Journal of Non - Neu~onrun Nurd Mechunm, 13 (1983) 309-323 Elsewer Science Publtshers B V , Amsterdam - Pnnted m The Netherlands

309

UNSTABLE CAPILLARY FLOW OF REINFORCED POLYMER MELTS

G AKAY*

School oj lndusrrral Scrence, Cranjreid lnsrrrure of Technolo@, Cranjreld, Bedford MK43 OA L (England)

(Recened February 16, 1983 m rewed form July 4. 1983)

Summaq

The rnJectlon capillary flow of various unfilled and glass flbre or calcium carbonate fllled polypropylene and nylon 6 6 melts IS studled using either a smgle capillary of five caplllanes m series, separated by small reservoirs Only unfilled nylon 6 6 yields mstablhty durmg flow through a smgle capillary due to mechanochermcal degradation m the capillary at extremely Hugh shear rates above 5 x lo5 s- ’ It IS found that only short glass flbre remforced polypropylene yields high frequency osclllatlons m the reservoir pressure and extrudate diameter and has dlscontrnulty m the flow curve when the apparent shear rate IS above 4 x lo5 s-’ and the flow IS through multlple caplllarles Further Increase m the shear rate restores the stable flow The mtenslty of the osclllatlons and the range of shear rate dunng wluch unstable flow occurs are Increased with mcreasmg melt temperature The mechanism of tkls unstable flow IS mvestlgated by studymg flbre orlentatlon at the capillary entrance and emt usmg mouldmgs slmulatmg capillary entry-exit flows

1. Introduction

Instablhty dunng the flow of unfilled polymer melts and solutions through a die has been studied extensively and excellent perIodIcal reviews are avallable [l-7] In contrast. the unstable flow of fllled polymer melts has received very little or no attention In ths work, we descnbe two types of flow mstablhty mvolvmg flbre remforced polypropylene (PP) and unfdled

* Present address Engmeenng and Corporate Technolog\ Dwslon, Port Sunhght Labora-

tory Umlever Research, Wlrral, Merseyslde L63 3JW. England

0377-0257/83/$03 00 Cb 1983 Elsevter Science Puhhshers B V

310

nylon 6 6 (PA) melts The flow mstablhty m short glass flbre remforced PP (SGFR-PP) appears to be untlated at the capillary entrance. wlule m unfilled PA melt, the source of mstablhty IS at the capillary wall Itself Some of the characterlstlcs of the unstable flow reported here are also observed for a large number of polymer melts and solutions under various flow condltlons. but not manifested by a single fluid Here. we summarlse bnefly the unstable flow charactenstlcs of unfilled polymeric fluids that are relevant to the present study

(a) Capillary entry flow pattern During the flat entry capillary flow of some polymer melts such as low density polyethylene (LDPE). a “flow cone” develops at the capillary entrance before the start of unstable flow The flow cone IS surrounded by a cuculatmg stagnant region which becomes larger with Increasing flow rate and eventually fractures perIodIcally as the flow rate 1s Increased even further Dunng the fracture of the flow cone. flow 1s sustamed from the stagnant reDon and “melt fracture” occurs We refer to this type of flow as “primary mstablhty” Unhke LDPE, primary mstablhty m HDPE 1s not accompamed by a marked region of crrculatmg flow although there IS some evidence of penodlc alternating flow as a result of slip between two flow re@ons m the reservou [3.5,8-lo] and the emergmg extrudate has a hehcal screw tread appearance

(b) Reservou pressure osclllatlons Dunng the unstable flow of some polymers (a notable example being HDPE) osclllatlons m the reservoir pressure develop If the flow IS brought about by the steady displacement of a plunger [5.11] The frequency of these osclllatlons may be as high as 1 cps [4.12,13]

