Indian Journal of Fibre & Textile ResearchVol. 18, March 1993, pp. 30-42

Review Article

Fibres and films from polyolefln blends

B L Deopura, S Mahajan & K N BhaumikDepartment of Textile Technology, Indian Institute of Technology, New Delhi 110016, India

Received 9 April 1992; accepted 22 July 1992

Among the various polymeric materials being developed, using the polymer blending concept, to ob-tain new materials with specific applications, polymer blends of various polyolefins have recently re-ceived increased attention. The present paper, which is an extension of our previous paper on fibres frompolymer blends [Indian J Fibre Text Res, 16 (1991) 65], describes the rheological and crystallization be-haviours of polyolefin b1ends and the role of interfacial agents in these blends with specific examples such

as polyethylene-polyethylene, polyethylene-polypropylene, polypropylene-polypropylene, polyethyleneor polypropylene with (ethylene-propylene) copolymers and ethylene-propylene diene terpolymers.

Keywords: Block copolymers, Compatibility, Co-crystallization, Interfacial agents, Polyolefin blends

1 Introduction ed, most combinations ate immiscible. In many in-Within the rapidly growing literature on polymer stances, phase-separated blends are preferred for

blends, crystalline polyolefins occupy only a very achieving useful results. Tailoring blends to achievesmall part. The reason for this lies in the molecular desired properties requires, among other things,incompatibility of the polymeric constituents which control over the spatial arrangement or morphologycauses these blends to separate into individual of phases, and some degree of stability once formed.phases. However, blends {)f olefins are now used Both these parameters are strongly dependent oncommercially because of their high impact strength, rheological behaviour and crystallization pheno-low temperature toughness and improved processa- mena. The most important way of improving orbility. Presently, the polyolefins constitute one of the manipulating the morphological behaviour of po-largest classes of macromolecules that are commer- lymer blends, through the modification of inter-cially produced each year because of the low pro- phase with the. help of comp~tibilizers, is al~o ~s-duction costs and wide range of properties. More cussed along WIth the rheological and crystallizationrecently, blends of various polyolefins have received behaviours in polymer blends.increased attention in order to obtain new materials 2.1 Rheological Aspectwith specific applications. Materials ranging from The key factor for control of phase morphology isbrittle plastics to elastomers can be obtained by the the skilful manipulation of polymer blend rheologyproper selection of blend constituents and it's com- during processing11-13. The ratio of the componentposition. In addition to developing new materials, rheological characteristics determines the results ofsome consideration has also been directed to recycl- dispersive mixing, i.e. the fineness of the dispersioning plastics where polyolefins constitute major of the minor componenp4. The elastic properties ofsources of such a potential waste problem. Polyole- the components determine, in turn, the formation offin blends which have been investigated extensively droplet-in-matriX or layer-in-matrix texture uponinclude binary and ternary blends prepared from completing the lamellar melt mixing stage of thevarious types of polyethylene (PE), polypropylene blending process 15. Reliable knowledge of the rheo-(PP), ethylene-propylene copolymer (EPR), and eth- logical characteristics over a wide range of shearylene-propylene diene terpolymer (EPDM). This rate or shear stress is of paramount importance inreview on polyolefin blends, complementary to predicting or modelling the blending process. Theseveral recent reports and monographs 1-1°, lays em- complementary physical characteristics required forphasis on control of phase morphology, crystalliza- these applications are molecular weight and its dis-tion behaviour and mechanical properties. tribution, chain branching, macromolecular struc-

ture, uniformity of components16-19, surface tension2 Control of Phase Morphology of molten polymers15, and a knowledge of the inter-

Although many polymer pairs have been identifi- phase layer behaviour8.

DEOPURA et aL: FmRES AND Fll.MS FROM POLYOlEFIN BlENDS 31

In preparing polyblend fibres or films, one would a better dispersion can normally be achieved. In ,aexpect the major component to form the matrix and shear environment, elastomers show orily a moder-the minor one to form the inclusions. This arrange- ate viscosity decrease as the temperature increases.ment is indeed the one usually observed in fibres Polypropylene, on the other hand, characteristicallyspun from intimate blends. However, phase reversal undergoes a rapid decrease in viscosity. In this tem-sometimes occurs if the two polymers differ in melt perature range (165-180°C), the viscosities of theviscosity8,2o-22. It is generally agreed that when the two components are very similar and the mechani-viscosity and elasticity of the minor component are cal shear can be effectively transferred to the niix-greater than that of the major component, the minor ture to enforce a good dispersion of the phases.component will be dispersed coarsely, and the ext- However, in addition to the finer dispersion level, itent of deformation would be less (Fig. la, b). A re- is also essential to control fluid flow in polymerlatively rigid element, such as a latex particle, would blend processing to preserve the dispersion levelnot deform at all. In general, to produce fibrils with obtained in the mixing step. After mixing, the po-larger length/diameter ratios by this route, it is im- lymer fluid must be delivered by laminar flow underportant that interfacial tensions be small and the rel- relatively small stresses to extrusion die for the sta-ative viscosity or elasticity of the dispersed phase be bility of state of dispersion achieved in the mixingsmaller as compared to that of the matrix23 (Fig. 1c). step. During this transport, opportunities exist forIt is also important to realize that fibrillar elements deleterious changes in the state of dispersion, suchof fluid are unstable24 and may tend to break up into as coalescence of small dispersed particles into larg-small dropletS if the dispersed phase is of low vis- er ones, deformation of dispersed domains and splitcosity and larger interfacial tension. The phase up of deformed domains into smaller ones; here,structure usually found in this type of blends is the coalescence is a competing process. At die entrance,matrix-fibril type when one component is in minor a rapid acceleration begins, which produces anquantity. The driving force for this arrangement is elongational and shear flow that deforms dispersedthe reduction in energy dissipation that occurs in the particles into fibrils. The extent of the particle elon-flowing polymers when the lower viscosity fluid gation depends on the relative rheological charac-moves to the higher shear rate regions. A classical teristics of the two phases, type and magnitude ofexample of how the polymer viscosity can control fluid motion, interfacial tension and die geometry.which component forms the matrix and which the fi-brils has been given by Tsuji25. According to Dane- 2.2 Crystallization Behavioursi26, the dispersion level in PP/elastomer blends can It is well known that crystallization of a blendeffectively be controlled by varying the temperature component can differ remarkably from that of theand mechanical shear during the extrusion process. corresponding homopolymers. As in pqlymers, inFor instance, when the niixture is held at the lowest general, the course of crystallization in blends is go-practical temperature above the melting point of PP, vemed by equilibrium thermodynamics and kinetic

