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Radiation Physics and Chemistry 79 (2010) 329–334

Contents lists available at ScienceDirect

Radiation Physics and Chemistry

0969-80

doi:10.1

� Corr

E-m

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

Processability improvement of polyolefins throughradiation-induced branching

Song Cheng, Ed 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

Polypropylene

Radiation

Long-chain branching

Melt strength

Processability

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

016/j.radphyschem.2009.08.046

esponding author.

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

a b s t r a c t

Radiation-induced long-chain branching for the purpose of improving melt strength and hence the

processability of polypropylene (PP) and polyethylene (PE) is reviewed. Long-chain branching without

significant gel content can be created by low dose irradiation of PP or PE under different atmospheres,

with or without multifunctional branching promoters. The creation of long-chain branching generally

leads to improvement of melt strength, which in turn may be translated into processability

improvement for specific applications in which melt strength plays an important role. In this paper,

the changes of the melt flow rate and the melt strength of the irradiated polymer and the relationship

between long-chain branching and melt strength are reviewed. The effects of the atmosphere and the

branching promoter on long-chain branching vs. degradation are discussed. The benefits of improved

melt strength on the processability, e.g., sag resistance and strain hardening, are illustrated. The

implications on practical polymer processing applications such as foams and films are also discussed.

& 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Modification of a polymer using ionizing radiation can be donebefore or after the polymer has been processed into formed parts.When formed parts made from polymers are modified byradiation, crosslinking is usually the desired reaction, with thepurpose to enhance the physical properties of the parts (Clelandet al., 2003). This technology has found many useful applications.For example, cable and wire insulation, tubing, films and fibersmade from polyethylenes are sometimes irradiated at radiationdoses of 100 kGy and more to induce crosslinking to improve suchproperties as mechanical strength, thermal resistance, flameretardancy and chemical resistance. After radiation crosslinking,polyethylene has significant gel content (i.e., 60–100%), andcannot be reprocessed.

For radiation modification of polymer resins prior to proces-sing into formed parts, various radiation-induced reactions,including crosslinking, degradation, branching and grafting, canbe taken advantage to add value to the polymer material. Anexample is radiation degradation of polytetrafluoroethylene(PTFE) to make ultra-fine polymer powders (Cleland et al.,2003). There have been few uses of pre-processing radiationcrosslinking of polymer resins, because significant crosslinkingcreates gels in the polymer causing the polymer to loseprocessability. However, it has been found that certain polyolefins,

ll rights reserved.

.

particularly polypropylenes and polyethylenes, can be modifiedwith ionizing radiation at low radiation doses without incurringsignificant crosslinking to improve the resin properties withoutaffecting recyclability. In this case, the main reaction that bringsabout the improvement is long-chain branching (defined asbranches containing more than 40 carbon atoms). The mostsignificant improvement is made on the rheological properties(e.g., melt strength) and hence the processability of the polymers(Sheve et al., 1986, 1990; DeNicola Jr. et al., 1996; Yoshii et al.,1996; Cheng and Phillips, 2006; Lugao et al., 2007).

In the late 1980s and early 1990s, Himont (which later becameMontell) developed ‘‘gel-free’’ high melt strength polypropyleneresins (PPs) for extrusion coating by radiation-induced long-chainbranching (Sheve et al., 1986), and Bradley and Phillips reportedthat the improvement of the melt strength enabled foaming usingthese novel high melt strength PPs (Bradley and Phillips, 1991).The commercial success of these PPs helped inspire continuedresearch and development on radiation modification of PP toimprove melt strength through long-chain branching (Yoshii et al.,1996; Lugao et al., 2007).

A similar approach was applied to polyethylene (PE) byMontell to make high melt strength PE in the 1990s (DeNicolaJr. et al., 1996). However, this high melt strength PE has not hadthe same commercial success as high melt strength PP. In recentyears, Sterigenics has developed a family of radiation-modified PEresins based on technology licensed from Gammatron (Du Plessiset al., 2006). By irradiating PE resins in air at relatively low dosesin combination with additional proprietary processing, resinswith significant long-chain branching but with insignificant gel

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content were made and used in various polymer processingmethods and applications. The modified PE resins offer signifi-cantly improved processability and end-use properties (Chenget al., 2005; Du Plessis et al., 2006; Cheng and Phillips, 2006).

