Indian Journal of Fibre & Textile Research Vol. 22, December 1997, pp. 236-245

Modification and long-range prediction of the creep behaviour of polypropylenea

V B Gupta

Department of Textile Technology, Indian Institute of Technology, New Delhi 110 016, India

Various approaches for the enhancement of creep resistance of polypropylene products prepared from fibres, filaments, tapes and films for use as technical textiles are reviewed. These include the right choice of the starting material; appropriate processing parameters; crosslinking; copolymerization; use of additives; and use of polymer mixtures. Following this, a model evolved for the long-range prediction of creep has been applied to the creep of oriented yam prepared from a polypropylene blend containing 5 wt.% of polystyrene.

Keywords :Creep, Creep resistance, Polypropylene

1 Introduction The growth of polypropylene as a technical

textile has been quite noteworthy and presently it commands a large share of the market, primarily because of its low cost and inert nature l

. Its low density, hydrophobicity, excellent resistance to chemicals and biological organisms, and wide range of physical properties make it particularly attractive for a fast growing geosynthetics market of which it has the largest share2

• However, the glass transition temperature of polypropylene is below room temperature (approximately-lO°C), as a result of which it has a tendency to creep at and above room temperature at rather low stresses3

•4

, a disadvantage it shares with polyethylene. This shortcoming of polypropylene acts as a negative factor in most applications but is of particular concern in applications like ropes, woven sacks, tarpaulins, polygrass, upholstery fabrics, filter fabrics and geotextiles for reinforcement. Taking the geotextile application as an illustration, it is interesting to noteS that compared to polyester and polyamide fabrics, polypropylene fabrics show higher creep deformation at times greater than one day after loading and with increasing time, the rate of creep shows a very large increase. Since the expected life of a geosynthetic is 50-125 years, this deficiency of

aA condensed version of this paper was presented at the Inter­national Seminar and Technomeet on Environmental Geo­technology with Geosynthetics held at Vigyan Bhavan, New Delhi. India, 31 July-3 August i 996.

polypropylene must be overcome to make it a strong ,candidate for applications involving reinforcement like road construction and earthwork6

. Some approaches for the enhancement of the creep resistance of polypropylene are reviewed in this article. Following this, a model evolved for the long-range prediction of creep has been applied to the creep of oriented yarn prepared from a polypropylene blend containing 5 wt. % of poly­styrene.

But before going into these aspects, it would be appropriate to first briefly consider the creep behaviour of a typical polymeric product and its analysis and then comment briefly on the possible molecular origins of creep and recovery from creep.

2 Creep Behaviour of a Polymeric Product and its Molecular Origin Creep is defmed as the time-dependent change in

strain following a step change in stress7• The tools

used for analysing creep data have evolved through pioneering efforts of metallurgists. It is noteworthy that while metals show creep at high temperatures as they begin to deform plastically even at stresses less than their yield stress, polymeric materials show significant creep at room temperature or even lower temperatures. The long chain structure of polymer is mainly responsible for this behaviour. As pointed out by Schwarzl8

, the long chains allow a variety of movements to the macromolecules: elastic changes of bond angles and bond distances; changes in

GUPTA: CREEP BEHAVIOUR OF POLYPROPYLENE 237

conformations of side groups and smaller parts of the chain; changes in the shape of the molecule's countour by internal rotations around the bond angles of its main chain; and finally, changes in the positions of neighbouring macromolecules relative to each other. Commenting on the resultant deformations, Schwarzl further states that rearran­gements) on a local scale, involving small parts of the molecule occur relatively fast and yield only a small contribution to the ' deformability of the material under stress. The larg§:r 'ili~ 'part of the molecule involved in a molecular, ~m\)vement, the more sluggish the movement and the larger its contribution to the deformation. These molecular movements occur over a huge scale of characteristic times covering many orders of magnitude. Thus, the increase in strain with time on the application of a stress to a polymeric sample is a manifestation of its visco-elasticity and is related to the enormous change in consistency (compliance) connected with these movements. The molecules in the amorphous regions of semi-crystalline polymers are believed to be in a state of vigorous motion above glass transition temperature (T J and rather limited motion in the glass-like state. On being subjected to a tensile load, the molecules in the amorphous phase are extended, perhaps by disentanglement of the structural elements and straightening of chain segments through rotation around single bonds. As a result, the system moves to a low entropy state wlrich would like to revert back to its original state to maximise entropy. As pointed out by McCrum et

aC, this generates a back stress which, on removal of the load, results in recovery from creep. The recovery is also aided by secondary valence bonds, entanglements, crosslinks (if any) and crystallites. The permanent deformation or secondary creep occurs mainly due to irreversible flow of polymer chains.