..boundary conditions, such as mutual dispersion ofthe components, super molecula~ structure, struc-

: ture of the interface, etc. These boundary conditionschange remarkably with blending, thus causing a lotof technically important and interesting effects. De-

(a ) pending on the blend components under considera-~ ~ ~ ~ -~ tion, a large number of physical and physico-chemi-

~ ~ ~":=" --cal phenomena may originate of which some may

~ ~-~ ~~ have non-equilibrium thermodynamic basis. For~ ~ -~ ~ melt-compatible polymer blends, among other ef-

( b fects, a variation of the crystallization process due to~ ~~~~~ ~- ---altered nucleation and growth conditions has been

;::~ ~ ~ -reportedl.6.27.28. The usual mixing-induced melting

: :=:::-_-=:::;;;- :~:~=::;;;-::::~;- point depression has been observed toOI.6.27.28. For: ' =-- -polymer blends with extended interfacial regions,~---=( c ) these phenomena were als<:> r~f~orted to. appl~ to ~he

F ' 1 Diff fd " f I (d k . ) bulk of the phase boundanes ..The mvestIgatIon19. -erent types 0 IsperSlon 0 a po yrner ar regIons ...in the matrix of an immiscible polymer. The spherical droplets ~f mcompa?ble polymer b~ends. rev~ed. th~ mduc-(a) are progressively extended into platelets (b) or fibrils (c) by tIon of specific crystal modificatIons, rejection, en-

deformation gulfing and deformation of the dispersed compo-

32 INDIAN J. FIBRE TEXT. RES" MARCH 1993

nent by the growing spherulites of the matrix materi- ,~ cal27,.and the nucleation at interface27,32.1n general, ,') -,' '\ n I'",~-

studIes on melt-extruded blends have suggested that 'a~~;b:l_ ~ ,1 I ,i -.I I ' , ' ' \,

the crystallization of one of the polymers is affected ' II, \. ' ~ .., ,

by the presence of other polymer. Even if the po- ,.i I ~ '""V

lymers are thermodynamically miscible, crystalliza- \ !. , ; Cor"'. ' ,

tion result~ in phase ~egregation u~ess the poly~ers ~ ~"'1;' J , ~j I' )can form ~somorphic blends. It IS also reco~ed ~/( ,/ I'i); l. ~ )that the differential rate of polymer crystallization -,--. V .-' ~

during extrusion affects the processing behaviour of ?? (~ ...J the polymer blends33.34. r "",' "\ i /:'" f\

The reported investigations concerning polymer ': \ : \. \ v -'v \ ,'"\crystallization in blends maybe categorized" in three ,-,' \... '.J ., ",._~,..."

main groups, viz. partially miscible, immiscible and ( a )

miscible blends, as shown schematically in Fig. 2. In

first case, partial molecular miscibility of compo- "

nents is possible. In such a case the blend may con- ~

sist of two phases, each containing two molecularly .,-

mixed components in different volume ratios35. ,~::i

Crystallization of components in such a system will '-'"be influenced by the composition of individual .' ,

phase and will involve demixing in the microscopic ,\.scale, Both the dispersion and the partial molecular ~"

miscibility of components in the blend can involve

altering of the crystallization kinetics and resultant ,.morphology of polymeri constituting the blend. The (' '

usual result being reduction in the rate and amount ;

of crystallinity35-37, slight melting point depression

due to imperfect crystal formation38, and appear- ( b )

ance of a single melting peak or a third melting peak

in addition to the two homopolymer peaks as a re- i "~,, C:;w:J-~~;:'~~\

suIt of co-crys.tals fo~ation. In immiscible blends, .,~~~~.., ~ i V.j'-:;~~.

due to lack of mteractlon between two components, ~I \.; \ -'; -\~, ,;there will be only two melting peaks of homopolym- .'\ ..

ers as no co-crystallization will take place. Here, the

melting temperatures of the component polymers

are important from the point of view of defining the

mode of nucleation. Depending on the melting point

difference, a component polymer may crystallize in

the presence of molten phase of the other compo-

nent or in the presence of solidified particles. Thus,

in case of blends with large differences in the melt~

ing points of component polymers, the crystalliza-tion of high melting polymer takes place in presence ( C )

of the molten phase of other component. The pres- F" 2 M h I " f bl d f I A ( I ' d I ' )19, -orp 0 ogles 0 a en 0 po yrner so 1 mes and

~nc~ of second molten phase may ~ect ~e crystal- polymer B (dashed lines) [(a) miscible, (b) immiscible, and (c)

lizatlon process through the effect of It'S VISCOSIty on partially miscible]

the mobility of the crystallizing polymer chains. On

the other hand, the low melting polymer crystallizes The other important aspect of crystallization be-

in the presence of the solidified particles of the other haviour in polymer blends is the rate of cooling.

component, which may act as nucleating agents. Grebowicz and Pakula4° studied the crystallization

Thus, the crystallization of the low melting polymer behaviour of PP/HDPE" blends at varying crystalli-

would take place in a heterogeneous nucleation zation rates and showed that the sequence of crys-

mode and at higher viscosity of the system as a result tallization of PP and PE depends on the cooling rate.

of solidified particles of other component39. At very slow cooling rate, PP crystallized at higher