In the following, radiation modification of PP and PE to causelong-chain branching without gels is reviewed. The effects ofradiation conditions and other important factors and theimplications on resin processability are discussed.

2. Polypropylene

As a commodity resin, PP has many good properties, e.g.,higher heat resistance than PE. However, PP’s poor melt strengthis a significant drawback that seriously limits the processability ofPP for applications such as foaming, blow molding and extrusioncoating. Melt strength of PP or PE is related to the polymers’molecular weight distributions and molecular structures, includ-ing the degree of long-chain branching. Increasing the degree oflong-chain branching is an effective way to enhance meltstrength. Normally, as shown in the study by Ghijsels and DeClippeleir, the greater the degree of long-chain branching, thehigher the melt strength, and the lower the melt flow rate(Ghijsels and De Clippeleir, 1994). Irradiation is one of the ways tocreate long-chain branching in PP.

2.1. Irradiation in oxygen-free or reduced oxygen atmosphere

Irradiation of PP is usually done in ‘‘oxygen-free’’ or reducedoxygen atmosphere to minimize the oxidative degradation thatoccurs when irradiated in air. In the example of the Himont highmelt strength PP invented by Sheve et al. the irradiation is done ina controlled atmosphere in which the active oxygen concentrationis less than 15%, preferably less than 0.004%, by volume (Sheveet al., 1986). Long-chain branching with a branching index of lessthan 0.9 is created by irradiation. Elongational rheology analysis(Fig. 1) shows that irradiated PP gains strain hardening (anincrease in resistance to thinning during elongation of thepolymer melt) and fails by fracture, while the un-irradiatedlinear PP had ductile failure and no strain hardening (Sheve et al.,1986).

Fig. 1. Strain hardening behavior of irradiated PP (1 Mrad=10 kGy) (Sheve et al.,

1986).

Sugimoto et al. irradiated PP in nitrogen atmosphere using a2 MeV electron-beam accelerator and compared the radiationbranching with chemical branching by using di-2-ethylhexylperoxy dicarbonate (EHPC) (Sugimoto et al., 1999). The meltstrength of the irradiated PP, measured by the Rheotens method(Wagner et al., 1998), increased significantly with increasingradiation dose (from 2 cN for un-irradiated to 12 cN at 80 kGy).The melt flow rate only had a small increase (from 2 g/10 min forun-irradiated to 4 g/10 min at 80 kGy). Higher melt strength wasachieved by radiation branching at 80 kGy (12 cN) than bychemical branching (5 cN). The degree of branching of theirradiated samples was characterized by the combination of gelpermeation chromatography (GPC) and low angle laser lightscattering (LALLS). These measurements show that the meltstrength increases with increased degree of branching, ascharacterized by the branching number for the first elutedfraction. Comparison of measured elongational viscosity vs. timefor PP irradiated at 80 kGy and the unmodified PP indicates thatradiation modification creates strain hardening behavior similarto that observed by Sheve et al., as shown in Fig. 1.

Auhl et al. studied the rheological properties of long-chainbranched PP irradiated by electron-beam under nitrogen atmo-sphere (Auhl et al., 2004). They found that the ratio of the zeroshear viscosities of branched PP and linear (un-irradiated) PP(Z0(br)/Z0(lin)) increased sharply with the increase of radiationdose before 10 kGy, and then decreased with the increase ofradiation dose after 10 kGy (as shown in Fig. 2). They attributedthe trend to the increase of the number of long-chain branchesand decrease of molecular weight with increasing dose, withbackbone scissioning overtaking the creation of long-chainbranching after 10 kGy.

Lugao et al. irradiated PP under acetylene atmosphere withelectron-beam and under acetylene/hydrogen atmosphere withgamma irradiation (Lugao et al., 2000). The molecular weightdecreases with increasing radiation dose for both atmospheres,indicating that degradation still predominates at such low doses.However, the samples irradiated in acetylene/hydrogen showincreased melt strength and increased polydispersity (ratio ofweight average molecular weight to number average molecularweight), indicating that there was also long-chain branching. Inanother study by Lugao et al., temperature rising elutionfractionation (TREF) was used to characterize the change ofmolecular weight distribution of PP irradiated under acetyleneatmosphere with gamma radiation (Lugao et al., 2002). There

0

1

2

3

4

5

6

0Radiation Dose (kGy)

η 0 (b

r)/η

0 (li

n)

20 40 60 80 100 120 140 160

Fig. 2. Ratio of the zero shear viscosities Z0 of branched and linear PP as a function

of radiation dose (Auhl et al., 2004).

