İSTANBUL TECHNICAL UNIVERSITY « INSTITUTE OF SCIENCE AND TECHNOLOGY
M.Sc. Thesis by Bülent ERİMAN
Department : Polymer Science and Technology Programme: Polymer Science and Technology
May 2008
EFFECT OF THE POLYETHYLENE GRAFT COPOLYMER TO STRUCTURES AND
MECHANICAL PROPERTIES OF
ACKNOWLEDGEMENT
First and foremost I would like to express my indebtedness to my advisor Prof. Dr. Nurseli Uyanık who has supported and encouraged me from the very beginning of this study and shared her deep knowledge and experience.
I would like to thank to Prof.Dr. Mine Yurtsever and her Ph.D. Student Erol Yıldırım during the study of our project.
I would like to thank to Prof. Dr. Hulusi Özkul to allow us to use tensile testing device.
I would like to thank to Gülnur Başer and my other lab. collaborator for their contribution during the study.
The authors wish to thank Süd-Chemie Inc. for supplying Na MMT, Nanofil 757 used. This research was supported from The Scientific and Technological Research Council of Turkey (Grant No. 105M049).
Finally, I would like to offer the most gratitude to my brother, my parents and my fiancee who have always supported me during my whole life.
TABLE OF CONTENTS
ACKNOWLEDMENT……… ii
TABLE OF CONTENTS……… iii
LIST OF TABLES ……….… vi
LIST OF FIGURES ……….……... vii
LIST OF SYMBOLS ……….…. ix SUMMARY ………. xi ÖZET ………..………….. xviii 1.INTRODUCTION...…… 1 2.THEORETICAL PART...… 3 2.1. Clay Minerals……….. 3
2.1.1. Slicate Mineral Structures………….………….………..…….. 4
2.1.2. Classification of Clay Minerals…...………..…..………...………… 5
2.1.2.1. Caolinit Group……….……… 6
2.1.2.2. Illit group …….……..………. 6
2.1.2.3. Clorit group…...….……….. 6
2.1.2.4. Smectite group………. 6
2.1.3. Cation Exchange Capacity…….……… 9
2.1.3.2. Inter layer Formation……..………….………... 10
2.2. Polyolefines……… 12
2.2.1. Polyethylenes……….……… 12
2.2.1.1. Low Density Polyethylene (LDPE) ..…..……… 12
2.2.1.2. Linear Low Density Polyethylene (LLDPE) ………..… 13
2.2.1.3. Other Polyolefins………. 14
2.2.2. Properties of Poliolefins……….…..……….…… 14
2.2.2.1. Mechanical Properties Of Polyolefins…………..……… 16
2.2.2.2. Dielectric………...………... 16
2.2.2.3. Density……….. 16
2.2.2.4. Melt Flow Index………...….………... 17
2.3. Compatibilizer……….………... 19
2.3.2. Classification According to Prep. and Prop. of Compatibilizers…... 21
2.3.3. Works on IA/MMI/MAH Grafting Polyolefins in Recent Years…... 22
2.4. Polymer Nanocomposites ……..……… 23
2.4.1. Polymer nanocomposite Synthesis Methods ……..………... 24
2.4.1.1. Melt Blending Synthesis…...………..……….. 24
2.4.1.2. Solvent Based Synthesis…….……….……… 25
2.4.1.3. In-situ Polymerisation……..……….………... 25
2.4.2. The structure of Nanocomposites …………..….….………. 26
2.4.3.Structural Characterization of PNCs ……….………… 29
2.4.4. Works on PNC including MAH/EVA Grafting Polyolefins ………. 32
3. EXPERIMENTAL PART……… 37
3.1. Chemicals Used………..……… 37
3.1.1. Low Density Polyethylene (LDPE) ……….………. 37
3.1.2. Linear Low Density Polyethylene (LLDPE)……….……… 37
3.1.3. Itaconic Acid ………..………... 37 3.1.4. Itaconic Monoesters ………. 37 3.1.5. Sodium Montmorillonite………..……….. 37 3.1.6. Dodecyl amine……… 38 3.1.7. Hexadecyl amine……… 38 3.1.8. Octadecyl amine………. 38
3.1.9. Dibenzoyl Peroxide (DBPO) ………. 38
3.1.10. Xylene……….. 38 3.1.11. Isopropyl Alcohol………. 38 3.1.12. Methyl Alcohol………. 38 3.1.13. Ethyl Alcohol……… 38 3.1.14. Potasiumhydroxide (KOH) ………. 39 3.1.15 Hydrochloric Acid (HCl) ……….. 39 3.1.16. Sodiumcarbonate (Na2CO3. H2O ) ……….. 39 3.1.17. Bromothymol Blue……….. 39 3.1.18. Methylene Red………. 39 3.2. Equipment Used……….……… 39
3.2.1. Magnetic Stirrer With Heater……….………... 39
3.2.3. Microwave Oven……….……….. 39
3.2.4. Extruder…………..……… 40
3.2.5 XRD Analysis………. 41
3.2.6. Mechanical Test Device……….………... 41
3.2.7. Shore-D Hardness Test Device……….. 41
3.2.8. Melt Flow Index device………..……… 41
3.3. Experimental Procedure………... 43
3.3.1. Preparation and Purification of Grafted Polyolefins……….… 43
3.3.2. Preparation of Organoclays……….……….. 44
3.3.3. Preparation of PNC……… 44
3.4. Tests and Analyses………. 45
3.4.1. Measurement of Grafting Ratio by Analytical Method……..……… 45
3.4.2. XRD Analysis……..……….. 45
3.4.3. Mechanical Test………..……… 45
3.4.4. Shore-D Hardness Test………..….. 46
3.4.5. Melt Flow Index Test………..……….. 46
4. RESULTS AND DISCUSSION………... 47
4.1. The Optimization of Reaction Conditions……..……….. 47
4.2. Synthesis Conditions For Characterization of Samples…………..… 48
4.3 XRD Analysis Results…….……… 48
4.4. Mechanical Characterization Test Results….………. 49
4.4.1. Tensile Tests Results……….………. 49
4.4.2. Hardness Tests Results………..………. 50
4.3. Melt Flow Index Test Results……….. 50
5.CONCLUSION……….. 52
REFERENCES………. 54
APPENDIX………... 58
LIST OF TABLES
Table 2.1: The most commonly observed coordination plyhedra for the common elements in silicate structures………..………..…...
