ARTICLE IN PRESS

Radiation Physics and Chemistry 78 (2009) 563–566

Contents lists available at ScienceDirect

Radiation Physics and Chemistry

0969-80

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/radphyschem

Improving processability of polyethylenes by radiation-inducedlong chain branching

Song Cheng, Edward Phillips, Lewis Parks �

Sterigenics Advanced Applications, 7695 Formula Place, San Diego, CA 92121-2418, United States

a r t i c l e i n f o

Keywords:

Polyethylene

Radiation

Long chain branching

Melt strength

Processability

6X/$ - see front matter & 2009 Elsevier Ltd. A

016/j.radphyschem.2009.03.043

esponding author.

ail address: lparks@sterigenics.com (L. Parks)

a b s t r a c t

Long chain branching (LCB) was induced on carefully selected grades of high-density and linear low-

density polyethylenes (LLDPEs) by electron-beam irradiation under ambient conditions at controlled,

relatively low-radiation doses. Although there was little or no crosslinking, the molecular weight

distribution became wider after irradiation. The melt flow indices (MFIs) of these polymers decreased

significantly with increasing radiation dose. However, differences in melt flow index changes were

observed with other irradiated polyethylenes, most likely because of differences in the polyethylene

resins, including those in additives, manufacturing processes, and types of copolymers. The melt

strength of the irradiated polymers from selected grades as measured by the Rheotens method was

enhanced considerably. Furthermore, the melt viscosity at low shear rates was higher after irradiation.

Because of long chain branching, these rheological properties in the irradiated polymers provide both

strain hardening at high extensibility and sag resistance at low shear rate to bring about significant

improvement of processability for certain polymer converting processes, such as blown film.

Preliminary blown film trials confirmed the processability improvement.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The use of ionizing radiation to modify polymers and toimprove material properties is well known (Charlesby, 1960;Woods and Pikaev, 1994). For example, electron-beam andgamma-ray irradiation are widely used to crosslink polyethylene(PE). Some formed parts made from PE, such as cable and wireinsulation, tubing, films, and fibers, are sometimes irradiated atdoses of 100 kGy and above to cause crosslinking of polymerchains to improve properties, such as mechanical strength,thermal resistance, flame retardancy, and chemical resistance.After radiation crosslinking, the PE has significant gel content (i.e.,60–100%), and cannot be reprocessed.

Irradiation of PE resins before conversion into formed parts isnot as common as irradiation of the formed parts themselves.Significant crosslinking from irradiation creates gels and resinstreated with doses similar to those used for formed parts wouldlose their processability in such processing methods as extrusion,film blowing, foaming, blow molding, etc. To modify PE withoutsignificant gel content, lower radiation doses must be used.

When PE is irradiated at relatively low dose and dependingupon the irradiation conditions (e.g., the atmosphere in whichirradiation occurs), the polymer may concurrently undergo

ll rights reserved.

.

different reactions, including degradation, crosslinking, long chainbranching (LCB, which is branching containing more than 40carbon atoms), and oxidation. In recent years, Sterigenics devel-oped a family of radiation-modified polyethylene resins withsignificant LCB but without significant gel content by irradiatingbase PE resins in air at relatively low doses using technologylicensed from Gammatron (Du Plessis et al.). Such radiationmodification of carefully chosen PE base resins can causesignificant improvement of processability and properties of theresins (Cheng et al., 2005; Du Plessis et al., 2006; Cheng andPhillips, 2006). The base resins were selected from commerciallyavailable linear PE resins, including HDPEs and linear low-densitypolyethylenes (LLDPEs).

Melt strength of PE is related to the polymers’ molecularstructures, including the degrees of branching, throughout thematerial and the molecular weight distribution. For example,low-density polyethylenes have higher melt strength compared toother types of PEs because the polymers inherently contain LCB.On the other hand, the more linear HDPEs and LLDPEs havecomparatively lower melt strength because they have no LCB.LCB created in originally linear PE can enhance its melt strength(DeNicola Jr. et al., 1996; Dealy and Wissbrun, 1990). In thispaper, evidence of LCB created by radiation modification of linearPEs is presented. Consequent melt strength improvement isdemonstrated and its implications on processability are discussed.The challenges of achieving LCB using irradiation are alsodiscussed.