DEOPURA et aL: FmRES AND FIlMS FROM POLYOlEFIN BLENDS 33

temperatures while at high cooling rate, the opposite phology, and (ill) addition of compatibilizing agents.sequence was observed. For the cooling rate of 2°CI The addition of compatibilizers is by far the mostmin, only a very small shift in the crystallization tem- popular method of alloying. The most frequently.perature of PP against PE was observed while for used compatibilizers are block, graft co-50°C/min cooling rate the shift was considerable. polymers, interacting copolymers and co-solvents.They also observed slight difference in the crystalli- The theory of compatibilization by addition of di-zation temperatures of blended and pure samples block copolymers has been developed by Hong andand related it to the early crystallization ofPP phase Noolandi41-43. The A-B block copolymer was as-in blended samples due to the availability of PE he- sumed to selectively dissolve block A in polymer A.terogeneous nuclei. However j some authors also re- and block B in polymer B, binding the two phases Aported38 that these blends segregate into two dis- and B via chemical bonds of copolymers as showncrete phases with a negligible influence of the PE in Fig. 3. It is well established that such bonding isphase on the crystallization behaviour of PP. In a si- strongly dependent on the presence of the blend ofmilar study on crystallization behaviour of PPI appropriate block or graft copolymers having chainLDPE and PP/HDPE blends, Plesek and Malac39 segments chemically Identical to that of homopo-observed two distinct crystallization peaks for PPI lymer blend components, type of blending processLDPE blends at slow as well as very high cooling and on the processing parameters employed. Therates. On the other hand, for PP/HDPE blend, the influence of these copolymers, generally referred tosame conclusion hold true only for the lowest cool- as "interfacial agetns", has been related to theiring rate. For higher cooling rates, only one crystalli- tendency to be preferentially located at the interfacezation peak was observed. between phases, and to the compatibility of their in-

dividual chain segments to penetrate into the phase2.3 Interphase Modification to which they are chemically identical or similar44,45.

It is well known that the mixing of two polymers Also important are-the interactions between the po-generally leads to heterogeneous system, since the lymers at their interfaces. The interfaces betweencompatibility between them is rather a rare event. the phases of polymer blends are sometimes calledThe formation of two-phase heterogeneous systems interphases since some localiz~ intermolecularis not neccssarily an unfavourable event, as many mixing or interdiffusion can occur in polymeruseful characteristics of single phase may be pre- blends which approach thermodynamic compatibil-served in the blend while other properties may be ity. In other words, small domains of a mixed phaseaveraged according to the blend composition. Prop- can exist between the "pure" phases. This molecularer control of overall blend morphology and good interdiffusion can increase the wetting of one po~adhesion between the phases are, in any case, re- lymer by the other, increases the adhesion betweenquired in order to achieve good end-use properties. the phases, or reduces the effective interfacial ten-This can be achieved by a physical or chemical ac- sion. Increased adhesion or interdiffusion betweention that results in the stabilization of polymer blend the phases would be expected to have improved ul-morphology and is generally referred to ascompa- timate properties of the blends. The decreased in-tibilization process. The compatibilization methods terfacial tension would be expected to give more ex-can be divided into three categories loosely labeled tensive subdivision of particles during melt mixing.as mechanical, chemical and physical. The aim of The smaller particles generally give rise to improvedmechanical compatibilization is development ofnon-equilibrium morphology which in one respect (0) Compatibilizer (b)is relatively stable and reproducible, yet in another Interphase1 interPhase.respect leads to enhancement of properties. Chemi- .cal compatibilization involves chemical reaction as ,51('6'1an essential factor for (i) .~ormatio~ of interpenetrat- {)I:!::ing network structure. (n) reversIble or permanent Ph Phase B Phase A Phase Bcrosslinkingivulcanization, (ill) exchange reaction ,""'creating in situ the compatibilizing copolymer, and ,~(iv) reactive processing. The last category of compa- {--' '-

tibilization method is physical compatibilization. Tothis category belong (i) ~~~ification ~f polym~ric Fig. 3-(a) Ideal configuration of a block copolymer at the inter-structure to e~ance mISCIbility (e.g. by mtroduction face between polymer phases A and B, and (b) Formation of anof ionic or hydrogen bonding groups), (ii) control of interphase between phases A and B promoted by acrystallization as a mean to lock-in developed mor- compatibilizer

--~ -

34 INDIAN J. FIBRE lEXT. REs., MARCH 1993

blend properties, as shown by Coran and Patel46, Other molecular parameters of AB copolymers,and depend upon the interfacial tension47. How- important for their emulsifying efficiency, are the -ever, the desired effect may still result if one of the molecular architecture, molecular weight of individ-arms (segments) of the block or graft is miscible with ual segments and the chemical composition. Theor adhere to one of the phases. best conditions are met when the interfacial agent is

Th tI.. ty f th tI.biliz.. ff ..simple in architecture and is able to concentrate ate ac VI 0 ese compa ers IS e ectlve m :very la f bl k I d h i phase boundanes. FIrst condItion can be satisfied byrge range 0 oc copo ymer an omopo- ...I I I .ght 1::' blk I usmg sImple copolymers of dl-block type, where ea-

ymer mo ecu ar wel s. ror OC copo ymer to. ...locate at bl d . t rf .t h Id h th sler penetration of segments mto respective homop-en me ace, 1 s ou ave epropens- ...ty t t . t tw has Thi t d .olymer phase IS possIble due to lower conforma-l 0 segrega e moo pes. s en ency m. ...

bl k I d d th . t tI. b tlonal requIrements, while segregation of copolymeroc copo ymers epen s on e m erac ons e- ...

tween th tw t d th . II at phase boundanes IS dIrectly dependent on thee 0 segmen s an elr mo ecu ar ....ght I ro . I th ts h Id b I copolymer composItion and the ratios of molecularwel s. n pa cu ar e segmen s ou e ong .

gh ~. weIghts of A and B segments of the copolymer toenou to have sufficIent cohesIve forces to anchor th fA d B h I ti. I 44th ..ose 0 an omopo ymers respec ve y .em firmly mto the domams they penetrate, but not Th h I f bl d f I A .thth ..

d " ffi . tiliz .e morp 0 ogy 0 en so po ymer Wi po-more an optimum requIre lor e clent u a- I B d I fAB h b tud .edtion44 ymer an copo ymer 0 type as een s 1