3 The Nature of Creep in Polymers and its Representation

The creep behaviour of polymer-based products may be considered under three sub-headings: (a) primary creep in which the rate of creep (or strain rate) decreases with increasing time (Fig. la, top) and the extension is fully recoverable with time, (b) secondary creep in which the strain rate is constant (Fig. la, middle) and the extension is irrecoverable

or plastic, leading eventually to sample failure, and (c) tertiary creep in which the strain rate increases with time (Fig. la, bottom), leading to failure. In a single creep experiment, a sample may show one or more of the above features (generally in a combined form) depending on the polymer material, stress applied, temperature and time.

The use of Sherby-Dorn plots lO (Fig. lb), in which the logarithm of strain rate is plotted against the total strain, is a useful way of establishing a steady state rate of creep and can thus assist in the avoidance of long-term instability provided the creep data is available over a long period for a range of tensile loads. In addition, the maximum load that may be sustained for any given time by the polymer without approaching creep rupture is simply estimated by multiplying the performance limit strain (as obtained from the Sherby-Dorn plot) by the isochronous stiffness at the given time. The isochronous stiffness is estimated from creep isochronals, which are plots of stress vs. creep strain at selected times based on isothermal creep data obtained at various stresses. The isochronals are also very useful in distinguishing between linear and non-linear viscoelastic behaviour and assist in identifying the limits of linearity and the nature of non-linearity of the viscoelastic responses. In

z ... cr t­V)

Q. ...

Prim ory creep

~ S econdor y creep

~ TNliory cr~ep

TIME flog scolt)

0 b

0' 0l-----~_----j

z « 0: t-V) r-- ------4 Q. .., ... cr w

CREEP STRAIN, %

Fig. I-{a) Creep curves to illustrate primary creep (top), secondary creep (middle) and tertiary creep (bottom), and (b) The corresponding Sherby-Dom plots

238 INDIAN 1. FIBRE TEXT. RES., DECEMBER 1997

addition to Sherby-Dorn plots and creep isochronals, curve fitting approaches have also been widely used, e.g. the empirical equation of Findley and O'Connor11 has been applied l2 to the creep of polypropylene and glass fibre-reinforced poly­propylene, and the multiple integral representation suggested by Grossl 3 and Kolskyl4 has been applied to represent the creep behaviour of oriented polypropylene fibres by Ward and Onatl5. The single integral representation of Bernstein et at. 16 has also been applied to the creep of polymers.

4 Enhancement of Creep Resistance As far as the enhancement of creep resistance is

concerned, researchers have worked more extensively on polyethylene than polypropylene. Ward 17 has pointed out that the creep and recovery behaviours of both ultra-high modulus polyethylene and polypropylene are very similar. The three principal ways in which creep resistance of ultra­high modulus polyethylene fibres has been enhanced l8 are by (i) increasing molecular weight, (ii) crosslinking, and (iii) copolymerization. It has been pointed oue9 that permanent deformation in tension (creep) can be regarded in polypropylene (and indeed in oriented polymers in general) as an extension of the drawing process; it diminishes with increasing draw ratio and should therefore approach a minimum for a given polymer where all the molecules are fully extended. Considering all these factors , the various approaches to the enhancement of creep resistance of polypropylene will be considered under the following six headings: the right choice 'of the starting material; appropriate processing parameters; crosslinking; copolymeri­zation; use of additives; and use of polymer mixtures.