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S. Cheng et al. / Radiation Physics and Chemistry 78 (2009) 563–566564

2. Experimental

2.1. Materials

Three commercially available polyethylene grades were se-lected as base resins for radiation modification. BR-201 is anun-irradiated HDPE homopolymer typically used for injectionmolding applications. BR-300 and BR-LLD are un-irradiated filmgrade LLDPEs and both are ethylene–octene copolymers. Table 1shows the melt flow indices (MFIs) and the densities of the baseresins. The resins were used as received from the supplier.

All base resins were irradiated at various doses using a 12 MeV,8 kW electron-beam accelerator. The irradiated resins are correspond-ingly called IR-201, IR-300, and IR-LLD. Additives, such as antio-xidants, were added to IR-300 through a masterbatch for some of themeasurements. The blend with the masterbatch is called R-300.

2.2. Gel content

Gel content of the polymers were determined by Soxhletextraction according to ASTM D2765-01 (2006) (ASTM D2765).

2.3. Melt flow index

Melt flow index values of the PE resins were measuredaccording to ASTM D1238-04c using a Dynisco instrument at190 1C, and 2.16 and 10 kg loads (ASTM D1238).

2.4. Capillary rheology

Melt viscosities of BR-201 and IR-201 were measured as afunction of shear rate using a capillary rheometer at 190 1C.

2.5. Rheotens tests

A Gottfert Rheotens instrument was used for Rheotens tests for BR-201, IR-201, BR-300, and R-300. Rheotens is a technology developed toquantitatively characterize the melt strength and extensibility ofpolyolefins (Wagner et al., 1998) by combining the capillary rheometerwith a device that pulls the melt from the capillary die at constantflow rate and at increasing drawing velocity. Drawing force ismeasured as the melt strand extruded from the die is accelerated ontake-away wheels. The maximum drawing force is directly related tomelt strength. The test conditions are listed in Table 2.

Table 1The base resins.

BR-201 BR-300 BR-LLD

Type HDPE LLDPE LLDPE

Melt flow index at 190 1C, 2.16 kg (g/10 min), I2 8.7 1.0 3.1

Density (g/cm3) 0.963 0.920 0.941

Table 2Test conditions for Rheotens.

Wheel position Approximately 114 mm below die

Wheel temperature Ambient

Barrel diameter 12 mm

Die entry angle 1801

Die inner diameter 2 mm

Die length 30 mm

Dwell time 6 min

Barrel temperature 190 1C

2.6. Blown film trials

R-300 and BR-300 were used for the blown film trials. AHofokawa Alpine blown film machine with a 63 mm screw and a120 mm diameter annular die was used. Thick films of 0.1 mmwere made using a die gap of 2 mm. The blow-up ratio (the ratioof the diameter of the bubble to the diameter of the die) wasvaried upward from 2.0 to 4.0.

3. Results and discussion

3.1. Evidence of negligible crosslinking

Table 3 shows the gel content measurement results for IR-201,BR-201, IR-300, and BR-300, which indicate that the gel content ofall of the irradiated resins is very low. No gels were visuallyobserved for any of the resins in their melt state, indicating nosignificant crosslinking after the radiation modification.

3.2. Evidence of long chain branching

Gel permeation chromatography (GPC) characterization in ourprevious study (Cheng et al., 2005) showed that the polydispersity(Mw/Mn) of irradiated HDPE (i.e., IR-201) increases with increasingradiation dose, indicating increasingly widened molecular weightdistribution that is a result of LCB. Additionally, the slight decreaseof Mn and the increase of Mw with increasing dose indicate thesimultaneous occurrence of chain scission and LCB.

The MFIs of IR-201 (HDPE) at various radiation doses measuredunder 2.16 kg load (I2) and 10 kg load (I10), both at 190 1C, areshown in Table 4.

Both I2 and I10 decrease with increasing radiation dose. Themelt flow rate ratio, defined as the ratio of I10/I2, increasessignificantly with increasing radiation dose. Decrease of the MFIsand increase of the melt flow rate ratios also indicate that LCBoccurred after the radiation modification (Rowland et al., 2000).