.by many researchers41-51. According to them, theIt has been found that block copolymers are size of domains in homopolymer blends were re-

usually better interfacial agents than graft copolym- duced by addition of a suitable block copolymer.ers because in the latter, multiple branches restrict ,This provides direct evidence that block copolym-the penetration into similar homopolymer phases. ers restrain the phiiSe segregation of the homopo-For the same reason, di-blocks are more effective Iymers in their macroscopic domains and, therefore,than tri-blocks. Block or graft copolymers have to facilitate mixing of the two immiscible homopolym-segregate into two phases in order to localize at the ers. There seems to be universal agreement thatblend interface. This specific behaviour of block stabilization of above type only occurs when theand graft copolymer as well as their immiscibility molecular weights of the homopolymers are lessonly in one of the homopolymer phases depends on than or comparable to the molecular weights ofthe interactions between two segments and their corresponding segments in the block copolymer48-51.molecular weights. The amount of compatibilizer When the homopolymer molecular weight isrequired depends on many factors of which confor- higher than that of the corresponding block seg-mation~d molecular weight are the most import- ment, the homopolymer forms a separate phase andant ones. Is is possible to estimate this amount for a is poorly solubilized into the domains of the blockgiven molecular weight M of the compatibilizer to copolymers.saturate all the interface in a blend. In this calcula-tion one takes into consideration the interfacial area 3 Selected Examples of Polyolefin Blendsper unit volume of the blend in correlation with thevolume fraction fjJ A of polymer A which is dispersed 3.1 Polyethylene-Polyethylene Blendsin the form of spherical particles of radius R in a ma- Various types of polyethylene are distinguished intrix of polymer B. When each of the compatibilizer terms of their density as high density (HDPE), lowmolecule occupies an area P at the interface, then density (IDPE) and linear low density (LLDPE)the ratio of the mass m of block copolymer required polyethylenes. These polyethylenes differ from oneper unit volume of the blend equals 3fjJAMIPRNA, another, mainly in the molecular structure, molecu-where N A is the Avogadro's number. When the va- lar weight distribution and chemical compositionlue of P is assumed to be 50 (A)2, one can calculate distribution as well as in the bulk structure includingfor fjJA = 0.2 and R= 1 .urn that one needs about 20% the spherulitic structure, lamellar size and crystal-of the block copolymer by weight with M= 105 to fill line/amorphous interphase structure52.53. The dif-up the interface. When the molecular weight of the ferent densities result from a variation in the crystal-compatibilizer is reduced to 104, then this amount line packing ability of polyethylenes due to a differ-drops to 2%. These ar~ents show the advantage ence in the degree and length of branching. For ex-of lower molecular weight compatibilizers but it ample, the density of LLDPE decreases with theshould be mentioned that higher molecular weight a-olefin comonomer content at an extent dependingcompatibilizers are needed in order to penetrate on the olefin chain length. Moreover, the shape anddeep enbugh into phases and to be anchored firm- size of the spherulites are strongly dependent on thely44.48. length and frequency of branching53. The spheru-

DEOPURA et al: FmRES AND FllMS FROM POLYOlEFIN BlENDS 35

lites in LLDPE are smaller and less regular than melt-compounded blends of IDPE of MFI-2 with'those in HDPE. However, they are markedly larger 10 grades of HDPE of MFI 0.2-0.9 by differentialthan those in IDPE of similar density and melt flow thermal analysis (DTA). The majority of blends ex-index52 (MFI). hibited three well defined melting peaks corre-

Several investigations have been performed to el- sponding to IDPE, mixed crystals of IDPE/HDPEucidate the mechanical properties, morphology and and HDPE crystals. It was observed that at lowcompatibility of various types of polyethylene HDPE content (less than 30%), blends of someblend54-81. These studies have examined binary and grades of HDPE exhibited only two peaks, corre-ternary blends prep~red from low, linear low and sponding to the melting of IDPE and co-crystals.high density polyethylenes which show that the mis- Thus, the blends of HDPE grades with IDPE couldcibility between two polyethylenes is not apparent. be divided into two groups: one exhibiting three wellIn principle, accounting for their behaviour in the defined melting peaks and the other exhibiting twomolten state, these blends should be considered as well defined melting peaks. Clampitt71 also studiedmiscible54,55. However, on cooling from the molten the blends ofHDPE with five grades ofIDPE basedstate, molecular fractionation occurs56-58. When on different MFI and degree of branching. Thesecrystalline polymers undergo crystallization, frac- blends also exhibited three melting peaks. It was ob-tionation has been known to occur which depends served that the co-crystals peak height was sensitiveon various parameters such as crystallization rate to both the degree of branching and the MFI ofand molecular weight and its distribution59-61. How- IDPE. It was found that a low MFI and a low degreeever, several studies involving different experimen- of branching of IDPE favoured the formation oftal techniques have shown that during isothermal large co-crystals. No co-crystals peak was observedcrystallization, excepting in very low molecular for blends of IDPE containing the highest d'egree ofweight species, co-crystallization also occurs among branching (30 methyl groups per 1000 carbon at-the different species62-64 and almost universally dur- oms) due to the decreased ability of IDPE chains toing rapid crystallization. Binary blends of IDPE and get packed in the crystallites. Also, the co-crystalHDPE and the ternary blends of low-, medium- ffild melting peak height decreased with increasing MFIhigh-density polyethylenes showed two endotherms ofIDPE.in differential scanning calorimetry (DSC) studies65. Similar three melting peaks have also been re-The lower peak is attributed to melting of co-crys- ported for LLDPEIIDPE blends 72 and HDPEItals of low- and medium-density polyethylenes, IDPE blends61,73. Studies74 on blend of LLDPEIwhereas the higher endotherm is associated with IDPE of identical density for 1:1 composition showmelting of HDPE crystals. On the other hand, pre- three shallow melting peaks around 1241 117 anddominant co-crystallization was observed by Kyu et 108°C while that of the composite film of identicalaL66,67, and Gupta et aL68 for blends prepared from composition show only two relatively sharp meltingLLDPE and HDPE, both at slow and rapid crystalli- peaks at 126 and 109°C corresponding to the melt-zation. No segregation was observed at the higher ing temperatures of the pure components. Hence,order structural levels of crystalline lamellae or the additional broad melting peak between the mainspherulite. The blend manifested intermediate me- peaks of pure components, observed for thechanical and optical relaxation in the. a- and p.. blended sample, indicates som:e level of co-crystalli-regions, Kyu et aL66,67 explained the phenomena by zation. Upon cooling from melt, these blended filmsconsidering that the crystallization of HDPE in the exhibited two crystallization exotherms at 111 andfirst stage of the process becomes the driving force 96°G, corresponding 'to pure polymer of LLDPEfor the crystallization of those segments within the and IDPE respectively, where the lower tempera-'.branched chains that are long enough to deposit in ture exotherm was found to be broad. This differ-the growing substrate. This means that although ence stems from the changing crystallization envi-short segments between branches are rejected, they ronment. LLDPE crystallizes out first from a moltenare bound to crystallize segments and complete blend, the composition of which changes durill'g thesegregation is, therefore, not possible. This is not the crystallization process, while IDPE mainly crystal-case for the blends of HDPE/IDPE where clear lizes from a blend in the presence of already crystal-segregation takes place for weight fractions of linear lized LLDPE. Moreover, some of the IDPE ge~polymers of 50 and 25%. Blends of high polydi- cocrystallized with LlDPE chains, mainly witlJspersity HDPEIIDPE were shown to be incompati- those containing more densely distributed branchesble by Norton and Keller69, while co-crystallization The latter may be responsible for broad lower tem-of the two PE samples was reported by Donatelli65 perature exotherm observed for blends. In anotherand Datta and Birley7O. Clampitt71 investigated the studt5 it has been shown that both binary low/high

