Processing degradation of polypropylene-ethylene copolymer-kaolincomposites by a twin-screw extruder Lip Teng Sawa, Du Ngoc Uy Lana,*, Nor Azura Abdul Rahima, Ab Wahab Mohd Kahara, a Sch
Trang 1Processing degradation of polypropylene-ethylene copolymer-kaolin
composites by a twin-screw extruder
Lip Teng Sawa, Du Ngoc Uy Lana,*, Nor Azura Abdul Rahima, Ab Wahab Mohd Kahara,
a School of Material Engineering, Kompleks Pusat Pengajian UniMAP, Taman Muhibbah, University Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia
b Faculty of Materials Technology, Ho Chi Minh City University of Technology, Vietnam
a r t i c l e i n f o
Article history:
Received 21 June 2014
Received in revised form
25 October 2014
Accepted 28 October 2014
Available online 1 November 2014
Keywords:
Polypropylene copolymer-kaolin
composites
Thermo-mechanical degradation
Processing degradation
Twin-screw extruder
a b s t r a c t Degradation resulting from the extrusion processing of polypropylene-ethylene kaolin composites (PPE/ kaolin) was investigated Degradation of the polymer matrix was evidenced by the formation of hy-droxyl, carbonyl and alkene groups, as detected by Fourier transform infrared spectroscopy measure-ments These measurements also confirmed that the filler loading accelerated the degradation process and resulted in scission of high-molecular weight chains These degradations resulted in significant reductions in the thermal stabilities of the composites, whereas the rheological behaviours and me-chanical properties of the composites were strongly influenced by the filler contents rather than by degradation
© 2014 Elsevier Ltd All rights reserved
1 Introduction
Processing degradation is an unavoidable thermo-mechanical
effect on polymers during processing, especially for
thermoplas-tics, which require heat and pressure to be melted for
compound-ing, shaping and shape stabilisation Heat is recognised as an agent
that initiate and accelerates degradation; pressure contributes to
the generation of heat through the friction that arises from the
shear activity between the polymer and equipment wall Because of
the chemical and physical changes caused by degradation, the
number of processing times must be limited to preserve the desired
properties of the polymer[1]
The effect of thermo-mechanical degradation of thermoplastic
due to melt processing has been studied by researcher over a
decade Most of these studies focus in manipulating of equipment
design, process parameter, number of recycling,filler content and
composition of the matrix These results agreed that the changes of
weight average molecular weight of a polymer is the fundamental
reason induce the changes of other physical and mechanical
prop-erties Gonzalez reported that low molecular weight compounds
formed by thermo-mechanical degradation induce a high meltflow
properties (low viscosity) [2] Nevertheless, the mechanical
properties of degraded product are difficult to be predicted Tochacek reported the tensile and flexural properties of degraded polypropylene co-polymer did not have substantially influences, but slightly decreased[1] Majority thermoplastics undergo reduc-tion in molecular weight after processing; however, there were some thermoplastics (e.g polyethylene) appear increment in mo-lecular weight due to reaction with bulky reactant or cross-linking Polypropylene-ethylene copolymer, which is used for commer-cial thermoplastics, is a block polymer with a combination of pro-pylene and ethylene repeating units Both types of repeating units undergo degradation through initial radical reactions; however, they produce different end products The polypropylene block contains tertiary carbon This carbon is easily transformed into the secondary radical state and to the less stable primary radical which could attract oxygen during processing These radicals also have a tendency to result in alkene end groups under oxygen-limited processing conditions Both degradations result in a weight reduction of polypropylene In contrast, the polyethylene block forms a primary radical, which requires higher activation energy to initiate compared to polypropylene The ethylene radicals are more susceptible to intermolecular radical transfer than to oxidation; subsequently, they undergob-chain scission and produce alkene end group products[3,4] So-although the reaction of this copol-ymer is complex, researcher believes that the polypropylene block
is involved in most reactions due to the low activation energy of the
* Corresponding author.
E-mail address: uylan@unimap.edu.my (D.N Uy Lan).
Contents lists available atScienceDirect
Polymer Degradation and Stability
j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / p o l y d e g s t a b
http://dx.doi.org/10.1016/j.polymdegradstab.2014.10.024
0141-3910/© 2014 Elsevier Ltd All rights reserved.
