, Mpa 440 590Glass Transition Temperature, C 78 121Thermal Resistance, C 120 150Thermal Shrinkage at 150 C, % 1.5 0.9Radiation Resistance, MGy 2 11Hydrolysis Resistance, h 50 200

* From Teijin Film Brochure

9

Table 2: Materials description and basic properties.

Material Description Intrinsic Viscosity

(dl/g)

Melting Point - Tm

(oC)

PET

PETN

PENT

PET/PENT

M&G, CLEARTUF 8406 – homopolymer

M&G, HIPERTUF 89010 – copolymer

(low naphthalate content )

M&G, HIPERTUF 86017 – copolymer

(low terephthalate content )

Blends of CLEARTUF 8406 – homopolymer with minor content of HIPERTUF 86017 - copolymer

0.84

0.85

0.55

------

251

241

248

--------

10

Table 3: DSC and DMA thermal/mechanical properties, Volume shrinkage at 900C, and O2TR for bottles made of PETN – copolymer, PENT – copolymer, PET/PENT – blends and PET – homopolymer; Blow mold wall temperature was kept at 1300C and cooling air at 350C.

Sample code Tg

(oC)

Tm

(oC)

Hf

(J/g)

Crystallinity

(%)

E’(25OC(

(MPa)

E’(90OC(

(MPa)

Top loadat

90oC

(N)

Volume Shrinkage at 90 C

(%)

O2TR

(cc/day.pack)

Homopolymer-

PET

Copolymer-

PETN

Copolymer-

PENT

Blend-PET/PENT (90/10)





84.7

88.8

118.5

87.0

89.9

89.2

91.3

92.1

248.1

238.8

248.2

238.4

235.3

232.4

228.7

223.7

45.2

38.7

45.5

39.9

39.5

39.1

37.0

33.9

33.2

34.5

-----

-----

-----

-----

-----

-----

2447

3025

3696

2691

2663

2724

3198

2690

2032

2724

3348

2248

2216

2299

2711

2318

224

282

345

235

232

245

294

219

9.2

2.8

0

4.6

4.2

4.2

3.5

4.5

0.096

0.066

0.021

0.081

0.080

0.070

0.051

0.046

11


Sample code Tg

(oC)

Tm

(oC)

Hf

(J/g)

Crystallinity

(%)

E’(25O

C(

(Mpa)

E’(90OC(

(MPa)

Top loadat

90oC

(N)

Volumeshrinkageat 900C

(%)

O2TR

(cc/day.pack)

Homopolymer-PET

Copolymer-PETN

Copolymer-PENT






85.1

91.8

119.0

89.9

90.8

91.5

92.4

94.4

248.7

239.4

258.0

238.8

234.9

232.5

229.1

224.9

46.9

41.3

46.0

42.5

41.6

40.8

39.8

34.4

34.4

36.8

-----

-----

-----

-----

-----

-----

2557

3304

3696

2733

2676

2812

3392

2781

2188

3098

3348

2351

2372

2447

3114

2393

247

308

358

262

258

266

318

244

4.9

0.6

0.0

1.2

1.3

1.2

0.9

1.4

0.086

0.042

0.021

0.061

0.066

0.056

0.030

0.039

12


Sample code Tg

(oC)

Tm

(oC)

Hf

(J/g)

Crystallinity

(%)

E’ (25OC(

(MPa)

E’ (90OC(

(MPa)

Top load

at 90oC

(N)

Volumeshrinkage at 900C

(%)

O2TR

cc/(day.pack)

Homopolymer-

Copolymer-PETN

Copolymer-PENT

Blend-PET/PENT

Blend-PET/PENT

Blend-PET/PENT

Blend-PET/PENT

Blend-PET/PENT

85.6

92.8

119.0

90.4

91.3

92.2

93.6

95.8

248.2

238.9

258.0

239.0

235.8

232.1

228.5

225.1

49.2

42.7

46.5

43.4

42.1

41.5

40.4

35.5

36.1

38.1

-----

-----

-----

-----

-----

-----

2644

3395

3696

2758

2703

2845

3428

2794

2350

3158

3348

2405

2417

2511

3146

2422

268

315

358

278

275

281

322

262

2.9

0.3

0.0

0.7

0.9

0.5

0.4

1.2

0.070

0.037

0.021

0.053

0.060

0.050

0.022

0.036

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Figures Captions

Figure 1: Picture of a PET Bottle.

Figure 2: Volume shrinkage vs. blow mold wall temperature

Figure 3: Tg vs. blow mold wall temperature

Figure4: ∆Hf vs. blow mold wall temperature

Figure 5: Storage Modulus (E' at 90C) vs. blow mold wall temperature

Figure 6: O2TR vs. blow mold wall temperature

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Figure 1: Picture of a PET Bottle.

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Lower panel of bottle

Figure 2: Volume shrinkage vs. blow mold wall temperature

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Figure 3: Tg vs. blow mold wall temperature

17

Figure4: ∆Hf vs. blow mold wall temperature

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Figure 5: Storage Modulus (E' at 90C) vs. blow mold wall temperature

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Figure 6: O2TR vs. blow mold wall temperature

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