36 INDIAN J. FIBRE 'IEXT. RES., MARCH 1993

and ternary low/medium/high density polyethylene UHMWPE fractions. This phenomenon is attribut-blends exhibit two melting peaks. For the binary sys- ed to better mixing of two components at highertems, the peaks were associated with the separate UHMWPE fractions. At lower Uill\IIWPE content,melting of the low- and-high-density crystalline re- the lower shear stress between the components re-gions. However, for the ternary systems, the low- suIts in inhomogeneous mixing of two components.temperature peak was assigned to the melting of co- This kind of phenomenon is commonly observed incrystals of the low- and medium-density polyethy- the blends where the viscosity of major componentlene. is signifjcantly lower than that of the minor compo-

Blends of LDPE/ilDPE have gained such im- nent8. On the other hand, Bhateja and Andrews85portance that they have been commercialized. showed that the melting point did not vary signifi-UDPE is added to LDPE owing to its superior me- cantly with composition, indicating no change in thechanical properties, e.g. higher tensile strength, perfection of crystallites. Further, the degree of crys-elongation at break, and impact strength. In addition tallinity in the blend was lower as compared to thatto this, it allows a higher degree of down-gauging of in homopolymer, because of the suppressed mobil-LDPE filmsI0.75-77. On the other hand, addition of ity and crystallizability of LDPE in the presence ofsmall amount of LDPE to UDPE modifies its ex- highly viscous melt ofUHMWPE.tensional viscosity and improves bubble stability, Termonia et al.86 investigated the mechanical pro-optical clarity and the productivity of tubular blown perties of highly oriented polyethylene blend fibresfilms. Hadjandreou et oL 78 studied the blends pre- prepared from binary mixtures of UHMWPE andpared from LDPE with HDPE or LLDPE and HDPE of 3 x 106 mId 59 x 103 molecular weightsshowed that improved solid-state mechanical pro- respectively. The gel-spun fibres prepared fromperties can be achieved from blended films of 20% these binary mixtures, covering the entire composi-HDPE or LLDPE with LDPE, particularly the creep tion range, showed systematic linear increase in ten-behaviour for short-term load applications: Similar sile strength and initial modulus with increasing highresults were also obtained by Yilmazer79 for blends molecular weight fraction. However, elongation atof LLDPE/LDPE. The mechanical properties for break was found to be dominated by the high molec-blended films followed additivity rule except in the ular weight component over a large compositionrange of 20-40% LLDPE content, for which the ten- range; it decreased only at high concentrations ofsile properties were higher, implying compatibility shorter chains.in this range. However, the tear strength of blendedfilms improved only in the transverse direction and 3.2 Polypropylene-Polypropylene Blendsit decreased in the machine direction. There are very few reports on studies of blends

As far as the ultra high molecular weight polyeth- prepared from different molecular weights of PP.ylene (UHMWPE) blends with conventional pol- Blends of different fractions of PP showed lower im-yethylenes are concerned, there are a limited num- pact resistance; however, the processing of suchber of publications available in the literature80-84. A blends was superior to that of the PP of comparablestudy of this kind is due to Dumoulin et aL 80 who in- intrinsic viscosity87. Blellds of PP resins were foundvestigated the properties of UHMWPE blends at to have better spinning performance for fine denierUHMWPE content less than 6%. The primary rea- fibres than those prepared from individual resins88.son for this is associated with the ultra high melt vis- Deopura et al.89.9° studied the crystallization behav-cosity of the material, which restricts the polymer iour of PP and high molecular weight PP (HMPP)processability by the conventional techniques. K yu blend system of various compositions. In case of 3%and Vadhar82, in their study on UHMWPE with HMPP addition to PP, increased crystallization rate,LDPE, LLDPE and HDPE, showed that co-crystal- compared to that for PP, was observed as a result oflization takes place in the UHMWPE/ilDPE and enhanced nucleation and gro,wth rate. However, atUHMWPE/HDPE blends. However, separate crys- higher HMPP compositions, the growth rate dec-tals are formed in UHMWPE/LDPE. They attribut- reased. The breaking strength and initial modulus ofed the formation of separate crystals in latter case to 6% HMPP blend fibres were 0.75 GPa and 7.34long chain branching ofLDPE. More or less similar GParespectively as compared to 0.65 GPa and 5.2results were also obtained by Vadhar and Kyu81 on GPa for PP fibres. Th.e improvement in mechanicalUHMWPE/ilDPE blends. The appearance of du- properties is attributed to the increased amorphousal peaks at lower UHMWPE contents and observa- orientation (Jam) and inter-crystalline tie molecules.tion of a single endotherm at higher UHMWPE On heat-setting, the rate of decrease of tam with heat-fractions indicate that co-crystallization takes place setting temperature was found to be less for 6%between UHMWPE/ilDPE blends at higher HMPP blended fibres as compared to that for PPfi-

DEOPURA et aL: FmRES AND Fll.MS FROM POLYOlEFIN BlENDS 37

bres. Similar observations were also made by Geleji not harmonize. A certain Qegree of compatibilityet aL 91 for fibres prepared from homologue mixtures has. been found in the selected combinations ofof polypropylene. The study showed improvement LDPE/PP, even though the thermodynamic compa-in tensile strength and modulus with increasing high tibility of these two polymers is insufficient to pro-molecular weight component. duce homogeneous melts in every ratio of the two

eompQnents; their structural similarity guarantees3.3 Polyethylene-Polypropylene Blends good adherence between both polymers, although