Polymer Degradation and Stability 111 (2015) 32e37
Trang 2ethylene copolymer during the melt-compounding process The
degradation mechanism can be estimated from the chemical
com-positions of the end products Moreover, the role of the filler in
processing degradation by comparing the end-product
concentra-tions determined using Beer's law The residual effect of processing
degradation on the composite performance is evaluated from the
perspectives of decomposition, rheology, and mechanical properties
2 Experimental
2.1 Materials
TitanPRO SM340 polypropylene copolymer (co-polyethylene)
with a meltflow rate of 4 g/10 min at 230C (ASTM D1238) was
supplied by Titan Polymer (M) Sdn Bhd Neat kaolin, with a linear
formula of ~Al2Si2O5(OH)4, was provided by SigmaeAldrich
(K7375) Polypropylene-grafted maleic anhydride (PPgMA) was
supplied by Uniroyal Chemical
2.2 Sample preparation
The samples were prepared using a twin-screw extruder, as
shown inTable 1 The twin-screw extruder used in this work was a
Benchtop 16e40 from the Labtech Engineering Company, which
has a 16 mm diameter and a 40 L/D screw ratio with an inter-mesh
co-rotational design The processing temperature was set at 190C,
and the screw speeds were set at 50 rpm The extrudates were
cooled in a water bath at room temperature and then pelletised
The kaolinfiller was dried at 80C for 12 h before compounding.
2.3 Attenuated total reflectance-fourier transform infrared
(ATR-FTIR) spectroscopy
A Perkin Elmer Spectrum RX1 PC Ready LX185256 equipped
with a PIKE Miracle™ Single Reflection Horizontal ATR accessory
was used in this study Spectra of the samples were recorded with a
2.4 Thermogravimetric analysis The thermal decomposition properties of the PPE/kaolin com-posites were measured using a Perkin Elmer Diamond TG/DTA in-strument under an air atmosphere The temperature range for testing was from room temperature to 600C with at a rate of
10 C/min The TGA thermograms for each formulation were plotted The initial decomposition temperature of thefirst 1 wt% drop (TD1%), the decomposition temperature at 50wt% (T50%), the end decomposition temperature (TEND), the onset decomposition temperature (TD), and the decomposition rate (Dslope) were used to evaluate the thermal performance of the composites The TDwas determined from the intersection point of two tangents from the region of the thermograms before and during decomposition Dslope
values were calculated from the slope of tangent at the decompo-sition region DTA thermograms were obtained by taking the first-order derivative of the thermogravimetric curve The decomposi-tion temperatures at the maximum degradadecomposi-tion rate, TDmax, were obtained from the peak values of each DTA curve for each sample 2.5 Constant plunger speed circular orifice capillary rheometer
A Dynisco LCR-7001 capillary rheometer was used in this experiment Shear rates of 50e5000 s1were introduced as vari-able parameters to determine the rheological responses of the PPE/ kaolin composites The shear stress and viscosity values were calculated using the built-in software Subsequently, both values were plotted in separate graphs against the shear rate, and the viscosity graph was plotted on a log10 scale to enable a visual comparison between formulations
2.6 Preparation and testing of tensile specimens Tensile specimens were prepared according to ASTM D638 type
IV with a thickness of 1 mm using a hot press machine The hot press temperature was set to 190C, with 4 min of pre-heat, 2 min
of full press (15 metric tons) and 6 min of cooling The tensile test was conducted at room temperature at a speed of 10 mm/min
3 Results and discussion 3.1 Degradation of polypropylene 3.1.1 FTIR analysis
Fig 1presents the FTIR transmission spectra of TSE 0, TSE 5 and TSE10 Six different functional groups were detected within the composites (magnified region): ester, ketone, tertiary alcohol,
Table 1
Formulations of PPE/kaolin composites.
Sample
notation
Polypropylene
co-polyethylene (wt%)
Kaolin loading (wt%)
PPgMA content (php)
TSE represents the as-prepared composites via twin-screw extruder.