(c) Dlscontmulty m the flow curve Dunng the extrusion of HDPE through a capillary at a constant pressure, and at a crltlcal flow rate. m addluon to extrudate dlstortlon. there IS dlscontmmty m the plot of output flow rate versus apphed pressure (flea curve). the output flow rate becomes a double-valued function of pressure and a hysteresis effect IS observed [l-4,14-18] The sue of the dlscontmulty Increases wrth Increasing length to diameter ratio of the capillary [1,2.4.5,16] wfule temperature decrease has the same effect [17] The crulcal shear stress at the dlscontmuny increases wuh temperature [l] The flow curve below the dlscontmmty IS sensltlve to molecular weight but above the dlscontmuny there appears to be no molecu- lar we@ t dependence [ 181

(d) RestoratIon of stable flow If the flow rate IS Increased further. after the establishment of the pnmary unstable flow. a second stable flow regime may be encountered m wmch the extrudate 1s agam smooth [1.2] Amongst the polymers wmch yield this second stable flow regime are HDPE [1,2] and PP [19]

(e) Extrudate osclllauons In general. polymers m solutions appear to have

311

the same unstable flow charactenstlcs as melts although polymenc solutions show a number of unstable flow characterlstlcs not reported for polymer melts Usmg ddute polyacrylanude solutions m a plunger type of rheometer, Ramamurthy [20] observed a longltudmally oscdlatmg exit stream which resulted m oscdlatmg flow rate when the shear rate Increased above a cntlcal value However. further Increase m shear rate restored the stable flow and the flow m the reservoir was confmed to a cyhndrlcal volume with diameter almost equal to that of the capillary Itself Although the emt stream was steady. the diameter of the Jet was less than that of the capillary, mdlcatmg negative die-swell, as Illustrated by Lenk [21] Tonuta. et al [22.23] usmg aqueous polymer solutions observed that when the shear exceeded the cntlcal shear rate (for the primary unstable flow) by a factor of ten, or so, the extrudate did not show any dre swell at the capillary exit. but the die swell was observed at some distance from the exrt and this distance changed perlodlcally with time These observations probably explam the reason for the drastic decrease m the swell at the capillary exit [20]

The charactenstlcs of the flow mstablhty reported In this study are compared and contrasted with those of the unfilled polymenc flulds as summarlsed above In so doing, we mvestlgate the charactenstlcs of flow curves, reservoir pressure osclllatlons. extrudate diameter osclllatlons and the capillary entrance and exit flow patterns by usmg mouldmgs stlmulatrng these flows Fmally, two separate mechanisms are described to account for the unstable flows mltlated at the capdlary entry or at the capillary wall

2. Experimental

Marerrals

Details of the materials used m ths study are avadable [24.25] Flbre remforced PP and PA are produced by ICI In all cases. the base material IS sutular to the unfilled grade A brief descrlptlon of these materials ar gtven below

GXM43 InJectIon mouldmg grade unfilled PP HW60 GR/20 Short glass flbre remforced PP contammg 20 percent by

weight of flbres with a modal length of 500 pm HW60 GR/30 Same as above but contams 30 percent by weight of flbres

with a modal length of 350 pm HW70 GR/lOmm Long glass flbre fllled PP contammg 25 percent by

we&t of poorly dispersed flbres with a well-defmed length of 10 mm CC-PP/40 A calcium carbonate fllled PP contammg 40 percent by

weight of sohd particles with an average size of 4 pm CC-PP/60 Same as above but contammg 60 percent by weight of

calcium carbonate

312

Maranyl Al00 InJectIon mouldmg grade unfilled nylon 6 6 Maranyl A190 Short glass flbre remforced nylon 6 6 contammg 32

percent by weight of flbres with a modal length of 300 pm These last two matenals are vacuum dried for 24 h at 100°C and kept

under vacuum at room temperature before bemg used In all cases. flbre diameter IS 10 pm

