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Polymer Degradation and Stability 111 (2015) 32e37 Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab Processing degradation of polypropylene-ethylene copolymer-kaolin composites by a twin-screw extruder Lip Teng Saw a, Du Ngoc Uy Lan a, *, Nor Azura Abdul Rahim a, Ab Wahab Mohd Kahar a, Cao Xuan Viet b a b School of Material Engineering, Kompleks Pusat Pengajian UniMAP, Taman Muhibbah, University Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia Faculty of Materials Technology, Ho Chi Minh City University of Technology, Vietnam a r t i c l e i n f o a b s t r a c t Article history: Received 21 June 2014 Received in revised form 25 October 2014 Accepted 28 October 2014 Available online November 2014 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 hydroxyl, carbonyl and alkene groups, as detected by Fourier transform infrared spectroscopy measurements 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 mechanical properties of the composites were strongly influenced by the filler contents rather than by degradation © 2014 Elsevier Ltd All rights reserved Keywords: Polypropylene copolymer-kaolin composites Thermo-mechanical degradation Processing degradation Twin-screw extruder Introduction Processing degradation is an unavoidable thermo-mechanical effect on polymers during processing, especially for thermoplastics, which require heat and pressure to be melted for compounding, 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 properties Gonzalez reported that low molecular weight compounds formed by thermo-mechanical degradation induce a high melt flow properties (low viscosity) [2] Nevertheless, the mechanical * Corresponding author E-mail address: uylan@unimap.edu.my (D.N Uy Lan) http://dx.doi.org/10.1016/j.polymdegradstab.2014.10.024 0141-3910/© 2014 Elsevier Ltd All rights reserved properties of degraded product are difficult to be predicted Tocha cek reported the tensile and flexural properties of degraded polypropylene co-polymer did not have substantially influences, but slightly decreased [1] Majority thermoplastics undergo reduction in molecular weight after processing; however, there were some thermoplastics (e.g polyethylene) appear increment in molecular weight due to reaction with bulky reactant or cross-linking Polypropylene-ethylene copolymer, which is used for commercial thermoplastics, is a block polymer with a combination of propylene 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 undergo b-chain scission and produce alkene end group products [3,4] So-although the reaction of this copolymer is complex, researcher believes that the polypropylene block is involved in most reactions due to the low activation energy of the L.T Saw et al / Polymer Degradation and Stability 111 (2015) 32e37 tertiary carbons within the backbones, and due to the low concentration of ethylene repeating units The combination of polypropylene-ethylene copolymer and fillers has extended the use of the raw material itself Kaolin (Al2Si2O5(OH)4) is generally used as an inert filler in polypropylene composites, but it still provides a limited improvement in certain specific applications, for example, as an anti-blocking agent in film production, to improve the oil-solvent resistivity of polymers, and to control viscosity and density [5] Kaolin is an inorganic filler with a high thermal conductivity, and it is hypothesised to provide better heat transfer but promote degradation of the matrix [6,7] It has also been hypothesised that the presence of kaolin increases the viscosity of the matrix, which might delay the heat exposure time of the matrix during processing and indirectly promote degradation The objective of this study is to determine the influence of the kaolin filler loading on the degradation of the polypropyleneethylene copolymer during the melt-compounding process The degradation mechanism can be estimated from the chemical compositions of the end products Moreover, the role of the filler in processing degradation by comparing the end-product concentrations 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 Experimental 2.1 Materials TitanPRO SM340 polypropylene copolymer (co-polyethylene) with a melt flow rate of g/10 at 230 C (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 in Table 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 190 C, and the screw speeds were set at 50 rpm The extrudates were cooled in a water bath at room temperature and then pelletised The kaolin filler was dried at 80 C for 12 h before compounding 2.3 Attenuated total reflectance-fourier transform infrared (ATRFTIR) 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 Table Formulations of PPE/kaolin composites Sample notation Polypropylene co-polyethylene (wt%) Kaolin loading (wt%) PPgMA content (php) TSE TSE TSE10 TSE15 TSE20 100 95 90 85 80 10 15 20 2 2 TSE represents the as-prepared composites via twin-screw extruder 33 cmÀ1 resolution, a cmÀ1 interval, and 40 scans for each spectrum over the wavenumber range from 4000 to 650 cmÀ1 All spectra were smoothed and baseline corrected using Perkin Elmer's Spectrum software Peak intensities were measured using the software for subsequent calculations 2.3.1 Calculation of the relative concentration using Beer's law Beer's law states that the