Trang 3alkene end group, and cis-trans C]C hydrocarbon These functional
groups and their specific wavenumbers are also listed inTable 2
The C]C functional group has a clear peak (1648 cm1) in all of the
samples, and other oxide products could only be minimally
observed in composites loaded withfiller[8] This result suggests
that degradation of the composite primarily occurs through chain
scission of the matrix; oxide products were only produced in
minimal amounts This circumstance can be attributed to the
lim-itation of oxygen/oxide species during the processing, and these
radical species eventually self-stabilise through chain scission[1]
3.1.2 Relative concentration
Fig 2indicates that increasing the kaolin loading resulted in a
higher concentration of ~C]C~ alkene groups This result also
in-dicates that more polypropylene radicals were initiated during
processing due to the presence of kaolin As an inertfiller, kaolin
does not undergo any chemical reactions with the
polypropylene-ethylene copolymer matrix However, physical interference (e.g.,
melt obstruction and increased heat transfer) by kaolin particles
may cause greater degradation to occur during extrusion
3.1.3 Degradation mechanism
The degradation in this experiment was confirmed to be
oxidative degradation due to the formation of oxide products A
similar study also reported that the alkyl radical of polypropylene is
more easily initiated by oxidative degradation due to its low
acti-vation energy (80e110 kJ/mol)[3] The degradation mechanism is
commonly agreed to be as follows:
Initiation:
Propagation:
Polypropylene underwent the same initiation as(R.1)and pro-duced primary and secondary radicals at this stage The mechanism
is redrawn into reaction (1), as shown inScheme 1 These radicals were further attracted by oxygen and became a peroxide radical species(R.2) However, partial primary and secondary alkyl radicals have the potential to rearrange into tertiary radicals[4]; therefore, three different types of peroxide radical could be formed, as shown
in (2), (3) and (4)
Further formation of alkyl radicals occurred via the reaction of the peroxide radical species and the polypropylene backbones to form hydrogen peroxide, which only requires an activation energy
of 30 kJ/mol Reaction (R.4) is the decomposition of hydrogen
Fig 1 FTIR spectra of (i) TSE 0, (ii) TSE 5, and (iii) TSE10.
Table 2
Detected and corresponding wavenumbers of functional groups in the degraded
products.
Functional group Detected wavenumber, cm1
(a) Ester, ReC(¼O)eO-R 1740, 1240
(b) Ketone, ReC(¼O)-R 1420, 1360
(c) Tertiary Alcohol, ReC(CH 3 ) (CH 3 )eOH 1195, 1034, 845, 720
(d) Cis ReCH]CH-R 1648, 700
(e) Trans ReCH]CH-R 1648, 1300, 1000, 975
(f) Alkene end group ReC(CH 3 ) ¼ CH 2 1775, 1648, 1415, 890
Fig 2 Peak intensity ratios of [~C]C~]/[~CH3].
L.T Saw et al / Polymer Degradation and Stability 111 (2015) 32e37 34
Trang 4peroxide, which requires an activation energy of 200 kJ/mol Jeffery
et al reported that the propagation step at this stage is more
in-clined towards reaction(R.5)than towards(R.4)because(R.5)only
requires an activation energy of up to 125 kJ/mol Primary,
sec-ondary and tertiary oxide radicals were formed through(R.5), and
then these radicals reacted with other alkyl sources and separately
formed into their respective alcohol species(R.6) [3]
Based on the FTIR analysis results, the degradation terminated
with alkene, ketone and ester functional group products This result
suggested that further oxidation occurred after propagation(R.6)
The mechanism for oxidation on alcohol species is proposed in
Scheme 2 Primary alcohol species have the potential to be oxidised
into carboxylic groups, as shown in (5) Moreover, secondary
alcohol oxidised into ketone (6), and no reaction occurred for
ter-tiary alcohol (7)
At the termination stage, carboxylic groups undergo esteri
fica-tion with other alcohol species Reacfica-tion (8) in Scheme 3 is a
reversible reaction However, there is no evidence of carboxylic
groups in the FTIR results; therefore, it is concluded that the
reaction ends with the formation of an ester The esterification of the carboxylic group also suggested that the kaolinfiller (alumina octahedra layer) functions as an adsorbent catalyst in this reaction [9] Finally, the alkyl radical group, which does not react with ox-ygen or with the radicals formed during propagation (R.5) and (R.6), may undergo chain scission to form alkene products, as shown in (9)[8]
3.2 Thermal stability of composites Fig 3presents the thermal stability behaviours of the compos-ites The TD1%, T50%, TD, TEND, Dslope, and TDmax decomposition temperatures for all composites are listed in Table 3 The TGA curves showed that the initial degradation temperatures were above 230 C for all samples This evidence proved that the degradation of PPE during extrusion process of 190C was origi-nated by thermo-mechanical activity, and was not a simple thermal degradation (just only heat) The thermal decomposition of com-posites containing kaolinfiller began at a lower temperature (TD1%) than that of the polymer matrix As discussed above, thermal degradation occurred during compounding, which induced the earlier initial degradation of the PPE/kaolin composites and the lower thermal resistance of TSE5 and TSE10 compared to TSE0 The by-products of processing degradation, such as alcohols, ketones, esters and alkenes, were found to degrade at approximately
230 Ce350C, whereas polypropylene degrades at higher
tem-peratures These degradations also exhibited a board range in the DTA thermogram, beginning at approximately 230 C This is a common temperature range for thermal degradation during studies of polyesters, which indicates the presence of oxygen[10] Kaolin was found to have a positive effect on thermal stability of PPE/kaolin composites, which becomes strong enough at high kaolin content to surmount the drawback of processing degrada-tion and enhance the thermal resistance of TSE15 and TSE20 This effect was similar to the report of M Guessoum and to other con-ventional inorganicfillers, such as talc[11,12] Thesefillers act as a heat barrier within the composite and cause the composite to require more energy to decompose[13] Because of its high thermal conductivity and heat capacity, kaolin will gain more heat compared to the polymer matrix Note that the temperature increased faster in the PPE/kaolin composites compared to the TSE0, which resulted in enhanced thermal stability and delay ma-trix decomposition rate as indicated by D values (as shown in
Scheme 1 Initiation and propagation of polypropylene radical.