Multr-stage rheomerer

Previously, a two-stage rheometer has been used to study the transient flow behavlour [24.25] and pressure dependence of vlscoslty [26,27(a. b)] of various fllled and unfdled polymer melts The design and operatmg proce- dure of this rheometer have been described previously [24.26] The capillary and reservoir assembly replace the standard nozzle m a medium-sized mjectlon mouldmg machme which 1s fltted with a closed-loop adaptIke controller In smgle-stage assembly. we use a 1 mm diameter and 20 mm long capillary. while m multi-stage assembly we use 5 capdlanes m series separated by 5 mm long cylmdrlcal spacers In ths case, the capdlary nearest to the pressure transducer IS 8 mm long so as to prevent damage to the capdlary at high pressures. whJe the lengths of the subsequent capdlarles are 4 mm, 3 mm, 3 mm and 2 mm, respectively The total length of the caplllanes IS 20 mm and they all have 1 mm diameters The pressure. P,, m the first reservoir (J = 1 for the smgle stage rheometer and 1 = 5 when 5 capdlanes are m series m the multl-stage rheometer) and the plunger position are recorded usmg a UV-recorder From the slope of the plunger posmon versus time, the volumetric drsplacement of the plunger per umt time. Qo, can be calculated Here. Q. IS called the Input flow rate Four different transducers are avadable to cover a pressure range 30 kpsl to 1 kpsl

(1 kpsl = lo3 PSI) The melt m the reservoir IS kept at a constant tempera- ture. T,. and the whole system can be kept at ths temperature

A cme-film (24 frames per second) of the emergmg stream durmg unstable flow IS taken and subsequently analysed frame by frame m order to deternune the vanatlon of the extrudate diameter with time The measure- ments are carned out at 3 cm from the last caplllary exit

InJectron mouldmg

To a large extent, flbre onentatlon m the mouldmgs are dlctated by the orlentatlon set-up dunng mould fllhng, although the packmg stage and the non-isothermal nature of the mould flllmg may alter ths flow Induced onentatlon Smce the fdled polymer melts are not transparent and the shear rates and pressures mvolved durmg capillary flow expenments are extremely

313

h@. conventional flow vlsuahsatlon techmques can no be used Therefore, mjectlon moulded samples are used m order to deternune flbre orlentatlon dunng flow [24,25]

A special mould Insert Mach consists of 12 cyhndncal expansions and contractions IS deslgned m order to study flow Induced phenomena m fllled and unfdled polymer melts The capillary section has a diameter of 3 mm and a length of 12 mm whde the cyhndncal expansion sectlon has a diameter of 12 mm and a length of 24 mm TUB Insert fits to a mould deslgned by Thomas [28] and the mouldmg IS cart-led out usmg the same mJectlon mouldmg machme InJectIon mouldmg condltlons are as follows Nozzle temperature (mcludmg the heatmg zones) T, = 210°C or 270°C (ths corre- sponds to T, m mjectlon capdlary flow) while the mould temperature T,, IS 30°C

Contact mrcro-radrography (CMR)

Flbre onentatlon at the entrance to a capillary or at the exit of a capdlary IS determmed by takmg X-ray pictures of thm shces ( = 65 pm) cut from the mouldmgs along a plane of symmetry so as to include both the capillary and the cyhndncal expansion Thus techmque IS known as contact micro-radlog- raphy. CMR [29]

3. Results and discussion

Transient flow m PP based melts [24] and PA based melts [25] has recently been studted In general, It IS found that a stable flow (m whch the reservoir pressure IS stable and the extrudate IS smooth) can be obtamed m PP based melts except m the case of long glass flbre filled melt. HW70 GR/lO mm In fllled or unfilled nylon 6 6, reservoir pressure fluctuations were confmed to a small range of shear rates In the present study. a new type of flow mstablhty has been observed for SGFR-PP melts due to entrance effects, whle the mstablhty m unfilled PA appears to be mltlated at the capillary wall None of the other melts. mcludmg calcium carbonate fllled PP’s ylelded flow mstablhty as marufested by pulsatrons m the re- servolr pressure and extrudate diameter and dlscontmulty m the flow curve In the followmg sections we therefore confme our dIscussIons to SGFR-PP and unfilled PA melts