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0

1

2

3

4

5

6

7

8

0EB Dose (kGy)

Mel

t Str

engt

h (g

f)

N2Air

2 4 6 8 10

Fig. 3. Melt strength comparison of PP with HDDA at a concentration of 1.5 mmol/

100 g PP irradiated under atmospheres of nitrogen and air atmosphere (Yoshii

et al., 1996).

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

0Radiation Dose (kGy)

TAIC

Con

tent

(wt%

)

Degradation

Long chain branching

Crosslinking, microgel

Croslinking, gel

20 40 60 80 100 120 140

Fig. 4. Effect of radiation dose and TAIC content on the PP radiation chemistry

(Schulze et al., 2003).

S. Cheng et al. / Radiation Physics and Chemistry 79 (2010) 329–334 331

were only changes in the high molecular mass (high elutiontemperature) fraction, indicating that there was probably Y-typebranching. However, these changes still brought about significantimprovement of melt strength.

2.2. Irradiation in molten state

Krause et al. electron-beam irradiated a PP in solid state in airatmosphere at room temperature (25 1C), and in molten state atan elevated temperature of 200 1C (Krause et al., 2006). Theirradiated samples were analyzed by a coupling of size exclu-sion chromatography and multi-angle laser light scattering(SEC–MALLS), and by differential scanning calorimetry (DSC)and elongational viscosity measurement. SEC–MALLS shows thatthe weight average molecular weight decreases with increasingelectron-beam dose, and there was more decrease for the moltenstate, indicating that there was degradation. However, the meansquare radius of gyration, /s2S, also decreases with increasingelectron-beam dose, indicating the occurrence of long-chainbranching. At 20 kGy dose, the decrease of /s2S was greater forirradiation in the molten state than at room temperature. Theelongational viscosity of the sample irradiated in molten state at10 kGy shows similar strain hardening behavior observed bySheve et al. (Fig. 1).

2.3. Irradiation with the addition of branching promoter

Oxidative degradation always competes with crosslinking orbranching when polymers are subjected to ionizing radiation.Although oxygen-free atmosphere is beneficial and necessary totilt the equilibrium towards long-chain branching in the case ofPP, such atmosphere requires special radiation processing techni-ques and specialized equipment in the irradiation facility. Sincethe 1990s, studies have been done to introduce long-chainbranching onto PP using radiation with the addition of radiationsensitizer in the PP composition, so that the desired long-chainbranching may be achieved when the PP is irradiated in air. Suchsensitizers, or ‘‘prorads’’, are normally multifunctional monomerscommonly used as radiation crosslinking promoters.

Yoshii et al. used various polyfunctional acrylates (PFAs) asradiation branching promoters for PP (Yoshii et al., 1996). The PFAswere combined with PP powder by stirring the mixture for 2 h andthen storing it for 2 days for penetration. Next, irradiation wasdone both in nitrogen and in air, and the results were compared.The melt strength of irradiated PP with the addition of variousPFAs at 2–5 wt% was measured at 230 1C and 20 mm/min take-upvelocity. The results show the most efficient PFAs are shorterchain bi-functional acrylates such as 1,4-butanediol diacrylate(BDDA) and 1,6-hexanediol diacrylate (HDDA). The melt strengthof the PP increased from 1.1 gf (grams force, for un-irradiatedlinear PP) to 6.6 and 7.0 gf for PP irradiated with BDDA and HDDA,respectively. Comparison of melt strengths for materials irra-diated under nitrogen and air atmospheres is shown in Fig. 3.HDDA at a concentration of 1.5 mmol/100 g PP was added for bothatmospheres.

Fig. 3 shows that it takes lower dose to achieve high meltstrength when the irradiation is done in nitrogen. It takes higherdose to reach the same level of melt strength when the sensitizedPP is irradiated in air, but the melt strength still increases withincreasing dose. Higher doses are needed for irradiation in airbecause of the competing oxidative degradation.