4.1 The Right Choice of Starting Material The right choice of the starting polymer,

particularly its molecular weight and molecular weight distribution, represents the first important step towards enhancement of creep resistance as these characteristics are important not only from the processing point of view but also in determining the properties of the product. Highly stereo-regular polypropylene (high isotactic content) with high molecular weight and narrow molecular weight distribution is considered to be the most appropriate

polymer for applications which demand high mechanical performance. Polypropylene is produced in a range of molecular weights (which are characterised by melt flow index or MFI) and molecular weight distributions (which are characterised by polydispersity). Developments in catalysts and polymerization methods have made it possible to offer products with a comparable MFI but with a narrow molecular weight distribution range, the so-called controlled rheology (CR) types. The older resins had rather broad molecular weight distributions with polydispersity of 10-40 or more while the newer CR resins with polydispersity of 3-4 are especially suited for high-speed spinning. The MFIs for textile applications are in the 15-25 giiO min range, while for the technical sector, polymers with higher molecular weight in the MFI range of 3-5 gllO min are used. However, for melt­blown structures, while the polypropylene in the MFI range of 12-35 gllO min was considered the right material, the current practice is to use polymers of low molecular weight with MFI in the range of 450-1500 gllO min; the resins with MFI of 1500 gllO min have been especially developed for this purpose.

Finally, it is worth noting20 that polypropylene with MFI of 5.0 gllO min and polydispersity 9.0 has

a Mw of 302,000, while that with MFI of 25.0 gil 0

min and polydispersity of 4.6 has Mw of 179,000.

4.2 Appropriate Processing Parameters

In a number of applications of technical textiles, high stiffness and high tenacity of the fibre along with high creep resistance are a prerequisite. This aspect has been discussed in relation to geosynthetics by Jones21 . These properties require a high orientation of the molecules in the fibre . The process parameters of interest from the viewpoint of enhancement of mechanical properties of polypropylene are those related to melt spinning, drawing, and post-drawing treatment.

During melt spinning, the spinlihe stress plays an important role in that higher the stress at the freezeline, the higher the orientation in the fibre. The increase in the rate of stress build-up along the spin line is caused by an increase in wind-up speed, decrease in extrusion temperature or extrusion rate and by an increase in resin molecular weight, which leads to higher melt viscosity. The stress level in the

GUPTA: CREEP BEHAVIOUR OF POLYPROPYLENE 239

melt ranges22 from 0.3 x 106 dynes/cm2 to 170x 1 06

dynes/cm2. Polypropylene products with high molecular

orientation show less creep. Polypropylene yarns which compare very well with commercially available polyester and polyamide high tech yarns can be produced by a proper choice of polymer and processing conditionsD

. It is well known that solid state drawing is much more effective in inducing orientation than melt drawing during the spinning process. It is reported24.25 that polypropylene fibres spun with low spin orientation are capable of attaining a higher degree of total orientation on solid state drawing than those spun with higher spin orientation. Similarly, rapidly quenched poly­propylene fibres allow much higher total orientation on cold-drawing than the slowly-cooled fibres. Sheehan and Cole26 prepared monofilaments from polypropylene by extruding them into cold water. This rapid quenching gave them a smectic structure instead of the usual monoclinic crystal form. When these fibres were drawn slowly in the oven at 130-135°C, very strong filaments with tenacity of 13 .1 g/den , elongation of 18% and modulus of 11 0 g/d~n were obtained. It has recently been

shown27•28 that as-spun fibres produced from a low

molecular weight controlled-rheology grade polymer (MF!, 35 gil 0 min) extruded at 280°C at a winding speed of 200 mlmin give paracrystalline fibres of low orientation which, on being subjected to a two-stage drawing process at 60 and 140°C respectively, are transformed into high oriented monoclinic fibres with tenacity of 85 cN/tex (9.6 g/den). Alternatively, such a yarn can be produced on spin-draw machine with the spinning speed being retained at 200 mlmin and the drawing rollers running at 800 mlmin. The industrial yarn will be expected to have high creep resistance; however, its creep behaviour has not been reported.