Processability and improvement in impact pr~- phase separation occurs on micro-scale during crys-perties of PP are often the reason for preparation of tallization. Film tapes made from blends of LDPEPE/PP blends. The morphology and properties of with PP in certain mixing ratios show remarkablyPE/PP blends have been studied extensively40.92-110. good end-use properties. Deanin and Sansone92 al-In particular, studies on these blends showed some so reported a significant improvement in maximuminteresting aspects from the theoretical and practi- tensile strength and work of rupture of 20:80cal point of view. Deanin and Sansone92 and Greco LDPE/PP blend. According to a patent99 of ICI Ltd,et aL93 found synergistic improvement in mechani- film tapes can be manufactured from a blend ofcal properties for some blend compositions. The 20:80 LDPEIPP having tensile strength of about 6.5reason for this synergism has been qualitatively cN/dtex (7.2 gpd) with improved fibrillation resist-ascribed to interfacial effects and partial miscibility ance. Undoubtedly, LDPE in blends with PP allowsof PE and PP chains in the molten state. Lack of co- a more effective reduction of the fibrillation tenden-crystallization, owing to the differences in the mo- cy than HDPE, especially when applied in the prop-lecular structure of PE and PP, supported by the er ratio and in good correspondence with the meltpresence of individual melting temperature of the index of the blending polymerloo.polymers in their blends indicate incompatibility ofthe system in the solid state. In liquid state, some in- Lovinger and Williamslol studied the morphologyteractions between chains can not be excluded ow- of HDPE/PP blends for the entire compositioning to the small differences in polarity and free vo- range. They found that structures having up to 50%lume. PP consisted primarily of interpenetrating networks

Detailed analyses of the PP blends, including of the two polymers, while the blends having 50%those with LDPE, have been carried out by many PP or more were typified by PE domains dispersedworkers27.94-98. The morphological studies clearly in a PP matrix. Addition ofHDPE to PP resulted insHow that these blends are heterogeneous two- reduction of average spherulite size of PP for all thephase systems, the components of which crystallize blend compositions, indicating increase in PP nuc-separately. The occlusions of the dispersed polyeth- leation density due to blending. Bartczak etaLIO2 re-ylene phase do not influence significantly the forma- ported that blends of HDPE/PP show PP spheru-tion and growth of PP spherulites, but the nucleation lites with a grain like structure constituted by HDPEdensity is decreased27.94.95. Martuscelli et aL96 re- inclusions even after the complete crystallizationported that isothermal radial growth rate of PP process. For the crystallization temperatures up tospherulites in the molten state remained unchanged 127°C, disappearance of heterogeneous nuclei fromby the addition of LDPE when measured at temper- PP matrix was observed due to their migration to-atures well above the melting point of LDPE. Still ward HDPE phase. However, an overall increase inthe overall crystallization rate was found to decrease heterogeneous nuclei was observed due to the pres-in isothermal studies by DSC. This implies that nuc- ence of interface of HDPE crystals growing at theseleation ofPP is affected adversely by the presence of temperatures and acting as nucleating agent for PP.LDPE in melt. Teh97 showed that the addition of For crystallization temperatures above 127°C, it wasLDPE enhanced the nucleation of small PP spheru- observed that presence of HDPE phase in moltenlite (a-form). The crystallinity of LDPE and PP in state adversely affects the homogeneous nucleationthe blends, as measured by DSC and WAXS (wide ofPP and thus decreases the nucleation density andangle X-ray scattering) was found to be unaffected increases the resultant spherulite size of PP.and followed the rule of additivity of mixture. How- Martuscelli et aL 103 extensively studied the isoth-ever, Dumoulin et aL98 reported increase in crystal- ermal crystallization kinetics of HDPE/PP blendslinity for LLDPE/PP blends containing up to 10 wt and showed that the crystallization of HDPE is% LLDPE.1n contrast to LLDPE/PP and HDPE/PP greatly delayed by the addition of small amount ofblends, which are compatible polymers and may be PP up to 10 wt %. The half crystallization time T 1/2mixed partially in the amorphous region over a wide for these blends increases to around three and fourrange of blend ratios, most other combinations do times larger than that ofHDPE andPP respectively.

.

38 INDIAN J. FIBRE 1EXT. RES., MARCH 1993

The retardation of crystallization rate, despite the The effects of EPR and other types of copolymerpresence of solidified PP for heterogeneous nuclea- on the crystallization behaviour of PP were investi-tion of HDPE, has been attributed to the increase in gated by Karger-Kocsis et all14. The blends contain-melt viscosity of HDPE phase due to presence of so- ing up to-40% EPR by weight were studied by usinglidified PP.1n another study on crystallization kinet- the techniques such as DSC, WAXS and SAXSics of HDPE/PP blends in non-isothermal mode, (small-angle x-ray scattering). The WAXS diffracto-Grebowicz and Pakula4° found that components in grams showed a significant change in structure ofblends, for all compositions, crystallize at higher PP. The intensity of peak at 28= 16°, which. corre-temperatures compared to homopolymers, and the sponds to (300) reflection of the hexagonal phase,sequence of crystallization of PP and HDPE de- decreased markedly with increasing conc~ntrationpends on the cooling rate. At very slow cooling rate, of EPR. It is also reported that the proportion of thePP crystallizes earlier while at higher cooling rate monoclinic and hexagonal phase altered in EPRthe opposite sequence is observed. blend samples as a result of large change in nuclea-