Flow curves

In the absence of any unstable flow, the reservoir pressure P, (J = 1 for smgle capdlary, J = 5 for multlple capdlanes) Increases monotomcally with

314

20

s

Kpsi

1c

0

/’ Open symbols=Stable flow

Closed symbols:Unstable flow

1 , I 1 1 I I 1

0 50 q&c/s1 100

Fig 1 Vanatton of reservoir pressure (P,) with Input flow rate ( Qo) durmg the capdIary flow

of various unfilled and glass flbre remforced polymers through a smgle capillary (1 = 1.

-) or multtple capdlanes tn senes (J = 5 - - - - - -) Maranyl AlOO. 7, = 300°C J = 1,

(0. l ) HW60 GR/20, Tl = 270°C J = 5, (v v) HW60 GR/30 Tl =18O”C, J = 5. (a).

T, = 270°C. J = 5, (0. l ), T, = 300°C. J = 5 (0 W) T, = 270°C. J = 1 (9) Stable flow IS

represented bq open symbols and unstable and unstable flow IS represented by filled svmbols

mcreasmg mput flow rate Q. as shown m Fig 1 If there IS unstable flow present, the reservou- pressure starts osclllatmg and the mean value of P, starts decreasmg (or remams nearly constant m the case of unfilled PA. Maranyl AlOO) with mcreasmg mput flow rate The unstable flow starts at a cntlcal mput flea rate Qz Dunng the flow of SGFR-PP melts through multlple capdlanes, stable flow IS restored when the mput flow rate reaches a second cntlcal value, denoted by Q,’ The mspectlon of Fig 1 mdxates that, for SGFR-PP. Qz IS Independent of melt temperature, flbre concentration and the aspect ratlo of the filler, and therefore unstable flow starts at progressively lower shear stress as the melt temperature 1s mcreased If the

315

melt temperature 1s low, unstable flow falls to set-m but the tendency towards mstablhty IS apparent m the flow curve correspondmg to r, = 180°C The range of Q. (Qt < Q,, < Q,‘) when the unstable flow persists m SGFR- PP depends on the matenal and smaller aspect ratlo of the flbres, and high melt temperatures appear to widen this range Temperature dependence of the unstable flow characterlstlcs of SGFR-PP 1s m direct contrast to those of unfilled polymers In HDPE, temperature Increase results m lugher crItIcal stress at the start of the unstable flow [l] (I e , also lugber shear rates) and the size of the dlscontmulty (whch occurs If the flow IS brought about by the apphcatlon of constant pressure) decreases [1.2,5,14-161

The only polymer melt whch yields mstablhty dunng flow through a smgle capillary IS the unfilled nylon 6 6 In tlus case. P, hardly Increases wrth mcreasmg mput flow rate Q. dunng unstable flow as shown m Fig 1

Pressure oscdiatrons

The transtent tnjectlon capillary flow behavlour of all the melts reported m ths work has been mvestlgated previously [24,25] In Fig 2, the develop- ment of reservoir pressure P,( J = 5) IS shown for SGFR-PP (HW60 GR/30) durmg stable and unstable flow When the flow IS steady, (Q,, -z Qg); P,

Increases to Its steady value monotomcally and remams constant after the mltlal transient has been completed Dunng stable flow, flow rates m and out of each capillary are equal to the mput flow rate Q. When Qg < Q. < Qc . flow rates m and out of each capillary become time dependent although Q, IS time Independent and as a result reservoir pressure also becomes time

20-

?