The drawn fibres ¥e generally subjected to heat­setting or annealing treatment so that the internal stresses are relieved. Such a treatment is generally given to the fibre in the constrained state to prevent large-scale disorientation of the molecules in the amorphous regions of the sample. Annealing can lead to an improvement in mechanical properties and certainly stabilizes the structure and dimensions of the fibre for use at elevated temperatures.

Besides the method described above, viz. rapid

quenching of melt-extruded fibre and subsequent drawing to high draw ratios, a number of other methods have been tried to produce high-modulus, high-strength polypropylene fibres which will be expected to have high resistance to creep. These techniques include (i) hydrostatic extrusion, and (ii) spinning the fibre in the form of a gel and subsequent hot-drawing29. By the latter technique, ultra-high modulus polypropylene films with Young ' s modulus of36 GPa (454 g/den) and tensile strength of 1.03 GPa (13 g/den) were prepared from polymer of moler:ular weight of 3.4 million from which a gel was made and cast into a film and then drawn at an elevated temperature to a very high draw rati030.

4.3 Crosslin king

A continuous increase in density (and crystallinity) was observed in polyethylene with

radiation dose up to 75 MRad of rirradiation31 while in polypropylene the density decreased at first up to a dose of 25 MRad before rising again. Many studies, which have been reviewed32

, have reported a decrease in crystallinity and melting point following irradiation and a number of such references are cited in the review. Irradiation results in complex morphological changes as, in addition to introducing cross links, it severs tie molecules in the amorphous regions, thus reducing the molecular weight.

It has been shown33 that it is possible to obtain dramatic enhancement in creep resistance of high' density polyethylene fibres either by electron irradiation in vacuum to a dose of 50 MRad or by irradiation in acetylene to a smaller dose of 20 MRad, both of which give a substantial degree of crosslinking. As a result, the strain rate effects are much reduced and a much more linear stress-strain curve is seen, i.e. the creep behaviour is quenched so that the material passes through a ductile-brittle transition.

Some results on the enhancement of the creep resistance of irradiated polypropylene tapes have been reported34. High energy irradiation induces both chain scission and crosslinking in isotactic polypropylene and, according to · the authors, invariably results in deterioration of mechanical properties, especially in drawn polypropylane. They found that crosslinking can be enhanced by carrying out the gal}11l1a irradiation in the presence of

240 INDIAN 1. FIBRE TEXT RES. , DECEMBER 1997

acetylene and reported enhancement of creep resistance at high temperatures by applying this technique to drawn tapes (draw ratio 7:1, drawn over hot rolls at 90°C) of polypropylene. Drawing results in a high degree of orientation of crystalline regions and lamellar unfolding occurs to a point where the polymer network has reached its natural extension. Further extension, say during creep, will be manifested initially In the interlamellar amorphous regions and later, if yielding occurs, by lamellar unfolding which will meet increasing resistance from the restraining effect of the network in the form of strain hardening. Irradiation affects the interlamellar amorphous regions, lamellar surface regions and the network.

The observed delayed yielding in the creep experiments at 30°C and the completely suppressed yield at 100°C in the irradiated samples strongly indicate preferential crosslinking in the lamellar surface regions.

4.4 Copolymerization

As stated earlier, copolymerization can enhance the creep resistance of high density polyethylene fibres . The creep behaviour of ethylene hexene-I copolymer polyethylene having approximately 1.5 butyl side branches per 1000 carbon atoms has been

studied35 and like gamma-irradiated polyethylene of

Mw 101 , 450, the copolymer of Mw 155,000 also

behaves like high molecular weight polyethylene of

Mw 312,000 as far as the creep is concerned.

Apparently, in the cross-linked and copolymer polyethylene samples of relatively low molecular weights, a comparable molecular network to the high molecular weight sample as a result of physical entanglements is created due to chemical cross links in the ca;e of gamma-irradiated sample and due to the branches in the copolymer sample .

Unmodified homopolymer grades of poly­propylene in tape form are known to fibrillate easily at high draw ratios and this effectively reduces their tenacity and thus makes them creep sensitive. The use of propylene-ethylene copolymers reduces the tendency to fibrillate36. Such copolymers are made by IPCL, Vadodara, India, as block and random copolymers.