Some authors have also reported the tensile pro- tion process. However, no significant difference wasperties of PE/PP blends. The earliest of such works observed in long period, as determined from SAXSare thos~ of Slonimskii et a[.l°4 and Plochocki 105 studies. It was also observed that the incorporationwho found a monotonic increase in tensile strength of EPR in PP matrix leads to significant changes inwith PP content. On the other hand, Noel and Car- the crystallization behaviour and morphology of PP.leyl06 obtained a maximum in ultimate strength and The d~gree of supercooling of PP decreased frominitial modulus at 90% PP, whereas Deanin and San- 60° to 42°C with increasing EPR content, indicatingsone92 found similar results at 80% PP. Recently, enhanced nucleation. This observation- was alsoGallagher et all07 have studied 1:1 HDPE/PP supported by a decrease in the spherulitic size ofPP.blends and homopolymers in highly oriented state. Similar results were also reported by Martuscelli etThey observed a clear phase separJ1tion of two a£27.94 for some isothermally crystallized PP/EPRblend components with nearly similar structure, blends. It was found that during crystal1fzation themorphology and crystalline c-axis orientation. The pre-existing copolymer particles are occluded,same study also indicates that mechanical response mainly in the intra-spherulitic regions, and such aof blend samples to the external macroscopic stress process is found to produce large structural modif-can be well described by a simple parallel model in ications in the spherulitic structure. Moreov~r, itwhich the two blend components, although mechan- was observed that the presence of copolymer drasti-ically linked, act more or less independently. cally influences the process of primary an<J secon-3.4 Blends of Polyethylene or Polypropylene with (Ethylene- dary nucleation which is a function of physical and

Propylene) Copolymer chemical properties of copolymer. The decrease inBlends based on PP and ethylene-propylene cop- the viscosity of elastomer and the increase in both

olymers, such as EPR, are gaining increasing indus- the elastomer content in blends and the ethylenetrial importance beyond the traditional applications content in the elastomer have been found to reduceof high-impact polymers. Moreover, the recent the spherulitic size in the blend samples 115. Recently,development of ethylene-propylene thermoplastic D'Orazio et al116 have also reported similar resultscopolymers has greatly i~creased the potential ap- for isothermally crystallized PP/EPR blends, as dis-plications of such blends. Polypropylene and EPR cussed earlier. They observed that for a given crys-are generally incompatible and their mixtures are tallization temperature, the thickness of the crystal-thus heterogeneouslll.112. Phenomena such as segre- line lamellae (Lc) of PP crystallized from its blendsgation, stratification and phase inversion are, there- decreases, whereas the thickness of the amorphousfore, to be expected as in other multiphase polymer interlamellar layer (La) increases with EPR contentsystems. The phase structure is complex and varies and crystallization temperature. They related thiswith PP content and processing conditions. With phenomenon to the hindering of PP crystal growthmore than 50% PP, i.e. in the plastic materials, the by low molecular weight EPR molecules which canPP forms a continuous phase. Below 50% PP, either diffuse easily in PP amorphous phase and form do-PP or the copolymer may be continuous. HoweveI;, mains, more or less interconnected with the amor-phase segregation is also reported in some cases phous PP phase.even at 8% PE content of the PP-PE copolymerl3.The correct choice of the grade of PP and copolym- In a detailed studyll7 on liquid and solid stateer is important in order to obtain the required phase thermodynamic compatibility of PP/EPR systemstructure and the mechanical properties of fibres carried out with the help of small-angle neutronand films. scattering (SANS) it was observed that phase separ-

DEOPURA et aL: FmRES AND Fll.MS FROM POLYOlEFIN BlENDS 39

ation occurred even when the ethylene content of tent and drawing temperature above 15Soc can pro-EPR was as low as 8%. Furthermore, the separated duce bimodal texture of PP crystals. They also ob-phase domains grew rapidly at melting tempera- served that under the similar crystallization andtu.res: On ~e other hand, ~e ~l.e~ds of atactic. PP drawing conditions, the index of anisotropy, Ia (cry-WIth ISOtaCtiC PP showed D1lsClbility upon melting. stallite C-axis orientation to the fibre axis), dec-The gross phase separation, however, was observed reases with increasing EPR content, and related it to~ both the cases during solidification or crystalliza- the disturbances caused by EPR molecules duringtion process. Wang and Huang!13 also investigated the orientation process. On the other hand, Coppolathe crystallization behaviour of EPR, PP-EPR block et al.120 showed that ductility for PP/EPR blendscopolymers and PP/EPR blends with the help of markedly increases up to 20% EPR content and is aDSC, WAXS and other techniques. The thermal ari- strong function of drawing and crystallizationalysis exhibited single endothermic melting peak temperatures of precursor films. The results alsoand single exothermic crystallization peak for EPR show that presence of EPR molecules decreasescopolymers corresponding to the ethylene sequence significantly the overall crystallinity and spherulite

, of the EPR copolymer. However, PP-EPR and PP- size and acts as a lubricating agent, decreasing theEPR-PP copolymers and their corresponding me- internal friction during morphological transforma-chanical blends showed two melting endotherms, tion from sphertllitic to fibrillar structure. The im-one at 154-162°C and the other at 119-121°C, attri- proved mechanical properties for PP and PP/EPRbuted to the crystallites of PP and PE sequences in blends were observed for higher drawing tempera-the EPR block respectively. Similar results were tures. However, at all the drawing temperatures, PP/found for the crystallization temperatures. Wang EPR blends showed lower modulus values than PP.and Huangll3 also observed significant lowering inmelting temperature as well as delayed crystalliza- 3.5 Blends of Polyethylene or Polypropylene with Ethylene-tion for PP and PE phases of copolymers as com- based Elastomerpared to that of blends and related it to the r- Blends of PP or PE with EPDM elastomers aremodification of PP, lowering of PP crystal sizes in the most studied of this group. The mechanical pro-( 110) direction, imperfect crystalline structure, and perties of these blends depend largely on the type ofreduced chain mobility due to hindrance caused by elastomer used for blending. Generally, a high mo-covalently bonded PP-EPR blocks in the mechanical lecular weight EPDM containing about 70-75% eth-blends. ylene with long sequences of ethylene units in the

