Indian Journal of Chemical TechnologyVol. I . July Il)lJ-t 1'1'. 199-2()."\

Effect of processing on the photo-oxidation of HDPE stabilised with LDPEUV masterbatch

o OgbobeDepartment of Polymer & Textile Technology. Federal University of Technology. PM B 1526. Owerri.

Imo Stale. NigeriaReceived 20 September 1993; accepted 17 December 1993

The photo stabilising action oflow density polyethylene UV masterbatch in slow cooled and quenchedcxtrudates of6.0 MFI and 0.2 MFI high density polyethylene obtained at varying processing conditions hasbeen examined using IR spectroscopy to monitor the build-up of volatile carbonylic oxidation productsafter artificial irradiation. Masterbatch dispersion in extrudates of the two grades of high densitypolyethylene is assessed by UV microscopy. The results show that although increased shear increases theeffectiveness ofthe UV absorber in the high melt-flow-index polymer while it has no significant effect on theperformance of the UV absorber in the low melt-flow-index polymer. Quenching the extrudates significantlyincreases the time to embriulcmcnt. The dispersion of low density polyethylene UV masterbatch is better inthe high melt-flow-index polymer when processed at equivalent extruder screw speeds.

As a result of the large quantities of polyolefins usedout-doors. their photostabilisation by theincorporation of suitable UV stabilising additiveswi\1 continue to be a centre of attention. Lowmolecular weight masterbatching has becomecommon place for the incorporation of additives intoplastics'. The problem of using low densitypolyethylene (LOPE) masterbatch for theincorporation of particulate additives intopolyolcfins have been discussed by Gale? and Boc.:'

In view of the fact that there is a wide range ofvarying melt-flow-index (MFI) of polyethylene inthe market compared to very few grades of LOPEused in masterbatch manufacture with the attendantmiscibility problem, the effectiveness ofUV absorberincorporated by masterbatching is of great interest.The photo-degradation of two grades of high densitypolyethylene(HOPE) photostabilised by the use ofLDPE UV masterbatch containing the activeingredient 2-hydroxy-4-n-octo-benzophenone hasbeen studied here. Moreover, to aid the interpretationof the artificial weathering result, a qualitativeassessment of absorber dispersion is also carried outusing an ultraviolet microscopical technique.

Experimental Procedure6.0 MFI HOPE (Rigidex HO-60-45P) of density

945 kg m - 3, 0.2 M FI HOPE (Rigidex 002-55) ofdensity 953kg m - J and LOPE UV masterbatchcontaining 1.5% 2-hydroxy-4-n-octo-benzo-

phenone used in the study were all supplied by ColePlastics, London.

UV masterbatch stabilised HOPE extrudates wereobtained by melt mixing the pelletised material withthe masterbatch in a laboratory extruder- Betol Maxmixing extruder, model CS-194 manufactured byInstron Scientific Instruments Inc.

Slow cooled and quenched extrudates processedunder low temperature/low shear regime (dietemperature 170°C/screw speed of 10 rpm) and lowtemperature/high shear regime (die temperature170°C /scrcw speed of 40 rpm) of the 6.0 MFI HDPEwith masterbatch were prepared. Similarly, slowcooled and quenched extrudates of 0.2 MFIHOPEjUV masterbatch blend were prepared.Extrudate quenching was done in an ice/watermixture maintained at a temperature between4- 100e. Die temperature/screw speeds of 190T/IOrpm, 190°Cj40 rpm and 200°Cj40 rpm were employedfor the 0.2 MFI HOPE. Extrudatcs were chipped andcompression moulded using a 200 urn thickaluminium sheeting cut to form a 15 ern x 20 emrectangular shape in the middle. Compressionmoulding was done at 160°C and rapidly cooledunder pressure.

The compression moulded HOPE films wereexposed in an apparatus utilizing a 500W high pressureHg/tungsten fluorescent lamp {MBTF) supplied bythorn EMI (A > 300 nrn, temperature 50°C andrelative humidity was ambient). The set-up which isshown in Fig. I is described in detail elsewhere."

200 INDIAN J. CHEM. TECHNOL., JULY 1994

Aluminium sheeling linin9'

--Drum

G.E.S.tillirg500 wa tt high presuremercury! turgstentlourescent 10mp(MBTF)

l"bolts to holdmetol ring onwhich films werehung.

Fig. I-Simulated sunlight equipment

Rates of polymer photo-oxidation were measuredby monitoring the build-up of non-volatilecarbonylic oxidation product absorbing at171Ocm- I in the infrared region using a Perkin ElmerModel 521 infrared spectrophotometer. Carbonylindex (CI) measurement was followed by theexpression (IOglO(Jo/lt)/d) x 10, where 10 = initiallight intensity, It = transmitted light intensity andd = film thickness in microns. Embrittlement isreached ' at CI '= 0.06.