It has been shown37 that the mechanical 'properties of polypropylene fibres are, in general, improved by grafting of methacrylic acid. The

authors suggest that during grafting, PMMA chains get attached to the polypropylene chains in the . amorphous regions and the PMMA has a reinforcing effect as its bulk reduces the mobility of the amorphous regions. It may, however, be stated that most of the available polypropylene copolymers have been designed to enhance the toughness of polypropylene and thus have a softening rather than a rigidifYing effect on the polymer.

4.5 Use of Additives

A wide variety of additives is used with polypropylene to give improved 3tability to heat, light and washing, flame resistance, printability and dyeability. However, in the context of the subject matter of this article, only those additives which lead to an enhancement of creep resistance will be considered.

A three-component modifY'ing additive has been reported to increase the tensile strength of isotactic polypropylene tapes significantly38 and will be briefly described. Polypropylene of MFI 3.6 gllO min and with 97% isotacticity was modified with the following three-component modifYing additive: CaC03, nucleus generator; stearic acid, plasticiser; and Ti02, polymer melt viscosity regulator. The total additive content was 2 wt %. The tapes produced from this compound were given a 7-fold stretch and a significant enhancement of tensile strength was observed. It was because large defects were not present in the modified polypropylene tapes and the authors suggest that the nuclei generated inhibit the development of microcrazes.

It has also been reported that mineral fillers like calcium carbonate reduce the tendency to fibrillate so that the tapes can be drawn to higher draw ratios and then slie6

. These tapes have enhanced creep resistance.

The surface modification of the filler particles with coupling agents, which contain pendent groups capable of reacting both with the resin and the filler surface, decreases the melt viscosity of CaC03 filled polypropylene and also increases the tensile strength of the melt-spun fibres . A significant improvement in spinnability is also observed39. These studies were made on heavily filled polypropylene.

4.6 Use of Polymer Mixtures .

More recently the advantages of using polymer mixtures have become so apparent that almost one

GUPTA: CREEP BEHAVIOUR OF POLYPROPYLENE 241

third of polymer usage is in terms of blends of polymers. They assist in various ways as the following examples illustrate. .Polyethylene and polypropylene are used extensively in sheath-core configuration to take advantage of the lower melting point of the former in thermally-bonded spunbonds; the polypropylene core fibre retaining its fibrous character and excellent physical properties. The use of blends of polypropylene with polyethylene also reduces the tendency of films or tapes to fibrillate, thus allowing high tenacity tapes to be produced. Another example relates to high molecular weight polypropylene of MFI 1.04, which normally requires high spinning temperature. When a low molecular weight polypropylene of MFI 20 is added to it, say to the extent of 25% or higher, the blend could be processed at much lower temperatures to give superior mechanical properties.

It has been shown that small quantities of commercial polystyrene in polypropylene can substantially reduce the shear viscosity of the melt of the mixture40

•41 as the local interactive forces

acting across the interface boundary of the incompatible blend system can result in ease of slippage of the matrix phase along the interface. A small amount of the polystyrene has also been found to reduce the crystallinity of the fibre and thereby

increase its shrinkage4Z•43 and also improve its

texturizability and dyeability44. Since creep resistance of polypropylene fibre increases with increasing orientation, it was considered worthwhile studying the drawability characteristics of polypropylene-polystyrene blends with the objective of producing a highly oriented fibre from the blend45

• Fibres were produced from mixtures of fibre grade polypropylene (MFI 20) with 5 wt. % of commercial polystyrene by first spinning a multi­filament yam at a wind-up speed of 750 mlmin. It was observed that under identical conditions of spinning, the spinline tension was lower in the case of the polymer mixture compared to that for the homopolymer. As a result, while the as-spun polypropylene sample displayed a predominantly monoclinic crystal form and high orientation and crystallinity, the as-spun blended sample had a predominantly pseudohexagonal smectic crystal form and low orientation and crystallinity. As stated earlier, this is an excellent precursor for a high modulus, high strength fibre. The presence of

polystyrene in the spun sample was found to substantially enhance the plastic deformation of the spun samples, as is seen from the stress-strain curves shown in Fig. 2. It is noteworthy that a mechanical stress-induced phase transition from f3 (hexagonal) to a (monoclinic) modification in polypropylene has been shown to result in a large increase in the specific plastic work, which is attributed to the high crystalline density of a-form

of isotactic polypropylene than the initial f3 modification along with the nature of the fJ-a transformation46