5
Table 2.2: Operating Conditions of LLDPE Processes………... 14
Table 2.3: The relationship between density and the various properties of the polymers………... ... 17 Table 2.4: Changes in polymer properties with melt index……….. 18
Table 2.5: Miciblity and immicibility of compatibilizers…………... 19
Table 3.1: Test concitions and standarts according sampe type……… 42
Table 3.2: Average MFI values with used weight usen in cylinder and measure time……… …………. 42 Table 6.1: G.D. of LDPE with IA and monoesters and % grafting. ([DBPO]=0.75 g/100g LDPE, T=1400C, MW power=100 W)…………...……….. 57
Table 6.2: G.D. of LLDPE with IA and monoesters and % grafting. ([DBPO]=0.75 g/100g LLDPE, T=1400C, MW power=100 W)……... 57
Table 6.3: Sample descriptions and contents of samples which contain LDPE……… 57
Table 6.4: Sample description and contents of samples which contain LLDPE …….. 59
Table 6.5: XRD measurement results of LDPE containing samples ….………... 60
Table 6.6: XRD Patterns of LLDPE containing samples……… 62
Table 6.7: Mechanical measurements of LDPE including samples……….. 63
Table 6.8: Mechanical measurements of LLDPE including samples……… 64
Table 6.9: Shore-D measurement results of PNC samples……… 65
Table 6.10: MFR, MVR, n-MFR and n-MVR values of all samples and control units………..………. 66
LIST OF FIGURES
Figure 2.1: Polyhedra and the corresponding atomic arrangement for the two
principal polyhedra of silicate mineral structures………... 5
Figure 2.2: Shematic represantation of clay mineralstructure………... 8
Figure 2.3: Structure of 2:1 phyllosilicates………... 11 Figure 2.4: Molecules of LDPE, LLDPE and HDPE……… 13
Figure 2.5: Schematic representation of an AB block copolymer compatibilizing a blend of polymers A and B……….……… 20
Figure 2.6: Schematic representation of a compatibilization reaction ……… 21
Figure 2.7: Method for creating intercalated polymer-clay architectures via direct polymer contact and via insitu polymerization ………. 25
Figure 2.8: Shematic representation of the various PNC architectures………. 28
Figure 2.9: Wide-angle and small-angle X-ray diffraction of polymer samples…….. 30
Figure 3.1: HAAKE MiniLab Micro Compounder ………. 40
Figure 3.2: Control panel of the MiniLab extruder... 40
Figure 3.3: Shore-D hardness measure device………... 41
Figure 3.4: “Melt Flow Index” MFI device... 42
Figure 3.5: Schematic representation of MFI device……… 43
Figure 6.1: XRD pattern of Nanofil 757………... 68
Figure 6.2: XRD patterns of polymer nanocomposite samples that contains LDPE as matrix and MMI in compatibility……….………. 69
Figure 6.3: XRD patterns of polymer nanocomposite samples that contains LLDPE as matrix and MMI in compatibilizer……..………. 70
Figure 6.4: XRD patterns of some polymer nanocomposite samples that contains LLDPE as matrix and IA in compatibility……… 71
LIST OF SYMBOLS λ = Wavelength, nm
Å = Ionic Radius θ = Diffraction angle d001 = Layerdistance
ABBREVIATIONS
CEC Cation Exchange Capacity PO Polyolefine
PE Polyethylene
PNC Polymer Nanocomposites
LDPE Low Density Polyethylene
LLDPE Lineer Low Density Polyethylene IA Itacnic Acid
MMI Mono Methyl Itaconate MBI Mono Buthyl Itaconate DDA Dodecyl amine
HDA Hexadecyl amine ODA Octadecyl amine MFI Melt Flow Index MVR Melt Volume Rate MFR Melt Flow Rate
EFFECT OF THE POLYETHYLENE GRAFT COPOLYMER TO STRUCTURES AND MECHANICAL PROPERTIES OF
POLYETHYLENE/CLAY NANOCOMPOSITES
SUMMARY
Polyolefins are the most widely used polymers. Preparation of PO-based polymer nanocomposites (PNC) is more difficult than that of any polymer, which contains polar groups in its backbone [1,2]. Since density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) are non polar polymers, homogeneous dispersion of polar clay can not be realized due to lack of PE miscibility with it or with organically-modified clay (organoclay) with the enhancement of the clay dispersion, the aspect ratio of the particle increases and the reinforcement effect improve. Strong interaction between a non-polar polymer and polar organoclay might be achieved with addition of a compatibilizer [2,4-7]. During melt-blending olefinic oligomers with polar functionality or PO grafted with polar group are intercalated into clay galleries, facilitating dispersion of silicates into PO [6]. Itaconic acid (IA) and its monoesters can be grafted onto polyolefins [8].
This study includes the effects of addition of polar groups containing compatibilizers onto the final properties of the PNCs. In these PNCs, PEs were chosen as matrix and organoclays and compatibilizers as additives. Polar group containing compatibilizer was prepared by grafting of itaconic acid (IA) and its monoesters (mono methyl itaconate and mono butyl itaconate) onto low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE). The layered silicate was Na montmorillonite (MMT) and organoclays were prepared by modification of MMT with surface active agents such as dodecyl amine (DDA), hexadecyl amine (HDA), and octadecyl amine (ODA). PNC materials will be synthesized by using different amounts of PEs, organoclays, and compatibilizers in Mini Lab Extruder.
xylene then was mixed together with dibenzoyl peroxide and the polar monomers in a certain proportion. In all experiments, the weight ratio of xylene to polyolefins was always 10/1.
The grafting ratios (GR) of were measured at standart analytical method. The determined GRs values are: 11.2x10-4 mole IA/100 g LDPE, 22.3x10-4 mole MMI/100 g LDPE, and 20.0x10-4 mole MBI/100 g LDPE, 5.8x10-4 mole IA/100 g LLDPE, 12.4x10-4 mole MMI/100 g LLDPE, and 11.8x10-4 mole MBI/100 g LLDPE.
The samples were prepared by single step melt mixing. Thus, 5-wt% organoclay and different concentration of compatibilizers (5, 10, or 15 –wt%) were mixed with PEs in Mini Lab extruder at 177°C set temperature, 87 rpm screw speed with 2 min. cycling time.
PNCs were examined by using the X-ray diffraction (XRD). The tensile 1% secant modulus; strength, strain and toughness at maximum and at break were determined. The processability of samples were investigated by melt flow measurements.
Each sample description refers to a specific composition involving the components used in the preparation of the samples.
XRD results of the original clay, modified clays and the nanocomposites prepared are provided in Table 1. X-Ray diffraction analyses were used to measure the separation of original clay layers by modifying and dispersion of organoclay in polymer matrix. It was observed that the compatibilizer content of 5 wt% does not effect the dispersion of organoclay in matrix while compatibilizer content of ≥10 wt% significantly increases the dispersion of the organoclay. The complete exfoliation was obtained for
LDPE-IA-OHDA 5-C10, LDPE-IA-OHDA 5-C15 ,LDPE-MMI-OHDA 5-C 15 and LLDPE-IA-OODA 5-C 15 samples.
Table 1: X-Ray diffraction analyses results of PNC samples
Tensile 1 % secant modulus, maximum strength, strength at break, strain at break, and toughness (W) at break of nanocomposites were measured and calculated values are listed in Table 2 and Table 3.
Maximum strength (σmax), strength at break, strain at break, and toughness at break (W) of the NC increase with increasing compatibilizer content. Mechanical tests revealed that increasing chain length of surface active modifying agent increases the dispersion of
of compatibilizers significiantly effect not only the dispersion of clay layers but also mechanical properties of PNC.
Table 3. Mechanical measurements of the samples including LLDPE
Compatibilizers might also influence the melt properties of the polymer matrix as observed during the MFR measurements. Calculation of “normalized” MFR values helps to see the interaction of clay with the continuous PO matrix.
The results show that MFR data are able to provide an indication of exfoliation and dispersion of clay in the PE matrix (Table 4). Increasing of n-MFR and n-MVR values with increasing compatibilizer content showed that, processability of nanocomposites was improved by addition of grafted PE compatibilizers.