Schulze et al. studied electron-beam irradiation of PP in airwith and without triallyl isocyanurate (TAIC) (Schulze et al.,2003), another widely used radiation sensitizer. It was meltblended with PP using a twin screw extruder before irradiation.

Rheometry analysis was used to determine if the sensitized PPwas degraded, crosslinked (to produce microgels or gels) or long-chain branched without significant gels at different radiationdoses, with results summarized in Fig. 4.

Fig. 4 indicates that long-chain branching can be obtained byusing 1–5 wt% TAIC when the sensitized PP is irradiated in airusing low radiation dose. Degradation is expected when there isno TAIC, even at low dose. The PP sensitized with 1–5 wt% TAICcrosslinks and creates microgels at 60–100 kGy. Measurable gelsare created when the dose reaches 130 kGy and the TAIC content is5 wt%.

Lugao et al. separately mixed a PP resin with TAIC and withtrimethylolpropane trimethacrylate (TMPTMA) sensitizers, andthen gamma irradiated the mixtures under nitrogen atmosphere

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at 20 kGy (Lugao et al., 2007). Gel content, melt flow rate, andmelt strength (by Rheotens) were measured at different sensitizerconcentrations. The changes of gel content, melt flow rate, andmelt strength are small when the PP is mixed with the sensitizersby stirring under room temperature. The changes are much moresignificant when the PP is blended with the sensitizers under meltby extrusion than when the PP is mixed with the sensitizers underroom temperature. Although the melt flow rate increases afterirradiation, the melt strength also increases, especially for thePP/TAIC blend. The relationship between the melt strength andthe melt flow rate is different from that shown in the work byGhijsels and De Clippeleir (Ghijsels and De Clippeleir, 1994)because of the concurrence of long-chain branching and degrada-tion. The gel content is larger after irradiation, indicating thatthere is partial crosslinking in addition to long-chain branching.The results also indicate that TAIC is more effective than TMPTMAas a branching/crosslinking promoter. TAIC is more toxic thanmultifunctional acrylates, which would limit its use in someapplications.

0

0.2

0.4

0.6

0.8

1

1.2

0Surface EB Dose (kGy)

Nor

mal

ized

MFI

(@19

0C, 2

.16k

g) IR-LLD

IR-300

4 8 12 16

Fig. 5. MFI vs. radiation dose for IR-300 and IR-LLD. The MFIs are normalized at

0 kGy for comparison purposes.

1.00E+02

1.00E+03

1.00E+04

1.00E+05

0.00E

+00

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

Shear Rate (s-1)

Mel

t Vis

cosi

ty (P

a.s)

BR-201

IR-201

Fig. 6. Capillary rheology of IR-201 and BR-201 (Cheng and Phillips, 2006).

3. Polyethylene

Compared with PP, PE is predominantly crosslinked and lessprone to degradation when irradiated by ionizing radiation. At lowradiation doses, however, it is possible to create long-chainbranching with radiation without creating significant crosslinkingor gels. Among the many PE grades, low density polyethylenes(LDPEs) have higher melt strength because the polymers inher-ently contain significant long-chain branching. On the other hand,the more linear high density polyethylenes (HDPEs) and linearlow density polyethylenes (LLDPEs) have comparatively lowermelt strength because they have no long-chain branching(although LLDPE may contain some short chain branching). Itcan be desirable to introduce long-chain branching onto HDPEsand LLDPEs to enhance the melt strength for processabilityimprovement.

3.1. Irradiation in oxygen-free or reduced oxygen atmosphere

In the Montell patent, DeNicola, Jr. et al. disclosed that PEs withhigh melt strength can be made by irradiating selected HDPEresins or ethylene copolymers in oxygen-free or reduced oxygenatmosphere (DeNicola Jr. et al., 1996). By irradiating an HDPE(Dow 4352N) in the same atmosphere as the inventors used fortheir high melt strength PP development (Sheve et al., 1986), themelt strength of the polymer significantly improves. The meltflow index (MFI) decreases and melt strength increases withincreasing radiation dose.