Onogi et OllIS reported that although the quench- copolymer chain is preferred. This leads to someed samples of virgin PP crystallized in the hexagonal ethylene crystallization in EPDM phase whichstructure ({J-form), the PP in the melt-blended PP/ seems to contribute to the physical cross-linking andEPR blends crystallized in a monoclinic structure hence greater elasticity of the blends with PP or PEunder the similar conditions of quenching. These re- phase. In contrast, a low molecular weight grade ofsults suggest that the addition of an elastomer re- matrix material such as PE or PP is preferred; thissuIts in a significant decrease in the proportion of helps to keep the melt viscosity low and assist crys-hexagonal ({J-form) PP crystallites. These results are tallizationI21-125. Due to continuing crystallizationin agreement with the findings of Karger-Kocsis ct process at temperatures below 1:" polyolefin plas-alll4, who have also reported a decrease in the hexa- tics/EPDM blend's are generally heterogeneous.gonal crystal form of PP when blended with elas- Thus, their properties will depend on the overalltomers. In addition, Karger-Kocsis et alll4 also morphology of the blend, i.e. the shape and size dis-observed a significant reduction in the degree of tribution of PE and PP spherulites and of elastomersupercooling of PP, indicating enhanced nucleation. domains, adhesion at interface, nature and structureThe studyllS also showed increase in deformation of the domains, and the kinetic factors such as theratio with increase in PP content of blends. The rates of crystallizatiOft and vitrification.blends having PP content more than 60% showed Starkweather26 extensively studied the structurenecking phenomena and larger increase in dichoric and morPhology of blends prepared from EPDMcrystallite orientation function ftP for PP crystallites, with LDPE and copolymers of ethylene-vinyl acet-while the blends containing less than 40% PP ate. When LDPE was added to EPDM having anshowed almost no increase in ftP. They related these ethylene-propylene ratio of 4.5 and a low level ofobservations to the large increase in viscous flow of polyethylene type crystallinity, crystalline interac-EPR molecules at room temperature. In a similar tions were seen through changes in the size of thestudy on deformation behaviour of PP/EPR blends, unit cell. As the EPDM was added to LDPE, theKammer et all19 showed that increase in EPR con- a-parameter of unit cell increased from 7.515 to

40 INDIAN J. FIBRE TEXT. RES., MARCH 1991

8.35 A while the b- and c-parameters were almostconstant. The unit cell dimensions for blends wereclose to those for methyl branched polyethyleneshaving the same overall concentration of methylgroups. The incorporation of this type of EPDMmolecules in LOPE unit cell interferes with thedevelopment of spherulites and improves the trans-parency of films. On the other hand, the blends ofHDPE with EPDM exhibited no significant effecton the spherulitic morphology of HDPE. Also, theunit cell parameters for HDPE remained un-changed. The results show that the interaction ofEPDM is more pronounced with LOPE, resultinginto miscible blend. It was also observed that the in-teraction between LOPE and EPDM increases withincreasing ethylene content of EPDM. Thus, blend-ing with EPDM significantly affected the crystalliza-tion of LOPE compared to HDPE. This effect wasattributed to the interaction between LDPE andEPDM having similar cohesive energy densities inthe amorphous phase. The dynamic mechanicalstudies also show a systematic shift in loss peak tow-ard ~relaxation of polyethylene with increasingLDPE content, indicating miscibility in amorphousphase.

Similar blend systems based on EPDM were stud-ied by Lindsay et ai'?' and others+":'?", In fact,these authors reported that commercially importantblends can be obtained with high ethylene EPDMand LDPE that have tensile strength higher than thetensile sJrength of either component polymers.Lindsay et al. also made DSC measurements thatshow significant decrease in the crystallization tem-perature of LDPE due to partial miscibility and mo-lecular level interaction between LDPE and high-ethylene sequences of EPDM.

The situation is different when the performanceof HDPE/EPDM blends is compared with PP/EPM(ethylene-propylene rubber) blends. Orazio et al.128found that EPM copolymers act as the interfacialagent for PE/PP/EPM ternary blends, improving,for example, the adhesion between HDPE dis-persed phase and PP matrix with reduction in do-main size. These observations suggest that a certainamount of EPM molecules can be dissolved in theamorphous phase of HDPE and PP. On the otherhand, the binary blends of PP/EPM showed a signi-ficant level of phase segregation compared toHDPE/EPM blend, indicating that the affinity ofEPM molecules is likely to be higher with HDPEmolecules than with PP.

Danesi and Porter+? studied the blends preparedfrom PP and EPDM with a view to establish rela-

tionship between morphology and physical propert-ies and to examine the principles which govern thedevelopment of morphologies. The influence ofblending conditions on dispersion states was alsoexplored. They reported that in the melt-blendedsamples of PP with EPDM and EPM, the domainsize of the dispersed phase varied with the blendcomposition, the difference in the melt viscosities ofthe component polymers and the annealing effectswhich take place during the residence period of themelt mixture inside the rheometer reservoir. How-ever, these changes in phase morphology did not re-sult in any significant change in the crystallinity ofPP, as evidenced by a marginal change in the valueof its heat of fusion, compared to virgin PP. Thestudies on crystallization behaviour of these blendsalso indicate that addition of EPDM to PP leads to asignificant change in the crystallization behaviour,nucleation density, spherulitic growth rate and size,overall morphology and the thermal properties 130.In addition, they also observed a significant depres-sion in equilibrium melting temperature, with re-spect to pure PP, for 20% EPDM addition and relat-ed it to the phenomenon of partial miscibility andmolecular fractionation process. Jang111 studied thesimilar system for crystallization behaviour and re-sultant morphology. He reported that addition ofEPDM phase results in an irregular spherulitic text-ure, smaller spherulites, loss in sharpness of spheru-litic boundaries and decrease in degree of super-cooling and melting temperature. However, no ap-preciable change in heats of fusion and crystalliza-tion other than a trivial volume effect was observed.The average spherulitic diameter of the sample con-taining 15% EPDM was about half of that of un-modified PP, indicating large increase in nucleationdensity for blends. Pukanszky et al.132 investigatedthe PP/EPDM blends in the entire compositionalrange and reported almost similar results as ob-tained by Martuscelli et al.130 and Jang 131.In addi-tion, they also observed formation of hexagonalmodification of PP crystal structure in 5-50 Vol %EPDM blend composition range under similar crys-tallization conditions as employed for PP. However,around 80 Vol % EPDM the crystallization processof PP changed significantly, reducing crystallizationrate, crystallinity, crystal size and crystal perfectionofPP.

The main advantages of film products made fromPP/elastomer blends having 5-15 wt % elastomerare excellent flexibility, high resistance to abrasionand fibrillation92.99.100.Unfortunately, it is difficultto mix these two polymers in the extruder. More-over, the elastomers have a tendency to stick to ex-truder screw in the compression zone, affecting the

DEOPURA et al.: FffiRES AND FILMS FROM POLYOLEFIN'BLENDS 41

dispersion level and the conveyance of the melt aftersome processing time. Due to these reasons, PP/elastomer blends find limited place in oriented filmproduct applications.

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