The microscope equipment employed has beendescribed in detail elsewhere":". Thin sections (20 J..Lmthick) of the compression moulded films were cutusing a Leitz 1400 base sledge microtome andmounted between quartz slide and cover-slip usingglycerol immersion liquid. The absorber distributionwas examined down the microscope and a magnifiedimage recorded on a 35mm Ilford FP4 film. Each filmwas developed for 6 min at 20°C in Acutol developerand fixed for 2 rnin in Hypam fixer.

Results and DiscussionFig. 2 shows the varia tion.of carbonyl index with

time in hours of irradiation in the weatheringequipment for a blend of6.0 MFI HOPE and LOPEUV masterbatch processed at die temperature/screwspeeds of 170°C/IO rpm, 170°C/40 rpm and 180°C/40rpm. It can be seen that processing has no significanteffect on carbonyl development in each case up to600h. However, carbonyl development is rapid in the

10

9

8

N 70

><)( 6•..'0C

5>.c

40.0~0u 3

200 400 600 800 1000 1200 1400

time,h

Fig. 2-Growth of carbonyl index with time in 6.0M FI HOPE:o =die temp. screw speed of 170'C/ IOrpm; D = l70'C/40rpm;

t'!. = 180'C/40 rpm.

extrudate obtained at a die temperature/screw speedof 170°C/ 10 rpm after 600h of irradiation, while thesame for extrudate obtained at a dietemperature/screw speed of 170°C/40 rpm and180°C/40 rpm progressed relatively slowly over thecorresponding time period. Thus, for a giventempera ture (170°C) increased screw speed improvesthe effectiveness of the UV absorber, while for a givenscrew speed (40 rpm), increased die temperaturedecreased the effectiveness of the UV absorber. Sincecrystallinity is expected to increase at this highertemperature, the observed result reflect the wellknown fact that UV absorber is rejected within thecrystal regions of crystalline polymers."

The photo-oxidation rates of two of the slowcooled films reported in Fig. 2 are compared withtheir quenched equivalents in Fig. 3. It can be seenfrom Fig. 3 that quenching of the extrudate obtainedat a die temperature/screw speed of 170°C/1O rpmdelays embrittlement by as much as 500 h whenviewed against the corresponding slow cooledextrudates. A similar decreased rate ofphoto-oxidation particularly at greater than 800 h ofexposure is observed in the quenched extrudateobtained at a die temperature/screw speed of170°C/40 rpm compared to its slow cooledequivalent. This suggests that the use of cold mouldmight improve the effectiveness of UV absorber inHOPE.

OGBOBE: PHOTO-OXIDATION OF HDPE

10

9

8

7N0

6)(

)(

o5u

c;;.. 4cB~ 3(3

2

o~~~~~~~ __~~~~ __~ __~200 400 600 800 1000 1200 1400 1600

Time,h

Fig. 3~Growth of carbonyl growth in quenched and slow cooledextrudates of 6.0MFI HDPE: • = 170"C/lOrpm (quenched)01= 170T/40 rpm (quenched), e= 170"CjlOrpm (slow cooled)

o = 170T/40rpm (slow cooled).

5r-----.--------------------------~

I, .

N 30

x

~.5 2:;:."0e8

o~~~~~~200 400 600 800 1000 1200 1400 1600

Time,h

Fig. 4---~Growth of carbonyl index with time in O.2MFI HDPEprocessed at die temperature screw speed of 0 = 190'Cj IOrpm;

o = 190'C/40rpm e = 200'C(40rpm

Figs 4 and 5 show the photo-decomposition ratesofextrudates of 0.2 MFI HOPE/LOPE UV absorberblend. It can be seen that unlike the high MFIpolymer, the low MFI polymer, increased screwspeed decreased the effectiveness of the UV absorberwhile quenched extrudates for the latter show trends

201

5

4

No)( 3K

'"UC

200 400 600 800 1000 1200 1400 1500

Time ,h

Fig. 5---Growth of carbonyl index with time in quenched and slowcooled ex truda tes ofO.2M FI H DPE; e = die temperature/screwspeed of IW'C! IOrpm (slow cooled) L'> = I9O'CjIOrpm (quenched).0= 190'Cj40rpm slow cooled 0 = 190'Cj40rpm quenched.

similar to what was observed for the former (6.0 MFIpolymer)-giving improved UV absorbereffectiveness for quenched extrudate. The differencein the effects observed on changing extruder screwspeed for the low and high MFJ polymers may well bebecause of the wider difference in molecular structureof LOPE (masterbatch base) and the low (0.2) MFIHOPE (wider structural mismatch).