KPS~

lo-

-------- /* -

~__~~____+-_-- - -_--- --c

, , -

,

, / ---

‘I /

Q$cgclsec)

50 I /

I I /I 65

--- I 100 , /)

‘n”‘-“J”‘l ‘)“‘I ‘I 8 ’ ‘L 0 1 tlmelsec) 2

Fig 2 Development of reservox pressure ( P5) wth mput flow rate ( QO) dunng the transient and ‘steady state” flow of 30 percent short glass flbre remforced PP (HW60 GR/30) through five caplllanes In senes, T, = 3OO’C

316

I ’ tlme(sec1 2

'a- q,=503

16 :-- 18- Q.153f.i

2

A ,6._"""J"' -

1 tlme(sec) 2

alunstable FLOW of HW60 GR120,TI=2700C

1 2

2 ot 1 2

1 time (set) 2

blunstable Flow of HW60 GR130, T, q 27O0C

t Qa=427cc/s

i2L1"""""'1"' 1 time (secl 2

Kpf[_ 1 time (set) 2

c) Unstable Flowof HW60 GR130, T, -3OOOC

Fig 3 Development of osallatlons m reservoir pressure ( fs ) with Input flow rate (Q,,) dunng the unstable flea of short glass flbre ranforced PP through hve caplllanes In senes (a)

HW60 GR/20. T, = 270°C (b) HW60 GR/30, 7-, = 270°C (c) HW60 GR/30. T, = 300°C

317

dependent Osclllatlons m P, start as soon as the mltlal transient IS complete When Q,, = Q,‘. the first pressure osclllatlon cycle falls to repeat Itself and

hence ths smgle Isolated peak m the pressure appears as “pressure over- shoot” However. unhke the pressure overshoots encountered m [24], thus overshoot does not grow with mcreasmg mput flow rate. and does not disappear with mcreasmg melt temperature In fact once the stable flow IS restored completely, the approach to a steady state does not mvolve any pressure overshoot, as seen m Fig 2 We Illustrate the development of the pressure osclllatlons with mcreasmg Input flow rate m Fig 3 where the mltlal transient IS not mcluded As seen m this. the amphtude of the osclllatlons Increases with mcreasmg melt temperature, whle the frequency of oscllla- tlons IS not affected The effect of flbre length (or flbre concentration) IS also Illustrated At ths stage, It IS not possible to attrlbute 0u.s difference either solely to flbre length or to flbre concentration

The frequency of pressure osclllatlons decreases lmtlally and then starts mcreasmg with mcreasmg mput flow rate Ths 1s also found m HDPE [12.13] Each pressure cycle can be dlvlded mto two regions, compresslon (pressure mcreasmg) and decompresslon (pressure decreasmg or steady pressure) regions These regions are illustrated m Fig 3 The fraction of time

12 t-

s KPS’ &=46-4 cc/set

8 1”““1”““““- 1 time (set) 2

!-

Fig 4 Development of osclllatlons m reservoir pressure ( I’,) with Input flop rate (QI,) dunng the unstable flow of unfilled PA (Maranvl AlOO) through a smgle caplllarv when T, = 300°C

318

spent m decompresslon (relative to the osclllatlon penod) Increases with

mcreasmg Q. dunng unstable flow Reservoir pressure osclllatlons dunng the capillary qectlon flow of

unfilled nylon 6 6 through a single contmuous capillary are shown m Fig 4 There are no pressure oscdlatlons present in SGFR-PA or when the flow IS through multlple capdlanes The amphtude of osclllatlons are small com- pared with those encountered m 30 percent SGFR-PP When Q. = Qg, development of the osclllatlons appears to take longer after the completion of the transient flow, compared with their development m SGFR-PP How- ever. as Q,, IS Increased further. the mductlon time for the start of oscillatory flow IS reduced These oscdlatlons were still present at the highest flow rate used m the present expenments

Ex trudate oscrllatrons

Dunng the unstable flow of SGFR-PP. the emergmg extrudate appears to have time penodlc expanding-contractmg behavlour The vanatlon of the extrudate diameter with time IS very smular to the vanatlon of the reservoir pressure as seen m Fig 5 where the dlmenslonless extrudate diameter B (B IS