Table 4. MFR, MVR, n-MFR and n-MVR values of PNC samples
Intercalated nanocomposites were prepared using functionalized PE as compatibilizers. It was shown that clay dispersion depends on the type and concentration of grafted polar groups. Thus, addition of wt% compatibilizer was not effective. Using compatibilizer content higher than 5-wt% can also increase all the mechanical properties. Increasing of n-MFR and n-MVR values with increasing compatibilizer content in the nanocomposites resulted in the improved processability of nanocomposites by addition of compatibilizers to the PNCs.
POLİETİLEN AŞI KOPOLİMERLERİNİN POLİETİLEN/KİL
NANOKOMPOZİTLERİNİN YAPISINA VE MEKANİK ÖZELLİKLERİNE ETKİSİNİN İNCELENMESİ
ÖZET
Poliolefinler fiziksel ve mekaniksel özelliklerinin iyiliği, düşük maliyetlerde kolay işlenebilirliği ve birçok uygulamada çok yönlü malzeme olarak kullanılabilmesinden dolayı çokça tüketilebilen termoplastik polimerlerdir. Poliolefin tabanlı polimer nanokompozitlerin hazırlanışı ana zincirinde polar grup taşıyan herhangi bir polimerin hazırlanışından daha zordur.[1,2] Alçak yoğunluklu polietilen (AYPE) ve doğrusal alçak yoğunluklu polietilen (DAYPE) polar polimerler olmadıklarından dolayı polietilenin kendisiyle veya bir oganokille karışma kabiliyetinin olmamasından dolayı polar killerle homojen dağılımı gerçekleşemez. Artan kil dağılımı katkı maddesinin etkinliğini artırır.[5] polar olmayan polimer ve polar organokil arasındaki kuvvetli etkileşim bir uyumlaştırıcının katılmasıyla sağlanabilir.[2,4-7] Polar fonksiyonelliği olan olefinik oligomerler veya polar grupla aşılanmış poliolefinler eriyik karıştırması sırasında silikatların poliolefinler içersine dispersiyonunu kolaylaştırarak kil galerileri içine dağıtılımı sağlanır.[6] İtakonik asit ve monoesterleri poliolefinler üzerine aşılanabilir.[8]
Bu çalışma polar grup içeren uyumlaştırıcıların eklenmesnin polimer nanokompozitlerin nihai özellikleri üzerine etkisinin incelenmesini içermektedir. Bu polimer nanokompozitler matris olarak polietilen, katkı olarak uyumlaştırıcı ve organokil içerir. Polar gurup içeren bu uyumlaştırıcılar itakonik asit (IA) ve monoesterlerinin (monometil itakonat ve mono butil itakonat) alçak yoğunluklu polietilen (AYPE) ve doğrusal alçak yoğunluklu polietilen (DAYPE) üzerine aşılanmasıyla elde edilmişlerdir. Organokiller, tabakalı silikat yapıdaki Na montmorillonitin, yüzey aktif modifiye bileşenleri olan dodesil amin (DDA), hekzadesil amin (HDA) ve oktadesil amin (ODA) ile modifikasyonundan elde edilmiştir. Polimer nanokompozit malzemeler değişik miktarlarda polietilen, organokil ve uyumlaştırıcının Mini Lab Ekstruderde işlenmesiyle elde edilmiştir.
Bu bağlamda, öncelikle uyumlaştırıcılar 140°C’ da, 100W mikrodalga boyunda, 10 dakika reaksiyon süresinde sentezlendi. Poliolefinler ksilen içersinde çözüldü ve ardından dibenzoil peroksit (DBPO) ve belli oranlarda polar monomerler ile karıştırıldı. Tüm deneylerde, ksilen ile poliolefin ağırlık oranı 1/10 olarak kullanılmıştır. Aşılanma dereceleri (AD) standart analitik metotlarla ölçülmüştür. Ölçülen aşılanma derecesi sonuçları: 11.2x10-4 mol IA/100 g AYPE, 22.3x10-4 mol MMI/100 g AYPE, ve 20.0x10-4 mol MBI/100 g AYPE, 5.8x10-4 mol IA/100 g DAYPE, 12.4x10-4 mol MMI/100 g DAYPE,ve 11.8x10-4 mol MBI/100 g DAYPE. Örnekler tek basamaklı eriyik karıştırma yöntemiyle hazırlanmıştır. Bu nedenle, %5 organokiller ve değişik konsantrasyonlarda uyumlaştırıcılar (%5,10 ve 15) minilab ekstruderde 177 °C proses sıcaklığında, 87 rpm vida hızında ve 2 dakika çevrim süresi kullanılarak polietilenler ile karıştırılmıştır.
Polimer nanokompozitler X-Ray difraksiyonu (XRD) ile incelenmiştir. 1% sekant modülü; Maksimum gerilim (σmax), kopma gerilimi, kopmada uzama ve tokluk (W) değerleri mekanik tesler ile hesaplanmıştır. Örneklerin işlenebilirliği eriyik akış ölçümleri ile irdelenmiştir.
Her örnek tanımlaması, örneklerin hazırlanılmasında kullanılan bileşenleri içerecek şekilde yapılmıştır.
Ojinal kil, modifiye kil ve hazırlanılan nanokompozitlerin XRD sonuçları Tablo 1’de verilmiştir. Orjinal kil tabakalarının modifikasyonla ayrılışını ve organokillerin polimer matris içersinde dağılımını incelemek üzere X-Ray difraksiyon analizleri yapıldı. %10’dan fazla olan uyumlaştırıcı yüzdesinin organokil dağılımını belirgin bir şekilde arttırdığı gözlemlenirken, %5 uyumlaştırıcı yüzdesinin organokilin matris içerisindeki dağılımını etkilemediği gözlemlendi. Kilin tam dağılımı AYPE-IA-OHDA
5-C10, AYPE-IA-OHDA 5-C15 ,AYPE-MMI-OHDA 5-C 15 ve DAYPE-IA-OODA 5-C 15 örneklerinde elde edildi.
Tablo 1: Polimer nanokompozit örneklerinin X-Ray difraksiyon analiz sonuçları
Nanokompozitlerin, ölçülüp hesaplanan, 1% sekant modülü, maksimum gerilim (σmax), kopma gerilimi, kopmada uzama ve tokluk (W) değerleri Talo 2 ve Tablo 3 de verilmiştir.
Maksimum gerilim (σmax), kopma gerilimi, kopmada uzama ve tokluk (W) uyumlaştırıcı miktarının artmasıyla arttığını gözlemledik. Mekanik testler, yüzey aktif maddenin zincir uzunluğunun artmasıyla matris içersindeki montmorillonit tabakalarının dağılımının arttığını göstermektedir. Diğer yandan uyumlaştırıcı tipi ve yüzdesi sadece kil tabakalarının dağılımını etkilemekle kalmayıp mekanik özellikleri
Tablo 3. Polimer matris olarak DAYPE içeren örneklerin mekanik ölçümleri.
Uyumlaştırıcı, eriyik akış ölçümlerinde görüldüğü üzere polimer matrisin eriyik özelliklerini etkilemektedir. Normalize eriyik akış indeksinin hesaplanması kil ile polimer matris arasındaki etkileşimin irdelenmesine yardımcı olmuştur.
Sonuçlar göstermiştir ki; n-MFR ve n-MVR değerlerinin uyumlaştırıcı yüzdesiyle artması nanokompozitlerin işlenebilirliğinin aşılanmış polietilen uyumlaştırıcılarının ilave edilmesiyle artmaktadır.(Tablo 4)
Tablo 4. Örneklerin n-MFR ve n-MVR sonuçları.