Scheme 2 Oxidation of alcohol species.
Scheme 3 Termination of radical and alcohol species.
Trang 5Table 3) This advantage turned to be significant at high kaolin
contents and induced higher TDvalues in TSE15 and TSE20, which
were 106% greater than that of TSE0 Nevertheless, the slight
decrease in the thermal stability of TSE20 could be due to the
greater amount of processing degradation in TSE20 compared to
that in TSE15, as discussed in Section3.1
3.3 Rheological behaviours of PPE/kaolin composites
Fig 4indicates that the rheological behaviours of the PPE/kaolin
composites are affected by the melt-compounding process and the
presence of the kaolinfiller Kaolin particles (discontinuous phase)
are theoretically resistant to flow The continuous phase of the
polymer melt is unable to carry these kaolin particles in the form of
a suspension; therefore, a higher shear stress is required by the
fluid to achieve the desired shear rate[14] Basically, processing
degradation could induce a lower molecular weight and increase
the melt mobility of the polymer chain The low molecular
com-pounds (short polymer chain) due to chain scission are easily to be
aligned by mechanical stress and results in lower shear stress
values[2] This effect can be observed on TSE0 which exhibits a slightly lower shear stress than virgin PPE at the high shear rate range However, this effect may be unobserved and may be sur-passed by the effect of kaolin on the rheological characteristics of the composites With respect to the similar results of TES15 and TES20, it could be stated that the effect of processing degradation
on the rheological behaviour was more significant in TES20 This result demonstrated that considerable processing degradation was caused by the high kaolin content, which was also evidenced by the TGA and FTIR results
Fig 5shows that the viscosities of the PPE/kaolin composites gradually decrease with increasing shear rate This is a common pseudo-plastic behaviour of a polymer matrix that possesses high flowability under high shear activity Notably, in the low shear rate range, the viscosity only differed at high kaolin loadings (TSE15 and TSE20) However, at the high shear rate range, distinct differences could be observed for each formulation This result implies that the flow resistance of the kaolin filler weakened under higher shear rates, at which point the high mobility of the polymer chain in the melt has the most prominent effect Furthermore, the severe pro-cessing degradation in TSE20 produced more chain scission and lower molecular weights, which decreased the viscosity of TSE20 to that of TSE15 despite the higher kaolin content
Table 3
Thermal decomposition temperatures of PPE/kaolin composites under a normal air
atmosphere.
T D1% ,C T D ,C T 50% ,C T END ,C D slope , wt%/C T Dmax ,C
TSE 0 310 373 415 458 1.547 435
TSE 5 267 335 389 434 1.183 407
TSE10 236 358 407 443 1.085 421
TSE15 295 398 443 474 1.058 452
TSE20 276 375 435 459 0.829 435
Fig 4 Shear stresses of PPE/kaolin composites.
Fig 3 Decomposition properties of PPE/kaolin composites (a) TGA and (b) DTA.