. 111 addition, the possibility of slippage of the matrix across the polystyrene phase would also contribute to improved drawability of the blended spun samples allowing them to be drawn to a draw ratio of 5.1 at room temperature, while the as-spun homopolymer sample could only be drawn up to a draw ratio of 4.4. This is most likely because the polystyrene is present as near-spherical particles of 0.5-1 f.1., as revealed by scanning electron microscopy, apparently in the amorphous regions of as-spun fibre and acts as plasticizer. In samples of draw ratio of 4.4 and above, the polystyrene is present in the oriented form and is trapped in the amorphous regions of the fibre. The creep and recovery curves for polypropylene yam of draw ratio 4.4 and for blended yams of draw ratio 4.4 and 5.1 are shown in Fig. 3, and the corresponding Sherby-Dorn plots are shown in Fig.4. The beneficial effect of higher draw is obvious as far as the creep and recovery are concerned.

The creep resistance of polypropylene can also be enhanced by combining it with glass fibres in

2.0.---------------------,

~ 1. 5 .., ~

co

Vl

~ 1.0 a: .... Vl

...J ... ~ 0. 5 :0: o z

100 200

Polypropylene

Blended Polyp~1tnt

300 400 500 600

STRAIN , %

Fig. 2-Room temperature stress-strain curves for as-spun samples

242 INDIAN J FIBRE TEXT. RES., DECEMBER 1997

various fonns and using different approaches but this is outside the scqpe of the present article and is, therefore, not discussed further.

5 Long-Range Prediction of Creep In a number of applications involving creep of

techTIical textiles, long-range prediction of creep can give valuable design infonnation. The Sherby-Dorn plots lO and the empirical equation of Findley and

4r-------------------~

3

~ 0 / /

2 / Z / ~ /

0:: /

I- ,-VI A ,-

,/

S ....

C ..... 0/'

0 10° 10 1

102 10

3 10

5 106

TIME , s

Fig. 3- Creep and creep recovery curves obtained for a stress of 25.3 MPa at 55°C [(A) Polypropylene DR 4.4, (B) Blended polypropylene DR 4.4, and (C) Blended polypropylene DR 5.1]

10°

B A " ,....

10-2 , ,

'1 , lit \

\ \

UJ \ l- e \ < 10'4

\ a::

\ \

\

Z " ,; a::

\ I-

10-0 V'l

\ 10-8

0 4

STRAIN, %

O'Connor I I , which have already been referred to earlier, can be used for this purpose. Two other approaches, viz. the time-temperature superposition principle47 and the time-stress equivalence48

.49 have also been used for the prediction of creep at long times. A combined time-temperature-stress super­position principle was evolved for predicting the long-range creep behaviour of polypropylene50

• This extends the time range of prediction to a significant extent and is particularly useful for predicting the creep behaviour of polymer-based products which can have a life span of 100 years or more, e.g. geotextiles. It will be shown that using this model, the experimental creep data on a drawn fibre made from polypropylene containing 5 wt. % polystyrene for a period of 4 h at temperatures of 35, 45, 55 and 60°C and at stresses of5.3, 10.5, 15.7 and 20.9 MPa can be reduced to creep data covering a period of over 125 years at a temperature of 35°C and a stress of 5.3 MPa. The theoretical background and the model for long-tenn prediction of creep in textile fibres have been dealt with in another pUblication50

.