Radiation-induced reactions of PE depend largely on theconditions of irradiation. The irradiation atmosphere and doserate impact the competition of degradation, crosslinking, and LCB.For example, when the PE is irradiated in inert gas or vacuum,the predominant reactions tend to be crosslinking and LCB(DeNicola Jr. et al., 1996). In contrast, when the PE is irradiatedin air, oxidative degradation becomes significant (Randall et al.,1983). Fourier transform infrared (FTIR) spectra of gamma- andelectron beam-irradiated HDPE samples in our previous study(Cheng et al., 2005) showed that gamma irradiation created highercarbonyl group concentration than electron beam irradiation,

Table 3Gel content of IR-201, BR-201, IR-300, and BR-300.

Resin IR-201 BR-201 IR-300 BR-300

Gel content 0.28% 0.05% 0.24% 0.02%

Table 4MFI and Melt Flow Rate Ratio of IR-201.

Electron beam

surface dose (kGy)

I2 (g/10 min) I10 (g/10 min) Melt flow

rate ratio (I10/I2)

0 8.65 48.0 5.55

8 3.70 31.7 8.58

16 0.85 13.6 16.0

24 0.12 7.77 64.7

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S. Cheng et al. / Radiation Physics and Chemistry 78 (2009) 563–566 565

indicating the possibility that gamma irradiation may yield moreoxidation and oxidative degradation.

We have found that radiation-induced reactions of PE alsodepend upon the specific grade of the PE resin. The change of MFIwith increasing radiation dose can vary from one PE grade toanother. For example, Fig. 1 illustrates the different relationshipsfor MFI vs. radiation dose for IR-300 and IR-LLD, both LLDPEs.

For IR-300, the rapid decrease of the MFI from the onset ofirradiation is a good indication that LCB is a predominant reactionthroughout the dose range. For IR-LLD, however, the MFI does notchange for the first few kGy of dose, and only starts to decreaseafter an ‘‘induction’’ dose. This makes the selection of the baseresin important to achieve the desired effects.

3.3. Effects of LCB on rheology

3.3.1. Increased melt viscosity at low shear rate

Capillary rheology results for IR-201 and BR-201, shown inFig. 2, show that the irradiated polymer and its base resindemonstrate typical shear-thinning behavior (decreasing viscositywith increasing shear rate). However, melt viscosity increased at

0

0.2

0.4

0.6

0.8

1

1.2

0 4 8 12 16

Nor

mal

ized

MFI

(@19

0C, 2

.16k

g)

IR-300

IR-LLD

Fig. 1. Melt flow index (MFI) vs. radiation dose for IR-300 and IR-LLD. The MFIs are

normalized at 0 kGy for comparison purpose.

1.00E+02

1.00E+03

1.00E+04

1.00E+05

Mel

t Vis

cosi

ty (P

a.s)

Shear Rate (s-1)0.0

0E+0

0

1.50E

-01

3.00E

-01

6.00E

-01

1.50E

+00

3.00E

+00

6.00E

+00

1.20E

+01

5.00E

+01

1.00E

+03

2.50E

+03

BR-201

IR-210

Fig. 2. Capillary rheology results for IR-201 and BR-201.

lower shear rates after the polymer was irradiated, demonstratingsag resistance.

3.3.2. Increased melt strength at high extensibility

Rheotens curves of R-300 and BR-300 are presented in Fig. 3.Comparison of the Rheotens curves of R-300 with LCB vs. the

more linear BR-300 is dramatic. At about 70 mm/s drawingvelocity, the BR-300 begins to exhibit melt instability in the formof a sinusoidal, undulating draw resonance. IR-300 does not havesuch melt resonance before the break at about 105 mm/s. Themelt strength of the R-300 was about 20 cN when it broke, whileBR-300, the linear precursor, began to exhibit draw resonance atthe drawing force of 4 cN.

Rheotens curves of IR-201 at various doses and BR-201base resin are shown in Fig. 4, which illustrate how the meltstrength evolves when the resin is irradiated at increasingdoses. The enhancement of melt strength with dose is veryevident.