L.T Saw et al / Polymer Degradation and Stability 111 (2015) 32e37 36
Trang 63.4 Mechanical properties of PPE/kaolin composites
Kaolin acts as a non-reinforcingfiller in the PPE system;
there-fore, decreases in tensile strength and elongation at break are
ex-pected to occur, as shown inFig 6andFig 7 The presence of kaolin
induced a decrease in tensile strength, which was less than 85% of
the tensile strength of PPE The elongation at break of the
com-posites considerably decreased to 7% of that of PPE, and this value
decreased further with additionalfiller loading In contrast, the
elastic moduli of the composites increased, up to 175% of the elastic
modulus of PPE This enhancement effect is common for inorganic
fillers With respect to processing degradation, the influence of
degradation on the tensile properties of the polymer was too
insignificant to be observed[1]
These changes in tensile properties are commonly agreed to be
due to the presence offiller particles hindering the alignment of
molecular chains in the matrix during deformation Maiti and
Lopez proposed that weak interactions between kaolin and the
matrix generated stress concentration points and agglomeration,
which led to low tensile strength and poor elongation[15] The
slight increase of tensile strength in TSE10 compared to TSE5 is
attributed to the high crystallinity of polypropylene in the
com-posites due to the optimum nucleation effect of kaolin at 10 wt%
[16]
This work is supported under the Fundamental Research Grant Scheme (FRGS: 9003-00326) provided by the Higher Ministry of Education Malaysia The authors also appreciate University Malaysia Perlis (Journal Incentive: 9007-00083) for sponsoring English grammar editing
Appendix A Supplementary data Supplementary data related to this article can be found athttp:// dx.doi.org/10.1016/j.polymdegradstab.2014.10.024
References
[1] Tochacek J, Jancar J, Kalfus J, Zborilova P, Buran Z Degradation of poly-propylene impact-copolymer during processing Poym Degrad Stab 2008;93(4):770e5
[2] Gonzalez-Gonzalez VA, Neira-Velazquez G, Angulo-Sanchez JL Polypropylene chain scissions and molecular weight changes in multiple extrusion Polym Degrad Stab 1998;60(1):33e42
[3] Peterson JD, Vyazovkin S, Wight CA Kinetics of the thermal and thermo-oxidative degradation of polystyrene polyethylene and polypropylene Mac-romol Chem Physic 2001;202(6):775e84
[4] Bockhorn H, Hornung A, Hornung U, Schawaller D Kinetic study on the thermal degradation of polypropylene and polyethylene J Anal Appl Pyrol 1999;48(2):93e109
[5] Wypych G, editor Handbook of fillers 3rd ed ChemTec Publishing; 2010 [6] Zhang LZ, Wang XJ, Quan YY, Pei LX Conjugate heat conduction in filled composite materials considering interactions between the filler and base materials Int J Heat Mass Transf 2013:64735e42
[7] Blasi CD, Galgano A, Branca C Modeling the thermal degradation of poly(-methyl methacrylate)/carbon nanotube nanocomposites Polym Degrad Stab 2013:98266e75
[8] Sclavons M, Laurent M, Devaux J, Carlier V Maleic anhydride-grafted poly-propylene: ftir study of a model polymer grafted by ene-reaction Polymer 2005;46(19):8062e7
[9] Oliveira ADND, Costa LRDS, Pires LHDO, Nascimento LASD, Angelica RS, Costa CED, et al Microwave-assisted preparation of a new esterification catalyst from wasted flint kaolin FUEL 2013:103626
[10] Dai K, Song L, Jiang S, Yu B, Yang W, Yuen RK, et al Unsaturated polyester resins modified with phosphorus-containing groups: effects on thermal properties and flammability Polym Degrad Stab 2013;98:2033e40 [11] Guessoum M, Nekkaa S, Fenouillot-Rimlinger F, Haddaoui N Effects of kaolin surface treatments on the thermomechanical properties and on the degra-dation of polypropylene Int J Polym Sci 2012;1:1e9
[12] Wang K, Bahlouli N, Addiego F, Ahzi S, Remond Y, Ruch D, et al Effect of talc content on the degradation of re-extruded polypropylene/talc composites Polym Degrad Stab 2013;98:1275e86
[13] Contat-Rodrigo L Thermal characterization of the oxo-degradation of poly-propylene containing a pro-oxidant/pro-degradant additive Polym Degrad Stab 2013;98:2117e24
[14] Ariff ZM, Ariffin A, Jikan SS, Rahim NAA In: Dogan F, editor Polypropylene, Chap.3: rheological behaviour of polypropylene through extrusion and capillary rheometry Croatia: InTech; 2012 p 29e48
[15] Maiti SN, Lopez BH Tensile properties of polypropylenekaolin composites.
J Appl Polym Sci 1992;44(2):353e60 [16] Ariffin A, Ariff ZM, Jikan SS Evaluation on nonisothermal crystallization ki-netics of polypropylene/kaolin composites by employing dobreva and kis-singer methods J Therm Anal Calorim 2011;103(1):171e7
Fig 6 Tensile properties of PPE/kaolin composites.
Fig 7 Elongation at break (log 10 scale) of PPE/kaolin composites during the tensile