3.2. Irradiation in air

Cheng et al. studied the rheology of selected HDPEs and LLDPEsirradiated in air using electron-beam irradiation (Cheng andPhillips, 2006). The base resins must be carefully screened andselected to efficiently achieve long-chain branching. The radia-tion-induced reactions and resulting properties can depend uponthe specific grade of the base PE resin. For example, Fig. 5illustrates the different MFI vs. radiation dose relationships fortwo irradiated LLDPEs, denoted as IR-300 and IR-LLD.

For IR-300, the rapid decrease of the MFI from the onset ofirradiation is a good indicator that long-chain branching is apredominant reaction throughout the dose range. For IR-LLD,however, the MFI does not change for the first few kGy ofabsorbed dose, and it only starts to decrease after an ‘‘induction’’

dose. A possible reason for the difference is that IR-LLD maycontain more antioxidant or other radical scavenging stabilizersthan IR-300. Other differences in PE grades, such as the catalyst inPE synthesis, the manufacturing process of the PE, and the co-monomer in LLDPE, may also cause the differences in theradiation chemistry and hence the MFI vs. dose relationship. Thismakes proper selection of the base resin essential for achievingthe desired long-chain branching.

Cheng et al. found other indications of the occurrence of long-chain branching with selected, appropriate base resins of bothHDPEs and LLDPEs. For example, there was an increase in thepolydispersity, an increase in the melt flow rate ratio (I10/I2, ratioof MFI under 10 kg load and MFI under 2.16 kg load), and anincrease in the area of the hysteresis loop for the large amplitudeoscillatory shear (LAOS) curves with increasing radiation dose foran irradiated HDPE resin (denoted as IR-201)—all indicators oflong-chain branching (Cheng et al., 2005; Cheng and Phillips,2006).

Fig. 6 shows the comparison of the melt viscosity of theirradiated IR-201 with that of the un-irradiated HDPE base resin

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0.0

2.0

4.0

6.0

8.0

10.0

12.0

0Velocity (mm/s)

Mel

t Str

engt

h (c

N)

0 kGy8 kGy10.8 kGy16 kGy

50 100 150 200 250 300 350

Fig. 7. Rheotens curves of IR-201 at various radiation doses.

0.0

2.0

4.0

6.0

8.0

10.0

12.0

0Dose (kGy)

MFI

(g/1

0min

.)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0M

elt S

tren

gth

(cN

)

Dow 4352N MFI, reduced oxygenR-201 MFI, in airDow 4352N Melt Strength, reduced oxygenR-201 Melt Strength, in air

. th

2 4 6 8 10 12 14 16 18

Fig. 8. Melt strength and MFI vs. radiation dose for IR-201 (in air) and for Dow

4352N (in reduced oxygen atmosphere) (DeNicola Jr. et al., 1996).

0

5

10

15

20

25

0Velocity (mm/sec)

Forc

e (c

N)

BR-300IR-300

100 200 300 400 500

Fig. 9. Rheotens curves of R-300 and BR-300 (Cheng and Phillips, 2006).

S. Cheng et al. / Radiation Physics and Chemistry 79 (2010) 329–334 333

(denoted BR-201) by capillary rheology analysis. The meltviscosity was greater at lower shear rate after radiationmodification, and the difference in melt viscosities is greaterwhen the shear rate is lower.

Rheotens curves of IR-201 from irradiation at various doses andof BR-201 base resin are shown in Fig. 7, which illustrate how meltstrength increases with increasing dose.

In Fig. 8, melt strength and MFIs (under 2.16 kg load) of IR-201are shown as a function of radiation dose and compared with themelt strength and MFIs of the irradiated Dow 4352N HDPE inthe work of DeNicola, Jr. et al. (DeNicola Jr. et al., 1996). Besidesthe difference in base resins, irradiation of IR-201 was done in air,while irradiation of Dow 4352N was done in reduced oxygenatmosphere. Fig. 8 shows that in both cases the melt strengthincreases while MFI decreases with increasing dose.

Fig. 9 shows the Rheotens curves of irradiated and un-irradiated LLDPEs (denoted as IR-300 and BR-300). The figureshows dramatic improvement of melt strength with irradiation.

4. Implications on processability

When the polymer processing conditions are properly set up,the changes of rheology from the radiation-induced long-chainbranching can improve processability, such as improving sagresistance and strain hardening.