1 2 time (set) 3

Fig 5 Vatlation of dlmenslooless extrudate diameter B (ratlo of the extrudate diameter to cap&q dmrneter) with time dunng unstable flow of short glass flbre remforced PP (HW60/30) melts through multiple caplllanes m senes when T, = 270°C

Fig 6 Flbre onentatlon at the entrance of a cap&q as obtamed from the mouldmgs of 30 percent short glass flbre remforced PP (HW60 GR/30) (a) T, = 2lOT (b) TN = 27OT Here T, IS the nozzle temperature

the ratlo of the extrudate diameter to capillary diameter) IS plotted agamst time Before the start of the unstable flow when Q. = 34 cc/s. B = 1 63 whle after the restoratlon of the stable flow when Q. = 60 cc/s (I e , Q. > Q,‘) B = 1 07 for HW60 GR/30 at T, = 270°C We have chosen these two flow rates as durmg unstable flow, the dlmenstonless extrudate diameter. B, changes approximately wlthm ths range Therefore. we can assume that the output flow rate from the last capillary cycles between these two flow rates dunng unstable flow These results are smular to those reported by Rama- murthy [20]

Flbre orrentatlon

Flbre onentatlons at the entrance and the exit of a capillary are Illustrated m Figs 6 and 7 at two mltlal melt temperatures TN = 210°C or TN = 270°C usmg 30 percent SGFR-PP When the melt temperature 1s low, there appears to be a stagnant repon near the corners of the reservoir and a flow cone can be Identlfled. with an apex angle of - 120” However, m the central regon of the flow cone, flbres appear to have onentatlon transverse to the flow dlrectlon, fornung an Inner cone with an apex angle of - 40”, therefore

320

Fig 7 Flbre onentatlon at the exit of a caplllq as obtamed from the mouldmgs of 30 percent short glass flbre remforced PP (HW60 GR/30) (a) T, = 210°C (b) T, = 270°C

reducmg the effective size of the flow cone to - 80” In the regon between these two cones, flbre onentatron IS parallel to the flow &rectlon If the melt temperature 1s Hugh. a flbre free regon develops at the comers of the reservous Flbre onentatlon lmmedlately next to ths flbre free region 1s parallel to the flow direction In both cases, flbre onentatlon m the capillary IS parallel to the flow direction, and there are hardly any transversely onented flbres m the capillary centre The flbre free layer propagates mto the exit reservoir when the melt temperature IS 27O”C, It does not do so when the melt temperature 1s 210°C. as seen m Figs 7(a, b) where flbre orientation IS shown at the exit of a capillary Although the flbre onentatlons are sumlar to those at the capillary entrance the size of the stagnant regons are reduced and the size of the outer flow cone (where the flbre onentatlon 1s parallel to the flow direction) 1s increased to - 160” whle the size of the Inner cone 1s reduced to - 30” Thus difference m the flbre onentatlon at the entrance and exit IS to be expected since m vlscoelastlc fluids such a difference 1s also present [30]

Various mechatusm which are hkely to produce ftbre free regions are discussed m [24,25,27(b), 311 The mechamsm proposed m [24,25,27(b)] 1s based on the exclusion of highly onented flbres m a melt when stress induced crystalhsatlon or molecular onentatlon occur This mechanism

321

requires a certam flow regme, shear rate. melt temperature and temperature gradlent and the disappearance of the flbre free regon upon expansion at the capillary exit IS to be expected, as seen m Fig. 7(a) However, the presence of the flbre free region at the capillary exit when the temperature IS Hugh (Fig 7(b)) suggests that at hgh melt temperature, the mecharusm of flbre segregation IS different and 1s probably due to particle exclusion dunng vortex flow It IS well known that red blood cells are excluded from the vortex during the capillary exit flow of blood [32] If there IS a strong vortex flow present at the entrance of the capillary, flbre free regions can develop Flbre exclusion from these regions requn-es low vlscoslty and, therefore, flbre free regions at the entrance and elut are only present at Hugh melt tempera- tures, whle the flbre free regon m the capillary may help the creation of a flbre free region at the exit Phase separation m polymer blends at a flat capillary entrance has also been reported [33]