Fonksiyonelize polietilen kullanılarak interclate nanokompozitler hazırlanılmıştır. Kil dağılımının aşılanmış polar grupların konsantrasyonuna ve tipine bağlı olduğu gösterilmiştir. %5 uyumlaştırıcı eklenmesinin etkili olmadığı görülmüştür. %5 den fazla uyumlaştırıcı kullanıldığında tüm mekanik özellikler artmaktadır. MFR ve n-MVR değerlerinin artan uyumlaştırıcı miktarlarına paralel olarak arttığı nanokompozitlerde, polimer nanokompozitlerin işlenebilirliğinin uyumlaştırıcı yüzdesindeki artışla arttığı saptanmıştır.
1. INTRODUCTION
Polymers have become one of the most important materials in our daily life. Increasing demand for using them forced the scientists to improve their properties. Therefore, in recent years, inorganic nanoparticle filled polymer composites have received increasing research interest, mainly due to their ability to improve properties of polymers.
In general, when composites are formed two or more physically and chemically distinct phases (usually polymer matrix and reinforcing element) are joined and the properties of the resulting product differ from and are superior to those of the individual components. The structures and properties of the composite materials are greatly influenced by the component phase morphologies and interfacial properties. Nanocomposites are based on the same principle and are formed when phase mixing occurs at a nanometer dimensional scale. As a result, nanocomposites show superior properties over their micro counterparts or conventionally filled polymers. Polymer nanocomposites are a class of reinforced polymers with low quantities of nanometric sized clay particles (generally), which give them improved properties. The reinforcing effect of nanoparticles is related to the aspect ratio (p) (ratio of the length or thickness to that of the diameter) and the particle-matrix interactions. Independent of the actual dimensions, for p > 500 the reinforcing effects are the same as those of any infinitely large particles. Because of the small size, the nanoparticles are invisible to the naked eye, so nanocomposite are transparent. Polyolefins (PO) are the most widely used polymers in preparation of polymer nanocomposites (PNC) and it is more difficult than that of any polymer, which contains polar groups in its backbone [1,2]. Since low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE) are non polar polymers, homogeneous dispersion of polar clay can not be realized due to lack of PE miscibility with it or with organically-modified clay (organoclay) with the enhancement of the clay dispersion, the aspect ratio of the particle increases and the reinforcement effect improves. Strong interaction between a non-polar polymer and polar organoclay might be achieved with addition of a compatibilizer [2,4-7]. During melt-blending olefinic oligomers with polar functionality or PO grafted with polar group are
intercalated into clay galleries, facilitating dispersion of silicates into PO. Itaconic acid (IA) and its monoesters and they can be grafted onto PO [8].
Homogeneous dispersion of nano-sized fillers in the matrix provides a large interfacial area; otherwise the loosely agglomerated nanoparticles would easily result in failure of the composites when they are subjected to force. A homogeneous product, incorporation of any additives requires a serious mixing in molten state, which is primarily provided by melt blending process by means of extrusion.
In this study, nanocomposites were produced by means of a corotating twin screw extruder in single step melt mixing method. This study was carried out to determine the effects of compatibilizer on the properties of PE-based PNC. In order to prepare the compatibilizers, LDPE and LLDPE were grafted with itaconic acid (IA), monomethyl itaconate (MMI) and monobutyl itaconate (MBI) by in a microwave assisting system. Organically modified Na-MMT was used as the nanofiller. Modifications of clays were done by using alkyl ammonium salts as the surface active modification agents (dodecyl amine (DDA), hexadecyl amine (HDA) and octadecyl amine (ODA)).
Dispersion of clay was characterized by using XRD tests, mechanical characterization of the samples was done with stress-strain measurements data, and the processabilities of PNCs were investigated by MFI measurements techniques.
2. THEORETICAL PART
In general, a composite is defined as two or more components differing in form or composition on a macroscale, with two or more distinct phases having recognisable interfaces between them. Nanocomposites (NCs) are materials that comprise a dispersion of nano meter (10-9) size particles in a single or multicomponent matrix. [1] The matrix may be metallic, ceramic or polymeric. Depending on the matrix nature NCs may be assigned into these three categories. The nano particles are classified as; 1) lamellar, 2) fibrillar, 3) tubular, 4) spherical, and 5) others. For the enhancement of mechanical and barrier properties of NCs lamellar particles are preferred. For rigidity and strength enhancement, fibrillar, for optical and electrical conductivity enhancement, spherical or other particles have been used. In polymer nanocomposites (PNC), matrix is a single or multicomponent polymer. In this work, LDPE and LLDPE with their grafted copolymers were used as multi component matrix and the organoclays as nano particle additives. To understand the PNC structure these main components will be discussed: clay minerals and polymer matrices.
2.1. Clay Minerals
The terms "clay" and "clay mineral" are used in various subjects. A common explanation of a clay substance is a material whose particles are very small. This is general engineering usage. The term "clay" now refers to any material which exhibits a plastic behavior when mixed with water, while "clay mineral" refers to materials which have a layer structure. Clay minerals typically form at low temperatures, at low pressures in the presence of much water, in nature. Under these conditions, perfection in the organization of the crystal structure is unlikely. The details of the crystal structure of these materials are of great importance in understanding the physical and chemical properties of clays. This also is true of the highly disordered or amorphous materials where there still exists short range order.
2.1.1. Silicate Mineral Structures
Silicate minerals are oxides of silicon and a small number of elements from the first three columns of the periodic table and the transition elements. As such they closely mirror the most of the elements in the crust of the Earth (Table 2.1). Since the number of different elements which play a major role in the structure of silicate minerals is small, it is not surprising that the fundamental building blocks of these minerals, and many other non-silicate minerals, are few. The basic building blocks are simple platonic polyhedra, largely tetrahedra and octahedra, which represent the placement of oxygen atoms and the smaller cations. The number of oxygen atoms arranged about a cation is termed the coordination number (CN); the smaller the cation, the smaller the CN. Figure 2.1 shows a tetrahedron and an octahedron in both aspects, i.e., as a polyhedron and as the arrangements of oxygen coordinating the central cation.
While these drawings show perfect polyhedra, in real mineral structures, these are rarely perfectly regular. For example, that the edges of the polyhedra are almost always of slightly different lengths and the central cation may be displaced from the geometric center. For the purposes of this discussion such deviations are not important. Such a view of crystal structures leads to a simplistic but nonetheless very useful concept of a silicate mineral, that is, a mineral is an arrangement of boxes in space (the coordination polyhedra), and we construct such a mineral by filling the boxes with appropriate cations.
In a simple structure there might be only two kinds of boxes, representing tetrahedra and octahedra, appropriately linked together. Thus, we can change the chemical composition of a mineral by replacing all or part of the cations in one type of box by another kind of cation, such that size and valence considerations are not violated. Two divalent cations which are not very different in ionic radius, magnesium and iron, for example, can readily substitute for each other in an octahedral site.[9]
Table 2.1: The most commonly observed coordination polyhedra for the common
elements in silicate structures in order of decreasing amount in the Earth’s crust, omiting which is the most abundant element.