Sag resistance is the ability of the polymer to exhibit highmelt strength at low shear rates and is important, for example,in extrusion blow molding of large parts with hangingparisons. As indicated in the example of Fig. 6, the radiation-modified PE with long-chain branching will show improved sagresistance.

The high melt strength behavior of radiation-modified poly-olefins with long-chain branching, such as that shown in Figs. 7and 9, means that these resins have strain hardening at highextensional rate, which would benefit various melt processes. Forexample, enhancement of melt strength can improve theprocessability in foaming applications (Bradley and Phillips,1991; Malwitz et al., 2005). In blow molding, after the moldcloses and forming pressure is applied, the polymer must resistthinning at high extensional rates as the material is distributedinto the far corners of the part. In extrusion coating, the meltcurtain is pulled from the die onto the substrate at high linespeeds. Polymers with strain hardening behavior would exhibitless neck-in (the difference between the die width and the finalcoating width) and less melt resonance. Strain hardening can alsoimprove bubble stability in film blowing.

5. Processing application examples

Sheve et al. used irradiated high melt strength PP for extrusioncoating (Sheve et al., 1990). The results indicate that coatings withthe irradiated PP or the blend of irradiated PP with linear PP hadsignificantly smaller neck-in than coatings with linear PP, and thuscould be coated at higher speed.

Bradley and Phillips used irradiated high melt strength PP andlinear PP to make foams using conventional foaming equipment(Bradley and Phillips, 1991). While foams could not be made withthe linear PP because of wall rupture from lack of melt strength,foams with uniform, closed cell structure were made using theirradiated high melt strength PP.

When IR-300 (LLDPE irradiated in air) was used for filmblowing, films were made with stable bubbles with blow-upratios ranging from 2.0 to 4.0. In contrast, the blow-up ratio couldonly go as high as 2.8 for un-irradiated BR-300 because the bubblebecame unstable for the lack of melt strength. Larger blow-upratios can improve the balance of film tear strength propertiesbetween the machine and transverse directions, while maintain-ing high production rate.

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Foaming experiments were also done with 100% IR-300 usingconventional foaming equipment. Isobutane was used as theblowing agent. After carefully optimizing the foaming conditions,good foams with low densities of 0.026–0.032 g/cm3 were madesuccessfully. This is notable because foams cannot typically beproduced from 100% LLDPE. Meanwhile, the foaming processa-bility of irradiated LLDPEs is comparable to that of LDPEs, oftenused for foams because of their melt strength. Good foams werealso made from blends of LDPE and IR-300.

In general, foams made from 100% irradiated LLDPE shouldhave better properties than LDPE foams. Malwitz et al. reportedthat foams made from irradiated HDPEs and irradiated LLDPEsshowed better mechanical properties, including higher tearstrength with better balance on the machine direction vs. on thetransverse direction than foam made from LDPE. Foams madefrom irradiated HDPE showed higher flexural modulus than foamsmade from LDPE. Foams with good properties were also madefrom blends of LDPEs and irradiated LLDPE scraps (Malwitz et al.,2005).

6. Conclusions

Long-chain branching can be introduced onto the polymerchains of polypropylenes and linear polyethylenes (HDPEs andLLDPEs), without incurring significant crosslinking, by ionizingirradiation with electron-beam or gamma ray at lower doses thanwhat is required for radiation crosslinking. The irradiation needsto be done in oxygen-free or reduced oxygen atmosphere if nobranching promoter is added for PPs. PPs with long-chainbranching can be made by radiation modification, even in ambientatmosphere, with the addition of a branching promoter such asTAIC or multifunctional acrylates. Oxidative degradation is alwaysa potential competition to long-chain branching and needs to beminimized for PPs.

PEs with long-chain branching can be created by radiationmodification in oxygen-free atmosphere, reduced oxygen atmo-sphere, or in air without a branching promoter. However, the baseresin must be carefully selected especially for irradiation in air.

The long-chain branching in the polymer structure for both PPsand PEs enhances melt strength, which improves melt processa-bility, such as sag resistance and strain hardening. The processa-bility improvements with the radiation-modified, high meltstrength PPs and PEs are beneficial for various melt processes,

including foaming, extrusion coating, and film blowing. Othermelt processes, such as blow molding and thermoforming, shouldbe investigated further.

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