4. Conclusions

The charactenstlcs of the flow mstablhty m SGFR-PP resemble those of various polymer melts and solutions, although m one Important aspect It differs from them If the melt temperature IS hgh, flow mstablhty becomes more pronounced, resultmg m hgher amphtude pressure osclllatlons m an extended shear rate range Thus IS due to the difference m the mechamsm of unstable flow m SGFR-PP and various unfilled polymenc flulds Flbre orlentatlon studies usmg InJectIon moulded parts which simulates the present multiple capillary flow mdlcate that flow mstablhty IS probably due to the formatlon and deformation of flbre free regons at the entrance and exit of the capillary Decompresslon m the reservoir occurs after the formatlon of the flbre free regions at the capillary entrance and melts feeds mto the capillary near the capillary wall thus causmg shp Compresslon m the reservoir starts when the flbre free region IS ehnunated due to feedmg through the caplllanes and flbre dlffuslon When the stable flow IS restored (Q. > Qi), It IS probable that these flbre free regions extend all along the capillary-reservoir assembly and are permanently estabhshed The absence of unstable flow m calcium carbonate fllled PP mdlcates that an optimum aspect ratio IS necessary for partrcle segregation at the capillary entrance The unstable flow mechamsm suggested here lmphes that durmg the second stable flow regime, the flow behavlour of SGFR-PP ~111 be very slnular to that of the base polymer In fact, It 1s found that [27(c)] durmg the unstable flow of SGFR-PP, the combmed entrance and exit (ends) pressure loss IS reduced by a factor of 2-3 and it becomes equal to that of the base polymer When the stable flow IS restored, ends pressure loss for SGFR-PP melts and unfilled PP melt Increases with mcreasmg apparent shear rate

322

The flow mstablhty encountered m unfdled PA (Maranyl AlOO) probably mltlates and propagates m the capillary It IS known that m HDPE. the source of stablhty IS the capillary Itself [l-5,16] due to the creation of a shp layer Ths shp layer m polymers may be created by stress-Induced dlffuslon [31,34], mechanochemlcal degradation [16,31], polymer-capillary mteractlon [l-5.16] and stress Induced molecular cham conformatlon at the sohd-melt Interface [35] Smce there IS no flow mstablhty present m PA melts dunng flow through multlple caplllanes, we can conclude that after passmg through each capillary, developmg shp layer IS destroyed upon entermg the next reservoir and the mdlvldual capillary lengths are not long enough to allou the developmg shp layer to cause mstablhty The absence of flow mstablhty m flbre fllled PA through a smgle capillary may be attributed to the reduction of mechanochemlcal degradation m these melts due to flbre orlentatlon (thus the modlflcatlon of the flow field) and/or the presence of sdane couphng agents used m the coatmg of the flbres. as shown recently

1361 In addltlon to Its Importance m the qectlon mouldmg of SGFR poly-

mers, the present multlple capillary experiments may be useful m studying flow induced phase separation m polymers and polymer blends. such as those studled by Kanu and Shaw [33]

Acknowledgements

I would hke to thank the Natlonal Physical Laboratory (N P L ) for supportmg this research and undertakmg the constructlon of the mould Insert used m tlus study Mr K Thomas (N P L ) for the use of his mould and for helpful dlscusslons, Mr A J Scott and Mr P Harrlson for their help m the expenmental work, ICI for the donation of the matenals used m the expenments and for helpful dIscussIons I am grateful to Professor D W Saunders for hs help and contmued Interest m ths project

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