Element C.N. Polyhedron Ionic Charge Ionic Radius (Å)
Slicon 4 Tetrahedron +4 0,26 Aluminum 4 6 Tetrahedron Tetrahedron +3 +3 0,39 0,54 Iron 6 6 Octahedron Octahedron +2 +3 0,78 0,65 Calcium 6 8 Octahedron cube +2 +2 1,00 1,12 Sodium 6 8 Octahedron cube +1 +1 1,02 1,10 Pottasium 6 8 Octahedron cube +1 +1 1,38 1,51 Magnesium 6 8 Octahedron cube +2 +2 0,72 0,89
Figure 2.1: Polyhedra and the corresponding atomic arrangement for the two principal
polyhedra of silicate mineral structures, i.e. octahedra (left) and tetrahedra (right). [9] 2.1.2. Classification of Clay Minerals
There are 4 types of clay minerals which are classified by their chemical formula; Caolinite, Smectide, Illite and Clorite.
2.1.2.1. Caolinit group
This group contains caolinit, dicit and nacrit. The general formula of the caolinit group is Al2O3·2SiO2·2H2O. There is no pure caolinit source in nature and generally they contain iron oxide, silica, silica types components. They are used as filler in ceramics paint, plastics and rubber and they are widely used in paper industry to product bright paper.
2.1.2.2. Illit group
These groups differ from smectite group clays by including potassium and can called as mica group. They are water included microscobic muscovit minerals and they are formation minerals which can be seperated to layers. The general formula of illit group is (K, H) Al2 (Si, Al)4 O10 (OH)2·xH2O. The stucture of this group is the same with slicate layered montmorillonite group. It can be used as filler material and in driling mud.
2.1.2.3. Clorit group
Clorit group clays have slim grain structure and green colour. This group clay includes a great deal of magnesium, Fe (II), Fe (III) and alumina. Clorit group minerals are generally known as fillosilicate group and they are not acceppted as one of clay group. This group has got a lot of members like amesite, nimite, dafnite, panantite and peninite. General formula of Clorit group is X4·6 Y4O10 (OH, O)8. In this formula, X shows Al, Fe, Li, Mg, Mn, Ni, Zn and rarely Cr elements, and Y shows Al, Si, B, Fe elements. They are not used in industry.[10,11]
2.1.2.4. Smectite group
The smectite minerals are classified according to the nature of the octahedral sheet (dioctahedral versus trioctahedral), by the chemistry of the layer and by the site of the charge (tetrahedral versus octahedral). The smectite minerals are very complex group, frequently having both octahedral and tetrahedral substitutions each contributing to the overall layer charge. The following formula are based on a layer of 0.33, with the value varying between approximately 0.2 and 0.6 and sodium is indicated as the interlayer
cation. There are three important dioctahedral smectites; two are aluminous and the third is iron rich. With a predominantly octahedral charge, the mineral is montmorillonite, Na0.33nH2O(Al1.67Mg0.33)Si4O10(OH)2 where, as before, the n indicates a variable amount of interlayer water coordinating (or not) the interlayer cation. The interlayer cation needs not be sodium, but this is the common occurrence. The term "montmorillonite" was frequently used as a group name for any swelling 2:1 clay mineral as well as the name of a specific mineral, clearly not a good situation. Presently smectite is the group name and montmorillonite is restricted to a mineral name belonging to that group. If the charge is predominantly tetrahedral and aluminous the mineral is beidellite with an ideal composition of Na0.33nH2O Al2(Al0.33Si3.67)O10(OH)2. Finally, if ferric iron substitutes for aluminum in the octahedral sites and the charge is tetrahedral, one has montmorillonite, Na0.33nH2O Fe2(Al0.33Si3.67)O10(OH)2. The trioctahedral equivalent of montmorillonite is the mineral hectorite, ideally Na0.33nH2O Mg3(Al0.33Si3.67)O10(OH)2. It should be noted that in contrast to montmorillonite, hectorites have lithium (1+) in some octahedral sites (not shown in the above formula) adding to the total layer charge. For saponite, aluminum substitutes for magnesium in the octahedral sites generating a positive contribution to the layer charge which reduces the negative contribution from the tetrahedral sites. The ideal formula without the aluminum substitution is Na0.33nH2O Mg3(Al0.33Si3.67)O10(OH)2. The interactions between adjacent smectite layers are not very strong and the interlayer material, hydrated cations, water, organics, are disordered, so that there is little coherence from one layer to the next. As a consequence, it is normally not possible to speak of a crystal of a smectite. There are some exceptions, saponite being one, where there is a greater degree of stacking regularity; it shows a degree of disorder. [9]
Montmorillonite is the mineral with the general formula of Na0,2 Ca0,1 Al2 Si4 O10 (OH)2 (H2O)10. Montmorillonite is a fine powder which has monoclinic-pyrismatic crystal structure, a colour from white to brown-green and yellow, average density of 2.35 g/cm3, molecular weight of 549.07 g/mol and hardness of 1.5–2 (Figure 2.2). Single montmorillonite crystals are quite fine, granulated and they got random outher lines. In general a montmorillonite crystal consists of 15–20 silicate units. This property
is so usefull for engineering projects. There are two different swelling types of montmorillonite according to expansion size of the basal space as crystallized and osmotic swelling. Crystillized swelling occurs when the water molecules enter in to the unit layers. First layer of the water molecules which are adsorbed occurs when they bind with hydrogen bonds to hexagonal oxygen atoms. Montmorillonitles whose cations are exchangable hydrates as Na+, Li+ can swell to 30–40 Å. Moreover, sometimes this swelling level increases up to hundred. This type ditance is called as osmotic swelling. Montmorillonits do not swell much when they got high valanced cations as exchangable cations.[12-16]
Figure 2.2: Shematic represantation of 2:1 clay mineral structure (red Al, small ones O,
light violet Ca, light purple Si).
The reason of this sittuation is that gravitational forces between silicate and cation layers are higher than ion hidration thurst force [17]. Montmorillonites enable polar or ionic organic molecules to penetrate between the layers. Adsorption of organical
mixtures causes to formation of organo-complex montmorillonites. Penetrating of big molecules in to layers of clay mineral could be determined by using XRD measurements. Montmorillonits have 2:1 type layered structure. Crystal like structure of the montmorillonite occures from, silicon-oxygen (Si-O) tetrahedral layer with (Al-O-OH) oktahedral layer which is between two Si-O layers. Silicon atoms are bonded with 4 oxygen atoms in (Si-O) layers. Oxygen atoms are placed regularly as one in centre of silicon atom and the other 4 atoms are on the corners of the tetrahedron (Figure 2.3). Layers are divided between every thirth neighbour tehrahedral layer structure from 4 oxygen atoms of tetrahedron layer. All of the fourth oxygen atom of the tetrahedron has condition as oriented to lower side of structure which can be seen in Figure 2.4 and they are at the same plane with the -OH groups of alumina octahedral layers. [18,19]
Figure 2.3: Structure of 2:1 phyllosilicates.