The approach evolved may be understood with reference to Fig. 5, where the creep curves A,B and C have been obtained at (stress, temperature)

combinations of (ai' T,), (a2, T,) and (a2, T2)

respectively. Curve C, obtained at temperature T2,

can be reduced to a curve for temperature T, and superposed on curve B using the time-temperature

superposition principle, with a shift factor log (aT)' The creep curve B and the shifted creep curve Care

Log (timll)

Fig. 5-Schematic illustration of the combined time­Fig. 4--Sherby-Dom plots corresponding to samples of Fig. 3 temperature-stress superposition principle

GUPTA: CREEP BEHAVIOUR OF POLYPROPYLENE 243

further reduced to a curve for stress a l to obtain an extended creep curve. The total shift of curve C to curve A is given by :

5.1 Experimental

Multifilament yarn was melt-spun on a Fourne Melt Spin Tester (SST 1207) using polypropylene with melt flow index (MFI) of 23 , in which 5 wt % of commercial atactic polystyrene had been dry mixed. It was wound at a speed of 750m/min and subsequently drawn on a two-zone drawing machine at a take-up speed of 110m/min to a total draw ratio of 4.4. The drawn fibre showed a density of 0.902g1cm3 and a birefringence of 0.036. The crystallinity, estimated from density usmg crystalline density of O.946g1cm3 and amorphous density of 0.854g/cm\ was ,53 .8%. The load­elongation tests were carried out at an extension rate of 100% min-I on an Instron 4101. The initial modulus of the yarn was found to be 40.6 g/den while its tenacity was 5.2g1den. The average extension at break was found to be 23.5%.

A Stanton Redcroft Thermo-Mechanical Analyser was suitably modified to carry out isothermal creep experiments. It allowed temperature control up to an accuracy of ±0.1 °C and had a very rapid but gentle loading procedure and an accurate device for measuring increase in length.

The samples were first conditioned by subjecting them to repeated cycles of loading and unloading51

until they gave reproducible results.

5.2 Results and Discussion

Using a combination of stress-time and time­temperature superposition, as described earlier, the creep data obtained at different stresses and temperatures were reduced to a curve for temperature of 35°C and stress of 5.3 MPa. It was possible to achieve this by a simple horizontal shift of the data on the logarithmic time axis, and the master creep curve so obtained is shown in Fig. 6. It is noteworthy that the predicted creep curve covers a period of over 120 years, which is the expected lifetime of some geotextiles used in reinforcement applications and could thus be of interest to designers of such products. It is interesting to note that after 1 year, the rate of creep becomes quite large. A comparison of the superposed creep data

3r

~ 0#

c~ 2 r · '"

0

a

~./ 0

~ ... VI

0

"" 1r .. ~

/ . .• -, ~ i ... "++ -

8 0 ... , ........

I I I

102 104 10

6 10

8 1dO

Tim~, s

Fig. 6--The master creep curve for fibre produced from a blend of polypropylene with 5 wt% of atactic polystyrene at a temperature of 35°C and strels of 5.3 Mpa [(e) 5.3 MPa, 35°C;(+) 10.5 MPa, 45°C; (*) \5 .7 MPa, 55°C; and (a) 20.9 MPa, 60°C)

z ... a: l­V!

4~---------------------------------.

3

~O~O'-------'O~2------~'O~'------~'O~6-------J,~

TIME, s

Fig. 7-Comparison of creep data for the three samples obtained by superposing the experimental data by reducing it to a stress of 25 .3 MPa and temperature of 35°C

for the polypropylene sample of draw ratio 4.4 and for the blended samples of draw ratio 4.4 and 5.1 (Fig. 7) shows the superiority of the blended sample.

6 Conclusions The creep resistance of polypropylene can be

enhanced by using a number of approaches outlined in this contribution. It is observed that a product made from high molecular weight polypropylene with narrow distribution and with high molecular orientation has high modulus and high tenacity;

244 INDIAN J. FIBRE TEXT. RES., DECEMBER 1997

consequently, its defonnation under load is small and the time-dependence of creep is much reduced in comparison to a sample with lower orientation. Such a sample is dimensionally more stable and therefore of high durability. The other methods like crosslinking, copolymerisation, use of additives and use of polymer mixtures are still in the development stage and considerable development work in these areas is needed to exploit their potential to make polypropylene more creep resistant. The creep data over a narrow range of time, temperature and stress can be transfonned to creep data for a fixed temperature and stress but for a wide time range through the use of time-temperature-stress super­position principle.

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