3.4. Processability advantages

3.4.1. Sag resistance

It is important to view melt strength over a broad shear raterange. In some processes the polymer melt is required to exhibit

0.0

2.0

4.0

6.0

8.0

10.0

12.0

0 50 100 150 200 250 300 350

Mel

t Stre

ngth

(cN

)

Velocity (mm/s)

0 kGy

8 kGy

10.8 kGy

16 kGy

Fig. 4. Rheotens curves of IR-201 at various radiation doses.

0

5

10

15

20

0 50 100 150 200 250 300 350 400 450

Forc

e (c

N)

Velocity(mm/sec)

BR-300

IR-300

Fig. 3. Rheotens curves of R-300 and BR-300.

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S. Cheng et al. / Radiation Physics and Chemistry 78 (2009) 563–566566

high melt strength at low shear rates, i.e., sag resistance. Forexample, in blow molding of large parts, the hanging parison willresist sagging if the polymer possesses such melt characteristics.As Fig. 2 indicates, the radiation-modified PEs with LCB possesssuch desirable rheological characteristics.

3.4.2. Strain hardening

The high melt strength behavior of the radiation-modified PEs,such as that shown in Fig. 3 and Fig. 4, means that these resinsdemonstrate strain hardening rheological behavior. Many meltprocesses can benefit greatly from strain hardening (Dealy andWissbrun, 1990). For example, enhancement of melt strength canimprove foamability in foaming applications (Bradley and Phillips,1991). As another example, after the mold closes and formingpressure is applied in blow molding, extensional rates areaccelerated and the polymer must resist thinning as material isdistributed into the far corners of the part.

Polymers exhibiting strain hardening or high-extensionalmelt strength deform uniformly as this stress is applied to themelt. In extrusion coating, the melt curtain is pulled from thedie onto the substrate at line speeds greater than 5000 mm/s.Linear polymers usually exhibit neck-in and sometimes showmelt resonance at draw ratios and extensibilities far below theprocess requirements for economic production rates. Radiation-modified high melt strength PEs can be expected to havesignificantly improved processability under these processingmethods.

Strain hardening can improve the bubble stability during filmblowing. In our blown film trials with R-300, films were madewith stable bubbles using blow-up ratios ranging from 2.0 to 4.0.In contrast, the blow-up ratio could only go as high as 2.8 for un-irradiated BR-300 because the bubble became unstable for lack ofmelt strength.

It is important that the processing conditions be optimized inorder for the processability advantages to be fully attained duringprocessing. For example, excessively high temperature and highshear should be avoided because some radiation-modified PEsmay be sensitive to these conditions.

4. Conclusions

Long chain branching can be introduced onto carefully selectedgrades of HDPE and LLDPE by irradiating the resins with electron-beam in air at low doses. The irradiated PEs with LCB demonstrateincreased melt viscosity at low shear rate and increased meltstrength at high extensibility. These rheological characteristicsmay be translated into processability advantages in terms of sagresistance and strain hardening, which can benefit various plasticprocessing methods, such as extrusion, blown film, foaming, andblow molding. Optimization of these advantages depends uponproper selection of the base resin, irradiation dose distribution,additives (such as antioxidants) added after irradiation, andproper setup of the processing equipment.

References

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ASTM D2765-01, 2006. Standard Test Methods for Determination of Gel Contentand Swell Ratio of Crosslinked Ethylene.

Bradley, M., Phillips, E., 1991. Novel polypropylene for foaming in conventionalequipment. Plastics Engineering 82 (March).

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Dealy, J., Wissbrun, K., 1990. Melt Rheology and Its Role in Plastics Processing. VanNostrand Reinhold, New York.

DeNicola Jr., A., Mayfield, J., McLaughlin, T., Beren, J., 1996. High Melt StrengthEthylene Polymer, Process for Making It, And Use Thereof. US Patent 5,508,319.

Du Plessis, T., Cheng, S., Suete, H., 2006. Radiation Treated Ethylene Polymers andArticles Made From Said Polymers. US Patent 7,094,472.

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Rowland, M., Turley, R., Hill, J., Kale, L., Kummer, K., Lai, S., Chum, P., 2000.Rheology-modified Polyolefins. US Patent 6,114,486

Wagner, M., Bernnat, A., Schulze, V., 1998. The rheology of the rheotens test.Journal of Rheology (42), 917–928.

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