2.1.3. Cation Exchange Capacity
Clay minerals get ability of pulling some ions and push them back again. In this case ions could replace each other. In the tetrahedron layer of montmorilonite Si+4 with Al+3 and in the octahedron layer of montmorilonite Al+3 with Mg+2, Fe+2, Zn+2 and Li+1 can
replace with each other. In tetrahedron this cation exchange capacity is low, despite of this it is significiantly high in octahedron. At the end of cation exchange, positive and negative charges occure. Two layered clays have natural surfaces according to their electrical charges but three layered clays have charged surfaces. Positive charge deficiency can be overcome by bonding of Na+, K+, Li+ or Ca+2 ions to crystal cage from their water layer of unit area.[12]
Despite of these conditions units can give these cations back, naturally. The ions captured by clay minerals in exchangable position are called as “exchangable ions”. Because these ions are mostly cations and their ion exchange ability or cation echange capacity is higher then certain values, these ions show properties such as clay minerals grade of swelling, gelation etc. Cation exhange capacity is defined as (meq.) Na2O in 100 gr clay. This charge is not locally constant, but varies from layer to layer, and must be considered as an average value over the whole crystal. Layered silicates have two types of structure: tetrahedrally substituted and octahedrally substituted. In the case of tetrahedrally substituted layered silicates, the negative charge is located on the surface of silicate layers and, hence, the polymer matrices can interact more readily with these than with octahedrally substituted material. The exchangable cations in clay minerals are H+, Na+, K+, Ca+2 and Mg+2. The exworks shows us cation exchange capacity of montmorillonite is between 80–150 meq.[10]
The general cation exchange capacity of natural or synthetic clay minerals is between 50–200 meq/100 gr. Because of the cation exchange capacity is higher than 200, the forces between layers prevent seperation of clay layers. On the other hand, clay minerals, which cation exchange capacity lower from 50 meq/100 gr clay, could not seperate the clay layers. Due to these reasons, montmorillonite which cation exchange capacity between 50–150 meq/100 g clay, are used as swelling agent in NCs.[12]
2.1.4. Inter Layer Formation
Several phyllosilicate minerals, either naturally or as the result of chemical treatment, have molecular species inserted between the silicate layers. Water is the most common interlayer species in nature, and water is normally found in smectites, vermiculites and
hydrated halloysites. The quantity of interlayer water is a function of relative humidity and the type of interlayer cation, in the case of smectites and vermiculites. There is a great interest in the nature of the interface between water and silicate minerals. Much of the chemical activity in soils, sediments and porous rocks occurs at such an interface. Experimentally, it is very difficult to examine this interface because it is such a small part of the liquid-solid system. Hydrated smectites and vermiculites have water between all of the silicate layers and therefore the percentage of the sample which is interface is enormously larger than the interface between a grain of quartz in contact and liquid water. Another way to look at this is that the surface are of a quartz sand is probably much less than 1 m2/gram while a typical smectite has a surface area of as much as 800 m2/gram.[9]
The surfaces of clay minerals present a number of potential sites at which organic molecules could attach themselves. These sites include the exchangeable cations. The oxygen atoms can occupy the surface of the silicate layer and at the edge of these layers. On the other hand, hydrogen atoms take part of surface hydroxyl groups. So there is a wide variety of interaction possible between the heterogeneous clay surface and the different functionalities of organic materials. If any organic material existing on external particle surfaces, intercalation of layers can occur.[9]
One can categorize the types of organic-clay interactions based on the bonding mechanisms between the organic and the clay surface/inter layer (inorganic) cations. • Cationic bonding: These involve organic cations such as the alkyl ammonium cations or amines and carbonyl groups which have become protonated, depending on the pH. • Ion-dipole and coordination bonding: This is particularly common for organic molecules having a permanent dipole, e.g., acetone.
• Hydrogen bonding: The organic molecule can be either the donor or the acceptor or both, depending on the nature of the clay surface and the organic molecule.
• Tetrahedral tetrahedral (TT) bonding: Molecules such as benzene can interact via their valance electrons with, for example, Cu2+ interlayer cations. Hydrogen bonding and TT bonding are both examples of Lewis electrondonor/ electron-acceptor interactions.
2.2. Polyolefins
A polyolefin, whose equivalent term is polyalkene, is a polymer produced from a simple olefin (also called an alkene) as a monomer. Their main members are polyethylenes and polypropylenes. Industrial production of polyolefins cover low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (i-PP), and together with some copolymers. [22-24]
2.2.1. Polyethylenes
Polyethylene is classified into several different categories based mostly on its density and branching (Figure 2.4). The mechanical properties of PE depend significantly on variables such as the extent and the type of branching, the percent crystalinity, and the molecular weight.
The types of polyethylene mostly consumed are LDPE (low density PE), LLDPE (linear low density PE), HDPE (high density PE), HMWPE (high molecular weight poly ethylene), UHMWPE (ultra high molecular weight polyethylene), HDXLPE (high density cross-linked PE), PEX (cross-linked PE), MDPE (medium density PE), and VLDPE (very low density PE).
2.2.1.1. Low Density Polyethylene (LDPE)
LDPE is defined by a density range of 0.910 - 0.940 g/cm3. LDPE has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure as well. This results in a lower tensile strength and increased ductility. LDPE is created by free radical polymerization. The high degree of branches with long chains gives molten LDPE unique and desirable flow properties. LDPE is used for both rigid containers and plastic film applications such as plastic bags and film wrap.[28]
LDPE is produced by a free-radical initiated reaction using oxygen or other free radical initiators such as organic peroxides or azo compounds. Synthesis conditions are usually 250–300 °C outlet temperature, 3000 atm. pressure. Nominal reactor residence times are about 10–50 seconds.
Figure 2.4: Molecules of LDPE, LLDPE and HDPE
Heat of polymerization is about 800 KCal/gm, which must be removed during the short residence time available. Only a small part of this heat can be removed through the reactor walls because of their comparatively limited area and necessary thickness. In addition, the polymer tends to deposit on cool surfaces. In practice, heat is removed by recirculating excess cool monomer and the system operates essentially adiabatically. Therefore, production rates vary directly with the ethylene recirculation rate and the allowable temperature rises through the reactor. Heat balance limits conversion to 15– 20% on each pass. Reactors are of two general types, autoclaves and high pressure tubes. Each of these types produces slightly different polymers, primarily because of differing temperature profiles through the reactors. [25-27]
2.2.1.2. Linear Low Density Polyethylene (LLDPE)
LLDPE is defined by a density range of 0.915 - 0.925 g/cm3 is a substantially linear polymer, with 4-6 carbon containing short branches (short-chain alpha-olefins) with approximately, every 100 carbons on the main chain. LLDPE has higher tensile strength, higher impact and puncture resistance than LDPE. Lower thickness films can be blown compared to LDPE, with better environmental stress cracking resistance compared to LDPE but is not easy to process in packaging. Cable covering, toys, lids, buckets and containers, pipe are some of the products can be made by LDPE. While
LDPE
HDPE
other applications are available, LLDPE is used predominantly in film applications due to its toughness, flexibility, and relative transparency.[22,23]
Linear low density polyethylenes are made with transition metals catalysts/initiator under 100-130 °C and up to 20 atm.(Table 2.2) Butene-1 is the usual comonomer, but either hexene-1, octene-1, or 4 methylpenetene-1 is employed to give enhanced physical and optical properties with higher production cost.
Table 2.2: Operating Conditions of LLDPE Processes Conditions Slurry Fluidized
Temperature 80-120 80-120
Pressure (Mpa) 4-7 2-3
Residence Time (hours) 0,75-1,05 4-7
Convension/pass (%) 95 1-4
2.2.1.3. Other Polyolefins
i-PP and HDPE which are stereospecific polymers can be produced under the same method given above for LLDPE. In the slurry process, MgCl2 supported TiCl4 is used as catalyst and Al(C2H5)3 as co-catalyst in the n-heptane solution for both production, generally. Atactic PP (a-PP) is the by-product of i-PP production.