quantitative content of a chemical functional group can be calculated by referring the peak intensity to another peak with a known concentration The relative concentration is used to determine the quantitative content of a polymer functional group by comparing two peaks with unknown concentrations Relative concentration is calculated by comparing the peak intensity ratio of inew/i2920, where inew refers to the intensity or peak height of newly formed peaks and i2920 refers to the peak height of the peak at 2920 cm-1, which corresponds to the molecular motion of eCH3 2.4 Thermogravimetric analysis The thermal decomposition properties of the PPE/kaolin composites were measured using a Perkin Elmer Diamond TG/DTA instrument under an air atmosphere The temperature range for testing was from room temperature to 600 C with at a rate of 10 C/min The TGA thermograms for each formulation were plotted The initial decomposition temperature of the first 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 TD was 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 decomposition region DTA thermograms were obtained by taking the firstorder derivative of the thermogravimetric curve The decomposition temperatures at the maximum degradation 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 sÀ1 were introduced as variable 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 mm using a hot press machine The hot press temperature was set to 190 C, with of pre-heat, of full press (15 metric tons) and of cooling The tensile test was conducted at room temperature at a speed of 10 mm/min Results and discussion 3.1 Degradation of polypropylene 3.1.1 FTIR analysis Fig presents the FTIR transmission spectra of TSE 0, TSE and TSE10 Six different functional groups were detected within the composites (magnified region): ester, ketone, tertiary alcohol, 34 L.T Saw et al / Polymer Degradation and Stability 111 (2015) 32e37 Fig FTIR spectra of (i) TSE 0, (ii) TSE 5, and (iii) TSE10 alkene end group, and cis-trans C]C hydrocarbon These functional groups and their specific wavenumbers are also listed in Table The C]C functional group has a clear peak (1648 cmÀ1) in all of the samples, and other oxide products could only be minimally observed in composites loaded with filler [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 limitation of oxygen/oxide species during the processing, and these radical species eventually self-stabilise through chain scission [1] R$ ỵ O2 /ROO$ (R.2) ROO$ ỵ RH/ROOH ỵ R$ (R.3) ROOH/RO$ ỵ $OH (R.4) ROOH ỵ RH/RO$ ỵ R$ ỵ H2 O (R.5) RO$ ỵ RH/ROH þ R$ (R.6) 3.1.2 Relative concentration Fig indicates that increasing the kaolin loading resulted in a higher concentration of ~C]C~ alkene groups This result also indicates that more polypropylene radicals were initiated during processing due to the presence of kaolin As an inert filler, kaolin does not undergo any chemical reactions with the polypropyleneethylene copolymer matrix However, physical interference (e.g., melt obstruction and increased heat transfer) by kaolin particles may cause greater degradation to occur during extrusion Polypropylene underwent the same initiation as (R.1) and produced primary and secondary radicals at this stage The mechanism is redrawn into reaction (1), as shown in Scheme 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 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 activation energy (80e110 kJ/mol) [3] The degradation mechanism is commonly agreed to be as follows: Initiation: RH/R$ ỵ H$ (R.1) Propagation: Table Detected and corresponding wavenumbers of functional groups in the degraded products (a) (b) (c) (d) (e) (f) Functional group Detected wavenumber, cmÀ1 Ester, ReC(¼O)eO-R Ketone, ReC(¼O)-R Tertiary Alcohol, ReC(CH3) (CH3)eOH Cis ReCH]CH-R Trans ReCH]CH-R Alkene end group ReC(CH3) ¼ CH2 1740, 1420, 1195, 1648, 1648, 1775, 1240 1360 1034, 845, 720 700 1300, 1000, 975 1648, 1415, 890 Fig Peak intensity ratios of [~C]C~]/[~CH3] L.T Saw et al / Polymer Degradation and Stability 111 (2015) 32e37 35 Scheme Termination of radical and alcohol species Scheme Initiation and propagation of polypropylene radical peroxide, which requires an activation energy of 200 kJ/mol Jeffery et al reported that the propagation step at this stage is more inclined towards reaction (R.5) than towards (R.4) because (R.5) only requires an activation energy of up to 125 kJ/mol Primary, secondary 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 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 tertiary alcohol (7) At the termination stage, carboxylic groups undergo esterification with other alcohol species Reaction (8) in Scheme is a reversible reaction However, there is no evidence of carboxylic groups in the FTIR results; therefore, it is concluded that the Scheme Oxidation of alcohol species reaction ends with the formation of an ester The esterification of the carboxylic group also suggested that the kaolin filler (alumina octahedra layer) functions as an adsorbent catalyst in this reaction [9] Finally, the alkyl radical group, which does not react with oxygen 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 presents the thermal stability behaviours of the composites The TD1%, T50%, TD, TEND, Dslope, and TDmax decomposition temperatures for all composites are listed in Table 