The embrittlement times for the various extrudatesamples are shown in Table 1. It is easily seen thatprocessing conditions have a significant effect on theeffectiveness of the masterbatch in HOPE of differingMFL For instance, quenching the extrudate of 6.0MFI HOPE obtained at a die temperature/screwspeed of 170°Cj 10 rpm increases the photo-oxidationstability by 50%. It is common practice now to useLOPE additive masterbatch in polyolefins withoutmuch regard to molecular weight differences betweenthe additive base and the polyolefin. This practiceassumes that the masterbatch mixes equally well inthe polyolefins.

Ultraviolet microscopy was used to investigate thequality of absorber dispersion in the two grades ofHOPE. Although variation of absorber dispersion atvarious processing conditions was not easily notablein a given grade, variation of dispersion in the twodifferent grades studied was obvious. Figs 6 and 7 areultraviolet micrographs of slow cooled extrudates of

202 INDIAN J. CHEM. TECHNOL.. JULY 1994

Table I Times to 50°;;, embrittlement* and times toembrittlement** of LDPE UV masterbatch/6.0 and 0.2 MFI

HDPE blends

Polymer Processing Time x 10 - 2.hcondition to SO'Yo(die tempera- ernbrittlernentture/screw speed)

Timex 10 2.hto embrittle-rnent

6.0 MFIHOPE

170T 10 rpm 7.00 10.00(slow cooled)

170TjlO rpm 13.00 15.00(quenched)

170T/40 rpm 13.00 15.00(slow cooled)

170T/40 rpm 7.00 10.00(quenched)

18()"C/40 rpm 12.00 13.00(slow cooled)

190T/IO rpm 1500(slow cooled)

190T/IO rpm > > 15.00(quenched)

I<)()"C 40 rpm 12.00(slow cooled)

190°(',40 rpm D.OO(quenched)

200T;40 rpm ~ 14.00(slow cooled)

0.2 MFIHOPE

*Timc to the development of 0.03 carbonyl index**Time to the development of 0.06 carbonyl index

6.0 MFI and 0.2 MFI HOPE blends, respectively,obtained at die temperatures/screw speeds ofl7(tC/l 0 rpm and 190°C; 10 rpm. The low M FIpolymer did not flow easily at a die temperature of170°C and was, therefore, processed at a dietemperature of 190°C. The dark areas show where theUV absorber is positioned while the light areasindicate parts of the polymer matrix not containingmuch UV absorber. For these materials to beadequately protected from UV light, the whole areashould have been uniformly dark. It may, therefore,be seen that the LDPE UV masterbatch is betterdistributed in the higher MFI polymer than in the lowMFI polymer.

Conclusion-- The result presented here show that theeffectiveness of LOPE UV masterbatch in HDPE

Fig. 6-UV Photomicrograph of slow cooled extrudate or6.0MFI HOPE/masterbatch blend.

Fig. 7--UV Photomicrograph of slow cooled extrudate of0.2MFI HOPE/masterbatch blend

depends not only on the processing conditions butalso on polymer structure. Whereas increased screwspeed enhanced absorber effectiveness in high MFIHDPE, effectiveness was diminished in the low MFIgrade. Quenched extrudates of HOPE stabilised byLDPE UV masterbatch have lower rates ofphoto-oxidation than their slow cooled equivalentsindicating that the use of cold moulds in HOPEextrudes containing LOPE UV absorbermasterbatch would enhance the effectiveness ofUV

OGBOBE: PHOTO-OXIDATION OF HDPE 203

absorber in crystalline polymer extrudates. Absorberdispersion and hence photo-oxidation stabilitydepend on structure of HDPE in which it has beenused, a higher MFI polymer grade gives a betterabsorber dispersion.

ReferencesI Modern Plastics International (1983) 54.

2 Gale G M, Plast Rubb Process Appl, 2 (1982) 347.3 Boes D, Kunstoffe, 64(11) (1974) 641.4 Ogbobe 0, Dispersion of LDPE masterbatches in polyolefine ,

Ph D Thesis, Institute of Polymer Technology, Loughborough,UK. 1985.

5 Adeniyi J B & Kolawole E G, Eur Polym J. 20 (1984) 43.6 Frank H P & Lehber H, J Polym Sci Ptc No. 31 (1970) 193.7 Billingham N C, Calvert P D, Knight J B & Ryan T G, Br Polym J

n,(1979) 155.8 Calvert P D & Ryan T G. Polymer. 19 (1978) 611.