2.2.2. Properties of Polyolefins
In today's competitive market place, prime grade commercial polyethylenes must be both processable and uniform. We use the term “processability” to describe the ease or difficulty with which an olefin can be handled during its fabrication into film, molded items, pipe, etc. Polyethylene with good processability is one which possesses the properties necessary to make it easy to convert the polyethylene pellets or powders into the desired products. The main characteristics or properties which determine an olefin's processability are molecular structure, uniformity, and additive content. However, processability is a property which is a result of the basic properties mentioned above. These characteristics include hot-melt extensibility, sensitivity to pressure and
temperature, smoking and odor, product stability during withdraw, and flow rate (which is an operating condition). Uniformity is a characteristic of critical importance. It is obtained only through rigorously controlled synthesis, densification, stabilization, blending, and handling, so that lot-to-lot variation is minimized. The customers expect to be able to process polymers during extended runs with, at worst, minor adjustments to their machinery between lots of material. It’s also expected a polymer to be free of contamination, dirt, discolored material, and other foreign matter and to be of light, uniform color, unless pigmented. Polyethylene must also be uniformly granulated to flow through the customers' handling and feeding systems. This means accurate and uniform pellet size with freedom from excessive fines or oversize particles, and no strings of agglomerated pellets or streamers. The polymer must also be free of excessive moisture.
The main structural factors that determine PE properties are the degree of short and of long chain branching, the average MW and the polydispersity. One of the most important characteristics that determine in the highest degree the properties and the behavior of different grades of PE is their branching. Branches prevent the polymer chains from packing together regularly and closely and have a predominant effect on the density of PE. The density can be considered a first indication of the degree of branching: the lower the density the higher the degree of branching. The presence of branches interferes with the ability of the polymer to crystallize. The degree of crystallinity of LDPE is usually of the order of 55-70% compared with that of HDPE which is 75-90%.
Other properties depending on crystallinity, such as stiffness, hardness, tear strength, yield point, Young’s modulus in tension and chemical resistance, increase with increasing degree of crystallinity (HDPE) whereas permeability to liquids and gases, flexibility and toughness decrease under the same conditions
Since PE is crystalline nonpolar hydrocarbon polymer it has no solvents at room temperature and dissolution takes place only on heating in solvents of similar solubility parameter such as hydrocarbons and halogenated hydrocarbons. The higher the degree
of crystallinity results the higher the dissolution temperature. LDPE dissolves at 60°C compared to 80-90°C for high density, more crystalline polymers.
The effect of branching also depends on the size of side chain branches. While short branches have a predominant influence on the degree of crystallinity and therefore on the density of the polymer, long branches affect more pronouncedly the polydispersity. The side chains may be as long as the main chain and like it may have a wide distribution of lengths. The higher the MW of the resulting polymer the wider the MWD, as chain transfer reactions may occur as well on side chains. Such a polymer may be made up of short chains grafted onto short chains, long chains onto long chains and a vast range of intermediate cases. Long chain branches also affect the flow properties. Long branched molecules are more compact and tend to entangle less with other molecules resulting in lower solution and melt viscosities as compared with unbranched polymers.[29,30]
2.2.2.1. Mechanical Properties of Polyolefins
Another factor that influences the properties of the melt, as well as those properties that involve large deformations, is the weight-average MW. Ultimate tensile strength, tear strength, low temperature toughness, softening temperature, impact strength and environmental stress cracking increase as the MW increases; on the contrary, the fluidity of the melt and the coefficient of friction decrease.
2.2.2.2. Dielectric
The electrical insulating properties of polyethylenes are excellent. The dielectric con-stant increases linearly with increasing density. As it is a non-polar material, dielectric constant and the power factor are almost independent of temperature and frequency.
2.2.2.3. Density
Polymer density is a rough measure of crystallinity and, therefore, of the physical and optical properties that are dependent on the degree of crystallinity. The relationship between density and the various properties of the polymers is illustrated in Table 2.3 LDPEs and LLDPEs of the same density have somewhat dissimilar properties. This
difference is largely because LDPE, being free radical initiated, contains a range of both long and short side chains attached to the main polymer backbone.
Table 2.3: The relationship between density and the various properties of the polymers.
LLDPE, on the other hand, contains only short branch lengths, those of the comonomer. Although the degree of crystallization is nearly the same, the morphology of the crystal is dissimilar. LLDPE possesses many improved solid properties, such as strength, toughness, and draw-down, but LDPE in general is easier to process, is softer, and yields films with better optical properties.[28-30]
2.2.2.4. Melt Flow Index
The melt index (MI) is a rough measure of average molecular weight and melt viscosity. It indicates how readily molten polymer will flow in processing machinery.
Table 2.4: Changes in polymer properties with melt index.
Because the melt index is measured at a single temperature at low shear, and because melt viscosities are highly non-Newtonian, the melt index alone does not adequately predict how processable a given polymer will be under higher shear conditions in commercial processing equipment. However, the melt index is often an adequate discriminant within a given group of resins produced under substantially similar
conditions. The various physical properties of the polymers will generally vary as the melt index varies, as illustrated in Table 2.4.
2.3. Compatibilizer
Polymeric compatibilizers serve as their name indicates to make compatible the different kinds of materials such as multi component structures. Before discussing the compatibilization of polymer pairs in multi component structures, the compatibility of polymer blends and the compatibilizers will be described.
2.3.1. Compatibility and Compatibilizers
When blending two polymers, the resulting behavior falls into three categories (Table 2.5). Either they are miscible and compatible or immiscible and incompatible, or they behave somewhere in between these two extremes.
The requirements are similar polarity and structure, and the result is a single-phase mixture. The materials have different polarities and structures, and the result is a two-phase mixture with poor properties, an undesirable state.
Rarer still is immiscibility and compatibility at which a mixture's constituents have different properties, but show some interaction, because of reactive groups, surface active agents, or compatibilizers.
Immiscibility and compatibility are not necessarily bad. Actually, the entire technology of toughened polymers is based on this approach, because it synergistically combines the properties of completely different polymers to form a blend with properties superior to those of the individual blend components.[34,35] The theoretical explanation for why such cases are observed has been extensively treated in the literature.[36]
Figure 2.5: Schematic representation of an AB block copolymer compatibilizing a
blend of polymers A and B.
Compatibilizers that compatibilize two polymers, A and B, consist of two parts (Figure 2.5). One part interacts with polymer A and the other part with polymer B, but to do so effectively, they must be concentrated at the interface between the two polymers.
The result is a better dispersion of the polymer blend as shown in Figure 2.6. Infinite dispersion, however, is not necessarily desirable since a minimum particle diameter exists for each system below which there will be no synergistic improvement in properties (e.g., fracture toughness)
Figure 2.6: Schematic representation of a compatibilization reaction
2.3.2. Classification According to Properties of Compatibilizers
Compatibilizers can be classified as follows: non-reactive, reactive compatibilizers, and random, graft, and block copolymers.