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 190 C was originated by thermo-mechanical activity, and was not a simple thermal degradation (just only heat) The thermal decomposition of composites containing kaolin filler 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 Ce350 C, whereas polypropylene degrades at higher temperatures 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 degradation and enhance the thermal resistance of TSE15 and TSE20 This effect was similar to the report of M Guessoum and to other conventional inorganic fillers, such as talc [11,12] These fillers 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 matrix decomposition rate as indicated by Dslope values (as shown in 36 L.T Saw et al / Polymer Degradation and Stability 111 (2015) 32e37 Fig Shear stresses of PPE/kaolin composites Fig Decomposition properties of PPE/kaolin composites (a) TGA and (b) DTA Table 3) This advantage turned to be significant at high kaolin contents and induced higher TD values 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 Section 3.1 3.3 Rheological behaviours of PPE/kaolin composites 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 surpassed 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 shows 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 processing 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 Fig indicates that the rheological behaviours of the PPE/kaolin composites are affected by the melt-compounding process and the presence of the kaolin filler 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 compounds (short polymer chain) due to chain scission are easily to be aligned by mechanical stress and results in lower shear stress Table Thermal decomposition temperatures of PPE/kaolin composites under a normal air atmosphere TSE TSE TSE10 TSE15 TSE20 TD1%, C TD, C T50%, C TEND, C Dslope, wt%/ C TDmax, C 310 267 236 295 276 373 335 358 398 375 415 389 407 443 435 458 434 443 474 459 1.547 1.183 1.085 1.058 0.829 435 407 421 452 435 Fig Viscosities of PPE/kaolin composites L.T Saw et al / Polymer Degradation and Stability 111 (2015) 32e37 37 Conclusion FTIR identified the oxidative by-products of polypropylene after twin-screw processing degradation The kaolin filler resulted in a greater formation of alkene groups, which could promote oxidative degradation Carbonyl groups from esters and ketones were also detected in the composites These oxidative by-products have low molecular weights, and they easily decomposed during TGA Therefore, thermal stability was only found in composites with filler contents of greater than 15 wt% Processing degradation did not have a notable influence on the rheological characteristics (compared to the kaolin content), but it is responsible for the reduction in mechanical properties Acknowledgements Fig Tensile properties of PPE/kaolin composites 3.4 Mechanical properties of PPE/kaolin composites Kaolin acts as a non-reinforcing filler in the PPE system; therefore, decreases in tensile strength and elongation at break are expected to occur, as shown in Fig and Fig 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 composites considerably decreased to 7% of that of PPE, and this value decreased further with additional filler 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 of filler 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 composites due to the optimum nucleation effect of kaolin at 10 wt% [16] Fig Elongation at break (log10 scale) of PPE/kaolin composites during the tensile test 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 at http:// dx.doi.org/10.1016/j.polymdegradstab.2014.10.024 References P, Bur Z Degradation of poly[1] Tocha cek J, Jan c ar J, Kalfus J, Zborilova an propylene impact-copolymer during processing Poym Degrad Stab 2008;93(4):770e5 lez-Gonz zquez G, Angulo-Sa nchez JL Polypropylene [2] Gonza alez VA, Neira-Vela 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 thermooxidative degradation of polystyrene polyethylene and polypropylene Macromol 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 polypropylene: ftir study of a model polymer grafted by ene-reaction Polymer 2005;46(19):8062e7 lica RS, [9] Oliveira ADND, Costa LRDS, Pires LHDO, Nascimento LASD, Ange 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 degradation of polypropylene Int J Polym Sci 2012;1:1e9 mond Y, Ruch D, et al Effect of talc [12] Wang K, Bahlouli N, Addiego F, Ahzi S, Re 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 polypropylene 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 kinetics of polypropylene/kaolin composites by employing dobreva and kissinger methods J Therm Anal Calorim 2011;103(1):171e7 ... process of 190 C was originated by thermo-mechanical activity, and was not a simple thermal degradation (just only heat) The thermal decomposition of composites containing kaolin filler began at a. .. products A similar study also reported that the alkyl radical of polypropylene is more easily initiated by oxidative degradation due to its low activation energy (80e110 kJ/mol) [3] The degradation. .. temperatures for all composites are listed in Table The TGA curves showed that the initial degradation temperatures were above 230 C for all samples This evidence proved that the degradation 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