Non-reactive compatibilizers, which compatibilize two polymers, A and B, consist of two parts: the first is soluble in polymer A, and the second is soluble in polymer B. The compatibilizer's effect is derived from their solubility. Therefore, the solubility parameters of both parts should be as close as possible to the solubility parameters of the polymer components in the polymer mixture.[37-39]
A block copolymer contains blocks of the polymer pairs. These blocks can be reactive or non-reactive polymers.[40]
In random copolymers, the components, the base polymer B and a comonomer A, are distributed randomly along the polymer chain. Random copolymers are usually produced in a high-pressure radical polymerization process.[41,42] Random copolymers only work well as compatibilizers when the comonomer A is reactive. In graft copolymers, either monomers or polymers are grafted onto each other. If only monomers are grafted to the backbone, the monomer should be reactive. An example is PP grafted with maleic anhydride. The exposure of the reactive monomers on the usually non-reactive base polymer backbone makes them more accessible to an attack by other polymers, transforming them into effective compatibilizers.
2.3.3. Works on IA/MMI/MAH Grafting Polyolefins in Recent Years
S.S.Pesetskii and his coworkers investigated grafting of itaconic acid on low density polyethylene in molten state via reactive extrusion many times in recently years. Pesetskii investigated firstly, initiator and stabilizator efficiency on grafting degree of LDPE with IA. It was shown that initiator solubility affects the grafting degree. The initiators which can be dissolve easily in LDPE, increases the grafting efficiency and the closer the thermodynamic affinity between the peroxide and the monomer, and decreases efficiency of grafting. The stabilizers (e.g.,1,4-dihydroxybenzene) with increased affinity toward the monomer reduce the grafting yield and inhibit crosslinking.[43,44] In another work, it was shown that thermomechanical and rheological properties of LDPE was changed. According to results, while unmodified PE exhibits two glass transition temperatures, modified PE with IA exhibits three glass transition temperatures and with increasing of grafting degree melting temperature increases 1-2 0C and melt flow rate (MFR) values decrease, it means that viscosity of polymer increases.[45] Pesetskii worked on thermal and photo oxidation of grafted LDPE and the functionalization of LDPE by grafting of itaconic acid to the macromolecules was found to accelerate its thermal and photo-oxidation in water.[46] Pesetskii made his last IA-g-LDPE work in presence of neutralizing agents. When the grafting takes place in the presence of neutralizing agents, the efficiency of the itaconic acid grafting onto macromolecules is found to increase. Neutralization of the grafted itaconic acid contributes to an increase in the mechanical and impact strengths of blends composed of functionalized low-density polyethylene and polyamide-6.[47]
M. Yazdani and his coworkers worked with monoesters of IA for grafting of PE and PP. Firstly, to improve the compatibility and properties of blends based on high-density
polyethylene (HDPE) and the ethylene-propylene copolymer (EPR), the
functionalization of both through grafting with an itaconic acid derivative, monomethyl itaconate (MMI), was investigated. The results show that the grafting reaction increases the toughness and elongation at break of all tested blends and they retained their strength and stiffness. Moreover, the grafted polymers behaved as nucleating agents, accelerating the HDPE crystallization.[48] In another work, Yazdani synthesized
functionalized polypropylene by radical melt grafting either with monomethyl itaconate and dimethyl itaconate to improve its compability of PP with PET. The use of PP grafted with MMI as compatibilizer resulted in even a better dispersion of PP as the minor phase increasing the components interface and there after to an improvement of the adhesion between the two phases. The noncompatibilized blend in this case also showed an even more pronounced two phase behavior as compared with PP/PET blends. The impact resistance of PET in noncompatibilized blend was hardly affected by incorporation of PP. However, when functionalized PP with either MMI or DMI was used as blend compatibilizers, there was an increase of the impact resistance of PET. This probably is due to spesific interactions and/or chemical reaction (transesterification) between the functional groups of the compatibilizer with the blend constituents resulting in a finer dispersion of the minor phase leading to improved interfacial adhesion.[49]
The first grafting reaction of LDPE in a solution medium was worked by Yu-Zhong Wang in recently year. The grafting reactions of MAH were carried out in microwave assisting system. The reaction of maleic anhydride (MAH) grafted onto low density polyethylene (LDPE) in xylene solvents in the presence of benzoyl peroxide (BPO) as an initiator by microwave irradiation has been investigated. The influence of reaction conditions such as initiator content, monomer content and irradiation time have been examined. IR spectra of PE and PE-g-MAH show that MAH is really grafted on the PE in a xylene solution by means of microwave. Moreover, the melting temperature of PE-g-MAH is lower than that of PE, but the melting enthalpy of PE-PE-g-MAH higher than that of PE. [50]
2.4. Polymer Nanocomposites
The structures and properties of the composite materials are greatly influenced by the component phase morphologies and interfacial properties. Nanocomposites are based on the same principle and are formed when phase mixing occurs at a nanometer dimensional scale. As a result, nanocomposites show superior properties over their micro counterparts or conventionally filled polymers.
2.4.1. Polymer Nanocomposite Synthesis Methods
Not all physical mixtures of polymer and silicate will form a nanocomposite. The compatibility between the two phases is important. Nanocomposites are synthesized from various polymers; nylon 6, polyimide, epoxy resin, polystyrene, polycaprolactone and acrylic. The exfoliated and homogeneous dispersion of the silicate layers, however, could be achieved only in few cases, such as polymers containing polar functional groups such as amides and imides. This is due to the fact that silicate layers of clay have polar hydroxy groups and are compatible with polymers containing polar functional groups.[48] Silicate clay layers are bound together by a layer of Na+ or K+ ions and are naturally hydrophilic.
Ion exchange reactions with cationic surfactants including primary, tertiary and quaternary ammonium ions render the normally hydrophilic silicate surface organophilic, which makes intercalation of many engineering polymers possible. The role of the alkyl ammonium cations in the organosilicates is to lower the surface energy of the inorganic host and improve the wetting characteristics and, therefore, miscibility with the polymer.[52]
Nanocomposites can be formed in one of three ways: • Melt blending synthesis.
• Solvent based synthesis. • In-situ polymerisation.
2.4.1.1. Melt Blending Synthesis
The melt blending process involves mixing the layered silicate under shear, with the polymer while heating the mixture above the softening point of the polymer. During this process, the polymer chains diffuse from the bulk polymer melt into the galleries between the silicate layers.
In some cases the polymer–silicate mixture can be extruded by using (a) static melt intercalation: by mixing and grinding dried powders of polymer and organic silicate in a pestle and mortar and then heating the mixture in vacuum, and (b) extrusion melt
intercalation: by extruding the mixture with twin screw extruder to produce a polymer nanocomposite from the polymer and modified clay.[54,71, 72]
2.4.1.2 Solvent Based Synthesis
The solvent based synthesis involves mixing a preformed polymer solution with clay. A polystyrene–clay hybrid can be prepared by mixing a polystyrene-toluene solution and silicate to yield a suspension and then evaporating the solvent. Polyimide–clay hybrids can be prepared by dissolving clay in dimethylacetamide (DMAC) and mixing with precursor solution of polyimide and then removing the solvent.[73]
2.4.1.3. In-situ Polymerisation
The clay/organoclay is dispersed in the monomer and the polymerisation reaction is carried out (Figure 2.7). Polystyrene clay nanocomposites can be prepared by the polymerisation of styrene in the presence of clay; chemical grafting of polystyrene onto montmorillonite interlayers have achieved by addition polymerisation reactions. Thermoset PNCs are prepared by using this method.[78]
Most thermoplastic polymer nanocomposites are produced by either of the first two methods.[74]
